Austenite Grain Size in Sinter Hardened Powder Metallurgy Steels

For quenched and tempered wrought steels it has been known for a long time that the prior austenite grain size plays a major role for the mechanical properties. Especially austenitizing for long times at high temperatures can cause a dramatic growth of the grains, resulting in high retained austenite content and poor mechanical properties. The behaviour of PM steels is not as well known, which is in part a consequence of the difficulties to clearly reveal the prior austenite grain boundaries. Especially for chromium containing low alloyed PM steels, it is a difficult task to visualize the austenite grains in a martensitic structure. In this work a simple method for revealing the grain boundaries by a thermal treatment exactly at the desired conditions was tested. Especially for sinter hardened samples, i.e. steel specimens that are gas quenched directly from the sintering temperature, which thus experience the same heat treatment for hardening that is necessary for the visualization, it is a very simple and reliable method to avoid any artefacts without any possible changes of the structure. The factors influencing the austenite grain size in sinter hardened steels as e.g. sintering temperature and time and the particle size of the starting powders were investigated for a low alloyed martensitic PM steel (Fe-3%Cr-0,5%Mo-0,5%C).