Thermogravimetry study of the effectiveness of different reducing agents during sintering of Cr-prealloyed PM steels

Successful removal of the surface oxides, present on the powder particle surface, during initial stages of sintering is the prerequisite for the development of strong inter-particle necks and so high mechanical performance of the sintered PM components. In the case of water-atomized powder prealloyed with chromium, surface oxide is composed of an iron oxide layer, covering about 90-95 % of the powder surface, with some presence of more stable fine particulate oxides. Sufficiently strong inter-particle necks require as minimum full removal of the iron surface oxide layer. This can be achieved by a number of gaseous reducing agents (H2, CO, or mixture of both) as well as by carbon typically admixed in the form of graphite. The present study is focused on the analysis of the reducing ability of the different sintering atmospheres (concentration of active gasses ≥10 vol%) as well as vacuum and their combined effect with graphite by means of thermal analysis. Number of effect characteristic for Cr-alloyed PM steels and their extent in different atmospheres were identified, namely risk of Cr-loss during sintering in high vacuum, risk of oxidation in CO-containing atmospheres, and nitrogen pickup, etc. When it comes to the oxidation/reduction processes, results indicate that the combination of the dry hydrogen-containing atmospheres and fine graphite grades allows successful sintering of chromium alloyed PM steels.