Sintering behaviour improvement of a low Cr–Mo prealloyed powder steel through Mn additions and others liquid phase promoters

Although its good hardenability, chromium has being avoided in powder metallurgy (PM) steels because of its high oxygen affinity and difficulties in sintering. For this purpose, a prealloyed iron powder Fe–3Cr–0.5Mo Astaloy CrM® grade, with different carbon content and some alloying elements additions have been studied. Mixtures were uniaxially compacted at 700 MPa. Sintering process was carried out at 1120° in dissociated ammonia atmosphere for 40 min. Due to previous detected problems in sintering it at low temperature (1120 °C) when is not possible to control properly the atmosphere, the item was focused on the behaviour improvement of Astaloy CrM. Trying to enhance the density and amount and extension of particles contacts should be interested introduce a transient liquid phase on mixtures. Moreover, if the objective is reduce the oxide layer of Astaloy CrM particles, avoiding carbon losses, some manganese additions can perform as oxygen traps. Two aspects are critical in this case: to achieve a good distribution of the mixture and, because of the secondary pores, to control de-dimension stability. Based on these investigations a brief overview of sintered densities and mechanical properties was done, as well as thermal analysis (performed by simultaneous thermal analysis with simultaneous thermo gravimetric (DTA–TG) test) and microstructural study by LOM.