Microstructural features limiting performance of PM steels

Abstract

Pressed and sintered structural steels exhibit mechanical properties that are markedly inferior to those of their wrought counterparts. This is due primarily to the unique microstructure of powder metallurgy products which includes porosity as the pivotal feature. It is shown here that porosity results in an out-of-proportion decrease in the effective load-bearing cross-section and this microstructural parameter is related directly to the mechanical properties. Interconnected porosity is more detrimental than isolated porosity, resulting in isolated sintered necks that are mostly small compared to the powder particles size and are thus unable to sustain higher loads. Also the distribution of porosity is rarely homogeneous, and the high porosity regions act as crack initiation sites particularly in cyclic loading. Fatigue fracture is also caused by singular defects such as secondary pores generated via a transient liquid phase or by slag inclusions. The effect of these inhomogeneities is more pronounced, the higher the strength of the material.