The toughness of Ni-Cr-Mo steels containing a small amount of V or/and Nb was estimated for as quenched conditions in relation to the austenite grain size. The toughness was quantified with the experimentally determined critical J-value, formulated by J. R. RICE, using small size three-point bending specimens.
The results obtained are as follows;
(1) The critical stress intensity factor calculated from the critical J-value is in satisfactory agreement with measured plane strain fracture toughness K_1c in the range of large scale yielding.
(2) The tensile properties deteriorate with coarsening of austenite grain. The toughness of the steels, on the other hand, remains nearly constant through a wide range of austenite grain size, showing a gradually decreasing tendency with the grain growth. However, all specimens with the largest grain in each series of the steels show a discontinuous deterioration in fracture toughness because of grain boundary fracture.
(3) The fracture toughness of the steels used is in a good correlation with the size of dimples, which originate at small inclusions. The toughness is, therefore, almost independent of the austenite grain size, as the austenitizing treatment results in no change in the mean spacing of the points responsible for dimple fracture.
(4) The steels with a small amount of V or/and Nb indicate the tendency to a better fracture toughness compared with those containing no Nb.