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Medium Mn steels have been considered as the next-generation materials for use in the automotive industry due to their excellent strength and ductility balance. To reduce the total weight and improve the safety of vehicles, medium Mn steels look forward to a highly promising future. However, hydrogen-induced delayed cracking is a concern for the use of high strength steels. This work is focused on the service characteristics of two kinds of medium Mn steels under different relative humidity conditions (40%, 60%, 80% and 100%). Under normal relative humidity (about 40%) at 25 °C, the hydrogen concentration in steel is 0.4 ppm. When exposed to higher relative humidity, the hydrogen concentration in steel increases slowly and reaches a stable value, about 0.8 ppm. In slow strain rate tensile tests under different relative humidity conditions, the tensile strength changed, the hydrogen concentration increased and the elongation decreased as well, thereby increasing the hydrogen embrittlement sensitivity. In other words, the smaller the tensile rate applied, the greater the hydrogen embrittlement sensitivity. In constant load tests under different relative humidity conditions, the threshold value of the delayed cracking of M7B ('M' referring to Mn, '7' meaning the content of Mn, 'B' denoting batch annealing) steel maintains a steady value of 0.82 σb (tensile strength). The threshold value of the delayed cracking of M10B significantly changed along with relative humidity. When relative humidity increased from 60% to 80%, the threshold dropped sharply from 0.63 σb to 0.52 σb. We define 80% relative humidity as the 'threshold humidity' for M10B. [ABSTRACT FROM AUTHOR]

The effect of hydrogen on mechanical properties of a twinning-induced plasticity austenitic steel was investigated by slow strain rate tensile tests and scanning electron microscopy after various hydrogen pre-charging conditions. A new method for evaluating hydrogen embrittlement of materials with low hydrogen diffusion coefficient has been proposed. Materials with low hydrogen diffusion coefficients can be regarded as composite materials after pre-charging, i.e., a hydrogen-rich brittle region and a hydrogen-lean ductile region. Based on the rule of mixtures, the influence of the hydrogen on the mechanical properties can be reflected by the contribution of the brittle zone to the overall mechanical properties, which is more realistic and universal compared to the characterization of the mechanical property loss by hydrogen content. This helps to simplify the selection of materials resistant to hydrogen embrittlement by directly comparison linear loss of mechanical properties when the "average" hydrogen content of the brittle region is similar. [Display omitted] • TWIP steel after hydrogen pre-charging can be regarded as composite material. • The contribution of brittle zone to the overall performance is basically linear. • Evaluation of HE sensitivity based on composite effect is closer to reality. • This novel method can evaluate the HE resistance of materials in different reports. [ABSTRACT FROM AUTHOR]

In this paper, extensive first-principle calculations were carried out to study the effect of the segregation of 16 alloying elements with different concentrations at α -Fe Σ 3(111)[110] Grain Boundary (GB) on the GB strength, trapping of H atoms and hydrogen-induced embrittlement of the GB. Studies shows: (i) The factors that control the segregation of alloy elements are mainly the GB microstructure (loose and compressed sites) and the radius of the alloy elements. (ii) The Electron localization function (ELF) values can be used as a key descriptor to evaluate the GB strengthening. (iii) Based on a new strengthening descriptor and the electronic structure between H and matrix, we elucidate the physical origin of H enrichment, H-induced decohesion and H-enhanced localized plasticity at GB. Combined with the strengthening descriptor and obtained database, we have screened out the HE-resistant alloying elements (B, C, Mo and W) for Fe GB. The available experimental data also show that the presence of B, C, Mo or W atom leads to increase of intergranular cohesion and HE-resistant. Because of the bonding mechanism independence, this study could be extended to more general GBs and provide profound guidelines for the design of HE-resistant high strength steels. [Display omitted] • Segregation behavior of alloying elements and H at GB. • A key descriptor to evaluate the GB strengthening and H enrichment. • The physical origin of H enrichment at GBs. • The B, C, Mo and W can enhance the GB strengthening and inhibit H enrichment. [ABSTRACT FROM AUTHOR]

