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The influence of grain growth inhibition by pearlite on hydrogen embrittlement (HE) behavior of ultra-low carbon ferritic steels was studied. The Fe-0.02C alloy has a considerable sensitivity to HE, while the Fe and Fe-0.1C materials have lower HE sensitivity. The high fraction of high-angle grain boundaries (HAGB) is responsible for the shift of the peak to a higher temperature in the Fe-0.1C alloy. The trapping of hydrogen by HAGBs was observed by hydrogen mapping. Higher density of HAGBs contributes to higher trap density in Fe-0.1C alloy and lower H diffusion coefficient. The coexistence of hydrogen enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) mechanisms was identified and discussed. The results indicate that HEDE is active in the initial stage of the tensile loading during crack initiation and HELP in the rest of tensile testing duration that controls the crack propagation. Due to the reduction in grain size caused by the addition of pearlite, the normalized hydrogen content per HAGB length is significantly lower than Fe-0.1C, which means that the critical amount of local hydrogen required for crack initiation is less likely to accumulate due to the weakening of cohesive interatomic strength. In Fe-0.02C alloy, the ciritcal local content was built up at HAGBs, and resulting in activation of HEDE mechanism for crack initiation. In Fe-0.1C, a lower possibility of accumulating the critical H concentration at HAGBs, resulting in limited activation of HEDE-based crack initiation at HAGB and less crack propagation events based on the HELP mechanism. [ABSTRACT FROM AUTHOR]

To avoid failures due to hydrogen embrittlement, it is important to know the amount of hydrogen absorbed by certain steel grades under service conditions. When a critical hydrogen content is reached, the material properties begin to deteriorate. The hydrogen uptake and embrittlement of three different carbon steels (API 5CT L80 Type 1, P110 and 42CrMo4) was investigated in autoclave tests with hydrogen gas (H2) at elevated pressure and in ambient pressure tests with hydrogen sulfide (H2S). H2 gas with a pressure of up to 100 bar resulted in an overall low but still detectable hydrogen absorption, which did not cause any substantial hydrogen embrittlement in specimens under a constant load of 90% of the specified minimum yield strength (SMYS). The amount of hydrogen absorbed under conditions with H2S was approximately one order of magnitude larger than under conditions with H2 gas. The high hydrogen content led to failures of the 42CrMo4 and P110 specimens. [ABSTRACT FROM AUTHOR]

• SSRT with Hollow Probe Specimens show a decrease in K55′s mechanical properties. • Charging with 200 bar H 2 leads to a loss of 63 % of fracture elongation. • The embrittlement tendency increases with increasing charging pressure. • Beneficial influence of pearlite inhibiting the crack propagation. The susceptibility of ferritic pearlitic steel K55 to hydrogen embrittlement in high pressure gas atmospheres was investigated using slow strain rate tests with hollow probe specimens. Therefore, the specimens were charged with an internal hydrogen pressure of up to 20 MPa according to planned conditions for underground gas storages. The results show a significant decrease in ductility caused by hydrogen embrittlement yielding a loss in fracture strain of 46 % at 10 MPa and 63 % at 20 MPa hydrogen partial pressure compared to a specimen tested in nitrogen atmosphere. Furthermore, observations on the cross-sections of the tested specimens reveal that pearlite acts as obstacle for hydrogen enhanced crack growth. [ABSTRACT FROM AUTHOR]

• Two-stage tempering methodology decreases the sensitivity of Q&T steel to HE. • Nanosized Mo carbides serve as hydrogen trapping sites and improve HE resistance. • Reduced dislocation density lowers the quantity of diffusible hydrogen. The objective of the present study was to enhance the hydrogen embrittlement (HE) resistance of the quenched and tempered martensitic steel via the interplay of heat treatment variance and precipitation of nanosized carbides. For this purpose, one-stage tempering and two-stage tempering methodologies were implemented, and steel was alloyed with Mo to instigate the precipitation of Mo carbides. The results revealed that two-stage tempered steel exhibited superior resistance to HE, as a result of reduced dislocation density and higher quantity of Mo 2 C. To discern the role and trapping behaviour of Mo 2 C carbides, Thermal Desorption Spectroscopy (TDS) combined with electrochemical hydrogen charging was utilized. Precipitated nanosized Mo 2 C exhibited the ability to trap hydrogen. On the contrary, an increase of dislocations and higher diffusible hydrogen content in one-stage tempered steel promoted deterioration of mechanical properties which was investigated by Slow Strain Rate Test (SSRT) and fracture surface morphology analysis. In addition, the effective diffusion coefficient for one-stage tempered steel was lower, as dislocations served as additional trap sites. [ABSTRACT FROM AUTHOR]

Austenitic stainless steel 1.4435, a candidate material for high-pressure tubing for hydrogen applications, was investigated in two different stages of cold deformation (0 and 27 %) regarding its hydrogen embrittlement tendency. The determination of effective diffusion coefficients revealed slightly increased diffusion kinetics at 60°C for the cold-reduced material (8.0 · 10−15 m²/s) compared to the solution annealed one (5.0 · 10−15 m²/s). High-pressure gaseous hydrogen charging shows no differences regarding the degree of cold deformation with a maximum hydrogen concentration of 112 wt.-ppm. Furthermore, a significant ductility loss expressed by the ratio of the reduction of area was observed. • Cold drawn material shows enhanced diffusion kinetics than solution annealed state. • Hydrogen contents of up to 112 wt.-ppm are present after gaseous charging. • No failure occurred in constant load tests at 90 % of the yield strength. • SSRTs show no significant decrease in fracture elongation due to hydrogen charging. • 1.4435 is applicable for hydrogen tank systems within the tested conditions. [ABSTRACT FROM AUTHOR]

Hydrogen uptake and embrittlement characteristics of a cold-drawn austenitic stainless steel wire were investigated. Slow strain rate testing and fracture surface analysis were applied to determine the hydrogen embrittlement resistance, providing an apparent decrease in resistance to hydrogen embrittlement for a 50% degree of cold deformation. The hydrogen content was assessed by thermal desorption and laser-induced breakdown spectroscopy establishing a correlation between the total absorbed hydrogen and the intensity of near-surface hydrogen. The sub-surface hydrogen content of the hot-rolled specimen was determined to be 791 wt.ppm. • TDS combined with LIBS was applied to quantify H-diffusion profiles. • Critical hydrogen concentrations for brittle fracture were obtained. • The degree of cold-drawing was correlated to hydrogen-induced damage. [ABSTRACT FROM AUTHOR]