压水堆控制棒驱动机构CANOPY密封焊缝服役环境与失效

综述了国际上对PWR核电站控制棒驱动机构密封焊缝(CSWs)结构、载荷、应力状态、失效分析的研究。研究认为CSWs的结构设计易造成高溶解氧服役环境,在CSWs死水区空腔存在微量氯离子污染的可能,导致较大应力腐蚀倾向。目前研究确认的失效模式有穿晶应力腐蚀(TGSCC)、沿晶应力腐蚀(IGSCC)、SCC+点蚀。计算出CSWs死水区空腔理论浓度可达230×10~(-6),分析认为溶解氧和低浓度水平的氯污染是可引起上述腐蚀的环境因素;CSWs应力腐蚀裂纹扩展模式与材料敏化、服役溶液环境、初始应变、应力状态等因素相关;点蚀可能成为应力腐蚀的起源也可发展成为独立的破坏形式。     

316不锈钢应力腐蚀断裂扫描电镜研究

应用扫描电镜和Ｘ射线能量色散谱仪对３１６不锈钢应力腐蚀断口形貌、腐蚀产物及裂纹扩展的晶体学特征进行研究。结果表明,断裂为穿晶断裂,断口形貌为台阶条纹和河流花样,并有腐蚀产物和腐蚀坑等。通过对腐蚀坑形貌的研究,提出了腐蚀坑形态与晶面之间的关系,证明了３１６不锈钢在氯离子环境下的应力腐蚀开裂主要沿｛１００｝、｛１１１｝、｛１１０｝晶面扩展的机制。     

Stress corrosion cracking of stainless steel bellows of satellite launch vehicle propellant tank assembly

Bellows made of austenitic stainless steel (AISI 304 grade) are being used as a conduit for liquid fuel and oxidizer in the propellant tank of a satellite launch vehicle. A few bellows were found leaky during re-pressure tests after 6 years of storage. A number of cracks were found originating from weld fusion lines. One of the leaking bellows was subjected to detailed metallurgical and chemical analysis. The synergistic effect of chloride ions and thermal stresses from welding was identified as the cause-a typical example of stress corrosion cracking (SCC).     

Crack initiation and propagation in torsional fatigue of circumferentially notched steel bars

Circumferentially notched bars of austenitic stainless steel, SUS316L, and carbon steel, SGV410, with three different notch-tip radii were fatigued under cyclic torsion without and with static tension. The torsional fatigue life of SUS316L was found to increase with increasing stress concentration under the same nominal shear stress amplitude. Electrical potential monitoring revealed that the crack initiation life decreased with increasing stress concentration, while the crack propagation life increased. This anomalous notch-strengthening effect was ascribed to the larger retardation of fatigue crack propagation by sliding contacts of fracture surfaces. The superposition of static tension on cyclic torsion causes notch weakening. The notch-strengthening effect in torsional fatigue was not found in carbon steels, SGV410. The difference in the crack path of small cracks near notch root between stainless steel and carbon steel gives rise to the difference in the notch effect in torsional fatigue. The factory-roof shape observed on fracture surfaces of SUS316L became finer with higher stress amplitude and for sharper notches. The superposition of static tension makes the factory-roof shape less evident. Under higher stresses, the fracture surface was smeared to be flat. The fracture surfaces of SGV410 became smoother with increasing stress amplitude and notch acuity. The three-dimensional feature of fracture surfaces clearly showed the difference of the topography of fracture surfaces. The topographic feature was closely related to the amount of retardation of crack propagation due to the sliding contact of fracture surfaces.     

Investigation of plastic zones near SCC tips in a pipeline after hydrostatic testing

Stress corrosion cracking (SCC) is an important failure mechanism for oil and gas pipelines. In the past, hydrostatic testing has been frequently used to assess and mitigate stress corrosion cracking. It is commonly agreed that an effective hydrostatic test not only eliminates critical crack-like flaws, but also blunts the sub-critical crack tip thereby suppressing further SCC propagation. However, little study has been done on the plastic deformation that results from the high stress intensity at the crack tip due to hydrostatic testing pressure and its possible role in subsequent SCC propagation. In this study, microstructural details were examined of an API 5L X52 SCC-containing pipe removed from field service. Plastic deformation generated by the hydrostatic testing pressure was revealed by using high-resolution imaging of a focused ion beam (FIB) microscope. The existence of the microscopic plastic zones around some crack tips suggests that caution should be taken when setting up pipeline hydrostatic tests.     

