高張力鋼硫化物腐食割れの温度依存性について

Abstract

Hydrogen diffusion in steels is temperature-dependent, and accordingly if incubation period to failure in sulfide corrosion environment were affected by hydrogen diffusion, the incubation period would be also temperature-dependent.In this investigation the temperature dependence of sulfide corrosion cracking in weldable high strength steels, especially of 80 kg/mm2 level, has been studied. Both constant strain method and constant load method were used in this investigation and the time to failure after loading was measured in the temperature range 0°C to 70°C. The results obtained are as follows.1). The susceptibility to sulfide corrosion cracking in high strength steels is temperature-dependent and the incubation period to failure is minimum at about 35°C.2). At temperatures lower than 35°C the relation between the reciprocal of testing temperature and the logarithm of ratio of incubation period to testing temperature is represented by a straight line with a positive incline. Activation energy of cracking calculated from the incline is stress-dependent and activation energy of cracking at no load is estimated at about 27, 000 cal/mol by extrapolating the relation between applied stress and activation energy. This value is markedly high comparing with activation energy of hydrogen diffusion in steel, i. e., about 9, 000 cal/mol.3). The susceptibility to sulfide corrosion cracking in high strength steels is anisotropic: the susceptibility in thickness direction is considerably higher than that in roll direction. It is supposed that the above results from the existence of thin and lamellar non-metallic inclusions extended in roll direction. Segregation of Mn, S, and small amount of Cr and C are recognized in the inclusions4). The incubation period to failure remarkably depends on applied load in roll direction, while in the thickness direction applied load dependence of the incubation period is scarcely recognized.