ASM Specialty Handbook Cast Irons Copyright © 1996 ASM International® J.R

Home , Austenite, Cast iron, Ductile iron, Gray iron, Malleable iron

Classification and B!asic Metallurgy of Cast Irons

THE TERM CAST IRON, like the term steel, carbon, silicon), minor (<0.1 %), and often alloy- iron exhibits a richer carbon phase than that of identifies a large family of ferrous alloys. Cast ing (>0.1%) elements. Cast iron has higher car- steel. Depending primarily on composition, cool- irons are multicomponent ferrous alloys, which bon and silicon contents than steel. Because of ing rate, and melt treatment, cast iron can solidify solidify with a eutectic. They contain major (iron, the higher carbon content, the structure of cast according to the thennodynamically metastable

Pearlite + Graphite (o:Fe + Fe3C) Fa Solid-state transformation Gray 'Y + Graphite cast (cooling through iron eutectoid interval) Slo Graphite shape+ depends Ferrite + Graphite on minor elements (uFe)

Flake~ Compacted Spheroidal

Liquid Solidification cast iron 'Y + Fe3C + Graphite Mottled cast iron (iron. carbon Graphitization alloy) potential

Solid-state transformation Pearlite + Fe,C .. White iron (cooling through eutectoid interval) Reheat above leutectoid interval "' + Fe3C--'Y + Graphite Hold above eutectoid interval eutectoidth~~:h l interval ~ Pearlite + Temper graphite Ferrite + Temper graphite

Malleable iron

Fig. 1 Basic microstructures and processing for obtaining common commercial cast irons 4 I Introduction

cleation potential of the liquid, chemical composition, and cooling rate. The first two factors determine the graphitization potential of the iron. Classification by A high graphitization potential will result in irons commercial name f------' with graphite as the rich carbon phase, while a or application low graphitization potential will result in irons with iron carbide. A schematic of the structure of the common (unalloyed or low-alloy) types of commercial cast irons, as weU as the processing required to obtain them, is shown in Fig. 1. The two basic types of eutectics, the stable austenite-graphite and the metastable austenite- iron carbide (Fe3C), have wide differences in their mechanical properties, such as strength, hardness, toughness, and ductility. Therefore, the basic purpose of the metallurgical processing of cast iron is to manipulate the type, amount, and High-temperature applications morphology 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 Flake (lamellar) surface because fracture occurs along the iron graphite carbide plates; it is the result of metastable solidification (Fe3C eutectic).

Compacted • Gray iron exhibits a gray fracture surface be- (vermicular) cause fracture occurs along the graphite plates graphite (flakes); it is the result of stable solidification (Gr eutectic), Spheroidal graphite With the advent of metallography, and as the body of knowledge pertinent to cast iron increased, other Temper classifications based on microstructural features be- carbon came possible:

Fig. 2 Classification of cast irons • Graphite shape: Lamellar (flake) graphite (FG), spheroidal (nodular) graphite (SG), compacted (vermicular) graphite (CG), and Table 1 Classification of cast iron by commercial designation, microstructure, and fracture temper graphite (fG). Temper graphite results from a solid-state reaction (malleabilization). Commercial designation Carbon-rich phase Matrix(a) Fracture Final structure after • Matrix: Ferritic, pearlitic, austenitic, martensi-

Gray iron Lamellar graphite p Gray Solidification tic, and bainitic (austempered). Ductile iron Spheroidal graphite F,P,A Silver-gray Solidification or heat treatment Compacted graphite iron Compacted {vermicular) graphite P,P Gray Solidification Another common classification scheme di- White iron Fc.1C P,M White Solidification and heat treatment(b) p vides cast irons into four basic types: white iron, Mottled iron Lamellar Gr + Fe3C Mottled Solidification Malleable iron Temper graphite F,P Silver-gray Heat treaunent gray iron, ductile iron, and malleable iron. As Austempercd ductile iron Spheroidal graphite At Silver-gray Heat treatment indicated above, white iron and gray iron derive their names from the appearance of their respec- (a) F, ferrite; P, pearlite; A, austenite; M, martensite; At, austempered (bainite). (b) White irons arc not usually heat treated, except for tive fracture surfaces. Ductile iron derives its stress relief and to continue austenite tmnsfonnation. name from the fact that, in the as-cast form, it exhibits measurable ductility. By contrast, neither Fe-Fe3C system or the stable iron-graphite sys- tioned that silicon and other alloying elements white nor gray iron exhibits significant ductility tem. When the metastable path is followed, the may considerably change the maximum solubil- in a standard tensile test. Malleable iron is cast as rich carbon phase in the eutectic is the iron car- ity of carbon in austenite (y). Therefore, in excep- white iron, then "malleabilized" (i.e., heat treated bide; when the stable solidification path is fol- tional cases, alloys with less than 2% C can solid- to impart ductility to an otherwise brittle mate- lowed, the rich carbon phase is graphite. Refer- ify with a eutectic structure and therefore still rial). ring only to the binary Fe-Fe3C or iron-graphite belong to the family of cast iron. Besides the four basic types, there are other system, cast iron can be defined as an iron-carbon The fonnation of stable or metastable eutectic specific forms of cast iron to which special names alloy with more than 2% C. The reader is cau- is a function of many factors, including the nu- have been applied. Classification and Basic Metallurgy I 5

