Search Results (76)

S355 steels are widely used in various applications. However, they may be affected by hydrogen, which can induce hydrogen-induced cracking (HIC). The effects of the quenching temperature (T_wq) on the microstructure variation and HIC susceptibility of S355 steel was investigated by microstructural characterization, hydrogen permeation (HP) test, slow strain rate tensile (SSRT) test, hydrogen microprint technique (HMT) test, and hydrogen-charged cracking test. The results indicate that the microstructure of the treated specimens consisted of predominantly lath martensite (LM) and small amounts of lath bainite (LB) for the T_wq of 950 °C and 1000 °C, while the microstructure of the treated specimens mainly consisted of LM for the T_wq of 1050 °C and 1100 °C. The results indicate that as the T_wq increased, the sample treated at 950 °C exhibited a minimum hydrogen embrittlement index (I_z), while the sample treated at 1050 °C exhibited the maximum I_z. The hydrogen diffusion coefficient was relatively low, while the hydrogen concentration and trap density were relatively high for the T_wq of 1050 °C. The lath interfaces in martensite were effective hydrogen traps with high hydrogen-trapping efficiency. Hydrogen-induced cracks were significantly affected by hydrogen trapping at martensitic lath interfaces, exhibiting a basically transgranular fracture. Full article

The hydrogen embrittlement of a typical Oil Country Tubular Good (OCTG) steel, API 5CT T95, was investigated through electrochemical hydrogen pre-charging followed by mechanical testing. J-integral and tensile tests were performed on electrochemically pre-charged samples, with varying charging conditions to simulate different hydrogen environmental exposure. Hydrogen concentration profiles during the electrochemical hydrogen charging process and subsequent mechanical testing in air were calculated with the support of hydrogen permeation tests and Finite Elements Method (FEM) mass diffusion analysis. This approach enabled a deeper understanding of the actual impact of hydrogen on the assessed mechanical properties. The results were compared with tests performed in air and with data available in the literature and were critically analyzed and discussed. A toughness reduction of up to 60% was observed under the most severe charging conditions; however, the alloy retained good ductility with a critical stress intensity factor of 124 MPa√m, well above the minimum values required for pipelines in high-pressure hydrogen gas and sour service applications, 55 MPa√m and 30 MPa√m, respectively, as specified by current ASME Standard and EFC Guidelines. Tensile tests on pre-charged specimens exhibited certain limitations due to the rapid hydrogen desorption rate with respect to the time required to conduct proper slow strain-rate tests. Full article

In this study, slow strain rate tensile tests (SSRT) were performed on T91 in lead-bismuth eutectic (LBE) with saturated oxygen to investigate the effects of temperature (350 °C, 450 °C, and 550 °C), strain rate (1 × 10⁻⁵/s and 2 × 10⁻⁶/s) and pre-exposure conditions (time, oxygen concentration) on the sensitivity to liquid metal embrittlement (LME). The results revealed that the embrittlement sensitivity of T91 in LBE is significantly influenced by temperature. LME was observed in T91 at 350 °C and disappeared when the temperature increased to 550 °C. Additionally, T91 exhibited increased sensitivity to LME at low strain rates, indicating that low strain rates promoted the occurrence of LME. Finally, through different pre-exposure conditions, it was found that the obvious LME phenomenon would only occur when the oxygen concentration was poor and the pre-exposure time was long (48 h), indicating that pre-exposure conditions have a crucial impact on the occurrence of LME. Full article

