Search

In interstitial-free (IF) steel with a certain microtexture, the micro-orientation of grains is essential to understand the occurrence of hydrogen-induced cracking in body-centered cubic (BCC) structural steels. In this study, the hydrogen embrittlement (HE) susceptibility of IF steels was determined by slow strain rate tensile (SSRT) tests and hydrogen microprinting (HMT) experiments from the perspective of crystal orientation. The strength of the specimen with hydrogen was slightly higher than that without hydrogen, while the ductility and toughness were drastically reduced by hydrogen charging during the SSRT test. The HE susceptibility was characterized by the loss of elongation (Iδ) and toughness (Iψ), with losses of 46.3% and 70%, respectively. The microstructural observations indicate that cracks initiated along grains oriented in the {100} || normal direction (ND), and grain boundaries (GBs) around {100}||ND were prone to be enriched in hydrogen atoms; that is, {100} || ND showed poor resistance to intergranular cracking and susceptible to hydrogen segregation. HMT was used to confirm the above viewpoints. Meanwhile, the statistical results showed those high-angle misorientations of 50–60° deviation are the locations most vulnerable to fracture. [ABSTRACT FROM AUTHOR]

The resistance of the Al0.5Cr0.9FeNi2.5V0.2 high-entropy alloy (HEA) to hydrogen embrittlement (HE) was investigated by a slow strain rate test (SSRT), and the fracture surface was examined through a scanning electron microscope. Compared with other high-strength steels, Al0.5Cr0.9FeNi2.5V0.2 showed insignificant strength loss after hydrogen charging. The fracture surface of the hydrogen-charged specimens mainly consisted of dimples, and no intergranular morphology was observed. The coupling effect of the dispersed nano-structured precipitates and high-density dislocations in Al0.5Cr0.9FeNi2.5V0.2 improves the resistance to hydrogen-induced strength loss. [ABSTRACT FROM AUTHOR]

The primary task of automotive industry materials is to guarantee passengers' safety during a car crash. To simulate a car crash, the influence of strain rates on mechanical properties and fracture behavior of medium Mn steels with different Si content (0Si without δ-ferrite and 0.6Si with about 20% δ-ferrite) was conducted using the uniaxial tensile test. The results show that ultimate tensile strength is higher, whereas total elongation is lower in 0Si than in 0.6Si. As the strain rate increases, ultimate tensile strength and total elongation decrease in both 0Si and 0.6Si; nonetheless, total elongation of 0.6Si decreases faster. Meanwhile, the area reduction of 0.6Si increases as the strain rate increases. The microcrack′s number on a rolling direction (RD)-transverse direction (TD) surface is considerably increased; nonetheless, the microcrack′s size is restrained in 0.6Si compared with 0Si. Microcracks start at γ(α′)/α-ferrite interfaces in both 0Si and 0.6Si, whereas little nucleation sites have also been found at (γ(α′)+α-ferrite)/δ-ferrite boundaries in 0.6Si. Meanwhile, δ-ferrite reveals a higher capacity for microcrack arrest. As the strain rate decreases, increased lower crack growth results in fine and even dimples on fractographs with abundant second cracks on fractographs; meanwhile, the small microcrack′s number increases, while the large microcrack′s number decreases on an RD-TD surface. [ABSTRACT FROM AUTHOR]

Hot-stamped steel has been widely used in automobile bumper and other safety components due to its high strength. Therefore, this paper investigates the effect of hydrogen content and strain rate on hydrogen-induced delay cracking (HIDC) behavior. The results showed that the plasticity of the steel significantly decreased with an increase in hydrogen content or a decrease in the strain rate. Fractography was analyzed after tensile tests. It was found that all of the pre-charged specimens cracked at large-sized inclusions when stretched at a strain rate of 1 × 10−3 s−1, which indicates that, in this case, the defect itself in the material had great influence on the extend properties. No inclusions were found at the main fracture origin area for hydrogen steady-state specimens, when stretched at a strain rate of 1 × 10−6 s−1, which demonstrated that a slower strain rate causes greater influence by hydrogen. However, for the non-pre-charged samples, the fractures surface showed that cracking originated from the defect near the sample surface, which was independent of strain rates. [ABSTRACT FROM AUTHOR]

