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1 Morphology of Exogenous Blowholes in steel castings (ISBN 978-969-8674-10-6) Dr. Engr. Pervaiz Habibullah, Ph.D dr[email protected] Cell:092-307-4428121
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1
Morphology of Exogenous Blowholes in steel castings
(ISBN 978-969-8674-10-6)
Dr. Engr. Pervaiz Habibullah, Ph.D
Cell:092-307-4428121
2
Dedicated to:
His Excellency
Late S.S. Iqbal Hussain Ambassador,
Islamic Republic of Pakistan in Romania (1982-85)
3
Acknowledgements
Author is grateful to scientific supervisor of doctorate’s dissertation, Prof. Dr.
Engr. L. Sofroni, (Ex) Head: Foundry and Forge Dept., Faculty of Metallurgy (now
faculty of metallurgy and materials science Engg.), Polytechnic University of Bucharest,
Romania and all other members of Dept., for their timely cooperation in elaboration of
the doctorate thesis from which extracts are presented in this monograph. I started
research in foundry department at Polytechnic University Bucharest Romania under
erudite guidance of Prof. Dr. L. Sofroni and with a wonderful group of friends that helped
me immensely. They guided me from very beginning of my involvement upto the final of
my research work of doctorate. Prof. Dr. Sofroni had always been pushing and
encouraging me with innovative ideas and have constantly vetting and verifying all of
my experiments and conclusions. It has dramatically increased the value and accuracy of
my work. I was also blessed with great friends from heavy metallurgical industry of
Bucharest, IMGB (Heavy Machine Parts Manufacturing Corporation Bucharest
Romania) & Uzina “23 Aug”
Author is also specially thankful for Prof. Engr. Alexanderu Rau, Prof. Dr. Fl.
Opera, Prof. Dr. Engr. Yanchu M., Mrs. Prof. Dr. Engr S. Vacu, Prof. Dr. Engr. S.
Buzila, for their valuable suggestion and for Dr. Engr. Elly Cohn, Prof. Dr. Engr.
Cocolash, Sl. , Engr. Iliescu, Engr. Miliataru, Engr. Baran, Engr. Palcau, for their
assistance in conducting some of the casting experiments.
Warm thanks are for late Prof. Engr. D. Briscan, Late Mrs. Prof. Vera Sofroni,
who have helped the author morally in overcoming some of the tedious bottle necks faced
in elaboration of doctorate dissertation.
4
Morphology of Exogenous Blowholes in steel castings Preface
Creating things is never easy! Casting is difficult to get right and exact. The first moments of creation of new casting are an explosion of interacting events! The liq. metal attacks and is attacked by its environment, impurities and gases.
The surging and tumbling flow of melt absorbs clouds of bubbles and large quantity of oxides. The mould shocks with the vicious blast of heat, buckling, fizzing and abrupt release of vapour which flood through the liq. metal. During subsequent cooling to room temperature the solidifying casting strives to contract and is resisted by the mould. The mould suffers and may crush and crack. The casting also suffers being stretched as on the rack due to the inter residual stresses. This is the story of casting from melt to the finish product!! [J. Campbell]
Surprisingly very few books have been written directly tackling, in detail the defects produced in casting: Their causes, mechanism, morphology and prevention. This book (more correctly ‘monograph’) is an attempt to examine one of the most tedious defect mainly concerned with dissolution of gas in casting coming from the crude mould.
Long ago, I had established myself as student and researcher in field of metal casting when I was awarded title of Ph.D. in Foundry at UPB – Polytechnic University of Bucharest – Romania in 1985. The topic of my Ph.D. was ‘A contribution to the physico chemical mechanism of blowholes formation in steel casting by steel- mould interaction’ My dissertation was comprised of study on sources of the gas formation in sand mould and mechanism of blowhole formation & their prevention by imposition of vacuum (depression) in sand mould containing some bonding material. It was originally written in Romanian language. In the beginning of 2011, I got some time and translated the specific portions of my thesis in English, reset and presented it in the form of short book and incorporated certain changes, additions and improvements. While writing this monograph, I had only one goal in my mind: providing students, researchers and foundry men, with broader survey of the subject title in easy understandable manner. Every attempt has been made to present the material insofar, with logic approach in clear expression and with practical solutions. Author strongly hope that monograph will prove highly beneficial as a reference book for researchers and Engineers working in ferrous and non-ferrous foundries. Finally, I owe an enormous debt of gratitude, reserved for my erudite professor of doctorate dissertation (1975-78 & 1983-85) Prof. Dr. L. Sofroni, who taught me to research independently in the field of Foundry and Prof. Dr. Manzoor H Khan (who was my Professor of Foundry in B.Sc. Met. Engg. at (WP) UET, Lahore in 1969, (now V.C.) who instilled in me a love for this discipline. May, 2011 Dr. Pervaiz Habibullah
Ph.D. Foundry [email protected]
Cell: 0307-4428121
5
Morphology of Exogenous Blowholes in steel castings
Contents
Chap No. Title Page No.
Acknowledgment
Preface
1 Introduction
2 Sources of Gases in Sand Mould
2.1 Gases from the sand mould
2.1.1 Evaporation of water
2.1.1.1 Condensation zone
2.1.1.2 Thermal interaction and Expansion defect
2.1.1.3 Thermal transpiration
2.1.2 Burning of Organic materials
2.1.3 Distillation of organic substances
2.1.4 Decomposition of minerals
2.1.5 Dissociation of gases
2.1.6 Dilation of air (present in the Pores of Mould)
2.1.7 Special additives
2.2 Gases which are evolved by physico chemical reactions at liq.
steel silica mould interface
2.3 Factors which affect gas forming and evolution
2.4 Mould gas analysis
2.5 Gas evolution test
3 Mechanisms of formation of Exogenous Blowholes
3.1 Energetic Interaction of Gas Bubble and liq. Metal
3.1.1 Kinematics of penetration of bubble in liq. metal
3.2 Appearance of Blowholes in castings
3.3 Nucleation of gas porosity
3.3.1 Homogeneous Nucleation
6
3.3.2 Heterogeneous Nucleation
3.4 Micro porosity model
4 Influence of Casting conditions on Blowhole Formation
4.1 Nature of Steel
4.1.1 Viscosity of Liq. alloy
4.1.2 Surface tension
4.1.3 Angle of wetting
4.1.4 Density
4.1.5 Chemical Composition of Steel
4.1.6 Thermo physical properties of alloys
4.2 Thermo physical properties of Mould and Cores
4.2.1 Organic and inorganic Binding Materials
4.2.2 Capacity of Filtration of gas (Permeability)
4.2.2.1 Mechanisms of gas Filtration to the Mould wall
4.2.3 Resistance to high temperature (high refractoriness, high
thermal stability)
4.2.4 Mould washes
4.2.5 Mould temperature
4.2.6 Coefficient of Heat accumulation
4.2.7 Effect of Condensation zone
4.3 Pouring conditions
4.3.1 Pouring temperature
4.3.2 Reynold Numbers
4.3.3 Sprue system
4.3.4 Velocity of pouring, rate of pouring
4.3.5 Length of flow of liq. metal
4.4 Pressure on the column of liq. metal
4.4.1 Metallostatic pressure
4.4.2 External pressure
4.5 Castign thickness
7
4.6 Height of liq. column
4.7 Periphery of casting
4.8 Inclination of casting
4.9 Vel. of solidification
5 Summary, Prevention and Conclusion
Photo No. Plates Page
1 Different stages in formation of a complete bubble in mould
cavity, filled with the molten metal
2 Sand sample from superficial layer of mould containing
mixture of silica fayalite and iron particles
3 Back scattered electron (BSE) image of the burn-in sample.
Album
Microporosity
Surface blowhole 1000x
Non metallic inclusion in blowhole, 8000x
Non-metallic inclusion in blowhole, 25000x
Interface of blowhole and metal, 8000x
Hydrogen blowhole in grey Iron, 8000x
Nitrogen fissure (from resin) in cast Iron, 8000x
CO blister (formed by reduction of MnS)
Macro shrinkage
Macro void
Dendrites, arm inside pinhole
Heavy oxide inclusions and Segregation of gas & slag
inclusions at some areas, in Steel casting. 100x Unetched
Shrinkage porosity, 50 x unetched
8
Chap 1 Introduction
Scope
The formation of blowholes in steel casting is considered the most controversial
topic of foundry. Wide difference of opinion exists among researchers in explaining the
mechanism of formation of gaseous porosity by steel mould reactions in casting.
Fundamental researches carried out and theoretical considerations have cleared to some
extent the concept of formation of this defect. Presently profound researches are being
carried out by eminent researchers at mondial level, to clarify the process of formation of
blowholes, filtration of gases through the mould and development of gas pressure in the
mould and many other related phenomena.
Gas porosity in casting is due to the bubbles being trapped during melting,
pouring and solidification. Porosity sources include entrapped air during filling gases
from unvented cores and moulds, reactions at the mould wall, dissolved gases from
melting and dross of slag are different crucial sources of blowhole formation.
Types and classification of blowholes
A dictionary* of foundry defines a blowhole in casting as “a hole in casting which
is usually round or oval, exists in small volume, single or in group in different parts of
casting”. This is the most prevalent casting defect which appears by evolution of gas at
metal-mould interface:
By decomposition of some materials added in moulding sand as special additives
to develop particular properties (water, volatile substances or those substances
which decompose on heating)
From liq. metal, the gases evolving from liq. metal during melting, casting and
solidification.
Considering the origin of gases, blowholes, can thus be widely grouped into two
categories as Exogenous and Indigenous.
1. Exogenous blowholes
Formed by gases produced by the mould or at metal mould interface.
*Vocabulary of foundry practice – Wdawnictwynukowo-techniczne Warszawa, 1963
9
2. Indigenous blowholes
Produced by reactions of atmospheric gases dissolved (usually oxygen) within the
vol. of liq. metal taking place during melting, casting and solidification.
3. Pin Holes
Pinholes form when gas dissolved in the liquid metal becomes less soluble during
solidification. The formation of small bubble is due to reduced solubility. The gas may be
from dissolved gases in the melting and metal handling procedure or the result of
exposure to gas evolved from the mould, core or coating. Wet coatings or chemical
binders formulations result in reactions with solidifying metal causing porosity at the
surface. Dirty or contaminated chills, poor sand binder mixing practices, failure to add
iron oxide in phenolic urethane binder systems, or condensation in the mould after
closing, are all well known causes of pinhole porosity.
To recognize pinholes and differentiate them from blowholes, their cross section
may be checked. Pinholes always has triangular cross section and are pointed towards
one end while blowholes are round in shape and their surface are not rough.
To avoid pinholes in steel, hydrogen contents in casting should remain under
6ppm or 0.0006%. Porosity occurs in the range of 9-13ppm. Pinhole porosity is always
present at levels in excess of 13 ppm. Hydrogen is picked up during melting. Hydrogen
creates problems in steel casting, in alloys containing nickel specially low temperature or
high strength grades. Hydrogen is responsible for loss of ductility and tensile strength by
hydrogen embrittlement. Hydrogen is thought to be responsible for wormhole like porosity
that originates at the surface and extends towards thermal centre of casting.
Pinholes are more prevalent in thinner casting with 10 to 30mm (0.4 to 1.2in) wall
thickness. Higher moisture contents in moulding sands and higher relative humidity gives
more pinholes. Pinholes are typically small, 1 to 2 mm (0.04 to 0.08in) in diameter.
Moisture on chills, inadequately dried coatings, improperly mixed sand can all cause
pinhole formation.
10
Approximate %age of rejection of casting due to blowhole formation*
Approximate %age of rejection of casting due to blowhole formation
Characteristics of the blowhole formed. Source of gas
Cast iron
Steel Non ferrous alloys
Gases dissolved in liquid metal
15 30 30 Blowholes formed are uniformly distributed in casting
Gases which are formed in liq. metal by intensive reaction.
5
15
20
Micro porosity and pores are observed in casting. These are usually sub cutaneous. If reactions are energetic, the bubble of gas can tear the hard skin and surface blowholes are formed.
Gases (air present in the pores of the mould surface)
20 15 15 Macro porosity of different types (localized or dispersed) is observed in this case.
Gases which are formed by late reactions by chemical interaction of metal and moulding mixture and cores at the interface of metal mould.
5
10
15
Internal (closed or semi closed) micro porosity is observed. Gases coming from the mould wall forms superficial blowholes of reticular shape.
Gases evolved by the substances present in the mould material by intense heat of liq. metal
50
25
10
Small, medium and large blowholes within the metal in groups localize at different locations in casting is observed.
Other gases 5 5 10 In this case size of the blowholes varies as small, medium and large.
* data collected from a large auto mobile industry.
11
Chap 2
Sources of gases in sand mould
[3-5] [11] [13] [29] [33] [71] [85]
When liquid steel flows on the mould surface, gases are evolved fiercely from the
surface of the mould, by thermal decomposition of volatile material present in the mould.
If pressure of the gas evolved is more than that of imposed on the mould surface by liquid
steel, the gases penetrate in liq. steel and form exogenous blowholes. The complex
physico chemical processes which play an important role in abrupt evolution of gases on
heating, are:
1. Evaporation of water and other volatile substances
2. Burning of organic materials present in the mould in the presence of
oxygen
3. Decomposition of minerals present in the moulding sand (e.g. carbonates
etc.)
4. Dissociation of gases produced by bonding material (CH4,NH3 etc.)
5. Dilation of air present in the pores of mould
6. Decomposition of organic substances (hydrocarbons, resins, oils etc.)
7. The coal added in the moulding sand swells, and an account of its large
expansion, is driven into the pores of the sand; the plastic phase of pore
addition appears to plasticize the binder temporarily.
8. Physico chemical reactions taking place at alloy-mould interface.
FeO + C = Fe + CO
FeO + 2H = Fe + H2O
Mg + H2O = MgO + H2
2Al + 3H2O = Al2O3 + 3H2
Ti+2H2O = TiO2 + 2H2
MgS + H2O = MgO + H2O
12
2.1 Gases coming from sand mould
2.1.1 Evaporation of water
Process of evaporation of water reaches to max. value when liq. steel comes in
contact with surface of green sand mould. Water is the principal constitute which
evaporates. Even in so called dry binder systems there is usually enough water to
constitute a major contribution to the total volume of gases which are liberated. Much of
the water is decomposed to hydrogen as is seen in the high hydrogen contents of analyzed
mould gases (Scott and Bates, 1975). Water exists in the moulding mixture in three
forms:
a. Chemically bound water
b. Physico chemical bound water and
c. Physico mechanical water.
Chemical bound water is constituted water, water of crystallization and zeolitic
water. Physico chemical water is colloidal and adsorbed water at capillaries while
physico mechanical water is the free water. Physico mechanical water bound is due to the
action of capillary forces by which water is being held in capillaries and due to the forces
appearing by the mixing and depend upon surface tension of liq. and pressure existing in
capillaries. Both of these pressures depend on mould surface which come in contact with
liq. steel (see table 2.1)
Process of evaporation of water (colloidal, of adsorption, in capillaries) is quite
different from free water present in the moulding material. Water present in capillaries
evolves in different ways than water held by chemical bound. Therefore, vapour pressure
in capillaries, on concave water meniscus is large, while on convex it is little, as
compared to the plain surface. An other factor which influences on mechanism and
velocity of evaporation of water from capillaries is thin film of water which forms on the
mould surface. It is different from other films (films of other materials added in the
moulding material such as, oils, resins and gases).
13
Table 2.1
Classification of water in sand mixture
Bond
Type
Form in which gas is produced
Temperature of
ejection of water oC
Specific density kg/dm3
Chemical
water
constituted water of
crystallization
Ionic Molecular
300-1300
200-500
2.3
2.4
Physico Mechanical
Adsorbed
Ionic & molecular
300-500
1.25
Free
Hygroscopic in capillaries of sand mould
Molecular Molecular
110
105
1.15
14
Fig. 2.1 Curves showing thermal analysis of (A)
kalonite (B) montimorilonite
Fig. 2.2 Formation of condensation zone I) H2O – H2O medium II) H2-H2O medium III) H2O – H2O medium
Condensation zone
Distance from cope surface
15
Water in crystals of minerals present in silica sand used for sand mould,
composed at high temperature and a strong source of blowhole formation. It is held in the
minerals in different bonds.
Process of evaporation of water is some time “endothermic”. Fig. 2.1 represent
thermal analysis of kaolinite and montimorilonite m[Mg3{Si4O10} (OH)2]. p[(Al,Fe,…)2
{Si4O10} {OH2}] nH2O. Their temperature decreases on heating. The thermic curve of
montimorilonite illustrates their endothermic peaks first at 150-180oC, corresponding to
evaporation of water of crystallization, nH2O, second at 500-700oC, evaporation of
constituted water (hydroxide in the veins in crystal) and third 800-900oC (separation of
hydroxide linked with magnesium item and also hydroxides from tetrahydrals, SiO4)
kalonite Al2 Si2O5 (OH)4, kalonite loses water of constituent at 550-610oC. Beside the
base minerals of moulding material, natural inorganic binders (kalonite of clay,
montimorilonite of bentonite), water is also found in impurities present in moulding
material. Some of these impurities which are found in large quantity (e.g. muscovite )
other have large capacity of evolution of gases (e.g. calcite, 1 kg calcium carbonate
evolve 0.44 kg or 22400 cm3 CO2 (for further details also see the table 2.2).
2.1.1.1 Condensation zone
When the liq. iron fills the last part of green sand mould, the pressure pulse
which accompanies the final instant of filling causes the metal to increase its local rate of
heat transfer to the mould. The enhanced heat transfer causes the water in the mould to
evaporate explosively. The high local pressures which are caused result in deeper sand
penetration.