Crack growth rates of Inconel 600 in aqueous solutions at elevated temperature

The effect of sulfate ions on the crack growth rates (CGRs) of notched specimens (CT) of Inconel 600 under constant load has been investigated in chloride containing aqueous solutions at 250 °C. The intergranular stress corrosion crack growth rates increased in chloride solutions while its hinder with increasing sulfate concentrations. Stress intensity factor (k) for stress corrosion cracking decreased with increasing of aggressivity of chloride ions while increased in the presence of sulfate solutions. At very low concentration of chloride (0.001 m), k-value retardation was observed. It is clear from the results that hydrogen in the aqueous solutions has a deleterious effect on crack propagation. The difference in crack growth rates in chloride ions and in chloride containing sulfate solution at high temperature can be recognized as caused by the difference in local environment conditions at a crack tip. The results indicate that the crack don't propagate under this conditions in the presence of sulfate ions. It is mainly due to a hinderance of chloride ions adsorption on active sites of the fracture surfaces and the formation of chromium oxide layer which is stable at higher temperature.     

铈离子对高强铝合金应力腐蚀开裂的缓蚀作用

基于慢应变速率拉伸实验(SSRT),采用恒电流极化、电化学噪声(ECN)与电化学阻抗(EIS)等方法,研究7A04铝合金在3.5%(质量分数)NaCl水溶液中的应力腐蚀开裂(SCC)行为以及Ce~(3+)对其SCC的缓蚀作用,探讨Ce~(3+)对裂纹孕育与发展过程的抑制机理。结果表明:无论是阳极还是阴极极化,均会促进7A04的SCC倾向,前者增加了裂尖的阳极溶解,后者则加速了裂尖的氢脆效应。Ce~(3+)的加入能延缓7A04的SCC断裂时间,但其有效性仅限于裂纹的萌生阶段。由于Ce~(3+)能够抑制铝合金表面的亚稳态点蚀发育和长大,因而使裂纹的孕育时间显著延长,降低了SCC的敏感性。不过一旦裂纹进入扩展阶段或者试样表面有预裂纹,则由于Ce~(3+)很难迁移到裂纹尖端或在裂尖区难以成膜,不能对裂纹的生长起到有效抑制作用,因而无法降低7A04的SCC发展速率。SEM分析表明7A04铝合金光滑试样SCC主要源于亚稳态或稳态点蚀的诱导作用。     

Root cause analysis of stress corrosion at tube-to-tubesheet joints of a waste heat boiler

Leakage at the tube-to-tubesheet joints occurred in a waste heat boiler. The mode and the root cause of the failure were investigated by chemical composition analysis of the tube material, metallographic structure and crack observation, and corrosion product analysis of the damaged tubes, as well as the operation condition examination of the waste heat boiler. Results revealed that failure of the tubes occurred due to the stress corrosion cracking (SCC), which was caused by tensile stress and chloride-buildup in the narrow and long gap between the tube and tubesheet hole. The gap formation was further analyzed by comparison of the minimum expansion pressure from the common formula provided by the manufacturer, with that from finite element method computations. It is found that the minimum expansion pressure used in manufacture is small and cannot eliminate the initial gap. Meanwhile, the enrichment of chloride in the gap was briefly discussed.     

Process and mechanical properties of carbon/carbon–silicon carbide composite reinforced with carbon nanotubes grown in situ

A hierarchical C_f/C–SiC composite was fabricated via in situ growth of carbon nanotubes (CNTs) on fiber cloths following polymer impregnation and pyrolysis process. The effects of CNTs grown in situ on mechanical properties of the composite, such as flexural strength, fracture toughness, crack propagation behavior and interfacial bonding strength, were evaluated. Fiber push-out test showed that the interfacial bonding strength between fiber and matrix was enhanced by CNTs grown in situ. The propagation of cracks into and in fiber bundles was impeded, which results in decreased crack density and a “pull-out of fiber bundle” failure mode. The flexural strength was increased while the fracture toughness was not improved significantly due to the decreased crack density and few interfacial debonding between fiber and matrix, although the local toughness can be improved by the pull-out of CNTs.     

奥氏体不锈钢波纹膨胀节失效分析

采用化学成分分析、硬度测试和金相检验等方法对某电厂多次发生泄漏的膨胀节进行了检测,对其形成原因进行分析。结果表明：膨胀节在服役环境下,其内侧蒸汽中携带的氯化钠,在膨胀节的波峰内侧部位发生富集和浓缩,并在工作条件下,腐蚀裂纹从膨胀节内侧启裂,以沿晶状特征逐渐向外壁延伸穿透,最终形成穿透裂纹和点蚀坑,使波峰处材质发生应力腐蚀开裂而导致泄漏。     

Stress corrosion cracking in an Al-bronze chlorine gas regulating valve

An analysis of the failure of a dry chlorine gas regulating valve made of Al-bronze is presented. The valve had a stainless steel spindle within its chamber. Cracks had initiated in the inner chamber walls and proceeded outwards resulting in leakage. The corrosion products on the crack surface were found to be a chloride–hydroxide complex of copper. This suggested the ingress of moisture along with the gas. The corrosion mechanism was identified as complex galvanic coupling between the dissimilar alloys which also resulted in depassivation of the spindle surface. The stresses for crack propagation had come from the contact stresses as well as gas pressure on engagement of the spindle with the valve body.     

