The following environments cause stress corrosion cracking of cast irons: (1)Д¬ast iron is prone to selective leaching in soft water, acidic water, brackish water and water containing low levels of hydrogen sulfide. Automotive applications for malleable iron continue to make large use of this material, including driveline yokes, connecting rods, and diesel pistons. Malleable iron section sizes larger than this will solidify with graphite flakes and would therefore have severely reduced ductility. 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 25вЂ“50В mm.

Clearly it is less costly to produce nodules in the as-cast condition than to require an additional lengthy heat treatment step. 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. Malleable cast iron has been produced for thousands of years, but it is slowly being replaced by ductile cast iron. Ferritic malleable iron is produced by slow cooling through the upper critical temperature, T UC, and pearlitic malleable iron by more rapid cooling, even cooling in air, after the malleablizing treatment ( Gilbert, 1968 Davis, 1996, pp. Fracture toughness values in the range 60вЂ“70В MPaВ m 1/2 have been reported ( Bradley and Srinivasan, 1990) for ferritic grades, values somewhat less than for a similar ferritic ductile iron. Pearlitic malleable irons would be stronger but less ductile than ferritic malleable cast irons for the same reasons as for ductile iron. The strength and ductility of malleable irons would overlap the as-cast properties of ductile iron shown in Figure 8. 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 final microstructure will be quite similar to ductile cast iron. The white iron castings are heated slowly into the Оі+Fe 3C two-phase field ( Figure 3) to a temperature in the range 800вЂ“1000В вЊ, and held for up to 24В hours allowing graphite (spheroidal вЂpopcornвЂ™-shaped) to nucleate and grow at the expense of the eutectic iron carbide.

It can be seen in the cooling curve that the temperature reached T C before nucleation of graphite, thus resulting in white iron. The phase diagram and the cooling curve characteristic of white cast iron are illustrated in Figure 3. Malleable cast iron has lower CE ( Table 1) than gray cast iron, solidifying white. Iacoviello, in Reference Module in Materials Science and Materials Engineering, 2016 14 Malleable Cast Iron, Processing, Microstructure, and Mechanical Properties

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