Considering the fig. 2.3 some of the transformation zones which are distinctive are as
follows:
1. The dry zone, where the temperature is high and all moisture has been
evaporated from the binder.
2. The vapour transport zone, essentially at a uniform temperature of 100oC, and
at a roughly constant content of water, in which steam is migrating.
16
Table 2.2
Temperature at which some of the minerals present in the sand are decomposed and produce gas
Mineral
Chemical formula
Temp at which
gas has generated (K)
Gas or vapour emitted
Quantity of gas generated in 1 kg of mineral
(kg) Mica
Muscovite
K2O. 3H2O 6 SiO2
x 2H2O
413-473
873-1133
3H2O
2H2O
0.104
0.070
Mag carbonate MgCO3. 3H2O 673-723 2H2O 0.130
Calcite CaCO3 1133-1283 CO2 0.44
Magnesite MgCO3 933-983 CO2 0.52
Dolomite CaCO3. MgCO3 1063
1213
CO2
CO2
0.24
0.24
Siderite FeCO3 858 CO2 0.38
Glauconite K,Mg(Fe.Al)3
SiO4 x 3H2O
773-1073 3H2O 0.13-0.16
Ferrous mineral
(hydrate)
nFe2O3.mH2O 398-573 mH2O 18m/160n+18m
Al hydro oxide nAl2O3.mH2O 523-793 mH2O
18m/102n+18m
17
Fig. 2.3 The structure of the heated surface of a greensand mould against a steel casting and the forms of silica (after Sosman, 1927) with solid lines denoting stable states, and dotted lines unstable states.
18
3. The condensation zone, where the steam re condenses. This zone was for many
years the subject of some controversy as to whether it was a narrow zone or
whether it was better defined as a front. The definitive theoretical model by
Kubo and Pehlke (1986) has provided a answer where direct measurement has
proved difficult; it is in fact a ‘zone’ [69].
4. The external zone where the temperature and water content are still
unchanged.
According to J. Campbell [69] evaporation front at 1s has traveled 1 mm, at 100s
has traveled 10mm, and requires 10000s (nearly three hours!) to travel 100mm. It is clear
that the same is true for aluminium, as well as steel. Measurements of the thermal
conductivity of various moulding sands by Yan et al. (1989) have confirmed that the
apparent thermal conductivity of the moisture-condensation zone is about three or four
times as great as that of the dry sand zone.
Moisture vaporizes not only at the evaporation front, but also in the transportation
and condensation zones. The pressure of water vapour at the evaporation front will only
be slightly above atmospheric pressure in a normal greensand mould. Kubo and Pehlke
(1986) confirm that gas in the dry sand and transportation zones consists of nearly 100
percent water vapour! In the condensation zone the percentage of air increases, until it
reaches 100 percent air in the external zone.
2.1.1.2 Thermal interaction and expansion defects
Water present in green sand mould from condensation zone. The water evaporated
from the mould surface, filters through the porous mould and collects on the upper or
lower side of mould, forming condensation zone containing more water (approximately
15-20%) (Fig. 2.2). Inner plain portion of the cope dries and dilates due to the heat
radiation coming from red hot liq. steel surface in the mould cavity. Dried portion of the
mould, due to dilation cannot support the weak ‘condensation zone’ rich in moisture and
due to loose adherence ruptures or drops causing some defects like rattail, buckle & scabs
etc.
19
2.1.1.3 Thermal transpiration
William (1970) described an experiment which demonstrated the effect of heated
gases diffusing away from the source of heat and allowing cooler gases to diffuse the up
temperature gradient. In this way oxygen from the air can arrive continuously at the
casting to oxidize the surface to a greater degree as compared to that, it would have been
expected normally. William took a sample of clay of 50mm length and 25mm dia in a
sand sampling tube. When one end was heated to 1000oC and the other was at the room
temp., he measured a pressure difference of 100mmHg if one end was closed or a flow
rate of 20mm /min if both ends were opened. Thermal transpiration is seen to be >1% of
the rate of vapour transport. However, thermal transpiration does seen to be a small
contribution to gas flow in the mould.
2.1.2 Burning of organic materials
The moulding mixture and cores are made from organic materials which, in the
presence of oxygen of the air, water vapour and CO2 may burn. Oxygen from the air may
penetrate in burning zone from the pores of the mould and cores and beside this air is
present in the pores of mould surface, even, before pouring of liq. metal.
Process of burning :
- incomplete burning 2C + O2 = 2CO……………. (2.1)
- Complete burning C = O = CO2…………….. (2.2)
- Final burning 2CO + O = 2CO2………….. (2.3)
- Dissociation 2CO = C + CO2………….. (2.4)
- CO2 gas and H2O may react with carbon
C + CO2 = 2CO ………… (2.5)
At 810 oC C + H2O = CO + H2…….. (2.6)
C + 2H2O = CO2 + 2H2…… (2.7)
Reactions 2.1 and 2.2 may take place at any temperature, reaction 2.3 occurs at
high temperature and 2.4 above 700oC. Reactions 2.1 to 2.3 are strong endothermic and
take place at high speed even at low temperature. Oxide of carbon forming CO2 creates
neutral atmosphere in the mould cavity. At 3% carbon, in form of coal or organic
products, in moulding material, oxygen present in pores of mould surface, is in a
sufficient quantity to oxidize and evolve 0.5 – 0.6 cm3 of oxide of carbon from 1cm3 of
20
volume of moulding mixture. This amount of gas is 5-10 times less than total volume of
gas evolved from moulding material in first 5-10 min. after filling of mould. This is due
to the fact that sources of oxygen are limited and cannot display intense processes of
burning of organic material in mould cavity. Filtration of air from pores of mould, after
filling the mould, introduces, in these moments, a strong current of gas inverse to the
atmosphere and content of air present in the pore is consumed in short interval of time.
In the mould cavity the gases from out gasing of mould may contain a number of
potentially flammable or explosive gases. These include a number of vapour such as
hydrocarbons and other organics such as alcohols and a number of reaction products such
as hydrogen, methane and CO.
2.1.3 Distillation of organic substances
Distillation of organic substances starts on heating and on gasification of organic
substances, in absence of air. In these conditions continuous and successive change in
structures of the products take place: At the beginning those fractions of organic
materials are affected which are most volatile (ethers) and on the increase of temperature
less volatile fractions are gasified (oils, hydrocarbons, resins) and finally coke soot is
distilled. Then, vapour of water and oils saturated hydrocarbons, hydrogen and some
quantity of carbon monoxide and CO2 and also liq. constituents are distilled. Products,
resulting from distillation do not burn due to the absence of oxygen. In process of
filtration of the products, through pores of the mould, some quantity of products
condenses and rest are evolved to the atmosphere.
21
2.1.4 Decomposition of minerals
Impurities present in the silica sand used for making the mould decompose and
evolve gases and vapour. Mineral which are found in silica sand are muscovite, calcite,
magnesium carbonate, siderite and others. Table 2.2 gives minerals found in silica sand and
clay, which can evolve gas or vapour on heating.
2.1.5 Dissociation of gases
Gas evolving from the mould and core on heating, may react with each other and with
ingredients of molten alloy. Some of these reactions take place with increase in volume (e.g.
dissociation of CH4).
CH4 = C + 2H2 – Q
CnH2n+2 = nC+ (n+1) H2
On dissociation of hydrocarbons process of carborization takes place at high
temperature and brings about separation of carbon which collects in the form black soot in
pores of mould.
When resins are present in the moulding material, NH3 is formed which decomposes
in atomic N and H2. These two gases are considered important sources of blowhole
formation.
In the case of phenolic urethane and similar binders, the thermal breakdown of the
binder assists the formation of a good surface finish to cast irons and other metals. However,
because of an excess of carbonaceous gases which are evolved, and which appear to compete
their breakdown at the surface of the advancing front of liquid metal, the actual advancing
meniscus becomes coated with a graphite film. For steel, the film dissolves quickly and
causes relatively little problems. For cast iron the film dissolves hardly at all, because the
temperature is lower, and the metal is already nearly saturated with carbon and can accept
little more. Thus the film is effectively permanent.
2.1.6 Dilation of air (present in pores of mould)
One of the source of gas porosity in casting is failure to exclude and eliminate all the
air from the mould cavity. In case if permeability of mould is too little, air present in mould
surface, play an important role in formation of exogenous blowhole. Depending upon the
porosity of mould, vol. of air in the mould is approximately 20-35% of the total vol. of
moulding material. On heating of mould, the vol. of air present in the pores will increase due
to dilation and it will enter in liq. metal in short period after pouring and filling of mould.
22
Table 2.3
Role of different processes in formation of gases ejected in mould (% with respect to total quantity of gas)
Process of evaluation of gases
Green sand mould Dry sand mould Moulding sand with mollasses
Evaporation of water from moulding sand mixture
50-60%
8-12%
3-5%
Burning of organic materials
2-3 5-8 2-10
Distillation of organic materials
30-40 45-65 80-90
Ejection of water from structure of minerals
0.5-8 15-25 0.5-2.0
Decomposition of mineral (impurities)
1-5 1-5 1-5
Dilation and dislocation of air
<0.5 <1 <0.5
Dissociation of gases
6-10 8-12 10-15
23
Casting temperature oC Fig. 2.4 Vol. of gas as a function of variation in heating temperatures of different binding materials 1. formaldehyde resins 2. sulphatic layee 3. coal tar 4. coal tar PSI 100
Time after pouring (min) Fig. 2.5 Effect of gaseous atmosphere in the mould cavity on vol. of gas.
of g
as
air
24
Table 2.4
Temperature of decomposition and formation from gas of different additives used in moulding sand
Additive
Approx. temp. of decomposition and formation of gas oC
Capacity of
formation of gas, cm3/g
Water 100 550
Urea formaldehyde resin 280-300 410
Dextrene 330-380 850
Molasses 380-420 540
Sulphatic layee 400-520 500
Bentonite (chemically bound water ) 200-420 20-100
Linseed oil 420-480 500
Phenol formaldehyde resin 650-750 460
Coal powder -- 200-600
Kaolin 600-900 10-50
25
But in majority of cases, for mould and cores made of silica sand, role of air
present in the core and in the mould, is insignificant as compared to the gas evolved from
the mould because vol of air, in core, at 500-700oC is little as compared to volume of the
mould gas formed in the mould cavity. Different processes which play an important role in
the formation of gas in mould and (percentage from total quantity of gas) is given table 2.3.
2.1.7 Special additives
Special additives present in moulding material decompos and produce gases.
Temperature at which different additives generate gas and quantity of the gas evolved
from the respective additive, is given in the table 2.4 and is illustrated in curves of fig.
2.4. Water vapour are formed at 100oC and 550cm3 vapour are formed per gram of water.
Kaolin is one of the most common additive which decomposes and evolves gas at high
temperature (600-900oC) but lowest volume. (10-50cm3 / gm)
2.2 Gases which are evolved by physico chemical reactions at
liq. steel-silica mould interface
Some gases are also evolved as the result of physico-chemical reactions which
take place in liq. steel, slag, different non metallic inclusions, moulding material,
refractory of the furnace used for melting of steel and atmosphere. Chemical reactions
which form gases start just from melting of steel in the furnace and continue upto the
complete solidification of casting in the mould. An active component produced during
melting of carbon steel is FeO, which participate in many chemical reactions. Liq. steel is
therefore deoxidized before pouring it in the mould cavity. During melting, casting and
solidification in the mould cavity, carbon from the cast steel reacts with FeO
FeO+ C = Fe+CO
CO produced form bubbles creating “boiling” during melting in the furnace and it
also continue upto pouring and filling of mould. On rapid cooling of casting and
decrease in solubility of gases in liq. steel, surplus gas is evolved but some of its bubbles
may remain in the solidified casting and are uniformly or randomly distributed
throughout the casting. FeO may also react with hydrogen and it is possible that hydrogen
may reduce FeO.
FeO + 2H = Fe + H2O
26
Vapour of water formed in such reaction do not have high speed of evaporation
and may form gaseous inclusion independently. Its size is further increased by diffusion.
Similar type of reaction may be possible in FeO and cementite (Fe3C)
FeO + Fe3C = 4Fe+CO
According to A.A. Gorshcov, such reactions provoke blowhole of large size. As
the metal temperature decreases, there is an increase in the amount of ladle slag that
forms on the surface of cast iron and in the amount of MnS that precipitates from the
melt. MnS, because it is lighter than cast iron floats to the surface where they mix with
the ladle slag, making slag more fluid. As a result, there is a greater chance for the slag to
be introduce in the mould to react with the graphite. This results in CO gas which interm
forms blowholes. Sources of forming gases are also the reactions that take place at steel
mould interface, between some ingredients of moulding material and liq. steel.
Fe + H2O = FeO + H2
Hydrogen formed dissolves partially in the liq. steel. FeO formed may dissolve in
the superficial layer of mould cavity during solidification and may react with the carbon
of liq. steel to form CO. Water vapour may enter in chemical reactions with Mg, Al, Ti
which are used in ferrous alloys as deoxidizer
Mg + H2O = MgO + H2
2Al + 3H2O = Al2O3 + 3H2
Ti + 2H2O = TiO2 + 2H2
Some researchers have indicated to possibility formation of monoxide of silicon
(SiO) at steel mould interface which is a product of interaction of SiO2 with organic
radical CnH22m evolved by decomposition of additives.
nSiO2 + CnH2m = nSiO + nCO + mH2
SiO2 may also be reduced by carbon from liq. steel, from moulding material and
mould washes and also some alloying elements. SiO gas forms bubbles in liq. steel but on
decrease of temperature dissociates (2SiO = SiO2 + Si) Si pure produced dissolves in liq.
steel while tiny SiO2 particle depicts on the surface of bubble and the bubble floats
carrying this small particle till it is captured by the solidifying dendrites forming
blowhole. In some cases particles of SiO2 are found on surface of blowhole. In case of
27
SG iron, the sulphide of magnesium may react with water.
MgS + H2O = MgO + H2S
H2S forms bubbles which cannot be ejected out of the casting due to rapid
solidification and form blowholes of particular type which are oriented and are observed
at the surface of casting. However SG iron cast in the dry mould do not show such defect.
In case of vermicular cast iron, titanium and iron oxidizes and forms ilmenite (Fe.TiO3).
Due to the transient nature of ilimenite radical, it is rapidly attacked by carbon present in
the molten alloy.
FeO + TiO3 = Fe.TiO3
Fe.TiO3 + 4C = Fe + TiC + 3CO
Bubble of CO having tiny inclusion of TiC raises through the volume of liq. alloy
and is caught by solidifying front forming blowholes. It has been confirmed by observing
the surface of blowholes by electron microscope and X-ray micro analyzer.
Iron composition can influence gas defects which indicates the importance
of chemical analysis of alloys. High levels of manganese and sulfur increase the amount
of slag and manganese sulfide. By keeping manganese and sulfur contents to 0.65% and
0.12% maximum, respectively, the chances of getting into this problem can be reduced.
Carbon monoxide blowhole defects in ductile iron exhibit a spherical or tear drop
shape. They are also the result of an oxide-graphite reaction. The oxide, which is a
combination of ladle slag and treatment dross, is often found to cause this. Good iron
cleaning practices and properly designed gating systems help to minimize this problem.
Aluminum levels as low as 0.005 in gray iron and 0.01% in ductile iron have been
found to encourage dissociation of water vapor, thus, increasing the hydrogen content of
the metal. Aluminum sources include steel scrap and ferroalloys used in iron treatment.
Excessive moisture in the mold, coatings, furnace or ladle refractories and alloys are all
sources of hydrogen. The magnesium used in ductile iron can encourage the metal to
absorb more hydrogen causing a source of porosity.
2.3 Factors which affect gas forming and evolution
Following factors play important role in gas generation from the sand mould:
1. Casting temperature
2. Low thermal conductivity of mould
28
Table 2.5
Chemical composition of the gases produced from moulding material *
Moulding material, containing:
Gas bakelite powder (%) Sulphatic layee (%)
H2S + CO2 0.2-1.0 0.4-1.3
CnH2n Upto 0.50 1.0-1.5
O2 0.4-1.0 0.3-0.9
CO 29.3-30.4 27.75-29.70
N2 3.96-8.66 4.09-6.57
CnH2n+2 5.34-6.83 10.96-14.06
H2 53.41-60.06 48.03-50.3
H2O 18 20
* Gas was taken from the mould at 40-100 seconds after pouring liq. metal in the
mould, at 1280-1300 oC
29
Table 2.6
Content of H2S. SO2 and HCN in the gas evolved by pyrolysis of moulding mixture [11]
Sample A Sample B
Gas Temp Scavenging gas Scavenging gas
Argon Air Argon Air
H2S
mg/kg
at:
400oC
700oC
1000oC
124
187
196
97
152
168
4
33
63
2
26
60
SO2
mg/kg
at:
400oC
700oC
1000oC
2.5
10.0
17.0
5.6
15.3
17.8
0.4
0.5
2.6
0.3
1.3
5.7
HCN
mg/kg
at:
400oC
700oC
1000oC
23
24
39
6
9
11
22
23
34
45
46
55
30
Table 2.9 Analysis of mould gas from green sand mould and sand mould made from
phenolic urethane, while casting steel [69].
Gas Green sand mould
(after Chechulin, 1965)
Sand mould with phenolic urethane
( after Bates and Monroe, 1981) O2 -- 1%
N -- 15
H 45% 45
CO 50 20
CO2 5 12
CxHy 0 12
31
3. Nature of atmosphere of mould cavity it affects chemical composition and volume of
gas formed (for ref. see fig. 2.5)
4. Quantity and quality of special additives
5. Physico mechanical state of mould surface, with which liq. steel comes in contact i.e.
permeability, grade of ramming etc.