敏化处理对Z3CN20-09M不锈钢高温水应力腐蚀的影响

为了研究敏化处理对Z3CN20-09M不锈钢高温水应力腐蚀行为的影响,使用敏化处理的Z3CN20-09M不锈钢制成U弯试样,并置于250、290及320℃的高温水中进行应力腐蚀开裂实验,采用扫描电镜观察了高温水实验后试样的氧化膜厚度以及应力腐蚀裂纹的萌生及扩展行为.结果表明:敏化处理增加了氧化膜的厚度,降低了耐蚀能力,使SCC敏感性增大;温度较高时,敏化处理的影响较大;铁素体相容易被侵蚀,大多数点蚀坑产生于铁素体中;SCC裂纹优先在点蚀坑底部和奥氏体/铁素体相界位置处形成;相界面对SCC裂纹的影响取决于SCC裂纹相对于相界面的取向,SCC裂纹扩展方向平行于相界面时裂纹易沿着相界扩展,SCC裂纹扩展垂直于相界面方向时相界面对裂纹扩展起阻碍作用.     

Failure analysis of 316L stainless steel tubing of the high-pressure still condenser

Sherritt International Corporation experienced corrosion failures with the 316L stainless steel tubing in a high-pressure still condenser employed for ammonia recovery. A detailed failure analysis was conducted on the condenser tubing to determine the mode and the root cause of the failure. The analysis included both optical and scanning electron microscopy (SEM) of the inner and outer surfaces of the tube as well as characterization of the corrosion products using energy-dispersive X-ray spectroscopy (EDX). Results revealed that the corrosion attack was confined to the first ～100 mm of the tubing at the inlet where the tube was connected to the top tubesheet. The tube suffered both external stress-corrosion cracking (SCC) and crevice corrosion from the shell side (water side), and wall thinning of the inner surface (the tube side) due to erosion corrosion. It was evident that failure of one of the tubes occurred due to SCC that penetrated the whole wall thickness and resulted in a leak failure. Some prevention measures are proposed to avoid this type of corrosion attack in the future.     

Stress corrosion cracking and hydrogen embrittlement cracking of welded weathering steel and carbon steel in a simulated acid rain environment

Abstract

The stress corrosion cracking (SCC) and hydrogen embrittlement cracking (HEC) characteristics of welded weathering steel and carbon steel were investigated in aerated acid chloride solution. The electrochemical properties of welded steels were investigated by polarisation and galvanic corrosion tests. Neither weathering steel nor carbon steel showed passive behaviour in this acid chloride solution. The results indicated that weathering steel had better corrosion resistance than carbon steel. Galvanic corrosion between the weldment and the base metal was not observed in the case of weathering steel because the base metal was anodic to the weldment. However, the carbon steel was susceptible to galvanic corrosion because the weldment acts as an anode. Slow strain rate tests (SSRT) were conducted at a constant strain rate of 7.87 × 10⁷ s^-1 at corrosion potential, and at potentiostatically controlled anodic and cathodic potentials, to investigate the SCC and HEC properties in acid chloride solution. The welded weathering steel and carbon steel were susceptible to both anodic dissolution SCC and hydrogen evolution HEC. However, weathering steel showed less susceptibility of SCC and HEC than carbon steel at anodic potential because of Cr and Cu compounds in the rust layer, which retarded anodic dissolution, and at cathodic potential due to the presence of Cr, Cu, and Ni in alloy elements, which inhibit the reduction of hydrogen ions. SEM fractographs of both steels revealed a quasicleavage fracture in the embrittled region at applied anodic and cathodic potentials.     

循环泵法兰螺栓断裂分析

通过金相检验、断口分析和化学成分分析等方法对循环泵法兰螺栓的断裂原因进行了分析。结果表明：螺栓早期断裂失效的主要原因是材料使用有误,加上受海水中的氯离子腐蚀引起奥氏体不锈钢螺栓产生应力腐蚀裂纹并扩展所致。     

LIFE PREDICTION BY SIMULATION OF CRACK GROWTH IN NOTCHED COMPONENTS WITH DIFFERENT MICROSTRUCTURES AND UNDER MULTIAXIAL FATIGUE

A modelling procedure was developed which is applicable to crack growth in notched components subjected to multiaxial fatigue for materials with different microstructures. An algorithm for crack growth, in a microstructure that was modelled as hexagons, was established as a competition between growth by crack linkages during the crack initiation and propagation stages and the propagation of a dominant crack as a single crack. Analytical results simulated by using the developed model were compared with experimental results from fatigue tests which had been conducted using notched specimens of pure copper, carbon steel and two kinds of titanium alloy. Cracking morphology, which was experimentally observed to depend on the microstructure and the loading mode, was well simulated using the present model. The fatigue failure life of a notched specimen was statistically estimated by a Monte Carlo procedure based on the model. The simulated life with a statistical scatter-band almost coincided with the experimental data.     

