COMPARISON OF STRESS CORROSION CRACKING BETWEEN CAST AND FORGED STEELS IN WATER

The stress corrosion behaviour in water of the cast steel in contrast with the forged steel of same heat has been examined comparatively. It was shown that the activation energy of crack growth under stress corrosion of both cast and forged steels is identical, i.e. Q= 5540 cal/mol, and is in agreement with the apparent diffusion activation energy of H_2 permeation in steel. The da/dt for cast or forged steel increases no matter whether cathodic or anodic polarization, but the former influences predominantly. Correspondingly, the steady-state H_2 permeation flux increases steeply with the increase of polarizing current either cathodic or anodic polarization. The influences of polarization upon da/dt and H_2 permeation flux are similar, The testing temperature is found in a less effect on K_(ISCC). The K_(ISCC) of the cast steel determined is larger than that of the forged steel. It may be due to that the steady-state permeation flux for the forged steel is twice as many as that of the cast steel. For both cast and forged steels, the fracture modes are clearly dependent upon K_1 at the crack tip and a transition from dimple to quasi-cleavage or intergranular is observed on the fracture surfaces with decreasing K_1.