2.4 Mould gas analysis
For determining the quantity of gas formed in the mould cavity, it is essential to know the
velocity of gas formation. Velocity of formation of mould gas is expressed in three modes:
a) per unit volume or on gravity of moulding material
b) per unit volume or on gravity of special additives
c) per unit area of surface of contact of liq. steel with mould surface.
According to V.K Dvorovoi, 1cm2 of green sand mould, on interface of liq. steel- mould,
in first three min. after pouring produces 30-40 cm3 of gas (it is 100 time greater than the content
of gas in liq. steel and 20 time greater than the max. limit of solubility of gas in liq. steel). The
chemical composition of gas taken from the mould made from baklite powder and sulphatic layee
is given in table 2.5. Content of H2S, SO2 and HCN in the gas evolved by the pyrolysis of
moulding material while using scavenging gas argon and air is shown in table 2.6. Analysis of
mould gas from greensand mould and mould made from phenolic urethane is given in table 2.7
[55]
Analysis of mould gas from green sand mould made from phenolic urethane is
given in table 2.9. In the case of steel being cast into green sand mould, the mould gas
mixture has been found to contain upto 50% H2. This increase in H2 content is almost
linear with respect to percentage water in sand binder / dry sand mould have practically
no hydrogen. The other changes brought about by increase moisture in the sand were a
decrease in O2, and increase in CO / CO2 ratio, and the appearance of a few percent of
paraffin. The presence of cereals in the binder was found to provide some oxygen, even
though concentration of O2 in the atm. feel because of dilution with other gases. (Locke
and Ashbrook, 1950).
2.5 Gas evolution test
Several tests have been developed to measure the gas evolution from different sand
mixtures by heating a small sample to relatively high temperature.
Total gas evolved and rate of the gas evolution from phenolic urethane no bake (PUNB)
and furan no bake (FNB) having loss of ignition LOI 2%, at different intervals from 15-120 sec,
32
after pouring steel, cast iron, brass and aluminium are shown in the table 2.8 and 2.9 respectively
while gas evolution from different no bake binders at 1010oC are given in the curve shown in fig.
2.6. Modified procedure given in ref [55] using a larger sand sample, a lower test temperature,
370oC, are consistent with casting and its results are shown in fig. 2.7. Similarly amount of gas
evolved from various binder systems (Self-set system: silicate ester, furan, alkyd oil isocyanate,
phenolic isocyanate, Vapour catalyzed: phenolic urethane, Heat cured: core oil, shell, hot box
(PF) using an improved test procedure, which includes contribution from water and other
volatile, after Naro and Pelfery (1983) is shown in fig. 2.8.
33
Fig. 2.7 Modified gas evolution test of no-bake binder systems at 370o (700oF)
Fig. 2.6
34
2.8
35
Table 2.8 Total G
as Evolved and Rate of G
as Evolution Calculated w
ith PUN
B 2.0%
LOI (Phenolic
urethane no bake) [55]
36
Table 2.9 Total G
as Evolved and Rate of G
as Evolution Calculated w
ith FNB
2.0% LO
I (Furan no bake)[55]
37
Chap 3
Mechanism of Formation of Exogenous Blowholes
[3-5] [9-17] [18-23] [27-29] [33] [55]
The process of formation of exogenous blowholes created by evolution of gases
from mould and cores completes in three steps.
i) Appearance of critical gas pressure at liq. metal mould interface,
ii) Penetration of gases in liq. metal and
iii) Fixation of bubble of gas in casting wall during solidification
All these stages of exogenous blowhole formation take place in sequence, one
after other. The epicenter, the most favourable sites, from which the bubble appears are
pours in the mould and roughness of the mould surface. It is confirmed by appearance of
blowholes on the surface of the mould made from silica sand and that of, made from
refractory materials [46]. Other sources like some type of nuclei, such as complex
inclusions which have liq. or solid phases of poor wetability facilitate and act as
“embryo” for nucleation of gas bubble. Exogenous inclusions which have pockets of
gases in the folds of films of inclusion are the most favourable sites for nucleation of
bubbles [51].
Germination of bubble starts when bubble appears in the pore, grows and
finally separates from the mould surface leaving small amount of the gas in the pores,
which forms new bubbles and in this way this very process of formation of bubble take
place (photo-1). Bubbles of gas penetrate in liq. metal when:
Pf > Pm + Pc + Ps …………………………… (3.1)
where
Pf is the gas pressure in the pore of the mould at liq. metal-mould interface.
Pm is the pressure exerted by the liq. metal at the corresponding point on the
mould surface
Pc is resistance given by the capillary forces
Ps supplementary pressure of gas on the column of liq. metal in the mould cavity.
38
Fig. 3.1 Variation in gas pressure during pouring liq. metal in mould cavity
Pres
sure
39
Fig. 3.2 Variation in gas pressure when (a) 6% (b) 4% (c) 2% (d) 0% carbon
daN
/m2
40
Pf : Pf, gas pressure in the mould cavity, results from the non accordance in vel. of
formation and evacuation of gas and acts on the surface of contact of liq. metal and
mould and as such, influences blowhole formation (and other defects). The gas has plenty
of opportunity to diffuse away through the bulk of the mould. The pressure build-up in a
greensand cavity during mould filling is normally only of the order of 100mm water
gauge (0.01atm) according to measurements by Locke and Ashbrook (1972).
Results of numerous researches of many researchers have shown that gas pressure
in the mould cavity attains two times the highest value: 1st time exactly after pouring,
because in initial moments of pouring, velocity of evolution of gas is very high therefore
it brings “boiling” of liq. metal and IInd time at 20-300 s (depending upon the casting
temperature and some other factors) after the 1st max. value of pressure, because boiling
of liq. metal reduces permeability of the mould, forms “condensation zone” in the cope
and other changes in the mould (fig. 3.1).
According to T.Viteza and Gy. Nandory [12] in moulding material containing 2-
6% carbon, 1st max. pressure appears 10 s after pouring liq. metal and IInd max pressure
may appear 50-60 s after 1st max pressure. Gas pressure increases with increase in
content of coal powder (fig. 3.2).
K. Suzuki and H. Yamaska [15] measured the gas pressure and metallostatic
pressure in the green sand mould, after 5 s of pouring liq. steel in it and found that
metallostatic pressure is almost 9-12 times greater then gas pressure at a particular point
on liq. steel mould interface. It is therefore concluded that mould gas enters into the liq.
steel before the liq. steel attains high metallostatic pressure due to its high density or it
may enter when jet of liq. steel enters in the mould cavity and metallostatic pressure is
not very high. When liq. steel level in the mould cavity reaches upto the cope surface,
fraction of gas which has not librated, remains in the casting as blowholes.
The variation in gas pressure while casting brass, cast iron and steel in the
greensand mould is shown in fig. 3.3 and is given in table 3.1. From these experiments, it
is concluded that gas pressure, during casting varies in four stages. (1) abrupt increase:
5-15 s, after pouring, manometer registers the highest gas pressure (1st max. pressure). (2)
rapid decrease: after attaining the max. value, gas pressure rapidly decreases due to the
41
filtration of gas through the mould wall which is permeable. (3) slow increase: for short
period the gas pressure remains constant and then it again slowly increases (IInd max.
pressure) due to the sand particles dilate by intense heat dissipated by hot metal and
decreases permeability of mould) (4) continuous decrease: after attaining IInd max.
pressure, gas pressure in the mould cavity decreases continuously and finally reaches to
zero.
Some of the modern test equipment have been designed to measure the gas
pressure in the mould cavity. One of such equipment have been shown in fig. 3.4. [55].
Its out fit have pressure transducer. The variation in gas pressure and mould temperature
while casting cast iron in mould and core made from furan is illustrated in fig. 3.5. The
temperature and the gas pressure profiles obtained confirms above mentioned
observation.
Pm : Metallostatic pressure on each point of mould or core surface is proportional to
the height of column of liq. metal and its specific gravity.
Pm = m. g. Hm …………………… 3.2
Hm is the height of the column of liq. metal. Height of the column of liq. metal
during casting vary from 0 at the beginning of pouring upto the max value at the filling of
the mould (fig. 3.1).
Pc : Pc pressure in the capillaries of the mould surface necessarily plays role in
formation and growth of blowholes and depends upon surface tension of the liq. metal ,
radius of the pore, r and angle of wetting .
Pc = (2 /r) (cos )………………… 3.3
This relation is viable in conditions of blowhole formation in the volume of liq.
metal (surface metal/gas). On contact with the mould wall, energy of “germination” of
bubble is little and will be greater as of the liq. metal increases (fig. 3.1)
42
Fig. 3.3 The variation in gas pressure while casting steel, cast iron and brass in green sand mould
Time (sec)
Cast iron steel
Brass
43
Table 3.1
Gas pressure in daN/m2 in mould cavity while pouring brass, cast iron and steel
Time after pouring (Sec)
Brass Cast Iron Steel
0 5 10 15 30 45 60 75 90
105 120 135 150 165 180 195 210 225 240 235 270 285
0 112.5 120 123* 87.5 75 17 15 17 24 24
24.5 25**
24 23 23 15
12.5 10 7 -- --
0 123.5 125* 112.5 95.5 75 62 50 51
52** 62 62 64 65 63
57.5 52 51 50 48 25 10
0 173* 125 112 50
52** 65 63 62 57 51 49 45 37 35 28 25 23 15 12 7.5 -
* 1st maximum gas pressure ** IInd maximum gas pressure
44
3.4 3.5
45
Table 3.2
Gas pressure in core made from dextrin, Pg.
Time after poring (sec)
Pg (daN/m2)
Time after pouring (sec)
Pg (daN/m2)
Time after pouring (sec)
Pg (daN/m2)
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135
0 0
24 340 480 600 700 750 776 780 780 772 772 764 750 716 656 620 596 550 530 520 500 488 460 472 458
140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270
438 428 410 400 392 380 370 368 356 350 344 340 332 328 320 312 306 300 292 286 274 270 260 250 240 230 220
275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350 355 360 365 370 375 380 385 390 395 400
-
210 200 192 190 170 158 148 136 128 112 106 98 92 86 82 78 72 70 68 66 62 62 60 56 52 50 -
46
Ps: Ps is value of supplementary pressure exerted by the gases in the mould cavity on
column of the liq. metal. It is influenced by heat and dilation of gases which are produced
in the mould cavity and on evolution of gases from upper structure (cope) during their
non ordinate flow during casting. In case of vent holes are provided or gas is evacuated,
during casting, value of Ps is negligible.
When condition given in expression 3.1 predominates liq. steel “boils” in mould
cavity, due to the abrupt effervescence of gases through molten steel. If penetrated
bubbles do not leave the molten metal, the ‘blowholes’ are formed. The blowholes are
formed during the period of “boiling” and continue upto the condition Pf > Pm + Pc + Ps
dominates.
According to I.I. Medvedeve [14] the “boiling” are of four types:
i) ‘Weak boiling’: Weak boiling is observed when Pf is slightly greater than
the sum of Pm +Pc + Ps. It lasts for shorter time and small number of
bubbles appear. This process of boiling is not clear (a, fig. 3.6).
ii) ‘Strong boiling’: It is produced when difference of Pf and (Pm+Pc+Ps) is
greater. In this case comparatively greater number of bubbles penetrate in
liq. steel (b, fig. 3.6).
iii) ‘Very strong boiling’: It is observed when difference in Pf and (Pm+Pc+Ps)
is high. Consequently large number of bubbles penetrate in liq. steel. This
type of boiling continues for larger period (c, fig. 3.6).
iv) ‘Double boiling’: Here boiling takes place two times: 1st time immediately
after pouring and IInd time after filling of mould (this time period may
concise with inception of dendrite formation) (d, fig. 3.6).
In the case of cores, ones they are covered by the liq. metal, the escape of the core
gases is limited to the area of core prints, if the metal is not be damaged by the passage of
bubbles through it. Furthermore, the rate of heating of core is often greater than that of
mould because it is usually surrounded on several sides by hot metal, and comparatively
volume of core is much less. All these factors contribute to internal pressure within the
core rising significantly.
47
Fig. 3.6 (a) Weak boiling (b) Strong boiling (c) Very strong boiling (d) Doubling boiling
48
Many authors have attempted to provide solutions to the pressure generated within
cores. However there has until recently been no agreed method for monitoring the rate or
quantity of evolved gases which corresponds with any accuracy to the condition of casting.
One method by Naro and Pelfrey, (1983) is an improvement in earlier methods in which the
water and other volatiles would condense in the pipe work of the measuring apparatus,
reducing the apparent volume of gases (Section 2.5 and fig. 2.8).
Some other experiments have also been carried out to measure the gas pressure
developed in the core dipped in liq. metal. Outfit of such an experiment is shown in fig. 3.7.
Results obtained for core from dextrin are tabulated in table 3.2 and graphically presented in
fig. 3.8. Like sand mould, the gas pressure in the core also rises high abruptly on pouring and
then decreases. It is due to the fact that gas pressure once attains high value and then
decreases due to the gas penetration in the liq. metal. Number of other researchers have
measured the gas pressure in the cores having variable permeabilities ( capacity of filtration
of gas) the results are given in table 3.3. Like sand mould, it is also confirmed, in the case of
cores, that gas pressure develops in the core attains two times the max values, e.g. for core
having filtration capacity 8.1x10-9 m5/gsec, 1st max pressure of 29.8 kgf/m2 is observed 5s
after pouring and IInd max. pressure of 23.8kgf/m2 at 60s after pouring.
J. Campbell (1950) devised a useful test which accessed the pressure in cores
compared to the pressure in the liq. metal. A modified version his test is shown in fig.3.9.
Right part of fig. 3.9 shows how the gas pressure in the core varies while filling the
experiment mould (a) rapidly (b) slowly. If the mould is filled quickly, then the hydrostatic
pressure due to depth in liq. metal is built up more rapidly than the pressure of gas in the
core. This dominance of metal pressure persist throughout filling and solidification. The
higher metal pressure effectively suppresses any bubbling of gas through the cores at point A
(fig. 3.9). Bubbles will form at A only in the exceptional circumstances where the core
pressure rises to extremely high levels, intersecting the horizontal part of the metal pressure.
If the mould is filled slowly, the gas pressure in the core exceeds the metal pressure at
B and remains higher during most of the filling of the mould. Gas escape through the metal
during the last stages of filling, so that, although the metal pressure in fig. 3.9 finally exceed
the gas pressure in this case.
49
Fig. 3.7 Measuring gas pressure in core of dextrin
Pressure measuring
Liq. metal
Core
50
Fig. 3.8 Gas pressure in the core made from dextrin 1. 1% dextrin, 3% linseed oil, 5% H2O 2. 1% dextrin, 3% linseed oil, 2% H2O
51
Fig. 3.9 The effect of fill rate on core out gassing (adapted from J. Campbell (1950)
52
Table 3.3
Gas pressure in core kgf/m2
Time Value of gas pressure when core has the capacity of filtration:
Sec 8.1x10-9 m5/g sec
12.7x10-9 m5/g sec
2.04x10-8
m5/g sec 4.08x10-8
m5/g sec 10.9x10-8 m5/g sec
5
15
30
45
60
75
90
102
120
180
29.8
13.1
12.5
16.8
23.8*
22
21.4
19.8
18
14.1
21.7
9.2
7.4
8.3
10.6
12.2*
11.8
11
10.5
9
14.2
5.5
4.4
4.5
5
6.8
7.3*
7.1
6.8
5
6.9
3
1.5
1.6
1.8
2
2.9
3.0*
2.8
1.8
2.5
0.8
0.4
0.5
0.7
0.8
0.8
0.9
1.0*
0.8
Observation: Max. pressure I was measured at 5 sec after pouring. * Value of IInd max. pressure I
53
As the filtration capacity of the core increases, value of both 1st max and IInd max
pressures decrease and IInd max pressure appears comparatively at the later time after
pouring.
3.1 Energetic gas bubble - liq. metal Interaction
Bubble which are evolved from the surface of mould penetrate in the liq. steel.
When bubble leaves the mould surface following forces act on it: Hydrostatic (buoyancy,
force of rising), vertical and horizontal forces caused by the concave and convex parts of
the mould. These are caused by the convection in the volume of liq. metal. Internal
friction of the liq. metal (viscosity ) which opposes the rising bubbles, adhesion (which
retains the bubble at the surface of the mould and core). Mechanical forces (parts of the
mould and core on which dendrites are formed which can catch the bubble). Due to the
influence of these forces, gas inclusion make depart from the casting during
solidification or may remain attached with mould wall, causing surface or subcutaneous
blowholes.
During rising of bubble in the vol. of the molten metal, following processes can
take place: Increase in vol. of bubble due to decrease in external pressure (metallostatic),
striking and interaction of bubble brings change in shape of bubble (from spherical to
elongated) and diffusion of gas, present in the molten metal, in the bubble and
agglomeration of number of bubbles. Vel. of rising of bubble of vol., (upto dia 1mm), in
case of laminar flow, can be calculated with the help of Stoke’s law:
v = 2/9 g r2 (m - i) 1/ ……………… 3.4
In which g is the accelerated due to gravity
r radius of bubble
m and I respectively densities of liq. metal and gas.
dynamic viscosity
for bubble > 1mm dia, vel. of rising of bubble can be calculated by:
m
imgv
38 ……………………….3.5
where is coefficient of opposite resistance of liq. metal.
54
The condition of separation of gas bubble from casting wall during solidification
is
tcr = h/vm …………………………….3.6
where tcr is duration of formation of hard skin which can be pierced by the gas bubble.
h height of solidified layer at the place (epicenter) on the mould wall, from where gas
bubble has ejected. vm average velocity of rising of bubble.
