A study of internal hydrogen embrittlement of steels

Abstract

A novel procedure for hydrogen charging and studying the Internal Hydrogen Embrittlement (IHE) of steels is described here. A cylindrical notched tensile sample with an extended end is employed for hydrogen charging. The extended portion of the sample forms the cathode in an alkaline bath and a constant uni-axial tensile load is applied during hydrogen charging. The stress gradient set up by the notch, which is not in contact with the electrolyte, enhances the hydrogen concentration at various trapping sites of the matrix beyond the solubility limit. Subsequent to charging, the specimen is kept under the same load as that during charging, for another 24 h to stabilize the population of hydrogen within the specimen matrix. At the end of this stage, the specimen is tensile tested to failure at room temperature. Two different steels namely maraging and mild steels have been chosen to study the effect of hydrogen ingress on mechanical properties. While an increase in tangent modulus (linear portion of the stress–strain diagram), yield strength, work hardening rate and ultimate tensile stress (UTS) has been observed on hydrogenation, a decrease in total elongation has been noticed for both the steels studied. Fractographic investigation has revealed that the fracture mode is predominantly ductile dimple (failure by micro-void coalescence) in both the materials and that the hydrogen reduces the size of the dimples. The observations of this investigation are significant in two respects: firstly, it demonstrates the efficacy of a hydrogen charging method for steels which can introduce hydrogen to a level much higher than its solubility limit and secondly, it reports for the first time enhancement of modulus and work hardening by hydrogen charging. These observations have been rationalized on the basis of current understanding on the effect of hydrogen on plastic properties and hypothesis of the models of IHE. It is suggested that the trapping of hydrogen by dislocations and other structural features of the matrix and the mutual interactions of their strain fields can account for the observed effects on yield strength, tangent modulus, work hardening rate, UTS and ductility.