Effect of highly pressurized hydrogen gas charging on the hydrogen embrittlement of API X70 steel

During the use of API X70 steel as a pipeline structural material for the transportation of natural gas, hydrogen embrittlement can occur due to the hydrogen contained in natural gas. The aim of this study is to investigate the effects of the hydrogen content under high-pressure hydrogen gas conditions on the hydrogen embrittlement of air-cooled API X70 steel. The air-cooled API X70 steel was manufactured by hot rolling and was then air-cooled to room temperature. Tensile test specimens were held for 0 h, 1000 h, and 2000 h within a pressure vessel filled with 100% hydrogen gas at a gas pressure of 10 MPa, with the tensile tests then performed at room temperature. The microstructure of the API X70 steel consists of coarse polygonal ferrite, coarse pearlite, and fine acicular ferrite. The yield and tensile strength increased and elongation decreased considerably after a holding time of 2000 h compared to those of 0 h and 1000 h within the pressure vessel. The morphology of the fracture surface changed from ductile to brittle upon hydrogen gas charging. Secondary cracks were observed in both of the hydrogen-gas-charged specimens. No external cracks were formed on the surface of the tensile-tested specimen with a 0 h holding time; however, many external cracks were observed on the specimen surface subjected to hydrogen gas charging.     

Experimental and finite element analysis for fracture of coating layer of galvannealed steel sheet

Mechanical properties of galvannealed (GA) steel sheet used for automotive exposed panel and predicted failure phenomenon of its coating layer were evaluated using finite element method. V-bending test was performed to understand better the fracture of coating layer of GA steel sheet during plastic deformation. Yield strength of the coating layer was calculated by using a relative difference between hardness of coating layer measured from the nano-indentation test and that of substrate. To measure shearing strength at the interface between substrate and coating layer, shearing test with two specimens attached by an adhesive was carried out. Using the mechanical properties measured, a series of finite element analyses coupled with a failure model was performed. Results reveal that the fracture of coating layer occurs in an irregular manner at the region where compressive deformation is dominant. Meanwhile, a series of vertical cracks perpendicular to material surface are observed at the tensile stressed-region. It is found that 0.26-0.28 of local equivalent plastic strain exists at the coating and substrate at the beginning of failure. The fracture of coating layer depends on ductility of the coating layer considerably as well.     

电化学充氢条件下X70管线钢及其焊缝的氢致开裂行为

采用电化学充氢的方法研究了X70管线钢在不同浓度硫酸溶液中的氢致开裂(HIC)行为．结果表明,增大充氢电流密度、延长充氢时间以及降低充氢溶液的pH值能够促进氢进入X70钢基体．微观观察表明,X70钢中的非金属夹杂物如氮化物和氧化物等对其氢致开裂行为有不同的影响,氮化物夹杂并不是充氢裂纹的必然形核位置,而Mg,Al,Ca等的氧化物是更为有害的氢致裂纹源．通过氢渗透实验测得室温下氢在X70钢中的有效扩散系数为3．34×10-9cm2／s．对XT0管线钢基体及焊缝试样电化学预充氢后拉伸,焊缝试样的拉伸塑性较差,各项塑性指标在充氢前、后均低于X70钢基体材料．     

Permeation measurements to study hydrogen uptake by steel electroplated with zinc-cobalt alloys

Hydrogen permeation measurements were performed to investigate hydrogen uptake by a steel substrate when electroplated with zinc-cobalt alloys. The influence of plating bath composition and pH were studied and the effect of absorbed hydrogen on embrittlement of high strength steel was measured in slow strain rate tests. It was shown that the majority of the hydrogen generated during electroplating was either evolved from the surface or trapped in the coating. Only a very small proportion, equivalent to 0.1-0.6% of the total charged passed, was absorbed by the steel and was responsible for causing embrittlement. The average amount of hydrogen required to cause hydrogen embrittlement failure of quenched and tempered AISI 4340 steel was 1.5 × 10⁻⁷ g H atoms cm⁻². There was a linear relationship between the rates of hydrogen uptake from the different coatings and the extent of hydrogen embrittlement produced in the high strength steel. In certain conditions, a cobalt-rich layer formed at the steel/coating interface and this layer was shown to be beneficial in restricting hydrogen uptake and embrittlement.     

Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test

The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentially-notched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel.     

渗铝氧化处理对CLAM钢组织和力学性能的影响

采用合适的渗铝氧化处理工艺在CLAM钢基体表面制备了铝化物涂层,然后利用XRD、EPMA、SEM、纳米压痕仪、室温拉伸试验机等手段研究了渗铝氧化处理前后组织和力学性能变化,尤其是涂层的相组成变化,进而详细分析了硬度变化和拉伸断口的断裂机制。结果表明,渗铝氧化处理后在CLAM钢表面形成了由约30.8 μm厚的FeAl相层和约70.7 μm厚的α-Fe(Al)固溶体层组成的铝化物涂层,最外层FeAl相的硬度最大为834.7 HV,由外向内硬度逐渐降低至315.1 HV,基体内部的硬度出现略微回升。CLAM钢在渗铝氧化前后的抗拉强度分别为581.38 MPa和555.83 MPa,断后伸长率分别为30%和28%,断裂模式由渗铝氧化前的韧性断裂变成准解理断裂。由于渗铝及氧化热处理导致的晶粒尺寸增大和第二相粒子聚集,CLAM钢在渗铝氧化后拉伸性能下降,同时在表面涂层处易产生裂纹源从而加速材料断裂。     

Determination of the critical hydrogen concentration for delayed fracture of high strength steel by constant load test and numerical calculation

The critical hydrogen concentration for hydrogen induced delayed fracture of the AISI 4135 steel at 1320 and 1450 MPa has been determined by constant load tests in combination with numerical calculations, and thus the concept of a critical hydrogen concentration has been verified. The time to fracture was obtained for circumferentially notched round bar specimens under a constant load after electrochemically pre-charged with various hydrogen contents. A numerical model was then developed for calculating the accumulated hydrogen concentration in the vicinity of the notch root, taking into account the driving effect of the hydrostatic stress on hydrogen transport. The results showed that the delayed fracture of the steel occurred when a critical hydrogen concentration at the location of the stress peak was reached by accumulation, and that the time to fracture was related to the stress-driven hydrogen accumulation process. The critical hydrogen concentration was dependent not only on the strength level, but also on the stress concentration factor of the specimens.     

42CrMo钢六角头螺栓热镀锌裂纹分析

42CrMo钢六角头螺栓在热镀锌工艺完成后的磁粉探伤时发现部分螺栓的圆角处存在裂纹。采用光学显微镜、扫描电镜和能谱仪等对螺栓进行了断口、显微组织和成分分析。结果表明,开裂螺栓经破断后的断口符合氢脆断口特征,断口附近氢元素的质量分数偏高,达4×10^-6。裂纹形貌特征和氢脆裂纹特征吻合,裂纹两侧组织为正常的调质组织且裂纹内还发现有锌液残余,可以排除裂纹是由于组织异常或在淬火时产生的。因此,可以推断42CrMo钢六角头螺栓开裂类型为氢致开裂,氢致开裂发生在热镀锌过程中。     

Susceptibility of a X80 steel to hydrogen embrittlement

The susceptibility to hydrogen of a X80 grade steel produced by a thermo‐mechanical control process (TMCP) has been investigated by keeping straining notched specimens under continuous charging conditions. Hydrogen charging was carried out either in synthetic seawater under potentiostatic control at ? 1000 mV vs. SCE or in sulphuric acid with an absorption promoter under galvanostatic control at ? 5 mA/cm². Results reported in terms of hydrogen effect on the ductility of the steel as a function of both cross head speed and root radius of the notch indicate that under the combined effect of cathodic charging, notch severity and very low strain rates the ductility of the TMCP X80 steel can be greatly affected by the presence of hydrogen. With notched specimens strained in air increasing loss of ductility in terms of reduction in area is observed as the notch severity increases. Notched specimens are fairly “more brittle” than smooth ones. As notched specimens are strained under cathodic charging at ? 1000 mV vs. SCE in the synthetic seawater, considerable decrease of reduction in area (RA) is observed. The same trend is observed for displacement and load at fracture both being connected with ductility even if a definite tendency is not always obtained. As the notched specimens are strained under cathodic charging in seawater the fracture morphology shows regions of mixed ductile and brittle fracture and zones where intergranular and/or transgranular fracture path are prevailing. Area of intergranular and transgranular fracture path, that can be more strictly associated with the presence of hydrogen, tends to increase as the strain rate decreases, which suggests a fracture behaviour influenced by hydrogen diffusion. Several mechanisms were involved in the rupture process in sulphuric acid depending on the notch geometry and, especially, on the cross head speed. Apparently, transgranular (quasi‐cleavage) rupture tends to prevail as the displacement becomes lower and lower. No evidence of intergranular fracture was observed.     

