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Classification and Basic Metallurgy of Cast Iron

Chapter · February 1990

DOI: 10.1361/asmhba0001001

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Classification and Basic Metallurgy of Cast Iron Doru M. Stefanescu, The University of Alabama

THE TERM CAST IRON, like the term steel, identifies a large family of ferrous alloys. Cast irons are multicomponent fer- rous alloys, which solidify with a eutectic. They contain major (iron, carbon, silicon), minor (<0.1%), and often alloying (>0.1%) elements. Cast iron has higher carbon and silicon contents than steel. Because of the higher carbon content, the structure of cast iron, as opposed to that of steel, exhibits a rich carbon phase. Depending primarily on composition, cooling rate, and melt treat- ment, cast iron can solidify according to the thermodynamically metastable Fe-Fe3C system or the stable Fe-Gr system. When the metastable path is followed, the rich carbon phase in the eutectic is the iron carbide; when the stable solidification path is followed, the rich carbon phase is graph- ite. Referring only to the binary Fe-Fe3C or Fe-Gr system, cast iron can be defined as an iron-carbon alloy with more than 2% C. The reader is cautioned that silicon and other alloying elements may considerably change the maximum solubility of carbon in austen- ite (7). Therefore, in exceptional cases, alloys with less than 2% C can solidify with a eutectic structure and therefore still be- long to the family of cast iron.

The formation of stable or metastable eu- tectic is a function of many factors including the nucleation potential of the liquid, chemi- cal composition, and cooling rate. The first two factors determine the graphitization po- tential of the iron. A high graphitization po- tential will result in irons with graphite as the rich carbon phase, while a low graphitization potential will result in irons with iron carbide. A schematic of the structure of the common types of commercial cast irons, as well as the processing required to obtain them, is shown in Fig. 1.

The two basic types of eutectics--the stable austenite-graphite or the metastable austenite-iron carbide (Fe3C)---have wide differences in their mechanical properties, such as strength, hardness, toughness, and ductility. Therefore, the basic scope of the metallurgical processing of cast iron is to

manipulate the type, amount, and morphol- ogy of the eutectic in order to achieve the desired mechanical properties.

Classification

Historically, the first classification of cast iron was based on its fracture. Two types of iron were initially recognized:

• White iron: Exhibits a white, crystalline fracture surface because fracture occurs along the iron carbide plates; it is the result of metastable solidification (Fe3C eutectic)

• Gray iron: Exhibits a gray fracture sur- face because fracture occurs along the graphite plates (flakes); it is the result of stable solidification (Gr eutectic)

With the advent of metallography, and as the body of knowledge pertinent to cast iron increased, other classifications based on microstructural features became possible: • Graphi te shape: Lamellar (flake) graphite

(FG), spheroidal (nodular) graphite (SG), compacted (vermicular) graphite (CG), and temper graphite (TG); temper graph- ite results from a solid-state reaction (malleabilization)

• Matr ix : Ferritic, pearlitic, austenitic, martensitic, bainitic (austempered)

This classification is seldom used by the floor foundryman. The most widely used

terminology is the commercial one. A first division can be made in two categories: • C o m m o n cas t irons: For general-purpose

applications, they are unalloyed or low alloy

• Spec ia l cas t irons: For special applica- tions, generally high alloy The correspondence between commercial

and microstructural classification, as well as the final processing stage in obtaining common cast irons, is given in Table 1. A classification of cast irons by their commer- cial names and structure is also given in the article "Classification of Ferrous Casting Alloys" in Volume 15 of the 9th Edition of M e t a l s H a n d b o o k .

Special cast irons differ from the common cast irons mainly in the higher content of alloying elements (>3%), which promote microstructures having special properties for elevated-temperature applications, cor- rosion resistance, and wear resistance. A classification of the main types of special cast irons is shown in Fig. 2.

Principles of the Metallurgy of Cast Iron

The goal of the metallurgist is to design a process that will produce a structure that will yield the expected mechanical proper-

Table 1 Classification of cast iron by commercial designation, microstructure, and fracture Commercial designation Carbon-rich phase Matrix(a) Fracture Final structure after

Gray iron . . . . . . . . . . . . . . . . . . . . . . . Lamellar graphite Ductile iron . . . . . . . . . . . . . . . . . . . . . Spheroidal graphite

Compacted graphite iron . . . . . . . . . Compacted vermicular graphite

White iron . . . . . . . . . . . . . . . . . . . . . . Fe3C

Mottled iron . . . . . . . . . . . . . . . . . . . . Lamellar Gr + F%C Malleable iron . . . . . . . . . . . . . . . . . . . Temper graphite Austempered ductile iron . . . . . . . . . Spheroidal graphite

P Gray Solidification F, P, A Silver-gray Solidification or

heat treatment F, P Gray Solidification

P, M White Solidification and heat treatmenttb)

P Mottled Solidification F, P Silver-gray Heat treatment At Silver-gray Heat treatment

(a) F, ferrite; P, pearlite; A, austenite; M, martensite; At, austempered (bainite). (b) White irons are not usually heat treated, except for stress relief and to continue austenite transformation.

ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys ASM Handbook Committee, p 3-11DOI: 10.1361/asmhba0001001

Copyright © 1990 ASM International® All rights reserved.

www.asminternational.org

4 / Cast Irons

Liquid cast iron

(iron - carbon- alloy)

Solidification Graphitization

potential

Solid-state transformation 7 + Graphite

/ G aoPnh i tme sh!Ple d e p t : (cooling through eutectoid interval) r ds

High

~Medi Flake Compacted Spheroidal

um k J ,.~ 7 + Fe3C + Graphite

~ , y + Fe3C Solid-state transformation

r (cooling through

eutectoid interval)

Slow X

Mottled cast iron

Pearlite + Fe3C

Reheat above eutectoid interval

Pearlite + Graphite (~xFe + Fe3C)

Ferrite + Graphite (~Fe)

White iron

Gray cast iron

3' + Fe3C.-~,-7 + Graphite .I[ Hold above

Cool eutectoid through interval

eutectoid interval

Pearlite + Temper graphite Ferrite + Temper graphite

41. Malleable iron

Fig. 1 Basic microstructures and processing for obtaining common commercial cast irons

7 + Fe3C

ties. This requires knowledge of the struc- ture-properties correlation for the particular alloy under consideration as well as of the factors affecting the structure. When dis- cussing the metallurgy of cast iron, the main factors of influence on the structure that one needs to address are:

• Chemical composition • Cooling rate • Liquid treatment • Heat treatment

In addition, the following aspects of com- bined carbon in cast irons should also be considered:

• In the original cooling or through subse- quent heat treatment, a matrix can be internally decarburized or carburized by depositing graphite on existing sites or by dissolving carbon from them

• Depending on the silicon content and the cooling rate, the pearlite in iron can vary in carbon content. This is a ternary sys- tem, and the carbon content of pearlite can be as low as 0.50% with 2.5% Si

• The conventionally measured hardness of graphitic irons is influenced by the graph- ite, especially in gray iron. Martensite microhardness may be as high as 66 HRC, but measures as low as 54 HRC conventionally in gray iron (58 HRC in ductile)

• The critical temperature of iron is influ- enced (raised) by silicon content, not carbon content

The following sections in this article dis- cuss some of the basic principles of cast iron metallurgy. More detailed descriptions of the metallurgy of cast irons are available in separate articles in this Volume describ-

ing certain types of cast irons. The Section "Ferrous Casting Al loys" in Volume 15 of the 9th Edition of Metals Handbook also contains more detailed descriptions on the metallurgy of cast irons.

Gray Iron (Flake Graphite Iron) The composition of gray iron must be

selected in such a way as to satisfy three basic structural requirements:

• The required graphite shape and distribu- tion

• The carbide-free (chill-free) structure • The required matrix

For common cast iron, the main elements of the chemical composit ion are carbon and silicon. Figure 3 shows the range of carbon and silicon for common cast irons as com- pared with steel. It is apparent that irons

Classification and Basic Metallurgy of Cast Iron / 5

I Graphite free

I Pearlitic

white iron

Wear resistant

Martensitic white iron (Ni-Hard)

Wear resistant

I I High-chromium iron Ferritic

(11-28% Cr)

Wear, corrosion, and heat resistant

f

ASTM A 532

Fig. 2

I 5O/o Si iron (Silal),

heat resistant

Classification of special high-alloy cast irons.

have carbon in excess of the maximum solubility of carbon in austenite, which is shown by the lower dashed line. A high carbon content increases the amount of graphite or Fe3C. High carbon and silicon contents increase the graphitization poten- tial of the iron as well as its castability.

The combined influence of carbon and silicon on the structure is usually taken into account by the carbon equivalent (CE): CE = % C + 0.3(% Si)

+ 0.33(% P) - 0.027(% Mn) + 0.4(% S) (Eq 1) Additional information on carbon equiva- lent is available in the article "Thermody- namic Properties of Iron-Base Alloys" in Volume 15 of the 9th Edition of Metals Handbook. Although increasing the carbon and silicon contents improves the graphiti- zation potential and therefore decreases the chilling tendency, the strength is adversely affected (Fig. 4). This is due to ferrite pro- motion and the coarsening of pearlite.

400

~. 350 Y/Ill/! I 50 ~"//11111!111

300 I 111,,, = =j == ' ""/, 40 ==

250 , i , , . "//,1111 30 a~ -~ 200 %~,, ?_ =~

# " ' 6 150 711 - 20

100 3.25 3.5 3.75 4.0 4.25 4.5 4.75

Carbon equivalent, %

General influence of carbon equivalent on I:I,T 4 H ~ . the tensile strength of gray iron. Source: Ref 2

I Graphite bearing

I Austenitic

I I

18O/o Ni Ni-resist

Corrosion and heat resistant

I Acicular

High strength i wear resistant

18% Ni, 5o/0 Si Nicrosilal

Heat and corrosion resistant

ASTM A 439

I High (15%)silicon iron,

corrosion resistant ASTM A 518 or A 518 M (metric)

, %C + 1/3% Si = Spheroi al irons

4.0 ~ ~ , ~ ,~.

m u 2.o IIIIIIIIIIIIIIIIIIIIIIIIIIH--White ,rons

7~,~ t ~ M a l l e a b l e irons

%(2 + %6% Si = 2.0 1.0

0 ~ - - S t e e l s

0 1.0 2.0 3.0 4.0

Silicon, %

,ts";". 3 Carbon and silicon composition ranges of common cast irons and steel. Source: Ref 2

Source: Ref 1

The manganese content varies as a func- tion of the desired matrix. Typically, it can be as low as 0.1% for ferritic irons and as high as 1.2% for pearlitic irons, because manganese is a strong pearlite promoter.

