Scanning Kelvin Probe Force Microscopy (SKPFM) was used to study the hydrogen diffusion through a surface designed for simultaneous mapping of multiple areas containing different thermal oxides, all ...covered with Pd. Potential maps were obtained simultaneously on an area of bare iron as the reference, an area covered with a bilayer oxide (inner magnetite and outer hematite) and an area covered with a magnetite layer (obtained by removing the outer hematite layer of a bilayer oxide). After hydrogen charging at the bottom side of the specimen, a contrast was obtained in the potential mapping on the covering Pd layer due to differences in hydrogen release through these distinct areas on the specimen surface. A finite element method (FEM) model of hydrogen diffusion across the different phases was developed to simulate the experiment. The modelling showed that both a lower diffusion coefficient and a lower solubility in the oxide can explain the contrast obtained in SKPFM. Cross diffusion in the ferritic bulk underneath the thermal oxide was found to have an influence on the spatial distribution of the hydrogen release.
•SKPFM was used to map the release of hydrogen through a Palladium covered surface.•The impact of thermal oxides on the hydrogen release was investigated.•Oxide layers resulted in a significant reduction in the release of hydrogen.•FEM modelling was used to model lower diffusion and/or lower solubility in the oxide.
The present work considers the effect of hydrogen on the mechanical properties of generic Fe-C-W alloys. Thermal treatment aimed at a martensitic microstructure and two conditions are compared; i.e. ...an as-quenched state and a quenched and tempered state with lower dislocation density and with W based carbides formed during tempering. The hydrogen induced mechanical degradation is evaluated by in-situ tensile tests, while thermal desorption spectroscopy, hot/melt extraction and permeation experiments are performed to understand the observed hydrogen embrittlement degree. The hydrogen induced ductility loss increases with increasing carbon content due to the higher amount of hydrogen trapped by the denser martensitic lath boundaries. Furthermore, the quenched and tempered condition shows a lower susceptibility to hydrogen. This is correlated to the reduced hydrogen content when tempered due to a decreased dislocation density and the fact that the tempered induced W2C particles did not trap hydrogen. Moreover, corresponding observations to this interpretation are perceived by permeation experiments since, on the one hand, the diffusion coefficient decreases with carbon content due to the increasing trapping ability of alloy A➔B➔C, and, on the other hand, the diffusivity increases when tempering is applied, which is also linked to the decrease in dislocation density and the disability of the carbides to hinder hydrogen diffusion by efficient trapping.
•The effect of H charging on tungsten alloyed steels is assessed by tensile tests.•The HE resistance increased for the Q&T condition compared to the as-Q state.•H trapping and absorption are determined by H characterization methods.•H diffusivity was higher in Q&T condition due to dislocation density decrease.•A correlation between the degree of HE and the amount of mobile H was established.
In the present work, the hydrogen/material interaction with two austenitic steel types, created via a completely different alloying strategy, is investigated. The two steels include 304L austenitic ...stainless steel (ASS) and 18Mn-0.6C twinning-induced plasticity (TWIP) steel. Constant extension rate tensile tests are performed to evaluate the influence of hydrogen on the deformation and fracture mechanisms. A reference condition without hydrogen is compared to a seven days electrochemically hydrogen precharged condition. 304L ASS shows transformation to α’-martensite without hydrogen, while hydrogen increases the α’-martensite fraction and additionally enables the transformation to ε-martensite. The fracture surface has a transgranular, quasi-cleavage appearance. The TWIP steel shows deformation twinning without hydrogen, while abundant ε-martensite transformation is observed in the presence of hydrogen. The fracture type is intergranular due to the high cracking sensitivity of the grain boundaries.
A virtually limitless amount of metals can be produced following the high entropy concept, of which at least some of them have shown to exhibit extraordinary properties such as high strength or ...toughness. Several compositions also seem to possess excellent resistance to hydrogen embrittlement. In this work, a near-equiatomic CoCrFeMnNi is investigated in interaction with hydrogen. The material is characterized by a higher hydrogen solubility and diffusivity as compared to an austenitic stainless steel due to the complex chemistry able to accommodate more hydrogen as well as providing fast hydrogen diffusion paths. The material is significantly embrittled after electrochemical hydrogen charging. A transitional fracture surface is observed through the thickness which can be linked to the hydrogen concentration gradient. High hydrogen concentration leads to intergranular fracture, while more moderate hydrogen concentrations lead to quasi-cleavage type of fracture.