The hydrogen embrittlement of 15Cr martensitic stainless steel, for steam turbine last stage blades, was systematically studied by using slow strain rate tensile (SSRT) test and constant loading tensile (CLT) test at room temperature and 80 °C to simulate the service conditions. It was shown that, despite the lower hydrogen concentration absorbed during SSRT, the hydrogen-induced fracture strength of 15Cr steel for SSRT was lower than the threshold fracture strength for CLT. This was due to the remarkable enhancement in local hydrogen concentration due to the transportation of hydrogen by mobile dislocation during SSRT. In addition, although the higher hydrogen concentration was absorbed during SSRT at 80 °C, the hydrogen embrittlement susceptibility of 15Cr steel for SSRT at 80 °C was lower than that at room temperature, because the degree of local hydrogen accumulation decreased at a higher temperature. • Hydrogen embrittlement behavior of 15Cr is studied at simulated service condition. • The risk of fracture is higher for slow strain rate test than for constant load test. • The accumulation degree of hydrogen at room temperature is greater than that at 80 °C. • Hydrogen embrittlement behavior is related to hydrogen accumulation. [ABSTRACT FROM AUTHOR]

This study investigates the hydrogen embrittlement (HE) behaviour of medium Mn steels with and without Nb precipitates under different strain rates. Through in-depth analysis of hydrogen diffusion, hydrogen trap sites, and hydrogen-induced cracking, the underlying reason for the inconsistent improvement of HE resistance induced by precipitates was unveiled. Notably, Nb precipitates did not improve HE resistance during slow strain rate tensile testing (SSRT) at low strain rates. The effectiveness of Nb precipitates in mitigating HE depends on hydrogen diffusion and hydrogen distribution, and especially the abilities of precipitates to reduce hydrogen accumulation at phase interfaces. These findings provide significant insights for designing high-performance medium Mn steels with better HE resistance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Multifractal provides an effective method for quantitative analysis of PLC effect in medium manganese steel. • The nonlinear characteristics of local stress jump in PLC effect are consistent with multifractal dynamic system. • The chaos of plastic instability system of medium manganese steel depends on strain rate and austenite content. The instability of plastic flow in medium manganese steel was investigated through uniaxial tensile tests conducted at room temperature across various strain rates. The Portvin-Le Chatelier (PLC) effect was analyzed using statistical methods and a multifractal approach. The findings indicate that the zigzag stress–strain curves associated with instability plastic flow exhibit typical multifractal characteristics. Furthermore, the multifractal analysis quantifies the influence of strain rate and initial phase composition on the instability characteristics of the stress–strain curves. Specifically, it was observed that lower strain rates and austenite content correspond to a chaotic state, whereas higher strain rates and austenite content result in self-organizing critical dynamics. Finally, the underlying physical mechanisms driving these differing modes of behavior are discussed, with a focus on their relationship to strain rate and the initial austenitic phase fraction. [ABSTRACT FROM AUTHOR]

In this study, the effect of segregation bands on the hydrogen embrittlement susceptibility of QP1180 was examined by slow strain rate tensile tests, and it was shown that almost all hydrogen-induced cracks originated at the segregation bands, which eventually led to the fracture. In addition, the hydrogen distribution characteristic of QP1180 was also visualized by hydrogen microprinting tests, showing that the local hydrogen concentration of QP1180 is as follows: Segregation band > Retained austenite > Prior austenite grain boundaries > Block boundaries > Martensite > Ferrite. [ABSTRACT FROM AUTHOR]

This study investigated the equivalence in evaluating hydrogen-assisted fracture strength undergoing slow strain rate tensile (SSRT) and constant load tensile (CLT) for three precipitation-hardened martensitic stainless steels containing second-phase particles of different sizes. For both PH13–8Mo and 17–4PH with nano-sized precipitates, the fracture strength was higher for SSRT than for CLT. However, for 15–6PH with both nano-sized precipitates and large-sized carbides, cracks at the larger-sized carbide/matrix interface led to the lower fracture strength for SSRT. Furthermore, we concluded that the hydrogen-assisted threshold fracture strength of CLT could be estimated by using SSRT if no large-sized particles were in high-strength steels. • The hydrogen-assisted fracture strength evaluated by SSRT and CLT was compared. • The fracture strength was higher for SSRT than CLT if no large-sized particles in steels. • The strain interface around nanoprecipitates during SSRT improved the HE resistance. • The cracks at the carbide interface during SSRT led to a lower fracture strength than CLT. • The threshold fracture strength for CLT can be approximated by using the SSRT. [ABSTRACT FROM AUTHOR]

