Precipitation rule of carbides in a new high speed steel for rollers

Abstract

The continuous cooling transformation (CCT) curve of a new type of high speed steel (HSS) for rollers was determined along with its typical microstructures. The CCT curve established shows that the Ac1 of the steel is 770∼830 °C, the Acm is 920 °C and the starting temperature of martensite transformation is 190 °C. The Fe–C isopleths, mass fractions of all the phases and mole fractions of the alloying elements in austenite with different tungsten (W) content have been calculated using Thermo-Calc. The isopleths analysis show that MCI, M6C, M7C3, M23C6 and MCII carbides exist in the HSS with 1–5 wt% of tungsten (W) element, whilst only MCI, M7C3 and M23C6 are present without W. Studies on HSS with different W content show that both the starting precipitation temperature of austenite and its maximum mass fraction decrease with increasing W content. The starting precipitation temperature of MCI remains almost invariant (1380–1390 °C), with its maximum mass fraction decreasing slightly with increase of the W content. Similarly, the starting precipitation temperatures of M7C3 and M23C6 remain almost unchanged at the high temperature region, while that of M7C3 decreases at the lower temperature region, and the maximum mass fraction of both carbide groups decreases with the W content. The effect of W content on the starting precipitation temperature and the maximum mass fraction of M6C and MCII were found to be different from that of other carbide groups, both increase significantly. With the W content changed from 0 to 5%, the maximum mole fraction of element Cr in austenite increases from 5.0 to 5.1 mol% while that of element Mo decreases from 1.7 to 1.3 mol%. The results show that MCI carbide contains elements V and Nb, and dissolves elements Mo, Cr, W and Fe; M6C carbide contains elements Fe, W and Mo; M7C3 and M23C6 carbides contain elements Fe and Cr; MCII carbide only contains elements W and Mo in addition to carbon.