and eutectoid points shift to lower carbon contents. Graphite free Graphite bearing Figure 5 represents the metastable equilibrium between iron and iron carbide (cementite), a metastable system. The silicon that is present remains in solid solution in the iron, in both ferrite Pearlitic Martensitic High-chromium iron Ferritic Austenitic Acicular and austenite, so it affects only the conditions and white iron white iron (11-28% Cr) the kinetics of carbide formation on cooling, not (Ni-Hard) High strength the composition of the carbide phase. The desig- wear resistant a,"(, Wear Wear Wear, corrosion, nations and Fe3C, therefore, are used in the resistant resistant and heat resistant ternary system to identify the same phases that 18% Ni 18% Ni, 5% Si occur in the Fe-Fc3C binary system. Some of the Ni-resist Nicrosilal silicon may precipitate along with the carbide, ASTM A532 but it cannot be distinguished as a different phase. Corrosion and Heat and The solidification of certain compositions occurs heat resistant corrosion resistant not in the metastable system. but rather in the stable system, where the products are iron and ASTM A439 graphite rather than iron and carbide. These compositions encompass the gray, ductile, and compacted graphite cast irons, To justify the usc of the ternary diagram at 2% 5o/o Si iron (Silal), High (15%) silicon iron, Si to trace phase changes, it must be assumed that heat resistant corrosion resistant the silicon concentration remains at 2% in all ASTM A 518 or A 518 M (metric) parts of the alloy under all conditions. This obviously is not strictly true, but there is little evi- Fig. 3 Classification of special high-alloy cast irons. Source: Ref 1 dence that silicon segregates to any marked de-. gree in cast iron. Thus it is only slightly inaccurate to use the constant-silicon section through the ternary diagram in the same manner • Chilled iron is white iron that has been pro- common cast irons mainly in the higher content as one would apply the Fe-Fe3C diagram to car- duced by cooling very rapidly through the so- of alloying elements (>3%), which promote bon steel. lidification temperature range. microstructures having special properties for ele- In summary, the addition of silicon to a binary • Mottled iron is an area of the casting that so- vated-temperature applications, corrosion resis- iron-carbon alloy decreases the stability of Fe3C, lidifies at a rate intermediate between those for tance, and wear resistance. A classification of the which is already metastable, and increases the chiHed and gray iron, and which exhibits main types of special high-alloy cast irons is stability of ferrite (the a field is enlarged, and the microstructural and fracture-surface features shown in Fig. 3. "{ is constricted). The equilibrium diagrams in of both types. Fig. 6 show that as the silicon content in the Compacted graphite cast iron • (also known as Fe-C-Si system increases, the carbon contents of vermicular iron) is characterized by graphite the eutectic and eutectoid decrease, while the that is interconnected within eutectic cells, as eutectic and eutectoid temperatures increase. is the flake graphite in gray iron. Compared with the graphite in gray iron, however, the The Iron-Carbon-Silicon System graphite in CG iron is coarser and more The metallurgy of cast irons has many similari- rounded, i.e., its structure is intermediate be- ties to that of steel, but the differences are impor- tween the structures of gray iron and ductile tant to the metallurgist who works with cast irons Carbon Equivalent (Ref 2) iron. (Ref 2). The amount of alloying elements present • High-alloy graphitic irons are used primarily in the most common grades of steel is relatively The upper dashed line in Fig. 7 indicates the for applications requiring corrosion resistance low, so these steels can basically be considered as eutectic composition for Fe-C-Si alloys. Without or a combination of strength and oxidation binary iron-carbon alloys. Thus the iron-carbon silicon the eutectic is at 4.3%. As the silicon resistance. They are produced in both flake content of iron is increased, the carbon content of diagram (Fig. 4) can be used to interpret their graphite (gray iron) or spheroidal graphite the eutectic is decreased. This is a linear relation structures under conditions of slow or near-equi- (ductile iron). and can be expressed as a simple equation: librium transformation. The cast irons, however,

