Hydrogen embrittlement in 304L (18wt.% Cr, 8-10wt.% Ni) austenitic stainless steel (ASS) fabricated by laser powder-bed-fusion (LPBF) was investigated by tensile testing after electrochemical hydrogen pre-charging and compared to conventionally available 304L ASSs with two different processing histories, (i) casting plus annealing (CA) and (ii) CA plus thermomechanical treatment (TMT). It was revealed that hydrogen-charging led to a significant reduction in ductility for the CA sample, but only a small effect of hydrogen was observed for the LPBF and CA-TMT samples. Hydrogen-assisted cracking behavior was found to be strongly linked to strain-induced martensitic transformation. In addition, the amount of alpha' martensite was much higher in the CA sample than in other samples, suggesting that severe hydrogen embrittlement can be correlated with the low mechanical stability of austenite. Detailed microstructural characterization showed that low austenite stability of the CA sample was mainly attributed to the retained content of delta ferrite and the chemical inhomogeneity inside the gamma matrix (gamma close to delta has ~2 wt.% higher Cr but ~2 wt.% lower Ni), but TMT enhanced the chemical homogeneity and thus austenite stability. By contrast, the LPBF process led directly, i.e. without any thermomechanical treatment, to a fully austenitic structure with homogeneous elemental distribution in the ASS. These results confirmed that the presence of delta and the chemical inhomogeneity inside gamma matrix, which promoted the deformation-induced martensitic transformation and the associated H enrichment at the gamma-alpha' interface, was the primary reason for the severe H-assisted failure.
The effect of crystallography on transgranular chloride-induced stress corrosion cracking (TGCISCC) of arc welded 304L austenitic stainless steel is studied on >300 grains along crack paths. Schmid and Taylor factor mismatches across grain boundaries (GBs) reveal that cracks propagate either from a hard to soft grain, which can be explained merely by mechanical arguments, or soft to hard grain. In the latter case, finite element analysis reveals that TGCISCC will arrest at GBs without sufficient mechanical stress, favorable crystallographic orientations, or crack tip corrosion. GB type does no…
In the present study, an attempt has been made to understand the effect of different competing mechanisms controlling the overall degree of sensitization (DOS) of deformed austenitic stainless steel at the early stage of sensitization. The Double Loop Electrochemical Potentiokinetic Reactivation (DL-EPR) studies were performed to characterize the Degree of Sensitization (DOS) as functions of pre-defined strain and sensitization temperature. X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were used to explain the phenomena qualitatively. A non monotonous behaviour in the variatio…
Irradiation Assisted Stress Corrosion Cracking (IASCC) is a material degradation phenomenon affecting austenitic stainless steels used in nuclear Pressurized Water Reactors (PWR), leading to the initiation and propagation of intergranular cracks. Such phenomenon belongs to the broader class of InterGranular Stress Corrosion Cracking (IGSCC). A micromechanical analysis of IGSCC of an irradiated austenitic stainless steel is performed in this study to assess local cracking conditions. A 304L proton irradiated sample tested in PWR environment and showing intergranular cracking is investigated. Se…
The mechanism of the evolution of the deformed microstructure at the earliest stage of annealing where the existence of the lowest length scale substructure paves the way to the formation of the so-called subgrains, has been studied for the first time. The study has been performed at high temperature on heavily deformed Ti-modified austenitic stainless steel using X-ray diffraction technique. Significant changes were observed in the values of the domain size, both with time and temperature. Two different types of mechanism have been proposed to be involved during the microstructural evolution …
Passivation mechanisms were investigated on (100)-oriented Fe-18Cr-13Ni surfaces with direct transfer between surface preparation and analysis by X-ray photoelectron spectroscopy and scanning tunneling microscopy and electrochemical characterization. Starting from oxide-free surfaces, pre-oxidation at saturation under ultra-low pressure (ULP) oxygen markedly promotes the oxide film Cr(III) enrichment and hinders/delays subsequent iron oxidation in water-containing environment. Exposure to sulfuric acid at open circuit potential causes preferential dissolution of oxidized iron species. Anodic p…