Different situations of interaction of bubble with solidifying casting are:
-- bubble of gas may pierce the thin solidified layer (a, fig. 3.10) This situation is
only possible when resistance of thin solidifying layer is low (10 KN/m3) If
bubbles of gas proceed one after other , they will pierce the solidified layer and
may come out of the cavity. (b,fig. 3.10)
The force ( Fg ) with which bubble acts of solidified layer is
Fg = r2. Pf …………………. 3.7
where r is radius of bubble
Pf is the gas pressure
Condition of piercing of hard skin is
Fg > Fm + R………………… 3.8
where Fm is the force which holds the hard skin.
Fm = (r + )2 (Pm + Pc) ……….. 3.9
where R is resistance of hard skin towards rupture.
R = [(r + )2 – r2] rt ……… 3.10
rt is resistance to rupture of metal at high temperature.
-- bubble of gas rises freely in the molten metal of casting wall with low viscosity.
-- bubble of gas which rises, meets with the hard skin on the open column of liq.
metal (c, fig. 3.10)
Force exerted on the hard skin is
Fg = (m - i) V …………………. 3.11
where
V is volume bubble of gas.
The intensity of this force is very little as compared to that of, in case of (a, fig.
3.10). Due to this, the bubble cannot leave and appears as subcutaneous blowhole.
55
Fig. 3.10 Interaction of gas bubble with liq. metal during solidification
56
Fig. 3.11 Penetration of gas bubble in liq. steel during solidification of different types of casting
57
-- bubble of gas meets to the hard skin formed on horizontal casting wall, which
cannot be pierced (in case thin casting wall or casting in metallic mould) (d, 3.10)
-- gas bubble reaches at the mould wall surface (upper part of the mould, cope,
before inception of solidification or increase in viscosity of the liq. metal (e, fig.
3.10)
-- rising of bubble in vertical casting wall (f, fig. 3.10).
Fig. 3.11 (a-l) shows penetration of gas bubble in liq. steel during solidification in
moulds of different configurations [4]. In case of permanent mould (metallic mould)
when hard skin has not formed, bubble of gas leave the vol. of liq. metal without any
blowhole (a, fig. 3.11). When hard skin is formed on the sides and not on the open part of
casting, almost similar situation will be observed (b, fig. 3.11). If hard skin is formed on
the open part of casting, the bubble will settle below it (c, fig. 3.11). If mould has vent
hole, the gas bubble will not have any resistance for leaving the mould cavity (d, fig.
3.11). Some of the bubbles, if retain in vol. of molten metal, they may stick with mould
surface within spaces of vent hole (e, fig. 3.11). In such a way, if number of bubbles
agglomerate and settle there large blowhole are formed (g, fig. 3.11). In such a case if
hard skin is formed, the bubble will stick with it (h, fig. 3.11). In case of sand mould, if
hard skin is not formed the bubble will rapidly leave the molten metal (h, fig. 3.11). If
bubble is rising with strong force and high speed, it may pierce the hard skin. (i, fig.
3.11). When hard skin of greater thickness is formed, the bubble may settle below this (j,
fig. 3.11). Rising bubble may stick with extruded mould wall (k, fig. 3.11). Bubbles
leaving from extruded portion may settle under hard skin (l, fig. 3.11).
3.1.1 Kinematics of penetration of bubble in liq. metal*
Bubble of gas ejected from a part of the mould enters in the vol. of the liq. metal
in the mould cavity and travels in it, on a specific path. It is subjected to the strokes of
number of forces e.g. intense resistance of waves of the flow of the liq. metal. The
equation of the path traced by the bubble within the liq. metal can be determined as
follows:
58
Archimedes force, Fa,
Fa + 4/3 R3 ( - I ) gj …………………….. 3.12
Resistance to the bubble according to Stoke’s law
= 6R v ……………………… 3.13
where,
R radius of the bubble (large in size)
density of the liq. metal
i density of gas
viscosity of liq. metal
v velocity of bubble
g acceleration due to gravity (j is superscript)
Newton equation (Newton’s second law of motion)
4/3 R3 i ṙ = 4/3 R3 ( - i )gj - 6 R ṙ ……. 3.14
(ṙ “second derivative” and ṙ is “first derivative”)
r(0) = 0 v (0) = 0 in initial conditions t = 0
on axes
2R2 i ẍ = -9 ẋ ………………………….. 3.15
2R2 i Ÿ = 2 R2 ( - i) g -9ý……………………. 3.16
Solution of the equation
x (t) = Al + A2
tR ie 22
9
y (t) = Bl + B2 t
R ie 22
9
tR i
9)(2 2
…..3.18
condition ṝ (0)= 0 A1 + A2 = 0
ŕ (0) = vo B 1 + B2 = 0
A1 CosvA
R oi
2229 ………………… 3.19
*Contributed specially by a Prof of Mathematics from Deptt of Mathematics, Polytechnic University of Bucharest, Bucharest, Republic of Romania
59
B1
sin
9)(2
29 2
22 oi
i
vgRBR
…………. 3.20
A1 = -A2 =
i
o
R
v
2291
cos
……………….. 3.21
Condition
B1 = -B2 =
i
io
R
gRv
2
2
291
9)(2sin
……… 3.22
Equation of the path traced by the bubble:
y =
)1(log81
)(41
2
2
1
1
AxRx
AB
ni
………………… 3.23
3.2 Appearance of blowhole in casting
Surface blowholes may be categorized in two groups, external and internal.
External blowholes may appear as a large crater (b, fig. 3.12) or as a porous roughness
(rugosity), on the surface of casting (a, fig. 3.12). The external type of blowhole, have
diameter 1-15mm while interior (subcutaneous ) have diameter equal to the diameter of
pores of mould wall. Pinholes (c, fig. 3.12) have triangular cross section and pointed
towards one end. Surface blowholes are affected by II max. gas pressure formed in the
mould cavity (curve showing the variation in gas pressure developed in mould cavity (fig.
3.13). As the rate of solidification is high (thick casting wall, high casting temperature
and high cooling rate) gas pressure in the mould cavity will be also be high. The
Tendency of formation of internal (sub surface) and external blowholes will also
increase. External blowholes are formed when gas pressure in slightly above the counter
pressure. Pf > Pm + Pc + Ps (fig. 3.13 ) domain 2 when Pf > Pm + Pc + Ps exists, the
internal blowholes are formed (fig. 3.13 domain 1) and when Pf < Pm + Pc, the metal
penetration defect will be observed (fig. 3.13 domain 3). Excellent surface finish and
sound casting will be obtained when Pf = Pm + Pc (fig. 3.13 domain 4). So, external
blowholes of crater type (macro blowholes ) are formed by the penetration of gases
60
evolved from the mould while external blowholes which appear as regosity of casting
surface (b, fig. 3.12) are the result of the chemical reactions of gases which take place at
the liq. metal mould interface.
While casting of an alloy of high melting point, which is super heated, the
superficial layer of the mould cavity may melt or soften (photo-2) and as such, it offers
almost 0 permeability to the mould gas. Bubble of gases ejected from the liq. metal and
from the layer of the mould which has melted (photo-3), concentrate on surface of
contact between metal-mould (photo-3) and are not evacuated, thus forms internal
blowholes and surface micro blowholes. These types of blowholes settle in metal or in
the molten layer of silicate (2FeO. SiO2) and have dimension 0.2 – 2cm (fig. 3.14).
Fig. 3.15 shows another mode of blowhole formation in steel casting. In phase I,
Pf gas pressure, in mould cavity is little and resistance of steel (W) is large i.e. Pf/W
limit, the result is that blowholes are not observed (Phase-II). When Pf /W > 1, the
process of formation of blowholes starts and when Pf/W >> 1, the blowholes grow
towards interior and process of blowhole formation is completed (Phase-III).
When the liq. iron fills the last part of a greensand mould, the pressure pulse
which accompanies the final instant of filling causes the metal to increase its local rate of
heat transfer to the mould. The enhanced heat transfer causes the water in the mould to
evaporate explosively. The high local pressures which are caused result in deeper sand
penetration.
In some cases, appearance of blowholes in the casting wall is brought about by
“explosion” of micro vol. of water vapour present in the mould wall coming in contact
with liq. metal (a, fig. 3.16). During casting, hot currents are formed by liq. metal and
spontaneously hot spots appear at some particular zones on the surface of mould wall (b,
fig. 3.16) which provoke brisk evaporation of micro volume of water or volatile metal,
present there, in a form of “explosion”. Vapour with the high surface tension penetrate in
the liq. metal and also exert pressure on the pores of the mould where they solidify
rapidly and form intimate contact with sand grains (c, fig. 3.16) In this way, the canals in
the mould surface, for evacuation of vapour, are blocked (permeability decreases rapidly)
and vapor remain in the casting wall in the form of blowholes (d, fig. 3.16). In such cases
vent holes created in this way are filled again by liq. metal (e, fig. 3.16).
61
Fig. 3.12 different types of surface blowholes (a) subcutaneous blowholes (b) external blowholes on the rough mould (c) micro pinholes
62
Fig. 3.13 Influence of gas pressure on tendency of blowhole formation (scheme)
(a) Pf large (b) Pf small value t1,t2,t3 time of solidification 1. zone of internal blowhole formation 2. surface blowholes 3. mechanical adherence 4. sound casting
63
Fig. 3.14 Surface blowhole formation (a) mode of formation in Mn steel (b)
1- liq. metal 2. blowholes 3. molten layer on mould surface 4. mould
64
Fig. 3.15 Mechanism of appearance of blowholes in steel castings
Blowhole not appeared Blowhole appeared Completion of appearance of blowholes
65
Fig. 3.16 Mechanism of appearance of superficial blowholes by “explosion” (Schematic)
66
Fig. 3.17 Appearance growth and completion of subcutaneous blowholes in casting (Sims & Zapffe )
sand
layer liq. steel
blowhole
67
At present, many hypotheses exist on origin of gases which form and feed bubble under
hard skin. Mechanism of appearance of fine types of blowhole is explained by diffusion of
hydrogen through hard skin formed during solidification towards interior of casting. Hydrogen is
produced by reaction of liq. metal with water vapour from the mould surface and condensation
zone (fig. 3.17). Diffusion of Hydrogen in solidified crust and dendrites occurs forming,
elongated blowholes. The hard skin formed on steel casting is thin and on heating, the gas canals
under the hard skin are exposed to the surface of casting, due to the evolution of gas on heating,
thus forming fine blowholes.
Turkdogan (1986), describes how sub surface porosity occurring in the cast irons and
steels poured into green sand mould is a sequence of metal mould interaction. ‘Gas bubbles
formed in crevices of the mould in contact with the metal, and the bubble into the metal, where
they trapped during the early stages of solidification. A molten metal of lower surface tension
allows bubbles to enter the metal more easily, thereby increasing the sub surface porosity’.
However some other researchers [72] do not reconcile with this widely accepted theory and have
suggested that sub surface porosity does not occur by penetration of the liq. surface by bubbles
from the mould, but is the consequence of normal segregation ahead of the solidification front,
and the normal processes of “nucleation” and growth of pores from gases in solution in the metal.
Segregation of H2 and N2 and CO and their concentration factor ahead of solidification front, is
calculated from their solubility in liq. iron.
Sr Gas Sol. in liq. Fe
Sol in solid Fe
Partition coeff. K
Factor: conc. ahead of
solid. front
Comments
1 H2 245ml/kg 69mg/kg 69/245=0.28 1/0.28=3.55 Modest contribution to gas pressure for
nucleation 2 N2 440ml/kg 129ml/kg 129/440=0.29 1/0.29=3.4 -do-
It shows that hydrogen and nitrogen have modest contribution to gas pressure for
“nucleation”. Similarly k for oxygen in iron is 0.05, and k 0.2 for carbon in iron,
with the concentration factor 20 and 5 respectively, so that in combination, the
equilibrium CO pressure at solidification front is 100 times higher in the bulk melt. It
confirms that CO has effective contribution for “nucleation” and is therefore considered
strong source of porosity in steel castings.
68
According to Sims & Zapfe, on decrease in solubility of hydrogen in liq. steel
during solidification, hydrogen bubble shifts to the region in liq. state, which is adjacent
to the solid. When hot liq. steel comes in contact with the mould wall having moisture, it
first forms hard skin. Moisture present in the mould surface is transformed into vapour.
The volume of vapour formed is 5000 times than the moisture present in the mould
surface. It creates strong oxidizing atmosphere in the mould cavity for steel. A part of the
vapour, present in pores of the mould, come in contact with liq. steel.
Fe + H2O = FeO + 2H
It is due to the fact that high concentration of hydrogen in layers of steel solidified
in green sand mould, adjacent to the mould wall, is observed. However some researchers
(E. Enipp [4]) consider the gas pressure developed in the mould cavity, which mainly
depends on the contents of moisture and permeability of mould, as strong source of
blowhole formation and steel even deoxidized properly, cast in sand mould, may also
show the gas defects.
The dissolution and diffusion of H2 in liq metal adjacent of solidifying front has
been explained by J. Campbell [70]. Hydrogen from a surface reaction can diffuse
sufficiently far in the time available during the solidification of an average casting to
contribute to the formation and growth of sub surface porosity. The salutes are rejected
ahead of the advancing solidifying front gradually building up to a concentration peak.
Thus conditions are exactly optimum for the creation of maximum gas pressure in the
melt at the point a millimeter or so under the surface of casting: the high peak will favour
conditions for nucleation of pores; the closeness to the surface will favour the transport of
additional gas. If there is enough gas already present in the melt, then contributions from
any surface reaction will only add to the already existing porosity.
Hydrocarbon can also be decomposed at the metal surface releasing C & H2. In most sand
casting both water vapour and hydrocarbons will be present in abundance. General mould
atmosphere often contains upto 50% hydrogen.
Beech (1974) showed that inner atmosphere of a long bubble is not necessarily
homogenous. The metal mould reaction will continue to feed the base of the bubble which is
within the diffusion distance of the surface. The front of the bubble may also gaining gas from the
melt if available. However, if the metal has low gas content, then the front part of the bubble
69
3.18
70
may be losing gas to the melt. The bubble is then effectively acting as a diffusing “short
circuit” for the transfer of gas from the surface reaction to the center of the casting.
According to some other researchers [44, 45, 49, 50, 56] blowholes are formed by
atomic H and N. these gases are evolved from special additives made from components
having nitrogen and hydrogen. The resins having urea decompose during casting form
NH3 which further decomposes into H and N native. Gases are dissolved in liq. steel and
are rejected when their solubility, during solidification, is reduced. These types of
blowholes are formed as minor cavities in superficial layer of casting near the hot spots.
Both nitrogen and hydrogen will have a similar influence nucleation of pore, but
hydrogen will be the major influence contributing to its growth. Nitrogen alone would not
have been particularly affected: even it may have successful in nucleating pores hydrogen
would have set the growth of the pore [70].
Hydrogen has a diffusion coefficient approx. ten times higher than that of any
other element in solution in liq. iron. The average diffusion distance d is approx. Dt1/2
where D is coeff. of diffusion, d is distance of advancing front at time t. In comparison
with other diffusing species, the radius over which hydrogen can diffuse into bubble is
(10/1)½ 3 greater. Thus the volume over which hydrogen can be collected by the
bubble, in comparison with other diffusing species, is 33 30 times greater. Thus it is
clear that hydrogen has a dominating influence over the growth of the bubble [72]
In case of austenitic steel castings, gas porosity is created by evolution of N from the
reactions between Ti nitride and oxide coming on the surface of casting during
solidification [5].
Fine macro porosity is observed in cast iron when 0.01 – 0.1% Al is added in it,
such porosity is observed when cast iron is cast in moulds having 5% water. While
casting same cast iron in the mould having special additive, 5% coal powder, the
blowholes are not observed. It is due to the fact that gas forms by the addition of coal
powder in mould cavity impedes the reactions between Al and H2O. In vermicular cast
iron, porosity is created by the reaction of the Ti. Ti is added in vermicular cast iron as a
“modifier” which oxides to TiO2, reacts with FeO and forms Fe TiO3. It is transient
radical and is rapidly attacked by carbon present in steel and creates CO, which a strong
71
source of porosity [7] P.P. Burg demonstrates that porosity is created by evaporation and
penetration of “water of constitution” which is present in clay, in liq. steel. Some
researchers consider (SiO) gas, formed by reduction of SiO2 with carbon as a possible
cause of blowhole formation.
3.3 Nucleation of gas porosity [71]
3.3.1 Homogeneous nucleation
If the pressure in the liq. is Pe then we need to carry out an amount of the work
PeV to push that the liq. far enough to create a bubble of volume. The formation and
stretching out of a new liq. gas interface of area A require work A, where is the
interfacial energy per unit area. The work required to fill the bubble with vapour or gas at
pressure Pi is negative and equal to –PiV. Thus the total work is:
G = A + PeV-PiV ………………… 3.24
= 4r2 + (4/3)r3 (Pe-Pi) ………… 3.25
where clearly (Pe – Pi) is the pressure difference between the exterior and the interior of
the bubble. Since bubbles growing from the bulk liq. will grow like an atom at a time, as
a result of statistical thermal fluctuation, small bubbles with radii less than critical radius
rcr with tend to disappear. A long chain of favourable energy fluctuations will produce a
bubble exceeding the critical radius rcr which will have the potential to grow to an
observable size. The pressures which are required for nucleation are extremely high and
make difficult the homogeneous nucleation of pores in the liq. metals.
The non metal oxygen, sulphur and phosphorous are particularly active in iron
melt: the presence only 0.2 weight percent of O2 reduces the surface tension of liq. iron
from 1.9 to approximately 1.0 N/m. This approx. half the estimate of pressure required
for nucleation.
