Researches on an Advanced Engineering Material for Automobile Industry: SG

iron with better Mechanical Properties and higher friction obtained by post

innoculation of tin and casting in vacuum (‘depression’) mould

By

*Prof. Dr. Pervaiz Habibullah

Spheroidal (nodular or ductile) graphite iron; SG iron, has an as-cast structure

containing graphite particles in the form of small rounded, “spheroidal”, “globular” or

“nodular” particles in a ductile metallic matrix. It has been long established that all of the

mechanical and physical properties of SG iron are a result of the graphite in the

spheroidal/nodular shape.

Late additions in SG iron of innoculants are nowadays well established in almost all

the ferrous foundries. Industrial products of SG iron are ductile, durable, resistant to wear and

fracture and their mechanical properties may further be altered, as per requirement, by post

innoculation, with different elements. We have innoculated the SG iron with a minute

amount of Sn, cast in vacuum („depression‟) mould and have studied the behaviour of tin in

SG iron.

1. Process of making SG iron

1.1 Melting

Return of SG iron scrap, cast iron having low sulphur and phosphorus and

manganese is melting in a ladle of 60kg capacity in a gas fired furnace for 3-4 hours and

ladle is taken out of the furnace and following ingredients are added:

Mg.FeSi = 1.3lg

Steel scrap = 3kg

Desulphurizer = 0.2kg

Fe.Si = 1kg

1.1.1 Charge

Composition of the charge is as follows:

Carbon = 3.36%

Silicon = 2.3

Manganese = 0.48

Phosphorous = 0.054

Sulphur = 0.022

Sn = 0.2

Fe = Balance

*Former G.M. Coord (Tech), Regional Headquarters (North), AIIP, CAA, Pakistan

1.2 Process

1.2.1 Spherdizing methods (Fe.Si)

i) Ladle transfer method (sandwich method)

ii) Covered ladle method

iii) Treatment with porous plug or other stirring method

The magnesium master alloy is placed on the bottom of the empty treatment ladle

and liq. iron is poured over it. A popular variation is the sandwich process where, small

steel pieces, about 2% of the total melt, are placed over the master magnesium alloy.

During heating, melting and superheating approx 2% steel is consumed.

We utilized sandwich process which is very much common for production of SG

iron and uses pouring of hot molten metal at about 1400-1500oC in ladle which contains

the nodulant under the innoculants coverage. The reaction with the molten metal is

violent. Magnesium acts as a deoxidizer (MgO) and desulphurizer (MgS). Si acts as

graphitizer and promotes nucleation sites for nodules. The phenomenon for which SG

iron (graphite instead of becoming flakes become nodule) is still unknown and various

theories have been put forth but none has sustained upto this time.

Desulpherization

These are the compounds that remove sulphur and are added in the melt

whenever required:

1. Caustic soda (NaOH)

2. Soda Ash (NA2CO3)

3. Burnt lime (CaO)

4. Limestone (CaCO3)

5. Calcium carbide (CaC2)

6. Calcium Cyanide (CaCN2)

Post innoculation (Sn):

Soon after the spherdizing treatment, when ladle is transfer to the mould line,

0.2% Sn is added in the ladle before pouring. Before addition the tin should be heated to

eject moisture and air from its pores and it should be red hot. Tin will flare and silvery

flames will appear. The liq metal then poured in the mould.

1.3 Moulding

An automobile part, Differential gear box (see fig. 1), was cast in the specially

designed mould box on which vacuum („depression‟) can be imposed. Specially designed

mould box shown in fig. 2, was used. This type of moulding box is identical to the cope

and drag commonly used in foundries except that on its one side a hole (1in. dia) is

drilled which is connected on its exterior side to vacuum pump (-1 kgf/cm2) and on the

interior side it is connected with ½” dia perforated pipe, which is, made rectangular,

giving sufficient space for pattern and encircling the mould cavity all around (see fig-2).

The gates and risers of proper size are provided. Soon as the liq. SG iron was poured in

the sand, the vacuum pump was turned on. In this way, the vacuum (depression or

negative pressure) is imposed on the sand mould. It absorbs all the gas formed at the

metal-mould interface mould and cores and reduces the temp. of the mould by 20 to 30%.

In this way, the casting produced is free of blowholes and pinholes.

1.4 Product

SG iron of the following composition is obtained.

Carbon = 3.4%

Silicon = 2.28%

Manganese = 0.3%

Magnesium = 0.05%

Phosphorous = 0.04%

Sulphur = 0.02%

Fe = Balance

2. Metallography and Mech. Properties*

The microstructure of SG iron produced is given in fig. 3 to 5.

