EFFECT OF POTENTIAL ON INTERACTION BETWEEN STRESS CORROSION CRACKING AND CORROSION FATIGUE OF AUSTENITIC STAINLESS STEEL IN BOILING 42% MgCl_2 SOLUTION

The environment-sensitive fracture behaviour of 0Cr18Ni9Ti austenitic stainless steel in boiling 42% MgCl_2 under the specific load of low frequency and high mean stress was inves- tigated from the relations and differences of crack growth rates and fractographs between stress corrosion fatigue and stress corrosion cracking.The interaction between stress corro- sion cracking and corrosion fatigue was also studied from fracture characteristics with empha- sis on the effects of applied potential on the interaction.

Mechanical properties and stress corrosion cracking behaviour of AZ31 magnesium alloy laser weldments

An AZ31 HP magnesium alloy was laser beam welded in autogenous mode with AZ61 filler using Nd-YAG laser system.Microstructural examination revealed that the laser beam weld metals obtained with or without filler material had an average grain size of about 12 μm.The microhardness and the tensile strength of the weldments were similar to those of the parent alloy.However,the stress corrosion cracking （SCC） behaviour of both the weldments assessed by slow strain rate tensile （SSRT） tests in ASTM D1384 solution was found to be slightly inferior to that of the parent alloy.It was observed that the stress corrosion cracks originated in the weld metal and propagated through the weld metal-HAZ regions in the autogenous weldment.On the other hand,in the weldment obtained with AZ61 filler material,the crack initiation and propagation was in the HAZ region.The localized damage of the magnesium hydroxide/oxide film formed on the surface of the specimens due to the exposure to the corrosive environment during the SSRT tests was found to be responsible for the SCC.

Review on Stress Corrosion and Corrosion Fatigue Failure of Centrifugal Compressor Impeller

Corrosion failure,especially stress corrosion cracking and corrosion fatigue,is the main cause of centrifugal compressor impeller failure.And it is concealed and destructive.This paper summarizes the main theories of stress corrosion cracking and corrosion fatigue and its latest developments,and it also points out that existing stress corrosion cracking theories can be reduced to the anodic dissolution（AD）,the hydrogen-induced cracking（HIC）,and the combined AD and HIC mechanisms.The corrosion behavior and the mechanism of corrosion fatigue in the crack propagation stage are similar to stress corrosion cracking.The effects of stress ratio,loading frequency,and corrosive medium on the corrosion fatigue crack propagation rate are analyzed and summarized.The corrosion behavior and the mechanism of stress corrosion cracking and corrosion fatigue in corrosive environments,which contain sulfide,chlorides,and carbonate,are analyzed.The working environments of the centrifugal compressor impeller show the behavior and the mechanism of stress corrosion cracking and corrosion fatigue in different corrosive environments.The current research methods for centrifugal compressor impeller corrosion failure are analyzed.Physical analysis,numerical simulation,and the fluid-structure interaction method play an increasingly important role in the research on impeller deformation and stress distribution caused by the joint action of aerodynamic load and centrifugal load.

EFFECTS OF STRESS RATIO AND FREQUENCY ON CORROSION FATIGUE CRACK GROWTH MECHANISM IN LOW ALLOY STEELS

Based on theoretical analysis about local strain,strain rate and dissolving rate at crack tip, the corrosion fatigue crack growth rate of steels ZG20SiMn and SM50B-Zc in fresh water and 3.5% NaCl solution were measured experimentally,and the PH and electrode potential within crack were also measured continuously along with crack propagating.It showed that the increase of crack growth rate,caused by both decreasing frequency and raising stress ratio,was mainly accelerated by hydrogen embrittlement.

Effects of laser shock peening on fatigue crack growth rate and fracture properties of AA2524 aluminum alloy

In order to prolong the service life of aircraft skin made from AA2524, the effects of laser shock peening(LSP) on fatigue crack growth(FCG) rate and fracture toughness(K_(c)) of AA2524 were investigated. Multiple LSP treatment was performed on compact tension(CT) specimen from single side and double sides. The surface integrity was measured with Vickers hardness tester, X-ray diffractometer and confocal laser scanning microscope, respectively. FCG rate test and fracture toughness test under plane stress were carried out after LSP treatment. The microstructure features of cross-sections were observed with scanning electron microscope. The results showed that the micro-hardness and residual stress of CT specimens were increased dramatically after LSP treatment. Compared to the base metal(BM), the fatigue life was prolonged by 2.4 times and fracture toughness was increased by 22% after multiple LSP.

