Hydrogen environment embrittlement of stable austenitic steels

Seven stable austenitic steels (stable with respect to γ → α′ transformation at room temperature) of different alloy compositions (18Cr–12.5Ni, 18Cr–35Ni, 18Cr–8Ni–6Mn–0.25N, 0.6C–23Mn, 1.3C–12Mn, 1C–31Mn–9Al, 18Cr–19Mn–0.8N) were tensile tested in high-pressure hydrogen atmosphere to assess the role of austenite stability on hydrogen environment embrittlement (HEE). The influence of hydrogen on tensile ductility was small in steels that are believed to have a high initial portion of dislocation cross slip (18Cr–12.5Ni, 18Cr–35Ni, 18Cr–8Ni–6Mn–0.25N), while the effects of hydrogen were significantly greater in steels with other primary deformation modes (planar slip in 18Cr–19Mn–0.8N and 1C–31Mn–9Al or mechanical twinning in 0.6C–23Mn and 1.3C–12Mn) despite comparable austenite stability at the given test conditions. It appears that initial deformation mode is one important parameter controlling susceptibility to HEE and that martensitic transformation is not a sufficient explanation for HEE of austenitic steels.     

Influence of macro segregation on hydrogen environment embrittlement of SUS 316L stainless steel

The objective of this work is to identify microstructural variables that lead to the large scatter of the relative resistance of 316 grade stainless steels to hydrogen environment embrittlement. In slow displacement rate tensile testing, two almost identical (by nominal chemical composition) heats of SUS 316L austenitic stainless steel showed significantly different susceptibilities to HEE cracking. Upon straining, drawn bar showed a string-like duplex microstructure consisting of α′-martensite and γ-austenite, whereas rolled plate exhibited a highly regular layered α′-γ structure caused by measured gradients in local Ni content (9.5–13 wt%). Both martensite and austenite are intrinsically susceptible to HEE. However, due to Ni macro segregation and microstructural heterogeneity, fast H-diffusion in martensite layers supported a 10 times faster H-enhanced crack growth rate and thus reduced tensile reduction in area. Nickel segregation is thus a primary cause of the high degree of variability in H₂ cracking resistance for different product forms of 316 stainless steel.     

Impact of chemical inhomogeneities on local material properties and hydrogen environment embrittlement in AISI 304L steels

This study investigated the influence of segregations on hydrogen environment embrittlement (HEE) of AISI 304L type austenitic stainless steels. The microstructure of tensile specimens, that were fabricated from commercially available AISI 304L steels and tested by means of small strain-rate tensile tests in air as well as hydrogen gas at room temperature, was investigated by means of combined EDS and EBSD measurements. It was shown that two different austenitic stainless steels having the same nominal alloy composition can exhibit different susceptibilities to HEE due to segregation effects resulting from different production routes (continuous casting/electroslag remelting). Local segregation-related variations of the austenite stability were evaluated by thermodynamic and empirical calculations. The alloying element Ni exhibits pronounced segregation bands parallel to the rolling direction of the material, which strongly influences the local austenite stability. The latter was revealed by generating and evaluating two-dimensional distribution maps for the austenite stability. The formation of deformation-induced martensite was shown to be restricted to segregation bands with a low Ni content. Furthermore, it was shown that the formation of hydrogen induced surface cracks is strongly coupled with the existence of surface regions of low Ni content and accordingly low austenite stability. In addition, the growth behavior of hydrogen-induced cracks was linked to the segregation-related local austenite stability.     

Hydrogen embrittlement and hydrogen diffusion behavior in interstitial nitrogen-alloyed austenitic steel

The susceptibility to hydrogen embrittlement and diffusion behavior of hydrogen were evaluated in interstitial nitrogen-alloyed austenitic steel QN1803 and 304 and 316 L stainless steels. The amount of transformed martensite and the activation energy of hydrogen diffusion were revealed via electron backscattering diffraction and thermal desorption spectroscopy. The austenite stability of QN1803 during the deformation process was higher than that of 304 and 316 L. However, the hydrogen content of QN1803 was high because of the small grain size and low activation energy of hydrogen diffusion. For the stable QN1803 and 316 L austenitic steels, martensite had no evident harmful effect because of its discrete distribution. A planar dislocation slip was observed in QN1803 during deformation. Hydrogen charging enhanced dislocation mobility, leading to severe strain localization. Thus, the severe strain in QN1803 promoted microcracking.     