2.1 SG iron without any post innaculation or alloying

Microstructure

Microstructure reveals well-formed SG iron nodules uniformly distributed

throughout the section thickness of the given sample. Nodule count ranges between

* Pakistan Standards & Quality control Authority (TSC) Lahore - Pakistan

Fig. 1 Differential gear housing of SG iron innoculated with Sn

cast in vacuum (depression) mould

New casting strategy for imposing vacuum “depression” in green sand mould

Moulding sand

Mould box

½” dia pipe

with orifices

(for imposing

vacuum) vacuum pump

Casting

Fig.2 “Specially designed mould box” for imposing vacuum “depression” in green sand

mould

Fig. 3 Microstructure of well formed SG iron, nodules uniformly distributed

through the section thickness (nodule counts 210 – 225 nodules /mm2,

average nodule size 20 ), with ferritic matrix

Fig. 5 Microstructure of SG iron, post innoculated by Sn = 0.2%.

Reveals fine pearlitic matrix with 10-20% ferrite dominantly

around nodules. (bull eye structure)

Fig. 4 Microstructure reveals well formed and evenly distributed graphite

nodules. The nodule count ranges 100-125 nodules/mm2, nodule size

ranges between 35-40 microns.

Etched 200 x

210-225 nodules/mm2, whereas average nodule size is about 20microns. Nodularization

is almost complete. Etched specimen shows all ferritic grains matrix (Fig.3).

Hardness = 128 BHN

Tensile strength = 70 ksi

Yield strength = 50 ksi

Elongation = 6%

2.2 SG iron innoculated with 0.2% Sn

Unetched: Microstructure reveals well formed and evenly distributed graphite

nodules. The nodule count ranges 100-125 nodules/mm2, nodule size ranges between

35-40 microns (fig. 4).

Etched with 2% Nital: Microstructure reveals fine pearlitic matrix with 10-20%

ferrite dominantly around the nodules i.e., bull eye structure (Fig.5)

Hardness : 234 BHN

Tensile strength: 58.2 ksi

Yield Strength: 48 ksi

Elongation: 1.419%

Comparison of cast iron, SG iron and SG iron post innoculated is given in the

table 1.

2.3 Electron Microscopy - SEM

i) Fig. 6: Scanning electron micrograph: as-cast specimen of SG iron graphite

morphology 3D (fully spheroidal; x 600, 20 m)

ii) Fig. 7: Scanning electron micrograph: as-cast specimen of SG iron graphite

morphology 3D (fairly spheroidal; x 600, 20 m)

iii) Fig.7: Scanning electron micrograph: as-cast specimen of SG iron graphite

morphology 3D (compacted / vermicular; x 200, 20 m)

(Courtesy by B.I. Imasogie and U. Wendz, 2004)

3. Discussion- Effect of post innoculants on SG iron[10-16]

and prevention of defects

Lanthanum

Post innoculation or alloying of some elements are nowadays practised in most of

the SG iron foundries. Lanthanum element promotes the Equi axial solidification

behaviour by (i) developing higher number of nucleation sites, thus providing more

solidification sites within the molten metal (ii) increasing viscosity of the liq. metal by

stirring motions within the molten iron (iii) restricting the growth of columnar grains.

At a given solidification stage, when lanthanum is added, the equi axial

solidification is developed. The thickness of the columnar solidification growth zone is

reduced leaving large free flowing passages for maintaining liq iron to travel within the

channels feeding casting areas. This phenomenon minimizes the shrinkage defect.

Tin & Copper

Effect of Tin additions upto 0.2% is SG iron were studied in the present research

work. These levels of additions were sufficient to produce pearlitic structure in casting

samples. Our researches have shown that tin additions promote pearlite as cast structure,

however the ultimate tensile strength and elongation drops when tin is added beyond the

point required to obtain a fully pearlitic matrix. However, hardness is increases (see

table-1). The sensitivity of tin additions is somewhat reduced on normalizing. Heat

treatment of SG iron increases the tensile strength of copper containing SG iron [10].

Further, tin and copper slow the response to ferritize annealing, when copper / tin

ratio is greater than 6 or 7, annealing response is substantially improved reflecting,

copper-tin interaction. Austinizing before annealing can improve mechanical properties

similarly normalizing can significantly raise hardness, strength and impact resistance of

castings containing tin and copper, with slight loss of elongation. Strength and hardness

decrease with decrease in austinizing temp. from 940 to 816oC; while elongation and

impact resistance increase. Copper increases hardness and corrosion resistance of SG

iron. In case of SG iron having more than 0.5% copper, increase in tin addition brings

about increase in hardness [11-12].

