Qualification of the in-situ bending technique towards the evaluation of the hydrogen induced fracture mechanism of martensitic Fe–C steels

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Abstract

This paper proposes a new in-situ hydrogen (H) charging bending technique to investigate the susceptibility to hydrogen embrittlement (HE) of high strength steels with limited ductility. The methodology is tested with generic martensitic Fe–C steels with a carbon (C) content of 0.2 wt%, 0.4 wt% and 1.1 wt%, respectively. The in-situ bending technique is developed to evaluate the hydrogen susceptibility of these brittle materials and is compared to uncharged samples as a reference. Moreover, as a crucial step in the validation of the technique, a comparison with conventional in-situ tensile testing for the most ductile material (i.e. Fe-0.2C) is performed. The bending results show that charging with H causes a significant ductility loss, which is characterized by a transition from a microvoid (Fe-0.2C), intergranular (Fe-1.1C) or mixed (Fe-0.4C) fracture surface for the uncharged samples to a hydrogen induced cleavage fracture appearance with additional cracking. The transition to the cleavage fracture type is found to be caused by the Hydrogen Enhanced Plasticity Mediated Decohesion mechanism, indicating that hydrogen is preferentially trapped at packet or block boundaries in high carbon steels without alloying additions. The fracture surface of the Fe-0.2C alloy after in-situ tensile testing was very similar to the fracture surface obtained after in-situ bending testing, which indicates that the fracture mode during bending is mainly dominated by the tensile field. This supports the applicability of the in-situ bending technique for intrinsically brittle materials.

Introduction

In a wide variety of industrial applications, high strength levels are required in order to increase the durability and performance of steel components. Particularly in the automotive industry, high strength steels allow for reducing the vehicle weight, which in turn improves the fuel efficiency, thus reducing the emissions assumed to cause global warming. However, both during the production process and the service life of the material, for instance when it comes into contact with lubricants, hydrogen (H) can ingress the metal lattice. This may lead to significant H induced mechanical degradation. Due to its very small dimensions, H is very mobile in the steel lattice, especially in a body centered cubic (bcc) crystal structure. Therefore, it can easily diffuse both interstitially and along interfaces or dislocation lines [1]. It can also accumulate at microstructural defects, and when a critical amount of hydrogen is present, cracks will initiate. The presence of both mobile and trapped hydrogen can result in ductility losses and unpredictable failure [[2], [3], [4]]. Although this embrittling effect was first discussed as early as 1875 by Johnson [5], no conclusive understanding on the underlying mechanisms has been obtained so far. This is mainly attributed to the fact that the hydrogen induced degradation depends on various factors such as chemical composition, microstructure, internal or applied stresses and H containing environment. The dependence on this wide variety of parameters complicates the understanding of the actual effect of introduced H on the mechanical properties of a specific steel grade. In literature can be found that the susceptibility to hydrogen degradation increases with increasing strength levels [[6], [7], [8]]. The martensitic phase, for example, enables to raise the strength level of steels but is also very susceptible to hydrogen embrittlement [9]. Several studies indicate that martensite contains more hydrogen after charging compared to more ductile bcc phases such as ferrite [[10], [11], [12]]. This is because martensite has a higher dislocation density and a lower hydrogen diffusivity coefficient [13,14]. However, understanding the hydrogen induced abrupt failure in martensitic steels is not straightforward. As suggested by Novak et al. [15], hydrogen embrittlement of martensitic steels is a complex interaction between several phenomena. Firstly, trapped hydrogen can reduce the cohesive strength in the steel lattice. Secondly, imposed stresses may cause decohesion of the martensitic structure. And lastly, hydrogen can cause dislocation pile-ups at the grain boundaries which influences the effective shear stress.