Water Hg Al Cu Fe Surface tension (N/m) 0.072 0.5 0.9 1.3 1.9
Atomic dia (nm) -- 0.30 0.29 0.26 0.25
3.3.2 Heterogeneous nucleation
Nucleation might occur on foreign substrate, solid surface of an impurity. This is
known as heterogeneous nucleation. The liq metal is considered to make an angle on
72
the mould surface. This contact angle defines the extent of wetting; = 0o means
complete wetting whereas = 180o is complete non wetting (see fig. 3.18) and 0< < 90o
partial wetting. All inclusions are good ‘nucleation’ sites for porosity. Those that are well
wetted will not be favoured. These include more metallic inclusions such as boride,
carbide and nitride. However, being well wetted they are mostly good nuclei for the solid
phase. The inclusions such as oxides contain minor amount of gases in their folds, thus
they offer the most favourable sites for “heterogeneous” nucleation. Good nuclei for
pores must be non wetting. These includes most oxides in most liq. metals. All the
surface oxides give problem when entrained by surface turbulence. The oxides retain air
as surface film and, even without the entrained air decohere easily from the melt, and
thus ‘nucleate’ porosity readily. As contact angle increases to 180o any difficulty of
heterogeneous nucleation falls to zero. However, max contact angle attainable in practice
is perhaps close to 160o. (Livingston & Swingley 1971). However, heterogeneous
nucleation on solid becomes favourable when wetting angle exceeds 65o.
Heterogeneous nucleation on the most non wetted solid known requires only
about one twentieth of the pressure required for nucleation in the bulk liquid [71].
According to Popel & Eisen (1956), FeO has surface tension only 0.6-0.5N/m depending
upon the oxygen content. According to J. Campbell [71] gas pore will heterogeneously
nucleate on FeO liquid inclusion at about critical pressure 17000 atm., which is almost
unattainable. But on heterogeneous nucleation it reduces largely, if a highly non wetting
inclusions were present inside the liq FeO inclusion. Then the lowest pressure for
nucleation of a pore in this complex inclusion would be approximately 17000/20 =
850atm which is attainable in cast iron and steel. Fisher has determined ratio of critical
pressure required for heterogeneous and homogeneous nucleation (P*het/P*hom) to whom
he has used to express as factor of nucleation.
(P*het/P*hom = 1.12 [2+cos) (1-cos)2/4]1/2 ……………… 3.26
o 0 65 90 120 160 180
P*het/P*hom 1.1 1 0.8 0.4 0.5 0.0
3.4 Micro porosity model [34]
Number of models of micro pore formation have been put forth. Here we are
discussing such a model given in ref [34] (Ch. Pequet & collaborators, 2002). It is based
73
on two main phenomena: pressure drop in mushy zone, and segregation of gas /
precipitation of gas bubbles (cavitations) only by H (neglecting other items).
The motion of the liq. metal in the mushy zone is supposed to be governed by the
pressure field only (in the absence of pore) the local pressure in the liq. pl(x,t), is made of
several components.
pl = pa + pm + pd ………………………………. 3.27
where pa is a atmospheric (or external ) pressure, pm is metallostatic contribution,
and pd is dynamic contribution responsible for the movement of the liq. in the mushy
zone.
In most metallic alloys, there is a certain amount of gas dissolved in the liq. or of
salute elements, having a fairly high vapour pressure. If gl is the volume of fraction of liq.
and gs is the volume fracture of solid and gp is the volume fraction of pore in a liq. alloy
then porosity formation will occur when
gl = 1 – gs – gp. ……………………….. 3.28
A. When no pore has formed!
Yet at a given location (gp = 0), there is only one variable, the pressure, appearing
in equation (3.28). Assuming lever rule and a locally close system for the segregation of
the gas, the mass balance of hydrogen can be written in this case.
[H]ol = [H]ssgs + [H]ll(1-gs) if gp = 0………… 3.29
where [H]o is nominal concentration of hydrogen in melt and [H]s and [H]l are H con. In
the solid and liq. phases respectively. The last two conc. are assumed to be related by the
partition coeff. KH i.e. [H]s = kH[H]l taking this value as equal to that of the saturated
solution (i.e. given by Sieverts’ law) by Eq. 3.29 the effective conc. in liq. [H]l(gs) as a
function of volume fraction of solid only can be calculated.
B. Soon as porosity formation occurs!
Hydrogen (or gas ) conservation can be written as
[H]ol = [H]ssgs + [H]ll(1-gs-gp) + gppp / T if gp 0…………3.30
is a gas conversion factor, pp is the pressure in the pores, and T is the
temperature (in K). The gas concentrations in each phase are given by Sieverts’ law.
[H]s*(T,pp, cl) = Ss (T,cl) pp / po and 3.31
[H]l*(T,pp, cl) = Sl(T,cl) pp / po 3.32
74
where Ss and Sl are the temperature and solute-dependent equilibrium constants,
po is the standard pressure, and cl is the solute concentration per unit mass. Sieverts’
constant for the liq. Sl(T, cl), is correlated with temperature and composition of the liq. by
the use of the following relationship.
Sl(T,cl) = 1/KlfH 3.33
with ln Kl = 5872/T = 3.284 and log10fH = eclH cl+ rcl
H cl2
where eclH and rcl
H are interaction solute coeff. on hydrogen of the first and second order.
The conc. in the liq. and solid phases are given by the pressure in the existing
pores and not in the liq. since the “reservoir” of gas (i.e, the bubbles) with which these
two phases are assumed to be in equilibrium is curved. The pressure in the pores is given
by
pp = pl + pr 3.34
where pr is the overpressure due to the capillarity effect. The radius of curvature of the
pore being r, pr is given by Laplace’s law.
pr = pp – pl = 2lg/ r 3.35
where lg is the interfacial tension between the liq. and the pore (gas).
C. Nucleation and growth of pore
When [H]l exceeds by certain amount, the equilibrium value given by Sievert’s
law, [H]*l (T, pp, cp), pores are assumed to nucleate with the given and fixed density o.
Defining the supersaturation for nucleation as [AH]nl , pores will form if
[H]l (gs) [H]ol (T, pl, cl )+[H]n
l 3.36
In fact, [H]nl can be converted to critical radius of curvature of pore nucleus, ro,
by mentioning [H]*l (T, pp, cp) + [H]n
l = [H]*l (T, pp, cp), thus pore pressure (pp ) can be
determined by pp = 2lg /r + pl
75
Chap – 4
Influence of casting conditions on blowhole formation
[4, 5, 17] [68-73]
The theoretical and experimental study carried out by author and number of other
researchers have shown that some factors concerned with casting and mould technology,
play an important role, directly or indirectly, in blowhole formation. These factors may
include: volatile matters and humidity present in moulding material, physico chemically
properties of the mould, nature of atmosphere in the mould cavity, composition of alloys
cast, casting temperature, pressure of column of liq. metal on mould wall, quantity of
gases dissolved in liq. steel during melting and grade of de-oxidation surface tension of
liq. metal, vel. of pouring and thermodynamics of reactions which take place at liq. metal
mould interface and some other factors.
4.1 Nature of Steel
4.1.1 Viscosity of Liq. alloy,
a. Kinematic viscosity and Dynamic viscosity
Mode of flow of liq. alloy depends on dynamic viscosity and specific gravity and
is expressed as kinematics viscosity,
= /s [m2 /s] …………. 4.1
In which is kinematics viscosity and s specific mass and is dynamic viscosity.
b. Kinematic viscosity and Reynolds number
Kinematic viscosity, in expression (4.1) is related with Reynolds number (Re) as
Re = vd/ = vd s / …………. 4.2
where
v is the velocity of laminar flow in m/s
d = hydraulic diameter in cm
c. Viscosity and fluidity
These are related according to the following expression.
v2/2g + 32 vL/g.d2 – H = 0 ………… 4.3
where L is the fluidity and H is the distance of flow.
76
d. Dynamic viscosity and temperature
Dynamic viscosity as function of temp. = o exp[E/RT]
where
o is dynamic viscosity at high temp. E energy of activation (80-120kg/atm)
R is the universal gas constant and T is absolute temp.
High viscosity of steel increases tendency of blow hole formation. Exogenous gases
produced in the mould penetrate in viscous alloy and find difficulty in rising through the vol. of
the liq. mass due to the high resistance of viscous alloy. Bubbles of gas captured by the advancing
front of dendrites, appear as blowholes after solidification.
Viscosity is influenced by certain elements. Elements which reduce density of alloys also
reduce viscosity at constant temperature. By this relation it may be explained why carbon, silicon
and phosphrous reduce viscosity of cast iron and at the same time, sulphur increases viscosity.
4.1.2 Surface Tension
Surface tension of liq. alloy plays an important role in casting, being strong relationship
exists in wettability / non wettability of mould wall by liq. alloy, and surface tension and
therefore, easy or difficult penetration of liq. alloy in capillaries of mould depends upon it.
Besides surface tension plays also an important role in crystallization and building structure of
cast iron, on germination, growth and separation / agglomeration of oxide non metallic inclusions
formed on de-oxidation. Influence of surface tension can be demonstrated as follows:
Alloys having high surface tension do not wet mould thoroughly (in canals, they present
convex meniscus and angle of wetting, > 90) have little capacity of filling intricate
parts of casting. This case exists for alloys having high melting point (e.g. alloys of iron,
copper etc.)
Alloys having low surface tension, wet mould wall thoroughly and penetrate in the pores
of the mould (angle of wetting, < 90 and with concave miscues in pores of mould
walls.) These alloys present better capability of filling mould cavity, because these have
better fluidity. These alloys are suitable for casting having thin wall and précised
geometrical configuration e.g. easily fusible alloys (lead, tin and antimony Sb) (see also
Section 3.3.1). Oxide particles which are formed in the liq. alloy increase capacity of
wetting of liq. alloys with mould wall. However films of oxides of Al and Mg. which
are formed during melting and casting of steel and cast iron greatly reduce capacity of
wetting of liq. alloy.
77
Fig. 4.1 Effect (%Mn/%Si) on surface tension of steel
dyne/cm
78
Richard, Toly and Poyet [17] have studied the influence of surface tension of liq. steel on
blowhole formation. According to these researchers, surface tension of steel depends on ratio
of Mn to Si (%Mn/%Si) in steel which is not de-oxidized. Fig. 4.1 and 4.2 illustrate variation
in surface tension () of different types of steel (which are not de-oxidized) with change in
(%Mn/%Si) of the steel. In both of these curves it may be observed that surface tension
increases with increase in (%Mn/%Si). Decrease in surface tension of liq. steel reduces the
chances of appearance of blowholes. It is illustrated in fig. 4.3 traced on the basis of results
observed from the alloys which are most susceptible to blowhole formation. It is confirmed
that for a fix content of Mn, increase in content of Si brings about decrease in surface tension
and elimination of blowholes. Elimination of blowhole is noted absolutely at a critical value
of surface tension (1180 dynes/cm) which differentiates in zone of appearance and non
appearance of blowholes. There value are almost constant for alloys with different
composition. Investigations have further proved that S and Si are more effective for surface
tension of steel. S = 0.2% and Si = 0.016% combate gas porosity by decreasing surface
tension.
Calculation have shown that (%Mn/%Si) = 0.92 to 0.95 is very susceptible for
appearance of blowholes and if this value is less than 0.8, the blowhole formation will be
totally eliminated.
Reduction in blowhole formation in case of stainless steel may occur as follows: The rate
of uptake of nitrogen in stainless steel is inhibited by the presence of silicon in the steel
which, at certain oxidation potentials, forms SiO2 on the surface in preference to Cr2O3
(Kirner, 1988). Even when the film consists only of a layer or so of adsorbed surface active
atoms, the presence of this layer will reduce the rate at which gases can diffuse across. This
happens, for instance, in the case of steels: sulphur and other surface active impurities hinder
the rate at which nitrogen can be transferred.
4.1.3 Angle of Wetting ()
Appearance of bubble at liq. metal / mould interface is always possible because
pores of the mould surface during pouring are full of mould gas. In case of complete
wetting of liq. metal with the mould surface ( = 0o or it tends to 0, 0) gas from the
pore, with radius rn form a bubble with concave surface towards liq. metal (a, fig. 4.4).
Bubble of gas will weak and can only grow if rn > rcr. Critical radius (rcr) of bubble can
only exist if:
79
dyne/cm
Fig. 4.2 Effect of (%Mn/%Si) on surface tension of steel
80
Fig. 4.3 Relationship between surface tensions of different types of steels (non-deoxidized) with tendency of appearance eoff blowholes
casting with blowholes
casting without blowholes
1.4 1.6 1.8
81
rcr = 2/P/n cos …………………………… 4.4
where P/n = critical pressure by which bubble of gas grows in vol. of liq. metal
and is the surface tension.
If liq. metal wets the mould partially (0< <90) (b, fig. 4.4), the stability of bubble
will be determined by :
rn rcr cos …………………………… 4.5
that is, by increase in angle , the pore radius will be reduced which can further
act as ‘germination’ for appearance of next bubble of gas. The rate with which bubble
will grow, at the same rate it will penetrate in the liq. metal. In this way on separation of
bubble from mould wall in the pore, ‘new’ bubble will form.
In conditions, when liq. metal does not wet totally the mould surface ( > 90o) (c,
fig. 4.4), bubble will form convex surface towards liq. metal in capillary of pore and
critical pressure pcr of gas in the pore, which displaces liq. metal from the pore, is
Pcr = Pm - 2/rn …………………… 4.6
That is, it will be little as compared to external counter pressure.
When bubble of gas reaches to the state of separation from gas / liq. metal
surface, transforms into the spherical form. Its pressures value is 0 at the initial stages and
then increases slowly to form other bubbles and so on.
4.1.4 Density
The alloys having little density are more susceptible for exogenous blowhole
formation. High density alloy have high metallo static pressure , so the gas evolved from
the mould have to over come the greater counter pressure, for penetration in liq. steel. In
case, for metals having high density, blowholes are formed by reactions taking place at
liq. metal – mould interface or in the vol. of liq. metal.
4.1.5 Chemical Composition of Steel [56]
From the elements present in steel, C, S and Mn play an important role in
blowhole formation. Variation in content of C, modifies the quantity of H in steel. It
affects the partial pressure of CO and liquidus – solidus interval.
When steel is not oxidized properly and is with high quantity of oxygen, increase
of content of C, increases partial pressure of CO and quantity of hydrogen necessary for
82
formation of blowhole decreases because total content of gas, necessary for formation of
bubble, remains constant. When steel is strongly deoxidized and have very little quantity of
oxygen, quantity of hydrogen necessary to form blowhole varies with length of liquidus-
solidus interval. So the steels with long liquidus-solidus interval, can tolerate high levels of
hydrogen before formation of blowholes. Fig. 4.6 & 4.7 show solubility of [H] in steel on
variation of content of carbon in steel alloyed with Cr-Mn and unalloyed carbon steel, at
1600oC and 700mmHg, respectively. In case of Cr – Ni steel, the critical value of hydrogen,
[H] content necessary for formation porosity in alloyed steel is:
Hcrcm3/100 g=1/0.9 (17.9-5.8 C) ………………… 4.7
Increase in content of Cr in steel increases the critical content of [H] for formation of
gas porosity [61] and interrelation of [H] and [N] absorbed and content of [C] and [Si]
present in the steel, is shown in fig. 4.5.
Reactions C- O is intense at low content of sulphur as compared to its high
concentration in steel. It is due to the fact that atoms of sulphur are absorbed at the surface of
bubble of CO and perturb the absorption of O: reaction which produces gas, taking place at
gas liq. steel interface, i.e. the surface of bubble. The process of reaction is complete in two
parts.
1. adsorption of atom of oxygen on the surface of the bubble, and
2. reaction between these atoms and carbon diffused from the solid front in the vol.
of liq. metal.
Sulphur being an element, superficially active, it influences reaction 1 rather than
reaction 2.
4.1.6 Thermo physical properties of alloys
Thermal conductivity, , specific heat, C and density can be expressed by
coeff. of thermal diffusivity, a
a = / C [m2/s] …………......... 4.8
Alloy with high coeff. of thermal diffusivity loses temperature rapidly brings
about increase in viscosity, so increase tendency of formation of exogenous blowhole.
Latent heat of crystallization (melting) L, influences fluidity, in the sense that great
quantity of heat librates on formation of primary crystals during laminar flow, which
brings about increase in temperature and so in, fluidity. The increase in fluidity decreases
tendency of blowhole formation.
83
Fig. 4.4 Formation of bubble in cylindrical pore of mould: (a) complete wetting (b) partial wetting (c) total non wetting with mould surface (bubble shown dotted).
gas gas gas
liq metal
84
Fig. 4.5 Absorption of [H] [N] (in ppm) in liq. steel on variation in [C] and [Si] of steel and its effects on blowhole formation
without blowholes
with blowholes
Fig. 4.6 Solubility of [H] in steel containing Mn and Cr
Fig. 4.7 Solubility of [H] in steel which do not contain Mn and Cr
%C
[C] 0.6
[C%]
85
4.2 Thermo physical properties of moulds & cores
4.2.1 Organic and inorganic binding materials
Researches conducted by author for observing influence of humidity in moulding
material on blowhole formation have shown that when content of water in moulding
material increases 6%, process of appearance of blowholes commences, when it is 8%,
frequency of blowhole increases. At 10-12% H2O blowholes appear in large scale and are
scattered throughout the casting (test bar). It shows different physico chemical reactions
which take place between humidity in the mould and liq. steel poured in it i.e. the
reaction which are responsible for gas porosity.
