Properties Of X80 Pipeline Steel Research Articles

Study on the mechanical properties of X80 pipeline steel under pre-charged high-pressure gaseous hydrogen

Hydrogen embrittlement (HE) is a significant challenge to the safe operation of hydrogen-blended natural gas pipelines. The mechanical properties of X80 steel were studied after H2 pre-charging followed by mechanical properties tests and fracture morphologies observation. Results indicated that the hydrogen saturation time of X80 steel was roughly 48 h. H2 pre-charging induced a decline in strength, plasticity, and fatigue properties, while hydrogen-assisted fatigue crack growth occurred during the early stage of fatigue crack growth rate tests. Additionally, the fracture morphologies transited from primary microvoid coalescence to a mixed mode characterized by ductile dimples and quasi-cleavage planes with increasing hydrogen. The HE susceptibility increased with increasing hydrogen partial pressure and decreasing loading frequency, but there existed a critical value (6 × 10−7 s−1) for the strain rate effect on the HE susceptibility. Under H2 pre-charging conditions, the predominant HE mechanism was the hydrogen-enhanced local plasticity (HELP) mediated hydrogen-enhanced decohesion (HEDE) mechanism.

Effect of electrochemical hydrogen-charging conditions on nanomechanical properties of X80 pipeline steel

In this work, nanoindentation tests, combined with hydrogen (H)-permeation testing, were used to study the effect of electrochemical H-charging conditions on H-permeating parameters and nanomechanical properties of X80 pipeline steel. The steady-state H-permeating current density, H-permeating flux, and the H subsurface concentration obtained in 0.5 M H2SO4 solution are apparently greater than the parameters obtained in near-neutral pH NS4 and 0.1 M NaOH solutions. The nanomechanical properties of X80 steel are heavily dependent on the electrochemical H-charging conditions (i.e., solution, time, and cathodic current density). The changes in nanohardness of the steel subjected to electrochemical H-charging depend on the charging solution used. In 0.5 M H2SO4 solution, as the electrochemical H-charging time increases, the nanohardness of the steel continues to decrease. However, in NS4 and 0.1 M NaOH solutions, the steel nanohardness increases with time. This phenomenon is attributed to the formation of corrosion product films on the steel surface. The softening and hardening transition phenomenon occurs on the steel subjected to electrochemical H-charging. Although measurement errors may cause variations in the nanohardness results, the evolution of nanohardness upon electrochemical H-charging exhibits a discernible tendency. H atoms could affect the elastic modulus of steel. When electrochemical H-charging promotes a uniform distribution of H atoms within the steel, achieved through either extended charging time or application of a cathodic current density, the elastic modulus of the steel tends to increase.

Influence of hydrogen in natural gas mixed hydrogen environment on mechanical properties of X80 pipeline steel

X80 pipeline steel is investigated for its mechanical performances within the natural gas/hydrogen mixtures containing hydrogen 0, 1.5, 3, 5, 10 and 15 vol% at the 12 MPa total pressure. According to the findings, hydrogen in natural gas/hydrogen mixtures has an essential effect on mechanical performances of X80 pipeline steel. Hydrogen does not significantly affect X80 pipeline steel strength, but has a large effect on elongation and a sharp effect on fracture toughness as well as fatigue crack growth rate. The fatigue crack growth rate elevates with addition of a small amount of hydrogen (1.5 vol%) by nearly one order of magnitude. As the hydrogen further increases from 1.5 vol% to 15 vol%, the fatigue crack growth rate continues to increase and gradually levels off. However, as the hydrogen increases, the fracture toughness KIC of X80 drops very sharply, from 251.73 MPa m0.5 when the hydrogen is 0 vol% to 95.7 MPa m0.5 when the hydrogen is 15 vol%, and the KIC falls by 61.98%. Therefore, when evaluating the hydrogen blending ratio, focusing on the role of hydrogen blending ratio in fracture toughness is more critical than fatigue crack growth rate.

Effects of CH4 and CO on hydrogen embrittlement susceptibility of X80 pipeline steel in hydrogen blended natural gas

The slow strain rate tensile experiments are carried out to investigate the tensile properties of X80 pipeline steel in hydrogen blended natural gas environments with different H2/CH4/CO contents. Mechanical properties and fracture morphologies are further analyzed. The results show that the hydrogen embrittlement susceptibility of X80 steel can be inhibited by the presence of CH4/CO, and the inhibition mechanisms are discussed. When the CH4 contents increase above 20 vol%, the inhibition on hydrogen embrittlement of X80 steel is stabilized. By comparison, the inhibitory effect of CO is more significant.