In order to get more fundamental insights on the hydrogen induced mechanical degradation of steels, simplified Fe–C structures are used in order to limit the number of influencing factors, i.e. the presence of precipitates, inclusions or second phases is eliminated. Depover et al. [10] performed a detailed study on martensitic Fe-0.2C and Fe-0.4C steels. They could accurately determine the degree of embrittlement of Fe-0.2C steels via in-situ tensile testing. However, the Fe-0.4C samples already broke upon clamping during in-situ hydrogen charging prior to tensile testing revealing a brittle fracture appearance. Hence, the evaluation of the degree of hydrogen embrittlement was not possible due to the unclear stress state during clamping. Consequently, the underlying mechanisms resulting in hydrogen embrittlement of materials, which are too brittle for tensile testing, are often not correctly evaluated by conventional tensile testing of flat sheet materials. Therefore, relatively brittle industrial components might experience unexpected crack development and failure due to an incorrect lifetime estimation. To be able to evaluate the effect of hydrogen on the mechanical integrity of brittle martensitic high strength steels, a different experimental technique is thus required. For brittle materials, bending tests are commonly favored over tensile tests since the positioning of the samples prior to testing is less complex [16]. However, existing research on the interaction between hydrogen and metal samples subjected to bending is scarce. There exist papers on bending experiments performed on samples which were gaseously hydrogen charged outside the set-up [17,18] or in-situ bending tests performed on microscales [19] but to date no reliable methodology on performing larger scale bending tests with simultaneous electrochemical H charging is available to our knowledge. Therefore, the goal of this paper is to merge the existing experience of in-situ H charging and bending experiments for brittle metals. In order to do so, a new hydrogen charging cell was developed and the in-situ bending method was explored by using generic martensitic Fe–C alloys. This testing methodology will also be applicable to other brittle materials which nowadays lack a reliable method for the evaluation and quantification of hydrogen induced mechanical degradation. The two objectives of this paper are (1) to qualify the in-situ bending method for brittle materials and (2) to investigate the HE of martensitic Fe–C steels.

Section snippets

Materials

Three generic Fe–C alloys with a different carbon content were cast and processed. The composition of the alloys can be found in Table 1, together with the austenitization temperature. Given the experience with Fe-0.2C and Fe-0.4C alloys, these materials can be used to develop an accurate and reliable in-situ bending methodology. The Fe-0.2C samples possess enough ductility to perform both bending and tensile experiments which allows comparing the two test methods. In addition, an Fe-1.1C steel

Microstructural characterization

The microstructures of the different alloys are given in Fig. 2. Each microstructure consists of a fully martensitic structure, although the Fe-1.1C samples contain about 10 vol% retained austenite which was only detectable with X-Ray Diffraction. The resulting prior austenitic grain size and Vickers hardness can be found in Table 2. Due to the high thermal stresses upon quenching, the Fe-1.1C samples contained quench cracks which mainly propagated along the prior austenitic grain boundaries

Conclusions

This paper proposes a new in-situ bending technique in order to investigate the fracture mechanism in the presence of hydrogen for high strength steels with limited ductility. The methodology was tested for generic Fe–C steels with a C content of 0.2 wt%, 0.4 wt% and 1.1 wt%. For these types of martensitic steels, H causes a significant ductility loss which is characterized by a transition from a microvoid (Fe-0.2C), intergranular (Fe-1.1C) or mixed (Fe-0.4C) fracture surface to a cleavage

CRediT authorship contribution statement

M. Pinson: Conceptualization, Investigation, Methodology, Validation, Visualization, Writing - original draft. H. Springer: Resources, Conceptualization, Supervision, Writing - review & editing. T. Depover: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Supervision. K. Verbeken: Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors wish to thank the senior postdoctoral fellowship of the Research Foundation - Flanders (FWO) via grant 12ZO420N and the Special Research Fund (BOF), UGent (grants BOF01P03516, BOF15/BAS/062 and BOF/01J06917) for support. The authors also acknowledge the help from Dr. Ives De Baere in the development of the in-situ bending setup.

References (30)

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