contain appreciable amounts of silicon in addi- 1 Figure 2 classifies cast irons according to their com- %C+ (~%Si=4.3 (Eq 1) mercial names, applications, and structures. tion to higher carbon contents, and they must be Lastly, a classification used frequently by the considered ternary Fe-C-Si alloys (Fig. 5). The It floor foundry worker divides cast irons intO two introduction of this additional constituent; sili- is convenient to combine the effect of the categories: con, changes the iron-carbon diagram. silicon with that of the carbon into a single factor, A section through the ternary Fe-Fe3C-Si dia- which is called the carbon equivalent (CE): • Common cast irons: For general-purpose ap- gram at 2% Si (which approximates the silicon (Eq2) plications, unalloyed or low alloy content of many cast irons) provides a convenient • Special cast irons: For special applications, reference for discussing the metallurgy of cast generally high alloy iron. The diagram in Fig. 5 resembles the binary The CE of a cast iron describes how close a Fe-Fe3C diagram in Fig. 4, but it exhibits impor- given analysis is to that of the eutectic composi- Table 1 gives the correspondence between tant differences characteristic of ternary systems. tion. When the CE is 4.3, the alloy is eutectic. A commercial and microstructural classification, as Eutectic and eutectoid temperatures change from CE of 3. 9 represents an alloy of lower carbon and well as the final processing stage in obtaining single values in the Fe-Fe3C system to tempera- silicon content (hypoeutectic) than the eutectic common cast irons. Special cast irons differ from ture ranges in the Fe-Fe3C-Si system; the eutectic composition, and aCE of 4.6 represents an alloy 6 I Introduction

1800 I 3270 eration, as well as knowledge of the factors that affect the structure. When discussing the metal- 1700 / 3090 Iurgy of cast iron, the main factors of influence on I the structure that one needs to address are: 1600 2910 I 1538 "C___....-1495 "C I • Chemical composition Solubility of Liquid - • Cooling rate 1500 ~ graphite in 2730 {8-~ liquid iron • Liquid treatment I-- • Heat treatment 1400 1'--- 2550 13~~ I ~ I In addition, the following aspects of combined car- 1300 ------2370 bon in cast irons should be considered: ------12:r "C ~~4.26% I -r---· f-·-t- 1200 t----- (y-Fe) 2190 • In the original cooling or through subsequent 2.08",{ 1154 "C - --~ Austenite w -- heat treatment, a matrix can be intemaJiy de- 1148 ".C 4.30% --- "2.11% 6.69% 1100 "" 2010 carburized or carburized by depositing graph- I ite on existing sites or by dissolving carbon 1000 1/ Cementite_ 1830 \;1- from them. Austenite (Fe,C) ~ + • Depending on the silicon content and the cool- "0 912 ° in ;; 900 .I cementite ing rate, the pearlite iron can vary in carbon •~ Oi6B% content. This is a ternary system, and the car- E ·y-3 17\. A,m bon content of pearlite can be as low as 0.50% ~ BOO 738 "C with 2.5% Si. b?o 'C :J:j -- • The measured hardness of graphitic irons is 700 t---0.77% 1290 influenced by the graphite, especially in gray A; (727 'C) iron. Martensite microhardness may be as high 1110 600 as 66 HRC or as low as 54 HRC in gray iron (58 HRC in ductile). 500 930 • The critical temperature of iron is influenced 1-- (a-Fe) Ferrite (raised) by silicon content, not carbon content. 400 Ferrite 750 + The following sections in this article discuss cementite 300 570 some of the basic principles of cast iron metallurgy. More detailed descriptions of the metal- 200 390 lurgy of cast irons are available in the articles in ! this Volume that describe specific types of cast 100 210 irons.

0 i ~ Fe 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Carbon, wt%

fig. 4 Iron-carbon diagram, where solid curves represent the metastable system Fe-Fe 3C and dashed curves represent the stable system iron-graphite · Gray Iron (Flake Graphite Iron) The composition of gray iron must be se- of greater carbon and silicon content (hypereutec- be noted, however, that irons of constant CE, but lected in such a way as to satisfy three basic tic) than the eutectic composition. Irons of the with appreciably different carbon and silicon structural requirements: same CE value may be obtained with different contents, will not have similar casting properties. carbon and silicon values. For example, carbon is more than twice as effec- • The required graphite shape and distribution Other values representing the CE of cast irons tive in preventing solidification shrinkage than • The carbide-free (chill-free) structure have also been suggested. When appreciable the CE equation indicates. However, silicon is • The required matrix amounts of phosphorus are present in the iron, the more effective in keeping thin sections from be- phosphorus content of the iron is included: coming hard. There are similar differences in For common cast iron, the main elements of the some of the use properties, and these limit the chemical compOsition are ·carbon and silicon. value of CE in specifications. Ftgure 7 shows the range of carbon and silicon for -CE=%C+ %Si+%P (Eq3) 3 common cast irons as compared with steel. It is apparent that irons have carbon in excess of the maximum solubility of carbon in austenite, which Thus, an iron with 3.2% C. 2% Si, and 0.4% P has is shown by the lower dashed line. A high carbon aCE value of 4.0 and is hypoeutectic. An iron with content increases the amount of graphite or Fe3C. 3.2% C, 2% Si, and I .3% P has a CE value of 4.3 Principles of the Metallurgy ffigh carbon and silicon contents increase the and is eutectic. An iron with 3.2% C, 2.9% Si, and of Cast Irons graphitization potential of the iron as well as its 1.3% P has aCE value of 4.6 and is hypereutectic. castability. Although increasing the carbon and The total carbon and silicon contents of the The goal of the metallurgist is to design a silicon contents improves the graphitization po- a11oy, as related in the CE value, not only estab- process that will produce a structure that will tential and therefore decreases the chilling ten- lish the solidification temperature range of the yield the expected mechanical properties. This dency, the strength is adversely affected (Fig. 8). alloy, but are also related to the foundry charac- requires knowledge of the structure-properties This is due to ferrite promotion and the coarsen- teristics of the alloy and its properties. It should correlation for the particular alloy under consid- ing of pearlite. Classification and Basic Metallurgy I 7

