Austenite stability and M2C carbide decomposition in experimental secondary hardening ultra-high strength steels during high temperature austenitizing treatments

The present study deals with the austenite stability and M2C carbide decomposition in three secondary hardening ultra-high strength (SHUHS) steels with varying levels of Cr and Mo (2Cr-1Mo, 2Cr-3Mo and 5Cr-5Mo) investigated using Vicker's hardness, optical and electron microscopy. These steels were subjected to high temperature austenitizing treatments at 1000, 1050, 1100 and 1150 degrees C. It has been established that increasing both Cr and Mo to 5 wt% as well as increasing the austenitizing temperature in this class of SHUHS steels is stabilizing the austenite such that almost 100% austenite is produced upon oil quenching. Further, higher Cr and Mo is also found to influence the stability of metastable M2C carbide formed during processing of the steels. While the M2C carbide in 2Cr-3Mo steel remained untransformed, it was found to transform partially to M6C during austenitization of 5Cr-5Mo steel. Hardness measurements on these steels revealed that hardness is relatively insensitive to austenitizing temperature in 2Cr-1Mo steel, decreased in 2Cr-3Mo and decreased more drastically in 5Cr-5Mo steel with austenitizing temperature. This dependence has been correlated to the influence of composition on M-s temperature and hence on retention of austenite as well as primary carbides. The experimental results were compared against theoretical calculations using ThermoCalc, which predict the presence of only M6C in both 2Cr-3Mo and 5Cr-5Mo steels. The apparent discrepancy between theoretical and experimental observations has been correlated to kinetic factors.