Evaluation of hydrogen trapping and diffusion in two cold worked CrMo(V) steel grades by means of the electrochemical hydrogen permeation technique
Graphical abstract
Introduction
Hydrogen embrittlement (HE) is a phenomenon responsible for premature failure of steel structures leading to the total loss of the structural integrity [1]. For the impending new hydrogen society, vessels and pipelines used to store and transport hydrogen must be able to provide a safe service during long periods of time in direct contact with hydrogen under high internal pressure, being then essential to ensure good resistance to hydrogen embrittlement (HE). Although Cr-Mo and Cr-Mo-V steel grades are commonly used in these type of facilities [1], it is necessary to understand the complex embrittlement phenomena that take place when hydrogen diffuses into these metallic components submitted to static and cyclic mechanical loads under hydrogen environments. In this regard, hydrogen diffusion through a steel wall is dominated by specific microstructural sites, called traps, which have hydrogen binding energies higher than regular interstitial sites (lattice sites). Accordingly, the definition of the different hydrogen states in steel (lattice hydrogen, reversible trapped and irreversible trapped hydrogen) is necessary to discuss the impact of hydrogen on damage [2], [3]. Hence, three type of sites (lattice, reversible and irreversible sites) can be distinguished in the steel microstructure as a function of the trapping energy. Although hydrogen reversibility process within the steel microstructure depends on the concentration and temperature, irreversible trapping character can be assumed when detrapping energy is higher than 30 kJ/mol [2], [4], [5], [6], [7]. Fig. 1 illustrates the detrapping activation energy, ‘ΔETL’, for each trapping site according to literature data.
Weak and medium energy trapping sites (reversible traps) can contribute to delay hydrogen transport towards the fracture process zone. Nevertheless, these trapping sites can act as a reservoir of diffusible hydrogen when they are saturated and no lattice hydrogen is available. Taking into account that hydrogen embrittlement process is dependent on the time, stress gradient (hydrostatic stress) and temperature [3], [4], [22], [23], [24], diffusible hydrogen (lattice hydrogen and reversible trapped hydrogen) can redistribute (trapping/de-trapping) within the steel microstructure during the service life of the structural components. Competition between trapping and de-trapping phenomena (diffusible hydrogen redistribution) is illustrated in Fig. 2. Indeed, de-trapped hydrogen atoms, motivated by the existing high hydrostatic stress, can diffuse toward a damage process zone (notch area, Fig. 2), promoting hydrogen embrittlement when a critical hydrogen concentration is reached, following the Oriani’s theory [25] according to equation 1, where: ‘CH-notch’ is the hydrogen concentration in the vicinity of the notch, ‘HD’ the diffusible hydrogen content (lattice and reversible hydrogen), ‘σH’ is the hydrostatic stress developed in the notch region, ‘VH’ is the partial molar volume of hydrogen in BCC Fe (VH = 2.1·10-6 m3/mol), ‘R’ the gas constant and ‘T’ is the testing temperature.
On the other hand, irreversible hydrogen traps (high energy traps, see Fig. 1) homogeneously distributed within the steel microstructure can notably contribute to delay hydrogen diffusion towards the mentioned damage process zone. Therefore, hydrogen accumulation on the process zone might be limited, contributing to improve the steel behavior in presence of internal hydrogen [3], [5], [23], [24]. At this respect, the addition of vanadium to chromium-molybdenum steels to estimulate the precipitation of vanadium carbides during the tempering was demonstrated to be a good practice in order to fight against hydrogen embrittlement [3], [5], [7]. Different authors have proved that vanadium carbides act as strong hydrogen traps [6], [7]. This highlights that the hydrogen states (i.e. type of hydrogen traps: weak, medium and high energy traps) are key factors for the hydrogen damage process, underlying the important role that microstructural units play on hydrogen embrittlement process.
Therefore, in order to study the interaction of hydrogen atoms with the steel microstructure, the purpose of this paper is to analyse hydrogen diffusion and trapping processes through two quenched and tempered low alloy steel grades (2.25Cr1Mo and 2.25Cr1MoV). The novelty of the work is addressed to the focus on the effect of plastic deformation rate (dislocations density) on hydrogen diffusion kinetics of different industrial steel grades, evaluating at the same time, the vanadium carbides effect (precipitated during the tempering treatments) on the hydrogen trapping and diffusion.
Section snippets
Materials, applied heat treatments and cold rolling process
Two different low-alloyed ferritic steels from the Cr-Mo family have been selected in this study, with or without vanadium. Table 1 shows the chemical composition of the steel grades. Pressure vessels employed in the petrochemical industry for hydrogen services are commonly manufactured with Cr-Mo-V steel grades. In order to achieve a good combination of strength and toughness, these grades of steel are commonly used in quenched and tempered conditions [1].
Heat treatments were applied on blocks
Microstructure characterization
The obtained microstructures of the 2.25Cr1Mo and 2.25Cr1MoV steels after the heat treatments described in Table 2 are shown in Fig. 7. They correspond to tempered martensite. The steel grades were composed of martensite laths, with similar average thickness, around 1 µm. Prior austenite grain boundary was around 25 µm. The profuse carbides precipitation that takes place during the tempering stage can be also seen. Fe-Cr-Mo mixed carbides, belonging to M7C3, M2C and M23C6 were identified in the
Discussion
Experimental results obtained from the hydrogen permeation tests, highlight that the microstructure and cold-work influenced the hydrogen diffusion and trapping processes. In order to discuss the hydrogen diffusion kinetics behavior observed in the 2.25Cr1Mo (V-free) and 2.25Cr1MoV (V-added) steel grades, thermal desorption analysis (TDA) analysis was also conducted. Therefore, hydrogen diffusion and trapping can be discussed on the basis of microstructural observations, results from TDA
Conclusions
This work evaluated hydrogen trapping and diffusion kinetics of 2.25Cr1Mo(V) steel grades subjected to different cold-plastic deformation ratios. Based on the results from a wide experimental campaign, these conclusions can be drawn:
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Plastic deformation, induced by means of cold-rolling process, contributes to delay the hydrogen diffusion kinetics, mainly thanks to the increase in the density of traps (i.e. dislocations density). However, the steady-state hydrogen current density remains
Author statement
All authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, data analysis, writing, or revision of the manuscript.
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Conception and design of study: I.Fernández-Pariente and L.B.Peral.
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Acquisition of data: L.B.Peral and Z.Amghouz.
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Analysis and/or interpretation of data: I.Fernández-Pariente and L.B.Peral.
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Drafting the manuscript: L.B.Peral, C.Colombo and I.Fernández-Pariente.
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Revising the
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgments
The authors would like to thank financial aid given by FEDER, the Asturias government, through the project FC-15-GRUPIN14-001 and Spanish MINECO (MAT2016-78155C2-1-R).
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