Effect of austenite instability on the hydrogen-enhanced crack growth of austenitic stainless steels

Fatigue crack growth tests were performed to assess the fatigue behavior of AISI 316L and 254 SMO stainless steels (SSs) in air and gaseous hydrogen. 254 SMO SS generally exhibited a greater resistance to fatigue crack growth than 316L. Sensitization treatment had only a marginal effect on the fatigue crack growth behavior of both alloys in air. Moreover, 316L SS exhibited significant hydrogen-enhanced crack growth but 254 SMO, even sensitized 254 SMO specimens, did not. A thin layer of strain-induced martensite was formed on the fatigue-fractured surface of the 316L SS, and its content increased when raising the stress ratio. The thin martensite layer was responsible for the hydrogen-enhanced fatigue crack growth of the 316L SS. By contrast, the extremely stable austenite was responsible for the low susceptibility of 254 SMO SS to hydrogen-accelerated crack growth. The trapping of hydrogen at the grain boundaries and the transformed martensite in the sensitized 316L specimens led to increased fatigue crack growth rates and intergranular fracture of the material.     

Hydrogen-assisted fatigue crack growth of AISI 316L stainless steel weld

The fatigue crack growth behaviors of AISI 316L stainless steel (SS) welds in air and gaseous hydrogen were evaluated, and further compared with the base plate. In air, the fatigue crack growth rate (FCGR) of the weld after heat-treatment at 1050 ^oC/1 h was similar to that of the base metal. Furthermore, all specimens became susceptible to hydrogen-accelerated crack growth. Mainly quasi-cleavage fracture related with the strain-induced martensite accounted for the accelerated crack growth in hydrogen. A smaller amount of martensite in the weld was responsible for the decreased susceptibility to hydrogen-enhanced fatigue crack growth relative to the base metal.     

Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen

Fatigue crack growth test was performed to evaluate fatigue behavior of 304 stainless steel specimens with or without laser processing (welding and surface treatment) in air and gaseous hydrogen. As the crack propagation normal to the laser welding or scan direction, the laser-processed specimens exhibited a higher resistance to crack growth in the low stress intensity factor range (ΔK) than the as-received steel plates regardless of testing environments. However, the marked retardation of crack growth behavior vanished for welded specimens subjected to a 850 °C/h stress relief treatment or with a shorter distance from notch tip to the weld centerline in the test.Fatigue-fractured appearance of the steel plate tested in air was composed of mainly transgranular fatigue fracture and some flat facets, along with a small amount of intergranular fracture. While quasi-cleavage fracture and few twin boundary separations were observed for the same specimen in hydrogen. On the other hand, the lower crack growth rate of laser-processed specimens in both air and hydrogen was accompanied with rubbed areas on the fracture surfaces. It was found that the extent of quasi-cleavage fracture was related to the formation of strain-induced martensite, which would contribute to an increased fatigue crack growth rate of all specimens in gaseous hydrogen.     

Effect of electron beam surface hardening on fatigue crack growth rate in AISI 4340 steel

This paper studies the effect of electron beam (EB) surface hardening on the fatigue crack growth rate in AISI 4340 steel. The heat treatment conditions were varied to consider the influence of microstructure and residual stress. The results show that increasing the EB heat input increases the compressive residual stress in the hardened layer. Thus EB surface-hardening treatment improves the fatigue crack growth resistance. This effect increases with increasing EB heat input but disappears as the ΔK value increases. The fracture mechanism of the hardened layer is intergranular fracture, while that of the base material is transgranular quasi-cleavage.     

15Mn钢在3.5%氯化钠水溶液中的腐蚀疲劳

研究了不同组织状态的15Mn钢在3.5％氯化钠水溶液中的疲劳裂纹扩展行为,测定了裂尖区的平均氢含量和裂纹闭合力,发现外部条件(力学及环境)和材料的显微组织对腐蚀疲劳有重要影响。结合断口和裂纹走向观察,提出了15Mn钢腐蚀疲劳断裂的复合机制和断裂机制图。     

The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions

The change in the mechanism of stress corrosion cracking with test temperature for Type 304, 310 and 316 austenitic stainless steels was investigated in boiling saturated magnesium chloride solutions using a constant load method. Three parameters (time to failure; t_f, steady-state elongation rate; l_ss and transition time at which a linear increase in elongation starts to deviate; t_ss) obtained from the corrosion elongation curve showed clearly three regions; stress-dominated, stress corrosion cracking-dominated and corrosion-dominated regions. In the stress corrosion cracking-dominated region the fracture mode of type 304 and 316 steels was transgranular at higher temperatures of 416 and 428 K, respectively, but was intergranular at a lower temperature of 408 K. Type 310 steel showed no intergranular fracture but only transgranular fracture. The relationship between log l_ss and log t_f for three steels became good straight lines irrespective of applied stress. The slope depended upon fracture mode; −2 for transgranular mode and −1 for intergranular mode. On the basis of the results obtained, it was estimated that intergranular cracking was resulted from hydrogen embrittlement due to strain-induced formation of martensite along the grain boundaries, while transgranular cracking took place by propagating cracks nucleated at slip steps by dissolution.     