From the minor elements, phosphorus and sulfur are the most common and are always present in the composition. They can be as high as 0.15% for low-quality iron and are considerably less for high-quality iron, such as ductile iron or compacted graphite iron. The effect of sulfur must be balanced by the effect of manganese. With- out manganese in the iron, undesired iron sulfide (FeS) will form at grain boundaries. If the sulfur content is balanced by manga- nese, manganese sulfide (MnS) will form, which is harmless because it is distributed within the grains. The optimum ratio be- tween manganese and sulfur for an FeS-free structure and maximum amount of ferrite is:

% M n = 1 .7(% S) + 0 .15 (Eq 2)

Other minor elements, such as aluminum, antimony, arsenic, bismuth, lead, magne- sium, cerium, and calcium, can significantly alter both the graphite morphology and the microstructure of the matrix.

The range of composition for typical un- alloyed common cast irons is given in Table 2. The typical composition range for low- and high-grade unalloyed gray iron (flake graphite iron) cast in sand molds is given in Table 3.

Both major and minor elements have a direct influence on the morphology of flake graphite. The typical graphite shapes for flake graphite are shown in Fig. 5. Type A graphite is found in inoculated irons cooled with moderate rates. In general, it is asso- ciated with the best mechanical properties, and cast irons with this type of graphite exhibit moderate undercooling during solid- ification (Fig. 6). Type B graphite is found in irons of near-eutectic composition, solid- ifying on a limited number of nuclei. Large eutectic cell size and low undercoolings are common in cast irons exhibiting this type of graphite. Type C graphite occurs in hyper- eutectic irons as a result of solidification with minimum undercooling. Type D graph- ite is found in hypoeutectic or eutectic irons solidified at rather high cooling rates, while type E graphite is characteristic for strongly hypoeutectic irons. Types D and E are both associated with high undercoolings during solidification. Not only graphite shape but also graphite size is important, because it is directly related to strength (Fig. 7).

Table 2 Range of compositions for typical unalloyed common cast irons [ Composition, % I

Type of iron C Si Mn P S

Gray (FG) . . . . . . . . . . . . . . . . . . . . . . . . . 2.5-4.0 1.0-3.0 Compacted graphite (CG) . . . . . . . . . . . . 2.5-4.0 1.0-3.0 Ductile (SG) . . . . . . . . . . . . . . . . . . . . . . . 3.0--4.0 1.8-2.8 White . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8-3.6 0.5-1.9 Malleable (TG) . . . . . . . . . . . . . . . . . . . . . 2.2-2.9 0.9-1.9

Source: Ref 2

0.2-1.0 0.002-1.0 0.02-0.25 0.2-1.0 0.01-0.1 0.01-0.03 0.1-1.0 0.01-0.1 0.01-0.03

0.25-0.8 0.06-0.2 0.06-0.2 0.15-1.2 0.02-0.2 0.02-0.2

6 / Cast Irons

Table 3 Compositions of unalloyed gray irons Carbon [ Composition, %

ASTM 48 class equivalent C Si Mn P S

20B . . . . . . . . . . . . . . . . . . . . . . . . 4.5 3 .1 -3 .4 2 .5 -2 .8 0 .5 -0 .7 0.9 0.15 55B . . . . . . . . . . . . . . . . . . . . . . . . 3.6 <-3.1 1 .4-1 .6 0.6--0.75 0.1 0.12

Alloying elements can be added in com- mon cast iron to enhance some mechanical properties. They influence both the graphi- tization potential and the structure and properties of the matrix. The main elements are listed below in terms of their graphitiza- tion potential:

High positive graphitization potential (decreasing positive potential from top to bottom)

Carbon Tin Phosphorus Si l icon A luminum Coppe r Nicke l

Neutral

I ron

High negative graphitization potential (increasing negative potential from top to bottom)

M a n g a n e s e C h r o m i u m M o l y b d e n u m Vanad ium

This classification is based on the thermo- dynamic analysis of the influence of a third element on carbon solubility in the Fe-C-X system, where X is a third element (see the section "Influence of a Third Element on Carbon Solubility in the Fe-C-X System" in the article "Thermodynamic Properties of Iron-Base Alloys" in Volume 15 of the 9th Edition of Metals Handbook). Although list- ed as a graphitizer (which may be true ther- modynamically), phosphorus also acts as a matrix hardener. Above its solubility level (probably about 0.08%), phosphorus forms a very hard ternary eutectic. The above classi-

t rE

I -

T 0 t c

Type D,E " ~

Time >

Characteristic cooling curves associated with F ig . 6 different flake graphite shapes. ;rE, equilibri- um eutectic temperature

fication should also include sulfur as a carbide former, although manganese and sulfur can combine and neutralize each other. The re- sultant manganese sulfide also acts as nuclei for flake graphite. In industrial processes, nucleation phenomena may sometimes over- ride solubility considerations.

In general, alloying elements can be clas- sified into three categories. Each is dis- cussed below.

Silicon and aluminum increase the graph- itization potential for both the eutectic and eutectoid transformations and increase the number of graphite particles. They form solid solutions in the matrix. Because they increase the ferrite/pearlite ratio, they lower strength and hardness.

