Compressive Hydrostatic Stress Research Articles

Insights into the formation of titanium hydrides from first principles calculations

Hydride induced embrittlement is one of the main causes for the stress corrosion crack of titanium alloys. The formation process of the hydride at atomic level remains controversial as it is not accessible directly by experiments. In the present work, the formation of titanium hydride is investigated by using a first principles method. By calculating the formation energies of TiHm with various types of H occupation state in HCP and FCC Ti matrix, we show that there exists phase equilibrium between α-Ti-H solid solution and δ/ε hydrides with χ and γ hydrides appearing as metastable phases. The energy barrier for the HCP → FCC structure transformation for the formation of the hydrides decreases with increasing H:Ti ratio m. The structure transformation is accompanied with spontaneous shift of half amounts of the H atoms from the octahedral to the tetrahedral interstices. The energy barrier for the migration of the remaining half of the H atoms from the octahedral to the tetrahedral interstices reduces with increasing m and approaches to a constant value. The hydrostatic compressive stress acting on TiHm, resulted from the H-induced lattice dilation tendency and the constraint of the surrounding matrix, does not facilitate the HCP → FCC transformation and the migration of the H atoms. The present work sheds light on the hydride formation process at atomic level.

Investigation on low hydrostatic stress extrusion technology for forming of large thin-walled components with high ribs

An increase in hydrostatic stress improves the plasticity of metallic materials. However, for some special components, such as large thin-walled components with high ribs, excessive hydrostatic stress can cause several problems that make it impossible to prepare such components using plastic forming methods. In this study, the effect of hydrostatic stress on the deformation behavior of large thin-walled components with high ribs was investigated using a combination of numerical simulations and theoretical derivations. The results indicated that excessive hydrostatic stress leads to die failure, causing metal flow difficulties, uneven deformation, inconsistent mechanical properties, and reduced forming accuracy. Therefore, a low-hydrostatic-stress extrusion method was proposed by adjusting the contact friction, metal flow direction and displacement, and force boundary conditions to reduce the hydrostatic compressive stress. These principles are mainly reflected in the following three aspects: First, the size of the difficult deformation zone was reduced by changing the friction conditions from dry friction to fluid friction or boundary friction to reduce the frictional resistance. Second, the high-stress zone was eliminated by changing the metal flow direction, adjusting the strain state from unidirectional to multi-directional flow, shortening the metal flow path, and reducing the metal flow resistance. Third, the strong compressive stress state in the three directions was weakened by regulating the force boundary conditions and changing the loading method from one-way extrusion to multi-directional loading. Based on these principles, a series of new forming technologies have been developed, such as actively counteracting frictional resistance, slotting, and ditching on dies for long-lasting lubrication, multi-directional short-range metal flow, drawing-assisted extrusion, and multi-directional active loading. The adoption of these technologies realizes the labor-efficient formation of large thin-walled components with high ribs and provides the formed members with high precision and uniform mechanical properties.

An investigation of fracture modes around a spherical cavity for understanding outburst propagation mechanism

AbstractOutburst of coal and gas is a major hazard in underground coal mining where coal seam gas content/pressure is high. A cone‐shaped cavity is often observed in the coal seam after bursting, implying the likely mechanical structure being formed during outburst propagation. Although the mechanism for outburst initiation and propagation is very complex, mining‐induced fracturing and damage of the coal material is an essential component. This paper investigates the mechanical conditions that are essential to initiate fractures around a spherical cavity, a simplified form of the cone‐shaped outburst cavity. The cavity is assumed to be situated in an infinite isotropic homogeneous elastic body under internal gas pressure and far‐field hydrostatic compressive stresses as a further simplification for analytical investigation. The analytical conditions under which circumferential and radial fractures can initiate around the cavity are formulated, based on extensional strain criterion for fracture initiation. Low internal pressure on the cavity surface and high far‐field compression lead to circumferential fractures. In contrast, high internal pressure and low far‐field compression lead to radial fractures. Subsequent numerical simulations with FRACOD3D and FRACOD, the 3D and 2D in‐house fracture simulation codes, were carried out to investigate fracture patterns around a cavity and inclusion. The simulation results confirmed the analytical results, including fracture shapes and the size of fractured zone. These results provide a new angle to understand the complex mechanism of outbursts.