The effect of hydrogen on the fracture behaviour of austenitic stainless steel has been investigated in the past [1][2]. It has been reported that fracture initiates by void formation at inclusions and regions of enhanced strain localisation [3]. There is experimental evidence that supports the fact that hydrogen influences void nucleation, growth and coalescence during material fracture [4]. This work investigates the effect of hydrogen on void growth and coalescence in austenitic stainless steel. The effect of hydrogen on void growth and coalescence for different stress triaxialities has bee…
A model has been developed which simulates the deformation of single crystal austenitic stainless steels and cap-tures the effects of hydrogen on stress corrosion cracking. The model is based on the crystal plasticity theory which relates critical resolved shear stress to plastic strain and the strength of the crystal. We propose an analytical repre-sentation of hydrogen interactions with the material microstructure during deformation and simulate the effects hydrogen will have on void growth prior to fracture. Changes in the mechanical properties of the crystal prior to fracture are governed …
Various mechanisms have been proposed for hydrogen embrittlement, but the causation of hydrogen-induced material degradation has remained unclear. This work shows hydrogen embrittlement due to phase instability (decomposition). In-situ diffraction measurements revealed metastable hydrides formed in stainless steel, typically declared as a non-hydride forming material. Hydride formation is possible by increasing the hydrogen chemical potential during electrochemical charging and low defect formation energy of hydrogen interstitials. Our findings demonstrate that hydrogen-induced material degrad…
Understanding the mechanism of martensitic transformation is of great importance in developing advanced high strength steels, especially TRansformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to enhanced work-hardening rate, postponed onset of necking and excellent formability. In-situ transmission electron microscopy has been performed to systematically investigate the dynamic interactions between dislocations and alpha martensite at microscale. Local stress concentrations, e.g. from notches or dislocation pile-ups, render free edges and grain boundaries favorable nucleation s…
This work addresses the early stages ($\le$1000 h) of the dissolution corrosion behavior of 316L and DIN 1.4970 austenitic stainless steels in contact with oxygen-poor (C$_O$ < 10$^-$$^8$ mass%), static liquid lead-bismuth eutectic (LBE) at 500°C for 600-1000 h. The objective of this study was to determine the relative early-stage resistance of the uncoated steels to dissolution corrosion and to assess the protectiveness of select candidate coatings (Cr$_2$AlC, Al$_2$O$_3$, V$_2$Al$_x$C$_y$). The simultaneous exposure of steels with intended differences in microstructure and thermomechanical s…
In order to predict InterGranular Stress Corrosion Cracking (IGSCC) of post-irradiated austenitic stainless steel in Light Water Reactor (LWR) environment, reliable predictions of intergranular stresses are required. Finite elements simulations have been performed on realistic polycrystalline aggregate with a recently proposed physically-based crystal plasticity constitutive equations validated for neutron-irradiated austenitic stainless steel. Intergranular normal stress probability density functions are found with respect to plastic strain and irradiation level, for uniaxial loading conditio…
Metallic materials, especially steel, underpin transportation technologies. High-manganese twinning induced plasticity (TWIP) austenitic steels exhibit exceptional strength and ductility from twins, low-energy microstructural defects that form during plastic loading. Their high-strength could help light-weighting vehicles, and hence cut carbon emissions. TWIP steels are however very sensitive to hydrogen embrittlement that causes dramatic losses of ductility and toughness leading to catastrophic failure of engineering parts. Here, we examine the atomic-scale chemistry and interaction of hydrog…
Time of Flight Secondary Ion Mass Spectroscopy, X-Ray Photoelectron Spectroscopy, in situ Photo-Current Spectroscopy and electrochemical analysis were combined to characterize the physicochemical alterations induced by electrochemical passivation of the surface oxide film providing corrosion resistance to 316L stainless steel. The as-prepared surface is covered by a ~2 nm thick, mixed (Cr(III)-Fe(III)) and bi-layered hydroxylated oxide. The inner layer is highly enriched in Cr(III) and the outer layer less so. Molybdenum is concentrated, mostly as Mo(VI), in the outer layer. Nickel is only pre…
A metastable austenitic Fe-Cr-Ni steel was processed by selective laser melting with low power, resulting in nearly random crystallographic texture. In-situ tensile loading and neutron diffraction experiments were undertaken and the results indicate high ductility, despite the presence of porosity, and pronounced strain-induced martensitic transformation. A secondary hardening is observed in the mechanical data due to the martensite carrying significant load, upon its formation. The pronounced martensitic transformation is discussed with respect to the initial and the evolving deformation text…