Nickel, copper, and tin increase the graph- itization potential during the eutectic transfor- mation, but decrease it during the eutectoid transformation, thus raising the pearlite/ ferrite ratio. This second effect is due to the retardation of carbon diffusion. These ele- ments form solid solution in the matrix. Be- cause they increase the amount of pearlite, they raise strength and hardness.

Chromium, molybdenum, tungsten, and vanadium decrease the graphitization poten- tial at both stages. Thus, they increase the amount of carbides and peadite. They con- centrate in principal in the carbides, forming (FeX),C-type carbides, but also alloy the ~xFe solid solution. As long as carbide formation does not occur, these elements increase strength and hardness. Above a certain level, any of these elements will determine the solidification of a structure with both Gr and Fe3C (mottled structure), which will have lower strength but higher hardness.

In alloyed gray iron, the typical ranges for the elements discussed above are as follows:

Element Composition, %

Chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .2 -0 .6 Molybdenum . . . . . . . . . . . . . . . . . . . . . . . . . . 0.2-1 Vanad ium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .1 -0 .2 Nicke l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .6-1 Coppe r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .5 -1 .5 Tin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .04-0 .08

The influence of composit ion and cooling rate on tensile strength can be estimated using (Ref 3):

TS = 162.37 + 16.61/D - 21.78(% C) - 61.29(% Si) - 10.59 (% Mn- 1.7% S) + 13.80(% Cr) + 2.05(% Ni) + 30.66(% Cu) + 39.75(% Mo) + 14.16 (% Si) 2 - 26.25(% Cu) 2 - 23.83 (% Mo) 2 (Eq 3)

v • -.~,',,, h ' ] , , ~ A ~ J t ~- t . - . - ~ - - ~ u I

! ( ." . - - , I , ~ (.,', ,s,~ t " m

Uniform distribution, Rosette Superimposed flake random orientation groupings sizes, random

orientation

Interdendritic segregation, Interdendritic segregation, random orientation preferred orientation

Typical flake graphite shapes specified in F ig . 5 ASTM A 247. A, uniform distribution, random orientation; B, rosette groupings; C, kish graphite (superimposed flake sizes, random orientation); D, interdendritic segregation with random orientation; E, interdendriUc segregation with preferred orientation

where D is the bar diameter (in inches). Equa- tion 3 is valid for bar diameters of 20 to 50 mm (0.8 to 2 in.) and compositions within the following ranges:

Element Composition, %

Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 .04-3 .29 Chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1--0.55 Molybdenum . . . . . . . . . . . . . . . . . . . . . . . . . 0 .03-0 .78 Sil icon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 .6-2.46 N icke l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .07-1 .62 Sul fur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .089-0 .106 Manganese . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.39--0.98 Coppe r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .07-0 .85

The cooling rate, like the chemical com- position, can significantly influence the as- cast structure and therefore the mechanical properties. The cooling rate of a casting is primarily a function of its section size. The dependence of structure and properties on section size is termed section sensitivity. Increasing the cooling rate will:

Maximum flake length, in.

0.006 0.010 0.015 0.020 0.025 0.030 0.035 415 k 60

345 50 -~ ~=

30 "~ "~ = 205 ~ ~ m

135 20 0.125 0.25 0.375 0.50 0.635 0.75 0.90

Maximum flake length, mm

Effect of maximum graphite flake length Fig. 7 on the tensile strength of gray iron. Source: Ref 3

Classification and Basic Metallurgy of Cast Iron / 7

Section thickness, in. 0.5 1.0 1.5

400500 liB-45B I IASTM~ 70

C 350 ~ 40B 50 300

=~ 2oO~o ~ ~ ~ #- 35B 20

100 5 10 15 20 25 30 35 40 45 50

Section thickness, mm (al _~

Section thickness, in. "~ 0.5 1.0 1.5

300 . , l, i l,i , ASTM A-48

m 250 Class I

,

35B I -- 150 - - - - 3 0 - - - 2 5 -

,00 i i 5 10 15 20 25 30 35 40 45 50

Section thickness, mm (b)

Influence of section thickness of the casting 8 "=~" on tensile strength (a) and hardness (b) for a series of gray irons classified by their strength as-cast in 30 mm (1.2 in.) diam bars. Source: Ref 2

• Refine both graphite size and matrix structure; this will result in increased strength and hardness

• Increase the chilling tendency; this may result in higher hardness, but will de- crease the strength

Consequently, composition must be tai- lored in such a way as to provide the correct graphitization potential for a given cooling rate. For a given chemical composition and as the section thickness increases, the graphite becomes coarser, and the pearlite/ ferrite ratio decreases, which results in low- er strength and hardness (Fig. 8). Higher carbon equivalent has similar effects.

The liquid treatment of cast iron is of paramount importance in the processing of this alloy because it can dramatically change the nucleation and growth condi- tions during solidification. As a result, graphite morphology, and therefore proper- ties, can be significantly affected. In gray iron practice, the liquid treatment used is termed inoculation and consists of minute additions of minor elements before pouring. Typically, ferrosilicon with additions of alu- minum and calcium, or proprietary alloys are used as inoculants. The main effects of inoculation are: • An increased graphitization potential

because of decreased undercooling dur- ing solidification; as a result of this, the chilling tendency is diminished, and

56

52

44

40

30

32 I-

24

20

3.4 3.6 3.8 4.0 4.2 4.4 4.6 Carbon equivalent, %

Influence of inoculation on tensile strength as 9 " ' 5 " a function of carbon equivalent for 30 mm (1.2 in.) diam bars. Source: Ref2

graphite shape changes from type D or E to type A

• A finer structure, that is, higher number of eutectic cells, with a subsequent in- crease in strength

As shown in Fig. 9, inoculation improves tensile strength. This influence is more pro- nounced for low-CE cast irons.

