Papers by Author: T. Gómez-Acebo

Abstract: Being an effective sintering enhancer boron is gaining relevance for obtaining high density PM steels. Thermodynamic calculations are an important tool for studying the roll of alloying elements in the formation of a liquid during sintering. In the present work, the system Fe-Cr-B was obtained by combining up to date thermodynamic descriptions for the subsystems Fe-Cr, Cr-B and B-Cr. The calculations were carried out with Thermo-Calc software to predict isothermal sections for the ternary diagram for 1210 and 1250°C. The analysis of the isothermal sections indicates that the solid phases in equilibrium with the liquid are M₂B and a-BCC solid solution. The generation of the liquid is based on a eutectic reaction (Lða+(FeCr)₂B) involving the mixed borides previously formed. On the other hand, simulations for PM steels with constant boron but higher chromium content allowed realising that the formation of the liquid may be completely inhibited, within the temperature range under consideration, as materials with too high Cr/Fe ratios are used. This study was also supported by selected experiments which were in excellent agreement with the thermodynamic calculations.

Abstract: A Mn-containing master alloy (MA) has been specially designed, through thermodynamic and metallurgical criteria, for obtaining high performance low alloy PM steels by SPSS or DPDS. This MA exhibits improved characteristics with respect to ferromanganese and other Mn carriers for alloying PM steels preventing oxidation, keeping a high compressibility of the powder mixture and providing opportunities for low temperature processing. The improved sinterability through the formation of a transient liquid phase leads to dimensional stability and high reproducibility of mechanical properties after sintering at 1120°C. The microstructural development of the PM steels was studied during the sintering cycles. The final microstructure of these PM steels, after defined sintering cycles, was characterised by LOM while the mechanical properties of the consolidated materials were determined by tensile testing.

Abstract: The use of boron for successfully obtaining high density PM stainless steels with improved mechanical properties and corrosion resistance is presented. Boron is added as part of master alloys which have been specifically designed to provide the formation of wetting liquid phases with excellent characteristics for producing controlled densification and alloying of 316L and 304L austenitic stainless steels. The as-sintered density and properties of these alloys is determined by the amount of master alloy, the chemical composition of the stainless steel powder, the sintering temperature and time. The microstructural development and alloy homogenisation are determined by the chemical composition of the Fe-based powder and the chemical reactions taking place between the basic powder and the master alloy particles during high temperature sintering. The use of this master alloy is shown to lead to stainless steels with outstanding combinations of strength and ductility. The influence of alloying and the sintering conditions on the final microstructure, density, corrosion resistance and tensile properties is also discussed.