From investigation concerning binding material used in moulds and pores
following conclusions can be drawn:
i) Baked core have low tendency of blowhole formation. The casting made in
moulds with low permeability are more susceptible to blowhole formation.
ii) Urethane additives are little susceptible to blowhole formation. Furan resin
and Sod silicate are most susceptible to blowhole formation even in non
favourable condition.
iii) Phenol urethane system which are catalyzed by amines are found less
susceptible to gas porosity than furanic and Sod. silicate / CO2 systems.
iv) Addition of FeO upto 2% in moulding material has shown increase in
tendency of blowhole formation.
4.2.2 Capacity of filtration of gas (permeability)
Filtration of gas from mould depends upon grade of ramming, permeability of
mould and number of vent holes. Casting experiments have shown that moulds rammed
to grade of ramming 80-90 Dieter Units (DU) and permeability 90-100 are least
susceptible to blowhole formation. Permeability is largely increased by adding cardboard,
powder of wood flour, horse dung, other materials which are burned by heat of liq. metal,
in mould and providing vent hole and by evacuation.
Permeability P is defined as the rate of gas flow Q (where the gas is usually air)
through a core material of area A and length L and driven by a pressure difference P
P = QA/LP
The SI units of P are quickly seen to be:
86
[P units = [litre/s][m2]/ [m] [Pa] = l/s mPa
4.2.2.1 Mechanism of gas filtration through the mould wall (put forth by
Bidulya)
Gas evolved in the mould cavity, forms a flux and filters through porous mould in
three principal mould: parallel, divergent and converging. Parallel flux of gas is formed
by plane mould and core surfaces and have constant transverse section (a fig. 4.8).
Divergent flux is produced by the convex parts of mould and transverse section of flux of
gas increases with the increase of distance of flux of gas from the source of generation of
gas (b fig. 4.8). Convergent flux of gas is produced from concave portion of mould and
transverse section of flux decreases with increase in distance of flux of gas from the
epicenter generating gas (c fig. 4.8).
From the epicenter, vapour travel like a wave through inter granular pores of
mould. When wave passes through the porous medium of the mould, on it Darcy formula
for determining quantity of gas which passes through the porous filter can be applied.
KRthPPdF
G f
192)(. 2
022
…………………. 4.9
where
G = quantity of gas in wt, which filters through the porous medium in unit time,
g/sec.
F = surface of porous filter
d = average dia of filter
Pf = gas pressure at epicenter
P0 = counter pressure (which in this case is pressure of height of column of liq.
steel on mould wall g/cm2
K = Darcy function of filtration
2
2
)1( mdmAK
…………………… 4.10
A = area of filter cross section
R = gas constant
t = abs. temp., K
h = thickness of the filtration column, in cm
87
m = V1-V2/V1 = grade of filtration, V1 = vol. filtered (grains and pores)
V2 = vol. of pores in filter
= coeff of hydraulic resistance
Considering a tube with cross section of 1cm2 in filtration zone and putting it in equation
4.9
htRm
mPP
G f
..)1(
.192
)(
2
20
2
…………………… 4.11
This equation may be utilized for studying the modes of filtration of gas from mould
surface. Following four cases are possible in case of casting:
1. Pf > Po
When gas pressure surpasses the pressure of liq. metal exerted on the mould wall bubbles
of gas may pierce the hard skin formed on the solidifying casting and enter in the liq. metal to
form blowholes.
2. Pf < Po
If counter pressure of liq. metal increases the gas pressure, molten metal and oxides
formed on the solidifying metal enter in pores between sand grains.
3. Pf = Po
If gas pressure is equal to counter pressure exerted by the liq. metal, blowholes and
surface imperfection are not formed. The is the most desirable condition.
4. Pf >> Po
During this condition, an explosion may occur and liq. metal may spurt out of the mould.
Transverse section of flux of gas passing through the core [Medvedev (1964,65)] is
Fnp = l / l0 dl/Fi
Where
Fi = transverse section of core = A(l)
l = length of the filter
A = surface area of transverse section of core
Filtration of gas through the mould wall depends on many factors mould and core
technology. These factors include permeability of moulding material, which depends on grain
size, grade of ramming, humidity, contents of special additives, time after mould preparation,
dynamic viscosity of gases and geometrical configuration of moulds and cores.
88
Fig. 4.18 Different types of gas fluxes which are generated in molud
a) Parallel
Fig. 4.9 Filtration of gas through different porous bodies with different geometrical configuration a , b = plain, parallel c, d = radial convergent & divergent e & f = plain and complicated
89
i. Grade of ramming
Grade of ramming influences the permeability while filtration of gas depends on
permeability. Increase in grade of ramming decreases permeability and in the same token,
capacity of filtration. Experiments carried out with 10% clay and 5% water have
demonstrated that when permeability decreases from 220-130, capacity of filtration decreases
1.4 – 1.5 times respectively (a, fig. 4.10).
ii. Grain size and density of moulding material:
Greater is the size of sand grain, more will be the capacity of filtration of mould.
Greater will be the density of the mould less will be its porosity and capacity of filtration. In
case of a typical moulding material which contain 6% bentonite, when apparent density of
mould is increased from 1.2 to 1.5g/cm3. The permeability increases from 200-425 (b, fig.
4.10).
iii. Moisture in moulding material
Increase in the water content of the moulding material which contains 5% clay from
7-11%, shows decrease in permeability from 600-300, which decreases the capacity of
filtration (c, fig. 4.10).
When mould temperature decreases from 10 to -70oC permeability for air (increases
approximately 60%). Decrease in the temperature of mould decreases the viscosity of mould
gases and effective volume of gas filtering, due to the increase in compression by cooling.
Under compression, green sand moulds permeability decreases upto 10 units as compared to
40-80 units in mould made of some special additives. The decrease in mould temperature
brings about increase in dia. Of the pore due to the shrinkage of sand grains. Thus increases
the permeability and capacity of filtration of mould.
iv. Special additives in the mould
Locke & Ashbrook [55] determined experimentally gas pressure developed in the
moulds having permeability 10-260 while casting liq. steel. The maximum gas pressure
approaches from 1.7 psi to 0 when permeability reaches 250 (fig. 4.11 dotted curve
similar experiment is conducted with samples having additives (0.5 to 3% starch and 0.5
to 3% core oil ) and maximum gas pressure is measured on flat sand / liq. steel. On
increase of the starch and core oil content in silica sand, the gas pressure developed in the
mould cavity is increased while on increase of permeability, it diminishes slowly to 0
(fig. 4.11 relevant curves).
90
Fig. 4.10 Factors those affect permeability of sand mould a) grade of ramming (sand with 10% clay 5% water ) b) apparent density of the mould having 6% clay c) humidity in sand mould with 5% clay d) variation in viscosity of gases with increase of temp. O2 , N2 , H2O ,H2 & CO e) time after pouring
Perm
eabi
lity
No
Perm
eabi
lity
No
DU
Perm
eabi
lity
No
apparent density
humidity
Vis
cosi
ty c
pois
Gas temp.
O2 N
CO
Water vapour H2
Perm
eabi
lity
No
50
0
25
Time after pourng (min)
1 2 3 4 5
oC
91
v. Geometrical configuration of moulds and cores
Geometrical configuration and cores largely influences the capacity and quantity
of gas filtering through the mould wall. Filtration of gas through different porous corps is
illustrated in fig. 4.9. Capacity of filtration through the plain porous parts of the mould
can be calculated by relation
M = K (A/l) …………… 4.13
where
M = capacity of filtration,
K = permeability,
A = transverse section of porous body through which gas filters and
l = thickness of the porous body
For radial parts, transverse section of flux of gas is calculated by formula 4.12 and
capacity of filter by formula 4.13. Exact value of capacity of filtration of complicated areas
fig. 4.9 ( c to f is difficult, due to simplification of function Fi. In these cases only the
approximative values can be determined.
Trajectory of gases formed the different zones of the mould is illustrated in fig. 4.12.
The mould gas passing through the plain surface of the mould have uniform flow while
through irregular and odd surfaces it will have non uniform flow of flux of gas. All the gas
formed in the mould cavity will converge towards the mould parts, in which ‘vent hole’ is
provided, while in such cores when no vent holes are provided, the flux of the mould gas will
travel towards the way, where it will find the lowest resistance (a, b, fig. 4.13).
Conditions of gas filtration through the cores are somewhat different from the
moulds. Mould acts as recipient for hot molten metal, the gas evolved from different
epicenters of the mould surface, on intense heating by liq. metal, scatters in the mould while
core is completely dipped in the molten metal and is subjected to intense heat from all sides
and special vent holes are provided for ejection of gas and releasing the gas pressure (a and b,
fig. 4.14). Convergent flux of gas increases the resistance forces of pore and reduces the
chances of evacuation of gas. Fig. 4.14 a to c illustrates process of filtration of gas through
cores of different configuration, provided with vent holes and dipped in the molten metal
upto the different heights. Fig. 4.14 d shows variation in gas pressure at different intervals
after pouring.
92
4.11 steel
93
A = uniform flow B = non uniform flow Fig. 4.12 trajectory of gases through the mould pieces
a = with vent holes b = without vent holes Fig. 4.13 Effect of vent holes on trajectory of gases
94
Some of the researchers have measured gas pressure at different points on sand core
dipped in the liq. metal and gas pressure developing in core of different permeability. Medvedeve
I.I. [4, 11] in an experiment dipped the core of a specific configuration in liq. metals in two
modes: from (a) “down to up” and (b) “up to down” and measured gas pressure (Pf) metallostatic
pressure (Pm) atmospheric pressure (Pa) and capillary pressure (Pc/) at different points on the core
A, B, C, D, E and K (see fig. 4.15). Value of Pa and Pc is same for all these points of these two
cores, Pm is greater in case of (a) then in case of (b) and depends upon height of column of liq.
metal in which the core is dipped. Pf varies almost in the same way for both of the cores and
values of (Pa + Pc + Pm – Pf) are different at different points and are not identical with each other.
In other experiment [Ref. 55] the core having different permeability numbers 50, 90, 205, 300 are
dipped in the liq. steel to different depths and maximum gas pressure is measured. Gas pressure
increases with increase in depth of the core for all the cores with different permeability number.
The Pf / permeability curve shows the linear and proportional increase (a, fig. 4.16). If core is
provided with vent hole the gas pressure will suddenly drop (d, fig. 4.16).
vi. Casting temperature
Increase in casting temperature influences the gas filtration in two modes: increases the
dynamic viscosity of mixture of mould gas and as such their capacity of filtration, but on the
other hand, it decreases permeability of the mould and so capacity of filtration. Fig. 4.10 d shows
that the viscosity of gases is function of temperature and variation in capacity of filtration with
time after pouring, is shown in fig. 4.10 e. It is divided in four stages: Immediately after pouring,
the change in permeability is shown by DC. In first mode capacity of filtration of gas is large due
to the increase in dynamic viscosity of gases and abrupt gas pressure developed in mould cavity.
After, the gas pressure attains highest value, it starts decreasing. Under effect of heat, particles of
sand dilate and permeability of sand decreases (CB portion of the curve). At this time the capacity
of filtration of gas is little. Gas temperature increases without any interruption, after pouring and
affects on physico chemical properties of gases, their chemical composition and viscosity. After
that the permeability of mould remains almost constant (curve BA)
In theory a lowering of the casting temperature will lower the internal core
pressure. However, this is quickly seen to be a negligible effect within the normal limits
of casting temperatures. For instance, a large change of 100oC in the casting temperature
of an aluminium
95
Fig. 4.14 Dipping of core in liq. steel and appearance of gas pressure
Time after pouring
96
Fig. 4.15 Gas pressure and contra pressure at different points in core with different configurations dipped in liq. steel (a) from down to up (b) from up to down
B E D
P
97
alloy will change the pressure by a factor of approximately 100/600. This is only 16.7 per
cent, and can be abandoned as a useful control measure. The effect of casting temperature is
seen to be important when moving from one alloy system to another, such as from aluminium
to steel. An enclosed core which would give no problems in an aluminium alloys casting may
cause blows when the same pattern is used to make an equivalent bronze or iron casting. (J
Campbell, 1993) [68].
4.2.3. Resistance to high temperature (high refractoriness, high
thermal stability)
Moulding material must have high resistance to the elevated temperature i.e. it should
have high thermal stability. As mentioned earlier, many times, that when liq. steel enter in the
mould cavity, volatile material decompose and gas formed filters through the porous mould.
If mould does not have high thermal stability and refractoriness, fissures and crevices may
appear on the active surface of mould which can act as canals for penetration of gases in the
liq. metal.
4.2.4 Mould washes
Mould and the core washes are considered very effective for elimination of certain
defects in casting. Refractory based washes penetrate in the superficial layer of the mould
and eliminate certain reactions which generate gases. Surface roughness of the mould and
mould washes (Rz) influences angle of wetting () and dimension of the pore and tendency of
blowhole formation.
4.2.5 Mould temperature
Mould temp. has also an important effect on porosity. According to Tedss increasing
temperature of mould from1000 to 1150oC reduces largely the central shrinkage porosity of
turbine blades. Bogdanov has confirmed an increase in density, corresponding, porosity increase
1 to 4%, when mould temp. was raised from 20 to 800oC. According the J Campbell, increase in
temp. modifies the morphology of porosity: from central macro porosity to dispersed porosity and
reduces total porosity by creating steep gradient of temp. in the mould wall.
4.2.6 Coefficient of heat accumulation
Capacity of mould to absorb heat, given by the liq. alloy can be calculated by
coeff. of accumulation on of heat bf
bf = . C.
98
4.16 [55]
(b)
(a)
99
where
= thermal conductivity
C = specific heat and
= density
For mould made by addition of fine materials, decrease in the vol. of pores brings
about increase in thermal conductivity and rate of coeff. Of accumulation of heat, bf when
thermal conductivity of the mould is large (in case of metallic mould) quantity of heat
transmitted through the mould wall will also large, metal loses large quantity of heat.
According the J. Campbell, in case of the mould having little value bf tendency of
formation of dispersed porosity will be increased.
4.2.7 Condensation zone
Condensation zone is formed in cope or drag, when water vapour formed under the action
of heat of liq. metal filter through the mould wall and rise upward, where they come in contact
with relatively cooler parts of mould walls. The hot water vapour are cooled and condense at the
cooler parts of the mould walls forming “condensation zone” which has more moisture content as
compared to the other parts of mould. Large number of bubbles of H2 are generated when heat
from the liq. alloys reaches in the condensation zone. Bubbles propagate in the molten metal due
to the high partial pressure and finally captured by the solidifying crystals to form blowholes or
pinholes (see detail in section 2.1.1.1).
4.3 Pouring conditions
4.3.1 Pouring temperature
High pouring temperature increases the tendency of indigenous blowhole formation in
steel castings. Liq. steel dissolve O2 from the atmosphere forming FeO, which reacts with carbon
of steel, forming CO
FeO + C = CO + FeO
Greater is the pouring temperature high will be rate of absorption of gases: O2 , N2 and H2
and also greater will be the frequency of formation of CO blister. Further, gases such as, H and N
diffuse in these “violent” blisters to give them final shape of blowholes.
4.3.2 Reynolds Numbers (Re) [57]
When liq. steel enters in the mould cavity and flows on the mould surface, laminar or
turbulent flow and oxygen in steel (Os) simultaneously influence appearance of blowhole.
100
In fig. 4.17 (a to b ), the effect of variation of values of Re and Os in upper and middle
portions of test bar, on appearance of blowholes, have been shown. When Os < Ocr and Re < Rcr,
grade of de-oxidation does not decrease below 1, result is, that casting is free of defect (a, fig.
4.17). (dotted line indicates Os while Os/ and Os// show respectively the content of O2 in middle
and upper parts of casting). When the flow of the liq. metal is laminar in all the casting length (Re
< Recr and Os < Ocr) blowholes will not appear (see fig. b, 4.17).
When Os// passes the value of Oscr and Re < Recr, blowholes will be formed in that part of
the casting where Os// > Ocr (c, fig. 4.17). When Re decreases below Recr laminar flow will be
observed and only upper portion is oxidized and interior remain non oxidized. When Re < Recr
and Os// are far greater then Ocr the blowhole will appear throughout the casting (d, fig. 4.17).
Influence of Re on blowhole formation in casting with casting walls at right angle
to each other is shown in fig. 4.18. When flow is turbulent (Re > Rcr) Os increases due to
the effect of oxidation in mould cavity but remains below Ocr thus blowholes do not
appear (a, fig. 4.18).
When upper portion of the mould is filled with laminar flow and lower portion by
turbulent flow, of the liq. metal value of Os will increase according to fig. 4.18 (a).
When Re increases than Recr, O2 in surface (Os//) and in the interior of molten metal (Os/)
will differ, but (Os//) will remain below Ocr, so the blowholes will not appear (b, fig.
4.18). If, at the beginning of pouring, liq. steel will have low “grade of de-oxidation”, the
level of de-oxidation increases very slowly in the mould (i.e. liq. steel has sufficient time
to react with the mould, “secondary oxidation in the mould cavity”). When Re > Recr and
Os// is greater than Ocr, blowholes will appear in all the casting.
4.3.3 Sprue System
Sprue system which reduces velocity of pouring and filling the mould cavity is
considered more favourable for appearance of gas defects. It is most probably due to the
fact that in such case, liq. steel takes relatively more time in filling the mould and absorbs
comparatively more quantity of gas from atmosphere and mould.
A correlation between gas defect and gates used for pouring is establish by
N.Yoshinaka & S.Uezima. Blowholes appear in that part of casting in which liq. steel
solidifies first or where slag formed by steel mould interaction collects and solidifies. Pin
holes appear in the zones away from gates and runner (fig. 4.19) In casting of thin walls,
101
maximum blowholes formation tendency is observed in casting within 13-25mm casting
wall thinkenss (fig. 4.20
4.3.4 Velocity of Pouring, rate of pouring :
Low velocity of pouring has recommended by some foundrymen as a proper
measure for eliminating porosity in casting. Low rate of pouring offers sufficient time for
gases to evolve.
