Interpretation of effect of matrix hardening on tensile and impact strength of sintered steels

Sintered steels in the density range 6.7-7 g cm(-3) and with a wide range of matrix hardnesses were produced with the main objective of understanding the interplay between porosity and matrix strength in determining their tensile and impact behaviour. The materials can be subdivided into two groups, characterised by different deformation and fracture mechanisms. The first group pertains to the as sintered materials (with a microhardness lower than 350 HV0.1). They are characterised by a strain hardening stage before fracture, and both tensile strength and impact fracture load increase as density and matrix strength are increased. The second group pertain to the heat treated materials with a higher matrix hardness. These materials are characterised by a macroscopically brittle behaviour, since fracture takes place before general yielding. Because of the high matrix hardness, the peak stresses at the pore edges are not relieved by plastic deformation, and the pores behave as internal cracks. Fracture is then attained once the local applied stress intensity factor becomes equal to the matrix fracture toughness. PM/0989 The authors (giovanni.straffelini@ing.unitn.it) are in the Dipartimento di Ingegneria dei Materiali, Universita di Trento, Trento, Italy. Manuscript received 15 November 2001; accepted 20 February 2002. (C) 2002 IoM Communications Ltd.