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Metal Casting Technologies : Dec 2009
28 www.metals.rala.com.au Introduction lloy White Irons, notably High Cr Iron and Ni-Hard, are extensively used as wear parts in many industries including mining, coal and mineral processing, cement production, dredging and slurry transport. These parts range from small diameter grinding cylpebs and balls to much larger castings such as pump bodies, and roller and table segments for roller crushers. The wear and fracture behaviour of Alloy White Irons depends on the type, proportion and morphology of hard eutectic and precipitated carbides within their microstructures and on the nature of the supporting matrix structures [1,2]. For many applications these irons need to be given hardening and/or tempering heat treatments to develop the optimum matrix structures for given service applications . Prior to hardening, some grades of High Cr Irons can be annealed to allow tool machining of wear parts. To achieve optimum performance in service the correct control of cast and heat treated microstructures in these irons is vital. An outline of the techniques that can be used to characterize microstructures using Alloy White Irons as examples has been given in a previous MCT article . This short paper revisits some important microstructural features of these irons. The form of carbides in alloy White irons Depending upon composition and thermal conditions, hard carbides to provide hardness and wear resistance can be formed during solidification and during solid state transformations as: ● Carbides which form via a eutectic reaction - Eutectic carbides. Microstructural Aspects of Alloy White Irons By John Pearce A 1a 1c 1b 1d Fig 1. The effect of %Cr content of the general as- cast microstructure of white irons (x200) (a) Continuous M3C eutectic carbide with a pearlite matrix in a 2.8%C- 0.11%Cr unalloyed iron. (Hardness 450Hv) (c) Fine fibrous M7C3 eutectic carbides in an austenite matrix in a 2.4%C-30%Cr iron, Cr/C ratio 12.5. (Hardness 498Hv) (b) Less continuous M7C3 eutectic carbide with a pearlitic matrix in a 3.1%C-14.7%Cr iron, Cr/C ratio 4.74. (Hardness 540Hv) (d) Eutectic M7C3 carbide in a ferrite matrix in a 2.1%C-36%Cr iron, Cr/C ratio 17. (Hardness 314Hv) TECHNICAL FEATURE
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