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 °C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 × 1012 cm−2 to 8 × 1010 cm−2. The films grown near room temperature showed large hydrostatic strain (∼6 × 10−3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 °C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality.

Experimental and theoretical investigation on the ductile removal mechanism in in-situ laser assisted diamond cutting of fused silica

Laser assistance and the use of a diamond tool with a negative rake angle are promising methods for machining hard and brittle materials. In this paper, a cutting force model and a smoothed particle hydrodynamics (SPH) model were constructed to investigate the ductile removal mechanism of fused silica. Subsequently, in-situ laser assisted diamond cutting (LADC) experiments were performed to demonstrate the critical depth of cut (DOC), cutting force, and surface quality. As expected, the developed theoretical model demonstrated satisfactory accuracy in predicting the cutting force, with an error rate confined to 12%. Results indicated that the ductile machinability was enhanced with laser assistance due to a decrease in hardness, and the initiation of brittle fracture was reduced by high hydrostatic compressive stress when using a tool with a negative rake angle. A smooth and homogenous surface of fused silica with a surface roughness (Sa) of 13.8 nm was achieved for the application of a diamond tool with a rake angle of −65°.

Shear and hydrostatic stress regulate fetal heart valve remodeling through YAP-mediated mechanotransduction.

Clinically serious congenital heart valve defects arise from improper growth and remodeling of endocardial cushions into leaflets. Genetic mutations have been extensively studied but explain less than 20% of cases. Mechanical forces generated by beating hearts drive valve development, but how these forces collectively determine valve growth and remodeling remains incompletely understood. Here, we decouple the influence of those forces on valve size and shape, and study the role of YAP pathway in determining the size and shape. The low oscillatory shear stress promotes YAP nuclear translocation in valvular endothelial cells (VEC), while the high unidirectional shear stress restricts YAP in cytoplasm. The hydrostatic compressive stress activated YAP in valvular interstitial cells (VIC), whereas the tensile stress deactivated YAP. YAP activation by small molecules promoted VIC proliferation and increased valve size. Whereas YAP inhibition enhanced the expression of cell-cell adhesions in VEC and affected valve shape. Finally, left atrial ligation was performed in chick embryonic hearts to manipulate the shear and hydrostatic stress in vivo. The restricted flow in the left ventricle induced a globular and hypoplastic left atrioventricular (AV) valves with an inhibited YAP expression. By contrast, the right AV valves with sustained YAP expression grew and elongated normally. This study establishes a simple yet elegant mechanobiological system by which transduction of local stresses regulates valve growth and remodeling. This system guides leaflets to grow into proper sizes and shapes with the ventricular development, without the need of a genetically prescribed timing mechanism.

Stress-adaptive design of 2D contact interfaces with uniform pressure: A bio-inspired approach

In this article, a bio-mimetic approach to the generative design of contact interfaces with uniform pressure distribution is explored. During the morphogenesis process, biological joints grow depending on the states of stress generating a shape adapted to the mechanical loads. This adaptation is driven by two main rules: shear stress inhibits growth, whereas cyclic hydrostatic compressive stress promotes it. In this work, we demonstrate that the stress-dependent growth rules of synovial joint morphogenesis can be applied to generate contact interfaces with uniform pressure in engineering applications. For that, we present a mathematical model that comprises a contact formulation and a bio-inspired growth function; the model is solved numerically using finite element methods. We analyse the impact of the growth rules of synovial joint development on the contact pressure distribution of two-dimensional contact interfaces. We study the parameter space of the bio-inspired growth function to fine-tune the model parameters. The model is tested in several cases with different boundary conditions and material properties. The results show that the proposed generative design process leads to contact interfaces that provide uniform contact pressure. A quantitative metric of the uniformity of the contact pressure is also defined. This metric indicates that the bio-inspired process generates geometries as good as those obtained with other methods present in the literature without the need of an initial shape close to the final one. Thus, this work demonstrates that synovial joint morphogenesis can be adapted for generative design in engineering.

