Cast irons rapidly cooled if the Si is above 3%. These elements,

9.3 Cast irons rapidly cooled if the Si is above 3%. These elements,

particularly Si, alter the eutectic composition which In the iron-carbon system (Chapter 3) carbon is ther-

may be taken into account by using the carbon equiv- modynamically more stable as graphite than cementite.

alent of the cast iron, given by [total %C C ⊲%Si C

Table 9.1 Compositions of some carbon and alloy steels

BS AISI-SAE designation

% Cr % others 040A20

0.12–0.2 S 527A19

0.20–0.30 Mo 708M40

0.90–1.20 0.15–0.25 Mo Ł Approximately equivalent composition.

Table 9.2 Compositions and properties of some stainless steels

Steel BS %C

% Condition designation

elongation (MN m ) (MN m )

50 Annealed Ferritic

22 Annealed Martensitic

16 and tempered

Precipitation hardening 17–4

10 Age- 17–7

0.15–0.45% Nb

0.75–1.25% Al

6 hardened

304 Modern Physical Metallurgy and Materials Engineering

but not enough to produce graphite flakes during cast- ing. White-heart malleable iron is made by heating the casting in an oxidizing environment (e.g. hematite iron ore at 900 °

C for 3 – 5 days). In thin sections the carbon is oxidized to ferrite, and in thick sections, ferrite at the outside gradually changes to graphite clusters in

a ferrite– pearlite matrix near the inside. Black-heart malleable iron is made by annealing the white iron in a neutral packing (i.e. iron silicate slag) when the cementite is changed to rosette-shaped graphite nod- ules in a ferrite matrix. The deleterious cracking effect of the graphite flakes is removed by this process and a cast iron which combines the casting and machinabil- ity of grey iron with good strength and ductility, i.e. TS 350 MN m and 5 – 15% elongation is produced. It is therefore used widely in engineering and agricul-

(a) ture where intricate shaped articles with good strength are required.

Even better mechanical properties (550 MN m ) can be achieved in cast irons, without destroying the excellent casting and machining properties, by the pro- duction of a spherulitic graphite. The spherulitic nod- ules are roughly spherical in shape and are composed of a number of graphite crystals, which grow radially from a common nucleus with their basal planes nor- mal to the radial growth axis. This form of growth habit is promoted in an as-cast grey iron by the addi- tion of small amounts of Mg or Ce to the molten metal in the ladle which changes the interfacial energy between the graphite and the liquid. Good strength, toughness and ductility can thus be obtained in cast- ings that are too thick in section for malleabilizing and can replace steel castings and forgings in certain

(b)

applications.

Heat-treating the ductile cast iron produces austem- Figure 9.7 Microstructure of cast irons: (a) white iron and

pered ductile iron (ADI) with an excellent combination (b) grey iron (400 ð). (a) shows cementite (white) and

of strength, fracture toughness and wear resistance for pearlite; (b) shows graphite flakes, some ferrite (white) and

a wide variety of applications in automotive, rail and a matrix of pearlite .

heavy engineering industries. A typical composition is 3.5 – 4.0% C, 2 – 2.5% Si, 0.03 – 0.06% Mg, 0.015%

%P⊳/3], rather than the true carbon content. Phospho- maximum S and 0.06% maximum P. Alloying ele- rus is present in most cast irons as a low melting

ments such as Cu and Ni may be added to enhance the heat-treatability. Heat-treatment of the cast ductile

point phosphide eutectic which improves the fluidity iron (graphite nodules in a ferrite matrix) consists of of the iron by lengthening the solidification period; this

C for 1 – 3 hours during which the favours the decomposition of cementite. Grey cast iron

austenization at 950 °

matrix becomes fully austenitic, saturated with carbon is used for a wide variety of applications because of

as the nodules dissolve. The fully austenized casting its good strength/cost ratio. It is easily cast into intri-

C and austempered cate shapes and has good machinability, since the chips

is then quenched to around 350 °

at this temperature for 1 – 3 hours. The austempering break off easily at the graphite flakes. It also has a high

temperature is the most important parameter in deter- damping capacity and hence is used for lathe and other

mining the mechanical properties of ADI; high austem- machine frames where vibrations need to be damped

C) result in high out. The limited strength and ductility of grey cast

pering temperatures (i.e. 350 – 400 °

ductility and toughness and lower yield and tensile iron may be improved by small additions of the car-

strengths, whereas lower austempering temperatures bide formers (Cr, Mo) which reduce the flake size and

C) result in high yield and tensile strengths, refine the pearlite. The main use of white irons is as a

high wear resistance and lower ductility and tough- starting material for malleable cast iron, in which the

ness. After austempering the casting is cooled to room cementite in the casting is decomposed by annealing.

temperature.

Such irons contain sufficient Si ⊲<1.3%⊳ to promote The desired microstructure of ADI is acicular ferrite the decomposition process during the heat-treatment

plus stable, high-carbon austenite, where the presence

Modern alloy developments 305

Figure 9.8 Microstructure and fracture mode of silicon spheroidal graphite (SG) iron, (a) and (b) as-cast and (c) and (d) austempered at 350 °

C for 1 h (L. Sidjanin and R. E. Smallman, 1992; courtesy of Institute of Metals) .

of Si strongly retards the precipitation of carbides.

to 40 MN m

When the casting is austempered for longer times ferrite– austenite boundaries must be avoided since this than that to produce the desired structure, carbides

leads to more brittle fracture. Generally, the strength are precipitated in the ferrite to produce bainite. Low

is related to the volume fraction of austenite and the austempering temperatures ⊲¾250 ° C⊳ lead to cementite

ferrite spacing. Figure 9.8 shows the microstructure of precipitation, but at the higher austempering temper-

Si spheroidal graphite (SG) iron and the corresponding atures ⊲300 – 400 ° C⊳ transition carbides are formed, ε

fracture mode.

higher. With long austempering times the high-carbon