Hydrogen embrittlement of X80 pipeline steel in H_2S environment: Effect of hydrogen charging time,hydrogen-trapped state and hydrogen charging–releasing–recharging cycles

This study investigated the susceptibility of X80 pipeline steel to hydrogen embrittlement given different hydrogen pre-charging times and hydrogen charging–releasing–recharging cycles in H_2S environment. The fracture strain of the steel samples decreased with increasing hydrogen pre-charging time; this steel degradation could almost be recovered after diffusible hydrogen was removed when the hydrogen pre-charging time was 8 d. However, unrecoverable degeneration occurred when the hydrogen pre-charging time extended to 16–30 d. Moreover, nanovoid formation meant that the hydrogen damage to the steel under intermittent hydrogen pre-charging–releasing–recharging conditions was more serious than that under continuous hydrogen pre-charging conditions. This study illustrated that the mechanical degradation of steel is inevitable in an H_2S environment even if diffusible hydrogen is removed or visible hydrogen-induced cracking is neglected. Furthermore, the steel samples showed premature fractures and exhibited a hydrogen fatigue effect because the repeated entry and release of diffusible hydrogen promoted the formation of vacancies that aggregated into nanovoids. Our results provide valuable information on the mechanical degradation of steel in an H_2S environment, regarding the change rules of steel mechanical properties under different hydrogen pre-charging times and hydrogen charging–releasing–recharging cycles.     

Hydrogen induced cracking of X80 pipeline steel

The hydrogen-induced cracking (HIC) behavior of X80 pipeline steel was studied by means of electrochemical charging, hydrogen permeation tests, tension test, and scanning electron microscopy (SEM). The experimental results indicate that the increase of charging time and charging current density or the decrease of the solution pH value leads to an increase of the hydrogen content in X80 steel, which plays a key role in the initiation and propagation of HIC. It is found that the majority of macro-inclusions within the as-used X80 steel do not constitute a direct threat to HIC except aluminum oxides, which directly or indirectly lead to HIC. The hydrogen trap density at room temperature is estimated to be pretty high, and this is an essential reason why the steel is sensitive to HIC. After hydrogen charging, the elongation loss rate and area reduction of X80 steel decline obviously, taking a noticeable sign of hydrogen-induced plasticity damages. It is demonstrated that the losses of these plastic parameters have a linear relation to the fracture size due to hydrogen.     

Environmental boundary and formation mechanism of different types of H<Subscript>2</Subscript>S corrosion products on pipeline steel

To establish an adequate thermodynamic model for the mechanism of formation of hydrogen sulfide (H₂S) corrosion products, theoretical and experimental studies were combined in this work. The corrosion products of API X60 pipeline steel formed under different H₂S corrosion conditions were analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. A thermodynamic model was developed to clarify the environmental boundaries for the formation and transformation of different products. Presumably, a dividing line with a negative slope existed between mackinawite and pyrrhotite. Using experimental data presented in this study combined with previously published results, we validated the model to predict the formation of mackinawite and pyrrhotite on the basis of the laws of thermodynamics. The established relationship is expected to support the investigation of the H₂S corrosion mechanism in the oil and gas industry.     

Effect of shot peening on hydrogen embrittlement of high strength steel

The effect of shot peening (SP) on hydrogen embrittlement of high strength steel was investigated by electrochemical hydrogen charging, slow strain rate tensile tests, and hydrogen permeation tests. Microstructure observation, microhardness, and X-ray diffraction residual stress studies were also conducted on the steel. The results show that the shot peening specimens exhibit a higher resistance to hydrogen embrittlement in comparison with the no shot peening (NSP) specimens under the same hydrogen-charging current density. In addition, SP treatment sharply decreases the apparent hydrogen diffusivity and increases the subsurface hydrogen concentration. These findings are attributed to the changes in microstructure and compressive residual stress in the surface layer by SP. Scanning electron microscope fractographs reveal that the fracture surface of the NSP specimen exhibits the intergranular and quasi-cleavage mixed fracture modes, whereas the SP specimen shows only the quasi-cleavage fractures under the same hydrogen charging conditions, implying that the SP treatment delays the onset of intergranular fracture.     