Effect of ball-burnishing on hydrogen-assisted cracking of a martensitic stainless steel

Slow strain rate tensile tests under hydrogen cathodic charging are conducted on 17-4 PH steel with two surface conditions: mirror polished and ball-burnished. In both cases, significant subcritical cracking initiating at the surface is observed leading to considerable reduction in elongation to fracture. However, ball-burnished specimens show better elongation and much less secondary cracking than the polished ones. Ball-burnishing introduces high compressive residual stresses in the specimen sub-surface. However, EBSD showed a very limited impact of ball-burnishing on the microstructure, so little effect on hydrogen trapping is expected. The beneficial effect of ball-burnishing on the resistance of the hydrogen assisted cracking is mainly explained by the high compressive longitudinal stress at the specimen surface, which makes crack initiation more difficult and hence delays specimen failure. In addition, it is estimated that the amount of hydrogen introduced at the specimen surface is decreased by approximately 30% due to the high compressive hydrostatic stress.

Cutting characteristics of monocrystalline silicon in elliptical vibration nano-cutting using molecular dynamics method

Based on the molecular dynamics method, the model of monocrystalline silicon in elliptical vibration nano cutting (EVNC) is established. The effects of amplitude ratio (ratio of amplitude in parallel cutting direction to amplitude in vertical cutting direction) on the mechanical properties, thermal properties, crystal phase transition and morphology are systematically investigated. The results show that with the amplitude ratio increasing from 0.0 to 3.0, the hydrostatic compressive stress during cutting in the contact area decreases from 12.31 GPa to 8.31 GPa, and the tensile stress on the machined surface decreases from 5.23 GPa to 3.07 GPa. The depth of tensile stress layer after cutting is reduced from 12.02 Å to 4.8 Å. During cutting the maximum temperature of the machined surface decreases from 1419 K to 1236 K, and the chip maximum temperature increases from 1730 K to 2994 K. The number of diamond structures (bond length is 2.35 Å) is reduced during the cutting process, and the moderately increasing in the amplitude ratio can inhibit the formation of the bct-5 phase. After cutting, the number of diamond structures (bond length 2.35 Å) on the workpiece surface increases. The scratch morphology quality is effectively improved. With increasing amplitude ratio, the extrusion impact of the tool increases, resulting in the length and direction of the atomic displacement vector in the cutting area changing periodically, and the atomic kinetic energy and potential energy also change periodically, which may be the reason for EVNC to obtain better surface integrity. The research results provide theoretical reference for the application of ultrasound vibration assisted process in the field of atomic or near-atomic scale manufacturing.

Martensite and nanobainite transformations in a low alloyed steel studied by in situ high energy synchrotron diffraction

Martensitic and nanobainite transformations are studied in situ in a low alloyed, high-Si steel by using in situ HEXRD, combined with dilatometry and SEM observations, and by considering the same steel composition and austenitization conditions. The martensitic microstructure presents a mixed lath-plate morphology with large scatter of sizes whereas the bainite microstructure shows finer laths with more uniform sizes. Recently introduced methods are used to track in situ by HEXRD, in one single experiment, the phase fractions, the distribution of the carbon and the evolution of the dislocation densities. The study of nanobainite revealed that about two thirds of the carbon partitions from the ferrite to precipitate into transition iron carbides or to enrich the austenite. Both processes occur very fast after the formation of each nanobainite lath, but the ferrite remains largely supersaturated in carbon. The dislocation density increases inside each new forming bainitic ferrite lath. It then decreases when recovery becomes preponderant, as described with a recovery model from the literature. After the martensitic transformation, the retained austenite ends up with high hydrostatic compressive stresses. Dislocation densities are higher than in nanobainite and probably more heterogeneous, because recovery is less significant. No carbides were detected, contrary to the nanobainite. The carbon mass balance is analyzed in the light of these new results and previous investigations on similar systems. • Same steel composition and austenitization treatment to study the transformations. • Transition-carbides precipitation kinetic was quantitatively investigated. • Dislocation recovery observed during the nano-bainitic transformation. • Carbon distribution and mass balance showed a deficit of ca. 0.1 wt%.

