Evaluation of hydrogen trapping and diffusion in two cold worked CrMo(V) steel grades by means of the electrochemical hydrogen permeation technique

Highlights

• Hydrogen diffusion coefficient decreases after cold-work.

• Dislocations core act as strong trapping sites for hydrogen atoms.

• V-addition contributes to reduce hydrogen diffusion coefficient.

• Hydrogen de-trapping probability can be reduced due to the V-addition.

• Nanometric Mo-V carbides act as strong traps for hydrogen.

Abstract

Hydrogen diffusion kinetics, which is influenced by the hydrogen trapping and de-trapping phenomena within the steel microstructure, plays an important role on the behaviour of steel components under hydrogen environments. Hence, the complex interaction between hydrogen atoms and steel microstructure must be analyzed in order to discuss the impact of hydrogen on the structural damage.

Quenched and tempered low-alloy ferritic steels from the Cr-Mo family, with and without vanadium, have been subjected to different plastic deformation ratios by cold rolling. Dislocation densities have been determined by the analysis of the peak broadening on X-Ray diffractograms. Hydrogen diffusion kinetics was characterized by means of hydrogen permeation transients. In addition, binding energies between hydrogen atoms and microstructure were also determined using thermal desorption analysis (TDA). The analysis of the results highlights the influence of dislocations density and vanadium carbides on the hydrogen diffusion kinetics. In the 2.25Cr1Mo steel grade, hydrogen apparent diffusion coefficient decreased after the cold-work due to the increase in the density of traps (mainly related to dislocation core, ΔE_TL = 55–60 kJ/mol). Nevertheless, after 10% of plastic deformation, apparent diffusion coefficient ‘saturates’ according to the ‘plateau’ determined in the dislocation density evolution at higher deformation levels. Due to the vanadium addition (+0.31%), hydrogen apparent diffusion coefficient was notably reduced (compared to that obtained in the V-free steel grade). Hydrogen trapping and diffusion are the result of the interplay between vanadium carbides (ΔE_TL = 35 kJ/mol) and dislocation core.

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, ‘ΔE_TL^’, 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: ‘C_H-notch’ is the hydrogen concentration in the vicinity of the notch, ‘H_D’ the diffusible hydrogen content (lattice and reversible hydrogen), ‘σ_H’ is the hydrostatic stress developed in the notch region, ‘V_H’ is the partial molar volume of hydrogen in BCC Fe (V_H = 2.1·10^-6 m³/mol), ‘R’ the gas constant and ‘T’ is the testing temperature.

$${\text{C}}_{\text{H}-\text{notch}}={\text{H}}_{\text{D}}\text{exp}\left(\frac{{\sigma }_{\text{H}}·{\text{V}}_{\text{H}}}{\text{R}·\text{T}}\right)$$

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.