Feasibility of new ladle-treated Hadfield steel for mining purposes

A debate has arisen over the possibility of using a new ladle-treated Hadfield steel instead of conventional heat-treated Hadfield steel in mining applications. This debate might be solved by identifying the differences between the mechanical properties and strain-hardening properties of conventional heat-treated Hadfield steel and its counterpart ladle-treated Hadfield steel. Tensile and compression tests demonstrated that the ductility of ladle-treated Hadfield steel is similar to that of conventional heat-treated steel. However, the strain-hardening property of the ladle-treated Hadfield steel is almost two times higher than that of the heat-treated Hadfield steel. The results of this study demonstrate that the improvement of the strain-hardening behavior is attributable to the low stacking-fault energy of the main austenite matrix, which results from the high segregation coefficient of carbon and manganese solutes of the main austenite matrix into the new eutectic phase. Superior wear abrasion resistance is a potential consequence of different strain-hardening properties under low and high loads.     

NbC纳米析出相对低合金高强钢氢致开裂的影响研究

We investigated the effect of nanosized NbC precipitates on hydrogen-induced cracking (HIC) of high-strength low-alloy steel by conducting slow-strain-rate tensile tests (SSRT) and performing continuous hydrogen charging and fracture analysis. The results reveal that the HIC resistance of Nb-bearing steel is obviously superior to that of Nb-free steel, with the fractured Nb-bearing steel in the SSRT exhibiting a smaller ratio of elongation reduction (I_δ). However, as the hydrogen traps induced by NbC precipitates approach hydrogen saturation, the effect of the precipitates on the HIC resistance attenuate. We speculate that the highly dispersed nanosized NbC precipitates act as irreversible hydrogen traps that hinder the accumulation of hydrogen at potential crack nucleation sites. In addition, much like Nb-free steel, the Nb-bearing steel exhibits both H-solution strengthening and the resistance to HIC.     

Variation in retained austenite content and mechanical properties of 0.2C–7Mn steel after intercritical annealing

The effects of annealing time and temperature on the retained austenite content and mechanical properties of 0.2C–7Mn steel were studied. The retained austenite content of 0.2C–7Mn steel was compared with that of 0.2C–5Mn steel. It is found that 0.2C–7Mn steel exhibits a similar variation trend of retained austenite content as 0.2C–5Mn steel. However, in detail, these trends are different. 0.2C–7Mn steel contains approximately 7.5vol% retained austenite after austenitization and quenching. The stability of the reversed austenite in 0.2C–7Mn steel is lower than that in 0.2C–5Mn steel; in contrast, the equilibrium reversed austenite fraction of 0.2C–7Mn steel is substantially greater than that of 0.2C–5Mn steel. Therefore, the retained austenite content in 0.2C–7Mn steel reaches 53.1vol%. The tensile results show that long annealing time and high annealing temperature may not favor the enhancement of mechanical properties of 0.2C–7Mn steel. The effect of retained austenite on the tensile strength of the steel depends on the content of retained austenite; in contrast, the 0.2% yield strength linearly decreases with increasing retained austenite content.     

Hot-corrosion behavior of Cr_2O_3–CNT-coated ASTM-SA213-T22 steel in a molten salt environment at 700°C

The present work investigates the hot-corrosion behavior of carbon nanotube(CNT)-reinforced chromium oxide coatings on boiler steel in a molten salt(Na_2SO_4–60 wt%V_2O_5) environment at 700°C under cyclic conditions. The coatings were deposited via the high-velocity oxygen fuel process. The uncoated and coated steel samples were subjected to hot corrosion in a silicon tube furnace at 700°C for 50 cycles. The kinetics of the corrosion behavior was analyzed through mass-gain measurements after each cycle. The corrosion products were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis techniques. The results revealed that uncoated steel suffered spallation of scale because of the formation of nonprotective Fe_2O_3 scale. The coated steel samples exhibited lower mass gains with better adhesiveness of oxide scale with the steel alloy until the end of exposure. The CNT-reinforced coatings were concluded to provide better corrosion resistance in the hot-corrosion environment because of the uniform dispersion of CNTs in the coating matrix and the formation of protective chromium oxides in the scale.     

