Fracture toughness of PM alloy steels

Macroscopically valid plane strain fracture toughness specimens of porous sintered alloy steels will generally fail in a microscopically ductile manner because the plane strain requirement is violated in the rather thin sinter necks between pores and a dimple fracture develops from microvoid coalescence. Therefore, in ordinary sintered steels, valid K _Ic values cannot be determined because a spontaneous brittle fracture does not occur on exceeding a critical crack opening displacement. This favourable behaviour is maintained in the as sintered condition with steels containing 0·5 or 0·65%C up to densities of 7·5 g cm⁻³. In order to come to valid K _Ic values, it is necessary to establish plane strain conditions between the pores, e.g. by approaching full density, or to embrittle the steel, e.g. by suitable heat treatments or alloying techniques. In the present study 0·3 or 0·6% phosphorus was added to a diffusion bonded alloy steel which is known to turn brittle in the presence of phosphorus when slowly cooled. With the experimental conditions chosen, true plane strain fracture toughness could be measured which was clearly lower than those from the regime of plastic sinter neck failure.