On the role of nitrogen on hydrogen environment embrittlement of high-interstitial austenitic CrMnC(N) steels

This work investigates the susceptibility of high-interstitial CrMn austenitic stainless steel CN0.96 to hydrogen environment embrittlement. In this context, an N-free model alloy of CN0.96 steel was designed, produced, and characterized. Both steels were subjected to tensile tests in air and in a high-pressure hydrogen gas atmosphere.Both steels undergo severe hydrogen embrittlement. The CN0.96 steel shows trans- and intergranular failure in hydrogen, whereas the N-free model alloy shows exclusively intergranular failure. The different failure modes could be related to different deformation modes that are induced by the presence or absence of N, respectively. In the CN0.96 steel, N promotes planar dislocation slip. Due to the absence of N in the model alloy, localized slip is less pronounced and mechanical twinning is a more preferred deformation mechanism. The embrittlement of the model alloy could therefore be related to mechanisms that are known from hydrogen embrittlement of twinning-induced plasticity steels.     

Hydrogen embrittlement behavior in interstitial Mn–N austenitic stainless steel

The susceptibility to hydrogen embrittlement behavior was investigated in an interstitial Mn–N austenitic steel HR183 and stainless steel 316L. Hydrogen was introduced by cathodic hydrogen charging at 363 K. HR183 has stronger austenite stability than 316L despite its lower nickel content, the addition of manganese and nitrogen inhibited martensitic transformation during the slow strain rate tensile deformation. Due to the diffusion of hydrogen being delayed by the interstitial solution of nitrogen atoms and the uniform dislocation slips, hydrogen permeates more slowly in HR183 than 316L, contributing to an 84.79 μm thinner brittle fracture layer in HR183 steel. Hydrogen charging caused elongation losses in both 316L and HR183 steels associated with the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanism. However, the hydrogen embrittlement susceptibility of HR183 is 3.4 times lower than that of 316L according to the difference in elongation loss between the two steel after hydrogen charging. Deformation twins trapped a lot amount of hydrogen leading to brittle intergranular fracture in 316L. The multiple directions of slip in HR183 steel suppressed the strain localization inside grains and delayed the adverse effects conducted by HELP and HEDE mechanism, eventually inhibiting server hydrogen embrittlement in the HR183 steel. This study is assisting in the development of low-cost stainless steel with excellent hydrogen embrittlement resistance that can be used in harsh hydrogen-containing environments.     

Development of a stable high-aluminum austenitic stainless steel for hydrogen applications

A novel high-aluminum austenitic stainless steel has been produced in the laboratory with the aim of developing a lean-alloyed material with a high resistance to hydrogen environment embrittlement. The susceptibility to hydrogen environment embrittlement was evaluated by means of tensile tests at a slow strain rate in pure hydrogen gas at a pressure of 40 MPa and a temperature of −50 °C. Under these conditions, the yield strength, tensile strength and elongation to rupture are not affected by hydrogen in comparison to companion tests carried out in air. Moreover, a very high ductility in hydrogen is evidenced by a reduction of area of 70% in the high-pressure and low-temperature hydrogen environment. The lean degree of alloying is reflected in the molybdenum-free character of the material and a nickel content of 8.0 wt.%. With regard to the alloy concept, a combination of high-carbon, high-manganese, and high-aluminum contents confer an extremely high stability against the formation of strain-induced martensite. This aspect was investigated by means of in-situ magnetic measurements and ex-situ X-ray diffraction. The overall performance of the novel alloy was compared with two reference materials, 304L and 316L austenitic stainless steels, both industrially produced. Its capability of maintaining a fully austenitic structure during tensile testing has been identified as a key aspect to avoid hydrogen environment embrittlement.     

Hydrogen embrittlement of Cr-Mn-N-austenitic stainless steels

Cr-Mn-N austenitic steels show a unique combination of properties, i.e. high strength, high ductility, non magnetic and good corrosion resistance at costs being much lower compared to Cr-Ni austenitic steels. Hydrogen environment embrittlement (HEE) was investigated by slow displacement tensile testing in hydrogen atmosphere at 10 MPa and −50 °C. The fracture appearance of stable Cr-Mn-N austenitic steels with lower Mn contents (12Mn-0.7N) was transgranular whereas higher Mn contents (18Mn-0.7N) resulted in twin boundary fracture. This change in fracture morphology was related to a modest change in macroscopic ductility. Such fracture behaviour is similar to what is known from metastable Cr-Ni austenitic steels, therefore, Mn and/or N cannot be used to replace Ni in stable austenitic high HEE resistant steels.     

Effect of alloying elements on hydrogen environment embrittlement of AISI type 304 austenitic stainless steel

The chemical composition of an AISI type 304 austenitic stainless was systematically modified in order to evaluate the influence of the elements Mo, Ni, Si, S, Cr and Mn on the material’s susceptibility to hydrogen environment embrittlement (HEE). Mechanical properties were evaluated by tensile testing at room temperature in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere. For every chemical composition, the corresponding austenite stability was evaluated by magnetic response measurements and thermodynamic calculations based on the Calphad method. Tensile test results show that yield and tensile strength are negligibly affected by the presence of hydrogen, whereas measurements of elongation to rupture and reduction of area indicate an increasing ductility loss with decreasing austenite stability. Concerning modifications of alloy composition, an increase in Si, Mn and Cr content showed a significant improvement of material’s ductility compared to other alloying elements.     

