Slow Strain Rate Tensile Research Articles

Investigating the Stress Corrosion Cracking (SCC) Susceptibility of Ti-6Al-4V Alloys Fabricated by Electron Beam Melting in Deep-Sea Environment

The stress corrosion cracking (SCC) susceptibility of electron beam melted Ti-6Al-4V alloy (ET) was compared with the conventional wrought alloy (WT). The electrochemical and slow strain rate tensile (SSRT) tests, as well as surface analysis, were conducted under simulated shallow and deep-sea environment. Under shallow conditions, the SCC susceptibility of both alloys was almost the same because of consistent passivation and repassivation performance of the passivating film. However, under deep-sea conditions, SCC susceptibility of ET was higher than that of WT due to stronger textured-like surface that appeared on ET alloy, where early developed passive film broke down, demonstrating lower passivation and repassivation rate.

Investigation of the Effect of pH on Stress Corrosion Cracking of API 5L X65 Steel by Impedance Spectroscopy and Slow Strain Rate Tensile Test

Investigation of the Effect of pH on Stress Corrosion Cracking of API 5L X65 Steel by Impedance Spectroscopy and Slow Strain Rate Tensile Test

Effects of composition and microstructure on oxidation and stress corrosion cracking susceptibility of stainless steel claddings in hydrogenated PWR primary water

Oxidation resistance and stress corrosion cracking (SCC) susceptibility of A508Ⅲ-309 L/308 L stainless steel (SS) claddings were investigated in simulated pressurized water reactor (PWR) primary water following a microstructure characterization. Preferential oxidation of slip bands was observed in 309 L SS. The oxidation resistance of 309 L SS was found to be lower than that of 308 L SS due to the higher Cr- and ferrite-content in 308 L SS than in 309 L SS after cladding. There was no indication of SCC for low alloy steel (LAS) after slow strain rate tensile (SSRT) tests. The presence of type-I and type-II boundaries, composition dilution and less δ-ferrite content synergistically contributed to increasing the SCC susceptibility of 309 L SS cladding. The effect of chemical composition and microstructure on SCC performance of the SS claddings was discussed.

Stress corrosion and mechanical properties of zinc coating on 304 stainless steel

Abstract The effect of stress corrosion on the mechanical properties of the coating in the zinc coating/304 stainless steel substrate system was investigated by three-point bending, slow strain rate tensile (SSRT) and nano-indentation tests. Studies have shown that fracture toughness was improved when the coating was thick but weakened when the coating was thin. At varied coating thicknesses (80 μm, 160 μm, 240 μm, 320 μm, 400 μm), the decline rates of the fracture toughness were 77.48 %, 71.82 %, 66.67 %, 55.48 % and 51.52 %, respectively, and those for the critical strain of crack initiation were 94.97 %, 91.88 %, 88.42 %, 76.19 % and 74.33 %, respectively. In addition, simulations were made to analyze the crack propagation of zinc coating in coating/substrate system under tensile loading by ABAQUS, which proved the accuracy of the experiment.

Atom probe tomography analysis of hydrogen distribution in laser peened Ti6Al4V alloy to control hydrogen embrittlement

Hydrogen embrittlement of Ti6Al4V alloy was investigated under electrochemical cathodic hydrogen charging. Advanced surface treatment and material characterisation, laser shock peening (LSP) or laser peening (LP), atom probe tomography, slow strain rate tensile (SSRT) test, and XRD residual stress are employed for analysis. Results obtained exposed the opportunity to implement surface treatment for controlling hydrogen embrittlement. Induced grain refinement and compressive residual stress restrict the hydrogen uptake in Ti6Al4Valloy. Furthermore, APT results gave wt. % of TiH (titanium hydride) in laser peened hydrogen charged and untreated hydrogen-charged specimens. Thus, it is concluded that compressive stress induced by laser peening process controlled uptake of hydrogen by Ti6Al4V alloy.