EDM and the resulting hydrogen embrittlement of maraging steel

The use of electrical discharge machining (EDM), or spark erosion as it is sometimes called, introduces hydrogen into maraging steel 250 such that brittle fracture surfaces result from embrittlement accelerated by slow-strain-rate tensile tests. Brittle fracture features indicate potential premature failure due to hydrogen embrittlement. Experimental control was provided by test specimens machined by abrasive waterjet, a machining technique that does not evolve hydrogen as a part of the machining process. Hydrogen concentration measurements indicate the hydrogen concentration of specimens fabricated by EDM is approximately 0.5 ppm, while that of specimens machined by abrasive waterjet is approximately 0.1 ppm. On the basis of constant-load tests, the time to failure of test specimens machined by EDM and loaded to 50% of the yield strength is estimated to be a minimum of 30 years.     

Effect of hydrogen on the properties and fracture characteristics of TRIP 800 steels

The paper describes effect of hydrogen on the properties and fracture characteristics of two variants of TRIP 800 C–Mn–Si steels. The effect of hydrogen was studied by means of tensile tests on specimens previously charged by hydrogen. Hydrogen provoked embrittlement in both variants but only for very high hydrogen content. Hydrogen embrittlement manifested itself mainly by a loss of plasticity. Both steel variants were able to absorb a large amount of hydrogen, up to 50 ppm. Concerning fractographic characteristics, steels containing higher hydrogen content displayed transgranular cleavage fracture. In exceptional cases, an irreversible embrittlement was revealed initiating on non-metallic inclusions.     

Salt Spray Corrosion Resistance of Aluminized Coatings on X70 Pipeline Steel by Laser Thermal Radiation

利用激光热辐射对X70管线钢表面进行渗铝处理,通过SEM观察了渗铝层表面和界面形貌,并对其结合界面的化学元素进行了EDS线能谱分析。对盐雾腐蚀后界面化学元素进行了EDS面能谱分析,用XRD分析了盐雾腐蚀前后渗铝层表面的物相,研究了渗铝层对X70管线钢抗盐雾腐蚀性能的影响。结果表明,激光热辐射渗铝处理后X70管线钢表层为氧化铝层,化学元素在表面和界面分布均匀,无富集现象,在结合界面处相互渗透,形成冶金结合形式;盐雾腐蚀后原始试样表面点蚀严重,形成全面腐蚀;渗铝处理后试样表面生成致密的Al2O3保护膜,阻止了腐蚀性介质Cl-离子和基体活性Fe的接触,提高了X70管线钢抗盐雾腐蚀能力。     

Characterization of hydrogen charging of 2205 duplex stainless steel and its correlation with hydrogen‐induced cracking

The effect of various experimental conditions (i.e., hydrogen charging current density, charging time, solution concentration, and temperature) on the embrittlement and cracking susceptibility of 2205 duplex stainless steel was studied by electrochemical hydrogen charging and slow strain rate tests. The results showed that the choice of the experimental conditions had obvious effect on the hydrogen concentration in the specimens. A relationship between the embrittlement and hydrogen charging conditions was established by the investigation of the fracture morphology. Under the free‐charging condition, the fracture surfaces were characteristic of dimples, while on the condition of the dynamic hydrogen charging, the hydrogen‐induced fracture showed the appearance of cleavage. Further examination of fracture cracks confirmed that the ferrite phase acts as a preferential path for crack propagation.     