In high-altitude mountainous areas, the phenomenon of rock frost damage under repeated freeze–thaw cycles are pronounced, with the deformation and failure processes of rock often accompanied by energy dissipation. To elucidate the energy evolution mechanism of rocks under freeze–thaw cycles, triaxial compression tests and numerical simulation tests were conducted under different freeze–thaw cycles. Results from indoor tests indicate that successive freeze–thaw cycles deteriorate the mechanical properties of rocks. Compared to conditions without freeze–thaw cycles, after 40 freeze–thaw cycles, the peak stress of the rock decreased by 42.8%, the elastic modulus decreased by 64%, and, with increasing confining pressure, the rate of decrease lessened, indicating that confining pressure can inhibit the decline in the mechanical properties of rocks. As the freeze–thaw cycles increase, the total absorption energy (TAE) of rocks gradually decreases. Meanwhile, with increasing confining pressure, the TAE, elastic strain energy (ESE) and dissipated energy (DE) of rocks all gradually increase. However, as the confining pressure increases, the TAE increases by 781%, the ESE increases by 449%, and the DE increases by 6381%. Numerical simulation results reveal that with an increase in the freeze–thaw cycles, shear failure phenomena gradually decrease while tensile failure phenomena gradually increase. During the compression process, the evolution of internal cracks in rocks demonstrates a trend of slow–steady–rapid development, with the number of cracks produced being positively correlated with the freeze–thaw cycles. The performance can provide valuable insights into the degradation mechanism of the mechanical properties of rocks and failure analysis in high-altitude mountainous areas. Full article

This study explored the influence of hydrogen on the tensile properties and fracture behavior of high-strength API X70 and X80 linepipe steels with bainitic microstructures under varying hydrogen charging conditions. The X70 steel exhibited a ferritic microstructure with some pearlite, while the X80 steel showed a bainitic microstructure and fine pearlite due to the addition of molybdenum. Slow strain rate tests (SSRTs) were conducted using both electrochemical ex situ and in situ hydrogen charging methods subjected to different current densities. The SSRT results showed that in situ hydrogen-charged SSRT, performed at current densities above 1 A/m², led to more pronounced hydrogen embrittlement compared to ex situ hydrogen-charged SSRT. This occurred because hydrogen was continuously supplied during deformation, exceeding the critical concentration even in the center regions, leading to quasi-cleavage fractures marked by localized cleavage and tearing ridges. Thermal desorption analysis (TDA) confirmed that a greater amount of hydrogen was trapped at dislocations during in situ hydrogen-charged SSRT, intensifying hydrogen embrittlement, even with a shorter hydrogen charging duration. These findings highlight the importance of selecting appropriate hydrogen charging methods and understanding the hydrogen embrittlement behavior of linepipe steels. Full article

With the rapid development of hydrogen pipelines, their safety issues have become increasingly prominent. In order to evaluate the properties of pipeline materials under a high-pressure hydrogen environment, this study investigates the hydrogen embrittlement sensitivity of X70 welded pipe in a 10 MPa high-pressure hydrogen environment, using slow strain rate testing (SSRT) and low-cycle fatigue (LCF) analysis. The microstructure, slow tensile and fatigue fracture morphology of base metal (BM) and weld metal (WM) were characterized and analyzed by means of ultra-depth microscope, scanning electron microscope (SEM), electron backscattering diffraction (EBSD), and transmission electron microscope (TEM). Results indicate that while the high-pressure hydrogen environment has minimal impact on ultimate tensile strength (UTS) for both BM and WM, it significantly decreases reduction of area (RA) and elongation (EL), with RA reduction in WM exceeding that in BM. Under the nitrogen environment, the slow tensile fracture of X70 pipeline steel BM and WM is a typical ductile fracture, while under the high-pressure hydrogen environment, the unevenness of the slow tensile fracture increased, and a large number of microcracks appeared on the fracture surface and edges, with the fracture mode changing to ductile fracture + quasi-cleavage fracture. In addition, the high-pressure hydrogen environment reduces the fatigue life of the BM and WM of X70 pipeline steel, and the fatigue life of the WM decreases more than that of the BM as well. Compared to the nitrogen environment, the fatigue fracture specimens of BM and WM in the hydrogen environment showed quasi-cleavage fracture patterns, and the fracture area in the instantaneous fracture zone (IFZ) was significantly reduced. Compared with the BM of X70 pipeline steel, although the effective grain size of the WM is smaller, WM’s microstructure, with larger Martensite/austenite (M/A) constituents and MnS and Al-rich oxides, contributes to a heightened embrittlement sensitivity. In contrast, the second-phase precipitation of nanosized Nb, V, and Ti composite carbon-nitride in the BM acts as an effective irreversible hydrogen trap, which can significantly reduce the hydrogen embrittlement sensitivity. Full article