EFFECTS OF HEAT TREATMENT AND ENVIRONMENT ON CRACK PROPAGATION RATE IN STEEL GC-4 UNDER CORROSION FATIGUE AND STRESS CORROSION

Comparative investigations were carried out of the effect of heat treatment regimes Jor steel GC-4(40CrMnSiMoVA)on its crack propagating rates,from corrosion fatigue, (da/dN)_(CF),or stress corrosion cracking,(da/dt)_(SCC),in media with various constituents and pH values.Both(da/dN)_(CF) and(da/dt)_(SCC) accelerate with the increase of yield stress of the steel,yet the former is far less than the later.In comparison with media,the (da/dt)_(SCC) in distilled water is slightly greater than that in 3.5% NaCl solution,and the (da/dN)_(CF) in distilled water is far less than that in 3.5% NaCl solution.With the pH value increasing in 3.5% NaCl solution,the(da/dN)_(CF) lowers down and the(da/dt)_(SCC) speeds up.An explanation was also proposed with concept of the cyclic hardening and softening at crack tip,as well as the crack closure and occluded cell effect.

Laser Shock Peening of Aluminum Alloy 7050 for Fatigue Life Improvement

The effects of laser shock peening (LSP) on improving fatigue life of aluminum alloy 7050 are investigated.Surface hardness is increased corresponding to a high dislocation density induced by LSP.The X-ray diffraction stress measurement shows that LSP results in prominent increase of surface compressive stress,quasi-symmetrically distributed in the laser peened region.The fatigue life of the alloy 7050 in rivet fastener hole structure is notably improved owing to LSP.The sequence of LSP and fastener hole preparation also influence the fatigue cycle life of the alloy.

Characterization of stress corrosion crack growth of 304 stainless steel by electrochemical noise and scanning Kelvin probe

The fatigue pre-cracking 304 stainless steel （SS） specimens with lengths of 1.002 mm （L-crack） and 0.575 mm （S-crack） were prepared. Their corrosion behavior was studied by electrochemical noise （EN） in 4 mol/L NaC1 ＋ 0.01 mol/L Na2S203 solution under slow-strain-rate-testing （SSRT） conditions. Moreover, the characteristics of L-crack＇s surface morphology and potential distribution with scanning Kelvin probe （SKP） before and after SSRT were also discussed. Compared with S-crack, L-crack is propagated and the features of crack propagation can be obtained. After propagation, the noise amplitudes increase with increasing stress and accelerating corrosion, the white noises at low and high frequencies （WE and WH） of the later stage are one order of magnitude larger than that at early stage in the current power spectral densities （PSDs）. The potential PSDs also increase, but WH disappears. In addition, the crack propagation can be demonstrated according to variation of probability distribution, surface morphology and potential distribution.

Corrosion Testing of a Heat Treated 316 L Functional Part Produced by Selective Laser Melting

Selective Laser Melting (SLM) shows a big potential among metal additive manufacturing (AM) technologies. However, the large thermal gradients and the local melting and solidification processes of SLM result in the presence of a significant amount of residual stresses in the as built parts. These internal stresses will not only affect mechanical properties, but also increase the risk of Stress Corrosion Cracking (SCC). A twister used in an air extraction pump of a condenser to create a swirl in the water, was chosen as a candidate component to be produced by SLM in 316 L stainless steel. Since the main expected damage mechanism of this component in service is corrosion, corrosion tests were carried out on an as-built twister as well as on heat treated components. It was shown that a low temperature heat treatment at 450℃ had only a limited effect on the residual stress reduction and concomitant corrosion properties, while the internal stresses were significantly reduced when a high temperature heat treatment at 950℃ was applied. Furthermore, a specific stress corrosion sensitivity test proved to be a useful tool to evaluate the internal stress distribution in a specific component.

