Changes in the surface chemistry of chromium-alloyed powder metallurgical steel during delubrication and their impact on sintering

Abstract

Changes induced in the surface chemistry of metal powder during the delubrication stage affect the subsequent sintering process and hence the final properties of the sintered compacts. The different stages during delubrication were characterized by means of in situ visual observation of components made by compacting chromium-alloyed water-atomized steel powder with lubricant additions. A strong correlation is found between this visual observation of changes of the compact appearance during the delubrication and the component chemistry depicted via the monitoring of the exhaust gases composition (CO2, O2 and H2O), thermogravimetric analysis, and changes in powder surface composition (X-ray photoelectron spectroscopy combined with scanning electron microscopy and energy dispersive X-ray analysis). The compacts were delubricated at different conditions and the effect of different parameters was studied, including temperature (300, 450 and 900°C) and processing gas (N2 or air). Delubricated compacts were further sintered at 1120°C in N2/3%H2 mix to evaluate the effect of delubrication on the properties of the sintered components. The mechanical properties, chemistry and microstructure obtained after sintering show a strong relation to the surface changes induced during delubrication. The results indicate that delubrication at 450°C in dry N2 preserves the original surface condition of the powder and upon sintering the compacts exhibit optimum properties. Sintered compacts delubricated at 450°C in air and at 900°C in N2 exhibited inferior properties. In the former case, this was related to the growth of Fe-oxide on the powder surfaces. In the latter case, the initial thin Fe-oxide layer, covering most of the powder surfaces, was reduced and oxygen made available was transferred into stable Cr–Mn–Si-rich oxide particles. Delubrication at 300°C resulted in insufficient lubricant removal along with soot formation contributing to inferior mechanical properties after sintering as well.