This study investigates the hydrogen embrittlement behavior of API X70 linepipe steel. The microstructure was primarily composed of a dislocation-rich bainitic microstructure and polygonal ferrite. Slow strain-rate tests (SSRTs) were performed under both ex situ and in situ electrochemical hydrogen charging conditions to examine the difference between hydrogen diffusion and trapping behaviors. The ex situ SSRTs showed almost the same tensile properties as air and a limited brittle fracture confined to near the surface. In contrast, the in situ SSRTs showed an abrupt failure after the maximum tensile load, leading to a brittle fracture across the entire fracture surface with stress-oriented hydrogen-induced cracking (SOHIC). The crack trace analysis results indicated that SOHIC propagation paths were influenced by localized hydrogen accumulation due to high-stress fields. As a result, the dominant hydrogen embrittlement mechanisms, such as hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE), changed. These findings provide critical insights into the microstructural factors affecting hydrogen embrittlement, which are essential for the design of hydrogen-resistant steels in hydrogen infrastructure applications. Full article

This study employs a custom hollow specimen setup to investigate the HE in API 5L X60 pipeline base and welded materials exposed to pure hydrogen and a 20% hydrogen–natural gas blend at 2.07 MPa. Results indicate embrittlement with increasing hydrogen concentration. The base material showed a hydrogen embrittlement index (HEI) of 11.6% at 20% hydrogen and 12.4% at 100% hydrogen. For the welded material, the HEI was 14.6% at 20% hydrogen and 18.0% at 100% hydrogen. Fractography analysis revealed that the base and welded materials exhibited typical ductile fracture features in the absence of hydrogen, transitioning to a mixture of quasi-cleavage and micro-void coalescence (MVC) features in hydrogen environments. Additionally, with hydrogen, increased formation of secondary cracks was observed. Notably, the study identified the Hydrogen-Enhanced Localized Plasticity (HELP) mechanism as a probable contributor to hydrogen-assisted fracture. Full article

X80 steel plays a pivotal role in the development of oil and gas pipelines; however, its welded joints, particularly the heat-affected zone (HAZ), are susceptible to stress corrosion cracking (SCC) due to their complex microstructures. This study investigates the SCC initiation mechanisms of X80 steel welded joints under practical pipeline conditions with varying levels of cathodic protection. The SCC behaviors were analyzed through electrochemical measurements, hydrogen permeation tests, and interrupted slow strain rate tensile tests (SSRTs) conducted in a near-neutral pH environment under different potential conditions (OCP, −1.1 V_SCE, −1.2 V_SCE). These behaviors were influenced by microstructure type, grain size, martensite/austenite (M/A) constituents, and dislocation density. The sub-zones of the weld exhibited differing SCC resistance, with the fine-grain (FG) HAZ, base metal (zone), welded metal (WM) zone, and coarse-grain (CG) HAZ in descending order. In particular, the presence of coarse grains, low dislocation density, and extensive M/A islands collectively increased corrosion susceptibility and SCC sensitivity in the CGHAZ compared to other sub-zones. The SCC initiation mechanisms of the sub-zones within the X80-steel welded joint were primarily anodic dissolution (AD) under open-circuit potential (OCP) condition, shifting to either hydrogen-enhanced local plasticity (HELP) or hydrogen embrittlement (HE) mechanisms at −1.1 V_SCE or −1.2 V_SCE, respectively. Full article

