Effect of high-pressure hydrogen environment in elastic and plastic deformation regions on slow strain rate tensile tests for iron-based superalloy A286

To investigate the effects of a high-pressure hydrogen environment in the elastic and plastic deformation regions, we performed slow strain-rate tensile tests of iron-based superalloy A286 at 150 °C by switching the atmosphere from 70 MPa hydrogen to air during the tests. The relationship between the nominal strain exposed to a hydrogen environment and the relative reduction in area (RRA) revealed that in the plastic deformation region, the RRA value decreased gradually depending on the nominal strain exposed to hydrogen, but in the elastic deformation region, the RRA value decreased rapidly. The RRA value further decreased when the stress cycle was applid in the elastic region. The fracture surface exhibited an intragranular slip plane fracture similar to that of the hydrogen-charged specimen. These phenomena suggest that the lattice decohesion theory is dominant in the elastic region, where hydrogen embrittlement occurs owing to an increase in the content of dissolved hydrogen.     

Crack growth rate in hydrogen pre-charged martensitic steels during slow strain rate tests

Crack growth rate in two high strength martensitic steels with the Mo contents of 0.43 wt.% and 1.06 wt.% was investigated by means of slow strain rate tests (SSRT) on compact tensile specimens after hydrogen pre-charging. It was found that the crack growth rate increased and the values of stress intensity factors K_IH and K_Imax decreased with the increase of pre-charged hydrogen concentration. The steel with higher Mo content showed much lower crack growth rate than the steel with lower Mo content. It could be attributed to more nano-sized precipitates that can act as the hydrogen trapping sites and mitigate hydrogen deleterious effects on crack growth rate and the K_IH and K_Imax values.     

Effect of residual hydrogen content on the tensile properties and crack propagation behavior of a type 316 stainless steel

The tensile properties and crack propagation rate in a type 316 austenitic stainless steel prepared by vacuum induction melting method with different residual hydrogen contents (1.1–11.5 × 10⁻⁶) were systematically investigated in this research work. The room temperature tensile properties were measured under both regular tensile (12 mm/min) and slow tensile (0.01 mm/min) conditions, and the fracture properties of the tensile fractures with both rates were analyzed. It shows that the hydrogen induced plasticity loss of stainless steel strongly depends on the tensile rate. Under regular tensile condition, there is no plastic loss even when the hydrogen content is up to 11.5 × 10⁻⁶ while in the slow tensile condition, the plastic loss can be clearly identified rising with the increasing H contents. The fatigue crack propagation rate was tested at room temperature, and the crack growth rate formula (Paris) of the 316 stainless steels with varied H contents were obtained. The fatigue crack propagation rate test shows that the crack growth rate of the 316 stainless steel with 8.0–11.5 × 10⁻⁶ hydrogen is significantly higher than that of benchmark steel.     

The experiment study to assess the impact of hydrogen blended natural gas on the tensile properties and damage mechanism of X80 pipeline steel

Environmental hydrogen embrittlement has become a non-negligible problem in the hydrogen blended natural gas transportation. To qualitatively study the degradation mechanism of X80 steel used in the natural gas pipelines, the slow strain tensile experiments are carried out in this work. The nitrogen and hydrogen are adopted to simulate the hydrogen blended natural gas to explore the tensile properties of X80 steel. According to the volume proportion of hydrogen, the test atmospheres are divided into the reference atmosphere and the hydrogen-contained atmospheres of 1%, 2.2% and 5%. The tensile experiments of the smooth and notched specimens are conducted in the above gas atmospheres. Mechanical properties and fracture morphologies after stretching are further analyzed. The results show that the hydrogen blended natural gas has little effect on the tensile and yield strengths. Distinguished from the hydrogen volume proportion of 1% and 2.2%, with the increase of hydrogen proportion, the effect of hydrogen on mechanical properties of specimens increases significantly. Moreover, the deteriorated mechanical properties of notched specimens are more seriously than those of smooth specimens. This work provides the basis for safe hydrogen proportion for X80 pipeline steel when transporting hydrogen blended natural gas.     

On the physical differences between tensile testing of type 304 and 316 austenitic stainless steels with internal hydrogen and in external hydrogen

Seventeen metastable austenitic stainless steels (type 304 and 316 alloys) were tested in tension both with internal hydrogen and in external hydrogen. Hydrogen-assisted fracture in both environments is a competition between hydrogen-affected ductile overload and hydrogen-assisted crack propagation. In general, hydrogen localizes the fracture process, which results in crack propagation of particularly susceptible materials at an apparent engineering stress that is less than the tensile strength of the material. Hydrogen-assisted crack propagation in this class of alloys becomes more prevalent at lower nickel content and lower temperature. In addition, for the tests in this study, external hydrogen reduces tensile ductility more than internal hydrogen. External hydrogen promotes crack initiation and propagation at the surface, while with internal hydrogen surface cracking is largely absent, thus preempting hydrogen-assisted crack propagation from the surface. This is not a general result, however, because the reduction of ductility with internal and external hydrogen depends on the specifics of the testing conditions that are compared (e.g., hydrogen gas pressure); in addition, internal hydrogen can promote the formation of internal cracks, which can propagate similar to surface cracks.     

