Qualification of the in-situ bending technique towards the evaluation of the hydrogen induced fracture mechanism of martensitic Fe–C steels
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)
- et al.
Hydrogen in metals
Eng. Fail. Anal.
(2001) - et al.
Combined thermal desorption spectroscopy, differential scanning calorimetry, scanning electron microscopy and X-ray diffraction study of hydrogen trapping in cold deformed TRIP steel
Acta Mater.
(2012) - et al.
On the synergy of diffusible hydrogen content and hydrogen diffusivity in the mechanical degradation of laboratory cast Fe-C alloys
Mater. Sci. Eng. -Struct. Mater. Prop. Microstruct. Process.
(2016) Thermal desorption spectroscopy study of the interaction between hydrogen and different microstructural constituents in lab cast Fe-C alloys
Corrosion Sci.
(2012)- et al.
Microstructural aspects upon hydrogen environment embrittlement of various bcc steels
Int. J. Hydrogen Energy
(2010) - et al.
Modelling of hydrogen diffusion and hydrogen induced cracking in supermartensitic and duplex stainless steels
Mater. Des. - Mater. Des.
(2008) - et al.
A statistical, physical-based, micro-mechanical model of hydrogen-induced intergranular fracture in steel
J. Mech. Phys. Solid.
(2010) - et al.
The effect of nanosized (Ti,Mo)C precipitates on hydrogen embrittlement of tempered lath martensitic steel
Acta Mater.
(2014) - et al.
“Hydrogen-enhanced-plasticity mediated decohesion for hydrogen-induced intergranular and ‘quasi-cleavage’ fracture of lath martensitic steels
J. Mech. Phys. Solid.
(2018) - et al.
In situ electrochemical microcantilever bending test: a new insight into hydrogen enhanced cracking
Scripta Mater.
(2017)
Revealing the former austenite grain boundaries of high-purity iron-carbon alloys
Metallography
Microstructural characterization of hydrogen induced cracking in TRIP-assisted steel by EBSD
Mater. Char.
The effect of TiC on the hydrogen induced ductility loss and trapping behavior of Fe-C-Ti alloys
Corrosion Sci.
Internal and surface damage of multiphase steels and pure iron after electrochemical hydrogen charging
Corrosion Sci.
Effect of hydrogen charging on the mechanical properties of advanced high strength steels
Int. J. Hydrogen Energy
Cited by (13)
A combined thermal desorption spectroscopy and internal friction study on the interaction of hydrogen with microstructural defects and the influence of carbon distribution
2022, Acta MaterialiaCitation Excerpt :The hydrogen embrittlement (HE) of steels has been causing serious issues for many engineering sectors over the last couple of decades, since the presence of atomic hydrogen (H) in a steel microstructure may lead to premature and unpredictable failure [1–9].
The role of cementite on the hydrogen embrittlement mechanism in martensitic medium-carbon steels
2022, Materials Science and Engineering: ACitation Excerpt :In order to introduce H in the steel microstructure, the samples are electrochemically charged by applying a current density of 0.8 mA/cm2 in a 0.5 M H2SO4 solution with 1 g/l thiourea. In previous work, it was shown that a current density of 0.8 mA/cm2 does not result in internal damage for generic medium-carbon martensitic steels [27,39]. To investigate the H uptake capacity of the materials after different charging time intervals, melt extraction is applied.
The effect of an Al-induced ferritic microfilm on the hydrogen embrittlement mechanism in martensitic steels
2022, Materials Science and Engineering: ACitation Excerpt :For this study, steel samples with a dominant martensitic microstructure are used. Martensite is not only the strongest microstructure constituent achievable with steels, but also considered to be the most susceptible to H embrittlement, and its interaction with H has been the subject of numerous scientific studies [13–16]. H is mainly trapped at the high angle grain boundaries (i.e. prior austenitic grain boundaries (PAGBs)), martensitic packet boundaries and martensitic block boundaries [14].
Evaluation of the cementite morphology influence on the hydrogen induced crack nucleation and propagation path in carbon steels
2022, International Journal of Hydrogen EnergyInvestigation of the effect of carbon on the reversible hydrogen trapping behavior in lab-cast martensitic Fe–C steels
2022, Materials CharacterizationCitation Excerpt :Where ɸ is the heating rate (°C/h), Tmax is the TDS peak temperature (°C), Ea is the detrapping activation energy (J/mol) and R is the universal gas constant (J K−1 mol−1). The microstructure of the used FeC alloys was already analyzed in detail in a previous study by the same authors where the mechanical degradation of the FeC alloys with and without hydrogen was studied [8]. In this study, special attention will be put on the effect of the carbon content on the microstructural characteristics and how this influences the hydrogen trapping behavior in martensitic FeC steels.