Hydrogen embrittlement of high-strength steel is an obstacle for using these steels in sustainable energy production. Hydrogen embrittlement involves hydrogen-defect interactions at multiple-length scales. However, the challenge of measuring the precise location of hydrogen atoms limits our understanding. Thermal desorption spectroscopy can identify hydrogen retention or trapping, but data cannot be easily linked to the relative contributions of different microstructural features. We used cryo-transfer atom probe tomography to observe hydrogen at specific microstructural features in steels. Direct observation of hydrogen at carbon-rich dislocations and grain boundaries provides validation for embrittlement models. Hydrogen observed at an incoherent interface between niobium carbides and the surrounding steel provides direct evidence that these incoherent boundaries can act as trapping sites. This information is vital for designing embrittlement-resistant steels.

Observation of hydrogen trapping at dislocations, grain boundaries, and precipitates

Hydrogen embrittlement is a complex phenomenon, involving several length- and timescales, that affects a large class of metals. It can significantly reduce the ductility and load-bearing capacity and cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Despite a large research effort in attempting to understand the mechanisms of failure and in developing potential mitigating solutions, hydrogen embrittlement mechanisms are still not completely understood. There are controversial opinions in the literature regarding the underlying mechanisms and related experimental evidence supporting each of these theories. The aim of this paper is to provide a detailed review up to the current state of the art on the effect of hydrogen on the degradation of metals, with a particular focus on steels. Here, we describe the effect of hydrogen in steels from the atomistic to the continuum scale by reporting theoretical evidence supported by quantum calculation and modern experimental characterisation methods, macroscopic effects that influence the mechanical properties of steels and established damaging mechanisms for the embrittlement of steels. Furthermore, we give an insight into current approaches and new mitigation strategies used to design new steels resistant to hydrogen embrittlement.

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Understanding and mitigating hydrogen embrittlement of steels: a review of experimental, modelling and design progress from atomistic to continuum.

The design of atomic-scale microstructural traps to limit the diffusion of hydrogen is one key strategy in the development of hydrogen-embrittlement–resistant materials. In the case of bearing steels, an effective trapping mechanism may be the incorporation of finely dispersed V-Mo-Nb carbides in a ferrite matrix. First, we charged a ferritic steel with deuterium by means of electrolytic loading to achieve a high hydrogen concentration. We then immobilized it in the microstructure with a cryogenic transfer protocol before atom probe tomography (APT) analysis. Using APT, we show trapping of hydrogen within the core of these carbides with quantitative composition profiles. Furthermore, with this method the experiment can be feasibly replicated in any APT-equipped laboratory by using a simple cold chain.

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Direct observation of individual hydrogen atoms at trapping sites in a ferritic steel

Hydrogen embrittlement of lath martenistic steels is characterized by intergranular and “quasi-cleavage” transgranular fracture. Recent transmission electron microscopy (TEM) analyses (Nagao et al., 2012a, 2014a, 2014b, 2014c) of samples lifted from beneath fracture surfaces through focused ion beam machining (FIB) revealed a failure mechanism that can be termed hydrogen-enhanced-plasticity mediated decohesion. Fracture occurs by the synergistic action of the hydrogen-enhanced localized plasticity and decohesion. In particular, intergranular cracking takes place by dislocation pile-ups impinging on prior austenite grain boundaries and “quasi-cleavage” is the case when dislocation pile-ups impinge on block boundaries. These high-angle boundaries, which have already weakened by the presence of hydrogen, debond by the pile-up stresses. The micromechanical model of Novak et al. (2010) is used to quantitatively describe and predict the hydrogen-induced failure of these steels. The model predictions verify that introduction of nanosized (Ti,Mo)C precipitates in the steel microstructure enhances the resistance to hydrogen embrittlement. The results are used to discuss microstructural designs that are less susceptible to hydrogen-induced failure in systems with fixed hydrogen content (closed systems).

Hydrogen-enhanced-plasticity mediated decohesion for hydrogen-induced intergranular and “quasi-cleavage” fracture of lath martensitic steels

The effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel

Hydrogen trapping and de-trapping behavior was investigated for steels with and without V. The de-trapping of hydrogen is very slow while the trapping presumably proceeds rapidly for steels containing VC precipitates. The activation energy for de-trapping is in the range of 33 to 35 kJ/mol. The trapped-hydrogen content and diffusible-hydrogen content in the steady state increase with increasing hydrogen entry rate into the steel. The density of hydrogen trapping sites decides the maximum trapped-hydrogen content; 9 ppm for 1 % V steel tempered at peak secondary hardening temperature. Analysis of hydrogen embrittlement cracking tests in terms of hydrogen contents such as the critical hydrogen content should be performed on the specimens with uniform hydrogen distribution and must consider the nature of hydrogen whether it is trapped or diffusible.

