Statistical approaches to PM steel process improvements

Abstract: Increased performance:cost ratios for PM steel components is vital if current growth rate of market shares of such components versus competing technology is to be sustained. Both performance and cost of components are affected by variation of component properties. This work focuses on variation; where major variation sources lie and measures to circumvent problems that could be taken.A design-of-experiments approach was used to identify the most important green compact variations and interactions of a Fe-P alloy. The results show that the highest impact energies are obtained for high green densities, and without graphite addition. It is concluded that sintered density is the reason for the observed impact energies. Variation sources controlling phosphorus content and added graphite content are of concern since they affect the dimensional stability. Prospects for a robust sintering operation of same alloy is investigated. The objective is to find a setting of the variables investigated to maintain high tensile strength and proof stress without loss of dimensional stability, tensile elongation or impact energy. In the study, an endothermic sintering atmosphere was better for the robustness of the tested properties. A mechanism for the occasionally brittle behavior of the system is concluded to be loss of grain boundary cohesion due to decarburization.Possibilities for improved particle size distribution of water atomized powder are studied experimentally and by simulations. The most important variable for all studied responses is water pressure. Higher melt temperatures reduce particle size due to decreased viscosity. Large sulfur additions reduce particle size and this is an effect of reduced surface tension. Narrower size distributions were obtained for powder atomized with aluminum additions and for increased melt temperatures. A model for particle size for water atomization is presented and used in simulations.