抄録
Fracture toughness tests of high speed steels were carried out to develop a method for evaluating fracture toughness. A precrack introduced by the BC (Bridge Compression) method, at the front of which the damage region with microcracks was formed, could not be assumed as an ideal crack. On the other hand, a precrack introduced by the FP (Fatigue Precracking) method, where the precrack was introduced under a sufficiently low level of Kfmax, could be assumed as an ideal crack. The KIC-values evaluated by the BC method were lower than those by the FP method. Therefore, it is necessary to introduce a fatigue precrack, which can be assumed an ideal crack, by the FP method for evaluating fracture toughness.
The size and volume fraction of carbides in the sintered high speed steel were smaller and higher than those in the melted high speed steel. The high volume fraction of carbides gave low fracture toughness, while the small size of carbides improved fracture toughness. Since in the present sintered high speed steel these effects by the size and volume fraction of carbides compensated each other, the fracture toughness of the sintered high speed steel was almost equal to that of the melted high speed steel.