Gaseous hydrogen embrittlement of T-250 laser welds

The tensile properties of laser-welded T-250 maraging steel are measured, with attention paid to the influence of strain rate and gaseous hydrogen on the fracture behavior of welded specimens. Post-weld heat treatments are performed on laser-welded specimens to obtain underaged (WU), peak-aged (WP), and overaged (WO) specimens. Hydrogen embrittlement (HE) affects the tensile fracture behavior of the welded specimens; HE changes not only the fracture mode but also the fracture location. Without the influence of hydrogen, the fracture location is at the softest region, the weld metal (WM), and the fracture appearance reveals a ductile dimple fracture. For welds sensitive to HE, the fracture is initiated at the heat-affected zone (HAZ) with coarse grain size, and the associated fracture surface exhibits intergranular and quasi-cleavage fractures that are brittle in nature. In addition, the HAZ with coarse grain size is more prone to HE, as compared to other regions in the welded specimens. The WU specimens are susceptible to HE in air under a low strain rate, while the WP specimens are only susceptible to gaseous hydrogen embrittlement (GHE). However, the WO specimens are immune to GHE and insensitive to strain rate.     

CrWMn钢喷涂NiFeCrBSi（G112）合金粉层的超塑焊合

ＣｒＷＭｎ钢火焰喷涂ＮｉＦｅＣｒＢＳｉ合金涂层，进行了恒温压缩超塑性试验，采用变截面法测定了ＣｒＷＭｎ钢的塑性流动应力对变形速率敏感性指数ｍ值．用涂层和基材的应变速率比Ｗ值，确定涂层和基材的最佳超塑条件．研究了涂层孔洞、涂层原颗粒间界及涂层与基材结合界面在不同变形温度下的焊合效果，指出在基材超塑温度范围内，Ｗ＝１时，涂层焊合良好，并具有较高的多冲抗力．     

The effects of sacrificial coatings on hydrogen embrittlement and re-embrittlement of ultra high strength steels

This paper describes an investigation of electrodeposited Zn-14% Ni and aluminium-based SermeTel 1140/962 coatings as possible replacements for cadmium. Slow strain rate tests were performed to measure the extent of direct hydrogen embrittlement of a high strength steel substrate as a result of the coating process and of hydrogen re-embrittlement caused by coating corrosion. The level of re-embrittlement was shown to depend on both the electrochemical potential of the coating and its barrier properties. Zn-14% Ni coatings caused the most re-embrittlement as they had the most active potential and contained through-thickness defects which left the steel exposed to hydrogen uptake. The microstructure of the high strength steel was also shown to be an important factor affecting the extent of embrittlement. AerMet 100 steel was more resistant than 300M steel and this was attributed to the presence of reverted austenite surrounding the martensite laths in AerMet 100, which trapped absorbed hydrogen and prevented a critical hydrogen concentration being reached in the more susceptible martensite phase.     

Corrosion behaviour of aircraft coating systems in acetate- and formate-based de-icing chemicals

Corrosion behaviour of four coating systems, which are commonly used in aircraft components; namely cadmium-coated and subsequently chromate-treated steel AISI 4340, aluminium-coated steel AISI 4340, anodised aluminium alloy 7075-T6 and chromate-treated aluminium alloy 7075-T6, were investigated in six commercial acetate- and formate-based de-icing chemicals. The results show that the aluminium-coated steel specimens experience least corrosion among the four coating systems; no corrosion is detected in potassium acetate-based de-icing chemicals (Safeway KA HOT, Safegrip and Safegrip+), although some corrosion is observed in the other three chemicals (Safeway SD, Safeway SF and Meltium). In contrast, the coatings in the other three coating systems are damaged in all six de-icing chemicals. The thickness of the coating does not necessarily predict how well the coating will protect the substrate; the thickest coating, several tens of micrometres, is apparent on the cadmium- and chromate-coated steel specimens, while only a few micrometres thin aluminium coating on the steel substrate is the only coating system that can survive the corrosion tests in some of the de-icing chemicals. Comparison of the results from the two test methods used in this research, polarisation measurements and chemical exposure tests, shows that, for each coating system, the extent of corrosion, as evaluated on the basis of weight changes calculated from corrosion current density values and those experienced during chemical exposure tests, is different: the weight changes that materialise during exposure tests are, at least, from one to two orders of magnitude higher than those predicted by calculations. In this paper, these observations are discussed and explanations for them are presented.     