Liq. metal may be poured in the mould at the optimum velocity so that defects other than
gas porosity may also not appear.
4.3.5. Length of flow of liq. metal (Lf)
If path of flow of liq. steel (Lf) is large, frequency of appearance of defects will
be high (fig. 4.21). On the entrance of steel in mould cavity, liq. steel is at high
temperature, so the mould is superheated and layer of slag (sand burn-on, 2FeO.SiO2)
appear on it (portion BN). Then it reaches to a zone where defect do not appear (portion
WD). This zone is followed by a zone where porosity may appear (portion PH) and at the
end, there is a portion of casting, where sub coetaneous (sub-surface) porosity is
observed (SB)
4.4 Pressure on the column of liq. metal
4.4.1 Metallostatic pressure
When liq. metal enters in the mould cavity, it exerts pressure, which is termed as
“metallostatic pressure” and it increases with increase in the height of the column of the
liq. metal on the mould wall and with increase in viscosity of liq. metal during
solidification. Bubbles which enter in the molten metal, shrink in size and volume, in an
irregular way, due to the metallostatic pressure. Greater will be the metallostatic pressure,
lower will be number of bubbles penetrating in the liq. metal and lesser will be the
tendency of blowhole formation.
102
Fig. 2.17 Effect of Re & Os on blowhole formation a) Os < Ocrt, Re < Recrt b) Re<Recrt, Os < Ocrt, O/s< Ocrt, O// < Ocrt c) O// > Ocrt, O/s<Ocrt , Re< Recrt d) O/s> Ocrt, O//s > Ocrt Re< Rcrt
Just after pouring
Just after solidification
Distance of flow of liq. steel in mould
103
Fig. 4.18 (B) Effect of type of flow on blowhole formation (a) Re > Recrt and Os < Ocrt (b) Upper portion Re < Recrit, lower portion Re> Recrit & Os/< Ocrit, O//s <Ocrit (c) At intersection Re & Rcrit, Os// > Ocrit
After filling of mould
After filling of mould
After solidification
After solidification
After filling of mould
Turbulent
Ocr Recr
Re
Re>Recr
Re<R
ecr
O//s
O//s
104
Fig. 4.19 Mode of formation of surface blowholes near the gates
a) appearance of blowholes in casting b) frequency of appearance in casting near
gate 1) gate 2. blowholes
Fig. 4.30 Effect of casting thickness on frequency of surface blowholes
Fig. 4.21 Effect of length of flow of liq. metal on appearance of blowhole formation
Length of path of flow of liq. metal
Freq
angle
Freq
defect
end
90
105
4.4.2 External pressure
Researches carried out by author with the help of a experimental model for
studying the effect of external pressure on size and volume of the bubble suspended in
the liq., has confirmed that blowholes may reduce in size and are eliminated by applying
external pressure on the column of liq. metal. Vol of bubble decreases 4 to 5% on each
pressure value 0.1daN/m2 (see fig. 4.22). The experiments carried out with test bar of
aluminium and bronze, vol of gas 2.71cm3/100g in aluminium and bronze at normal temp
& pressure decreases to 0.95cm3/100g at 1daN/cm2. Size of bubble diminishes gradually
on the application of external pressure on the column of liq. metal and finally bubble
disappears at 500daN/cm2. The vol. of porosity is inversely proportion to the pressure
applied to it during its growth. The solidification under reduced pressure enhances the
size of pore, for instant, in reduced pressure test, percentage of porosity is expanded by a
factor of 10 by freezing at 76mmHg (0.1 atm) residual pressure rather than at 760mmHg
(normal atm pressure) [72].
4.5 Casting thickness
According to E.P. Babici, V P Sebrov and V.C. Postika, the process of formation
of porosity in castings may be divided in four periods:
I) First period starts when liq. metal enters into the mould and continues upto the
formation of “hard skin” on casting surface – a period in between pouring and
inception of solidification.
II) Second period starts with reaction between water vapour and liq. metal and
terminates when mould is unable to transfer water at the mould / liq. metal
interface. A part of the vapour enters in the reactions with the liq. metals and in
the case of liq. steel it forms FeO and H. During this period, gas pressure in the
mould reaches to maximum value but after filtration of vapour through the mould
wall, their pressure reaches to the level of atmospheric pressure.
III) FeO and H dissolve in the liq. steel, gas pressure in the mould is equal to the
atmospheric pressure.
IV) Solidification
106
Fig.
4.2
2 ef
fect
of e
xter
nal p
ress
ure
on v
olum
e of
gas
bub
ble
susp
ende
d in
col
umn
of tr
ansp
aren
t liq
.
Ver
y sm
all b
ubbl
e sm
all b
ubbl
e M
ediu
m b
ubbl
e La
rge
bubb
le
6x8m
m
107
Fig. 4.23 shows quantity of H and FeO dissolved in the outermost hard skin
formed on casting test bars with different wall thickness.
1. In case of casting with medium thickness, period II & III will terminate before
termination of period I. Content of hydrogen in the superficial hard skin is
normal. Content of FeO decrease below critical value during period III. So, sound
castings are obtained. But a content of FeO is greater than critical value (due to
insufficient de-oxidation) small blowholes of micro size may appear (curve, a/,
a//, fig. 4.23)
2. In case of the casting with thin walls (curves c/, c// fig. 4.23), period I is so short
that it finishes before period II. Period III is totally absent and exists high
concentration and of H and FeO. Surplus FeO and H is consumed in blowhole
formation.
3. In case of casting with walls thickness less than (a) and greater than (b), fig. 4.23
c, d/, d//, superficial hard skin will have high con. of FeO and H
Zuithoff (1964, 1965) [43] showed that aluminium de-oxidation would control the
appearance of pores. Clearly if oxygen was high, then pores could nucleate, but they would
not necessarily grow unless sufficient hydrogen was present. Conversely, if hydrogen was
high, pores might not form at all, if no oxygen was present to facilitate nucleation. The
hydrogen would therefore simply remain in solution in the casting.
Results of the casting experiments carried by Pribyl and Starosta [37] are
presented in fig. 4.24. In case of thick casting, and quantity of C is as little (C<Ccr) that it
could not enter in reaction with FeO the blowholes will not appear (a, fig. 4.24). In case
of castings with thick walls being cast in the oxidizing moulds, blowholes are formed
when C > Ccr (b, fig. 4.24, curve C, section C)
FeO + C = Fe+CO
Original CO blisters are elongated and occur at the margins of casting (b, fig.
4.24, section C/) . H diffuses in violent blisters formed by CO and give them final shape
of blowholes. In case of thin castings, in a mould which is slightly oxidizing, blowholes
are formed in the zones where C > Ccr (c, fig. 4.24, curve C/, Section C). When C > Ccr
and steel is again slightly de-oxidized, blowholes can also be found in all the casting (c,
fig. 4.24, curve C, section C/). If steel is strongly de-oxidized casting will be sound and
free from blowholes (c, fig. 4.24, curve C//, section C//).
108
Fig. 4.23 Formation of FeO
and H2 in steel castings w
ith different casting thicknesses (a) medium
(b) thin (c) in betw
een a & b
Time
Time
Time
Time
Time
Time
%H
2 crit %
H2 nor
Pres. atm
%
FeO crit
%FeO
nor V
ol of pors
109
According to J Turton [45] decrease in the mould section thickness increases ratio
of surface area of mould wall in contact with the liq. steel, which increases the rate of
cooling at the steel mould interface, particularly, in case of green sand mould. So
decrease in mould wall section thickness directly affects on blowhole formation, i.e.,
greater is the surface at which the reactions are taking place, more will be absorption of
gas in casting.
According to J Campbell [52], when section thickness of casting decreases,
tendency of formation of porosity transforms from mechanism of ‘nucleation’ to ‘non
nucleation’ (fig. 4.25). In case of large castings, strong film (hard skin) is formed on the
exterior of casting after solidification, while its interior part remain in the liq. state.
Pressure of liq. metal in the interior of casting remains at reasonably high level, during
the formation of hard skin, therefore, conditions are not favourable for initiation of any
pore. Similarly if internal pressure decreases due to the contraction, inception of porosity
by process of non nucleation is also not possible due to the formation of solid crust. In
casting whose internal portion is isolated from atmosphere, the only possibility of
formation of porosity in such cases, is its initiation by nucleation i.e. any inclusion can
act as “embryo” for nucleation of a pore and initiate interlinked porosity. In case of
casting of intermediate dimension, surface of casting may remain in condition of
incomplete solidification and internal portion may feed the liq. metal. Due to the decrease
of casting temperature, shrinkage takes place and pores are developed on the surface
(internally connected shrinkage porosity) - (mechanism by non-nucleation) various
stages of pore development are shown in fig. 4.25 a, b & c.
4.6 Height of liq. column
Greater is height of the liq column, lesser are the chances of formation of
blowholes in casting. Column of liq. metal exerts metallostatic pressure on mould surface
and bubbles of gases ejected from the mould cannot enter in it till gas pressure increases
and overcomes the counter pressure exerted by the liq. metal.
4.7 Periphery of casting
It has been indicated R. Ferry that periphery of round castings are more
susceptible to the appearance of blowholes. These blowholes are the result of reaction
taking place at liq. metal mould interface.
110
Fig. 4.24 Blowhole formation in steel castings with different casting thickness
111
Fig. 4.25 Porosity appearance in castings (a) thin (b) intermediate (c) thick
a) thin surface blowhole b) intermediate interconnected porosity c) thick casting
air
nuclei
Fig. 4.26 Effect of inclination on porosity in steel (0.25% C)
Poro
sity
Inclination
112
Fig. 4.27 Different types of breakthrough exhibited by the bubbles
Breakthrough towards exterior
Breakthrough towards interior
without breakthrough
113
Fig. 4.28 Fixation of gas bubble in solidifying metal
Crust
W=vel. of gas bubble F = vel. of advancement of solidifying front
W/F large
W/F less
W/F equal
114
Fig.4.29 Correlation between gas pressure developed in mould cavity (Pf) and physical morphology of gas defect (schematic)
Velocity of solidification High Pg
Temp
Medium
Low
Hard skin
Liq metal Liq metal
Liq metal
Hard skin
Sold. front
115
4.8 Inclination of casting
In some casting experiments, J Campbell cast test bars of 100 x 30 x 5 mm thick
with 2, 4, 6 & 8 o inclination. Pouring was carried out directly from the furnaces to get
same casting temperature, steel composition and amount of the gas dissolved in test bars.
Porosity was detected by NDT- radiography. Results obtained have shown that frequency
of porosity decreases largely with inclination of 2oC and then remains same (fig. 4.26).
A useful simple test for taper steel samples was proposed by Denisov (1965): a
sample test piece was developed 110mm high, and 30x 15mm at the top tapering to 25 x
12mm at the base. A metal pattern of the sample is then moulded in the sand.
Immediately after casting, the sample is knocked out and quenched in water. It is then
broken into three pieces in a special tup. The entire process takes 1-2min. It was found
that the tapered test piece gave an accurate prediction of the risk of subsurface porosity
and it was concluded if such defect is observed in the sample, it will also be experienced
in the castings.
4.9 Vel. of solidification
When bubble is ejected from the solidifying casting, it leaves behind a passing
path, trajectory or “breakthrough”. Bubble, with “breakthrough towards exterior”,
“breakthrough towards interior” and leaving “no breakthrough”, is shown in fig. 4.27 a, b
& c, respectively. If vel. of propagation of bubble (W) is less than vel of solidifying front
(F), the bubble will appear as blowhole in solidified casting (b, fig. 4.28) (Casting
without any breakthrough). If both of these velocities are equal, the bubble will have
breakthrough towards exterior (a, fig. 4.28) while advancement of solidifying front is at
the lower velocity than that of the bubble, the bubble will be ejected from the solidifying
front leaving trajectory, breakthrough towards interior of casting (c, fig. 4.28).
Correlation between gas pressure developed in the mould cavity ( Pf ) and
physical morphology of gas defects is illustrated in fig. 4.29. At the max. gas pressure I,
the bubbles of gas will rapidly leave the vol. of molten metal even it has high vel of
solidification (a, fig. 4.29). In the mean time thin hard skin will be formed and if still (Pf
> Pm + Pc + Ps condition exists (see text) the bubble will pierce the hard skin forming
external blowholes (b, fig. 4.29). With passage of time the hard skin formed is thickened
116
and the gas pressure attains value of max. pressure II, bubble entered during this
condition, in the mould cavity, will not be so energetic to pierce hard skin and will settle
under it (c, fig. 4.29) and will appears as blowhole after solidification (d, fig. 4.29)
In case, if vel. of solidification is medium (say equal to the vel. of propagation of
gas bubble ) the bubble will float on the solidifying front (e, fig. 4.29) (max gas pressure I
and will grow in size by agglomeration with other bubbles) (f, fig. 4.29). In the mean
time the thickness of hard skin is also increased and gas pressure attain value of max. gas
pressure II. Its pressure will not be enough to pierce thick hard skin so it will settle there
and after solidification of casting, it will appear as blowhole. In case, vel of solidification
is lower (i, fig. 4.29) than that of vel of bubble, at max pressure I, the bubble will eject
rapidly. Some time after pouring the bubble will still not be captured by the solidifying
front (j, fig. 4.29). Because bubble is proceeding at higher speed than vel of advancement
of solidifying front. The bubble will leave the solidifying metal leaving a path (k, fig.
4.29) and after solidification it will appear as breakthrough towards interior (l, fig. 4.29).
117
Chap 5 Summary, prevention and conclusion
One of the most important casting defects is blowholes, which are formed by
evolution of gases from:
- substances present in the moulding material (water, volatile matters, minerals in
silica sand etc. ) and liq. metal / mould interaction.
- Liq. metal, gases which are absorbed during melting and casting.
Depending on the origin of the gases, the blowholes are grouped in:
- Exogenous blowholes which are formed by gases produced from the mould by
metal and mould interaction.
- Indigenous blowholes which are produced by the atmospheric gases dissolved in
liq. metal and reactions in vol. of liq. metal during melting, pouring and
solidification.
According to the dictionary of Foundry, Blowhole is defined as “a sphere shaped cavity,
which occupies small volume and is found single or in group, in different parts of casting”
Being the most tedious and frequent casting defect foundrymen are pondering over this and
valuable studies are carried out on this most controvercial topic of foundry.
Sources of mould gas
1. Evaporation of water and other volatile substances
2. Burning of organic materials present in the mould in the presence of oxygen
3. Decomposition of minerals present in the moulding sand (e.g. carbonates)
4. Dissociation of gases produced by bonding material (CH4,NH3 etc.)
5. Dilation of air present in the pores of mould
6. Decomposition of organic substances (hydrocarbons, resins, oils etc.) added in
the moulding mixture.
7. Physico chemical reactions taking place at alloy-mould interface.
FeO + C = Fe + CO
FeO + 2H = Fe + H2O
Mg + H2O = MgO + H2
2Al + 3H2O = Al2O3 + 3H2
Ti+2H2O = TiO2 + 2H2
MgS + H2O = MgO + H2
118
Composition of mould gas
Moulding material contains Gas Bakelite, % Sulphatic lye %
H2S + CO2 0.2-1.0 0.4-1.3
CnH2n Upto 0.50 1.0-1.5
O2 0.4-1.0 0.3-0.9
CO 29.30-30.4 27.75-29.70
N2 3.96-8.66 4.09-6.57
CnH2n+2 5.34-6.83 10.96-14.06
H2 53.41-60.06 48.03-50.03
H2O 18 20
Temp of gas formation and capacity of gas generation of different bonding materials
Material Temp. oC Capacity of gas generation cm3/g
- Water 100 550
- urea formaldehyde 280-300 410
- dextrin 330-380 850
- molasses 380-420 540
- sulphatic layeee 400-420 500
- bentonite (chemically
bound water)
200-420 20-100
- linseed oil 420-480 500
- phenol formaldehyde 650-750 460
- coal tar 600-850 450
- coal powder -- 200-600
- kaolin 600-900 10-2
119
Mechanisms of Exogenous Blowhole Formation
Exogenous blowhole are formed in following three steps:
- appearance of supplementary gas pressure at the liq. alloy-mould interface.
- Penetration in gas bubble in the liq. alloy.
- Fixation of gas bubble in casting wall forming blowhole
The bubble of gas penetrates in liq. alloy when
Pf > Pm + Pc + Pext
where
Pf is gas pressure at alloy-mould interface
Pm – metallostatic pressure
Pc – pressure necessary for overcoming capillary forces
(Pc = 2 cos/r where = surface tension of liq. alloy, = contact angle, r =
radius of pore in mould surface)
Pext – external pressure (atmospheric pressure)
Ascending velocity of bubble (<1mm) in liq. alloy, v,
v = 2/9 g r2 (a - g) 1/
where
g = accel. due to gravity
r = radius of bubble
a = density of alloy
g = density of gas
= dynamic viscosity of alloy
In case bubble >1mm
a
gagV
1
38
= resistance coff. of liq. alloy
Energetic interaction of gas bubble with liq. alloy during its solidification in
mould cavity is explained by Medeev
a) If gas bubble is rising at high pressure and hard skin formed is very thin, the
bubble will pierce through it. The force with which bubble will act on hard
skin (Fg),
120
Fg = r2 Pg
condition of piercing
Fg > Fm + R
where
Fm = force which has supported the hard layer
R= resistance of layer to piecing
b) gas bubble rises without any obstacle
c) bubble meets with hard skin formed on the upper surface and acts with force Fg
Fg = (a - g) V
a - g are sp. gr. of alloy and gas respectively.