Effect of finish rolling temperature on microstructure and mechanical properties of X80 pipeline steel by on-line quenching

A Ti–V microalloyed X80 seamless pipeline steel was prepared using on-line quenching and off-line tempering technology. The influence of finish rolling temperatures (FRTs∼950 °C and 900 °C) and tempering temperatures (600°C–700 °C) on microstructure and mechanical properties was investigated. Fully recrystallized austenite was formed at 950 °C FRT, but partially at 900 °C FRT. The quenched microstructure consisted of lath bainite, granular bainite, cementite and TiC precipitates. With the decrease in FRT, more granular bainite and strain-induced TiC precipitates were obtained in the quenched microstructure. The original quenched structure significantly impacted the subsequently tempered microstructure. The tempered microstructure was composed of polygonal ferrite, tempered bainitic laths, cementite and (Ti, V)C precipitates. As FRT decreased, the tempered microstructure exhibited more polygonal ferrite, smaller grain size and better precipitation characteristics of (Ti, V)C precipitates, simultaneously improving the strength and impact toughness. An ideal combination of the yield strength (∼632 MPa), tensile strength (∼707 MPa), impact value (∼249 J at −20 °C) and shear-fracture area (∼97% at −20 °C) was achieved at 900 °C FRT and 680 °C tempering temperature, which satisfied the API requirements of X80 pipe. Therefore, 900 °C FRT and 680 °C tempering temperature were suitable process parameters for on-line quenching technology to produce X80 seamless pipeline steel.

Evaluation of Mechanical Properties and Microstructure of X70 Pipeline Steel with Strain-Based Design

The microstructure and mechanical properties of X70 pipeline steel with a ferrite/martensite dual-phase microstructure produced by thermo-mechanical controlled processing were investigated by tensile tests, Charpy V-notched (CVN) impact tests, drop-weight tear tests, guided-bend tests, scanning electron microscopy and transmission electron microscopy combined with thermodynamic simulation analysis. All the mechanical properties met the strength, ductility, toughness and deformability properties requirements of X70 grade pipeline steel with strain-based design. The shear fracture area and absorbed energy of CVN at −10 °C were >97% and >205 J in base metal (BM), weld metal (WM) and heat affected zone (HAZ) with low transition temperature, indicating adequate resistance to propagating fracture. The microstructure of WM was mainly intragranular acicular ferrite that can guarantee high strength, toughness and over matching requirements of the welded joint. Because of being exposed to successive heat inputs, the ferrite plus martensite/bainite microstructure of BM was heated between Ac1 and Ts forming the HAZ. However, a high CVN impact toughness of 345 J at −10 °C in HAZ was obtained, which indicated that the excellent mechanical properties of BM would not be seriously deteriorated during the welding thermal cycles with the reasonable addition of Ti and Nb.

Comparison of microstructures and properties of X80 pipeline steel additively manufactured based on laser welding with filler wire and cold metal transfer

Additive manufacturing (AM) with low heat input is a crucial technology of on-line maintenance of long-distance oil & gas transmission pipeline. By utilizing two methods, i.e. laser and cold metal transfer (CMT) arc, with low heat input and a homemade filler wire, the AM test was separately carried out on the widely used X80 steel. Parameters and the overlap ratio of AM processes were optimized at first; afterwards, the additively manufactured parts (AMPs) were prepared on the surface of the X80 pipeline steel substrate. Subsequently, the microstructures of the AMPs were observed and also microhardness test, impact test and tensile test were carried out. The result showed that the heat affected zones (HAZs) of the substrate were both not softened by applying the two methods. The microhardne values of the laser- and CMT-based AMPs were separately 140% and 119% that of the substrate, respectively. The impact energies of the laser- and CMT-based AMPs at room temperature were 53% and 67% that of the substrate, respectively. The average tensile strengths of both laser- and CMT-based AMPs were higher than that of X80 substrate, while the average percentage elongations after fracture of laser- and CMT-based AMPs are similar, which both accounted for about 66% that of the substrate. It is noted that the mechanical properties of the laser- and CMT-based AMPs both showed anisotropy, in which the anisotropy of the former was more significant.