Heat treatment can considerably alter the matrix structure, although graphite shape and size remain basically unaffected. A rather low proportion of the total gray iron produced is heat treated. Common heat treatment may consist of stress relieving or of annealing to decrease hardness.

Ductile Iron (Spheroidal Graphite Iron)

Composition. The main effects of chemical composition are similar to those described for gray iron, with quantitative differences in the extent of these effects and qualitative differ- ences in the influence on graphite morpholo- gy. The carbon equivalent has only a mild influence on the properties and structure of ductile iron, because it affects graphite shape considerably less than in the case of gray iron. Nevertheless, to prevent excessive shrink- age, high chilling tendency, graphite flotation, or a high impact transition temperature, opti- mum amounts of carbon and silicon must be selected. Figure 10 shows the basic guidelines for the selection of appropriate compositions.

As mentioned previously, minor elements can significantly alter the structure in terms of graphite morphology, chilling tendency,

3.5

\ ~ -High impact transition temperature

3 0 "OX ~ iGraphite flotation

-zx- ":!!iEiEiE ~EE~EZh.. ~

=xcesslve . ~ i!~ii~!~ii:~:i:~:~:i:~:~:!:~:i:~: :::::::::::::::::::::7::::::::::::::::::::

shrinkage " ~iiiiiiiii;iiiiiiiiiiiiiiiiiii !iii!iiiiiiiiii!iiiiiiiii!iiiiiiiiii ~ 2.0 X "

1.5 ,Tendency to form white iron 3.4 3.5 3.6 3.7 3.8 3.9

Total carbon, %

''~r;"" 1 0 Typical range for carbon and silicon con- tents in good-quality ductile iron. Source:

Ref 2

and matrix structure. Minor elements can promote the spheroidization of graphite or can have an adverse effect on graphite shape. The minor elements that adversely affect graphite shape are said to degenerate graphite shape. A variety of graphite shapes can occur, as illustrated in Fig. 11. Graphite shape is the single most important factor affecting the mechanical properties of cast iron, as shown in Fig. 12.

The generic influence of various ele- ments on graphite shape is given in Table 4. The elements in the first group--the spheroidizing elements--can change graphite shape from flake through com- pacted to spheroidal. This is illustrated in Fig. 13 for magnesium. The most widely used element for the production of sphe- roidal graphite is magnesium. The amount of residual magnesium, Mgre~id, required to produce spheroidal graphite is generally 0.03 to 0.05%. The precise level depends on the cooling rate. A higher cooling rate requires less magnesium. The amount of magnesium to be added in the iron is a function of the initial sulfur level, Sin, and the recovery of magnesium, -q, in the par- ticular process used:

0.75 Sin+ Mgresid MgaddeO - (Eq 4)

A residual magnesium level that is too low results in insufficient nodularity (that is, a low ratio between the spheroidal graphite and the total amount of graphite in the structure). This in turn results in a deterio- ration of the mechanical properties of the iron, as illustrated in Fig. 14. If the magne- sium content is too high, carbides are pro- moted.

The presence of antispheroidizing (dele- terious) minor elements may result in graph- ite shape deterioration, up to complete graphite degeneration. Therefore, upper limits are set on the amount of deleterious elements to be accepted in the composition of cast iron. Typical limits are given below (Ref 6):

8 / Cast Irons

Element Composition, %

Aluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1 Arsenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.02 Bismuth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.002 Cadmium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.01 Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.002 Ant imony . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.002 Selenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.03 Tel lur ium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.02 Ti tan ium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1 Z i rconium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1

These values can be influenced by the combination of various elements and by the presence of rare earths in the composition. Furthermore, some of these elements can be deliberately added during liquid pro- cessing in order to increase nodule count.

Alloying elements have in principle the same influence on structure and properties as for gray iron. Because a better graphite morphology allows more efficient use of the mechanical propert ies of the matrix, alloy- ing is more common in ductile iron than in gray iron.

Cooling Rate. When changing the cooling rate, effects similar to those discussed for gray iron also occur in ductile iron, but the section sensitivity of ductile iron is lower. This is because spheroidal graphite is less affected by cooling rate than flake graphite.

