偏心泵螺栓断裂失效分析

针对偏心泵上多个螺栓发生断裂的情况,进行宏观断口形貌观察、扫描电镜微观断口分析和能谱仪断口表面成分分析。同时进行螺栓化学成分、硬度分析,以及利用光学显微镜进行金相分析,讨论得出螺栓断裂的原因。结果表明:Na~+和Cl~-离子引起应力腐蚀造成螺栓退刀槽应力集中处裂纹源的形成,偏心泵的交变载荷导致螺栓发生疲劳裂纹扩展,而最终断裂。     

超高强钢延迟裂纹的研究

采用四点弯曲试验及表面裂纹法试验分别研究了37SiMncrMoV 钢焊接接头的氢致裂纹和应力腐蚀开裂。高湿度能增大氢致裂纹和应力腐蚀开裂倾向。扫描电镜显示了氢致裂纹断口的沿晶断裂特征.用扫描电镜及光学显微镜研究了热影响区的局部熔化区及淬火区显微组织对裂纹倾向的影响。采用改进的焊接工艺,防止了高压容器的低应力破坏。     

23CrNi3Mo钢钎尾断裂失效分析

利用化学分析、金相显微镜、SEM和TEM等手段,从化学成分、组织结构、显微硬度、裂纹形态和断口形貌等方面对失效钎尾进行了分析,对钎尾失效过程中裂纹的萌生与扩展机理以及疲劳失效行为进行了研究。结果表明,钎尾渗碳后外表面碳含量为0.65%,组织为高硬度的孪晶马氏体;心部碳含量为0.23%,组织为韧性较好的下贝氏体。钎尾内外表面均存在应力腐蚀裂纹,裂纹萌生于腐蚀坑底部,扩展方式为穿晶型,腐蚀介质的存在是裂纹萌生与扩展的主要原因,而应力状态、显微组织和夹杂物等对裂纹扩展的影响作用较小。裂纹扩展到一定长度后,在钎尾中心水孔处发生开裂,进而由内向外扩展,最终导致钎尾的断裂失效。     

单晶高温合金损伤与断裂特征研究

研究了单晶高温合金在持久、拉伸和低周疲劳条件下的损伤与断裂特征。结果表明:单晶合金高温持久微观断裂方式为沿原始微孔洞扩展的微孔聚集型断裂,中温持久微观断裂方式为微孔聚集型断裂与滑移剪切断裂共存的混合型断裂;高温拉伸首先在内部以微孔聚集型模式开裂,最后阶段以滑移剪切的方式发生断裂,微孔聚集型断裂过程占主要地位,中温拉伸以纯滑移剪切的方式发生断裂,断口由一个平面组成;低周疲劳断裂由裂纹萌生、裂纹稳定扩展和裂纹失稳扩展3个阶段组成。断口呈现多源开裂特征,疲劳裂纹一般萌生于表面。疲劳裂纹扩展初期断口基本与主应力方向垂直,随着疲劳裂纹扩展,断口表现为与主应力约成45°的平面特征。     

TC11钛合金高温持久异常断裂分析

为分析TC11钛合金高温持久试样异常断裂原因,通过光学显微镜(OM)、扫描电子显微镜(SEM)观察分析高温持久异常断裂试样的断口及表面微观形貌,并用能谱仪进行微区成分分析。异常断裂试样表面发生明显氧化和多处开裂,试样断口边部存在多处深褐色氧化凹坑,断口外圆周凹坑处多发生沿晶断裂,心部为韧性断裂。试样表面裂纹区域含有Cl、Mg、Na等元素,是导致试样异常断裂的直接原因。Mg、Na、Cl等元素是由捆绑热电偶的石棉绳引入,在高温环境下试样表面发生热盐应力点腐蚀,随着高温持久试验应力的持续加载试样发生形变,点腐蚀凹坑处产生裂纹并迅速延伸导致试样异常断裂。采用镍铬丝捆绑热电偶时试样表面未发生热盐应力腐蚀,其对TC11钛合金持久性能的影响很小。     

