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.     

The dependence of hydrogen embrittlement on hydrogen transport in selective laser melted 304L stainless steel

The mechanical property and hydrogen transport characteristics of selective laser melting (SLM) 304L stainless steel were investigated by tensile tests and thermal desorption spectroscopy (TDS). The heat treatment affected the hydrogen embrittlement (HE) susceptibility and the treatment at 950 °C showed the larger HE effects. Cellular structures and melt-pool boundaries were dissolved at 850 and 950 °C, respectively. TDS results indicate that the hydrogen diffusivity of the as-received SLM 304L was lower than that of wrought 304L and the hydrogen diffusion activation energy increased with the recrystallisation degree, which was related to the dislocation density. Dislocations, rather than strain-induced martensite, were the main cause of HE owing to the high austenite stability of the samples. The pre-existing dislocations in the SLM 304L sample heat-treated at 950 °C for 4 h affected the hydrogen transport behaviour during sample stretching and led to severe HE.     

The dependence of fatigue crack growth on hydrogen in warm-rolled 316 austenitic stainless steel

The fatigue crack growth rate of warm-rolled AISI 316 austenitic stainless steel was investigated by controlling rolling strain and temperature in argon and hydrogen gas atmospheres. The fatigue crack growth rates of warm-rolled 316 specimens tested in hydrogen decreased with increasing rolling temperature, especially 400 °C. By controlling the deformation temperature and strain, the influences of microstructure (including dislocation structure, deformation twins and α′ martensite) and its evolution on hydrogen-induced degradation of mechanical properties were separately discussed. Deformation twins deceased and dislocations became more uniform with the increase in rolling temperature, inhibiting the formation of dynamic α′ martensite during the crack propagation. In the cold-rolled 316 specimens, deformation twins accelerated hydrogen-induced crack growth due to the α′ martensitic transformation at the crack tip. In the warm-rolled specimens, the formation of α′ martensite around the crack tip was completely inhibited, which greatly reduced the fatigue crack growth rate in hydrogen atmosphere.     

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.     

Effect of strain-induced martensite on hydrogen embrittlement of austenitic stainless steels investigated by combined tension and hydrogen release methods

The hydrogen transport behavior together with hydrogen embrittlement (HE) in hydrogen-charged type 304 and 316 stainless steels during deformation was investigated by combined tension and outgassing experiments. The specimens were thermally hydrogen-charged in 30 MPa hydrogen at 473 K for 48 h. HE of hydrogen-charged type 304 steel decreases with increasing prestrain and almost no HE is observed in hydrogen-charged type 316 steel. Prior strain-induced α′ martensite formed by the prestrain at 208 K has little relation with HE, while dynamic α′ martensite formed during deformation after the prestrain shows obvious HE. The differences in hydrogen diffusivity and solubility between α′ martensite and austenite (γ) induce hydrogen diffusion from dynamic α′ martensite and then its accumulation at the boundary between the α′-rich and γ-rich zones, resulting in crack initiation at the boundary between the α′-rich and γ-rich zones.     

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.     

Low-temperature tensile and impact properties of hydrogen-charged high-manganese steel

Low-temperature mechanical properties of a high-manganese austenitic steel were evaluated with and without hydrogen pre-charging to examine the applicability of the alloy as a material for hydrogen infrastructure. The high-manganese steel, along with the conventional 304 and 316 L austenitic steels, was examined for hydrogen-related properties including hydrogen content after gas-phase pre-charging, tensile properties, and Charpy impact toughness at different temperatures ranging from room temperature to −80 and −196 °C, respectively, and the resultant fracture surfaces. Under hydrogen-charged conditions, the high-manganese steel showed low-temperature mechanical properties comparable to those of conventional austenitic steels, suggesting the potential of the alloy for structural applications in hydrogen environment.     

Effective hydrogen diffusion coefficient for CoCrFeMnNi high-entropy alloy and microstructural behaviors after hydrogen permeation

Herein, the first observation of the effective hydrogen diffusion coefficient of CoCrFeMnNi high-entropy alloy (HEA) was performed using electrochemical hydrogen permeation; further, it was compared with those of stainless steels (SS) 304 and 316L. HEA and SS 316L showed similar effective hydrogen diffusion coefficient of 1.75 × 10⁻¹¹ m²/s and 1.91 × 10⁻¹¹ m²/s, respectively. SS 304 showed the smallest that of 0.58 × 10⁻¹¹ m²/s in the study. Hydrogen diffusion through the grain boundary was dominant in face-centered cubic metals. Hydrogen permeation resulted in no change in the microstructure of HEA and SS 316L; however, it caused a martensitic transformation in SS 304.     

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.     

Combined effects of prior plastic deformation and sensitization on hydrogen embrittlement of 304 austenitic stainless steel

Austenite stainless steels (ASSs) may suffer from both cold deformation and sensitization prior to hydrogen exposure. There is scant data in literature on the combined effect of prior deformation and sensitization on the hydrogen embrittlement (HE) of ASSs. The present study investigated the combined effects of tensile plastic prestrain (PS) and 650 °C sensitization (ST) on the HE of 304 steel by hydrogen pre-charging and tensile testing. The results are explained by terms of pre-existing α′ martensite content. PS higher than 10% can enhance HE significantly by inducing severe α′ transformation prior to hydrogen exposure. Prior ST also enhances HE, but submitting the prestrained and α′-containing 304 steel to short-time ST can diminish the enhancement of HE by prestraining, as ST can cause the reversion of α′ to austenite, reducing pre-existing α′ content. It is inadvisable to make 304 steel be sensitized/welded firstly and deformed subsequently, even if the ST time is short such as what happens during welding, because this treating sequence can induce more α′ than prestraining alone, enhancing HE more significantly. Apparent hydrogen diffusivity can be related quantitatively to pre-existing α′ content, proving directly that α′ platelets can act as diffusion “highways” in ASSs. It is indicated that pre-existing α′ can enhance subsequently the HE of ASSs is because it can lead to a large amount of hydrogen entering the ASSs during hydrogen exposure by acting as diffusion “highways”. HE is enhanced by increasing hydrogen amount rather than by pre-existing α′ itself.     