The liquid treatment of ductile iron is more complex than that of gray iron. The two stages for the liquid treatment of ductile iron are:

• Modification, which consists of magne- sium or magnesium alloy treatment of the melt, with the purpose of changing graph- ite shape from flake to spheroidal

• Inoculation (normally, postinoculation, that is, after the magnesium treatment) to increase the nodule count. Increasing the nodule count is an important goal, be- cause a higher nodule count is associated with less chilling tendency (Fig. 15) and a higher as-cast ferrite/pearlite ratio

Heat treatment is extensively used in the processing of ductile iron because better advantage can be taken of the matrix struc- ture than for gray iron. The heat treatments usually applied are as follows:

• Stress relieving • Annealing to produce a ferritic matrix • Normalizing to produce a pearlitic matrix

Table 4 Influence of minor elements on graphite shape Element category Element

Spheroidizer . . . . . . . . . . . . Magnesium, calcium, rare earths (cerium, lanthanum, etc.), yttrium

Neutral . . . . . . . . . . . . . . . . Iron, carbon, alloying e lements

Ant i sphero id izer (degenerate shape) . . . . Aluminum, arsenic, bismuth,

tellurium, titanium, lead, sulfur, antimony

I II III

IV V VI

VII

Typical graphite shapes after ASTM A 247. I, spheroidal graphite; II, imperfect spheroidal graphite; III, 1 1 " 5 " temper graphite; IV, compacted graphite; V, crab graphite; VI, expJoded graphite; VII, flake graphite

• Hardening to produce tempering structures • Austempering to produce a ferritic bainite

The advantage of austempering is that it results in ductile irons with twice the tensile strength for the same toughness. A compar- ison between some mechanical properties of austempered ductile iron and standard ductile iron is shown in Fig. 16.

Compacted Graphite Irons Compacted graphite irons have a graphite

shape intermediate between spheroidal and flake. Typically, compacted graphite looks like type IV graphite (Fig. 1 I). Consequent- ly, most of the properties of CG irons lie in between those of gray and ductile iron.

The chemical composition effects are sim- ilar to those described for ductile iron. Carbon equivalent influences strength less obviously than for the case of gray iron, but

420 ~ 61

360 ~ Spheroidal 52

300 / L 43.5

Compacted 240 35 =

/ / ° '~ 180 ~ F l a k e _ 26 --'~

'~ / 17.5 120

oo///// oV o

0 0.1 0.2 0.3 0.4 0.5 Strain, %

Influence of graphite morphology on the I:|a 1 2 ='~5" stress-strain curve of several cast irons

Classification and Basic Metallurgy of Cast Iron / 9

100

o~ 6O

~; Flake - -Compa Spheroidal "Z ; +0 j~ 20

o 0 0.01 0.02 0.03 0.04

Residual magnesium, %

Influence of residual magnesium on graph- I:;,1, 1 3 "'~5° ite shape

more than for ductile iron, as shown in Fig. 17. The graphite shape is controlled, as in the case of ductile iron, through the content of minor elements. When the goal is to produce compacted graphite, it is easier from the standpoint of controlling the structure to com- bine spheroidizing (magnesium, calcium, and/ or rare earths) and antispheroidizing (titanium and/or aluminum) elements. Additional infor- mation is available in the article "Compacted Graphite Irons" in Volume 15 of the 9th Edition of Metals Handbook.

The cooling rate affects properties less for gray iron but more for ductile iron (Fig. 18). In other words, CG iron is less section sensitive than gray iron. However, high cooling rates are to be avoided because of the high propensity of CG iron for chilling and high nodule count in thin sections.

Liquid treatment can have two stages, as for ductile iron. Modification can be achieved with magnesium, Mg + Ti, Ce + Ca, and so on. Inoculation must be kept at a low level to avoid excessive nodularity.

1600 ! ! .0 1400 ; :! i~ ....~ 200 + t . r i e p e d

1200 i 180

160 = 1000 == \ ++++++++++++ +~ ~ m ~++++++++++++++++i 120 _= 600 ~ \ .£ "~ ~ Quenched and m ~ ~ tempered 100 i~ ~- 600 ~ 80

400 ~ ~ - 60

ASTM grades ~ 40 200 I I

0 5 10 15 20 Elongation, %

Properties of some standard and austere- 1 6 " 5 " pered ductile irons. Source: Ref 8

E 100 E t ~ E ~ 80

o ~

~ 60

g I---

++iiii+ ~ ++++++++i+++i+++

,o ++D % + 20 0.01 0.02 0.03 0.04 0.05 0.06

Residual magnesium, %

(a)

0+07

600 8

500 --Tensi le s t r e n , , ~ " ~ ' ~ ' ~ - 7 . j " 6

m 400 / ~ :~ "Elongation

300 . - 4 t ~

3 ~ (n 200 '~

100 ~ rength - 1

0 0 0 20 40 60 80 100

Nodularity, %

(b)

Influence of residual magnesium (a) and 14 " ' ~ " nodularity (b) on some mechanical proper- ties of ductile iron. Sources: Ref 4, 5

Heat treatment is not common for CG irons.

M a l l e a b l e I r o n s

Malleable cast irons differ from the types of irons previously discussed in that they have an initial as-cast white structure, that is, a structure consisting of iron carbides in a pearlitic matrix. This white structure is then heat treated (annealing at 800 to 970 °C, or 1470 to 1780 °F), which results in the

600 87 , i = i I

Spheroidal graphite (SN~G 500/7)

500 ;+~ii;~;ii~;~;i+~iP~;i;i+iii;~;i;i~i;;!i~;;~i~;~!~i~i~;ii~i~;i!i!i+:~k:~ii;iiii;ii;~ii~i~i~;Ii;~;~i~++ n.5

400, O O . . . . "~, 58 . . . . ,o . . 2 . b .

