Microstructural Development in Rapidly Solidified Powder of M2 Type High Speed Steel During Annealing

Abstract

Rapidly solidified (RS) powder of M2 type high speed steel (HSS) prepared by nitrogen spraying was annealed in the temperature range 150–1200°C. A minor decrease in microhardness of the powder particles was determined after annealing below 400°C. Beyond this temperature, the microhardness increased, principally because of the hardening effect of carbide precipitates in the material. The RS powder material became overtempered above a temperature of approximately 550°C, which was determined as the maximum precipitation hardening temperature and the structure typical for soft annealed HSS origins in powder particles. The main structural constituents were the bcc phase, M₆C, and MC carbides. No other carbides were found. Overheating the RS powder above the critical temperature A ₁ resulted in almost complete austenitising of the material, partial dissolving of M₆C carbides, and complete breakdown of the as solidified microstructure. PM/0751