Chromium

Chromium raises strength of ferritize- annealed casting and reduces elongation

and impact resistance. When SG iron containing 0.09% Cr is normalized, its strength is

reduced, while normalizing 0.18% Cr brings about increase in strength and reduction in

elongation, impact resistance.

Lead (Pb)

Pb has low melting point and transforms into fumes just after addition in the

molten SG iron. However it may effect the properties. With 0-0.03% tin and 025-0.5%

copper, the addition of lead upto 200ppm reduces strength and elongation; max. at 100-

130 ppm lead. With no tin, impact resistance decreases above 100ppm lead. With varied

content of tin, copper and chromium, wide varieties of SG iron can be produced for

various as cast or heat treated conditions.

Other elements

A modified thermal analysis was presented in ref [7] for observing the effect of

alloy elements. Surface areas of nodular graphite on austenite decomposition of SG iron

during isothermal holding 250-400oC completes in three stages: Developing ferrite plate

in austenite matrix were observed on the basis of the calculated heat evolution of phase

transformation i.e. nucleation and early growth, sidewise growth and branching of ferrite

plates. Silicon addition increases the surface area of nodular graphite. However Mo, Cu,

Ni, and Mn suppress ferrite formation in the austempered ductile iron. Mo is the most

effecting alloying element in suppressing the decomposition of austenite during

isothermal holding.

Ductile iron containing V and Co when heat treated, the microstructure of the

alloyed iron consisted of graphite nodules in ferrite matrix with fine dispersion. (20 –

80nm in dia ). These carbide particles improve strength and refine the grain size of

ferrite, resulting in an iron of intermediate strength and high ductility.

Investigation on ductile iron produced in the commercial foundries with varying

contents of Si, Mn, Cu, Ni, Mo, and P have also concluded that tensile properties,

hardness and microstructure impact, toughness are correlated with the composition and

content of elements added.

Defects in SG Iron

Compacted graphite within the structure

The most common cause of appearance of compact graphite within the structure

of SG iron, is the failure of complete nodulisation process. Use of unsuitable noduliser or

while using correct noduliser, it has been added in an inadequate quantity, are main

causes of this defect.

Low Nodule count

Nodule count depends upon the quantity of the innoculant. Avoiding long holding

times in the furnace and prolonged pouring time, post-innoculation will result in

consistent nodule counts. Nodule count depends upon the additions of innoculant in a

proper quantity.

Exploded graphite

The exploded graphite is apparently appears similar to a nodule in SG iron but it

is split and blown apart (fig. 9). Some rare earth, cerium, lanthanum, neodymium,

praseodymium etc are beneficial in that they neutralize the effects of some detrimental

trap elements such as Pb, Bi, Sb, Ti etc. Rare earth elements are also good nudularisers

and promote the nudularization. However, they may not be utilized in excess because

otherwise, they are an energetic source of exploded graphite. This is more especially

when high purity charges are used which are low in impurities. Exploded graphite is

normally found in thicker section castings with slow cooling rates or at very high carbon

equivalent levels.

Shrinkage

The casting defect, shrinkage, in SG iron is mainly caused by sand systems and

feeding & gating systems. Some shrinkage defects are contributed by the metallurgical

factors such as composition, casting temp. innoculation and high magnesium residuals

Gas porosity

In the present experiment, we have cast differential gear box of SG iron

innoculated with Sn, in the sand mould on which the vacuum was imposed (see text).

Depression or negative pressure imposed on the sand mould absorbs all the mould gas

formed at the metal-mould interface, mould and cores and reduces the temp. of the mould

by 20 to 30%. In this way, the casting produced is free of blowholes and pinholes.

4. Conclusion

1. The results obtained showed clearly that the properties of the SG irons not only

depend largely on the form and/or morphology of graphite precipitated in the

casting but also on the alloying elements. However, the common defining

characteristics of this group of materials is the morphological structure of graphite

2. Pearlite and ferrite ratio affects the tensile strength, yield strength and elongation

of SG iron. The ferrite and pearlite ratio can be controlled through alloying, shake

out temp. controls or post-casting heat treatment.

3. Effect of tin additions (upto 2%) was studied. This level of addition was sufficient

to produce pearlitic structure in casting samples. Our researches have shown that

tin additions promote pearlite as cast structure, however the ultimate tensile

strength and elongation drops when tin is added beyond the point required to

obtain a fully pearlitic matrix. The sensitivity of tin additions is somewhat

reduced on normalizing.

