Effects of aging treatment on the susceptibility to stress corrosion cracking(SCC) of HSLA-100 steel were investigated in a synthetic sea water + 0.1 N $H_2SO_4$ + 1 g/L thiourea solution at room temperature under various cathodic current density. SCC test were conducted using a slow strain rate test(SSRT) with evaluating the influences of aging treatment, cathodic current density, and strain rate on the fracture mode of the alloy. The susceptibility to hydrogen embrittlement(HE) of the alloy was determined in terms of the ratio of strain to failure in environment to that in air($\varepsilon_{f,env}/\varepsilon_{f,air}$), measured from the slow strain rate tests.
When the alloy was aged for 1 hour in the range of aging temperature of 500 to 650℃, the resistance to HE increased with aging temperature. This is due to both the decrease of yield strength and the increase of hydrogen trap sites such as matrix/precipitates interfaces with aging temperature increased.
The susceptibility to hydrogen embrittlement(HE) of the alloy was significantly dependant on the cathodic current density. When the susceptibility to HE was evaluated in terms of strain to failure ratio, the results show three region with applied cathodic current density. In region I obtained at a low cathodic current density, the alloy was immune to HE, showing the strain to failure ratio exceeding 0.85 fractured in the ductile fracture mode of microvoid coalscence(MVC). In region II, the susceptibility to HE significantly increased with the applied cathodic current density, where the fracture occurred in a mixed mode of tearing topography surface(TTS) and intergranular fracture with micro-deformation. In region III obtained at a high cathodic current density, the alloy was very susceptible to HE with the strain to failure ratio remained at a low value less than 0.15, and was fractured in a brittle intergranular fracture(BIF) mode. In the range of strain rate of $2.5\times10^{-7}$ to $2.5\times10^{-6}$, the resistance to HE increased with increasing the strain rate, which was attributed to the decrease of stress enhanced diffusion of hydrogen into the alloy.
From the above results, fracture mechanism maps were proposed showing the effects of cathodic current density, yield strength, and strain rate on the fracture mode. With decreasing the yield strength, the region for ductile fracture mode(regionⅠ) expanded at the expense of the region Ⅲ for BIF. As the strain rate increased, the region for the ductile fracture mode was expanded, at the expense of the region Ⅱ for the mixed mode of Mode Ⅱ BIF and Mode Ⅲ.