Development and Application of Laser Peening in Japan

This paper reviews the current status of research and development on laser peening without protective coating(LPPC) in Japan. LPPC is an innovative process since it does not require any surface coating that is formed prior to laser irradiation in conventional laser peening for preventing the surface from melting. Surface residual stress of various metal materials was converted from tensile to compressive by LPPC. High cycle fatigue property was remarkably improved for steels, aluminum alloys and titanium alloys. Accelerating stress corrosion cracking (SCC) tests showed that LPPC completely prevents SCC of sensitized austenitic stainless steels, nickel-based alloys and their weld metals. LPPC has been utilized to combat against SCC in Japanese nuclear power reactors since 1999.

激光高速应变对双态组织近α型Ti-6Al-3Nb-2Zr-1Mo合金组织与性能的影响

利用激光冲击强化使Ti-6Al-3Nb-2Zr-1Mo(Ti80)合金表面产生严重塑性变形,借助SEM、TEM和XRD等方法深入研究了表面严重塑性变形对Ti80合金显微组织、力学性能以及耐腐蚀性能的影响。结果表明:激光冲击强化使Ti80合金表面产生无相变的严重塑性变形层,表层晶粒内部产生大量位错胞、位错墙、位错缠结等晶体缺陷。此外,通过位错间的相互作用,表面粗大晶粒细化至纳米级。晶粒细化和晶粒内高密度位错严重阻碍了位错的运动,从而增加了样品的硬度和强度。与原始试样相比,LSP处理试样的表面显微硬度提高了16.5%,产生了386.4 MPa的高幅残余压应力,在不牺牲延展性的前提下提高了强度。电化学和静态浸泡实验结果表明,LSP处理后试样的自腐蚀电位增加了77.7 mV vs.SCE,腐蚀电流密度和腐蚀速率分别降低了1.358μA·cm^(-2)、0.08 mm·year^(-1),点蚀和晶间腐蚀现象减弱。这是由于LSP处理后样品晶粒得到细化,增强了表面结构的稳定性并提供了更多钝化膜的形核位点,从而提高了Ti80合金的耐腐蚀性能。

Grain boundary character and stress corrosion cracking behavior of Co-Cr alloy fabricated by selective laser melting

In this work,we used the selective laser melting(SLM)fabricated Co-Cr alloy with prominent residual strain,extremely non-equilibrium microstructures,and low stacking fault energy as a precursor to fabricate materials with the optimal grain boundary character distribution.The grain boundary engineering(GBE)of the Co-Cr alloy was achieved by a simple heat treatment of the SLM-fabricated Co-Cr alloy.The obtained GBE Co-Cr alloy exhibited 81.47%of special grain boundaries(∑3^(n)n=1,2,3),while it substantially disrupted the connectivity of the random high-angle boundaries,successfully reducing the propensity of intergranular degradation.Slow strain rate tests(SSRTs)showed that the GBE Co-Cr alloy possessed lower stress corrosion cracking(SCC)susceptibility and higher ductility in the corrosive environment(0.9%Na Cl solution)than in the air.The high fraction of special boundaries,coupled with the stress-induced martensitic transformation(SIMT)in the GBE Co-Cr alloy yielded these results,which unique and rarely simultaneously satisfied for common structural materials.The current"SLM induced GBE strategy"offers a novel approach towards customized GBE materials with high SCC resistance and ductility in the corrosive environment,shedding new light on developing high-performance structural materials.

某高架桥高强度螺栓断裂失效分析

高架桥用42CrMoA高强度双头螺栓在服役期间发生螺纹断裂现象,通过外观检查、化学成分分析及扫描电镜分析等手段,对其断裂原因进行了分析。结果表明,螺栓在冶炼过程中成分控制不严,在安装中质量不达标,紧固力不均匀或预紧力不足造成螺栓在螺纹处出现缝隙或间隙,在腐蚀环境下发生缝隙腐蚀。在闭塞腐蚀电池及应力集中作用下加速溶解金属基体,在缝隙腐蚀的阳极端部形成裂纹源。裂纹在腐蚀和疲劳的双重作用下,使裂纹不断向纵深扩展,直至螺栓断裂。安装质量不达标且冶炼工艺控制不严是引发螺栓断裂的关键因素。

