A study of internal hydrogen embrittlement of steels
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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