Advances in Sintering of Powder Metallurgy Steels

Abstract

In comparison to traditionally fabricated steels that can undergo extensive processing to produce a complex-shaped component, the powder metallurgy (PM) technique can provide a more efficient approach as it is capable of producing intricately-shaped components that require little to no additional processing and machining [1], [2]. A key factor in being able to do so pertains to quenching and utilizing an appropriate quenching agent that can provide dimensional stability to the part being quenched [3], [4]. To ensure that a PM component can perform equally well when being quenched by a quenchant of reduced cooling capability, the PM component should be if not more, then just as hardenable. Steel hardenability can inevitably be improved with the increase of overall alloying content [5], however, if overall alloying content is to be kept at a minimum, the concept of lean PM steel design is one worth investigating; where a lean steel entails that each and every alloying addition is utilized to its maximum potential.

This study evaluates the homogenization behaviour of alloying elements in PM steels during sintering as well as the efficiency of wide-spread industrial practices involving the use of various master alloys and ferroalloys, and investigates the realm of liquid phase sintering to understand and optimize the homogenization behaviour of alloying elements and mechanical properties of PM steels. In the context of this work, multi-component master alloys contain at least three of non-ferrous metals as alloying elements and ferroalloys are master alloys containing iron in addition to typically a maximum of two other non-ferrous alloying additions. Part one of this study discusses a combination of thermodynamic software (DICTRA and Thermo-Calc), incremental sintering experiments and scanning electron microscopy (SEM) - wavelength dispersive spectroscopy (WDS) that were used in order to form a deeper understanding of the homogenization behaviour of alloying elements within PM steel during sintering. Electron microscopy analyses on partially and industrially sintered components provide elemental maps to track the evolution of alloying elements as they relax to homogeneity. Electron microscopy analyses for this portion of the study were conducted on an industryproduced automotive component that was sectioned and sintered industrially as well as experimentally at 1280°C for 30 minutes and 13.4 hours.

DICTRA simulations carried out for this research provide a 1-D insight into the evolution of concentration profiles and phases throughout various sintering times for systems involving Cr, Mn, C and Fe. DICTRA simulation results of alloying sources were studied alongside alloying element profiles obtained by compiling point quantification from wavelength dispersive spectroscopy maps for the sintered automotive component. Computational results provided conservative, semi-quantitative recommendations on optimal alloy addition forms that lead to an improvement in homogenization. Part two of this study involves the approach of fabricating and testing multi-component master alloy additions. As these materials are widely employed in PM and are typically fabricated by solidification, their states are non-equilibrium and therefore have regions containing phases precipitating in the beginning of freezing which have higher melting temperatures than regions with phases forming later on. During heating, it is hypothesized that Scheil’s solidification path backtracks and as a result, a fraction of liquid in the ferroalloy can be estimated at sintering temperature. If the fraction is significant, the utilization of this ferroalloy implies liquid phase sintering. Through a combination of Thermo-Calc and Fortran softwares, multi-component ferroalloys with promising compositions were discovered in Fe-C-Cr-Mn, Fe-C-Cr-Mn-Ni, FeC-Mn-Mo, Fe-C-Mn-Mo-Ni and Fe-C-Cr-Mn-Mo-Ni systems for low temperature liquid phase sintering. Those of the Fe-C-Cr-Mn-Mo, Fe-C-Cr-Mn-Mo-Ni and Fe-Mn-Mo-Ni system were fabricated and tried in practice. Compositional maps and mechanical properties of PM steels made with variations of this specially tailored multi-component master alloys were compared with those for which traditional alloy additions were used.