A study on martensitic phase transformation in 9Cr-1W-0.23V-0.063Ta-0.56Mn-0.09C-0.02N (wt.%) reduced activation steel using differential scanning calorimetry

Abstract

The salient characteristics of martensite formation kinetics in 9Cr-1W-0.23V-0.063Ta-0.09C-0.02N (wt.%) reduced activation steel have been investigated using differential scanning calorimetry and supplemented by microscopy characterisation. Accurate determination of the martensite start (M_S) and finish (M_f) temperatures as a function of cooling rate (1-99 K min⁻¹) and holding time (1-120 min) in the γ-austenite phase field have been made. The critical cooling rate to form nearly 100% martensite is found to be of the order of 5-6 K min⁻¹. The measured latent heat for the martensite transformation is found to be in the range 62-75 J g⁻¹. In the case of austenitisation at 1323 K, it is found that the grain size of the parent austenite exhibited significant growth and the observed M_S temperature too exhibited a systematic increase with austenite grain size. However, for those samples that are solution treated at a slightly lower temperature of 1253 K, the M_S is found to be initially insensitive to the increase in holding time for up to about 30 min. This trend is followed later on by the usual increasing character of M_S for further rise in the holding periods, up to about 120 min. This unusual initial behaviour is also reflected by the corresponding variation in the hardness values of the quenched in martensite. Moreover, the growth of parent austenite grains has also been restricted during the initial phase of solution annealing at 1253 K. This is attributed to the presence of undissolved carbide particles, which exert a pinning influence on the movement of austenite grain boundaries. The observed athermal martensite formation kinetics is described well by the Koistinen-Marburger relation.