Abrasion wear behavior of hypoeutectiv 16 Mass% chromium cast iron containing molybdenum

Abstract

Hypoeutectic 16%Cr cast irons without and with Mo were prepared in order to investigate the abrasion wear behavior. After annealing, the cast irons were hardened from 1323 K and tempered at three levels of temperatures, before HTmax (B-HTmax), at HTmax and over HTmax (O-HTmax). The abrasion wear resistance was evaluated using Suga abrasion wear test (SAT) and Rubber wheel abrasion wear test (RWAT). In the tempered state, hardness curve showed a secondary hardening due to the precipitation of secondary carbides and the transformation of destabilized austenite retained after hardening into martensite. The degree of secondary hardening was high in Mo containing specimen. The volume fraction of retained austenite (Vγ) decreased remarkably as the tempering temperature was increased. The Vγ value at HTmax was less than 6%. A linear relation was obtained between wear loss and wear distance. The lowest wear rate (RW) was obtained in the HTmax specimen for SAT and in the as-hardened (As-H) specimen for RWAT. The highest RW was obtained in the O-HTmax specimen. Under the same heat treatment condition, the RW in SAT was much larger than that in RWAT. The RW increased proportionally with an increase in the applied load and it decreased with an increase in the macro-hardness. The lowest RW seems to be obtained in the specimen with certain amount of retained austenite, 15%Vγ for SAT and 10 to 15%Vγ for RWAT, respectively. In SAT, the RW was lowered with increasing of Mo content, and the Vγ at the lowest RW shifted to the high Vγ side. In RWAT, however, the RW did not show a clear difference. An increase of cooling rate in the hardening by oil quench leaded to improve the wear resistance due to the perfect hardening. The wear mechanism was quite different between SAT and RWAT. In SAT, at the early state, the matrix was preferably cut off or worn and removed more than the eutectic carbides. After this phenomenon continued, cracks occurred in the eutectic carbides and resultantly, spalling of carbides could take place. In RWAT, the surface was quite smooth at the beginning of wear. When wearing proceeded, the surface got uneven by wearing with abrasives. At this point, the primary matrix was worn more than the eutectic region. Then the eutectic areas projected from the worn surface were taken away.