Effects of stress concentration on the mechanical properties of X70 in high-pressure hydrogen-containing gas mixtures

This study aims to investigate the mechanical properties of X70 pipeline steel under the synergistic influence of hydrogen and stress concentration. Slow strain rate tensile tests and low-cycle fatigue tests were performed on the specimens with different stress concentration factors (Kt) in 10 MPa nitrogen/hydrogen mixtures. Results show that the degradation degree of the ductility and fatigue life of X70 steel induced by hydrogen increases with the increase of Kt, and as the hydrogen partial pressure in mixtures increases, the influence of Kt on hydrogen-induced degradation increases as well. In addition, finite element analysis was performed via a modified hydrogen diffusion/plasticity coupled model to study the effect of Kt on hydrogen distribution in the specimens, which can influence the mechanical properties of X70. The maximum hydrogen concentration consistently appears at the notch tip of the specimen and increases with the increase of Kt, which is proposed to be one of the reasons for the severe hydrogen embrittlement of the specimens with large Kt. As the axial tensile force on the specimen increases, the maximum hydrogen concentration at the notch tip begins to be dominated by hydrogen in the normal interstitial lattice sites and, subsequently, in the trapping sites.

Environment hydrogen embrittlement of pipeline steel X70 under various gas mixture conditions with in situ small punch tests

A series of small punch (SP) tests was conducted to investigate the effect of natural/hydrogen gas mixture conditions on the mechanical properties of X70 pipeline steel. The various gas mixture conditions included 0.1, 0.5, 1, 3, and 5 vol% hydrogen blends and three typical pipeline pressure levels (5, 7, and 10 MPa). The hydrogen embrittlement (HE) susceptibility grades depend on the hydrogen concentration and tend to increase with hydrogen concentration. The hydrogen gas mixture conditions have an insignificant effect on the yield load of the elastic bending region, whereas the maximal load decreases significantly when the hydrogen concentrations increase. Moreover, the quantitative parameters obtained from the SP load displacement curves and fracture morphologies were analyzed to characterize the HE behavior. By evaluating the SP absorption energy as a function of the hydrogen pressure, the ductile–brittle transition and saturated hydrogen pressures of the HE were determined. According to the results, the strength degradation grades of the mechanical properties of X70 pipeline steel in the gaseous hydrogen environment were not affected by the exposure time.

Effect of low partial hydrogen in a mixture with methane on the mechanical properties of X70 pipeline steel

In this study, the effect of a low partial hydrogen in a mixture with natural gas on the tensile, notched tensile properties, and fracture toughness of pipeline steel X70 is investigated. An artificial HE aging is simulated by exposing the tested sample to the mixture gas condition for 720 h. In addition, a series of tests is conducted in ambient air and 10 MPa of 100% He and H2. Overall, 10 MPa of 100% H2 significantly degrades the mechanical properties of an X70 pipeline steel. However, it is observed that the 10 MPa gas mixture with 1% H2 does not affect the mechanical properties when tested with a smooth tensile specimen. In the notched tensile test, a significant reduction in loss in the area is observed when tested with a notched specimen with a notch radius of 0.083 mm. It is also confirmed that a 10-MPa gas mixture with 1% H2 causes a remarkable reduction in the toughness. The influence of the exposure time to 1% hydrogen in a mixture with natural gas was found to be minor.

Macro- and microscale investigations of hydrogen embrittlement in X70 pipeline steel by in-situ and ex-situ hydrogen charging tensile tests and in-situ electrochemical micro-cantilever bending test

The effect of hydrogen on the mechanical properties of X70 pipeline steel was investigated by a combination of macro- and microscale approaches. Various tensile tests under vacuum, in-situ H-plasma charging (IHPC), and ex-situ electrochemical H-charging (EEHC) conditions were conducted to elucidate the hydrogen effect in the macroscale approach. All tensile tests were performed inside an environmental scanning electron microscopy (ESEM) chamber. The IHPC, as a novel hydrogen charging technique, was compared with conventional EEHC while uncharged tensile specimens were used as a reference. The results demonstrated that the variations in the IHPC condition were negligible compared to the vacuum condition, whereas the differences in the ex-situ condition were more prominent. Furthermore, susceptibility to hydrogen in the vacuum-ex situ regime was manifested in the reduction of yield strength and ultimate tensile strength. These findings were confirmed by the fractographic analysis, where some of the effects of hydrogen (e.g. the formation of secondary cracks by detrimental inclusions (MnS and Al2O3) and the transition of fracture features from ductile dimples to cleavage patterns) were well illustrated. On the other hand, micro-cantilever bending tests were performed in the air to avoid hydrogen effects and applied inside a miniaturized electrochemical cell to promote hydrogen uptake. The bending results and post-mortem analysis of the tested cantilevers indicated that the hydrogen-reduced flow stress and hydrogen-induced cracking occurred for the H-charged bent cantilever, while only increased plastic behavior occurred for the cantilever bent in the air.

