Castability, Heat Treating and Post-casting Operations
Like other similar irons with the carbon formed into spherical or nodular shapes, malleable iron exhibits good ductility. Incorrectly considered by some to be an "old" or "dead" material, malleable iron still has a legitimate place in the design engineer's toolbox. Malleable iron is a good choice for small castings or castings with thin cross sections (less than 0.25 inch, 6.35 mm). Other nodular irons produced with graphite in the spherical shape can be difficult to produce in these applications, due to the formation of carbides from the rapid cooling.
Malleable iron also exhibits better fracture toughness properties in low temperature environments than other nodular irons, due to its lower silicon content. The ductile to brittle transformation temperature is lower than many other ductile iron alloys.
In order to form properly the spherical-shaped nodules of graphite in the annealing process, care must be taken during the casting process to ensure the iron casting will solidify with an entirely white iron cross section. Heavier sections of a casting will cool slowly, allowing some primary graphite to form. This graphite forms random flake-like structures and will not transform in heat treatment. When stress is applied to such a casting in application, the fracture strength will be lower than normal, and the large particles of primary graphite can be seen. Such iron is said to have a 'mottled' appearance. Some countermeasures can be applied to enhance forming the all white structure, but malleable iron foundries often avoid producing heavy sections because of the risk of forming primary graphite through slow cooling.
After the casting and heat treatment processes, malleable iron can be shaped through cold working, such as stamping for straightening, bending or coining operations. This is possible due to malleable iron's desirable property of being less strain rate sensitive than other materials.
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