铀矿用不锈钢泵轴断裂原因分析

目的找到不锈钢泵轴断裂原因。方法通过对断裂的泵轴进行失效分析,利用扫描电镜、金相显微镜、直读光谱仪、显微硬度计等测试方法和手段,对失效泵轴的断口形貌、组织、化学成分、显微硬度等进行分析。结果断口形貌呈明显的脆性疲劳开裂特征,且裂纹源呈现多源特征,有疲劳辉纹和二次裂纹存在。316L泵轴材质成分和组织问题不大,在近表面存在大量夹杂物,同时泵轴表面观察到点蚀和微裂纹存在。结论近表面夹杂物在酸性环境中极易引起点蚀,同时泵轴与联轴器根部结合处存在变截面,形成应力集中。当泵轴受到腐蚀、应力以及电机交变载荷作用时,形成腐蚀疲劳裂纹源,裂纹扩展造成瞬断是此次不锈钢泵轴断裂的主要原因。     

AZ31镁合金摩擦搅拌焊在慢拉伸应力腐蚀过程中的组织演化和氢致延迟开裂

研究AZ31镁合金摩擦搅拌焊组织的演化,包括织构和断口变化。利用中子衍射仪测量织构。利用光学显微镜和扫描电子显微镜观察应力腐蚀开裂（SCC）样品的金相及断口形貌。利用X射线衍射仪研究SCC样品的断口表面结构。结果表明,在母材表面形成了明显的基面织构。然而,搅拌区晶粒发生了基面旋转,大多数晶粒的基面沿着焊接方向倾斜25°。在慢拉伸应力作用下,在空气和侵蚀性溶液中分别形成了羽毛状孪晶和氢化物。在溶液中,穿晶裂纹扩展,最终断裂在回转侧。在断口表面存在的氢化物表明镁合金应力腐蚀可能存在氢致延迟开裂机制。     

60Si2Mn弹簧钢的断裂失效分析

60Si2Mn弹簧钢在正常服役过程中出现断裂,断裂后部分原件损失,根据材料力学原理,并结合断口特征,确定断裂起源位置;采用光学显微镜、扫描电镜、体视显微镜、直读光谱仪等对残留端口的显微组织、宏微观断口形貌和化学成分等进行观察和检测分析,以确定断口起源和断口特征。结果表明:由于生产过程操作不当,弹簧表面存在微裂纹,同时由于喷丸处理的工艺不恰当,造成弹簧表面形成较深的应力线,应力线发展成为沿应力线的开裂和脱落。这些微裂纹、开裂以及脱落,作为弹簧的疲劳断裂源,造成了弹簧的断裂。     

硫化氢环境中17-4PH钢抗氢致开裂与应力腐蚀开裂性能

根据美国NACE标准研究了17-4PH钢在酸性H2S水溶液中的抗氢致开裂(HIC)和应力腐蚀开裂(SCC)的性能,利用光学显微镜及扫描电镜(SEM)观察了裂纹及组织形貌,并结合理论分析了材料的氢致开裂与应力腐蚀开裂行为。结果表明:17-4PH钢在标准NACE试验溶液中会产生氢致裂纹,试样内部微裂纹主要在晶界、夹杂等缺陷处成核并扩展;标准C型环试样在0.8σs的恒应力作用下,浸泡于饱和硫化氢溶液中,720h内3组试样均发生断裂,表明其SCC敏感性较大,试样的宏观裂纹由边缘向内部扩展;扫描电镜结果显示,SCC断口有明显的脆性断裂(解理断口)特征,应力腐蚀开裂是由HIC引起,且裂纹扩展形式多为穿晶型。     

电站锅炉水冷壁管腐蚀疲劳断口分析

对锅炉水冷壁开裂管样断口特征和断裂机理进行分析,结果表明裂纹源区于腐蚀坑,在扩展区可见疲劳弧线、腐蚀产物和腐蚀坑,局部区域可见疲劳条带和二次裂纹;在锅炉启停过程中,准解理型和韧窝型断口形貌特征交替出现;水冷壁管的腐蚀疲劳失效为交变温差应力与腐蚀介质共同作用的结果.     

空气滤网不锈钢丝开裂原因分析

对某电厂开裂失效的不锈钢丝编制的空气滤网的分析表明：其开裂是由晶间腐蚀和应力腐蚀联合作用引起的开裂.主裂纹是应力腐蚀裂纹，不锈钢丝冷加工残余拉应力及环境中Cl离子的存在是不锈钢丝发生应力腐蚀开裂的主要原因.而用材不当，导致发生了晶间腐蚀.晶间腐蚀作为裂纹的起源，诱发并促进了不锈钢丝的应力腐蚀开裂.      