Effects of heat treatments on the microstructure and environment-induced cracking of CF8A steel in simulated BWR water

The effects of heat treatments on the susceptibility to environment-induced cracking (EIC) of the CF8A stainless steel (SS) in simulated water of a boiling water reactor (BWR) were investigated by slow strain-rate tensile (SSRT) tests. Different microstructures were resulted by subjecting the as-casted CF8A to stress relief treatment (1050 °C/2 h, C sample), thermal aging (385 °C/20000 h, A sample), recovery annealing (500 °C/1 h, R sample) or solution annealing (1200 °C/2 h, S sample). The results indicated that after aging treatment (A sample), a decrease in tensile ductility and impact toughness was resulted relative to the un-aged specimens. Recovery annealing (R sample) caused an improved tensile ductility and impact toughness of the embrittled CF8A. Excessive solution (S sample) led to an obvious decrease in tensile strength but increase in ductility and toughness of the CF8A. The EIC susceptibility of the tested samples in simulated BWR water increased in the order: S < C < R < A samples. The cracks in distinct CF8A specimens were found to initiate at and propagate along δ/γ interfaces in simulated BWR water. The lowered ferrite content and change in ferrite morphology accounted for the superior EIC resistance of the S sample relative to that of the other ones in simulated BWR water.

Improved Stress Corrosion Cracking Resistance of High‐Strength Low‐Alloy Steel in a Simulated Deep‐Sea Environment via Nb Microalloying

Microstructure observations, thermal desorption spectroscopy (TDS) tests, hydrogen permeation measurements, electrochemical tests, and slow strain rate tensile (SSRT) tests are conducted investigate the beneficial effect of Nb microalloying on the stress corrosion cracking (SCC) behavior of high‐strength low‐alloy (HSLA) steels in a simulated deep‐sea environment. The results show that the addition of Nb significantly decreases the SCC susceptibility of HSLA steel in a simulated deep‐sea environment, and the role of Nb is attributed to microstructure optimization and the strong hydrogen‐trap function of nanosized NbC precipitates with hydrogen activation energy up to 100.5 kJ mol−1. In the simulated deep‐sea environment, the SCC resistance of HSLA steel is enhanced by microstructure optimization and the hydrogen‐trap function via Nb microalloying, and both anodic dissolution (AD) and hydrogen embrittlement (HE) are weakened. Moreover, the improvement of SCC resistance of HSLA steel with a higher hydrogen concentration via Nb microalloying is more obvious, in which the decrease of hydrogen‐induced anodic dissolution (HIAD) and HE effects is more distinct, and the strong hydrogen‐trap effect of NbC precipitates is the predominant mechanism to improve SCC resistance.

Stress corrosion cracking behavior of welding joint of high strength steel

Nowadays, high-strength steel structures are increasingly being used in marine or soil environments, but low attention has been paid on the corrosion and stress corrosion cracking problem. In this paper, the susceptibility to stress corrosion cracking (SCC) of butt-welding joints of Strenx® 700 (S690QL high strength steel) both in marine and acid-polluted marine atmospheres was studied, by using slow strain rate tensile (SSRT) test. The seawater corrosion environment was considered and it was prepared following the ASTM D1141. To study the influence of the environment pH, NaOH 0,1M solutions and pure H2SO4 were used to adjust the pH of the simulated seawater solution to 8 and 4, respectively. The slow strain rate tensile tests (SSRT) with a crosshead speed of 0.000017 mm/s were carried out at room temperature (approximately 25 °C). After failure, the SCC susceptibility was evaluated acquiring the elongation loss rate and the reduction-in-area loss rate. The fracture surfaces were analyzed by SEM observation.

The effect of low temperature heat treatment on stress corrosion crack initiation in machined 316L stainless steel in high-temperature hydrogenated water

A notable reduction of the stress corrosion cracking (SCC) initiation susceptibility was identified for machined cold-rolled 316L stainless steel that received a 650°C/10 hours heat-treatment prior to slow strain rate tensile (SSRT) testing in high-temperature hydrogenated water. The cracks per unit area of the machined surface of heat-treated specimens decreased by ~50% compared to as-machined surfaces, and by >70% with respect to polished surfaces. The results were ascribed to recrystallization of the ultrafine-grains present in the outer deformation layer of the machined surface, which resulted in a reduction of the tensile residual stress and nano-indentation hardness.

