Effect of Hydrogen on the Notch Tensile Strength of JIS SNCM439 Steel

Abstract

The notch tensile tests of JIS SNCM439 steel were conducted using various notch root radii at room temperature in a 20 MPa high-pressure hydrogen gas atmosphere to investigate the hydrogen embrittlement of the steel specimen at the notch root. The maximum load in hydrogen gas decreased with the increasing stress concentration factor (K_t) as the notch root radius decreased, and the value was reduced compared to that in air at any K_t, beyond which the load abruptly decreased and the specimen broke. Moreover, the local strain value of the notch root at the maximum load point increased with an increase in K_t in air, but that value remained almost constant in hydrogen gas regardless of K_t. Furthermore, the fracture strain observed during the notch tensile test in high-pressure hydrogen gas exceeded the local deformation during the smooth tensile test. In other words, cracks initiation in hydrogen gas were significantly affected by strain value, cracks were generated when voids began to form in the material, and lead to fracture. The specimens in hydrogen gas had quasi-cleavage fracture surfaces with no cracks directly below the notch root. Cracks developed in these fracture surfaces near the center of the specimen, and the propagation of these cracks resulted in the formation of ultimate fracture surfaces with dimples. The quasi-cleavage fracture surface directly under the notch root was considered as the area that yielded before cracking, and the fracture surface accompanied by cracks was considered as the area where the cracks had grown under the effect of high-pressure hydrogen gas. The fracture surface with cracks depended on the load at the time of crack growth, and the percentage of intergranular surfaces increased as the load decreased.

Fig. 10 Relationship between the stress concentration factor and the equivalent plastic strain at the notch root at the maximum load point in air and in hydrogen gas.