University of Regensburg（雷根斯堡大学）；Sharif University of Technology（谢里夫理工大学）；Friedrich-Alexander-Universität Erlangen-Nürnberg（埃尔朗根-纽伦堡大学）；Harvard Medical School（哈佛医学院）；Harvard University（哈佛大学）
During the strip rolling process, a considerable amount of the forces of the material pressure cause elastic deformation on the work-roll, i.e., the deflection process. The uncontrollable amount of the work-roll deflection leads to the high deviations in the permissible thickness of the plate along its width. In the context of the Austenitic Stainless Steels (ASS), due to the instability of the Austenite phase in a cold temperature, cold deformation leads to the production of Strain-Induced Martensite (SIM), which improves the mechanical properties. It leads to the hardening of the ASS 316L du…
The behavior of hydrogen in iron and iron alloys is of interest in many fields of physics and materials science. To enable large-scale molecular dynamics simulations of systems with Fe-H interactions, we develop, based on density-functional theory calculations, an interatomic Fe-H potential in the Tersoff-Brenner formalism. The obtained analytical potential is suitable for simulations of H in bulk Fe as well as for modeling small FeH molecules, and it can be directly combined with our previously constructed potential for the stainless steel Fe-Cr-C system. This will allow simulations of, e.g.,…
Leakage-Safe Machine Learning for Hydrogen Embrittlement Detection in 316L Stainless Steel: A Region-Held-Out Evaluation of Texture and Deep Features in SEM Micrographs
Institute of Physics, Bahauddin Zakariya University（巴哈丁·扎卡里亚大学物理研究所）
The main purpose of this study is to determine, via a three dimensions Finite Element analysis (FE), the stress and strain fields at the inner surface of a tubular specimen submitted to thermo-mechanical fatigue. To investigate the surface finish effect on fatigue behaviour at this inner surface, mechanical tests were carried out on real size tubular specimens under various thermal loadings. X ray measurements, Transmission Electron Microscopy observations and micro-hardness tests performed at and under the inner surface of the specimen before testing, revealed residual internal stresses and a…
Unlike the vast amount of irradiated material data that exists for stainless steel internals from LWRs, with high fast neutron flux and an irradiation temperature of~330oC, the CANDU reactor is unique with a high thermal spectrum stainless steel components peripheral to the core. In particular, the CANDU design contains an austenitic stainless steel calandria vessel, which contains the heavy water moderator at a temperature of 60-80oC. This article explores the effects of low (60-80oC) and moderate (300-360oC) irradiation temperature on irradiation induced defects and defect sinks, both in ter…
Hydrogen embrittlement mechanisms of steels have been studied for several decades. Understanding hydrogen diffusion behavior in steels is crucial towards both developing predictive models for hydrogen embrittlement and identifying mitigation strategies. However, because hydrogen has a low atomic mass, it is extremely challenging to detect by most analytical methods. In recent years, cryogenic-transfer atom probe tomography (APT) of electrochemically-deuterium-charged steels has provided invaluable qualitative analysis of nanoscale deuterium traps such as carbides, dislocations, grain boundarie…
Electrochemical polarization measurements were combined with surface analysis by Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), X-Ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM) to study the alterations of the passive film on 316L austenitic stainless steel induced by the presence of chlorides in sulfuric acid electrolyte. The work was performed at a stage of initiation of localized corrosion preceding metastable pitting at the micrometer scale as verified by current transient analysis and AFM. The results show that Cl-ions enter the bilayer structure of the …
This thesis was focused on the surface modification with plasma discharge. Austenitic AISI 316L stainless steel sample was carburised under different experimental conditions and mechanical properties have been studied (thickness, lattice parameter, elemental composition, hardness, wear resistance and corrosion resistance). After that, steel substrates have been nitrided or carburised in order to analyse their stability under ion bombardment using a plasma focus device. Helium and deuterium were the gases used in 0, 1, 5, and 10 discharges. Optical and X-ray characterisations were used. Finally…