Effect of solution pH on stress corrosion cracking behavior of modified AZ80 magnesium alloy in simulated body fluid

Magnesium (Mg) and its alloys are widely used as biodegradable materials in orthopedic applications. However, the poor corrosion resistance of Mg alloys in physiological environments largely limits their application in medical implants. In this study, the surface of AZ80 Mg alloy were modified by two treatment methods, micro-arc oxidation (MAO) and the composition of laser shock penning/micro-arc oxidation technology (LSP/MAO). Slow strain rate tensile (SSRT) testing was performed on untreated (BM) and treated (MAO and LSP/MAO) specimens in simulated body fluid (SBF) with various pH values (4, 7.4, and 9) at a strain rate of 5.3 × 10−7 s−1. The scanning electron microscopy (SEM) was used to observe the fracture morphology of corroded specimen. The energy-dispersive X-ray spectroscopy (EDS) was used to analyses elements on corroded surface. The corrosion resistance of different specimens was investigated by electrochemical testing. The results indicated that the MAO specimen itself had the best stress corrosion cracking (SCC) resistance in pH 7.4 SBF, and the worst in pH 4 SBF. The LSP/MAO specimen possessed superior SCC resistance to MAO specimen in SBF with various pH values. The LSP/MAO composite coating can effectively improve the corrosion resistance and mechanical properties of AZ80 Mg alloy, which helps to extend the service life of Mg alloy implants and promote the development of Mg alloys in medical and industrial fields. The SCC mechanisms of three specimens were evaluated in SBF with various pH values.

Modeling and Numerical Validation of Stress-Strain Curves of Maraging Steels, Grades 300 and 350 Under Hydrogen Embrittlement

Maraging steels are ferrous alloys with Ni, Co, Mo, and Ti additions. These materials are a special class of ultra high mechanical strength steels with wide and special applications in strategic areas, which makes their knowledge very valuable. Computational advances allowed to analyze the behavior of these materials numerically, using the finite element method and developing mathematical models that can represent numerically its mechanical behavior. The present work has the objective of surveying the mechanical properties of maraging steels 300 and 350 by slow strain rate tensile (SSRT) tests, after the solution treatment at 1183K for 1h. Additionally, it was evaluated the hydrogen embrittlement in samples tested by SSRT under cathodic protection with a potential -1.2 VSCE in 3.5% NaCl solution. The study was complemented with detailed fractographic analysis. This work also presents the analysis of representative models by use of Hollomon, Swift, Voce and coupled Swift-Voce equations to describe the strain-hardening behavior. Compared to the others, the Voce’s model was the one which best fitted the experimental results, with values of R2 higher than 0.992. Through the variation of the chemical composition found in the different grades of maraging steels, this work contemplates the creation of a generalized Voce model based on the variation of the Ti content. The work concludes presenting the generalized Voce model proposed and a numerical analysis of the SSRT results with a good accuracy of the strain-hardening response.

Inhibiting stress corrosion cracking by removing corrosion products from the Mg-Zn-Zr alloy pre-exposed to corrosion solutions

Magnesium and its alloys are susceptible to stress-corrosion cracking (SCC), which can manifest itself during the slow-strain rate tensile (SSRT) testing in air if the specimens were pre-exposed to corrosive media. This phenomenon is generally associated with hydrogen embrittlement (HE) which is believed to be related to diffusible hydrogen penetrating into the metal during the pre-exposure. In the present study, we show that the corrosion product layer deposited on the surface of the pre-exposed specimens is crucial in the SCC mechanism. The specimens of the alloy ZK60 were SSRT tested in air, in corrosive media, in air after pre-exposure to corrosive media as well as after pre-exposure and removal of corrosion products. To vary the severity of SCC, four NaCl-based corrosion solutions were utilised. The embrittlement resulted in a marked decrease in ductility and the concurrent appearance of multiple side-surface cracks as well as brittle fragments on the fracture surface. The most striking finding of the present study is that the removal of corrosion products from the surface of the pre-exposed specimens results in complete recovery of the mechanical response and in the elimination of all harmful embrittling features regardless of the corrosive solution used. This effect is found to be inconsistent with the common viewpoint where the SCC mechanism is governed entirely by diffusible hydrogen. The obtained results shed new light on the role of diffusible hydrogen, corrosion products and irreversible corrosion damage in the SCC mechanism of Mg alloys pre-exposed to corrosive media.

Pearlite-driven surface-cracking and associated loss of tensile ductility in plain-carbon steels under exposure to high-pressure gaseous hydrogen

Four, hot-rolled, plain-carbon steels with varying carbon content were subjected to slow strain-rate tensile (SSRT) tests in a 95-MPa gaseous hydrogen environment at ambient temperature. The influence of pearlite volume fraction on the magnitude of hydrogen-induced degradation of the materials’ strength and ductility was thereby determined. Hydrogen was seen to significantly affect strain-to-failure and reduction-in-area in all four materials, wherein such a loss of tensile ductility was ascribed to the premature initiation and subsequent propagation of surface micro-cracks as revealed by the quantitative damage evolution analyses on the post-fractured specimens. The pearlite grains on sample surfaces manifestly served as the preferential origins of hydrogen-induced micro-cracks, resulting in more considerable embrittlement in materials possessing a higher percentage of pearlite, due to the rapid coalescence of discrete embryonic damage during tensile straining.