A thermodynamic approach for the development of austenitic steels with a high resistance to hydrogen gas embrittlement

The CALPHAD method was employed to assess the austenite stability of model alloys based on the Cr–Mn–Ni–Cu system. Stability was evaluated as the difference in Gibbs free energy between the austenite and ferrite phases. This energy difference represents the chemical driving force for the martensitic transformation and is employed as a design criterion. Six novel alloys featuring a lower driving force compared to the reference material AISI 316L were produced in laboratory. The susceptibility of all alloys to hydrogen gas embrittlement was evaluated by slow strain-rate tensile testing in air and hydrogen gas at 40 MPa and −50 °C. The mechanical properties and ductility response of four of the six alloys exhibited an equivalent performance in air and hydrogen. Thermodynamic calculations were in agreement with the amount of α′-martensite formed during testing. Furthermore, a 4.5 wt.% reduction in the nickel content in comparison to 316L promises a cost benefit for the novel materials.     

Hydrogen absorption of different welded duplex steels

In this study, hydrogen absorption and storage was investigated for various high-alloyed ferritic–austenitic duplex stainless steels. On account of the specific transformation and solidification behaviour, respectively, of duplex stainless steels as compared to single-phase ferritic and austenitic steels, special conditions have to be considered concerning hydrogen absorption which may ultimately lead to microstructure-dependent hydrogen-assisted weld metal cracking. Hydrogen absorption during welding may occur via the shielding gas, moisture from the surroundings or via the welding filler material. As a contribution to the interpretation and prediction of hydrogen-induced cracking in welded duplex stainless steels, the actual hydrogen absorption via the arc as well as the weld metal hydrogen diffusion was investigated in a duplex stainless steel DSS (1.4462) and in a lean-duplex stainless steel LDS (1.4162). Isothermal heat treatment using carrier gas hot extraction enabled quantification of the amounts of hydrogen trapped in the respective microstructures. The total hydrogen concentrations were found to be nearly identical. Trapped hydrogen was however observed to be dependent on the material and on the microstructure condition. The influence of hydrogen on the mechanical properties of the weld metal was characterized with the help of tensile tests. In addition, hydrogen embrittlement was detected in scanning electron microscopic analyses.     

Notched-tensile properties under high-pressure gaseous hydrogen: Comparison of pipeline steel X70 and austenitic stainless type 304L, 316L steels

The effect of high-pressure gaseous H₂ on the fracture behavior of pipeline steel X70 and austenitic stainless steel type 304L and 316L was investigated by means of notched-tensile tests at 10 MPa H₂ gas and various test speed. The notch tensile strength of pipeline X70 steel and austenitic stainless steels were degraded by gaseous H₂, and the deterioration was accompanied by noticeable changes in fracture morphology. The loss of notch tensile strength of type 316L and X70 steels was comparable, but type 304L was more susceptible to hydrogen embrittlement than the others. In the X70 steel, hydrogen embrittlement increased as test speed decreased until the test speed reached 1.2 × 10^?3 mm/s, but the effect of test speed was not significant in 304L and 316L steels.     

Origin of deformation twins and their influence on hydrogen embrittlement in cold-rolled austenitic stainless steel

The effect of cold rolling on hydrogen embrittlement in stable 18Cr–1Mn–11Ni-0.15 N austenitic stainless steels was investigated. Alloy plates were cold-rolled to 15% or 30% reduction, then pre-charged with hydrogen and subjected to tensile testing with slow strain rate. Hydrogen-induced degradation of tensile elongation became increasingly severe with the increase in the degree of cold rolling. During cold rolling, deformation twins with various orientations were actively generated, and twins with specific orientations were vulnerable to hydrogen-induced cracking. Cold rolling also increased the density of defects, and thereby facilitated penetration of hydrogen into the steels. The combination of cracks generated at the twin boundaries, and the promoted hydrogen diffusion caused severe hydrogen embrittlement in the cold-rolled steels.     

Suppression of hydrogen absorption into 304L austenitic stainless steel by surface low temperature gas carburizing treatment

Effect of low temperature gas carburizing (LTGC) on hydrogen absorption and hydrogen embrittlement (HE) susceptibility of 304L metastable austenitic stainless steel was investigated. The LTGC treatment imparted carburized layer on the steel surface with supersaturated solute carbon atoms (namely expanded austenite or S-phase) and more than 1 GPa surface compressive stress. Carburized layer thickness, carbon concentration level, residual compressive stress and hardness increased but hydrogen absorption decreased with increasing LTGC treatment time. Carburized surface layers had much higher austenite stability. The HE susceptibility of carburized steel was reduced due to the reduction of hydrogen absorption and the increment of austenite stability. The specimens whose residual compressive stresses were eliminated by tensile plastic straining also exhibited low hydrogen absorption during hydrogen charging, indicating that, besides the residual compressive stress, the supersaturated solute carbon atoms also have the ability to reduce hydrogen absorption. In addition, the results indicate that the supersaturated solute carbon atoms in the LTGC case can suppress hydrogen solubility without affecting diffusivity.     