300 - " Compacted g r a p h i t e 43.5 _~

~- 200 ~ 29 Flake graphite

~00 I t I 145 3.9 4.0 4.1 4.3 4.3 4.4 4.5 4.6

Carbon equivalent, %

Effect of carbon equivalent on the tensile F ig . 1 7 strength of flake, compacted, and spheroi- dal graphite irons cast in 30 mm (1.2 in.) diam bars. Source: Ref 9

700 50

E No ule c iii~i~iiii!. ............. V E

+ + 400 20 2

300 10 Chill width

200 0.4

0 0.6 0.8 1.0 1.2 1.4

75% FeSi added as postinoculant

Influence of the amount of 75% ferrosilicon E:+r,$. 15 added as a postinoculant on the nodule count and chill depth of 3 mm (0.12 in.) plates. Source: Ref 7

decomposition of Fe3C and the formation of temper graphite. The basic solid state reac- tion is:

Fe3C -~ 7 + Gr (Eq 5) The final structure consists of graphite and pearlite, pearlite and ferrite, or ferrite. The structure of the matrix is a function of the cooling rate after annealing. Most of the malleable iron is produced by this technique and is called blackheart malleable iron. Some malleable iron is produced in Europe by decarburization of the white as-cast iron, and it is called whiteheart malleable iron.

The composition of malleable irons must be selected in such a way as to produce a white as-cast structure and to allow for fast annealing times. Some typical compositions are given in Table 2. Although higher car- bon and silicon reduce the heat treatment time, they must be limited to ensure a graphite-free structure upon solidification. Both tensile strength and elongation de- crease with higher carbon equivalent. Nev- ertheless, it is not enough to control the carbon equivalent. The annealing time de- pends on the number of graphite nuclei available for graphitization, which in turn

~. 500

.rE 400

300

--~ 200 £

Section thickness, in. 0.6 0.8 1.2 1.6 2.4 4 6 8 12

600 I I I I I I 87 ,..Compacted graphite cast iron

+++++i+i+i+i+i+ ~

43.5 ~

~'+:~"~:~:~:+iii:~ii:~iii+i::i::++ ~ . 29 .i + ~ i+!+i!++i++i++i++ ii++i++i++i++i++i++i++ 100 - ~ ~ i 14.5 "-

0 L o 15 20 30 40 60 100 150 200 300

Section thickness, mm

Influence of section thickness on the ten- E:-r,~. 18 sile strength of CG irons. Source: Ref 10

10 / Cast Irons

250

i E E 200 ~2

150 -'G

~00

~ so E (11

I - -

0

CE = constant

0.5 1.0 1.5 C/Si ratio

Influence of C/Si ratio on the number of r . ' ~ 1 9 H ~ . temper graphite clusters at constant carbon equivalent. Source: Ref 10

depends on, among other factors, the C/Si ratio. As shown in Fig. 19, a lower C/Si ratio (that is, a higher silicon content for a constant carbon equivalent) results in a higher temper graphite count. This in turn translates into shorter annealing times.

Manganese content and the Mn/S ratio must be closely controlled. In general, a lower manganese content is used when fer- ritic rather than pearlitic structures are de- sired. The correct Mn/S ratio can be calcu- lated with Eq 2. Equation 2 is plotted in Fig. 20. Under the line described by Eq 2, all sulfur is stoichiometrically tied to manga- nese as MnS. The excess manganese is dissolved in the ferrite. In the range delim- ited by the lines given by Eq 2 and the line Mn/S = I, a mixed sulfide, (Mn,Fe)S, is formed. For Mn/S ratios smaller than 1, pure FeS is also formed. It is assumed that the degree of compacting of temper graphite depends on the type of sulfides occurring in the iron (Ref I I). When FeS is predominant, very compacted, nodular temper graphite forms, but some undissolved Fe3C may persist in the structure, resulting in lower elongations. When MnS is predominant, although the graphite is less compacted, elongation is higher because of the com- pletely Fe3C-free structure.

The Mn/S ratio also influences the num- ber of temper graphite particles. From this standpoint, the optimum Mn/S ratio is about 2 to 4 (Fig. 21).

Alloying elements can be used in some grades of pearlitic malleable irons. The manganese content can be increased to 1.2%, or copper, nickel, and/or molybde- num can be added. Chromium must be avoided because it produces stable car- bides, which are difficult to decompose during annealing.

Cooling Rate. Like all other irons, mallea- ble irons are sensitive to cooling rate. Nev- ertheless, because the final structure is the result of a solid-state reaction, they are the least section sensitive irons. Typical corre- lations between tensile strength, elongation, and section thickness are shown in Fig. 22.

08 ~ / ~ o ~

[Mn] = 0 I "~ 41k X.~.~ o., - - F e S + IMnFelS~g" F ~

~ 0.3

c~ (Mn 0.2

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Manganese, %

"=151:"~" 2 0 Influence of the MnlS ratio on the shape of temper graphite. Bracketed elements are

dissolved in the matrix.

