XPS ANALYSIS OF OXIDE TRANSFORMATION DURING SINTERING OF CHROMIUM ALLOYED PM STEELS

摘要

Water atomized PM steels pre-alloyed 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 which brings a risk of formation of stable oxides that are difficult to reduce and hence inhibit the development and 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 the 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 were estimated on the fracture surfaces of the compacts. These were based on iron prealloyed with 1.8 wt. % Cr, in the delubricated condition, and were sampled during the heating stage at 700, 900 and 1120 °C. The sintering atmosphere was dry 90% N_2/10% H_2 atmosphere (dew point -50°C). The results show an enrichment of surface oxides in Cr and Mn at around 900°C during the heating stage, followed by their significant reduction close to the sintering temperature. This study provides important details for tailoring the sintering process, in terms of proper atmosphere and process control, for chromium alloyed powder.