Effect of hydrogen on the friction and wear of Ni-P coatings

The effects of charging and outgassing on the friction and wear of Ni-P amorphous and nanocrystalline coatings were studied under ambient humidity and temperature. The results indicate that hydrogen charging can greatly decrease the volume loss during friction and increase the wear durability. However, the wear durability can be restored after outgassing. There are many cracks on the brittle nanocrystalline before charging, and no crack appears after charging, but there are only a few cracks on the outgassed sample. This indicates that hydrogen charging can inhibit the crack formation during friction for the brittle Ni-P nanocrystalline. For the charged sample, after the surface layer was removed by polishing, the wear track and the friction coefficient are consistent with those of an uncharged sample. The friction coefficient of the charged amorphous coating is very small within the first 140 s and then increases suddenly to the value corresponding to the uncharged sample. The absorbed surface layer lubricates the surface, reduces the friction coefficient, and improves the wear durability.     

Experimental and numerical study on plasma nitriding of AISI P20 mold steel

In this study, plasma nitriding was used to fabricate a hard protective layer on AISI P20 steel, at three process temperatures (450°C, 500°C, and 550°C) and over a range of time periods (2.5, 5, 7.5, and 10 h), and at a fixed gas N₂:H₂ ratio of 75vol%:25vol%. The morphology of samples was studied using optical microscopy and scanning electron microscopy, and the formed phase of each sample was determined by X-ray diffraction. The elemental depth profile was measured by energy dispersive X-ray spectroscopy, wavelength dispersive spectroscopy, and glow dispersive spectroscopy. The hardness profile of the samples was identified, and the microhardness profile from the surface to the sample center was recorded. The results show that ε-nitride is the dominant species after carrying out plasma nitriding in all strategies and that the plasma nitriding process improves the hardness up to more than three times. It is found that as the time and temperature of the process increase, the hardness and hardness depth of the diffusion zone considerably increase. Furthermore, artificial neural networks were used to predict the effects of operational parameters on the mechanical properties of plastic mold steel. The plasma temperature, running time of imposition, and target distance to the sample surface were all used as network inputs; Vickers hardness measurements were given as the output of the model. The model accurately reproduced the experimental outcomes under different operational conditions; therefore, it can be used in the effective simulation of the plasma nitriding process in AISI P20 steel.     

Effects of elemental Sn on the properties and inclusions of the free-cutting steel

A new environment-friendly free-cutting steel alloyed with elemental Sn (Y20Sn) was developed to meet the requirements of machinability and mechanical properties according to GB/T8731-1988. The machinability of the steel is enhanced by the segregation of elemental Sn at grain boundaries. The effect of Sn segregation on intergranular brittle fracture at normal cutting temperature from 250℃ to 400℃ is confirmed. The formation mechanism of main inclusions MnS is influenced by the presence of Sn and the attachment of Sn around MnS itself as a surfactant, and this mechanism also explains the improvement in machinability and mechanical properties of the steel. In the steel, the relevant inclusions are mainly spherical or axiolitic, and are uniformly distributed in small volume. Such inclusions improve the machinability of the steel and do not impair the mechanical properties as well. Experimental results demonstrate that the appropriate content of Sn in the steel is 0.03wt% to 0.08wt%, and the remaining composition is close to that of standard Y20 steel.     

Effect of in-situ nanoparticles on the mechanical properties and hydrogen embrittlement of high-strength steel

We investigated the critical influence of in-situ nanoparticles on the mechanical properties and hydrogen embrittlement(HE) of high-strength steel.The results reveal that the mechanical strength and elongation of quenched and tempered steel(919 MPa yield strength,17.11% elongation) are greater than those of hot-rolled steel(690 MPa yield strength, 16.81% elongation) due to the strengthening effect of insitu Ti_3O_5–Nb(C,N) nanoparticles.In addition, the HE susceptibility is substantially mitigated to 55.52%, approximately 30% lower than that of steels without in-situ nanoparticles(84.04%), which we attribute to the heterogeneous nucleation of the Ti_3O_5 nanoparticles increasing the density of the carbides.Compared with hard TiN inclusions, the spherical and soft Al_2O_3–MnS core–shell inclusions that nucleate on in-situ Al_2O_3 particles could also suppress HE.In-situ nanoparticles generated by the regional trace-element supply have strong potential for the development of high-strength and hydrogen-resistant steels.     