X70管线钢电化学充氢后的力学行为研究

应用拉伸试验和慢拉伸试验，研究X70管线钢电化学充氢后材料拉伸性能的变化．结果表明：电化学充氢对X70管线钢的强度没有显著的影响，主要降低了材料的塑性，从而降低了材料的断裂延性和断裂强度．在静态电化学充氢条件下，材料的塑性随充氢时间的增加，依次降低．在慢拉伸条件下，动态电化学充氢显著降低材料的塑性．断口分析表明：静态电化学充氢后的断口以韧窝为主要特征，但韧窝直径变小；慢拉伸的动态电化学充氢断口出现准解理断裂．     

Corrosion fatigue behavior of a steel with sprayed coatings

This paper describes the corrosion fatigue behavior and fracture mechanisms of a steel with different sprayed coatings. Rotating bending fatigue tests were conducted in 3% NaCl solution using specimens of a medium carbon steel with sprayed coatings of a ceramic (Cr₂O₃), a cermet (WC-12%Co) and two metals (Ni-11 % P and Al-2% Zn). The corrosion fatigue process was basically the same for ceramic, cermet, and Ni-11 % P sprayed specimens. That is, the corrosive media could be supplied from the specimen surface to the substrate through cracks initiated during fatigue cycling and/or pores in the coatings, and thus corrosion pits were generated followed by subsequent crack initiation and growth in the substrate. The corrosion fatigue strength of ceramic sprayed specimens was slightly improved compared to that of the substrate steel because the under-coating (Ni-5%A1) could impede the penetration of the corrosive media although the ceramic coating had a poor resistance to cracking under cyclic loading. Cermet sprayed specimens also exhibited improved corrosion fatigue strength because of the high resistance to cracking and the low volume fraction of pores of the coating. In Ni-11 % Psprayed specimens, cracks were initiated in the coating even at low stress levels; thus the corrosion fatigue strength was the same as that of the substrate. Anodic dissolution took place in Al-2 % Zn coating because the coating was electrochemically poor, and thus the substrate was cathodically protected. Therefore, the corrosion fatigue strength of Al-2 % Zn sprayed specimens was enhanced to as high as the fatigue strength of the substrate in room air. Based on the experimental results, a dual-layer coating consisting of WC-12%Co and Al-2%Zn was fatigue tested. The coating was effective at low stress levels and exhibited long life under conditions where corrosion fatigue strength was critical.     

Fatigue behavior of a SAE 1045 steel coated with Colmonoy 88 alloy deposited by HVOF thermal spray

An investigation has been conducted in order to study the fatigue behavior of a SAE 1045 steel substrate coated with a Ni-base alloy known commercially as Colmonoy 88, deposited by HVOF spray technique. Fatigue tests were conducted under axial conditions (R = 0.1), employing samples of the substrate material in the as-polished condition, after grit blasting with alumina particles and after grit blasting and coating with a deposit of about 250 μm thick. The fatigue tests were conducted at maximum stresses in the range of 380-533 MPa, depending on the condition of the material. A detailed fractographic analysis of some selected samples tested at different stresses was carried out, aimed mainly at determining the crack nucleation and propagation sequence. The results indicate that the deposition of such a coating leads to a fatigue strength debit of the substrate in the range of 10-20% and a similar debit in fatigue limit of ∼ 11-13%. It has been found that grit blasting is the process responsible for the fatigue strength debit observed in the coated samples. Fatigue cracks have been observed to initiate at the substrate-coating interface and at the free surface of the coating, mainly close to alumina particles embedded on the substrate and sharp notches produced during the process. The fractographic analysis of the fracture surface of the coated specimens points out the characteristic heterogeneous nature of the coating, particularly regarding some of its mechanical properties, such as fracture toughness.