V = vol. of bubble
d) Due to decreasing vis of liq. alloy the bubble settles under the hard skin and
forms subcutaneous blowholes
e) If bubble reaches at the surface of mould, before the decrease of viscosity of
alloy, the bubble ejects out.
f) In vertical parts of casting the bubble rises through the liq. alloy when
solidifying front are far off.
When liq. alloy has contact angle = 0 or 0 bubble will appear in pore, with
concave surface towards liq. steel and it will find difficulty in penetration due to the
complete wetting of mould surface with liq. alloy. In case of partial wetting (0 < < 90o)
the bubble will depart from the mould surface, leaving tiny bubble in the pore which will
act ‘embryo’ for new bubble.
In case of non wetting ( > 90o) bubble will form convex surface towards column
of liq. metal. When bubble reaches to the state of separation, it appears in spherical form.
Factors Effecting Exogenous Blowhole Formation
Principal factors which influence on exogenous blowholes are: content of volatile
material, humidity in the moulding sand, physico mechanical property of the mould,
nature of the gaseous atmosphere in the mould cavity, composition of alloy cast, casting
temperature, casting thickness, surface tension of liq. alloy, pouring rate,
thermodynamical conditions of reactions which take place at the steel-mould interface
and some other factors. Gas pressure at steel mould interface, Pf :
121
ltP
PV
taP
o
p
gf
2
where
is coeff. of nature of mould
ag - coeff. f evolution of gas
t - time after casting
Vp – vol. of pores
Po – atm. Pressure
P – permeability of mould
l – thickness of filtration column
The gas pressure reaches two times max. values. First time immediately after
pouring and secondly after almost 20 to 200 sec. after pouring, due to the decrease in
permeability of mould, when hot liq. metal dissipates heat to the mould wall.
Factors concerning pouring conditions
- casting temp.
- Reynolds Numbers,
- Spruce system
Factors concerning configuration of castings
- casting thickness
- height of the liq. column
- velocity / rate of solidification
Factors concerning nature of steel
- surface tension and angle of wetting
- chemical composition
- density of the liq. metal
- viscosity of liq. metal
Factors concerning nature of mould
- Organic and inorganic. binders and special additives
- permeability and capacity of filtration
- temperature of mould
- hot strength of mould
122
Sr. Factors: increase in:
Tendency of blowhole formation
1. Surface tension Increases
2. Sp heat of steel Increases
3. Latent heat of mould Increases
4. Density Decreases
5. Viscosity Increases
6. Humidity Increases
7. Permeability Decreases
8. Temp. of mould Decreases
9. Hot strength Decreases
10. Casting temp. Increases
11. Reynolds Numbers Decreases
12. Extern. Pressure Decreases
13. Casting thickness Increases
14. Height of col. of liq. alloy Decreases
15. Casting taper Decreases
16. Angle of wetting () Increases
Quantity of gas (G) filtering through the porous media can be determined by
Darcy formula
Fd2 (Pf2 – Pm
2) G = 192 KRh where
F is thickness of porous media
d – diameter of pores
Pf – gas pressure
Pm – metallostatic pressure
- coeff. of hydraulic resistance
K – filtration factor
R – gas constant
h – thickness of filter
123
Prevention
Modern moulding and casting technology has offered a number of methods for
prevention of blowholes in casting. Some of these are: drying of mould and cores, use of
refractory powders and washes, increasing the external pressure, reducing the quantity of volatile
materials in moulds, increasing permeability of moulds and cores etc.
One of the most efficient method recently introduced for prevention of gaseous porosity
is the absorption of gases from the moulds and cores during casting with the help of vacuum
pump. In this way, the mould gas is absorbed toward outside, eliminating the danger of their
penetration in the liq. alloy. It has been concluded from the data available in the concerned
literature that while using the sand moulds with bonding materials, following principal
possibilities exist for reducing the tendency of blowhole formation in castings.
1. Reducing the gas pressure Pf at metal -mould interface for obtaining following inequality
Pf < Pm + Pc + OPext
It has been successfully obtained by absorbing air and gases from the pores of the mould
and cores during casting by using specially designed mould box connected with the vacuum
pump.
2. Decreasing the radius of capillaries of mould surface, thus reducing the possibility of
penetration of bubble of mould gas. It has been carried out by impregnating the active
surface of the mould cavity with different solutions, ethyl silicate, polystyrene solution in
CCl4
3. Changing the contact angle of liq. alloy mould and reducing the volume of pores in the
active surface of mould cavity by applying mould washes an impregnants .
4. Decreasing the temp. of grains of sands from the superficial layer of the mould cavity for
diminishing chances of appearance internal tension in the mould, which creates fissure.
These fissures give way to the mould gas to enter into liq. metal . Imposition of vacuum
continuously sucks air through the pores of the sand thus decreases temperature of the
mould and hot layer on metal-mould interface.
5. Elimination or removal of condensation zone away from the liq. alloy mould interface. It
is done by: (a) vacuum created in the moulds and cores (b) active surface of the mould
cavity which comes in contact with liq. alloy, is impregnated.
For further details, please, see my next book “Miracle of Imposing Vacuum
(‘depression’) in Sand Mould” which will be published by end of 2011!
124
Album
of
Blowholes in iron and steel castings
125
126
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43. A.J. Zuidhof- Formarea suflurilor si porozitatilor in formade impunsaturi de ac in piesele turnate din otel in function de continuturile de hydrogen si oxygen ale garjei. Giesserei, 9/1965, p.820-827 (traducere)
44. K.E. Honer – Effect of nitrogen on the formation of gas cavities in steel castings. Giesserei, 62 (1975), p. 6-12.
45. J. Turton – Gas porosity in steel castings. The British Foundry man, Jan. 1967, vol. IX, part I, p.23-32.
46. Chen, F., Keverian, J. Effect of nitrogen on surface pinholes in steel castings. Modern Casting, 1966. 50, No. 1 p. 95-103
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47. Sanders, M.A.S. –D’on viennent les pigures? Founderie, Fev. 1958, No. 45, p.53-63. 48. Heiser, F.A., Slawsky, M.L., Child, M.J. –Factors affecting gas-metal surface defect in
shell molded steel castings AFS Trans, 1962. 49. Volker, W. – Gasalegabe und Randblasenbildung beim Giessen and Erastarren von
beruhigtem stahl. Stahl and Eisen, 26 Dec. 1968, 26, p. 1455-1462. 50. Dawsow, J.W. Kilshan, J.A., Worgah, A.D. –Wature et origine des defauts dus aux gas
dens les pieces en fonte. Founderie, 287, p. 77-81 et 288, p. 122-125. 51. M.I. Clifford-Metal penetration into pore, nitrogen/hydrogen pinholes and resin defect.
The British Foundry men Nov. 1967, LX, Part-II, p. 447-456 52. J.Campbell – On the origin of porosity in long freezing range alloyes. The British
Foundry men. April 1969, vol. IXII, Part 4, p. 147-148. 53. R.A. Rijkof, P.S. Spesski – Regimal de guze al formei de turnare. Litoinoe proizvodstvo,
1961, 4. p21-23. 54. Mackawa, S., Suzuki, K. – Contact angles between moulds and molten steel. Imono, Vol.
43, 8, Aug. 1971, p. 659-666. 55. AFS Transactions 2005 AFS, Schaumburg IL USA (paper 05-245) (04) pdf 5 of 28. 56. Ferri, R. – Hydrogen pinholes in grey cast iron, Fonderie (italiana), No. 9, 1972, p 273-
280. 57. Popov, A.D. Sufluri de suprafata si sub coajii in piesele turnate din otel. Lit.
Proizvodstvo, iul. 1975, p. 24-26 58. Colland, A. –Role de L’humidite da vent dans 1’elaboration metallurgious des fantes au
cubilot. Fonderie, 1964, 222, p. 233-245. 59. Lipovsky, R. –Aparitia suflurilor la piesele cu pereti subtiri turnata din otel (I, II).
Slevanestvi, 20/8, 1972, 327-327, Slevanestvi, 10/73, p. 396-402. 60. Nilles, P., Pesch, R. –La formation de gas lors de la solidification d’acier effervescent.
International Iron & Steel Congress, Dusseldorf, 1974. 61. **Cercetarea infl. constin. De carbon in otel, la micsorarea porozitatii de H. Express
informatia, 18, 1968, p. 30. 62. Pribyl, J. Chemiche Einwirkung der umegebungseinfluuses suf den flussign stahl nach
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stahlgusstuchen. Giesserei, 16 April 1964, 51, p.205-214. 64. Setov, S.I, Druian, N.A. Ubaltov, Iacovlov-Condille de formare si eliminare a porozitatii
cu aspect de cita. Lit. Proizvodstvo, 8, 1974, 32. 65. E.Guenzi, M. Degois-Contribution to teh researches of influence of gases in cast iron on
pinhole formation.38th International foundry congress, Dusselderf, 1971. 66. Rabinovici, B.V. Spiski-Untersuchung des Mecanismus des Bil-dungsprozesses von
gasblasen mit hilfe von durch-sichtogen modellen. Giessereitechnik, 21, 5, 1975, p. 149-159.
67. Stefanescu, C. Materale si amestecuri de formare pentru turnatorii. Bucuresti, 1971, p. 79-91.
68. J. Campbell – Casting – Pub. Butterworth & Heineman, 1st, 1991 (reprinted 1993) p. 105-110.
69. Ibid, p. 116-117 70. Ibid, p. 119-121 71. Ibid, p. 162-164 72. Ibid, p. 168-170
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Selected Glossary AQL Acceptable Quality Level A quality level established on a prearranged system of inspection using samples selected at random. As-cast condition Casting without subsequent heat treatment. Automatic High Pressure Moulding Line Moulding process which works automatically Backing sand The bulks of the sand in the flask. The sand compacted on top of the facing sand that covers the pattern. Binder The bonding agent used as an additive to mold or core sand to impart strength or plasticity in a “green” or dry sate. Burn on sand Sand adhering to the surface of the casting that is extremely difficult to remove. Chaplet A small metal insert of spacer used in molds to provide core support during the casting process. Charge A given wweight o fmetal introduced into the furnace. Chill A metal insert in the sand mold used to produce local chilling and equalize rate of solidification throughout the casting. Cleaning Removal of runners, rises, flash, surplus metal and sand from a casting. Cold setting Moulding material contain resin which has high formability and sets immediately after moulding. Cold shut
A surface imperfection due to unsatisfactory fusion of metal. Cope The top half of a horizontally parted mold. Core A sand or metal insert in a mold to shape the interior of the casting or that part of the casting that cannot be shaped by the pattern. Core assembly An assembly made from a number of cores. Corebox The wooden, metal or plastic tool used to produce cores. Coreprint A projection on a pattern that leaves an impression in the mold for supporting the core Core wash A liquid suspension of a refractory material applied to cores and dried (intended to improve surface of casting) Crush The displacement of sand at mold joints. Cupola A cylindrical, straight shaft furnace (usually lined with feretories) for melting metal in direct contact with coke by forcing air under pressure through openings near its base. Cure to Harden Die A metal form used as a permanent mold for die casting or for a wax pattern in investment casting. Dowel A pin of various types used in the parting surface of parted patterns or dies to assure correct registry. Draft Taper on the vertical sides of a pattern or corebox that permits the core or sand mold
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to be removed without distorting or tearing of the sand.. Draft Refers to the taper of the pattern, which allows it to be extended from a compacted sand mold. Drag The bottom half of a horizontally parted mold. Ejector pins Movable pins in pattern dies that help remove patterns from the die. Facing sand The sand used to surround the pattern that produces the negative draft. Draft refers to the taper of the pattern, which allows it to be extended from a compacted sand mold. Feeder Sometimes referred to as a "riser," it is part of the gating system that forms the reservoir of molten metal necessary to compensate for losses due to shrinkage as the metal solidifier Finish allowance The amount of stock left on the surface of a casting for machining Finish mark A symbol appearing on the line of a drawing that represents the edge of the surface of the casting to be machined or otherwise finished. Flask A rigid metal or wood frame used to hold the sand of which a mold is formed and usually consisting of two parts, cope and drag. Foundry returns Metal (of known composition) in the form of gates, sprues, runners, risers and scrapped castings returned to the furnace for remeltingr Gas porosity A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting.
Gate (ingate) The portion of the runner where the molten metal enters the mold cavity The gating system, which brings the molten metal to the mold cavity, is illustrated in its simplest form. The design of this system is critical in the introduction of clean metal to trye mold cavity. Green sand Moist clay-bonded molding sand. Heat A single furnace charge of metal . Heat treatment A combination of heating and cooling operations timed and applied) a metal or alloy in the solid state in a manner that will produce desired mechanical properties. Hotbox process A resin-based process that uses heated metal coreboxes to produce cores. Hot tear Irregularly shaped fracture in a casting resulting from stresses set up by steep thermal gradients within the casting during solidification". Inclusions Particles of slag, refractory materials, sand or deoxidation products trapped in the casting during pouring solidification Investment casting A pattern casting process in which a wax or thermoplastic pattern is used. The pattern is invested (surrounded) by a refractory slurry. After the mold is dry, the pattern is melted or burned out of the mold cavity, and molten metal is poured into the resulting cavity. Ladle A container used to transfer molten metal from the furnace to the mold. Locating pad A projection on a casting that helps maintain alignment of the casting for machining operations/Locating surface A casting surface to be used as a basis for
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measurement in making secondary machining operations. Master pattern The object from which a die can be made; generally a metal model of the part to be cast with process shrinkage added. Matchplate sand moulding Moulding which is carried out with the help of pattern which is fix on a plate (matchplate) Mechanical properties Those properties of a material that reveal the elastic and inelastic properties/ when force is applied. This term should not be used interchangeably with "physical properties." Mechanized sand moulding Sand moulding which is carried out by the machine (jolting, squeezing, gilting) etc. Metal Lot A master heat that has been approved for casting and given a sequential number by the foundry. Mold Normally consists of a top and bottom form, made of sand, metal or any other investment material-. It contains the cavity into which molten metal is poured to produce a casting of definite shape.-Mold cavity The impression in a mold produced by removal of the pattern. It is filled with molten metal to form the casting. Mold coating Nobake process Molds/cores produced with a resin-bonded air-setting sand. Also known as the airset process because molds are left to harden under normal atmospheric conditions? No bake raw mold Parting line The line showing the separation of the two halves of the mold. Pattern
The wood, metal, foam or plastic shape used to form the cavity in the sand. A pattern may consist of one or many impressions ant would normally be mounted on a board or plate complete with a runner system Matchplate patterns feature impressions on both the cope (top) and drag (bottom) sides, and typically are used to produce molds for small parts, such as this elbow casting. Pattern draft The taper allowed on the Vertical faces of a pattern to permit easy withdrawal of the pattern from the mold or die. (See draft) Pattern layout Full-sized drawing of a pattern showing its arrangement and structural features. Patternmaker's shrinkage The shrinkage allowance made on all patterns to compensate for the change in dimensions as the solidified casting cools in the mold from freezing temperature of the metal to room temperature. The pattern is made larger by the amount of shrinkage characteristic of the particular metal in the casting and the amount of resulting contraction to be encountered) Permeability The property of a mold material to allow passage of mold/core gases during the pouring of molten metal; Physical properties Properties of matter such as density, electrical and thermal conductivity, expansion and specific heat. This term should not be used interchangeably with "mechanical properties." Pig iron Blocks of iron to a known metal chemical analysis that are used for melting (with suitable additions of scrap, etc.) for the production of ferrous castings. Pilot or sample casting A casting made from a pattern produced in a production die to check the accuracy of
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dimensions and quality of castings that will be made. Porosity Holes in the casting due to: gases trapped in the mold, the reaction of molten metal with moisture in the molding sand, or the imperfect fusion of chaplets with molten metal Recovery rate Ratio of the number of saleable parts to the total number pf parts manufactured, expressed as a percentage. Refractory Heat-resistant ceramic material. "Reject rate Ratio of the number of parts scrapped to the total number of parts manufactured, expressed as a percentage. Riser Runner system or gating The set of channels in a mold through which molten metal is poured to fill the mold cavity. The system normally consists of a vertical section (downgate or sprue) to the point where it joins the mold cavity (gate) and leads from the mold cavity through vertical channels (risers or feeders) inclusions Cavities or surface imperfections on a casting caused by sand washing into the mold cavity Scrap (a) Any scrap metal melted (usually with suitable additions of pig iron or ingots) to produce castings; (h) reject castings Shakeout The process of separating the solidified casting from the mold materials. Shrinkage Contraction of metal in the mold during solidification. The term also is used to describe the casting defect, such as shrinkage cavity, which results from poor design, insufficient metal feed or inadequate feeding.' Slag A fused nonmetallic material/that protects molten metal from the air and extracts certain impurities from the melt. Slag inclusions
Casting Surface imperfections similar to sand inclusions but containing impurities from the charge materials, silica and clay eroded from the refractory lining, and ash from the fuel during the melting process. May also originate from metal-refractory reactions occurring in the ladle during pouring of the casting. Slurry A flowable mixture of refractory particles suspended in a liquid. Sodium silicate/CO2 process. Molding sand is mixed with sodium silicate and the mold is gassed with CO2 gas to produce a hard mold or core Sprue (downsprue-downgate) The channel, usually vertical, that the molten metal enters. Test bar Standard specimen bar designed to permit determination of mechanical properties of the metal from which it was poured Test lug A lug cast as a part of the casting and later removed for testing purposes. Vent An opening or passage in a mold or core to facilitate escape of gases when the mold is poured.
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