Effect of recrystallization annealing treatment on the hydrogen embrittlement behavior of equimolar CoCrFeMnNi high entropy alloy

Equimolar CoCrFeMnNi high-entropy alloy (HEA) could be effectively strengthened by grain refinement process. However, the dislocations, grain boundaries and twins determined by the grain refinement process worked as hydrogen diffusion paths and trapping sites may obviously affect the hydrogen induced cracking. In this study, in combination with microstructures and fracture surface morphologies analysis, the HE behavior of CoCrFeMnNi HEA after different recrystallizing annealing processes were studied using slow strain rate tensile (SSRT) tests during in situ electrochemical hydrogen charging. The results indicated that the hydrogen trapping sites such as dislocations and grain boundaries controlled the HE behavior of the alloys. “Low temperature-short time” recrystallizing annealing refined the grains and increased the strength, but heavily decreased the hydrogen embrittlement (HE) resistance due to the residual dislocations along the grain boundaries. However, “High temperature-long time” recrystallizing annealing alloy showed lower strength but very well HE resistance due to the decreasing of grain boundaries and dislocations.

Mechanism of hydrogen-induced hardening in pure nickel and in a copper–nickel alloy analyzed by micro Vickers hardness testing

It has often been observed that solute hydrogen triggers a remarkable increase in flow stress in pure Ni. As reported in previous studies, this H-induced hardening has been attributed to the solid-solution strengthening and/or H-enhanced evolution of the dislocation structure. However, it remains unclear which of those two factors actually dominates such macroscopic hardening-behavior. Via two mechanical tests at different scales, i.e. , slow strain-rate tensile (SSRT) and micro Vickers hardness (MVH) tests, the H-enhanced evolution of the dislocation structure and its contribution to H-induced hardening in pure Ni and in Cu–Ni alloy were investigated in this study. In pure Ni, the SSRT test at room temperature (RT) demonstrated an increase in flow stress with a greater work-hardening rate, indicating the H-enhanced evolution of the dislocation structure, as supported by the MVH test. Nevertheless, the SSRT and MVH tests revealed that the H-enhanced evolution of the dislocation structure was significantly suppressed at 77 K. On the other hand, with respect to Cu–Ni alloy, no H-enhanced evolution of dislocation structure was observed during the SSRT and MVH tests, regardless of the deformation temperatures. This difference in the evolution of the dislocation structure between pure Ni and Cu–Ni alloy was consistent with the experimental results obtained beforehand, using scanning transmission electron microscopy (STEM). A series of experimental results indicated that both the solid-solution strengthening and H-enhanced evolution of the dislocation structure could contribute to the increase in flow stress, with the degree of contribution dependent on the respective material and the deformation temperature.

Effect of applied potential on stress corrosion cracking behavior of X80 steel in alkaline soil simulated solution with sulfate-reducing bacteria

In this study, the effect of applied potential on stress corrosion behavior of X80 pipeline steel in Dagang soil containing sulfate-reducing bacteria was comprehensively analyzed by means of the AC impedance technique and slow strain-rate tensile tests and scanning electron microscope observation of fracture morphology. The results indicate that, with decreasing electrode potential, the corrosion degree of the sample first decreases and then increases, and the stress corrosion cracking sensitivity exhibits a certain rule. Under the Ecorr and a potential of −725 mV(vs.CSE), the stress corrosion cracking mechanism of metal is anodic dissolution. The synergistic effect of sulfate-reducing bacteria and stress results in the initiation and propagation of cracks on the surface of the metal electrode. At −1275 mV(vs.CSE), the mechanism of stress corrosion cracking is hydrogen-induced cracking. The sulfate-reducing bacteria exacerbates the cathodic hydrogen evolution at this potential. Under the combined effects of hydrogen and stress, stress corrosion cracking behavior occurs in metals.