An Fe–Ni–Cr–H interatomic potential and predictions of hydrogen-affected stacking fault energies in austenitic stainless steels

While Fe–Ni–Cr austenitic stainless steels exhibit relatively good resistance to hydrogen embrittlement, they still suffer from significant degradation of ductility, fatigue and fracture properties in gaseous hydrogen environments. Experimental studies in the literature suggest that hydrogen reduces stacking fault energy in austenitic stainless steels. This phenomenon causes a large separation of partial dislocations and lower propensity for cross-slip. Whereas lower stacking fault energy does not correlate well with loss of ductility in the absence of hydrogen, lower stacking fault energy trends toward greater loss of ductility when hydrogen is present. Calculations of stacking fault energy are challenging for austenitic stainless steels. One main issue is that in alloys, stacking fault energy is not a single value but rather varies depending on local composition. Herein, we first report an Fe–Ni–Cr–H quaternary interatomic potential and then use this potential to perform time-averaged molecular dynamics simulations to calculate stacking fault energies for tens of thousands of realizations of local compositions for selected stainless steels alloys with and without internal hydrogen. From statistical analyses, our results suggest that hydrogen reduces stacking fault energy, which likely impacts deformation mechanisms of Fe–Ni–Cr austenitic stainless steels when exposed to hydrogen environments. We then perform validation MD simulation tests to show that hydrogen indeed statistically increases the stacking fault widths due to statistically reduced stacking fault energies.     

Hydrogen transport in solution-treated and pre-strained austenitic stainless steels and its role in hydrogen-enhanced fatigue crack growth

Hydrogen solubility and diffusion in Type 304, 316L and 310S austenitic stainless steels exposed to high-pressure hydrogen gas has been investigated. The effects of absorbed hydrogen and strain-induced martensite on fatigue crack growth behaviour of the former two steels have also been measured. In the pressure range 10–84 MPa, the hydrogen permeation of the stainless steels could be successfully quantified using Sieverts' law modified by using hydrogen fugacity and Fick's law. For the austenitic stainless steels, hydrogen diffusivity was enhanced with an increase in strain-induced martensite. The introduction of dislocation and other lattice defects by pre-straining increased the hydrogen concentration of the austenite, without affecting diffusivity. It has been shown that the coupled effect of strain-induced martensite and exposure to hydrogen increased the growth rate of fatigue cracks.     

Influence of machining parameters on surface roughness and susceptibility to hydrogen embrittlement of austenitic stainless steels

In the present work, an investigation on the susceptibility to hydrogen embrittlement of AISI 304 and 310 austenitic stainless steels was performed. The hydrogen embrittlement process leads to degradation of mechanical properties and can be accelerated by the presence of surface defects combined with elevated surface hardness. Tensile test specimens of the selected materials were machined by turning with different cutting parameters in order to create variations in surface finish conditions. The samples thus prepared were submitted to tensile tests before and after hydrogen permeation by cathodic charging. Regarding the AISI 304 steel, it was possible to notice that the presence of strain-induced martensite on the material surface led to severe hydrogen embrittlement. In the case of the AISI 310 steel, due to its higher nickel amount, no martensite formation could be detected, and this steel was found to be less susceptible to embrittlement in the tested conditions.     

Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe–Mn–Al–C light weight austenitic steel

Hydrogen embrittlement of a precipitation-hardened Fe–26Mn–11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.     

Effect of grain size on hydrogen embrittlement in stable austenitic high-Mn TWIP and high-N stainless steels

Two stable austenitic steels, 20Cr-11Ni-5Mn-0.3N (wt%) stainless steel (STS) and 18Mn-1.5Al-0.6C (wt%) twinning-induced plasticity steel (TWIP), were investigated to understand the effect of grain size on hydrogen embrittlement (HE). Grain refinement promoted HE in the STS but suppressed HE in the TWIP. These opposite effects occurred because the steel composition affected deformation mechanism. Cr-N pair enhanced short-range ordering (SRO) in STS, which promoted planar slip and delayed mechanical twinning. In contrast, TWIP exhibited mechanical twinning which was more active in coarser grains. Final dislocation density after tensile deformation was increased by grain refinement in STS, but was decreased in TWIP. The damaging effects of hydrogen on strain energy at interfaces and on interfacial bonding strength were controlled by dislocation density; therefore, increase in dislocation density led to increase in susceptibility to HE.