Mechanism of hydrogen embrittlement cracking produced by residual stress from indentation impression

Abstract

Abstract This study investigated the mechanism of hydrogen embrittlement (HE) cracking, produced by residual stress that develops around an indentation impression crater. A spherical indenter was used to indent a high strength steel, forming locally compressive plastic deformation (i.e., an impression crater). Subsequently, hydrogen was charged into the specimen with the impression. It is found that HE cracking occurs around the impression, and propagates radially. The radial crack morphology was changed by varying the applied indentation force and time of hydrogen charging. In fact, two types of cracks were observed, short cracks (less than 50 μm in length) and long ones. The impression due to a small indentation force produced only short cracks, while a large force yielded both short and long cracks. In order to clarify the mechanism of crack morphology formation, the finite element method (FEM) was used to compute the residual stress field surrounding the impression. It was found that circumferential stress with a tensile component was developed around the impression, leading to initiation and propagation of HE cracks. For the smaller indentation force, the potential area for crack initiation is at the rim of the impression (on the material surface). However, a larger force induced crack initiation additionally beneath the surface leading to longer crack formation. Crack propagation was simulated using a cohesive zone model (CZM) in FEM in order to elucidate the crack morphology showing both short and long cracks. Multiple crack propagation was simulated using two CZM elements in order to investigate the interaction between two cracks. It was found that the crack length becomes shorter when the distance between the two cracks is closer (the crack angle is small). This is due to the fact that the stress as the driving force decreases owing to neighboring crack generation. On the contrary, when the distance between the two cracks is large (the cracks are away from each other), the lengths of the two cracks are identical, since there is no interaction between them. This study also investigated the relationship between crack growth resistance (i.e., threshold stress intensity factor, Kth) and simulated crack length. From the length of the long crack, the HE crack growth resistance, Kth, can be estimated. It was found to be strongly dependent on hydrogen content, while there is little influence of the indentation force. This study thus established the computational framework for the prediction of HE damage (crack morphology) surrounding the indentation impression for various applied indentation forces and hydrogen content. The findings will be useful for predicting HE cracking from an indentation impression crater, when the crater formed, for instance, due to shot peening or foreign object contact, is exposed to a hydrogen environment.