150 0 %Mn= 1.7 (% S)+O.l5 (Eq4)

. (l:i-Fe + -y-Fe + L) Other minor elements, such as aluminum, antimony, arsenic, bismuth, lead, magnesium, cerium, and cal- I ~ 600 ' cium, can significantly alter both the graphite mor- 140 0 -r 1'.. phology and the microstructure of the matrix. The range of composition for typical unalloyed common cast irons is given in Table 2. The typi- L cal composition range for low- and high-grade 1\~(li-Fe) unalloyed gray iron (flake graphite iron) cast in Delta ferrite ~ 400 sand molds is given in Table 3. 130 0 - ' \ 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. 9. Type A graphite is found in inoculated irons cooled with moderate rates. In ~ 200 general, it is associated with the best mechanical 120 0 t\. / \ ' properties, and cast irons with this type of graphite exhibit moderate undercooling during solidifi- (7-Fe) cation (Fig. 10). Type B graphite is found in irons Austenite \ v -!' of near-eutectic composition, solidifying on a fr-Fe + Fe3C + L) limited number of nuclei. Large eutectic cell size 1100 I and low undercoolings arc common in cast irons 000 ' exhibiting this type of graphite. Type C gmphite occurs in hypereutectic irons as a result of solidification with minimum undercooling. Type D graphite is found in hypoeutectic or eutectic irons solidified at rather high cooling rates, while type 1000 I E graphite is characteristic for strongly hypoeu- 1800 tectic irons. Types D and E are both associated with high undercoolings during solidification. 1/ Not only graphite shape but also graphite size is important, because it is directly related to 900 strength (Fig. 11 ). Alloying elements can be added in common 600 cast iron to enhance some mechanical properties. ~1 They influence both the graphitization potential and the structure and properties of the matrix. The main elements are listed below in terms of their 800 \I ~ graphitization potential: v ~e +-y-F~Fe3Cl - -- 1400 High positive graphitization potential (decreasing positive ~{o:-Fe) potential.from top to bottom) Ferrite Carbon T'm 700 Phosphorus Silicon Aluminum 1200 Copper Nickel Neutral 600 kon 0 4 Carbon content, wt % High negative graphitization potential (increa<;ing negative potential from top to bottom) Fig. 5 Section through the Fe-Fe3C-Si ternary equilibrium diagram at 2% Si Manganese Chromium Molybdenum Vanadium The manganese content varies as a function of pacted graphite iron. The effect of sulfur must be the desired matrix. Typically, it can be as low as balanced by the effect of manganese. Without This classification is based on the thermody- 0.1% for ferritic irons and as high as 1.2% for manganese in the iron, undesired iron sulfide namic analysis of the influence of a third element pearlitic irons, because manganese is a strong (FeS) will form at grain boundaries. If the sulfur on carbon solubility in the Fe-C-X system, where pearlite promoter. content is balanced by manganese, manganese X is a third element (Ref 6). Although phosphorus From the minor elements, phosphorus and sul- sulfide (MnS) will form, but this is hannless is listed as a graphitizer (which may be true ther- fur are the most common and are always present because it is distributed within the grains. The modynamically), it also acts as a matrix hardener. in the composition. They can be as high as 0.15% optimum ratio between manganese and sulfur for Above its solubility level (probably about for low-quality iron and are considerably less for an FeS-free structure and maximum amount of 0.08%), phosphorus forms a very hard ternary high-quality iron, such as ductile iron or com- ferrite is: eutectic. The above classification should also in- 8 I Introduction

1500

0 1200 ~ :;;0 0 ~ E r-0

a a+ Fe3C 600 0 2 3 4 (a) Carbon content,%

6+L 6.0%Si / 1500~ ~- ~ 10:<+y+L L / y 0 0 \ ~~/L+Fe,C 1200H~,-t'-"-+""''-l-+---i ~ :;;~ \j ~ 1300H---~~----~'----+-----4 0 1i E !" !"~ 900~~~$:a~+2y~+~F~e~,~c~~~~~~ ; a•Fe'jc~ / y +C, 600!;----~--+--~------o 11001J_-~-L---;':---7------o 0 2 3 4 0 2 3 4 (b) Carbon content,% (c) Carbon, content,% (d) Carbon content, wt%

