Relationships between homogeneity, structure and mechanical properties in certain sintered iron, nickel, carbon materials.

The commercial production of low alloy steels from blended mixtures of powders normally results in a product which is chemically heterogeneous. Although reference has often been made to the effects of heterogeneity in sintered steels, few investigations have included an assessment of homogeneity and none of these has related the quantitative measurement of homogeneity to the mechanical properties of the product.A study was made of three classes of sintered material, each containing 3% nickel and 0.337% 0.03% carbon but with different initial degrees of homogeneity. This was achieved by the use of blended elemental powders, a nickel-plated iron powder produced in a specially developed apparatus and a prealloyed powder. Electron probe micro-analysis was employed for the quantitative measurement of the degree of homogenisation that had occurred in each of these materials during sintering. The homogenisation parameter so determined was related to the mechanical and physical properties and microstructure of the materials after these had undergone various sintering treatments.In each class of material, the degree of homogenisation that was developed during sintering times of up to eight hours was given by an exponential relationship which was similar to that predicted theoretically by other workers. The established equation [mathematcial formula], in which 0 s and Q o are the tensile strengths of sintered and fully dense materials respectively, P is the fractional porosity and F is theempirically determined stress concentration effect of the pores, was found to predict tensile strength more accurately than other published equations, provided that the materials concerned possessed homogenisation parameters below a certain value. Although these materials of high homogeneity consisted mainly of lath martensite, the presence in the microstructure of isolated regions of acicular martensite, bainite and ferrite was not detrimental to tensile strength provided that the above criterion was maintained. The tensile strength of alloys of low homogeneity was not predictable by the use of the above equation; the presence in the microstructure of nickel-rich austenite, especially in the form of continuous interparticle networks, was particularly deleterious in this respect. The impact fractures of all three classes of material were ductile despite the low values of impact strength which were due, not to the type of microstructure associated with material of low homogeneity, but mainly to pore shape and distribution. It was considered, on the basis of metallog-raphic evidence, that homogenisation in the materials investigated proceeded by a complex mechanism that involved both surface and volume interdiffusion.