Hydrogen diffusivity and tensile-ductility loss of solution-treated austenitic stainless steels with external and internal hydrogen

The effects of external and internal hydrogen on the slow-strain-rate tensile (SSRT) properties at room temperature were studied for ten types of solution-treated austenitic stainless steels containing a small amount of additive elements. The hydrogen diffusivity and solubility of the steels were measured with high-pressure hydrogen gas. The remarkable tensile-ductility loss observed in the SSRT tests was attributed to hydrogen-induced successive crack growth (HISCG) and was successfully quantified according to the nickel-equivalent content (Ni_eq), which represents the stability of the austenitic phase. The relative reduction in area (RRA) of the steels with a larger Ni_eq was influenced by the hydrogen distribution, whereas that of the steels with a smaller Ni_eq was not. This unique trend was interpreted with regard to the hydrogen distribution and fracture morphology (HISCG or microvoid coalescence).     

Effect of electrotransport treatment on susceptibility of high-strength low alloy steel to hydrogen embrittlement

Electrotransport theory is defined as mass transportation of solute such as hydrogen in metal under the influence of an electrostatic force field. In this study, electrotransport treatment was applied to remove the accumulated hydrogen inside of the high-strength low alloy steel. The effectiveness of the electrotransport treatment was evaluated by hydrogen concentration measurement, slow strain rate test, and fracture surface analysis. The efficiency of electrotransport treatment is improved with increasing applied current and time, and the highest efficiency was obtained as 88.7% at 450 A for 40 min. The ultimate tensile strength and elongation of specimen after electrotransport treatment was enhanced dramatically in comparison with that of specimen under hydrogen charging condition. The brittle fracture mode was observed on the hydrogen charged specimen, but a clear ductile fracture mode was observed on the specimen after electrotransport treatment. These results confirm that the electrotransport treatment is effective to remove the accumulated hydrogen inside of the high-strength low alloy steel.     

Influence of surface martensite layer on hydrogen embrittlement of Fe-Mn-C-Mo steels in wet H2S environment

This study investigated the effect of thermally induced surface martensite layer on hydrogen embrittlement of Fe-16Mn-0.4C-2Mo (wt.%) (16Mn) and Fe-25Mn-0.4C-2Mo (wt.%) (25Mn) steels through slow strain rate stress corrosion cracking testing and proof ring testing in wet H₂S environment. The 16Mn steel had a surface layer of less than 150 μm in depth containing ε-martensite, α′-martensite and austenitic twins. The martensite layer is found to reduce the hydrogen embrittlement resistance of the steel. In comparison, the 25Mn steel developed a full α′-martensite surface layer, which exhibited practically nil effect on the hydrogen embrittlement resistance of the steel. The ε-martensite provides much larger interface areas with the mechanical twins of the austenite in the 16Mn steel than the α′-martensite/austenite interfaces in the 25Mn steel. These interfaces are hydrogen trapping sites and are prone to initiate surface cracks, as observed in the scanning electron microscope. The formation of the cracks is attributed to hydrogen concentration at the ε-martensite and austenitic twin interfaces, which accelerates material fracture.     

Establishment of an in-situ small punch test method for characterizing hydrogen embrittlement behaviors under hydrogen gas environments and new influencing factor

The hydrogen embrittlement (HE) behaviors of structural materials used to handle hydrogen must be tested at their use environments. In this study, an in-situ small punch (SP) test method was established to characterize HE behaviors of SA372 and STS304 steels under hydrogen gas environments at high pressures and low temperatures. In addition, a new influencing factor, relative reduction of thickness (RRT) was proposed to quantify the HE sensitivity of structural steels. Under 10 MPa H₂ gas environment, load-displacement curves obtained and fractographic morphologies of the recovered specimens were analyzed to investigate the HE behaviors based on punch velocity and test temperature. When the HE sensitivity was evaluated by RRT, STS304 steel appeared to be vulnerable to HE as compared to SA372 steel across the tested temperature range. As a result, it was found that in-situ SP test and RRT can be used to quantify the HE sensitivity for structural materials screening regardless of test environments.     

Simulation of the impact of internal pressure on the integrity of a hydrogen-charged Type-316L stainless steel during slow strain rate tensile test

A hydrogen-charged Type-316L austenitic stainless steel represents a slight loss of tensile ductility and cup-and-cone fracture accompanied by small-sized dimple. The reduction in the dimple size is interpreted to be attributed to void sheets caused by localized slip deformations by hydrogen. This paper aims to clarify the contribution of an internal pressure to the characteristic void growth of a hydrogen-charged Type-316L stainless steel during slow strain rate tensile (SSRT) test in air at room temperature. The internal pressure of pre-existing voids in the specimen charged by 100 MPa hydrogen gas at 270 °C for 200 h was simulated by diffusion-desorption analysis of hydrogen with the finite differential method (FDM). The subsequent impact of the internal pressure on the void growth was simulated by fracture-mechanics approach with the finite element method (FEM). The simulations performed under various void morphologies and fracture toughness suggested that the internal pressure in the voids was significantly low, hardly affecting the void growth.