1500 1500 5.2% Si 7.9%Si

1500 1400~ 1400 3.5% Si a+'0 0 a t:-.. 0 1400 K ~ 130 1300 ~~ 0 0 '" ,..-- " ~ 0 I t\+y+L" 0 ~ a ~a+'(~ :;; :;; \ a+L y+ L 0 0 I ~+y y+~ I 0 ~ I I ~ :;; 1300 E 120 1200 1'\ 0 E 0 a+y I I y+L+C,.._ " a ~ !" !" I a+y+Lj'X / E ~ I I y y y+ L 1 !" I I :--a+ y a+ y+ C1 I 1200 ~ 110 o 1100 \ y+L+C1 y + c, a+ C, ".L I I ' c,/ y+LJc, I I I /a+ I a+ C 1 a+C,+Ic, I y+ c, a+ y + C 1 f-J +c, I 1100 1000 I 1000 1/ I I 0 2 3 4 0 '' 2 3 4 0 2 3 4 (e) Carbon content, % (I) Carbon content, wt% (g) Carbon content, wt%

Fig. 6 Influence of silicon content on the solubility lines and equilibrium temperatures of the iron-carbon system. (a) to (c) Source: Ref 3. (d) to {g) Source: Ref4

elude sulfur as a carbide former, although manga- transformations and increase the number of the matrix. Because they increase the amount of nese and sulfur can combine and neutralize each graphite particles. They form solid solutions in pearlite, they raise strength and hardness. other. The resultant MnS also acts as nuclei for the matrix. Because they increase the fenite-pear- Chromium, molybdenum, tungsten, and vana flake graphite. In industrial processes, nudeation lite ratio, they lower strength and hardness. dium decrease the graphitization potential at both phenomena may sometimes override solubility Nickel, copper, and tin increase the graphitiza- stages. Thus, they increase the amount of car- considerations. tion potential during the eutectic transformation bides and pearlite. They principally concentrate In general, alloying elements can be classified but decrease it during the eutectoid transforma- in the carbides, forming (FeX)nC-type carbides, into three categories, discussed below. tion. thus raising the pearlite-ferrite ratio. This but also alloy the aFe solid solution. As long as Silicon and aluminum increase the graphitiza- second effect is due to the retardation of carbon carbide formation does not occur, these elements tion potential for both the eutectic and eutectoid diffusion. These elements form solid solution in increase strength and hardness. Above a certain Classification and Basic Metallurgy I 9

400 • Increase the chilling tendency; this may result ~/1/. in higher hardness, but will decrease the n.• 350 111 50 .• ~ ' /111;;)(~ 1;; strength 111 ~ " 300 It/; a, 4.0 ~ It/tit. 40 c c I tift ~ Consequently, composition must be tailored in such ~ 250 "' 11t; a way as to provide the correct graphitization poten- 11 30 • 11 • • 200 i' 1;; ~ tial for a given cooling rate. For a given chemical ferrosilicon with Element Compooition, % additions of aluminum and calcium, or proprie- Fig.7 Approximate ranges of carbon and silicon for steel tary alloys, are used as inoculants. The main and various cast irons. Sourc-e: Ref 2 Carbon 3.04-3.29 Chromiwn 0.1-0.55 effects of inoculation are: Molybdenum 0.03-0.78 Silicon 1.6-2.46 • An increased graphitization potential because level, any of these elements will determine the Nickel 0.07-1.62 of decreased undercooling during solidifica- Sulfur 0.089-0.106 tion; as a result of this, the chilling tendency is solidification of a structure with both Gr and Manganese 0.39-0.98 Fe3C (mottled structure), which will have lower Coppcr 0.07-0.85 diminished, and graphite shape changes from strength but higher hardness. type D or E to type A In moderately alloyed gray iron, the typical The cooling rate, like the chemical composi- • A finer structure (i.e., higher number of eutec- ranges for the elements discussed above are as tion, can significantly influence the as-cast struc- tic cells), with a subsequent increase in follows: ture and therefore the mechanical properties. The strength

Element Composition,% cooling rate of a casting is primarily a function of its section size. The dependence of structure and As shown in Fig. 13, inoculation improves tensile Chromium 0.2-0.6 properties on section size is tenned section sensi- strength. This influence is more pronounced for Molybdenum 0.2-1 tivity. Increasing the cooling rate will: low-CE cast irons. Vanadium 0.1-0.2 Heat treatment can considerably alter the ma- 0.6-1 Nickel • Refine both graphite size and matrix structure; trix structure, although graphite shape and size Coppcr 0.5-1.5 Tin 0.04-0.08 this will result in increased strength and hard- remain basically unaffected. A rather low propor- ness tion ofthe total gray iron produced is heat treated. The influence of composition and cooling rate on tensile strength can be estimated using Table 2 Range of compositions for typical unalloyed common cast irons