The l i qu i d t r e a t m e n t of malleable iron increases the number of nuclei available for the solid-state graphitization reaction. This can be achieved in two different ways, as follows:

• By adding elements that increase under- cooling during solidification. Typical ele- ments in this category are magnesium, cerium, bismuth, and tellurium. Higher undercooling results in finer structure, which in turn means more 3,-Fe3C inter- face. Because graphite nucleates at the -,/-Fe3C interface, this means more nucle- ation sites for graphite. Higher under- cooling during solidification also prevents the formation of unwanted eutectic graphite

• By adding nitrite-forming elements to the melt. Typical elements in this category are aluminum, boron, titanium, and zir- conium

50{

A 40c \

30C ~ , ,

200

101 /

O

"--L

B ~ v

b 2 4 6 8 10 12 Mn/S ratio

Influence of the Mn/S ratio on the number r-'_rlg. 21 of temper graphite clusters after annealing. A, low-temperature holding for 12 h at 350 °C (660 °F); B, no low-temperature holding

The hea t t r e a t m e n t of malleable iron de- termines the final structure of this iron. It has two basic stages. In the first stage, the iron carbide is decomposed in austenite and graphite (Eq 5). In the second stage, the austenite is transformed into pearlite, fer- rite, or a mixture of the two. Although there are some compositional differences be- tween ferritic and pearlitic irons, the main difference is in the heat treatment cycle. When ferritic structures are to be produced, cooling rates in the range of 3 to 10 °C/h (5 to 18 °F/h) are required through the eutectoid transformation in the second stage. This is necessary to allow for a complete austenite- to-ferrite reaction. A typical annealing cycle for ferritic malleable iron is shown in Fig. 23. When pearlitic irons are to be produced, different schemes can be used, as shown in Fig. 24. The goal of the treatment is to

Bar diameter, in. Bar diameter, in. 0.32 0.48 0.64 0.80 0.96 1.12 0.32 0.48 0.64 0.80 0.96 1.12

400 58 16

380 iiiiiiiiiiiiiiiii i iii,i!ii 86 1,

..,=" o~ ................

.... %iiiiiiiiil 49 ,= .I lO

- - 0

.... 46 m 8 . . . . . . . ' " . . . . . . . . . . . .

43 6

360

.E

Q 340

o~

~- 320 t -

300

280 40 4 8 12 16 20 24 28 8 12 16 20 24 28

Bar diameter, mm Bar diameter, mm

(a) (b)

Influence of bar diameter on the tensile strength (a) and elongation (b) of blackheart malleable iron. 17[a 2 2 " b ° Source: Ref 13

Classification and Basic Metallurgy of Cast Iron / 11

1250

-<--First > < Second -->- stage stage

960 1760 o o

~- 720 1330 ~-

I--- I--

500 930

250 0 12 24 36 48 60

Time, h

Heat treatment cycle for ferritic blackheart 23 " ' ~ " malleable iron. Source: Ref 1

achieve a eutectoid transformation accord- ing to the austenite-to-pearlite reaction. In some limited cases, quenching-tempering treatments are used for malleable irons.

Special Cast Irons Special cast irons, as previously dis-

cussed, are alloy irons that take advantage of the radical changes in structure produced by rather large amounts of alloying ele- ments. Abrasion resistance can be im- proved by increasing hardness, which in turn can be achieved by either increasing the amount of carbides and their hardness or by producing a martensitic structure. The least expensive material is white iron with a pearlitic matrix. Additions of 3 to 5% Ni and 1.5 to 2.5% Cr result in irons with (FeCr)3C carbides and an as-cast martensitic matrix. Additions of 11 to 35% Cr produce

940 °C / 7 5 0 °C

940 °C

705°C I

530 °C 34

940 °C ~. 770 °C / < 25 15

> ' V~/~ ' v >lr<:

940 °C ~ 800 *C / 24 35

Time, h

Heat treatment cycles for pearlitic black- I::;,r, 24 H ~ . heart malleable irons

(CrFe)7C 3 carbides, which are harder than the iron carbides. Additions of 4 to 16% Mn will result in a structure consisting of (FeMn)3C, martensite, and work-harden- able austenite.

Heat resistance depends on the stability of the microstructure. Irons used for these applications may have a ferritic structure with graphite (5% Si), a ferritic structure

with stable carbides (ll to 28% Cr), or a stable austenitic structure with either sphe- roidal or flake graphite (18% Ni, 5% Si). For corrosion resistance, irons with high chro- mium (up to 28%), nickel (up to 18%), and silicon (up to 15%) are used.

REFERENCES I. R. Elliott, Cast Iron Technology, But-

terworths, 1988 2. C.F. Walton and T.J. Opar, Ed., Iron

Castings Handbook, Iron Castings So- ciety, 1981

3. C.E. Bates, AFS Trans., Vol 94, 1986, p 889

4. R. Barton, B.C.I.R.A.J., No. 5, 1961, p 668

5. R.W. Lindsay and A. Shames, AFS Trans., Vol 60, 1952, p 650

6. H. Morrogh, AFS Trans., Vol 60, 1952, p 439

7. D.M. Stefanescu, AFS Int. Cast Met. J., June 1981, p 23

8. J.F. Janowak and R.B. Gundlach, AFS Trans., Vol 91, 1983, p 377

9. G.F. Sergeant and E.R. Evans, Br. Foundryman, May 1978, p 115

10. D.M. Stefanescu, Metalurgia, No. 7, 1967, p 368

II. K. Roesch, StahlEisen, No. 24, 1957, p 1747

12. R.P. Todorov, in Proceedings of the 32nd International Foundry Congress (Warsaw, Poland), International Com- mittee of Foundry Technical Associa- tions

13. K.M. Ankab, O.E. Shulte, and P.N. Bidulia, Isvestia Vishih Utchebnik Zavedenia-Tchornaia, Metallurghia, No. 5, 1966, p 168

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