46 - Optimal Intrusive Force for a Periodontally Compromised Tooth: A Finite Element Analysis Strategy

The present study evaluated the optimal force for orthodontic intrusive mechanics of a tooth with different levels of periodontal support reduction using a finite element analysis (FEA) approach. An anatomical 3D model was constructed representing a second unirradicular premolar inserted into a maxillary bone segment. Based on the control model, three horizontal bone resorption conditions were simulated with alveolar bone height loss of 2 mm, 4 mm, and 6 mm (R2, R4, and R6, respectively). An 25 cN intrusive force was used for the two simulated mechanics: bilateral mini-implant intrusion and conventional intrusion. A root resorption risk index (RRRi) was calculated by dividing the maximum compressive hydrostatic stress in the periodontal ligament by the hydrostatic stress of the capillaries (4.7 kPa). It was assumed that the optimal intrusive force of reduced periodontal premolars was reached when the compressive hydrostatic stress distributions pattern of the reduced models was similar to those found in the control models of the corresponding mechanics. The FEA optimal force values for each model were compared with those obtained by the analytical formula (Force = stress x area). A linear trendline was observed between the control force reduction percentage and the bone loss height for both mechanics. For each millimeter of bone height loss, the control force had to be reduced by 9% for conventional intrusion, and by 8% for bilateral mini-implant intrusion. The root resorption risk index and the optimal intrusive force depends not only on the remaining root area with ligament support, but also on the intrusive mechanics used. The use of bilateral mini-implants in orthodontic intrusion reduced the external apical root resorption risk index and increased the optimal intrusive force. The proposed FEA strategy provided an easy, precise and feasible approach to suggest the optimum force in a clinical situation.

Use of the AE Effect to Determine the Stresses State in AAC Masonry Walls under Compression.

Safety and reliability of constructions operated are predicted using the known mechanical properties of materials and geometry of cross-sections, and also the known internal forces. The extensometry technique (electro-resistant tensometers, wire gauges, sensor systems) is a common method applied under laboratory conditions to determine the deformation state of a material. The construction sector rarely uses ultrasonic extensometry with the acoustoelastic (AE) method which is based on the relation between the direction of ultrasonic waves and the direction of normal stresses. It is generally used to identify stress states of machine or vehicles parts, mainly made of steel, characterized by high homogeneity and a lack of inherent internal defects. The AE effect was detected in autoclaved aerated concrete (AAC), which is usually used in masonry units. The acoustoelastic effect was used in the tests described to identify the complex stress state in masonry walls (masonry units) made of AAC. At first, the relationships were determined for mean hydrostatic stresses P and mean compressive stresses σ3 with relation to velocities of the longitudinal ultrasonic wave cp. These stresses were used to determine stresses σ3. The discrete approach was used which consists in analyzing single masonry units. Changes in velocity of longitudinal waves were identified at a test stand to control the stress states of an element tested by the digital image correlation (DIC) technique. The analyses involved density and the impact of moisture content of AAC. Then, the method was verified on nine walls subjected to axial compression and the model was validated with the FEM micromodel. It was demonstrated that mean compressive stresses σ3 and hydrostatic stresses, which were determined for the masonry using the method considered, could be determined even up to ca. 75% of failure stresses at the acceptable error level of 15%. Stresses σ1 parallel to bed joints were calculated using the known mean hydrostatic stresses and mean compressive stresses σ3.