激光冲击强化对FV520B钢疲劳寿命的影响

为提高叶轮的使用寿命,对叶片的抗疲劳性能提出了更高的要求,激光冲击强化(LSP)处理是提高材料抗疲劳性能的重要途径。针对FV520B钢棒状试样进行LSP试验和不同应变幅值下的单轴低周疲劳试验,并进行疲劳寿命预测。结果表明,LSP后试样的表面硬度由330 HV提升至490 HV,且LSP后试样表面产生约−90 MPa的残余压应力。相比于未冲击试样,LSP试样的疲劳寿命均有所提高,±0.5%应变幅值下试样的疲劳寿命提高132.2%。SEM结果表明,LSP后试样表面产生的残余压应力抑制了疲劳裂纹的萌生和扩展,裂纹萌生位置由试样表面向次表面转移,且疲劳条纹的间距和韧窝尺寸减小,从而延长了试样的疲劳寿命。采用Manson-Coffin方程针对光滑试样和LSP试样进行疲劳寿命预测,总的来说,对于光滑试样预测结果与试验结果吻合较好;对于LSP试样,预测的疲劳寿命偏保守。考虑残余压应力的影响针对Manson-Coffin方程进行修正,得到了较好的预测结果。研究结果可为FV520B材料LSP处理工艺和疲劳失效研究提供理论依据。

喷丸残余应力对轨道车辆齿轮齿根裂纹应力强度因子的影响分析

高周循环应力工况下齿根裂纹所引发的疲劳断裂是轨道车辆传动齿轮强度失效的主要原因之一。为了分析喷丸残余应力和齿根弯曲应力耦合下对轨道车辆传动齿轮齿根裂纹应力强度因子影响规律与作用机制,针对实际喷丸过程中弹丸位置的随机特性,基于有限元二次开发了20结点奇异元1/4结点位移法,联合建立了FEM-DEM(有限元-离散元)耦合喷丸模型和三维单齿齿根裂纹模型,制定了一套多步骤联合数值模拟方法,并对齿根裂纹应力强度因子在内部残余应力场和外部载荷共同作用下的变化进行了分析。研究结果表明:喷丸强化引入的残余压应力场使得裂纹尖端应力强度因子减小约12%,有效地抑制了疲劳裂纹扩展;喷丸对裂纹扩展的抑制作用在表面与近表面区域最为明显,随着裂纹深度和外加载荷的增大而减弱;残余应力对应的应力强度因子与外载荷无关,仅与初始应力状态有关。

多因素耦合作用下高强钢焊缝连接疲劳性能研究进展

焊接是钢结构广泛采用的连接形式。在提高钢材强度等级后,焊接接头在服役过程中对蚀坑、裂纹等缺陷的敏感度增强,且焊接会劣化钢材成分、组织性能进而导致焊接接头产生初始缺陷。因此,在环境腐蚀、应力集中、超载等因素作用下,钢结构焊缝连接未达到设计服役年限就发生疲劳断裂,造成严重的资源浪费。基于对多因素耦合下高强钢焊缝连接疲劳性能研究现状的总结,介绍了腐蚀疲劳机理及腐蚀预测模型,分析了疲劳设计方法及疲劳寿命评估理论,归纳了焊接残余应力、点蚀坑以及初始裂纹对焊缝连接疲劳性能的影响。结果表明:焊接工艺、焊接参数以及焊接几何尺寸等对焊接接头的力学性能影响显著,焊接残余应力会削弱高强钢焊接接头的疲劳强度,焊接缺陷会引起应力集中现象,加快裂纹萌生速度;高强钢在腐蚀初期形成点蚀坑并逐渐发展成裂纹,进而导致焊缝连接的腐蚀疲劳性能降低。随着高强钢焊缝连接在工程中的应用增多,建议开展不同强度等级及焊接形式的焊缝连接疲劳试验,并将理论成果和经验方法在实际工程中推广应用。

激光增材制造体育器材用TC4钛合金疲劳裂纹扩展行为研究

目的揭示应力比对增材制造TC4钛合金疲劳裂纹扩展行为的影响规律。方法采用紧凑型拉伸试样,在恒载荷幅条件下对激光增材制造TC4钛合金进行了应力比为0.1、0.3和0.5的疲劳裂纹扩展实验,定量评价了不同应力比下合金的疲劳裂纹扩展速率和变化规律。基于Paris公式对裂纹扩展速率进行了拟合,分析了应力比对各参数的影响规律。最后通过扫描电镜对断口表面形貌进行了观察,分析了应力比对断裂模型的影响。结果在相同的∆K条件下,疲劳裂纹扩展速率随着应力比的增大而增大。在Paris公式中,参数C随应力比的增大而减小,参数m随应力比的增大而增大,并且m和lgC呈现线性关系。随应力比的增大,断口表面的河流花样增多、疲劳辉纹变浅、二次裂纹数量增加。结论应力比引起的裂纹尖端闭合效应和平面应力比变化是导致裂纹扩展速率发生改变的主要原因。

