Malleable iron

Malleable iron is cast as White iron, the structure being a metastable carbide in a pearlitic matrix. Through an annealing heat treatment the brittle as cast structure is transformed. Carbon agglomerates into small roughly speherical aggregates of graphite leaving a matrix of ferrite or pearlite according to the exact heat treat used. Three basic types of malleable iron are recognized within the casting industry, Blackheart malleable iron, Whiteheart malleable iron and Pearlitic malleable iron.^[1]

History

Malleable iron was discovered by Réaumur about 1720. He discovered that iron castings which were too hard to be worked could be softened by packing them into iron ore or hammer slag and exposing them to high temperature for a number of days. Creating malleable iron begun in the United States in 1826 when Seth Boyden started a foundry for the production of harness hardware and other small castings.^[2]

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 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, during the casting process care must be taken to ensure the iron casting will solidify with an entirely white iron cross section. Heavier sections of a casting will cool slowly and with the slow cooling some primary graphite may form. This graphite will form random flake-like structures and will not transform in heat treatment. When stress is applied to such a casting in application the fracture 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 due to the constraint of slow cooling times causing the formation of the primary graphite.

After the casting and heat treat process malleable iron can be shaped through cold working, such as stamping for straightening, bending or coining operations. This is possible due to malleable iron property of being less strain rate sensitive than other materials.

Applications

It is often used for small castings requiring good tensile strength and the ability to flex without breaking (ductility). Electrical fittings, hand tools, pipe fittings, washers, brackets, fence fittings, power line hardware, farm equipment, mining hardware, and machine parts.^[3]

See also

• Ductile iron
• Wrought iron

References

1. ^ Foseco Ferrous Foundryman's Handbook, by Robert R. Brown, published by Butterworth-Heinemann, ISBN 075064284X
2. ^ Journal and Proceedings of the Hamilton Scientific Association, 1927
3. ^ Malleable Iron Castings, Published by Malleable Founders Society, by Harvey E Steinhoff (Author), Gordon B Mannweiler (Author), 1960