Orthodontically induced root resorption: A critical analysis of finite element studies’ input and output

Orthodontically induced inflammatory root resorption (OIIRR) constitutes an undesirable risk connected to orthodontic treatment. Finite element analysis (FEA) is a powerful tool to study the risk of OIIRR. However, its efficiency in predicting OIIRR depends on the insertion of the correct inputs and the selection of an output coherent with the clinical failure mechanism. By combining a systematic review with a 3-dimensional FEA, this article discusses which are the implications of using certain periodontal ligament (PDL) properties (linear and nonlinear models) and failure criteria. Six orthodontic loading regimes were simulated in a maxillary premolar: pure intrusion, buccal tipping, and their combination applied with either a light (25cN) or a heavy (225cN) force. Three stress parameters in the PDL were compared: von Mises stress, minimum principal stress, and hydrostatic stress (σH). The comparison between linear and nonlinear models showed notable differences in stress distribution patterns and magnitudes. For the nonlinear PDL, none of the light-force models reached the critical compressive hydrostatic stress of 4.7kPa, whereas all the heavy-force models reached it. In addition, the regions of critical compressive σH matched with the regions with resorption craters in clinical studies. In linear models, the σH critical value of 4.7kPa was reached even in the light-force scenario. Only compressive hydrostatic stress in PDL satisfied the requirements to be used as an FEA indicator of OIIRR. However, the requirements were satisfied only when a nonlinear PDL model was considered.

Effects of Peening Direction on Reverse Transformation Induced by Shot-Peening for Fe-33%Ni Alloy

Effects of peening direction on the reverse transformation induced by the shot-peening for the Fe-33 mass%Ni alloy with large amount of martensite (α’) are investigated. When the angle between the peened surface and the peening direction (Hereafter, peening angle) is 90 o, the reverse transformation occurs and subsequently martensitic transformation is induced by the shot-peening. On the other hand, in case of the peening angle of 30 o, only reverse transformation occurs. Furthermore, the volume fraction of austenite (γ) in the specimen after the shot-peening increases as the peening angle decreases. This means that the reverse transformation induced by the shot-peening is enhanced by decreasing the peening angle. Moreover, residual compressive stress around the peened surface increases as the peening angle decreases. Since the hydrostatic compressive stress decreases phase transformation temperature, the phase transformation temperature around the peened surface would be decreased by the shot-peening. Therefore, the reverse transformation behavior depending on the peening angle can be explained by the residual compressive stress due to the shot-peening.

On the Role of the Reserve Zone and Mechano-Regulatory Stimuli in the Development and Maturation of the Growth Plate: Observations and Models

Background. The subchondral epiphyseal bone plate between the epiphysis and the growth plate cartilage is formed by an unknown process. Objective. To examine if reserve zone chondrocytes could be involved in the development of the subchondral epiphyseal bone plate, by relating their intracellular stress-strain state to mechanobiological tissue differentiation theories. Methods. Multiscale elastic finite element models of the porcine proximal femoral physis at 20, 35, and 480 days after birth were created, based on histological observations. Findings. Simulating 15% compression of the growth plate produced intracellular maximum principal strains of 0.1–15% and compressive hydrostatic stresses of 0.15–0.4 MPa, depending on age and depth within the reserve zone. These values are within the range known to correspond with endochondral bone formation in fracture healing. Near the epiphyseal bone border the values for cellular compressive hydrostatic stress (> 0.15 MPa) and maximum tensile strain (0.1%) fall in the same range as found for hypertrophic chondrocytes at the metaphyseal bone growth front. Conclusion. The evidence suggests that growth plate cartilage has a secondary growth front at the reserve zone-epiphysis border that contributes to forming the subchondral epiphyseal bone plate.

Quantifying hydrogen concentration in the vicinity of zirconium hydrides and deformation twins

This study focuses on investigating the effects of the localized stresses that develop during formation of zirconium hydrides on the redistribution of hydrogen atoms in the zirconium matrix. A coupled mass diffusion and crystal plasticity finite element approach is used. The effects of the transformation strain associated with the formation of hydrides within various combinations of soft-hard grains and tri-crystals are studied. In addition, the interaction between intergranular and intragranular hydrides with grain boundaries is investigated. It is shown that hydrogen atoms tend to diffuse towards the tips of hydrides and away from their sides due to the tensile and compressive hydrostatic stresses that respectively develop at these two locations. This results in the propagation of a hydride along its axial direction. It is further shown that under a tensile applied stress, hydrides tend to form and propagate within hard grains, where stress hotspots with high tensile hydrostatic stresses develop. Lastly, it is shown that hydrogen atoms tend to diffuse towards twin tips, particularly when the twin tip is located near a harder grain.