Heterogeneous plastic strain is essential for understanding hydrogen-induced crack (HIC) initiation in twining-induced plasticity (TWIP) steel with relatively no microtexture. In this study, electron backscattered diffraction technology was used to quantitatively characterize the heterogeneous plastic strain after a slow strain rate test with hydrogen charging, and the deformation mode and HIC initiation mechanism were clarified. From a statistical analysis of more than 100 grain boundaries, the cracked grain boundary and its vicinity have high heterogeneous strain; that is, the geometrically necessary dislocation density is high, and vice versa. Therefore, high strain incompatibility is the key factor for HIC initiation. [Display omitted] • The hydrogen-induced cracks (HICs) initiation depends on heterogeneous plastic. • The heterogeneous plastic strain existing between grains was quantitatively characterized. • The larger geometrically necessary dislocations is, the larger the heterogeneous strain is. [ABSTRACT FROM AUTHOR]

The initiation and propagation of hydrogen-induced cracks (HICs) in twining-induced plasticity (TWIP) steel were quantificationally explored based on the HIC statistics at a macro level and a micro level by slow strain rate tensile (SSRT) with in-situ hydrogen-charging and electron microanalysis. The heterogeneous deformation and grain boundary character determines a HIC initiation. The slip transfer parameter m ′ < 0.93 and the minimum residual Burgers vector of residual dislocations on a grain boundary b r > 0.19 were the critical condition for HIC initiation. Moreover, HICs prone to initiate and propagate along random grain boundaries (RGBs), while the Σ 3 grain boundaries and vice. [Display omitted] • The heterogeneous deformation character determines hydrogen-induced cracks (HICs) initiation. • The critical condition for HIC initiation were m′ < 0.93, b r > 0.19 and Miso. between 35–47°. • The HICs propagation is strongly dependent on grain boundary type, HICs prone to propagate along RGB, while Σ3 grain boundary and vice. • The RGB fractal character at a micro scale determine the HICs distribution at a macro scale. [ABSTRACT FROM AUTHOR]

The uptake and accumulation of hydrogen in materials during a service process is the primary prerequisite of hydrogen embrittlement. However, the actual hydrogen concentration that causes brittle fracture is unknown. In this study, slow strain rate tensile tests combined with finite element simulations (FESs) were used to study the influence of stress variation and initial hydrogen content C 0 (0.91, 1.70, 2.90, and 3.41 ppm) on the hydrogen redistribution and fracture behavior of smooth tensile specimens of Q960E steel for the first time. Furthermore, the actual threshold hydrogen content for hydrogen-delayed cracking was also obtained. Results showed that the presence of hydrogen significantly increases the elongation loss but has a minimal effect on strength loss. The occurrence of brittle cracking depends on the duration and the quantity of accumulated hydrogen. Brittle fracture occurs only at low strain rates under low C 0 of 0.91 and 1.70 ppm. However, a high strain rate can result in brittle fracture at high C 0 of 2.90 and 3.41 ppm. FES and fracture behavior indicate that the critical hydrogen content that causes brittle fracture is about 1.8 ppm. If the C 0 is less than 1.8 ppm, brittle fracture is also observed at the center of the specimen by stress-induced hydrogen diffusion to achieve this critical value at low strain rate. A logarithmic relationship is observed between C 0 and the brittle zone size with a constant strain rate. [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]

• The initiation of HICs is mainly dominated by planar dislocation slips at a slower strain rate for high-Mn TWIP/TRIP steels with the lower PVFM. • The initiation of HICs is mainly dominated by deformation twins at faster a strain rate for high-Mn TWIP/TRIP steels with the lower PVFM. • The inconsistent deformation of ε-martensite promotes the initiation of HICs at the two strain rate for high-Mn TWIP/TRIP steels with the higher PVFM. • The ε/γ interfaces with the orientation of the N-W orientation play a role in arresting the propagation of HICs. • The ε-martensite with <0001>||RD microscopic crystallographic orientation promotes the formation of cleavage or quasi-cleavage fracture morphology. The initiation and propagation of hydrogen-induced cracks (HICs) in high-Mn twinning/transformation-induced plasticity (TWIP/TRIP) steels were explored by slow strain rate tension test. For TWIP/TRIP steels with 10% phase volume fractions of ε-martensite (PVFM), the initiation of HICs is dominated by dislocation slips at 1 × 10−6 s−1, but, at 1 × 10−5 s−1, it is dominated by deformation twins. For TWIP/TRIP steels with 45% PVFM, inconsistent deformation of ε-martensite promotes HICs initiation. Both Σ3 grain boundary and γ/ε phase interface with Nishiyama-Wassermann orientation could prevents HICs propagation. And ε-martensite with <0001>||RD microscopic orientation at the crack tip is more likely to induce transgranular crack propagation. [ABSTRACT FROM AUTHOR]