Critical Aspects of Delubrication and Sintering of Chromium-alloyed Powder Metallurgy Steels

Doctoral thesis, 2015

The cost efficiency and performance of Powder Metallurgy (PM) steels can be improved by replacing conventionally used alloying elements such as copper and nickel with chromium. Utilizing chromium imposes a challenge in terms of processing due to its effect on powder compressibility and high oxygen affinity, the latter of which, to a certain extent, is dealt with by introducing chromium in prealloyed form that decreases its activity. Therefore, each stage in the PM processing route has to be reconsidered to satisfy the thermodynamic and kinetic requirements to appropriately sinter components based on chromium alloyed steel powder for manufacturing high-performance PM parts. For conventional PM steels, consolidation is typically achieved by compaction, where the final dimensions of the part are achieved, followed by sintering, where metallurgical bonding between the powder particles takes place. To facilitate compaction, powder is mixed with lubricant, which also improves the tool life; but the lubricant has to be removed prior to sintering. Among the problems encountered during sintering, those concerning delubrication are frequent but are difficult to detect. In the present study, a reliable approach for in-situ monitoring of the delubrication of PM steel compacts is presented. The method is based on continuous monitoring of the process atmosphere using sensors commonly used in the industry (CO2 and O2). Using this method, the effect of various process parameters on lubricant removal has been investigated and the changes occurring in the surface chemistry of the compacts during delubrication and their impact on sintering have been assessed. Based on these studies, delubrication at 450 °C in dry N2 with dynamic gas flow conditions around the sample and a low heating rate were proposed to be optimum for the delubrication of chromium-alloyed PM steels. Sintering of water-atomized chromium-alloyed powder compacts is typically performed in hydrogen-containing atmospheres with nitrogen as the carrier gas. However, attaining good carbon control in these atmospheres is challenging. Hence, the effect of different active constituents in the atmosphere, such as hydrogen, carbon monoxide and propane, on the reduction-oxidation and carburization-decarburization processes during the sintering of chromium-alloyed PM steels was investigated. It was shown that concentration of carbon monoxide above 1 vol.% in the sintering atmosphere results in significant oxidation of the compacts, whereas lower concentrations contribute to counteract the carbon loss and provide for the possibility of carburization during a continuous sintering process. Furthermore, lean atmospheres containing carbon monoxide, hydrogen and hydrocarbons as active constituents, with their total concentration not exceeding 5 vol. %, were shown to be potential candidates for sintering of chromium-alloyed PM steels, since they provide carburization while confining oxidation to acceptable levels.