Effect of plastic deformation at room temperature on hydrogen diffusion of S30408

Understanding the influence of plastic deformation on diffusion is critical for hydrogen embrittlement (HE) study. In this work, thermal desorption spectroscope (TDS), slow strain rate test (SSRT), feritscope, transmission electron microscope (TEM) and TDS model were used to study the relation between plastic deformation and hydrogen diffusion, aiming at unambiguously elucidating the effect of plastic deformation on hydrogen diffusion of austenitic stainless steel, S30408. An effective method was developed to deduce apparent hydrogen diffusion coefficient of austenitic stainless steel in this paper. Results indicate apparent hydrogen diffusion coefficient decreases firstly and then increases with increasing plastic deformation at room temperature. Hydrogen diffusion effected by plastic deformation is a complicated process which is suggested to be divided into two processes controlled by dislocation and strain-induced martensite, respectively, and the transition point is about 20% strain demonstrated by experiments in this case.     

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.     

Ductility and fatigue properties of low nickel content type 316L austenitic stainless steel after gaseous thermal pre-charging with hydrogen

The susceptibility of low nickel content type 316L austenitic stainless steel to hydrogen was quantified using low strain rate tensile tests and strain-controlled low-cycle fatigue life measurements. Both tests were performed under air condition after charging with high-pressure 10-MPa hydrogen gas at 300 °C for eight days. No significant influence of hydrogen was recognized in 0.2% proof stress, but the strain at fracture and reduction area was decreased significantly in both hydrogen pre-charged and in gaseous hydrogen conditions compared to companion tests conducted in air. The decrease of fatigue life in the high strain amplitude region was related to a significant decrease in the plastic component while the effect of hydrogen on the elastic component was negligible. Highly localized deformation and a pronounced martensite transformation occurred near the site of the fracture surface in the high strain amplitude regime, resulting in the early formation of abundant micro-surface cracks in this regime of the hydrogen pre-charged samples.     

Hydrogen diffusivity in different microstructural components in martensite matrix with retained austenite

We elucidate the hydrogen diffusivity in martensite matrix with retained austenite (RA). Two aspects are focused: effect of microstructure on hydrogen diffusion behavior; hydrogen diffusivity calculation for different microstructural components. Quenched martensite (QM) had the highest effective hydrogen diffusion coefficient because of high dislocation density. Effective hydrogen diffusion coefficient decreased with the increase of intercritical annealing temperature because of decrease in dislocation density and increase of RA. According to the principle of Maxwell-Garnett equation, the hydrogen diffusion coefficient for grain boundary (GB) was 7.99 × 10^?8 m²/s and hydrogen diffusion coefficient of tempered martensite (TM) was 7.84 × 10^?11 m²/s.     

Effect of hydrogen and strain rate on nanoindentation creep of austenitic stainless steel

The effect of hydrogen and strain rate on nanoindentation creep of austenitic stainless steel was investigated by nanoindentation loading and creep tests. The loading segment mainly reflects dislocation nucleation, but creep mainly indicates dislocation movement. The strain rate significantly influences dislocation nucleation, resulting in increased creep displacement with strain rate. The interaction between hydrogen and dislocation is responsible for the increased creep displacement in 310S steel. Hydrogen greatly enhances the displacement burst after small strain rate due to increased dislocation nucleation rate. Hydrogen slightly enhances creep of 304 steel as hydrogen diffuses through strain-induced α′ martensite to accumulate under the indenter before creep and enhances dislocation movement.     

Influence of copper as an alloying element on hydrogen environment embrittlement of austenitic stainless steel

A Cu alloyed (18Cr–10Ni–3Cu) and a Cu free (18Cr–12.7Ni) austenitic stainless steel were tensile tested in gaseous hydrogen atmosphere at 20 °C and −50 °C. Depending on the test temperature, the Cu alloyed steel was extremely embrittled whereas the Cu free steel was only slightly embrittled. Austenite stability and inherent deformation mode are two main criteria for the resistance of austenitic stainless steels against hydrogen environment embrittlement. Based on the well known austenite stability criteria, the austenite stability of both steels should be very similar. Interrupted tensile tests show that martensite formation upon plastic deformation was much more severe in the Cu alloyed steel proving that the influence of Cu on austenite stability is overestimated in the empirical stability equations. When tested in high pressure H₂, replacing Ni by Cu resulted in a fundamental change in fracture mode atmosphere, i.e. Ni cannot be replaced by Cu to reduce the costs of SS without compromising the resistance to hydrogen environment embrittlement.     

Effect of nitrogen-addition on the absorption and diffusivity of hydrogen in a stable austenitic stainless steel

An austenitic stainless steel (SUS316L) was prepared with and without addition of solute nitrogen. The effect of cold-working and nitrogen addition on hydrogen solubility and hydrogen diffusion were investigated. High-pressure hydrogen gas and thermal desorption techniques were used. Increasing dislocation densities were related to a higher hydrogen content and higher nitrogen content related to a lower hydrogen content. Both dislocations and nitrogen had a negligible effect on hydrogen diffusion. The different hydrogen contents in the dislocations and the metal lattice, as well as trapping and diffusion activation energies explained the lack of effect of dislocations on hydrogen solubility.     

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.     

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.