Lean Atmospheres for Sintering of Chromium Alloyed Powder Metallurgy Steels

Abstract

Thermodynamic evaluation of the reducing and carburizing activities of N2‐based lean sintering atmospheres containing CO, H2, and small amounts of hydrocarbons with their total content not exceeding 5 vol% was performed with regard to sintering of chromium alloyed PM steels. Based on the thermodynamic evaluation, three sintering atmosphere compositions were chosen for experimental studies. For the chosen sintering atmospheres, the effect of graphite addition, sintering temperature, holding time and cooling rate on the microstructure, and final oxygen and carbon contents was studied. The compacts without admixed graphite exhibited a microstructure with pearlitic, and in some cases bainitic regions, with their content varying according to the temperature profile and increasing with increase in CO concentration in the atmosphere. These results are indicative of the carburizing ability of the atmospheres which was also supported by the results from chemical analysis. Pearlitic–bainitic microstructures were observed in the compacts admixed with graphite after sintering which suggests that the proposed atmospheres provide conditions sufficient for at least partial reduction of the surface iron oxide during sintering. The final oxygen content obtained after sintering in the proposed lean atmospheres was higher than that obtained in the case of nitrogen/hydrogen mix but still was within acceptable limits. The results obtained suggest that the proposed lean CO and H2 containing sintering atmosphere compositions are promising candidates for robust sintering of chromium alloyed PM steels.