BRITTLENESS OF STRUCTURAL PM STEELS ADMIXED WITH MANGANESE STUDIED BY ADVANCED ELECTRON MICROSCOPY AND SPECTROSCOPY

Artikel i vetenskaplig tidskrift, 2008

Increasing mechanical properties requirements for structural parts direct modern powder metallurgy towards improved performance by adding such expensive alloying elements as Ni and Mo. The strong tendency to price increase during last decade lead to decreased competitiveness of Ni-Mo-alloyed PM parts compared to products of conventional metallurgy. Therefore, cheaper alloying elements as Cr and Mn, commonly used in cast and wrought steels, are of great interest. However, the use of these alloying elements faces some difficulties in compaction and sintering, especially in the case of high-oxygen affinity manganese. The present study deals with reasons of brittleness of PM steels admixed with manganese as medium-carbon ferromanganese powder. Two systems with the same composition of Fe-0.8Mn-0.5C, but sintered at different conditions (different sintering temperatures, atmosphere purity and cooling rates) were studied. Specimens were sintered in 90%N2/10%H2 atmosphere but of different purities (DP = -40°C and -60°C) for low and high temperature sintered specimens, respectively. The results obtained indicate that the brittleness is caused by a complex effect of microstructure heterogeneity around oxidized large-sized manganese carrier residues. The highest negative impact on the mechanical properties is produced by the weakness of the boundaries of the base matrix particles around manganese carrier residuals due to segregation and reaction product formation at these boundaries. Intensive study of inter-granular decohesion facets on the fracture surface close to admixed particles by advanced characterisation techniques (XPS, Auger spectroscopy, SEM+EDX) indicates that reaction products are mostly complex refractory oxides and manganese sulphide, the composition of which is dictated by type of the manganese carrier used and sintering conditions (temperature profile, sintering atmosphere purity, etc.). Specimen sintered at lower temperature and poorest atmosphere purity show ten times higher oxygen content and worse mechanical properties due to the large ferromanganese residuals and higher portion of the inter-granular decohesion facets.