The effect of heat treatment on the gouging abrasion resistance of alloy white cast irons
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I.
INTRODUCTION
A L L O Y white cast irons find wide application in a number of industries where good resistance to abrasive wear is required. They are generally considered to be the most abrasion-resistant class of ferrous alloys, with a microstructure consisting of a high volume fraction of very hard eutectic carbides in a strong supporting matrix. These same microstructural features that impart such good abrasion resistance are also responsible for giving white cast irons poor toughness. Accordingly, their use tends to be restricted to applications where resistance to impact is not as important as resistance to the surface damage caused by abrasive wear, such as in pump components and chute liners. However, following the development of a greater understanding of the factors responsible for controlling their properties, and with concomitant improvements in production practices, white cast irons now find application in significantly more arduous conditions in the mining and mineral processing industries, l~j These uses include liner plates in ball, rod, and autogenous mills and swing hammers in impact crushers. Alloy white cast irons are based upon the Fe-Cr-C ternary system, the liquidus surface of which is shown in Figure 1. t2j Almost all commercially important grades are hypoeutectic in composition and solidify from the melt initially as dendrites of austenite, followed by the formation of an austenite/carbide eutectic. In the lowchromium content, nickel-bearing grades (commonly termed Ni-Hard 1 and Ni-Hard 2), the eutectic carbide is (Fe, Cr)3C, whereas in the grades with chromium contents in excess of about 12 pct the eutectic carbide is ( F e , Cr)TC 3. At intermediate chromium contents, as in the 9 pct chromium, nickel-bearing grade (commonly I.R. SARE, Assistant Chief, and B.K. ARNOLD, Experimental Scientist, are with the CSIRO Division of Manufacturing Technology, Adelaide, South Australia. Manuscript submitted April 22, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
termed Ni-Hard 4), solidification also involves formation of the austenite-(Fe, Cr)7C3 eutectic. A subsequent quasi-peritectic reaction causes the partial transformation of (Fe, Cr)7C3 to (Fe, Cr)3C. t3j The final microstructure in this case consists of duplex eutectic carbides with an inner core of (Fe, Cr)7C3 and an outer shell of (Fe, Cr)3C. In the as-cast condition, the matrix in alloy white cast irons is primarily austenite. Some martensite is also usually present in the microstructure in the as-cast state, predominantly in localized regions adjacent to the eutectic carbides where depletion of chromium and carbon from the austenite has raised the Ms temperature sufficiently to permit transformation to martensite. If the cooling rate of a casting is slow enough, partial transformation of austenite to pearlite or some related form of ferrite-carbide transformation product may take place. Although alloy white iron castings are sometimes used in the as-cast condition, they are commonly heat-treated to give a predominantly martensitic m
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