Hydrogen embrittlement of high strength steels: Determination of the threshold stress intensity for small cracks nucleating at nonmetallic inclusions

The objective of this study is to determine the threshold stress intensity factor for small cracks in high strength steels in a hydrogen environment by studying the failure of hydrogen pre-charged cylindrical specimens loaded in uniaxial tension. Fracture of these specimens under tension usually initiates at the largest nonmetallic inclusion contained in the specimen and such typical inclusions are Al₂O₃ (CaO)_X and TiN. The onset of the failure process is the crack initiation and propagation from a cavity forming either through debonding along the inclusion/matrix interface or through cracking of the inclusion. By analyzing the stress intensity factor for planar cracks emanating from inclusions, we calculated the threshold stress intensity by using experimental measurements of the applied tensile stress at the failure of the specimen. The results indicate that the threshold stress intensity is a linear function of the size of the inclusion and the hydrogen concentration in the specimen upon failure. The size of the inclusion is calculated as √area, where area denote the area of the domain defined by projecting the inclusion surface on a plane normal to the cylindrical axis of the specimen. Analysis of the experimental data indicates that the threshold stress intensity decreases as the inclusion size decreases. The estimates of KTH obtained by this method through fracturing uniaxial tension specimens can be used as a lower bound of the resistance to hydrogen embrittlement (HE) of component of high strength steel containing small defects and cracks.