Sharing: The influence of temperature and hydrogen pressure on the fracture behavior of X80 steel in a pure hydrogen environment
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0. Introduction Hydrogen transportation is an important aspect in the utilization of hydrogen energy. However, since the presence of hydrogen atoms causes steel to become brittle, when using pipelines to transport hydrogen or hydrogen-containing media, the plasticity and fatigue strength of the pipeline steel decrease, and hydrogen embrittlement cracking may occur, leading to serious safety accidents. There are many factors that affect the hydrogen embrittlement sensitivity of steel used for pipelines, including metallurgical factors and environmental factors. For hydrogen-blended natural gas pipelines, operating conditions such as temperature and hydrogen pressure have a significant impact on the interaction between hydrogen and the steel used in the pipelines. Studies by MENG et al. have found that X80 pipeline steel is susceptible to hydrogen-induced embrittlement in natural gas/hydrogen mixtures; at a total pressure of 12 MPa, as the hydrogen partial pressure increases, the hydrogen embrittlement index rises and fatigue crack propagation accelerates significantly. Studies by AMARO et al. have found that, compared to an air environment, the elongation after fracture of both X52 and X100 steels under a high-pressure hydrogen environment of 13.8 MPa is significantly reduced, with a notable loss in toughness. ZHOU et al. found that in a hydrogen-blended natural gas environment, a high hydrogen partial pressure reduces the mechanical properties of X80 pipeline steel. The mechanism by which temperature affects hydrogen embrittlement is complex; generally, an increase in temperature promotes the generation and diffusion of hydrogen atoms on the steel surface, thereby exacerbating the hydrogen embrittlement effect. However, at the same time, an increase in temperature also enhances the fracture toughness of the steel. It is generally believed that hydrogen embrittlement can occur over a wide range of temperatures, and the external ambient temperature is crucial for the reaction between hydrogen and the material surface, the solubility of hydrogen, and its diffusion. *Studies by NG et al. have found that at current densities of 10 and 20 mA·cm−1, the temperature thresholds for hydrogen embrittlement in X70 steel are 293 and 283 K respectively; whereas when the current density reaches 30 mA·cm−1, no distinct critical temperature for hydrogen embrittlement was observed. Studies by WU et al. have found that both the hydrogen embrittlement sensitivity and the thickness of the hydrogen embrittlement layer in 2205 duplex stainless steel decrease as the temperature increases after hydrogen charging. MOMOTANI et al. found that as the temperature decreases within the range of 0–100 °C, the hydrogen embrittlement sensitivity of low-carbon martensitic steel increases; however, when the temperature drops further below 0 °C, the steel’s hydrogen embrittlement sensitivity decreases instead. ZHANG et al. found that the hydrogen embrittlement effect of SUY, S15C, and S35C carbon steels first increases as the temperature decreases, reaches its maximum around 200 K, and then declines rapidly. LI et al. found that as the temperature increases, the hydrogen diffusion rate in steel and the subsurface hydrogen concentration increase. Currently, research has mainly focused on the hydrogen embrittlement behavior of steel in liquid environments. There is little research on the effect of temperature in gas environments on the hydrogen embrittlement behavior of pipeline steel, especially high-grade pipeline steel. The safe operating temperatures for different types of steel when used in hydrogen transport still need to be determined. Moreover, the relationship between hydrogen pressure and the hydrogen embrittlement sensitivity of steel used in pipelines, as well as the critical pressure threshold at which hydrogen embrittlement occurs, remain unclear. X80 steel, with its high strength and low cost, is widely used in oil and gas transmission pipelines. The author conducted slow strain rate tensile tests on X80 steel in a pure hydrogen environment at various hydrogen pressures and temperatures, in order to study the effects of hydrogen pressure and temperature on the tensile properties and hydrogen embrittlement susceptibility of X80 steel. The mechanism behind these effects was analyzed by examining the fracture morphology. This research aims to provide valuable experimental data for assessing the safety of pipelines transporting hydrogen or hydrogen-containing fluids at different operating temperatures, as well as to offer theoretical support for the evaluation of hydrogen embrittlement in such pipelines.
