Influence of tempering temperatures on the microstructure, secondary carbides and mechanical properties of spray-deposited AISI M3:2 high-speed steel

Abstract

The evolution of microstructure and mechanical properties of a spray-deposited AISI M3:2 high-speed steel (HSS) are investigated using a series of tempering temperatures. The results demonstrate a strong correlation between secondary precipitates and tempering. With increasing tempering temperature, secondary precipitates exhibit a transformation from coarse strips of M3C type to a refined and dense coherent precipitates of types MC and M2C. Highly dispersed fine secondary precipitates contribute to an enhanced bending strength and the secondary hardening peak without reduction of its impact toughness. The dimension of MC and M2C precipitates increased while the volume fraction decreased with the increasing temperature. Tempering at 650 °C can result in the generation of ellipsoidal M3C carbides at the cost of the fine MC and M2C precipitates. Overaging at 700 °C lead to the transformation of the fine precipitates to coarse MC, M3C and M7C3, and finally to M23C6 and M6C, along with the matrix structure change from plate martensite to recrystallized ferrite, which enables the toughness to increase significantly. The strengthening effects of secondary precipitates are also quantitatively analyzed and assessed.