Effect of Hydrogen on Fracture of Austenitic Fe-Mn-Al Steel

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.

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.

Journal of the Association of Arab Universities for Basic and Applied Sciences, 2012

Fracture morphologies of tested specimens, by means of the slow strain rate tensile testing, were examined to study fracture modes by using scanning electron microscopy. This study has led to the determination of three categories of specimens: unembrittled specimens, slightly embrittled specimens and severely embrittled specimens. The first category of unembrittled specimens includes specimens tested in air, at potentials of À0.80 and À0.85 V (SCE) with embrittlement ratio (ER) P 0.97. In the category of specimens slightly embrittled, only the specimen tested at À0.9 V with an embrittlement ratio (ER) of 0.69 is included. The last category of severely embrittled specimens includes the specimens with embrittlement ratio (ER) < 0.69. The fractographic studies are consistent with the embrittlement of steel being due to hydrogen embrittlement. The intergranular and transgranular quasicleavage fracture surface morphologies observed with embrittled specimens are typical and characteristic of hydrogen embrittlement.

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.

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.

Corrosion Science, 2011

The paper describes effect of hydrogen on the properties and fracture characteristics of two variants of TRIP 800 C–Mn–Si steels. The effect of hydrogen was studied by means of tensile tests on specimens previously charged by hydrogen. Hydrogen provoked embrittlement in both variants but only for very high hydrogen content. Hydrogen embrittlement manifested itself mainly by a loss of plasticity.

Hydrogen embrittlement (HE) of TM210 maraging steel was studied by slow strain rate tensile and constant load tests. The over-aged sample exhibited the best resistance to HE, since HE susceptibility of the maraging steel does not depend on the strength, but rather on the reverted austenite content. The hydrogen concentration, observed by scanning Kelvin probe force microscopy, was enriched in the reverted austenite at the grain boundaries and martensite lath boundaries, resulting in hydrogeninduced cracks propagating along the grain boundaries and martensite lath boundaries.

Hydrogen redistribution under stress-induced hydrogen diffusion and corresponding fracture behaviour of a 960 MPa grade martensitic steel were studied. Slow strain rate tensile (SSRT) tests after hydrogen pre-charging were performed and the fracture surface was observed and analysed. The strain rate ranged from 10 −6 to 10 −4 s −1. In the pre-charged sample with a certain hydrogen content of 0.62 ppm, hydrogen distribution was homogeneous before the SSRT test. After tensile testing, brittle fracture features appeared in the centre of the fracture surface, while ductile features appeared in the surrounding area. Brittle region size increased with the strain rate slowing down in the range from 10 −4 to 5 × 10 −6 s −1 , while it stabilised at the strain rate slower than 5 × 10 −6 s −1. Relationship between the strain rate and the brittle region size was established and discussed based on the present data of hydrogen content in the material.

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, 2020

Hydrogen embrittlement is a well-known problem with high-strength steels. An important aspect of hydrogen embrittlement research is the effect of the prior austenite grain (PAG) structure on hydrogen-induced fracture. The microstructural anisotropy of PAG structure depends on the steel manufacturing process. In this study, 500 HBW martensitic steels with different PAG structures are investigated with a novel tuning-fork test that utilizes an integrated loadcell system. The loadcell clamping system is used during hydrogen charging, allowing tracking of the applied force throughout the tests, which enables detection of separate phases of cracking and time-to-fracture. The elongated PAG morphology produces different results depending on the crack path direction in relation to the rolling direction, whereas the equiaxed PAG morphology does not manifest an orientation dependence. Depending on the PAG shape, also the fracture morphology differs. Time-to-fracture results show that elongate...