Fracture toughness of PM alloy steels

AbstractMacroscopically 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 s...