Ni-Cr-Mo鋼の遅れ破壊における粒界き裂の枝分かれ

Abstract

The effects of tempering temperature, prior austenite grain size, stress intensity factor and testing temperature on the micro-branching frequency λ were investigated on a Ni-Cr-Mo high strength steel (SNCM 439) quenched and tempered, λ being defined as the ratio of the number of intergranular crack branching to the number of grains per unit length.λ takes a maximum value in the tempering temperature range of temper brittleness, and increases with increase in the prior austenite grain size. λ increases also with increase in the stress intensity factor K up to 40 MPa·m1⁄2 and then decreases for the material tempered at 623 K, whereas for the material tempered at 473 K it is almost constant independent of K. λ increases also with increase in the testing temperature for the material tempered at 473 K, whereas it is almost constant independent of the testing temperature for the material tempered at 623 K.The experimental results can be interpreted by considering the change in the binding strength of two grain boundaries at the triple point due to the segregation of impurity atoms, the dislocation pile-up and the corresponding hydrogen concentration on grain boundaries.