(Ref 5): Com ositiou, % Typcofiron c ~ Mn p s TS ~ 162.37 + 16.61/D- 21.78 (%C) Grny 2.5-4.0 1.0-3.0 0.2-1.0 0.002-1.0 0.02-0.25 -61.29 (% Si)- 10.59 (% Mn- 1.7% S) Compacled graphite 2.5-4.0 1.0-3.0 0.2-1.0 0.01-0.1 O.Ql-O.Q3 + 13.80 (% Cr) + 2.05 (% Ni) + 30.66 (% Cu) Ductile 3.0-4.0 1.8-2.8 0.1-1.0 0.01-0.1 0.01-0.03 0.06-0.2 0.06-0.2 + 39.75 (% Mo) + 14.16 (% SiJ' White 1.8-3.6 0.5-1.9 0.25-0.8 Malleable 2.2-29 0.9-1.9 0.15-1.2 0.02-0.2 0.02-0.2 - 26.25 (% Cu)2 - 23.83 (% Mo)2 Source: Ref2 (Eq5)

Type A Type B Type D Type E

Uniform distribution, Rosette grouping, Superimposed flake size, lnterdendritic segregation, lnlerdendritic segregation, random orientotion random orientation random orientation random orientation preferred orientation Fig. 9 Types of graphite flakes in gray iron (AFS-ASTM). In the recommended practice (ASTM A247), these charts are shown at a magnification of lOOx. They have been reduced to one- third size for reproduction here. 10 I Introduction

Section th'1ckness, in. 400 ,--,--,---,,----,---,---, 56 0.5 1.0 1.5 500 70 AST~- 450 52 ~• A-48 -CI~"_I, 60 .. :;; 400 ~ ~ 508 -458 ~ ~- ~ '-l.l. ' £ 0, 350 40B 50 0, 48 t 0 0 ~ ~ t-- ~ 30 0~ l7 -, ~ ~ TE t; ..... 40 t; 250 r- • •~ .. ~ E 0 200 " " 30 0 (". (". 1-• ~• ~ :;; 150 -3TB 20 30B 258./ £ 100 0, 5 10 15 w 25 ~ ~ ~ ~ ~ ~ 250 1--~-l-_: Section thickness, mm t; {a) ...• c Section thickness, in. (". Fig. 10 Characteristic cooling curves associated with 0 5 1 0 1 5 different flake graphite shapes. TE, equilibrium 30 0 I eutectic temperature 1' I ~ST~ A·~B ID 250 Class I ' ,; ~ t-k 45B I /40B Maximum flake length, in. ~ 20 of@ 1' 0005 0010 0015 0020 0025 0030 0035 • I 15 2;~ ~5 50 Ot-ti_ 3~r ~• -~ :;; 1\ ] 10 0 100 L_l__l__L__L__ 345 50 £ £ 10 15 20 25 30 35 40 45 50 3.4 3.6 3.8 4.0 4.2 4.4 4.6 ~ ~ Carbon equivalent,% 0 \ l..n 0 Section thickness, mm 275 40 ~ ~ {b) Fig. 13 Influence of inoculation on tensile strength as a .. I~ ~ .. fundion of carbonequivalentfor 30 mm (1.2 in.) .• 205 30 .• 0 0 Fig 12 Influence of section thickness of the casting on diam bars. Source: Ref 2 (". • • (a) tensile strength and (b) hardness for a series of t-- o- 1- 135 20 gray irons classified by their strength as-cast in 30 mm (1.2 0.125 0.25 0.375 0.50- 0.635 0.75 0.90 in.) diam bars. Source: Ref 2 Maximum flake length, mm Fig. 11 Effect of maximum graphite flake length on the graphite shapes can occur, as illustrated in Fig. tensile strength of gray iron. Source: Ref 5 15. Graphite shape is the single most important factor affecting the mechanical properties of cast * iron, as shown in Fig. 16. ,g 2.5 Common heat treatments are stress relieving or The generic influence of various elements on " 00 mmealing to decrease hardness. graphite shape is given in Table 4. The elements in the first group, the spheroidizing elements, can 2.0~~~ change graphite shape from flake to compacted to spheroidaL This is illustrated in Fig. 17 for mag- Ductile Iron (Spheroidal nesium. The most widely used element for the Graphite I ron) production of spheroidal graphite is magnesium. 3.5 3.6 3.8 3.9 Total carbon,% The amount of residual magnesium required to Composition. The main effects of chemical produce spheroidal graphite, Mgresid, is genera1ly Fig. 14 Typical range for carbon and silicon contents in composition are similar to those described for 0.03 to 0.05%. The precise level depends on the good-quality ductile iron. TC, total carbon. Source: Ref 2 gray iron. with quantitative differences in the cooling rate. A higher cooling rate requires less extent of these effects and qualitative differences magnesium. The amount of magnesium to be in the influence on graphite morphology. The CE added in the iron is a function of the initial sulfur between the spheroidal graphite and the total has only a mild influence on the properties and level, Sin. and the recovery of magnesium, 11· in amount of graphite in the structure). This in tum structure of ductile iron, because it affects graph- the particular process used: results in a deterioration of the mechanical prop- ite shape considerably less than in the case of erties of the iron, as illustrated in Fig. 18. If the gray iron. Nevertheless, to prevent excessive 0.75 Sm + Mgresict magnesium content is too high, carbides are pro- (Eq6) shrinkage, high chi11ing tendency, graphite flota- Mgaddcd moted. tion, or a high-impact transition temperature, op- The presence of antispheroidizing (deleterious) timum amounts of carbon and silicon must be minor elements may result in graphite shape de- selected. Figure 14 shows the basic guidelines for A residual magnesium level that is too low terioration, up to complete graphite degeneration. the selection of appropriate compositions. results in insufficient nodularity (i.e., a low ratio Therefore, upper limits are set on the amount of As mentioned previously, minor elements can significantly alter the structure in terms of graphite morphology, chilling tendency, and matrix Table 3 Compositions of unalloyed gray irons structure. Minor elements can promote the spheroidization of graphite or can have an ad- Carbon Com osition,% ASTM A 48 class equivalent c Si Mn p s verse effect on graphite shape. The minor elements that adversely affect graphite· shape are 20B 45 3.1-3.4 2.5-2.8 0.5-0.7 0.9 0.15 said to degenerate graphite shape. A variety of 55B 3.6 .0.1 1.4-1.6 0.6-0.75 0.1 0.12 Classification and Basic Metallurgy I 11

