Chemical reactions during sintering of Cr-Mo prealloyed powder compacts in inert and reducing atmospheres

摘要

Cr-Mo prealloyed PM steels are increasingly being used for highly stressed precision parts. The most critical stage of manufacturing is the sintering process since the Cr content not only makes the materials sensitive to oxygen pickup, but it results in natural surface oxide layers more difficult to reduce during sintering. In this work, the chemical reactions in the early stage of sintering of Cr-Mo prealloyed compacts containing admixed graphite have been studied through dilatometry combined with mass spectrometry. It is shown that oxygen removal is attained both in Ar and H_2, i.e. in inert and reducing atmospheres, although in different temperature intervals: when sintering in high purity H_2, part of the oxygen is removed at about 400°C, indicating that the oxides thus reduced are formed by Fe and Mo. The reducing agent here is H_2. Removal of the remaining oxides, however, requires temperatures well in excess of 1100°C, and there carbon is the reducing agent also in H_2. When sintering in argon, the reduction starts at significantly higher temperatures, at about 1000°C, being purely carbothermic, which indicates that in this case all the oxide layers are present as more stable Cr based oxides, Fe and Mo oxides having been converted accordingly during the heating process. Generally, sintering in inert atmosphere is regarded preferable, being more reproducible and less dependent on the purity of the protective atmosphere, and in any atmosphere sintering temperatures >1200°C are recommended due to the more favourable thermodynamics.