不同热处理态的304和321奥氏体不锈钢在氯化铵环境中的应力腐蚀行为对比研究

目的 对比研究原始、固溶和敏化态的304和321奥氏体不锈钢在模拟加氢催化氯化铵环境中的应力腐蚀（SCC）行为及机理。方法 将304和321奥氏体不锈钢经过热处理制备成固溶和敏化态试样,采用U形弯试样在模拟加氢催化氯化铵环境中浸泡的应力腐蚀试验方法对其进行研究,通过观察U形弯弧顶的腐蚀形貌和开裂时间,并结合腐蚀及裂纹的SEM照片和电化学测试结果进行分析。结果 原始和固溶状态304不锈钢U形弯试样在氯化铵溶液环境中开裂时间为25 d左右,断口形貌分别为穿晶断口和沿晶断口;敏化态试样18 d后发生开裂,断口形貌为穿晶和沿晶的混合断口。原始和固溶态321不锈钢U形弯试样在该环境中经过39 d均无应力腐蚀裂纹;敏化试样经30 d后产生宏观开裂。电化学测试结果显示,不同热处理态的304不锈钢在氯化铵溶液中均具有明显的点蚀敏感性,321不锈钢在该环境中耐点蚀和应力腐蚀的能力优于304不锈钢。结论 不同状态的304不锈钢在高温氯化铵环境中具有较强的应力腐蚀倾向,特别是敏化态试样;321不锈钢在该环境中的应力腐蚀敏感性相对较小,但敏化处理显著增加了其沿晶应力腐蚀倾向,而固溶态试样具有明显的沿晶腐蚀特征。     

CRYOGENIC MECHANICAL BEHAVIOUR OF MARAGING STEEL CONTAINING HYDROGEN

Within the range of 77 to 295 K,the strength and plastieity of 18Ni maraging steel increasewith decreasing temperature and decrease with increasing hydrogen content.The susceptibilityto hydrogen embrittlement of the maraged specimens is superior to solid solution ones.Thedislocations after plastically deforming will tangle into dislocation cells which reducethermselves in size as the temperature decreases,and coarsen as the hydrogen content in-creases.No cryogenie deformation twin was found.The tensile fracture surfaces of the solu-tion and maraged specimens containing 5.90 ppm hydrogen under 295 to 223 K are revealedas quasi-cleavage and intergranular features respectively and as transgranular ones underother experimental conditions.The influence of hydrogen on the mechanical behaviour ofsteel and the mechanism of hydrogen induced deformation and fracture are discussed. Research Assistant,Institute of Metal Research,Academia Sinica,Shenyang 110015,China     

含氢马氏体时效钢低温力学性能行为与断裂机制SCIEI

两种状态的钢的强度和塑性均随温度降低而升高,随氢含量的增加而降低。时效样品虽比固溶样品的氢脆敏感性高,但即使在77K下前者仍有良好的抗氢性能。塑性变形后位错缠结成位错胞,其胞尺寸随温度下降而减小,随氢含量的升高而增大。没有发现低温形变孪晶。295和223K下,氢含量为5.90ppm的固溶和时效态试样拉伸断口分别为准解理和沿晶断裂,其它实验条件下的皆为穿晶断裂。并讨论了氢对材料力学性能行为的影响及断裂机制。     

Fatigue crack growth behavior of laser-annealed IN 718 alloy in hydrogen

The effect of hydrogen embrittlement on the fatigue crack growth of IN 718 plate and laser-annealed specimens in hydrogen containing environment were investigated. Although the differences in tensile strength and impact toughness between solution-annealed (S) and aged (A) IN 718 specimens were significant, the experimental results indicated that both specimens within the low ΔK regime exhibited a similar fatigue behavior. As the ΔK increased above 30 MPa , the solution-annealed specimen revealed a higher fatigue crack growth rate (FCGR) than the aged one. In general, the IN 718 alloy had a low sensitivity to hydrogen-enhanced fatigue crack growth, independent of hydrogen sources. Residual compressive stresses ahead of the crack tip were responsible for the improved resistance to fatigue crack growth in a laser-annealed specimen. For alloys with similar strength, IN 718 alloy trapped a huge amount of hydrogen in the matrix showing a less susceptibility to hydrogen-enhanced fatigue crack growth in comparison with the maraging steel. Additionally, fatigue-fractured appearance near crack initiation sites reveals quasi-cleavage fracture in embrittled specimens.     