A New General Paradigm for Understanding and Preventing Li Metal Penetration through Solid Electrolytes

Summary The use of lithium (Li) or sodium (Na) metal anodes together with highly ion-conductive solid electrolytes (SEs) could provide batteries with a step improvement in volumetric and gravimetric energy densities. Unfortunately, these SEs face significant technical challenges, in large part because Li and Na dendrites can penetrate through SEs, leading to short circuits. The ability of such a soft material (Li or Na metal) to penetrate through ceramic is surprising from the point of view of models widely used in the Li-battery field. We introduce a concept, new to the battery field, for preventing penetration of lithium dendrites through SEs by putting the SE surfaces into a state of residual compressive stress. For a sufficiently high compressive stress, cracks have difficulty forming, and cracks that do form are forced to close, inhibiting dendrite penetration. This approach is widely used to solve commercially important stress corrosion cracking problems in metals and static fatigue problems in ceramics and glasses (e.g., Gorilla Glass). However, the technique will not be useful for SEs if the Li-ion transport rate through a SE is substantially reduced when the SE is under compression. Our molecular dynamics calculations for Li-ion transport through a common SE demonstrate that the introduction of even very high residual compressive stresses (∼10 GPa) has only a modest effect on Li-ion transport kinetics, suggesting that this approach is viable and capable of providing a new paradigm for developing high-performance and mechanically stable SEs.

Microstructure and Mechanical Properties of AM60 Magnesium Alloy Processed by a New Severe Plastic Deformation Technique

The mechanical properties and microstructure of AM60 Magnesium alloy processed by a new cyclic extrusion channel angular pressing (CECAP) technique were investigated in this research. In this novel method, a low extrusion ratio stage was added at the end of the CECAP process to enhance the hydrostatic stresses and equivalent strain. The structure was refined from the average grain size of ~ 76 µm for the annealed samples to 2.63 µm in the new CECAP after four passes. While for the CECAP and ECAP processes, it was 3.43 and 6.98 µm, respectively. Experimental results of the proposed CECAP process showed an improvement in the mechanical properties after the second pass that were stopped for the ECAP process. However, the hardness of the CECAP-processed and proposed samples was constantly increasing. This increase in hardness was 196% and 175% for the proposed CECAP and CECAP processes after four passes compared to the unprocessed sample, respectively. Moreover, the compressive yield stress and ultimate compressive strength of the CECAP-processed samples were higher in all passes; however, they were smaller than those of the proposed CECAP process. The hydrostatic compressive stresses and higher plastic strain in the proposed technique helped in further refining of the grains, which can effectively improve the mechanical properties.

A New Modified Cyclic Extrusion Channel Angular Pressing (CECAP) Process for Producing Ultrafine-Grained Mg Alloy

In this study, a new modified cyclic extrusion channel angular pressing (CECAP) process with multi-pass capability is presented for producing ultrafine-grained (UFG) Mg alloy. In this method, an additional extrusion stage with a low extrusion ratio is added at the end of the CECAP process to enhance hydrostatic stresses, equivalent strain, and facilitation of the process implementation. The experimental results showed a remarkable grain refinement with a relatively uniform distribution of microstructure and β-phase in the new CECAP processed sample. The average grain size decreased from ~75 µm in annealed condition to 4.5 µm and 3.5 µm in CECAP and new CECAP processes, respectively. Also, more dispersion of the β-phase with smaller sizes was observed in the processed microstructure. The hardness and its uniform distribution were increased significantly. The compressive yield stress (CYS) was enhanced to about 153.3 MPa from the initial value of ~85 MPa. Also, both the strength and ductility of the new CECAP processed sample were higher than those of CECAP processed one. These excellent properties might attribute to higher hydrostatic compressive stresses resulting from the last low ratio extrusion stage which was almost 50% higher in the new CECAP process.

Comprehensive effect of hydrostatic compressive stress in retained austenite on mechanical properties and hydrogen embrittlement of martensitic steels

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.