Effect of Nb addition and Pre-strain on Hydrogen Embrittlement of Low-carbon Steels with Ferrite-pearlite Structure

The effect of pre-strain on the hydrogen embrittlement of Nb-free and Nb-added low-carbon steels with ferrite-pearlite structure was investigated in this study. After the steels were electrochemically charged with hydrogen, slow-strain rate tensile (SSRT) tests were conducted on them to examine hydrogen embrittlement behavior. The SSRT test results revealed that the Nb-added steel had a lesser decrease of elongation and reduction of area than the Nb-free steel. The formation of NbC carbide and grain refinement caused by the Nb addition improved resistance to hydrogen embrittlement. The loss of elongation and the reduction of area after hydrogen charging occurs when pre-strain is increased. The pre-strain increases dislocation density and thus increases the amount of reversible hydrogen trap sites associated with hydrogen embrittlement. 10% pre-strained specimens exhibited a significant loss in elongation and reduction of area, regardless of Nb addition. Based on the results of electron back-scatter diffraction, fractographic, and silver decoration analyses for Nb-free and Nb-added steels, the hydrogen embrittlement mechanism in low-carbon steels with different amounts of pre-strain is discussed in terms of dislocation density and hydrogen distribution.

The effect of heat treatment on environment assisted cracking susceptibility of 9% Ni steel

ABSTRACT The influence of temper temperature on the environment assisted cracking of seamless 9% Ni steel tubes were investigated using scanning electron microscope (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), slow strain rate tensile (SSRT) tests, hydrogen permeation tests and electrochemical measurements. Mechanical and electrochemical tests were conducted in sodium thiosulphate (Na2S2O3 10−3 mol L−1) solution at open circuit potential (OCP) and at cathodic potential. Results showed that 9% Ni steel had a higher ductility loss at OCP when compared with the results obtained under cathodic potential. The lower ductility loss at cathodic potential was attributed to the inhibition of the anodic dissolution. After the tests, the specimens were examined using scanning electron microscopy to characterise the fracture morphology. The fracture surface exhibited mixed morphology, with a ductile morphology in the centre and a brittle morphology near the edges.

Inhibiting Stress Corrosion Cracking by Removing Corrosion Products from the Mg-Zn-Zr Alloy Pre-Exposed to Corrosion Solutions

Magnesium and its alloys are susceptible to stress-corrosion cracking (SCC), which can manifest itself during the slow-strain rate tensile (SSRT) testing in air if the specimens were pre-exposed to corrosive media. This phenomenon is generally associated with hydrogen embrittlement (HE) which is believed to be related to diffusible hydrogen penetrating into the metal during the pre-exposure. In the present study, we show that the corrosion product layer deposited on the surface of the pre-exposed specimens is crucial in the SCC mechanism. The specimens of the hot-extruded alloy ZK60 were SSRT tested in air, in corrosive media, in air after pre-exposure to corrosive media as well as after pre-exposure and removal of corrosion products. To vary the severity of SCC, four NaCl-based corrosion solutions were utilized. The embrittlement resulted in a marked decrease in tensile ductility and the concurrent appearance of multiple cracks on the side surface as well as brittle fragments on the fracture surface. The most striking finding of the present study is that the removal of corrosion products from the surface of the pre-exposed specimens results in complete recovery of the mechanical response and in the elimination of all harmful embrittling features regardless of the corrosive solution used. This effect is found to be inconsistent with the common viewpoint where the SCC mechanism is governed entirely by diffusible hydrogen. The obtained results shed a new light on the role of diffusible hydrogen, corrosion products and irreversible corrosion damage in the SCC mechanism of Mg alloys pre-exposed to corrosive media.

A layer-by-layer assembled coating for improved stress corrosion cracking on biomedical magnesium alloy in cell culture medium

Magnesium (Mg) alloys have shown great potential for biomedical implant materials due to their biodegradability and suitable mechanical strength. Recent works have shown that biomedical Mg alloys suffered from accelerated degradation rates when exposed to a stressed condition in biological media. Therefore, it is necessary to endow Mg alloys with enhanced stress corrosion cracking resistance. In this study, phytic acid (PA), branched polyethylenimine (bPEI) and graphene oxide (GO) were introduced onto Mg1Zn alloy via layer-by-layer (LbL) assembly technique. The characteristics of the multilayer films were investigated by SEM, FTIR and XPS. Biodegradability Corrosion resistance of the samples was measured by electrochemical and immersion tests. The stress corrosion cracking (SCC) behaviour of the LbL coated Mg alloys was performed using slow strain rate tensile (SSRT) tests. The results showed that the multilayer coating with smooth and uniform morphologies could enhance the corrosion resistance of Mg alloys due to the physical barrier. The LbL coating exhibited a remarkable ability to improve the SCC resistance of Mg1Zn alloy in Dulbecco's modified eagle medium.