Effect of Strain Rate on Stress Corrosion Cracking of 316L Austenitic Stainless Steel in Boiling MgCl2 Environment

Stress corrosion cracking behaviors of AISI 316L austenitic stainless steel at slow strain rates in two environments of air and MgCl₂ at ambient temperature and 154 °C were investigated. The results revealed that a decrease in strain rate, during testing in boiling MgCl₂ environment, led to a rigorous deterioration of the mechanical properties of the material, causing brittleness of the steel. The results obtained from fractography indicated that the samples tested in air had typical ductile fracture surface appearances, while the fracture surfaces of the samples tested in a corrosive environment showed a combination of intergranular and transgranular fracture modes, having a brittle macroscopic appearance. The transgranular mode became predominant as strain rate decreased. The results suggested that the presence of deformation bands in front of crack tips were responsible for transgranular cracking caused by stress corrosion.     

Stress corrosion cracking of gas-tungsten arc welds in continuous-cast AZ31 Mg alloy sheet

AZ31 Mg alloy sheet was welded using a gas-tungsten arc (GTA) process over inserts containing 2.3–9.3 wt.% Al. The welded specimens were susceptible to SCC in distilled water, with susceptibility increasing with decreasing weld metal Al (or β particle) concentration. Primary stress corrosion cracks initiated at the weld metal–HAZ interface by stress-assisted localised dissolution and propagated through the weld and base metals by transgranular and intergranular H-assisted fracture (TG-HAF and IG-HAF) respectively. The IG fracture mode may be intrinsic to the texture imparted upon the base metal by rolling. The increase in SCC susceptibility with decreasing weld metal Al concentration is contrary to the purported roles of β particles in promoting localised corrosion and as crack nucleation sites, but corresponds with increases in weld – base metal galvanic current density and weld metal localised corrosion susceptibility.     

A hydrogen embrittlement mechanism for sensitized types 304, 316 and 310 austenitic stainless steels in boiling saturated magnesium chloride solutions

We have already proposed a mechanism for intergranular hydrogen embrittlement (IG-HE) for solution annealed austenitic stainless steels (types 304, 316 and 310) in HCl solutions and in boiling saturated magnesium chloride solutions. The proposed IG-HE mechanism was based on martensite transformation, hydrogen-enhanced local plasticity (HELP), grain boundary sliding (GBS). Recently, it was reported that the fracture susceptibility and fracture mode for sensitized steels in boiling saturated magnesium chloride solution under an open-circuit condition were significantly different from those observed for solution annealed steels. In the present paper, the hydrogen embrittlement behavior of sensitized types 304, 316 and 310 in boiling saturated magnesium chloride solutions was explained in more details in terms of an inhibiting effect of chloride ions, martensite transformation, Cr depletion, HELP, the degree of corrosiveness through the comparison with those for the solution annealed steels. Furthermore, a transgranular HE (TG-HE) cracking mode that was not observed for the solution annealed steels was discussed as well as IG-HE. Then a TG-HE mechanism for sensitized austenitic stainless steels was proposed, while the IG-HE mechanism for solution annealed austenitic stainless steels which was discussed in details was applied to IG-HE of sensitized austenitic stainless steels. It was also pointed out that the occurrence of both TG-HE and IG-HE was explained with an identical concept.     

激光喷丸强化对电化学充氢316L奥氏体不锈钢振动疲劳性能的影响

研究了激光喷丸强化对电化学充氢316L奥氏体不锈钢振动疲劳性能的影响.测试分析了不同激光功率密度喷丸316L不锈钢充氢试样的残余应力、显微硬度和微观组织结构,并对比研究其振动疲劳寿命和断口形貌.结果 显示,激光喷丸诱导材料表层位错密度增加,并有效细化晶粒,抑制了氢原子的入侵,同时复杂晶界和高密度位错增殖结构阻碍了氢原子...     