420 61

360 l:tl- 52 / 300 - 43.5 ~• v :; Compicted ] u) 240 I / 35 "•~ •~ t; t; .!!! 180 I I 26 • ~ .. c c I / ~·- ~ / ~ Ill 120 / 17.5 (/; / 60

·~ If .. -~, ~ 0 0 ~l 0 0.1 0.2 0.3 0.4 0.5 '';. ;·"\' $ Strain,% ( ~ ( ...- Fig. 16 Jnfl~ence of graphite morp.hology on the stress- .. ~ 4 stram curve of several cast trans '\ t ,.. ( - •• • 100 80 -·--~·--- f \ IV v VI

;/!. 60 I \ ,g Flake 1/ \ Sphemid•l ~ 0 ~ 40 Compacted

0 o\ 0 ) ;:-..,..o 0 0.01 0.02 0.03 0.04 Residual magnesium,% Fig. 17 Influence of residual magnesium on graphite VII shape

fig. 15 Typical graphite shapes after ASTM A 247. I, spheroidal graphite; II, imperfect spheroidal graphite; Ill, temper graphite; IV, compacted graphite; V, crab graphite; VI, exploded graphite; VII, flake graphite

Table 4 Influence of various elements on deleterious elements to be accepted in the compo- In general, alloying elements have the same graphite shape sition of cast iron. Typical maximum limits are influence on structure and properties of ductile Element category Element (Ref9, 10): iron as for gray iron. A better graphite morphology allows more efficient use of the mechanical Spheroidizer Magnesium, calcium, rare earths (cerium, lanthanum, etc.), yttrium Element Composition, % properties of the matrix, so alloying is more com- Neutral Iron, carbon, alloying elements mon in ductile iron than in gray iron. Antispheroidizer Aluminum, arsenic, bismuth, tellurium, Almninum 0.05 Cooling Rate. When the cooling rate is (degenerate shape) titanium, lead, sulfur, antimony Arsenic' 0.02 Bismuth 0.002 changed, effects similar to those discussed for Cadmium O.ot gray iron occur in ductile iron, but the section 0.002 sensitivity of ductile iron is lower. This is because nodule count. Increasing the nodule count is an Antimony 0.001 ""' spheroidal graphite is less affected by cooling important goal, because a higher nodule count Selenium 0.03 rate than flake graphite. Tellurium 0.02 is associated with less chilling tendency (Fig. Titanium O.o3 The liquid treatment of ductile iron is more 19) and a higher as-cast ferrite/pearlite ratio Zirconium 0.10 complex than that of gray iron. The two stages for the liquid treatment of ductile iron are: Heat treatment is extensively used in the processing of ductile iron. Better advantage can These values can be influenced by the combi- 1. Modification, which consists of magnesium or be taken of the matrix structure than for gray iron. nation of various elements and by the presence of magnesium alloy treatment of the melt, with the The heat treatments usually applied are: rare earths in the composition. Furthermore, purpose of changing graphite shape from flake some of these elements can be deliberately added to spheroidal • Stress relieving during liquid processing in order to increase nod- 2. Inoculation (nonnally, postinoculation, i.e., af- • Annealing to produce a ferritic matrix ule count. ter the magnesium treatment) to increase the • Nonnalizing to produce a pearlitic matrix 12 I Introduction