The infiltration of hydrogen can weaken the strength and toughness of materials, especially in high-strength steel. Considering the inherent trade-off between strength and toughness in steel design, balancing these two factors with the material's hydrogen embrittlement sensitivity has become a huge challenge in steel design. The long cycle nature of hydrogen embrittlement testing makes the iteration of traditional trial and error methods infeasible for steel development. This work adopts active learning algorithms based on Bayesian and Pareto optimization to iteratively optimize the chemical composition and processing technology of high-strength steel, in order to simultaneously improve its hydrogen filled tensile strength (UTS_H) and hydrogen filled total elongation (TEL_H). After two rounds of iterative optimization, the iterative optimization algorithm significantly improved the strength prediction of materials after hydrogen charging. The machine learning model found 16 types of steel from over 15 million unknown materials. Eight materials showed better comprehensive performance than known data, with one material having a tensile strength exceeding 1800 MPa and an elongation of 12.6%. Subsequent experiments showed that the optimization algorithm demonstrated an understanding of the impact of austenite content on hydrogen embrittlement sensitivity in multiple experiments, and designed high-strength steel resistant to hydrogen embrittlement through the austenite content of martensitic steel. • ML model accurately screens high-strength steels for hydrogen embrittlement resistance. • Iterative use of Bayesian modeling to minimize trial cycle time. • 8 excellent hydrogen embrittlement-resistant materials are screened from 1500 candidate spaces through 2 rounds of iteration. [ABSTRACT FROM AUTHOR]

The effect of hydrogen on the twinning evolution in a twinning-induced plasticity (TWIP) austenitic steel was investigated at different strain levels. The presence of hydrogen decreased the twin thickness and increased the twin density in individual twin bundles, in comparison with the hydrogen-free condition. This mechanism, reported for the first time, was termed hydrogen-enhanced densified twinning (HEDT). Based on these experimental observations, the effect of hydrogen on twinning was analyzed from the three stages of nucleation, growth and stability. HEDT may elevate the local stress, and thus significantly affect the hydrogen embrittlement (HE) of TWIP steels. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This study investigated the effects of hydrostatic compressive stress in retained austenite (RA) on mechanical properties and hydrogen embrittlement (HE) of the martensitic steels. It was found that a high hydrostatic compressive stress in RA, which was produced by the martensitic transformation, lead to an improvement of ductility for the steels without hydrogen. However, the mechanical properties were different after hydrogen charging. The low hydrostatic compressive stress in RA had little effect on HE resistance, but the high hydrostatic compressive stress (231 MPa) induced the twinning of RA and resulted in the aggravation of the HE sensitivity. Moreover, the detwinning of RA via tempering improved the HE resistance. • The nanotwins retained austenite formed via sub-zero treatment. • High hydrostatic stress leads to an excellent ductility without hydrogen. • The nanotwins retained austenite steel had low hydrogen embrittlement resistance. [ABSTRACT FROM AUTHOR]

The twinning and hydrogen embrittlement (HE) behavior of twinning-induced plasticity (TWIP) steels with various gradient twin structures were investigated by slow strain rate tensile testing. The results revealed that the gradient design can be very beneficial for alleviating the strength-ductility trade-off dilemma in TWIP steels, especially under hydrogen environment. Among the different structures after cathodic hydrogen charging, the most outstanding was the inverse gradient-like structure material, which comprehensively utilized fine recrystallized grains at sample's edge region to effectively lower HE (EI loss : 4.01% and UTS loss : 1.61%) and gradient-design of the central coarse grain to improve the comprehensive mechanical properties. [Display omitted] • Gradient design can make steels have superior comprehensive mechanical properties. • The greater the degree of pre-torsion, the better the HE resistance. • Inverse gradient structure can be obtained by heat treatment after pre-torsion. • Inverse gradient structure materials exhibit the best HE resistance. • Refined grains are more potent for HE resistance vs increased twin volume fraction. [ABSTRACT FROM AUTHOR]

The effect of deformation twins on the hydrogen embrittlement (HE) of a twinning-induced plasticity (TWIP) steel with various pre-strain levels, coupled with subsequent annealing treatment was systematically investigated by a slow strain rate tension test after cathodic hydrogen charging. The results revealed that specimens with high twin volume fraction had relatively better HE resistance. The high-density twin boundary not only plays an important role in hindering hydrogen diffusion, but also can refine the grains and inhibit the accumulation of hydrogen, so as to relieve stress concentration and avoid premature fracture of the specimen. [Display omitted] • The larger the pre-strain, the higher the strength, and the better the HE resistance. • Sample annealing after high pre-strain still maintains good HE resistance. • Twins have an advantage over dislocations in hindering hydrogen diffusion. • High density of twin boundaries can disperse hydrogen to prevent strain localization. [ABSTRACT FROM AUTHOR]