1. Sample preparation and testing methods: The test materials were obtained from domestically produced X80 steel straight-seam pipes with specifications of ϕ1,219 mm×22 mm. The main chemical components (by mass percentage/%) are 0.053C, 0.14Si, 1.65Mn, 0.0004S, 0.009P, 0.23Cr, 0.11Mo, 0.117Ni, 0.034Al, 0.012Cu, 0.016Ti, 0.044Nb, and 0.007V, meeting the requirements of API SPEC 5L—2018 «Specification for Line Pipe». Samples of size 10 mm×15 mm×20 mm were cut from straight-seam pipes using wire cutting. After rough grinding, fine grinding, and polishing, they were etched with an ethanol nitric acid solution having a volume fraction of 3%~4%, dried with a hair dryer, and their microstructure was observed using an Imager.M1m optical microscope. As can be seen from Figure 1, the microstructure of X80 steel is relatively uniform, consisting mainly of acicular ferrite (AF) and granular bainite (GB). Acicular ferrite possesses high toughness, which can effectively inhibit crack propagation, thereby endowing X80 steel with good resistance to hydrogen embrittlement. Figure 1. Microstructure of X80 steel. According to GB/T 34542.2—2018 \"Hydrogen storage and transmission systems – Part 2: Test methods for the compatibility of metal materials with hydrogen environments\", samples were taken along the axial direction of the X80 steel pipes to prepare tensile specimens as shown in Figure 2. A JD350 surface roughness meter was used to measure the surface roughness of both the outer and inner surfaces of the specimens, which was found to be 0.8 μm. Slow strain rate tensile tests were conducted using a WDML-100 slow strain rate corrosion testing machine. Use a vacuum pump to remove the air from inside the test chamber, then replace the gas in the test chamber and the hydrogen supply pipeline system with nitrogen. After that, use another vacuum pump to extract the nitrogen. Repeat this process 3 times to ensure that impure gases such as oxygen and water vapor are completely removed from the test chamber. Finally, fill the chamber with high-purity (99.999% pure) hydrogen, using pressures of 2.5, 6.0, and 10.0 MPa respectively ； After the specimens were left in an environment of high-purity hydrogen gas at different pressures for 24 hours, slow strain rate tensile tests were conducted at a stretching rate of 0.01 mm·min−1, with a holding time of 30 minutes. The test temperatures were –2, 23, and 48 °C, and three parallel tests were performed for each temperature. Slow strain rate tensile tests were conducted in a nitrogen atmosphere with a purity of 99.999% in order to calculate the hydrogen embrittlement index and quantify the sensitivity to hydrogen embrittlement. The formula for calculating the hydrogen embrittlement index is as follows:
As the temperature increases, the hydrogen embrittlement index of X80 steel first increases and then decreases, and its sensitivity to hydrogen embrittlement also first increases and then decreases. At a temperature of 23 °C, the hydrogen embrittlement index is at its maximum, and the sensitivity to hydrogen embrittlement is at its strongest. This is because hydrogen atoms diffuse slowly at low temperatures, dislocation movement is restricted, and the sensitivity to hydrogen embrittlement is low ； As the temperature rises initially, hydrogen atoms diffuse more rapidly and their interaction with dislocations increases, leading to the formation of hydrogen-dislocation clusters that cause dislocation pile-up, thereby promoting crack initiation and propagation and increasing susceptibility to hydrogen embrittlement ； When the temperature exceeds 23 °C, hydrogen atoms tend to escape, resulting in a decrease in the effective hydrogen concentration. Moreover, the increased plastic deformation capacity of steel at higher temperatures helps to reduce stress concentration and inhibit crack propagation, thereby lowering the susceptibility to hydrogen embrittlement. 2.3 Fracture surface morphology of tension testing: As shown in Figure 6, at a temperature of 48 °C and a gas pressure of 10 MPa, circumferential cracks could be clearly observed at the tension fracture site; this indicates that the presence of hydrogen increases the brittleness of steel, facilitating the initiation and propagation of cracks at notches.
As can be seen from Figure 7, at different temperatures and hydrogen pressures, cracks in the hydrogen environment occur directly at or near the notches; the fiber regions are distributed along the circumference, and the cracks extend from the surface of the tensile fracture inward, showing clear characteristics of hydrogen-induced damage, indicating that these areas are those in direct contact with hydrogen and where aggregation is accelerated. At a constant temperature, under a hydrogen pressure of 2.5 MPa, numerous microholes are present on the fracture surface ； As the pressure increased to 10.0 MPa, secondary cracks appeared on the outer surface of the fracture edge; this was likely due to the increased pressure facilitating the penetration of hydrogen into the steel and its accumulation at grain boundary intersections or internal defects, which accelerated the initiation and propagation of microcracks and ultimately led to fracture. At a constant hydrogen pressure, at a temperature of −2 ℃, the tensile fracture surface is uneven, with distinct shear lips present ； As the temperature rose to 23 °C, cracks originated at the stress concentration site at the root of the notch and expanded rapidly ； When the temperature rises to 48 °C, the initiation sites of fracture cracks are relatively dispersed, with no typical stress concentration; this is because further increasing the temperature accelerates hydrogen diffusion but reduces its segregation at grain boundaries, thereby decreasing the susceptibility to hydrogen embrittlement.