THE EFFECT OF COOLING CONDITION ON THE δ-TO-γ TRANSFORMATION IN SELECTED FERRITIC STAINLESS STEELS

摘要

In high chromium ferritic stainless steel, the δ-phase transforms to the γ-phase in the temperature range 1200-900°C and the cooling condition in the continuous casting process greatly affects the formation of crack-related defects. In order to study these phenomena, solidification and the subsequent phase transitions were observed in-situ using a high-temperature laser-scanning confocal microscope (HT-LSCM) and several metallographic investigations followed. With an increase in cooling rate, the nucleation of γ-phase at grain boundaries was suppressed at the expense of intra-granular nucleation, thereby contributing to an increase in the total fraction of the γ-phase and hence, eventually an increase in the fraction of martensite. The hardness of the δ-phase increased at higher cooling rates. It is proposed that with higher cooling rates, the propensity to cracking increases due to the enhanced role of martensite as crack initiation sites with the concomitant propagation of cracks through the brittle δ-matrix.