“INTERNAL GETTERING” – METALLOTHERMIC REDUCTION PROCESSES IN THE EARLY STAGE OF SINTERING

One of the aspects of modern material systems for high strength sintered steel parts is the presence of alloy elements with widely different oxygen affinity. Compared to the base iron, the “new” alloy elements Cr, Mn and Si form oxides with much higher stability. It is shown here that in case of powder mixes this leads to oxygen transfer from the base iron particles to the alloy elements during heating up to sintering temperature, i.e. metallothermic reduction of the iron surfaces. With prealloyed powders, the surface oxides, which are originally mostly iron oxide, are transformed into alloy element oxides during heating unless the iron oxides can be reduced at low temperature with H2. In any case, the heterogeneity of the oxygen affinity is a parameter that has to be considered when defining alloy systems for sintered steels