The effect of grain size and aging on hydrogen embrittlement of a maraging steel

Notched tensile tests were conducted under a slow displacement rate to evaluate the influences of grain size and aging on hydrogen embrittlement (HE) of T-200 maraging steel. In addition, an electrochemical permeation method was employed to measure the effective diffusivity (D_eff) and apparent solubility (C_app) for hydrogen of various heat-treated specimens. The results indicated that the aged (482 °C/4 h) specimens comprised of numerous precipitates led to a raised C_app and a decreased hydrogen diffusivity as compared to those of the solution-treated ones. The solution-treated specimens were resistant to gaseous HE, whereas aged specimens were susceptible to it, implying the strength level was the controlling factor to affect the HE susceptibility of the specimens. Nevertheless, all specimens suffered from sulfide stress corrosion cracking (SSCC) severely but to different degrees. The aged specimens were more likely to form intergranular (IG) fractures in H₂S but quasi-cleavage (QC) in H₂. For the solution-treated specimens, a fine-grained structure was susceptible to HE in H₂S and revealed mainly QC that differed from the IG fracture of the coarse-grained one. The fracture mode of the specimens could also be related to the transport path and / or the supply of hydrogen to the plastic zone of notched specimens in hydrogen-containing environments.     

Fracture behavior of high strength Mg-Gd-Y-Zr magnesium alloy

The fracture behavior of a permanent mould casting Mg-8.57Gd-3.72Y-0.54Zr(mass fraction,%)(GW94) alloy was investigated under different thermal conditions,including as-cast,solution-treated,peak-aged,and over-aged states.Scanning electron microscopy(SEM) and optical microscopy(OM) were employed to examine the crack nucleation and fracture model.The results indicate that the GW94 alloy shows different behaviors of crack initiation and fracture under different thermal conditions. During tensile test at room temperature,the fracture model of the as-cast GW94 alloy is quasi-cleavage,while that of the solution-treated alloy is transgranular cleavage.It is a mixed pattern of transgranular and intergranular fracture for both the aged conditions.Large cavities formed at grain boundaries are observed in the peak-aged sample tested at 300℃,corresponding to the intergranular fracture.Localized plastic deformation at grain boundaries is also observed and corresponds to the high elongation at 300℃.     

Fatigue crack growth behavior of heat affected zone in P92 steel weldment

In this study, the effect of microstructure on fatigue crack growth behavior of HAZ was investigated using newly developed P92 steel weldment. Fatigue crack growth rates of the fusion line and fusion line+2 mm in HAZ were found to be faster than that of welds or base metal, while fatigue crack growth rate of the fusion line+1 mm was the slowest. The fracture appearance of the fusion line and fusion line+2 mm revealed mainly transgranular, cleavage-like faceted fracture surfaces and the fusion line+1 mm showed quasi-cleavage fractures with scattered dimples, which increased resistance to fatigue crack growth.     

Fatigue Crack Propagation Behavior of as-Extruded AZ31B Mg Alloy Welded Joint

对AZ31B 镁合金焊接接头和热影响区的疲劳裂纹扩展行为进行研究,分析了焊接接头[L-T(W)]和热影响区的紧凑拉伸试验[C(T)],其中热影响区的C(T)试验包括焊缝平行于挤压方向[T-L(H)] 和垂直于挤压方向 [L-T(H)]两种。结果表明:对于L-T(W) 试样,裂纹沿挤压方向扩展,裂纹扩展经历先快后慢的扩展过程;T-L(H) 试样裂纹平行于缺口方向扩展,L-T(H)试样裂纹为平行于缺口方向和与缺口成一定角度两种扩展方向,裂纹扩展经历先慢后快的扩展过程。裂纹尖端扩展为穿晶和沿晶的混合模式,疲劳断口为准解理特征的脆性断口。     

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.     

A comparison of hydrogen embrittlement susceptibility of two austenitic stainless steel welds

Slow displacement rate tensile tests were carried out to assess the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of AISI 316L and 254 SMO stainless steel (SS) plates and welds. 254 SMO generally exhibited a better resistance to hydrogen embrittlement than 316L. The strain-induced transformation of austenite to martensite in the 316L SS was responsible for the high hydrogen embrittlement susceptibility of the alloy and weld. Sensitized 254 SMO (i.e., heat-treated at 1000 °C/40 min) base plate and weld comprised of dense precipitates along grain boundaries. Interfacial separation along solidified boundaries was observed with the tensile fracture of 254 SMO weld, especially the sensitized one. Dense grain boundary precipitates not only reduced the ductility but also raised the susceptibility to sulfide stress corrosion cracking of the sensitized 254 SMO plate and weld.