XPS Analysis of Oxide Transformation During Sintering of Chromium Alloyed PM Steels

Water atomized PM steels prealloyed with chromium are becoming more and more widely used due to their high performance characteristics and sinter-hardening capabilities. Due to its low cost and recyclability, chromium is a cost effective alloying alternative to replace expensive and toxic Ni and non-recyclable Cu that are traditionally used in PM. Chromium has higher affinity towards oxygen that brings risk of formation of stable oxides that are difficult to reduce and hence inhibit the development and the growth of inter-particle necks during sintering. This in turn directly influences the strength of the compacts. In this study, the analysis of the change in oxide state after delubrication and during heating stage until sintering temperature was performed by means of surface sensitive analytical techniques such as XPS and high resolution SEM combined with EDX analysis. Composition, morphology and distribution of the oxides was estimated on the fracture surface of the compacts, based on powder prealloyed with 1.8 wt. % Cr, in the as-delubricated condition and sampled during the heating stage at 700, 900 and 1120 °C. Sintering atmosphere applied was dry 90% N2/10% H2 atmosphere (dew point ~-50 °C). The results show enrichment of surface oxides in Cr and Mn at around 900 °C during heating stage followed by their significant reduction close to the sintering temperature. This study provides the important details for tailoring the sintering process in terms of proper atmosphere and process control for chromium alloyed powder