喷丸强化对车辆传动齿轮裂纹扩展影响的研究综述

疲劳断裂是重载车辆传动齿轮的主要失效形式之一,齿轮底部疲劳裂纹的扩展将缩短车辆传动系统的服役寿命,严重时会导致车辆发生安全事故。延缓裂纹扩展的主要方法是在传动齿轮的表面引入一定大小的残余压应力。喷丸技术是一种冷加工表面强化处理工艺,该技术利用高速弹丸冲击材料表面,使零件表层产生塑性应变的同时,在表面和内部引入残余压应力,从而使裂纹闭合的能力得到强化,达到延缓裂纹扩展的强化效果。为了更好地揭示喷丸引入的残余压应力对疲劳裂纹扩展的影响,首先综述了传动齿轮表面疲劳裂纹产生的原因以及疲劳裂纹的扩展行为对重载车辆服役的影响。从强度因子、J积分以及裂纹闭合效应出发,介绍了传动齿轮表面疲劳裂纹扩展的理论以及残余压应力与疲劳裂纹扩展速率之间的关系。其次概述了目前国内外常用的新型有益于将残余拉应力转化为残余压应力的微粒子喷丸、激光喷丸、超声喷丸方法,并与传统机械喷丸技术相比较,阐述了新型喷丸表面强化技术的优缺点。此外,从数值模拟和试验结果两方面,论述了喷丸速度、喷丸角度、弹丸直径、弹丸材质和覆盖率5个工艺参数对在传动齿轮表面引入残余压应力的改善影响。最后对喷丸强化技术在传动齿轮上的多目标参数优化以及多尺度残余压应力与疲劳性能进行了展望,并结合重载车辆的使用需求,强调需要创新设计一种效率高、价格低、适用性广的喷丸技术,以进一步推动喷丸强化在延缓疲劳裂纹扩展方面的持续发展。

Nb微合金化对Cr-Co-Ni-Mo系超高强度不锈钢腐蚀疲劳性能的影响

为探讨超高强度不锈钢的应力腐蚀开裂行为,采用Cr-Co-Ni-Mo系超高强度不锈钢为研究对象,利用OM、XRD、TEM等测试手段,结合腐蚀疲劳试验,研究了Nb微合金化对Cr-Co-Ni-Mo系超高强度不锈钢腐蚀疲劳性能的影响。结果表明,试验钢在3.5%NaCl溶液中具有一定的应力腐蚀敏感性,其应力腐蚀开裂机理为氢致开裂和阳极溶解的混合机制。Nb微合金化提高了钢的腐蚀疲劳性能,钢中添加0.11%的Nb后,钢的腐蚀疲劳强度由440 MPa提高至495 MPa,其主要原因是,Nb微合金化可以细化钢的晶粒尺寸,促进钢中不可逆氢陷阱NbC的析出,增加了钢中原奥氏体晶界总量、小角晶界所占比例、Σ3晶界数量、奥氏体体积分数等。

激光冲击强化对Ti-6Al-4V榫齿结构高低周复合疲劳性能的影响

为了探究激光冲击强化对Ti-6Al-4V合金榫齿结构高低周复合疲劳行为的影响,本文对经过激光冲击强化处理的Ti-6Al-4V榫齿结构的表面完整性特征和高低周复合疲劳性能进行测试。结果表明:相比于未经过激光冲击强化处理的原始榫齿结构,激光冲击榫齿结构的高低周复合疲劳寿命提高729%。激光冲击改变了榫齿结构接触区域的微动磨损特征。原始榫齿结构表现为大尺寸的脱层磨损坑,坑底部诱发微裂纹并向基体内扩展,而激光冲击榫齿结构表现为小尺寸脱层薄片。此外,激光冲击还降低榫齿结构的疲劳裂纹扩展速率。激光冲击强化抗高低周复合疲劳作用显著,这主要归因于激光冲击强化层改变疲劳裂纹萌生方式,并抑制裂纹早期扩展。