SURFACE AND IN-DEPTH CRACK PROPAGATION IN HIGH TEMPERATURE LOW-CYCLE FATIGUE OF SUS 316 STAINLESS STEEL

Abstract— The low-cycle fatigue crack propagation behaviour of surface cracks in SUS316 stainless steel at 700°C, in both the surface direction and the in-depth direction, has been studied with special emphasis on the role of oxidation. The coalescence behaviour of surface cracks is essential for the process of crack propagation in high temperature low cycle fatigue, irrespective of the existence of oxidation effects. For sub-surface cracks the process of crack propagation is divided into two stages characterized by differences in fracture mode. In both stages, the in-depth crack propagation rate in air is higher than that in vacuum. This difference in crack propagation rate is the main reason for the decrease of fatigue life in air compared with that in vacuum. The crack propagation behaviour in the in-depth direction can be estimated from the conversion of the surface crack length into the subsurface depth by the use of an aspect ratio.     

Mixed-mode quasi-static failure criteria for adhesively-bonded pultruded GFRP joints

The strain energy release rates of adhesively-bonded pultruded GFRP joints were determined experimentally. The crack propagated in the adherend along paths outside the symmetry plane accompanied by fiber bridging. A new method, designated the “extended global method”, was introduced to facilitate mode partitioning in the mixed-mode experiments. Non-linear finite element models were developed in order to quantify the effect of the observed fiber bridging on crack propagation. An exponential traction-separation cohesive law was used to model the fiber bridging zone and calculate the energy release rate due to the fiber bridging, while the virtual crack closure technique was used for calculation of the fracture components at the crack tip. Experimental, analytical and numerical analyses were used to establish quasi-static mixed-mode failure criteria for crack initiation and propagation. The derived mixed-mode failure criteria can be used for simulating progressive crack propagation in other joint configurations comprising the same adhesive and adherends.     

碳纤维-玻璃纤维混杂增强环氧树脂三维编织复合材料薄壁圆管压溃吸能特性与损伤机制

基于三维编织成型及真空辅助树脂传递成型技术,制备了编织纱和轴纱不同混杂方式(编织纱/轴纱:碳纤维-碳纤维(CF-CF)、碳纤维-玻璃纤维(CF-GF);玻璃纤维-碳纤维(GF-CF))增强环氧树脂(EP)的三类三维编织复合材料薄壁圆管,通过准静态轴向压溃及详细的破坏断面观察,研究了纤维混杂方式对薄壁圆管的能量吸收性能和破坏模式的影响。研究发现:CF-CF/EP样品的比能量吸收值分别比GF-CF/EP大36%,比CF-GF/EP大12%。编织纱为碳纤维时(CF-CF/EP及CF-GF/EP),圆管的破坏模式均为折叠破坏模式,编织纱采用碳纤维能有效地遏制中央裂纹的轴向扩展,折叠变形的三维结构内部发生了较多细小的微观破坏。而编织纱为玻璃纤维的GF-CF/EP,破坏模式则为开花内外弯曲式,中央裂纹产生,三维结构呈现分层并向圆管内外弯曲。      

Characterization of failure modes for different welding processes of AISI/SAE 304 stainless steels

Weld joints manufactured with a welding electrode type 308L and by three different arc welding processes shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and flux cored arc welding (FCAW) in a AISI/SAE 304 were studied in order to compare the failure mechanisms associated with their mechanical and microstructural properties. Chemical compositions were analyzed by optical emission spectroscopy and the ferrite numbers (FN) of the welds were also identified. Relevant microstructural characteristics of the different processes were analyzed by microscopy techniques. Finally, fatigue tests were performed to study the variations in the mechanical properties of each process and to analyze their most probable failure modes by means of a fractographic study, in which the characteristic morphologies of each one (nucleation, propagation, final fracture) were identified by means of optical stereoscopy and scanning electron microscopy (SEM). Three different fracture modes were found at the welding joints that showed correlations with microstructural changes produced during the welding process. The first failure mode displayed that the nucleation of the crack was at the weld root. The second failure mode was generated at the heat affected zone (HAZ), where the crack nucleated due to a variation in the grain size produced by the process and then further propagated through the edge of the weld. The third failure mode appeared due to the presence of exogenous inclusions generated by the welding process, which acted as stress concentrators in the weld and produce the initiation and further propagation of the crack. Lastly, some welding processes presented a combination of the previous failure modes and consequently multiple sites of crack nucleation.