Effects of sinter hardening technology on homogeneous and heterogeneous microstructures

Abstract

Owing to high performance/cost ratio, sinter hardening process is gaining more popularity towards liquid quench hardening of PM sintered steels. Effect of cooling rate during sinter hardening on the microstructure and properties of sintered steel grade Distaloy HP with heterogeneous and Astaloy CrM with homogeneous microstructure has been studied. The samples were sinter hardened at different cooling rates of 0·5–3°C s⁻¹ in order to investigate the influence of homogeneity towards sinter hardening technology. The microstructure and hardness of the samples were measured carefully. In addition, the chemical analysis of the samples after sinter hardening was measured. It has been shown that sintering process and cooling rate change the microstructure and hence the hardness of the materials. The most differences between samples were detected at a cooling rate of 3°C s⁻¹. At this cooling rate, the homogeneous microstructure was fully martensitic, while the heterogeneous microstructure had lesser amount of martensite. Effect of martensite formation on mechanical properties was also investigated.

Keywords:

• Powder metallurgy
• Sinter hardening
• Distaloy HP
• Astaloy CrM
• Cooling rate

The financial support of this work by Höganäs AB (Sweden, Höganäs) and SAPCO (Iran, Tehran) is gratefully acknowledged. Many thanks to Professor A. Bergmark and P. Johansson at Höganäs AB for their valuable contribution to this work.