An Investigation on the Strengthening Mechanism Based on Grain‐Boundary Misorientation of High‐Strength Pipeline Steel

The microstructure and grain‐boundary misorientation of X80 high‐strength pipeline steel are analyzed using the scanning electron microscope, the electron backscatter diffraction, and the transmission electron microscope, and the effect of grain‐boundary orientation on slip dislocation is investigated by constructing a grain‐boundary dislocation model. The results show that the anisotropy of the mechanical properties of X80 pipeline steel is related not only to grain size and microstructure type but also to grain‐boundary orientation, which the obstruction effect of the high‐angle grain boundary on slip dislocation is significantly greater than that of the low‐angle grain boundary. The larger the orientation difference is, the more disorderly the arrangement of atoms on both sides of grain boundary is, and the more difficult it is for dislocations to slip across grain boundary, so the strengthening effect is more remarkable. Also, when the grain‐boundary misorientation is greater than 15°, the strengthening effect tends to stabilize due to the influence of the interfacial superposition.

Influence of H2S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel

Tensile and fracture properties of X80 pipeline steel were studied in a mimic petrochemical environment. X80 pipeline steel specimens were firstly exposed to air or H2S corrosive medium. Then their tensile properties and δ-∆a resistance curves were obtained in experiments. The influence of H2S corrosion on the X80 pipeline steel’s crack growth resistance curve, fracture toughness, and plastic work loss was analyzed. The comparison between the test results after two pretreatments indicates that there was no significant difference in the X80 pipeline steel’s ultimate tensile strength before and after H2S corrosion. But fracture elongation, area reduction and fracture toughness varied greatly (a substantial decrease in elongation and reduction of area). The crack growth resistance curve of the specimen in air was obviously higher than the crack growth resistance curve of the corroded one. Under stable crack growth stage, the crack initiation toughness δ 0.2BL of the specimen in air was 0.732 mm, 2.02 times that of the corroded one (0.364 mm). In the case of similar crack growth ∆a, the plastic work required in the crack growing process (UP) of the specimen in air was 2.29 times that of the H2S-corroded specimen ( $$ {U}_P^{\hbox{'}} $$ ). H2S corrosion resulted in a significant reduction of the fracture toughness of X80 pipeline steel. Hence, H2S corrosion should be avoided in the process of natural gas transportation by pipelines, so as to protect the pipeline steel from toughness degradation.

Effect of Annealing Process on the Mechanical Properties of X70 Pipeline Steel

The influence of the mechanical properties of X70 pipeline steel under different annealing temperature was studied. The corresponding microstructure was investigated by the Field Emission Scanning Electron Microscopy. The results showed that the yield strength and the tensile strength both experienced from rise to decline with the increase of annealing temperature. The grain sizes were coarse and a large amount of cementite precipitated due to preserving temperature above 550 °, which induced matrix fragmentation and deteriorate the -10 ° DWTT Toughness. There were little changes on the microstructure and mechanical properties when the annealing temperature was under 500 °.

Effect of Ultra Fast Cooling Coiling Temperature on Microstructures and Properties of X80 Pipeline Steel

Effect of ultra fast cooling coiling temperature on microstructures and properties of X80 pipeline steel was studied adopting the detection means of energy dispersive spectrometer, scanning and transmission electron microscopy. Result shows that the toughness increases with the increasing of ultra fast cooling coiling temperature. The yield strength decreases firstly and then increases with the increasing of ultra fast cooling coiling temperature. The yield strength reaches the minimum value at the temperature of around 350 °C.

Hydrostatic pressure effects on stress corrosion cracking of X70 pipeline steel in a simulated deep-sea environment

