Influence of Sintering Cycle and Chemical Composition on the Amount of Martensite Obtained After Cooling of PM Nickel Steels

摘要

Despite numerous reports on sintered PM steels in the Fe-Mo-Ni-Cu-C system the dependence of vol% martensite for a given chemical composition as a function of sintering parameters and cooling rate, requires further understanding and characterisation. Considering sinter-hardening cycles, thus avoiding extreme cooling rates, martensite formation directly relates to chemical gradients generated during sintering which consequently define microstructural areas of distinct local hardenability. Using a wide range of cooling rates and sintering time-temperature combinations, as well as Fe-based powders with and without prealloyed Mo, this work quantitatively shows that the relative amounts of equilibrium and non-equilibrium transformation products after cooling are a direct consequence of the redistribution of elements during processing where Ni and C play a dominant role. An in-depth characterisation of the relationship between chemical composition and maximum martensite vol% formed was carried out aided by EBSD, XRD and SEM-EDS analyses.