Medium-carbon, high-strength steels are widely used in the field of hydrogen energy because of their good mechanical properties, and they can be readily tailored by heat treatment processes such as the normalizing–tempering (N&T) and quenching–tempering (Q&T) methods. The hydrogen embrittlement (HE) susceptibility of a medium-carbon, high-strength steel was investigated utilizing microstructural characterization with scanning electron microscopy (SEM), the electron backscatter diffraction (EBSD) technique, and transmission electron microscopy (TEM). A study was also conducted on the steel’s hydrogen transport behavior as affected by the N&T and Q&T treatments. The steel contained more hydrogen traps, such as dislocations, grain boundaries, lath boundaries, and carbide interfaces, after the Q&T process, which was associated with a lower HE sensitivity when comparing the two treatments. In comparison, the N&T process produced larger-size and lesser-density carbides distributed along the grain boundaries, and this resulted in a relatively higher HE susceptibility, as revealed by the slow-strain-rate tensile (SSRT) tests of the hydrogen-charged steels and by the fractographic study of the fracture surface. Full article

Hydrogen embrittlement (HE) of 1.4614 and 1.4543 maraging stainless steels is characterized with fine structural TEM-ASTAR, hydrogen electrochemical permeation, thermodesorption and tensile testing (with slow strain rate during H cathodic charging) experiments. Both solutions annealed and 550 °C aged states are studied. When aged, 1.4614 grade contains nanometric hexagonal N_i3Ti precipitates whereas 1.4543 grade contains three families of nanometric precipitates, namely, hexagonal Ni₃Ti, non fcc-rich Cu and G-phase rich in Si, Ti and Ni with cube-cube relations with martensite. From the permeation and thermodesorption results, it appears that austenite and nanometric precipitates seem to constitute deep traps for hydrogen in both grades. From tensile tests with in situ H charging, 1.4614 and 1.4543 maraging grades show a high susceptibility to HE with relative reduction of area (RRA) strictly superior to 80% for both metallurgical states. The presence of retained austenite does not seem to bring any HE resistance because of its transformation into martensite during deformation. Full article

In this paper, the influence of different fiber materials on the dynamic splitting mechanical properties of concrete was investigated. Brazil disc dynamic splitting tests were conducted on plain concrete, palm fiber-reinforced concrete, and steel fiber-reinforced concrete specimens using a split Hopkinson pressure bar (SHPB) test device with a 100 mm diameter and a V2512 high-speed digital camera. The Digital Image Correlation (DIC) technique was used to analyze the fracture process and crack propagation behavior of different fiber-reinforced concrete specimens and obtain their dynamic tensile properties and energy dissipation. The experimental results indicate that the addition of fibers can enhance the impact toughness of concrete, reduce the occurrence of failure at the loading end of specimens due to stress concentration, delay the time to failure of specimens, and effectively suppress the expansion of cracks. Steel fibers exhibit a better crack-inhibiting effect on concrete compared to palm fibers. The incident energy for the three types of concrete specimens is roughly the same under the same impact pressure. Compared with plain concrete, the energy absorption rate of palm fiber concrete is decreased, while that of steel fiber concrete is increased. Palm fiber-reinforced concrete and steel fiber-reinforced concrete have lower peak strains than plain concrete under the same loading duration. The addition of steel fibers significantly impedes the internal cracking process of concrete specimens, resulting in a relatively slow growth of damage variables. Full article

