Cast irons

8.3 Cast irons

In the iron–carbon system (Chapter 2), carbon is thermodynamically more stable as graphite than cementite. At the low carbon contents of typical steels, graphite is not formed, however, because of the sluggishness of the reaction to graphite. But when the carbon content is increased to that typical of cast irons (2–4% C), either graphite or cementite may separate depending on the cooling rate, chemical (alloy) composition and heat treatment (see Figure 8.7). When the carbon exists as cementite, the cast irons are referred to as white because of the bright fracture produced by this brittle constituent. In gray cast irons the carbon exists as flakes of graphite embedded in the ferrite–pearlite

Ph ysical

Metallurgy

Table 8.2 Compositions and properties of some stainless steels. and Steel

Condition Adv designation

Austenitic Mater 304

9 Cold worked ials 316

50 Annealed Ferritic 430

22 Annealed Martensitic

18 Quenched and tempered 431

Precipitation hardening 17–4

10 Age hardened 17–7

0.15–0.45% Nb

0.75–1.25% Al

Advanced alloys 457

(a) (b) Figure 8.7 Microstructure of cast irons: (a) white iron and (b) gray iron ( ×400). (a) shows

cementite (white) and pearlite; (b) shows graphite flakes, some ferrite (white) and a matrix of pearlite.

matrix and these impart a dull gray appearance to the fracture. When both cementite and graphite are present a ‘mottled’ iron is produced.

High cooling rates, which tend to stabilize the cementite, and the presence of carbide formers give rise to white irons. The addition of graphite-forming elements (Si, Ni) produces gray irons, even when rapidly cooled if the Si is above 3%. These elements, particularly Si, alter the eutectic composition which may be taken into account by using the carbon equivalent of the cast iron, given by [total %

C + (% Si + %P)/3], rather than the true carbon content. Phosphorus is present in most cast irons as a low melting point phosphide eutectic, which improves the fluidity of the iron by lengthening the solidification period; this favors the decomposition of cementite. Gray cast iron is used for a wide variety of applications because of its good strength/cost ratio. It is easily cast into intricate shapes and has good machinability, since the chips break off easily at the graphite flakes. It also has

a high damping capacity and hence is used for lathe and other machine frames where vibrations need to be damped out. The limited strength and ductility of gray cast iron may be improved by small additions of the carbide formers (Cr, Mo), which reduce the flake size and refine the pearlite. The main use of white irons is as a starting material for malleable cast iron, in which the cementite in the casting is decomposed by annealing. Such irons contain sufficient Si (<1.3%) to promote the decomposition process during the heat treatment, but not enough to produce graphite flakes during casting. 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 nodules 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 machinability of gray iron with good strength and ductility, i.e.

TS 350 MN m −2 and 5–15% elongation is produced. It is therefore used widely in engineering and agriculture, where intricate-shaped articles with good strength are required.

Even better mechanical properties (550 MN m −2 ) can be achieved in cast irons, without destroying the excellent casting and machining properties, by the production of a spherulitic graphite. The spherulitic nodules 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 normal to the radial growth axis. This form of growth habit is promoted in an as-cast gray iron by the addition of small amounts of Mg or Ce to the molten metal in the ladle, which changes the interfacial energy between the

458 Physical Metallurgy and Advanced Materials graphite and the liquid. Good strength, toughness and ductility can thus be obtained in castings

that are too thick in section for malleabilizing and can replace steel castings and forgings in certain applications.

Heat treating the ductile cast iron produces austempered ductile iron (ADI), with an excellent combination of strength, fracture toughness and wear resistance for a wide variety of applications in automotive, rail and heavy engineering industries. A typical composition is 3.5–4.0% C, 2–2.5% Si, 0.03–0.06% Mg, 0.015% maximum S and 0.06% maximum P. Alloying elements such as Cu and Ni may be added to enhance the heat treatability. Heat treatment of the cast ductile iron (graphite nodules in a ferrite matrix) consists of austenization at 950 ◦

C for 1–3 hours during which the matrix becomes fully austenitic, saturated with carbon as the nodules dissolve. The fully austenized casting is then quenched to around 350 ◦

C and austempered at this temperature for 1–3 hours. The austempering temperature is the most important parameter in determining the mechanical properties of ADI; high austempering temperatures (i.e. 350–400 ◦

C) result in high ductility and toughness and lower yield and tensile strengths, whereas lower austempering temperatures (250–300 ◦

C) result in high yield and tensile strengths, high wear resistance, and lower ductility and toughness. After austempering the casting is cooled to room temperature.

The desired microstructure of ADI is acicular ferrite plus stable, high-carbon austenite, where the presence of Si strongly retards the precipitation of carbides. When the casting is austempered for longer times than that to produce the desired structure, carbides are precipitated in the ferrite to produce bainite. Low austempering temperatures ( ∼250 ◦

C) lead to cementite precipitation, but at the higher austempering temperatures (300–400 ◦

C) transition carbides are formed, ε-carbides at the lower temperatures and η-carbides at the higher. With long austempering times the high-carbon austenite precipitates χ-carbide at the ferrite–austenite boundaries. The formation of bainite does not result in any catastrophic change in properties but produces a gradual deterioration with increasing time of austempering. Typically, ADI will have a tensile strength of 1200–1500 MN m −2 , an elongation of

6–10% and K 1c ≈ 80 MN m −3/2 . With longer austempering the elongation drops to a few percent and the K 1c reduces to 40 MN m −3/2 . The formation of χ-carbide at the ferrite–austenite boundaries must

be avoided, since this leads to more brittle fracture. Generally, the strength is related to the volume fraction of austenite and the ferrite spacing. Figure 8.8 shows the microstructure of Si spheroidal graphite (SG) iron and the corresponding fracture mode.