The stress corrosion cracking (SCC) properties of X70 pipeline steel in simulated deep-sea environments was investigated in a high-pressure vessel with a constant loading device. The results reveal that the threshold stress, σSCC, for SCC is 0.89σb, 0.78σb and 0.68σb in a 3.5% NaCl solution (pH = 7) of 0.1 MPa, 5 MPa and 10 MPa, respectively. When the 3.5% NaCl solution pH value decreased to 3, σSCC decreased further to 0.83σb, 0.68σb and 0.53σb, corresponding to 0.1 MPa, 5 MPa and 10 MPa, respectively. These results indicate that the hydrostatic pressure (HP) promotes SCC, particularly in an acidified sodium chloride solution. Scanning electron microscopy (SEM) and confocal scanning laser microscopy showed that on the sample surface, the HP promoted pitting propagation and linkage, which then initiated microcracks in SCC. The hydrogen concentration was 0.10 ppm, 0.17 ppm and 0.23 ppm after a 200-h immersion in a 3.5% NaCl solution (pH = 7) of 0.1 MPa, 5 MPa and 10 MPa, respectively, revealing that the hydrogen concentration increased approximately linearly with the HP. This result indicated that much more hydrogen was involved in the SCC process in a solution under a higher hydrostatic pressure. The SCC mechanism is controlled by both anodic dissolution and hydrogen-induced cracking in deep-sea environment.

Microstructure, texture evolution and mechanical properties of X70 pipeline steel after different thermomechanical treatments

The evolution of microstructure, texture and mechanical properties of API X70 pipeline steel after different thermomechanical treatments has been studied using a combination of X-ray diffraction and electron backscatter diffraction (EBSD). Our investigations revealed that different microstructure consisting of polygonal ferrite, bainite, coarse and fine acicular ferrite grains was obtained with a centre line segregation traversing through and parallel to the rolling direction. EBSD investigations confirmed that both dynamic recovery and partial recrystallization occurred during hot rolling requiring further annealing for a more homogenous equiaxied grain structure. X-ray macro-texture measurement showed relatively a random texture components comprising of Cube, Goss, Brass, S, Copper, R cube and {331}<1−10>. Texture inhomogeneity during hot rolling were observed for the different processing parameters. The grains at the surface are oriented towards {110}|| ND while the mid thickness has grains mostly oriented in the {001}|| ND with a spread towards the {111}|| ND. We observed that after accelerated cooling, the fast cooling rate and low temperature interruption allows the formation of more bainite which in turn increased the tensile strength of the steel.

Synergistic action of hydrogen and stress concentration on the fatigue properties of X80 pipeline steel

The objective of this study is to quantify the synergistic action of hydrogen and stress concentration on the fatigue properties of pipeline steel. Notch tensile tests and fatigue-life tests were performed on X80 specimens under both a hydrogen partial pressure of 0.6MPa and a nitrogen environment. The notch tensile results indicate that hydrogen had little influence on the notch strength of the specimens with various stress concentration factor (Kt) values; however, as the Kt values increased, there was a decrease in the reduction of area (RA). The results of the fatigue-life tests suggest that the stress concentration severely impaired the fatigue properties, and that further deterioration would occur with hydrogen. The finite element analysis (FEA) results show that the weakest mechanical properties were obtained when Kt was equal to 3.3, due to the synergistic action of the strain and its distribution. This matter should be further considered before hydrogen is blended into existing natural gas pipelines that have been in service for a long period.

Influence of hydrogen pressure on fatigue properties of X80 pipeline steel

The low-cycle fatigue and fatigue crack growth (FCG) properties of X80 pipeline steel in hydrogen atmosphere were determined to investigate the variation of hydrogen pressure and its influence on fatigue life. The test environment was switched to a hydrogen atmosphere after 1000, 3000, or 5000 cycles of pre-fatigue testing in a nitrogen atmosphere. Notch tensile tests were conducted in nitrogen and hydrogen atmospheres after the specimens were pre-fatigued for 3000 or 5000 cycles. The results showed that the cycles to failure of X80 decreased exponentially with increasing hydrogen pressure. When the displacement amplitude (DA) values remained steady (below 3000 cycles), the X80 steels showed no noticeable deterioration in the fatigue properties with or without hydrogen. When the DA values increased (above 5000 cycles), cracks propagated slowly and fatigue properties were strongly reduced in the hydrogen atmosphere, but not in nitrogen. Hydrogen-accelerated crack growth dominates the reduction of fatigue life below 0.6 MPa of hydrogen pressure. Hydrogen-accelerated crack initiation plays a more important role than FCG in the reduction of fatigue life with increasing hydrogen pressure.

Study on Improving the Performance of X80 Pipeline Steel

This paper analyzes the methods of improving the performance of X80 pipeline steel. In the process of manufacturing piping, the right temperature of coiling the steel can improve the comprehensive mechanical properties of X80 pipeline steel. The proper cooling rate can make heat affected zone get the good performance. Quenching can improve the performance of X80 pipeline steel, but the temperature can not be too high. At the same time, the appropriate tempering treatment can make X80 pipeline steel obtain good hardness, strength, plasticity and toughness.