1600 Section thickness, in. 220 0.8 1.2 1.6 2.4 6 1400 200 500 ~• 180 2 1200 400 ~• .• .s' ~ :; 160 ~ £ c 0, ~ 300 £ 1000 t; ~ 140 c c , ...• ~ .• 200 c ~ 120 c BOO .. ~ .• f-• 0.05 0.06 0.07 ...• c 100 c .100 Residual magnesium,% • ~ f- 600 80 Ia) Section thickness, mm 400 60 600 Fig. 22 Influence of section thickness on the tensile ~ 4{1 strength of compacted graphite irons. Source: 500 ,. - Ref14 f-T,nL""~ ,. 6 10 15 20 400 Elongation,% ~• "c' :; v,. ~~ongation 0 Properties of some standard and austempere 25 0 -~ 300 Fig.20 ~ ductile irons. Source: Ref 12 ' • c ' CE = clnstant ~ j/ 0 E \ u; 200 w E 200 ~Fatigue ~~rength 600 8 100 & \_ 1 I Spheroidal graphite (SNG 50017) 150 0 500 72.5 -~ 20 40 60 80 100 /{' ] 'E. IOD Nodularity,% 2 o- a-" --0 1"-- £ ~ ..c:' 40 -- - 5 0, ' (b) 0, '~- -- c • c 1-"' - 0 ~ E 5o ~ ~ --o ro-· --::--d) t; t; Influence of (a) residual magnesium and (b) Compacted graphite_ 4 ~ ~ ""' ""-... Fig. 18 ~ 300 3.5 modularity on some mechanical properties of ..... ·;;; 0 -~ c ductile iron. Source: Ref?, 8 1'---- 0 0.5 1.0 ~ ~ 1.5 ~ 20 0 29 C/Si ratio llake raphil;:----. Fig 23 Influence of C/Si ratio on the number of temper 700 50 100 14.5 • graphite clusters at constant carbon equivalent. 3.9 4.0 4.1 4.2 4.3 4.4- 4.5 4.6 Source: Ref 14 'E 600 40 Carbon equivalent,% ~ E Fig. 21 Effect of carbon equivalent on the tensile strength magnesium, Mg + Ti, Ce + Ca, and so on·. Inocu- '3• of flake, compacted, and spheroidal graphite irons ~ 500 30 E lation must be kept at a low level to avoid exces 0 cast in 30 mm (1.2 in.) diam bars. Source: Ref13 c £ sive nodularity. i 15_ , ~• Heat treatment is not common for CG irons . 0 400 20 0 i' u 15), and most of the properties ofCG irons lie in '3• Malleable Irons ~ between those of gray and ductile iron. z0 300 10 The chemical composition effects of CG Malleable cast irons differ from the types of irons are similar to those described for ductile irons previously discussed in that they have an 200 iron. The CE influences strength less obviously initial as-cast white structure, that is, a structure 0.4 0.6 0.8 1.0 1.2 1.4 than for the case of gray iron, but more than for consisting of iron carbides in a pearlitic matrix. 75% FeSi added as postinoculant ductile iron, as shown in Fig. 21. The graphite This white structure is then heat treated (anneal- Fig 19 Influence of the amount of 75% fcrrosilicon shape is controlled, as in the case of ductile iron, ing at 800 to 970 °C, or 1470 to 1780 °F), which ' added as a postinoculant on the nodule count results in the decomposition of Fe3C and the and chill depth of 3 mm (0.12 in.) plates. Source: Ref 11 through the content of minor elements. When the goal is to produce CO, it is easier from the stand- formation of temper graphite. The basic solid- point of controlling the structure to combine state reaction is: • Hardening to produce tempering structures spheroidizing (magnesium, calcium, and/or rare earths) and antispheroidizing (titanium and/or (Eq7) • Austempering to produce a ferritic bainite aluminum) elements. Additional information is available in the article "Foundry Practice for Cast Austempering results in ductile irons with twice the The fmal structure consists of graphite and pearlite, Irons" in this Volume. tensile strength for the same toughness. A compari- pearlite and ferrite, or ferrite. The structure of the The cooling rate affects properties of CG son between some mechanical properties of austem- matrix is a function of the cooling rate after anneal- irons less for gray iron but more for ductile iron pered ductile iron and standard ductile iron is shown ing. Most of the malleable iron is produced by this (Fig. 22). ln other words, CG iron is less section in Fig. 20. technique and is called blackheart malleable iron. sensitive than gray iron. However, high cooling Some malleable iron, called whiteheart malleable in Compacted Graphite Irons rates are to be avoided because of the high pro- iron, is produced Europe by decarburization of pensity of CO iron for chilling and high nodule the white as-cast iron. Compacted graphite (CG) irons have a graphite count in thin sections. The composition of malleable irons must be shape intermediate between spheroidal and flake. Liquid treatment can have two stages, as for selected in such a way as to produce a white Typically, CG looks like type IV graphite (Fig. ductile iron. Modification can be achieved with as-cast structure and to allow for fast annealing