Hydrogen Trapping Behavior in Vanadium-added Steel

Thermal desorption analyses (TDA) were conducted in high strength martensitic steels containing carbon from 0.33 to 1.0 mass pct, which were charged with hydrogen at 1223 K (950 °C) under hydrogen of one atmospheric pressure and quenched to room temperature. In 0.33C steel, which had the highest Ms temperature, only one desorption peak was observed around 373 K (100 °C), whereas two peaks, one at a similar temperature and the other around and above 573 K (300 °C), were observed in the other steels, the height of the second peak increasing with carbon content. In 0.82C steel, both peaks disappeared during exposure at room temperature in 1 week, whereas the peak heights decreased gradually over 2 weeks in specimens electrolytically charged with hydrogen and aged for varying times at room temperature. From computer simulation, by means of the McNabb–Foster theory coupled with theories of carbon segregation, these peaks are likely to be due to trapping of hydrogen in the strain fields and cores of dislocations, and presumably to a lesser extent in prior austenite grain boundaries. The results also indicate that carbon atoms prevent and even expel hydrogen from trapping sites during quenching and aging in these steels.

Thermal Desorption Analysis of Hydrogen in High Strength Martensitic Steels

A computer model was developed to simulate the thermal desorption flux of hydrogen from steel specimens by an explicit finite difference method, which is convergent and stable for all time increment The influences of experimental parameters, eg ramp rate, specimen size and duration of exposure at ambient temperature etc, on the desorption spectrum, concentration profile of hydrogen in the lattice and fractional occupancy at the trapping sites, were studied assuming that the major trapping sites were dislocations The increase in ramp rate caused a marked shift in the desorption peak to a higher temperature, whilst the increase in the specimen size shifted the peak to higher temperatures to a moderate extent The duration of exposure may have a considerable influence on the thermal desorption spectra when the desorption peak is associated with a lower binding energy trap site

Modeling thermal desorption analysis of hydrogen in steel

A steel which is excellent in delayed fracture resistance containing, by mass%, C: 0.10 to 0.55%, Si: 0.01 to 3%, and Mn: 0.1 to 2%, further containing one or more of Cr: 0.05 to 1.5%, V: 0.05 to 0.2%, Mo: 0.05 to 0.4%, Nb: 0.001 to 0.05%, Cu: 0.01 to 4%, Ni: 0.01 to 4%, and B: 0.0001 to 0.005%, and having a balance of Fe and unavoidable impurities, the structure being a mainly tempered martensite structure, the surface of the steel being formed with (a) a nitrided layer having a thickness from the surface of the steel of 200 µm or more and a nitrogen concentration of 12.0 mass% or less and higher than the nitrogen concentration of the steel by 0.02 mass% or more and (b) a low carbon region having a depth from the surface of the steel of 100 µm or more to 1000 µm or less and having a carbon concentration of 0.05 mass% or more and 0.9 time or less the carbon concentration of the steel.

High-strength steel and high-strength bolt with excellent resistance to delayed fracture, and manufacturing method therefor

1) Steel Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 1-8, Fuso-cho, Amagasaki, Japan. 2) Formerly Sophia University. 3) Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japan. 4) Neturen Co., Ltd., 1586-1, Higashi-une, Ako, Hyogo, 678-1185 Japan. 5) Steel Research Laboratory, JFE Steel Corporation, 1-1, Minamiwatarida-cho, Kawasaki-ku, Kawasaki, 210-0855 Japan. 6) National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047 Japan. 7) Mitsubishi Steel Mfg. Co., Ltd., 1-6, Yawatakaigandori, Ichihara, Chiba, 290-0067 Japan. 8) Steel Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 20-1, Shintomi, Futtsu, Chiba, 293-8511 Japan. 9) Nippon Steel & Sumikin Technology Co., Ltd., 12 Nakamachi, Muroran, 050-0087 Japan.

https://www.jstage.jst.go.jp/article/isijinternational/56/7/56_ISIJINT-2015-565/_pdf

Daisuke Hirakami

Papers

Hydrogen Trapping Behavior in Vanadium-added Steel

Thermal Desorption Analysis of Hydrogen in High Strength Martensitic Steels

Modeling thermal desorption analysis of hydrogen in steel

High-strength steel and high-strength bolt with excellent resistance to delayed fracture, and manufacturing method therefor

Comparison of Constant Load, SSRT and CSRT Methods for Hydrogen Embrittlement Evaluation Using Round Bar Specimens of High Strength Steels