The influence of heat treatment on the high-stress abrasion resistance and fracture toughness of alloy white cast irons
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INTRODUCTION
IN a recent article, the authors reported the results of a study of the effect of heat treatment on the gouging abrasion resistance of a series of alloy white cast irons. Iu The alloy compositions selected for the study were four of the widely used grades from the ASTM standard for abrasion-resistant cast irons. I21 It was shown that optimal gouging abrasion resistance was exhibited at intermediate levels of retained austenite, the position of the minimum in gouging abrasion weight loss vs retained austenite content depending on alloy composition. Of particular note was the observation that subcritieal heat treatment at 500 ~ following hardening at the conventionally used austenitizing temperature for each alloy, which is commonly utilized to reduce the austenite content to very low levels in order to improve spalling resistance under conditions of repetitive loading, 13 6] caused a significant fall in both hardness and gouging abrasion resistance. The undesirable effects of subcritical heat treatment at 500 ~ were attributed to the loss of mechanical strength of the matrix, and therefore its inability to provide adequate support to the eutectic carbides as a result of the reduction in the carbon content of the martensite caused by secondary carbide precipitation. There have been very few investigations that have addressed "pure" abrasion resistance (in the absence of any repeated impact loading) following subcritical heat treatment. Those that do exist have generally reported no deleterious effect. Zum Gahr and Doane, 171using a lowstress abrasion, wet sand rubber wheel test, found that I.R. SARE, Assistant Chief, and B.K. ARNOLD, Experimental Scientist, are with CSIRO Division of Manufacturing Technology, Woodville, South Australia 5011, Australia. Manuscript submitted November 17, 1994.
METALLURGICALAND MATERIALSTRANSACTIONS A
the lowest volume loss in a CrMoCu 20.2.1 type of alloy occurred in samples tempered for 3 hours at 500 ~ after standard hardening at 1000 ~ although the retained austenite content was, inexplicably, still 40 pct. In related work on the same alloy, using a high-stress abrasion pin test on garnet, Zum Gahr and Eldis t81 showed slight deterioration with tempering the hardened structure at 500 ~ for short times, and slight improvement when tempering times in excess of 10 hours were employed. In all instances, the change in hardness with tempering at 500 ~ was minimal. The principal beneficial effect of the subcritical heat treatment was seen as being the reduction in the amount of retained austenite. Contrasting with these published results, however, is a study by Blickensderfer et al.,[6l which reported pin test results after subcritical heat treatment of alloy white cast irons at temperatures in the range 450 ~ to 540 ~ They found the best abrasion resistance in alloys that had not been tempered after hardening and that contained retained austenite contents of 20 pct or more, with the amount of wear increasing as the martensitic matrix became increasingly tempered. The g
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