The twinning behavior and hydrogen embrittlement (HE) of a twinning-induced plasticity (TWIP) steel were investigated for different pre-strains at different strain rates. The lower pre-strain rate led to thinner twins and higher twin volume fraction. The influence of the twin characteristics on HE sensitivity was analyzed from statistical data on the evolution of surface crack characteristics during secondary straining period after cathodic hydrogen charging. Based on the ratio of crack initiation sites caused by the intersection of twins and grain boundaries, it was found that thinner twins in twin bundles of similar thickness promoted the generation of hydrogen-induced cracks (HICs). [Display omitted] • Lower strain rate led to thinner twins and higher twin volume fraction. • Specimens pre-strained with higher strain rate possessed greater resistance for HE. • The statistical evolution of secondary cracks was investigated. • Thinner twins are more likely to cause hydrogen-induced cracking. [ABSTRACT FROM AUTHOR]

• The hydrogen behavior of 13Cr-5Ni-2Mo supermartensitic stainless steel was comprehensively investigated through two aspects. • Precipitated austenite can increase the hydrogen embrittlement resistance of the martensitic matrix. • A mechanism of the role of precipitated austenite on HE behavior under different hydrogen concentrations is proposed. • The higher hydrogen adsorption in 13Cr supermartensitic stainless steel leads to a lower hydrogen resistance compare to that of 960QC steel. • We propose a few strategies to drive high strength hydrogen resistant steels. This work presents a comprehensive study of the hydrogen embrittlement (HE) behavior of a 13Cr-5Ni-2Mo supermartensitic stainless steel through two aspects: hydrogen diffusion and hydrogen concentration. Precipitated austenite reduced the effective hydrogen diffusion coefficient and hydrogen permeation flux, and thus increased the HE resistance. Further increasing the austenite content could not provide additional HE resistance as a martensitic transformation is prone to occur when a high hydrogen concentration and stress are coupled, leading to hydrogen induced crack initiation. The HE susceptibility of 13Cr-5Ni-2Mo steel is much higher than that of 960QC steel because of the higher adsorbing hydrogen concentration. [ABSTRACT FROM AUTHOR]

A 13Cr austenitic stainless steel with a good combination of high yield strength, ductility and hydrogen embrittlement resistance was designed by combining hetero-deformation induced strengthening and martensitic transformation induced plasticity effects. Room temperature metastable austenite is promoted by adding 8 wt% Mn to a 13Cr–5Ni–2Mo supermartensitic stainless steel. A heterostructure consisting of remaining martensite grains located between lath austenite grains and micrometer-sized austenite grains embedded inside ultrafine grains, was fabricated by cold rolling (44% reduction) and annealing at 973 K through a shear reversion process. The novel 13Cr austenitic stainless steel exhibits a yield strength of 923 MPa, tensile strength of 1085 MPa and total elongation of 33.2%. The hydrogen embrittlement resistance of the novel steel was determined by incremental step loading technique testing and was increased 40% compared with that of the 13Cr–5Ni–2Mo supermartensitic stainless steel. This study provides an approach for the design and industrial fabrication of high strength (125 ksi grade level), high ductility and hydrogen embrittlement resistant stainless steels that can be used in harsh service environments containing hydrogen. [ABSTRACT FROM AUTHOR]

• Austenite content is the most influential factor on the hydrogen diffusion rate. • Dissolved H in austenite can diffuse to the interface during the transformation. • Hydrogen promotes local deformation of ferrite by promoting dislocation emission. To balance good strength and ductility, Medium Mn steels have been considered as the new generation materials used in automotives. However, they show delayed fracture, which is thought to relate to hydrogen. Therefore, in order to understand their mechanical properties with various hydrogen contents, it is very important to assess their safety. In this study, the effect of hydrogen on the mechanical properties of several medium Mn steels (MMSs) with different Mn content and treatments were investigated. For medium Mn steels, the stability of the retained austenite (RA) was examined and the hydrogen permeation test was performed. Austenite, as a barrier to hydrogen diffusion, leads to a tortuous diffusion path, which reduces the diffusion rate of hydrogen in MMSs. Increasing the Mn content increases the volume fraction of RA in the specimen. The stress-strain curves of MMSs were divided into three parts, determined by the stability and content of RA. Slow strain rate tensile tests with hydrogen charging were performed to investigate the behaviour of hydrogen diffusion and the mechanism of hydrogen embrittlement (HE). After the tests, the hydrogen content was measured and the fracture surface was observed and analysed. The phenomenon of HE in steels has been mainly explained based on the mechanisms of hydrogen-enhanced localised plasticity (HELP), hydrogen-enhanced decohesion (HEDE), and hydrogen assisted micro ovoid coalescence (HDMC). Due to the existence of hydrogen, fracture morphology changes from ductile mode to brittle mode. It was also found that under the same strain rate, tensile strength and elongation decrease gradually as hydrogen concentration increases, resulting in final failure. [ABSTRACT FROM AUTHOR]