Environmentally-assisted Cracking Behavior Of Two Structural Steels And The Monitoring Technology

Various components made of structural steels may fail due to crack initiation and propagation during service under synthetic action of environment and stress, which is called environmentally assisted cracking (EAC).It is of great academic and engineering significance to study the EAC propagation behavior of structural steels and related monitoring technology. In the present work, pre-cracked specimens and some materials testing methods such as optical microscopy (OM) and scanning electron microscopy (SEM) were used to study the crack propagation behavior of pipeline steel X60 and high strength steel 35CrMo in various environments, also trying to establish a direct current potential drop (DCPD) system to monitor the crack propagation.The crack propagation behaviors of pipeline steel X60 in a typical soil environment in Shanghai area and in dynamic hydrogen-charging solution were studied. The results showed that there were no cracks in the steel in the soil environment under high static loading together with various applied potentials in the range from -1000 to -2000 mV(SHE), indicating that dynamic loading may be the major factor resulting in stress corrosion cracking (SCC) of X60 in soil environment. The EAC only appeared under severe conditions like dynamic hydrogen-charging, and tended to propagate along the direction parallel to the rolling direction of the plate for the pipe. The fracture mode was transgranular quasi-cleavage with a little intergranular cracking.The crack propagation behavior of high strength steel 35CrMo with different heat treatments including high temperature and low temperature tempering was studied in 3.5% NaCl solution, soil solution and static hydrogen-charging environment respectively. The results showed that the steel in these three environments exhibited high sensitivity to EAC, mainly in an intergranular fracture mode. The susceptibility to cracking could be decreased by increasing the tempering temperature, resulting in an increase of stress intensity threshold and a decrease of crack propagation rate. In 3.5% NaCl solution, the susceptibility also decreased with the increase of environment temperature from room temperature to 50℃. At room temperature and under similar loading conditions, the low temperature tempering steel in the soil solution environment showed a lower cracking susceptibility than in 3.5% NaCl solution.Tests showed that the DCPD method was feasible to monitor crack propagation. The potential drop change could be consistent with the crack growth trend. However, in the testing process, there was relatively big error between crack length values calculated by the Johnson formula and determined by visual reading measurements respectively due to various disturbances resulting in fluctuation of measured potentials. Consequently, the method needs further improvement.