Malleable Cast Iron - an overview

07 Sep.,2022


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14 Malleable Cast Iron, Processing, Microstructure, and Mechanical Properties

Malleable cast iron has lower CE (Table 1) than gray cast iron, solidifying white. The phase diagram and the cooling curve characteristic of white cast iron are illustrated in Fig. 1. It can be seen in the cooling curve that the temperature reached TC before nucleation of graphite, thus resulting in white iron. The white iron castings are heated slowly into the γ+Fe3C two-phase field (Fig. 1) to a temperature in the range 800–1000 °C, and held for up to 24   hours allowing graphite (spheroidal ‘‘popcorn’’-shaped) to nucleate and grow at the expense of the eutectic iron carbide. The final microstructure will be quite similar to ductile cast iron. Like ductile cast iron the matrix is the continuous phase and so the final structure will have similar properties to ductile cast iron, excellent strength, ductility, and toughness. The strength and ductility of malleable irons would overlap the as-cast properties of ductile iron shown in Fig. 6. Pearlitic malleable irons would be stronger but less ductile than ferritic malleable cast irons for the same reasons as for ductile iron. Fracture toughness values in the range 60–70   MPa   m1/2 have been reported (Bradley and Srinivasan 1990) for ferritic grades, values somewhat less than for a similar ferritic ductile iron. Ferritic malleable iron is produced by slow cooling through the upper critical temperature, TUC, and pearlitic malleable iron by more rapid cooling, even cooling in air, after the malleablizing treatment (Gilbert 1968, Davis 1996, pp. 94–106).

Malleable cast iron has been produced for thousands of years, but it is slowly being replaced by ductile cast iron. Two important reasons why malleable iron is decreasing in favor is cost and limitations on section size that can be produced in the malleable state. Clearly it is less costly to produce nodules in the as-cast condition than to require an additional lengthy heat-treatment step. In addition ductile cast iron can be produced in very large section sizes, while the requirement to start with white iron limits malleable iron to practical section sizes of the order of 25–50   mm. Malleable iron section sizes larger than this will solidify with graphite flakes and would therefore have severely reduced ductility. Automotive applications for malleable iron continue to make large use of this material, including driveline yokes, connecting rods, and diesel pistons.