Key Aspects of Sintering Powder Metallurgy Steel Prealloyed with Chromium and Manganese
The powder metallurgy (PM) process is a cost efficient near net-shape technology suitable for sustainable manufacturing of structural steel components. A drawback with the PM technology is the difficulty to use oxidation sensitive alloying elements, such as Cr and Mn. These are effective alloying elements and their cost is significantly lower than the cost of the most commonly used alloying elements (Cu, Ni and Mo) in PM steel. Hence, increased usage of Cr and Mn as alloying elements in PM steel is crucial for retaining the competitiveness of the PM technology versus other metal forming processes. Sintering is the most critical process step when manufacturing PM steel parts from oxidation sensitive powder. The research work presented in this thesis was aimed at building a fundamental knowledge platform regarding key aspects of sintering PM steel prealloyed with Cr and Mn. These aspects are sintering atmosphere requirements, oxide reduction mechanisms, and effects of residual oxides on mechanical properties of the sintered material.
The test materials used in the research studies were four different water-atomized steel powder grades prealloyed with 3%Cr-0.5%Mo, 1.5%Cr-0.2%Mo, 1.8%Cr and 0.8%Cr-0.4%Mn. Compacted specimens based on these powder grades were used for sintering experiments in N2/H2 (90/10) atmosphere at temperatures in the range of 1120-1300°C. Oxide reduction reactions during sintering were studied by the means of photoacoustic spectroscopy. Mechanical properties of the sintered specimens were evaluated through hardness measurements, tensile tests and impact tests. The analysis techniques used for evaluation of specimen microstructures and fracture surfaces were light-optical microscopy and scanning electron microscopy combined with energy dispersive X-ray spectroscopy.
The research work shows that the critical oxygen partial pressure in the sintering atmosphere can be well-predicted by thermodynamic calculations of oxide stabilities in the steel. Stable Cr-Mn spinel oxides on the powder surfaces are reduced via carbothermal reactions at temperatures above 1000°C. Sintering for 30 minutes at 1120°C in a reducing atmosphere gives incomplete reduction of these oxides, which does not affect the sinter neck formation in the PM part. Increased sintering temperature to 1200-1250°C enables nearly complete oxide reduction, although high PM part density may obstruct the reduction of the stable oxides due to less effective CO diffusion in the pore system of the part. Residual oxides after sintering are in the form of micrometer size particulate features and these have no detrimental effect on the evaluated mechanical properties of the PM steel.