4. Further, tin and copper slow the response to ferritize annealing, when copper / tin

ratio is greater than 6 or 7, annealing response is substantially improved

reflecting, copper-tin interaction. Austinizing before annealing can improve

mechanical properties similarly normalizing can significantly raise hardness,

strength and impact resistance of castings containing tin and copper, with slight

loss of elongation.

5. Ductile iron containing V and Co when heat treated, the microstructure of the

alloyed iron consisted of graphite nodules in ferrite matrix with fine dispersion.

(20 – 80nm in dia ). These carbide particles produce dispersion strength and refine

the grain size of ferrite, resulting in an iron of intermediate strength and high

ductility.

6. Rare earth lanthanum element promotes the Equi axial solidification behaviour

by (i)developing higher nucleation power, thus producing more solidification sites

within the molten metal (ii) modifying the molten iron viscosity favouring stirring

motions within the molten iron (iii) restricting the growth of columnar grains.

7. SG irons, post innoculated by 0.2% Sn and some other elements, such as Cu and

V, were first time carried out in Pakistan, at WMZA Foundry, Lahore the leading

specialist in founding cast iron grades 14, 17, SG iron and non ferrous metals. The

alloy made by post inoculation of SG iron with Sn was applied in the automobile

industry and deferential gear box and diff. housings were cast and fitted with the

heavy vehicles for their testing. As reported by the WMZA Foundry, no such

complaint of failure of said parts, was reported.

Acknowledgement

Author is grateful to Ch. Ikram, Dy. Director, Pakistan Standards & Quality

control Authority (TSC) Lahore, for his valuable cooperation in studying the

metallography of newly proposed SG iron. In the same token, I am grateful to Mr.

Waheed, Manager WMZA Foundry for extended the facility for using his ferrous foundry

for casting experiments.

Bibliography

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McGraw Hill, ISBN 0-07-295358-6

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Copper on Properties of Ductile Iron

16. *wikipedia, ductile iron

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by creating vacuum in sand mould during pouring of liquid alloys”. 52nd

International Foundry Congress, Melbourne, Australia, 1985.

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antique of early Islamic period, in vacuum mould – 69th

World Foundry Congress,

China, Oct. 10-16, 2010.

Fig. 6 Scanning electron micrograph of as cast specimen of SG iron

graphite morphology 3D (fully spheroidal; x 600, 20 m)

(B.I. Imasogie and U. Wendz, 2004)

Fig. 7 Scanning electron micrograph of as cast specimen of SG iron

graphite morphology 3D (fairly spheroidal; x 600, 20 m)

(B.I. Imasogie and U. Wendz, 2004)

Fig. 8 Scanning electron micrograph of as cast specimen of SG iron

graphite morphology 3D (compacted / vermicular; x 200, 20 m)

(B.I. Imasogie and U. Wendz, 2004)

Fig. 9 Exploded graphite unetched x 100

Table 1 Comparison of Mechanical properties of cast iron, SG iron and innoculated with tin (0.2% Sn)

Type of iorn

alloy

Chemical

Comp

Condition Microstructure Tensile strength Yield

strength

Elongation

(%)

Applications

Gray iron

Ferritic C = 3.4%

Si = 2.2%

Mn = 0.7%

Annealed Ferritic matrix 26ksi

(179MPa)

-- -- Cylinder blocks, heavy

gear box diesel engine

casting

Pearlitic C = 3.2%

Si = 2.0%

Mn = 0.7%

As cast pearlitic

matrix

36ksi

(252MPa)

-- -- -do-

Pearlitic C = 3.3%

Si = 2.2%

Mn = 0.7%

As cast Pearlitic

matrix

42ksi

(293MPa

-- -- -do-

SG Irons

Ferritic C = 3.5%

Si = 2.2%

Annealed Ferritic 60ksi

(414MPa)

40 ksi

(276MPa)

18 Pressure castings such as

valves and pump

Pearlitic C = 3.5%

Si = 2.2%

As cast Ferritic

Pearlitic

80ksi

(552MPa)

55 ksi

(379MPa)

6 -do-

Martensitic

(120 90 2)

C = 3.5%

Si = 2.2%

Martensitic Quenched &

tempered

120ksi

(828MPa)

90 ksi

(621MPa)

2 -do-

SG iron post inoculated with Sn*

Pearlitic C = 3.34%

Si = 2.2%

Mn=0.3%

Sn =0.2%

As cast Pearlitic

matrix 10-

20% ferrite

58.2 ksi

(570.941N/mm2)

48kN

(48ksi)

1.419 Differential gear box,

diff. housing

Ref: Data of cast iron and SG iron by courtesy of “Foundations of Mat. Sci. and Engg.” Ed. II by Smith W.F. p 492

* Result of the present researches have been incorporated in the table.