Microstructure Evolution of HP40 and HK40 Steels After Isothermal Aging

Abstract

Thermo-Calc and experimental analyses were carried out for the as-cast and heat-treated HK40 and HP40 steels. The as-cast steel specimens of about 1 × 1 × 1 cm were aged at 900 °C for times between 100 and 1000 h. As-cast and heat-treated specimens were characterized by X-ray diffraction, SEM, and Vickers hardness. Thermo-Calc Scheil analysis indicated an austenite matrix with the presence of Cr-rich M₇C₃, and M₇C₃ and (Nb,Ti)C carbides in the as-cast HK40 and HP40 steels, respectively, which is in agreement with X-ray diffraction, optical microscope, and scanning electron microscope results of present work. The aging treatment at 900 °C produces the precipitation of M₂₃C₆ carbides, and M₂₃C₆ and (Nb,Ti)C carbides in the austenite matrix of the as-cast HK40 and HP40 steels, respectively. This precipitation process promotes an increase in hardness. Thermo-Calc Prisma also predicted the precipitation of the same phases, and the fastest growth kinetics of precipitation was at 900 °C for the as-cast HK40 and 960 °C for the as-cast HP40 steel. In general, the aging response is better for the as-cast HK40 steel.