Fatigue crack growth behaviour of 4130X steel in H ₂ S environment

Fatigue crack growth rates of 4130X steel used for compressed natural gas vessels were investigated in this paper. Considering the operating conditions, corrosion fatigue tests at a low frequency of 0·0067 Hz, in H2S saturated, H2S unsaturated and air environments were conducted on modified wedge opening load specimens by using a home made low cycle fatigue test system. Curve fitting was applied to the fatigue test data of da/dN–ΔK according to Paris formula. A correlation study between fracture surface and stress intensity factor range was conducted and K values for three stages in different environments were characterised quantitatively. The results show that da/dN in H2S environment is more than 20 times faster than in an air environment. When the H2S concentration reaches a certain range, the increase of da/dN becomes slower than that of the H2S concentration. da/dN differs by 2·4 times while the concentration differs by 11 times. The corrosive environment accelerates the fatigue failure.