APPLICATION OF FRACTOGRAPHY FOR INVESTIGATION OF SURFACE OXIDE REDUCTION/TRANSFORMATION AND INTER-PARTICLE NECKS FORMATION DURING SINTERING OF PREALLOYED WITH Cr AND Mn PM STEELS

Journal article, 2014

Typical purpose of microfractography is the determination of the reasons and mode of component failure with the aim to avoid similar type of failure in the future. However, in case of powder metallurgy (PM) steels micro-morphology of the fracture surface contains significant information concerning consolidation process of PM steel from the point of view of efficiency of surface oxide reduction and following inter-particle necks development, carbon dissolution, etc. Powder surface is inevitably covered by surface oxide layer which composition and thickness are determined by powder manufacturing and alloy composition. In order to develop strong inter-particle necks, surface oxide has to be efficiently reduced during initial stages of sintering (heating stage). On the other hand, thermodynamics and kinetics of surface oxide reduction/transformation is determined by number of factors, including sintering process (sintering atmosphere composition, purity and flow, temperature profile, furnace load, etc.), alloy composition (type and content of alloying elements as well as activity of carbon source used), sintering component properties (geometry, mass, density, etc.). Present work summarizes some of the examples of application of fractography for investigation of the surface oxide reduction/transformation and carbon dissolution and their effect on the quality of inter-particle necks developed during sintering of prealloyed with chromium and manganese PM steels. Application of modern microscopy techniques widens application of the fractography from typical use as technique for failure analysis to the efficient tool for estimation of the quality of the consolidation process and evaluation of the required processing conditions to reach optimum properties from PM alloys.