A study of internal hydrogen embrittlement of steels

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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.

Introduction

The effects of hydrogen on the mechanical properties of iron and steels have been widely investigated [1], [2], [3]. As regards the embrittling effect of hydrogen it is well known that the chief effects are a decrease in ductility and true stress at fracture. Hydrogen embrittlement (HE) of steels can be classified into three main types [4], [5].

  • 1.

    Hydrogen reaction embrittlement arises because of the generation of hydrogen on the surface as a result of a chemical reaction [6], [7]. The resulting hydrogen can either form blisters in the sub-surface region or gaseous methane in the interior. Precipitation of hydrogen as hydride in hydride forming elements such as zirconium and titanium [8], [9] are other examples where chemical reaction aids the hydrogen-induced embrittlement of the matrix.

  • 2.

    Environmental embrittlement takes place in the hydrogen containing atmospheres through adsorption of molecular hydrogen on the surface and its absorption within the lattice after dissociation into atomic form [5].

  • 3.

    Internal hydrogen embrittlement (IHE), in contrast takes place in the absence of a hydrogenated atmosphere and is brought about by hydrogen which has entered the lattice during processing or fabrication of steel [10]. Having entered the lattice, hydrogen embrittles the steel over a period of time which is a function of concentration, temperature and state of stress within the matrix.

The details of the effect of hydrogen on mechanical properties and the mechanism of degradation are reasonably well understood in the case of hydrogen reaction embrittlement. However, generally accepted mechanisms for the other two classifications of HE, have not yet been established because of the contradictory experimental results obtained by various researchers [2], [3], [5]. This is more so in the case of IHE and one of the main reasons for the discrepancies in the reported results has been attributed to the possible structural damage caused by hydrogen charging which masks the intrinsic effect of hydrogen on mechanical properties [11], [12].

This paper presents the results of experiments designed to elucidate the intrinsic effect of hydrogen on the mechanical properties of ferritic steels by employing a new technique of hydrogen charging. Two different steels namely mild steel and maraging steel with widely different strength levels have been chosen for this study.

Section snippets

Experimental procedure

As received materials in tempered condition have been employed in this study. The nominal compositions of these two steels are given in Table 1.

The three essential steps in the experimental procedure in this study on internal hydrogen embrittlement are:

  • 1.

    The electrolytic charging of specimen with hydrogen under uniaxial loading conditions.

  • 2.

    After charging hydrogen for a specified number of hours, the charging cell is removed and the specimen is held in a fixture, shown in Fig. 1, under a

Results

The results of tensile tests on mild steel and maraging steel are given in Fig. 4, Fig. 5. Fig. 4 shows the tensile test results for mild steel with and without notch. For maraging steel, the tests were performed only on notched specimens with and without hydrogen. The purpose of such tests was to estimate the limiting elastic load to be applied during hydrogen charging. Additionally, such tests also yield information on the deformation behaviour of the material in the absence of hydrogen. A

Discussion

This investigation suggests a new method of charging hydrogen and studying the intrinsic effect of hydrogen on mechanical properties of steel as during IHE. Hydrogen is charged into the test specimen by stress assisted diffusion of hydrogen generated remotely by electrolytic charging. Hydrogen charged in this manner is not expected to cause any damage to the microstructure as the test piece is not in direct contact with the electrolyte and is thus not exposed to high fugacity hydrogen that

Conclusions

A novel technique for charging hydrogen has been demonstrated. This method permits hydrogenation to a very high level without causing damage to the microstructure and facilitates studying the intrinsic effect of hydrogen on the mechanical properties of steel.

The intrinsic effect of hydrogen on both the steels studied are the enhancement of yield strength, ultimate tensile strength, tangent modulus, work hardening rate and reduction in ductility. The fracture mode was ductile dimple and hydrogen

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