Development and progress on hydrogen metallurgy

Hydrogen metallurgy is a technology that applies hydrogen instead of carbon as a reduction agent to reduce CO_2 emission, and the use of hydrogen is beneficial to promoting the sustainable development of the steel industry. Hydrogen metallurgy has numerous applications,such as H_2reduction ironmaking in Japan, ULCORED and hydrogen-based steelmaking in Europe; hydrogen flash ironmaking technology in the US; HYBRIT in the Nordics; Midrex H_2TM by Midrex Technologies, Inc.(United States); H_2FUTURE by Voestalpine(Austria); and SALCOS by Salzgitter AG(Germany). Hydrogen-rich blast furnaces(BFs) with COG injection are common in China. Running BFs have been industrially tested by AnSteel, XuSteel, and BenSteel. In a currently under construction pilot plant of a coal gasification–gas-based shaft furnace with an annual output of 10000 t direct reduction iron(DRI), a reducing gas composed of 57 vol% H_2 and 38 vol% CO is prepared via the Ende method. The life cycle of the coal gasification–gas-based shaft furnace–electric furnace short process(30 wt% DRI + 70 wt% scrap) is assessed with 1 t of molten steel as a functional unit. This plant has a total energy consumption per ton of steel of 263.67 kg standard coal and a CO_2 emission per ton of steel of 829.89 kg, which are superior to those of a traditional BF converter process. Considering domestic materials and fuels, hydrogen production and storage, and hydrogen reduction characteristics, we believe that a hydrogen-rich shaft furnace will be suitable in China. Hydrogen production and storage with an economic and large-scale industrialization will promote the further development of a full hydrogen shaft furnace.     

原位纳米增强高强韧钢氢脆性能研究

We investigated the critical influence of in-situ nanoparticles on the mechanical properties and hydrogen embrittlement (HE) of high-strength steel. The results reveal that the mechanical strength and elongation of quenched and tempered steel (919 MPa yield strength, 17.11% elongation) are greater than those of hot-rolled steel (690 MPa yield strength, 16.81% elongation) due to the strengthening effect of in-situ Ti₃O₅–Nb(C,N) nanoparticles. In addition, the HE susceptibility is substantially mitigated to 55.52%, approximately 30% lower than that of steels without in-situ nanoparticles (84.04%), which we attribute to the heterogeneous nucleation of the Ti₃O₅ nanoparticles increasing the density of the carbides. Compared with hard TiN inclusions, the spherical and soft Al₂O₃–MnS core–shell inclusions that nucleate on in-situ Al₂O₃ particles could also suppress HE. In-situ nanoparticles generated by the regional trace-element supply have strong potential for the development of high-strength and hydrogen-resistant steels.     

Effects of solution environments under disbonded coatings on the corrosion behaviors of X70 pipeline steel in acidic soils

A rectangle crevice assembly was used to study the effects of cathodic protection (CP) potential, crevice thickness, holiday size, bubbling CO₂, and surface condition on the chemical and electrochemical environment of the local solution under disbonded coatings. It is found that the cathodic protection removes dissolved oxygen from the crevice and thus shifts the solution to a more alkaline state. Furthermore, the potential of the steel reaches the protected potential range. The available protection distance increases with the negative applying potential. The steady potential and pH distribution are easily achieved, but the polarization degree is not satisfied within the thinner crevice. The difference in the solution environment is found to correlate to the holiday size. The smaller the holiday, the smaller the difference is. The presence of CO₂ inhibits the formation of an alkaline environment. It is also found that the rust layer dramatically decreases the polarization rate in the crevice.     

Effect of nanosized Nb C precipitates on hydrogen-induced cracking of high-strength low-alloy steel

We investigated the effect of nanosized NbC precipitates on hydrogen-induced cracking(HIC) of high-strength low-alloy steel by conducting slow-strain-rate tensile tests(SSRT) and performing continuous hydrogen charging and fracture analysis. The results reveal that the HIC resistance of Nb-bearing steel is obviously superior to that of Nb-free steel, with the fractured Nb-bearing steel in the SSRT exhibiting a smaller ratio of elongation reduction(I_δ). However, as the hydrogen traps induced by NbC precipitates approach hydrogen saturation, the effect of the precipitates on the HIC resistance attenuate. We speculate that the highly dispersed nanosized NbC precipitates act as irreversible hydrogen traps that hinder the accumulation of hydrogen at potential crack nucleation sites. In addition, much like Nb-free steel, the Nb-bearing steel exhibits both H-solution strengthening and the resistance to HIC.     

