焼結合金鋼の破壊靭性

Abstract

Effects of alloying elements and heat treatment on the fracture toughness of sintered alloy steels have been investigated by means of three-point bending test, optical and scanning electron microscopies.The fracture toughness (KO) of sintered steels increased with an increase of carbon content. In Fe-Cu-1%C, the KO was somewhat increased by an addition of 1%Cu, but it was constant with Cu above 1%. This is explained in terms of the formation of pores at the original location of the Cu powders. On the other hand, the KO of Fe-Ni-1%C showed a peak with an addition of 3%Ni. This is attributed to the appearance of Ni rich martensitic phase with Ni above 3%, although Ni is effective to the densification of iron compacts. The KO of these compacts were also increased with yieled strength by heat treatment.A remarkable feature of the obtained results was that the KO increased simultaneously with the yield strength, which was contrary to the general behaviour of wrought materials. The origin of this mechanism is clarified from SEM observation, that is, the fracture process of sintered alloy steels is the necking down between the crack tip and adjacent pores.