A study of the mechanism of stress‐corrosion crack propagation in AISI 4340 steel in aqueous 3.5 wt.% NaCl solution

Stress‐corrosion (SC) crack propagation in AISI 4340 steel has been studied with 2 mm thick single edge‐notched (SEN) specimens under constant load conditions as a function of applied potential and tempering conditions in an aqueous 3.5 wt.% NaCI solution at 30°C. The SC crack lengths were estimated by using the electrical potential method. As the amount of cathodic polarization increased, the SC crack propagation rate increased. Anodic polarization yielded opposite results. These polarization effects on the SC crack propagation are discussed in terms of absorbed hydrogen resulting from a cathodic reaction on the specimen surface. SC cracks propagated by intergranular fracture through most of the inner region, but shear lips were formed at the near subsurface, irrespective of applied potential and tempering temperature. This is explained in terms of the stress state dependency of hydrogen behaviour. Above experimental evidence well supports the theory that SC crack propagation is controlled by the hydrogen embrittlement (HE) process. The SC crack propagation rate decreased in the sequence of 300, 200, and 400°C‐tempered specimens. This is discussed as being related to the microstructural and yield stress effects.