An Integrated Experimental and Numerical Investigation of Hydrogen Embrittlement Susceptibility and Mechanism(s) in Martensitic Steels

Book excerpt: "Hydrogen Embrittlement (HE) is a serious engineering problem for a wide range of industries starting from fastener, oil & gas to aerospace and nuclear. After several decades of extensive research, the HE problem has not been mitigated to a satisfactory level. HE in general, comprises of numerous layers of complexities involving hydrogen metal interactions, hydrogen diffusion and fracture, where each of these phenomena are entangled with one another to a certain degree. High strength martensitic steels which are known for structural and critical engineering applications suffer premature failure due to HE, to a great extent. The interactions of the hydrogen with the complex microstructure of these martensitic steels further enhances the challenges of mitigation. Therefore, the aim of the current research is to develop a better understanding on the susceptibility and mechanism(s) of HE failure of these materials, by studying the key factors affecting their embrittlement. In order to carry out the investigation, a combined approach based on experiments and numerical modeling has been adopted. As a first step, the HE susceptibility of high strength martensitic steels was evaluated using conventional slow strain rate testing methodology, in bending. A stress coupled hydrogen diffusion finite element analysis (FEA) model integrated with a cohesive zone model was developed to simulate the HE test. The primary factors influencing material susceptibility to HE were studied using the model, and the evaluation of critical hydrogen concentration as a metric of material susceptibility was demonstrated. Following the first study, a new approach involving rapid fracture test in four-point bending was proposed to assess HE susceptibility and mechanism(s). Stress coupled hydrogen diffusion FEA was also performed to calculate both stress and hydrogen concentration distributions in the domain, while simulating the test. A mechanistic description rooted in hydrogen enhanced decohesion (HEDE) mechanism was used to corroborate the mechanical test results, and fundamental understanding on the role of strength, microstructure and plasticity influencing HE susceptibility of materials, was also developed.The difference in susceptibility obtained from the rapid HE test, for two different quench and tempered martensite steels with similar strength level and microstructural features were explained using advanced microstructural characterization techniques, FEA and nanoindentation. The role of local microstructure affecting the micromechanics of HE fracture was discussed.Finally, hydrogen diffusion along the interface boundaries in a typical martensitic microstructure was investigated using centroidal Voronoi based FEA model. The influences of packet boundaries and prior austenite grain boundaries on the output hydrogen flux and concentration were studied. The presence of retained austenite in the microstructure affecting the diffusion of hydrogen was also studied. An overall understanding on hydrogen diffusion characteristics in a martensitic microstructure was demonstrated for better prediction of HE fracture.Thus, the current research provides fundamental understanding on the HE susceptibility of martensitic steels, as well as mechanistic insights, that could be instrumental in tackling the HE problem"--