In the study of the liquid metal embrittlement (LME) of the T91/lead-bismuth eutectic (LBE) system, it is observed that LME occurs in a temperature interval which is similar to the temperature range where dynamic strain aging (DSA) is observed. However, the potential correlation between DSA and LME has not yet been satisfactorily investigated. This investigation for the T91/LBE system is exactly the topic of this work. For the evaluation of DSA and LME, slow strain rate tensile tests are conducted in the temperature range between 200 °C and 450 °C with strain rates of $5\times {10}^{-5}$ s${}^{-1}$ and $5\times {10}^{-6}$ s${}^{-1}$ in reference and a molten oxygen-depleted LBE environment. The resulting tensile properties, as well as the fracture surfaces and lateral surfaces of the failed samples, suggest a correlation between DSA and LME in the T91/LBE system. The maximum mechanical degradation of T91 is observed in the case where the effects of both DSA and LME on material properties are found to be at maximum. However, the observation of DSA was not identified as a prerequisite for LME to take place. Therefore, these results may indicate that DSA partly contributes to the ductility minimum observed in the T91/LBE system. In addition, the results of this work show that changes in the fracture surface and lateral surface are more sensitive features to claim for the potential occurrence of LME than the changes in total elongation. Full article

The treatment with hydrofluoric acid (HF-treatment) was suggested to be an effective way of improving the corrosion resistance of Mg alloys, including Mg-Zn-Ca (ZX) ones used for biodegradable implants. However, the effect of the HF-treatment on the stress corrosion cracking (SCC) susceptibility of ZX alloys has not been reported yet, although this phenomenon can induce premature brittle failures of the metallic medical devices, and thus, it is critical for their in-service structural integrity. In the present study, the effect of the HF-treatment on the microstructure, cytotoxicity, corrosion rate, mechanical properties, and fracture and side surface characteristics of the as-cast ZX20 alloy were investigated with the use of scanning electron microscopy, immersion, and slow-strain rate tensile testing in Hanks’ solution and indirect cell viability tests. It is found that the HF-treatment exerts no cytotoxic effect and results in a significant reduction in corrosion rate (up to 6 times of magnitude) and SCC susceptibility indexes (up to 1.5 times of magnitude). The observed improvement of corrosion and SCC performance of the alloy by the HF-treatment is found to be attributed to three effects, including (i) formation of the protective surface film of MgF₂, (ii) removal of surficial contaminations originating from sample preparation procedures, and (iii) dissolution of surficial secondary phase particles. The mechanism of corrosion and SCC in the specimens before and after the HF-treatment are discussed. Full article

To study the corrosion characteristics of high-strength sucker rods in high-salinity well fluids under alternating stresses, a single-factor stress corrosion test was designed. The slow strain rate tensile test (SSRT) was carried out for four kinds of high-strength sucker rods under different Cl⁻ and HCO₃⁻ concentrations and with different service strengths, and the variable stress corrosion cracking susceptibility was analyzed. The results show that the elongation loss and absorbed work loss of the H-grade ultra-high-strength 4330 sucker rod after stress corrosion are greater than those of both the high-strength 4142 sucker rod and the high-strength 20CrMoA sucker rod. The elongation and absorbed work loss of the 30CrMoA and 20CrMoA sucker rods are less affected by the changes in Cl⁻ and HCO₃⁻. With the increase in use strength, the elongation and absorbed work loss of the high-strength sucker rod increase. The change in the surface of the sucker rod during the corrosion process is inconsistent with the actual elongation of the sucker rod and the absorbed work loss. It can be concluded that the stress corrosion cracking susceptibility of the sucker rod is not necessarily related to the tensile strength of the sucker rod. The 4330 sucker rod is not suitable for applications in wells with a high concentration of Cl⁻, but it is suitable for operation in alkaline conditions where corrosive media such as HCO₃⁻ and Cl⁻ coexist. Under highly corrosive and highly mineralized conditions, the 30CrMoA sucker rod is less susceptible to stress corrosion. The stress corrosion cracking susceptibility of the 20CrMoA sucker rod is lower than that of the 4142 sucker rod. In high-salinity well fluids, the higher the use strength, the higher the stress corrosion cracking susceptibility of the high-strength sucker rod is. The test results for the weight-loss-based corrosion rate and plastic loss may contradict the determination of the corrosion susceptibility of the material under working conditions. Full article