Hydrogen redistribution under stress-induced diffusion and corresponding fracture behaviour of a structural steel (original) (raw)
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Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test
Corrosion Science, 2007
The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentiallynotched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel.
The influence of hydrogen flux on crack initiation in martensitic steels
Understanding hydrogen transport and trapping phenomena is a key feature to revisit the hydrogen embrittlement (HE) models proposed in the literature. Both aspects can be affected by stress-strain states at different microstructural scales. Elastic distortion and plastic strain are both aspects of the mechanical states associated with defects (vacancies, dislocations), metallurgical elements (grain boundaries, precipitates), internal stresses and applied stresses, which can modify the diffusion and solubility of hydrogen. In the present work we first explore the effects of a tensile stress applied on martensitic steel membrane on the hydrogen concentration and mobility. In a second part, we analyse the impact of mobile and trapped hydrogen on HE using local approach of fracture under hydrogen flux.
Corrosion Science, 2021
A rapid fracture test in four-point bending is proposed to assess hydrogen embrittlement (HE) susceptibility of high strength martensitic steels. The novelty of this technique is the rapid rate of loading, whereas conventional approaches require prolonged slow strain rate testing. The essential fractographic features required to identify the mechanisms of HE failure remain evident, despite the fast loading conditions. To demonstrate these attributes, two quenched and tempered steels at two different strength levels were tested, with and without pre-charging of hydrogen. Stress coupled hydrogen diffusion finite element analysis was performed to calculate both stress and hydrogen concentration distributions. In addition to fractographic analysis, a mechanistic description rooted in hydrogen enhanced decohesion (HEDE) mechanism was used to corroborate the mechanical test data. The study shows that the approach is capable of quantifying HE susceptibility by being responsive to key factors affecting hydrogen induced fracture, thus developing further understanding on the HE of martensitic steels.
Effect of Hydrogen on Fracture of Austenitic Fe-Mn-Al Steel
ISIJ International, 1994
Bombay 400 076, India. / 993) In this investigation a stable high manganese austenitic steel, 0.45C-1 7Mn-2.8Al, has been studied for hydrogen embrittlement using cathodically precharged specimens. Tensile testing of axisymmetric and p[ane strain specimens precharged with hydrogen show an appreciable loss of 8-100/o reduction in area (RA) whereasthe loss in o/o elongation is lesser. Thetrue fracture strain decreased from 0.88 to O.73 for axisymmetric and from O.79 to 0.60 for plane strain specimens, Hydrogen precharging is observed to result in decrease of CTOD at crack initiation by about 0.07 mm and a decrease in crack tip fracture strain for crack initiation from O.53 to 0.34. The greater effect of hydrogen precharging thus observed is attributed to existence of higher stress triaxiality in CTOD and plane strain tensile testing in comparison to axisymmetric one. OnSEM examination of fracture surfaces the uncharged tensile specimensshowedon]y dimpled fracture, the precharged specimensshowedtransition from dimpled to quasicleavage and intergranular fracture near the surface. Regions close to the pre-fatigue tip in CTOD specimens depict intergranular fracture. The fractographic changes are attributed to the combined role of stress intensity and hydrogen concentration variation arising out of hydrogen transport inside the specimen.
2014
The hydrogen embrittlement (HE) may sometimes affect the Martensitic Steels following surface treatments. This damage appears to be dependent on baking time during which hydrogen can be desorbed from the sample and/or be redistributed within the material. The present study aims to identify the evolution of different hydrogen states in a quenched and tempered martensitic steel during the desorption phase and to evaluate their effects on the mechanical behavior in a simple tensile test on smooth specimens. The present work shows that during baking phase at specific temperatures, a real competition exists between desorption and deep trapping on specific defects (vacancies and/or dislocations) of diffusible hydrogen. The transition between both the regimes implies a times range for which the initially ductile rupture becomes a quasi-cleavage process. This transition of the fracture mechanisms is directly correlated to a time at which the flux of hydrogen is maximized. The critical stress for quasi-cleavage fracture was assessed at 435 MPa and ductile fracture criterion follows a Beremin form, which suggests a predominance of decohesion at inclusion and/or precipitate interfaces less affected by hydrogen (critical stress around 600 MPa).
Corrosion Science, 2006
The critical hydrogen concentration for hydrogen induced delayed fracture of the AISI 4135 steel at 1320 and 1450 MPa has been determined by constant load tests in combination with numerical calculations, and thus the concept of a critical hydrogen concentration has been verified. The time to fracture was obtained for circumferentially notched round bar specimens under a constant load after electrochemically pre-charged with various hydrogen contents. A numerical model was then developed for calculating the accumulated hydrogen concentration in the vicinity of the notch root, taking into account the driving effect of the hydrostatic stress on hydrogen transport. The results showed that the delayed fracture of the steel occurred when a critical hydrogen concentration at the location of the stress peak was reached by accumulation, and that the time to fracture was related to the stress-driven hydrogen accumulation process. The critical hydrogen concentration was dependent not only on the strength level, but also on the stress concentration factor of the specimens.
Fracture criterion for hydrogen embrittlement of high strength steel
Materials Science and Technology, 2006
The notch tensile strength of a boron bearing steel at 1305 MPa has been investigated by means of slow strain rate tests after electrochemical hydrogen charging. Results show that the notch tensile strength of the steel decreased with increasing diffusible hydrogen content and the decrease in the notch tensile strength was more pronounced for specimens at a higher stress concentration factor. Finite element analysis results show that the dependence of the notch tensile strength on stress concentration factor cannot be accounted for by the local peak stress and local peak hydrogen concentration; however, the equivalent plastic strain at the notch root at an applied stress equal to the notch tensile strength was independent of stress concentration factor. The equivalent plastic strain at the notch root can be used as a fracture criterion for hydrogen embrittlement of the steel.
Influence of hydrogen on plasticity around the crack tip in high strength steels
Engineering Fracture Mechanics, 2017
Fracture mechanics concepts applied to tests in aggressive environments are a challenge for integrity analysis. Specifically about hydrogen, the concentration of this element in defects or in trapping sites can cause unexpected failure. The present paper presents results showing the influence of hydrogen in the reduction of fracture toughness and a discussion about how to deal with it in high strength alloys. The results show that the hydrogen reduces the plasticity and consequently the applications of CTOD concepts are questionable for the studied materials.
The Effect of Hydrogen on Failure of Complex Phase Steel under Different Multiaxial Stress States
Metals
The demand for advanced high-strength steel (AHSS) in the automotive industry has increased over the last few years. Nevertheless, it is known that AHSSs are susceptible to hydrogen embrittlement. Therefore, the influence of hydrogen on the localization and damage behavior of a CP1000 steel sheet was investigated in this work. The sheet metal was electrochemically charged to a hydrogen content of about 3 ppm (by weight). Tensile tests were performed at different nominal strain rates between 0.00004 s−1 and 0.01 s−1 to investigate the effects of strain rates on their susceptibility to hydrogen embrittlement. Nakajima tests were utilized to investigate the hydrogen effects on the steel’s formability under different stress states. Three different Nakajima specimen geometries were employed to represent a uniaxial stress state, a nearly plane strain stress state, and an equibiaxial stress state. Further, forming limits were evaluated with the standardized section line method. Hydrogen em...