烧结铁矿中FeO含量对其在富氢高炉中还原行为的影响

Japan started the national project “COURSE 50” for CO₂ reduction in the 2000s. This project aimed to establish novel technologies to reduce CO₂ emissions with partially utilization of hydrogen in blast furnace-based ironmaking by 30% by around 2030 and use it for practical applications by 2050. The idea is that instead of coke, hydrogen is used as the reducing agent, leading to lower fossil fuel consumption in the process. It has been reported that the reduction behavior of hematite, magnetite, calcium ferrite, and slag in the sinter is different, and it is also considerably influenced by the sinter morphology. This study aimed to investigate the reduction behavior of sinters in hydrogen enriched blast furnace with different mineral morphologies in CO–CO₂–H₂ mixed gas. As an experimental sample, two sinter samples with significantly different hematite and magnetite ratios were prepared to compare their reduction behaviors. The reduction of wustite to iron was carried out at 1000, 900, and 800°C in a CO–CO₂–H₂ atmosphere for the mineral morphology-controlled sinter, and the following findings were obtained. The reduction rate of smaller amount of FeO led to faster increase of the reduction rate curve at the initial stage of reduction. Macro-observations of reduced samples showed that the reaction proceeded from the outer periphery of the sample toward the inside, and a reaction interface was observed where reduced iron and wustite coexisted. Micro-observations revealed three layers, namely, wustite single phase in the center zone of the sample, iron single phase in the outer periphery zone of the sample, and iron oxide-derived wustite FeO and iron, or calcium ferrite-derived wustite 'FeO' and iron in the reaction interface zone. A two-interface unreacted core model was successfully applied for the kinetic analysis of the reduction reaction, and obtained temperature dependent expressions of the chemical reaction coefficients from each mineral phases.     

淬火分配处理对高碳钢组织、力学性能和磨损性能的影响

The present work employed the X-ray diffraction, scanning electron microscopy, electron backscattered diffraction, and electron probe microanalysis techniques to identify the microstructural evolution and mechanical and abrasive behavior of high carbon steel during quenching-partitioning treatment with an aim to enhance the toughness and wear resistance of high carbon steel. Results showed that, with the increase in partitioning temperature from 250 to 400°C, the amount of retained austenite (RA) decreased resulting from the carbide precipitation effect after longer partitioning times. Moreover, the stability of RA generally increased because of the enhanced degree of carbon enrichment in RA. Given the factors affecting the toughness of high carbon steel, the stability of RA associated with size, carbon content, and morphology plays a significant role in determining the toughness of high carbon steel. The analysis of the wear resistance of samples with different mechanical properties shows that hardness is the primary factor affecting the wear resistance of high carbon steel, and the toughness is the secondary one.     

Abrasive wear behavior of cast iron coatings plasma-sprayed at different mild steel substrate temperatures

Three kinds of cast iron coatings were prepared by atmospheric plasma spraying. During the spraying, the mild steel substrate temperature was controlled to be averagely 50, 180, and 240℃, respectively. Abrasive wear tests were conducted on the coatings under a dry friction condition. It is found that the abrasive wear resistance is enhanced with the substrate temperature increasing. SEM observations show that the wear losses of the coatings during the wear tests mainly result from the spalling of the splats. Furthermore, the improved wear resistance of the coatings mainly owes to the formation of oxides and the enhancement in the mechanical properties with the substrate temperature increasing.     

Azo dye degradation behavior of AlFeMnTiM (M=Cr,Co, Ni) high-entropy alloys

Because of the potential carcinogenic effects and difficult degradation of azo dyes, their degradation has been a longstanding problem. The degradation of azo dye Direct Blue 6(DB6) using ball-milled(BM) high-entropy alloy(HEA) powders was characterized in this work. Newly designed AlFeMnTiM(M = Cr, Co, Ni) HEAs synthesized by mechanical alloying(MA) showed excellent performance in the degradation of azo dye DB6. The degradation efficiency of AlFeMnTiCr is approximately 19 times greater than that of the widely used commercial Fe–Si–B amorphous alloy ribbons and more than 100 times greater than that of the widely used commercial zero-valent iron(ZVI) powders. The galvanic-cell effect and the unique crystal structure are responsible for the good degradation performance of the BM HEAs. This study indicates that BM HEAs are attractive, valuable, and promising environmental catalysts for wastewater contaminated by azo dyes.