Thermal desorption spectroscopy (TDS) was performed on ultra-low carbon (ULC) steel with various degrees of hydrogen-induced damage and deformation-induced defects. First, the extent to which ...hydrogen-induced damage manifests itself in TDS measurements was evaluated. Application of multiple test conditions on cold deformed ULC steel with and without the presence of hydrogen-induced damage showed that such damage did not appear as a hydrogen signal on the TDS spectra. Second, interesting features observed on the TDS spectra of cold deformed ULC steel were further investigated by assessing the TDS spectra of recrystallised and annealed ULC steel. As such, the four peaks in the TDS spectra were linked to microstructural features: interstitial lattice positions, grain boundaries, dislocations and microvoids.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Hydrogen embrittlement leads to mechanical degradation of metals. Hence, hydrogen sorption/desorption properties of metals need to be characterized. An electrochemical procedure based on cyclic ...voltammetry (CV) and potentiostatic polarization is elaborated on plain-carbon steel. The procedure consists of first two consecutive CV cycles (pretreatment and reference CV), followed by cathodic H-charging, and subsequent CV scans to study and quantify the H-sorption/desorption. Best practice in this procedure is to perform all steps consecutively without interruption or sample manipulations between steps to avoid spontaneous H-loss. The H-related interaction with the steel is clearly identified in the CV and can be differentiated from the electrolyte contribution coming from thiourea. The study confirms the role of thiourea as H-recombination poison in alkaline solution, and also demonstrates that it contributes to the CV response. Additionally, various charging times are investigated to study the time to H-saturation, and also the scan rate during the CV procedure is varied to study time-related phenomena. Dedicated discharging experiments were included in the study to complement the CV data, giving additional insights in the H-steel interaction. Moreover, hydrogen related findings are successfully verified by using a complimentary method, i.e. hot extraction. The better understanding of the peaks in the CV and the continuous procedure result in a reliable methodology to characterize the H-sorption/desorption in steel.
Advanced (ultra)high-strength steels that utilise bcc-fcc microstructures are appealing solutions for producing a combination of high strength and deformability. However, they are also susceptible to ...hydrogen embrittlement (HE). As larger less stable retained austenite (RA) can impair mechanical performance, its size and morphology are critical factors for achieving and maintaining the desired properties. Here, we present a combined experimental–density functional theory (DFT) study on HE with medium-carbon direct-quenched and partitioned (DQ&P) martensitic steels with varying vol% and film-thickness of RA, showing significantly improved HE resistance as a function of bcc-enrichment and increasing RA film-thickness. DFT reveals low attraction of hydrogen in bcc-Fe with Al, implying a stronger push towards fcc. Furthermore, the solubility of hydrogen increases dramatically with the carbon-enrichment of fcc-Fe when hydrogen and carbon are second neighbours. The theoretical results explain the observed differences in hydrogen diffusion, trap density, and the consecutively lower HE in Al-DQ&P over Si-DQ&P. Our combined experimental and theoretical study thus highlights the important interplay of bcc and fcc phases and H-uptake within austenite-containing carbon steels.
•Improving HE resistance as a function of RA film-thickness and bcc-alloying with Al.•Insights into the interplay of bcc/fcc and H-uptake for austenite-containing carbon-steels.•Outstanding performance of Al-DQ&P with thickest RA-films under in situ hydrogen-charged SSRT tests.•DFT reveals H-solubility in bcc-Fe lower with Al-alloying over Si-alloying.•H-solubility in fcc increases significantly when H and C are second neighbours.
Herein, a numerical solution of Fick's second law in one dimension with experimentally determined diffusion coefficients at different constant loads is combined with slow strain rate tensile tests ...and subsequent fractography on dual‐phase steel. From the latter, the depth of the hydrogen embrittled region is determined and correlated to concentration profiles determined by the numerical model. The concentration profiles indicate that incorporating stress dependency of the diffusion coefficient results in different concentration profiles compared to using a constant diffusivity. Additionally, these allow to more accurately determine a critical local hydrogen concentration based on the total diffusible hydrogen concentration at saturation.
This study presents a numerical solution of Fick's second law with experimental stress‐dependent diffusion coefficients and slow strain rate tensile testing results as input data. Comparing the modeled concentration profiles with fractographic scanning electron microscopy images allows to determine a critical local hydrogen concentration for brittle fracture.
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