Correlation between fracture mechanisms and microstructure in Mn powder metallurgy steels susceptible to intergranular failure

This study was undertaken to thoroughly characterize the relationship that exists between microstructure, strength and fracture mechanisms of Fe-Mn-C powder metallurgy steels. Indeed, up to now the peculiar fracture behaviour of PM components made from this metallurgical system was not clearly understood by the PM community. A design of experiments (DOE) approach was used to determine the effect of chemical composition on tensile properties of Fe-Mn-C PM steels components and therefore on the fracture mechanisms leading to failure. Quantitative data describing fracture mechanisms as a function of sintered microstructure were obtained and compared against tensile strength measurements. Results led to the identification of microstructural conditions for which Fe-Mn-C PM steels are susceptible to the detrimental effect of Mn-oxide inclusions. It was found that the presence of martensite having a hardness higher than 661 HV exacerbates the detrimental effect of inclusions present in sinter necks, which explains the decreasing tensile strength observed with increasing carbon concentration. The beneficial effect of the divorced eutectoid phase as a secondary microstructural constituent is also discussed. Knowledge generated regarding the fracture behaviour of components made from this family of PM alloys allowed for the optimization of the volume fraction of microstructural constituents to maximize tensile properties. This work provides information that was missing up to now regarding the capabilities and limitations of admixed Mn PM steels in terms of tensile properties and allows the formulation of design guidelines to alleviate the adverse effects of Mn oxidation.