Impact notch toughness of high-strength porous steels

Instrumented impact testing was used to investigate the effect of both notch severity and matrix strength on the impact fracture behaviour of different Fe–Mo–C high-strength steels produced by powder metallurgy, in the as-sintered as well as in the heat-treated condition. As the matrix strength and/or the notch severity are increased, two fracture mode transitions were observed: from a ductile to a very localized (but still ductile) type of fracture, and from a very localized to a mixed type of fracture. The fracture behaviour was found to be directly correlated with the shape of the impact curves. In the case of the ductile type of fracture, two types of load-deflection curves were encountered. The first type shows the presence of a distinct yield point and a strain hardening stage up to the fracture load. In the second type (encountered in specimens with increased matrix strength and/or notch severity) the distinct yield point disappears and the load–deflection curve shows a continuous yielding up to fracture. In the case of the ductile but very localized and the mixed type of fracture, the load–deflection curve is linear up to fracture. The deformation and fracture behaviour of the specimens which display this type of impact curve is explained using fracture mechanics arguments and it is concluded that the surface pores act as crack precursors.