A study of internal hydrogen embrittlement of steels

A novel procedure for hydrogen charging and studying the Internal Hydrogen Embrittlement (IHE) of steels is described here. A cylindrical notched tens ile sample with an extended end is employed for hydrogen charging. The exte nded portion of the sample forms the cathode in an alkaline bath acid a con stant uni-axial tensile load is applied during hydrogen charging. The stres s gradient set up by the notch, which is not in contact with the electrolyt e, enhances the hydrogen concentration at various trapping sites of the mat rix beyond the solubility limit. Subsequent to charging, the specimen is ke pt under the same load as that during charging, for another 24 h to stabili ze the population of hydrogen within the specimen matrix. At the end of thi s 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 st rength, work hardening rate and ultimate tensile stress (UTS) has been obse rved 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 coales cence) in both the materials and that the hydrogen reduces the size of the dimples. The observations of this investigation are significant in two resp ects: firstly, it demonstrates the efficacy of a hydrogen charging method f or steels which can introduce hydrogen to a level much higher than its solu bility limit and secondly, it reports for the first time enhancement of mod ulus and work hardening by hydrogen charging. These observations have been rationalized on the basis of current understanding on the effect of hydroge n on plastic properties and hypothesis of the models of IHE. It is suggeste d that the trapping of hydrogen by dislocations and other structural featur es of the matrix and the mutual interactions of their strain fields can acc ount for the observed effects on yield strength, tangent modulus, work hard ening rate, UTS and ductility. (C) 2000 Elsevier Science S.A. All rights re served.