Tailoring of Liquid Phase Sintering and Microstructure of Sintered Steel

Doctoral thesis, 2004

The present study intends to interpret the effects of alloying elements on microstructure and mechanical properties via liquid phase sintering of iron and steel powders. The tools involved are thermodynamic modelling, tailoring of powder properties and processing, and optimisation of sintering. Alloy systems such as the Fe-P-C system are alternative candidates for enhanced sintering shrinkage by liquid phase sintering generation. Alloys of this system are prone to form brittle grain boundary precipitates, which can be a drawback concerning mechanical properties. Control of alloy composition as well as sintering and post-sintering procedures are therefore key parameters. In this study, Fe-P-C material is sintered to near full density (95%-99%) starting from mixtures of standard water-atomised base powders and liquid forming additives. Post-sintering heat treatment is successful in breaking up the continuous phase decoration (e.g. Fe3P) on grain/prior particle boundaries. Still, segregation of P during cooling and associated embrittliment is difficult to avoid. Higher level of C in starting powder mixtures in combination with post-sintering decarburisation is successfully developed as a means of avoiding high P concentration on grain/particle boundaries. The mechanical properties of such material, sintered to 85-95% gave elongation to fracture of 2-12 % and ultimate tensile strength of 400-600 MPa. Another means of achieving acceptable high sintered density was to make use of the Fe-P-C-Cu-Si system. Sintering of such material leads to grey iron like microstructure with relative density of up to 95 % and mechanical properties corresponding to elongation to fracture of 1-3 % and UTS of 200-400 MPa. Another concern is the size and carbide morphology in the liquid phase sintering of high speed steel powder. Although, full density is easily achieved, the final microstructure in principle contains large bulky carbide precipitates. The formation of M6C carbides is of specific concern. It is shown here that addition of elemental Si, V and Ni can be used to enhance the microstructure and having near full density material with improved distribution of M6C in particular.