疲劳损伤对核电汽轮机焊接转子接头应力腐蚀开裂敏感性的影响

采用慢应变速率拉伸试验(SSRT)研究了不同程度疲劳损伤对核电汽轮机焊接转子接头应力腐蚀开裂敏感性的影响,利用扫描电子显微镜等观察手段讨论了疲劳损伤对汽轮机焊接接头应力腐蚀开裂敏感性和二次裂纹的作用机理. 结果表明,疲劳损伤增强了核电汽轮机焊接转子接头应力腐蚀开裂敏感性. 此外,疲劳损伤的作用使空气中试样的塑性提高而在腐蚀溶液中塑性降低,也影响了试样内部二次裂纹的产生和扩展.     

Numerical modeling of general cracks from the viewpoint of eddy current simulations

This study discusses numerical modeling of fatigue and stress corrosion cracking in eddy current simulations. Ten fatigue crack specimens and another 10 stress corrosion crack specimens are prepared for this purpose. The specimens are made of type 316 stainless steel and measure 10 mm in thickness for a general evaluation of the model. Eddy current inspections of the specimens are performed using a differential type plus point probe; the specimens then undergo destructive tests to confirm the true profiles of the cracks. Subsequent numerical simulations are conducted to evaluate the equivalent conductivity and width of the cracks. The simulations demonstrate that a fatigue crack can be modeled as a non-conductive region, and it is not necessary to know exactly how wide the opening of a fatigue crack is. They also revealed, in contrast, that stress corrosion cracking needs to be modeled as a conductive region with a certain width.     

Influence of Twinning Texture on the Corrosion Fatigue Behavior of Extruded Magnesium Alloys

Bio-magnesium alloys have received great attention due to their degradability and biocompatibility.Corrosion fatigue failure is a huge challenge in vivo for bio-magnesium alloy implants.Understanding the eff ects of twinning textures on the corrosion fatigue of magnesium alloys is meaningful for the applications.In the current study,pre-compression strains of 2% and 4% were carried out on extruded rods.The effects of twinning texture on the corrosion performance and corrosion fatigue resistance were investigated.The hydrogen evolution tests indicated that twinning texture enhanced the corrosion resistance of longitudinal cross section by improving uniformity of surface energy.The results of corrosion fatigue tests indicated that the differences in mechanical damage caused by twinning texture dominated the corrosion fatigue behavior under high stress amplitude.The secondary cracks of surface deteriorated the corrosion fatigue resistance of the original specimens under low stress amplitude.The compact corrosion film and the re-passivation of matrix suppressed the hydrogen induced cracking,thereby improving the corrosion fatigue resistance of the pre-compression specimens.     

The corrosion fatigue and stress corrosion cracking of α-brass in ammoniacal solutions

A study of the corrosion fatigue and stress corrosion cracking under a constant strain-rate in 70 Cu?30 Zn brass as a function of pH has been made. Correlation between the two has been discussed. Corrosion fatigue was very dependent on pH both with respect to the lifetime and to the nature of cracks as well as those of stress corrosion cracking. A minimum in fatigue life and a large fatigue crack growth rate were observed in neutral tarnishing solutions of pH 6.5–7.0. The fatigue crack propagation rate in neutral tarnishing solution was about twice as great as that in an acidic non-tarnishing solution. Fractographic examination on the fracture surfaces of corrosion fatigue and stress corrosion cracking showed that intergranular cracking predominated both in neutral tarnishing and acidic non-tarnishing solutions, and transgranular cracking predominated in alkaline solution. It is concluded that the tarnish film is not a primary factor controlling the cracking mode in the acidic to neutral pH range and that intergranular cracking may result from the localized dissolution of alloy at grain boundaries.     

The Effects of Hot Corrosion Pits on the Fatigue Resistance of a Disk Superalloy

The effects of hot corrosion pits on low-cycle fatigue life and failure modes of the disk superalloy ME3 were investigated. Low-cycle fatigue specimens were subjected to hot corrosion exposures producing pits, then tested at low and high temperatures. Fatigue lives and failure initiation points were compared to those of specimens without corrosion pits. Several tests were interrupted to estimate the fraction of fatigue life that fatigue cracks initiated at pits. Corrosion pits significantly reduced fatigue life by 60 to 98%. Fatigue cracks initiated at a very small fraction of life for high-temperature tests, but initiated at higher fractions in tests at low temperature. Critical pit sizes required to promote fatigue cracking were estimated based on measurements of pits initiating cracks on fracture surfaces.     

TC2钛合金弹性片裂纹原因分析

针对航空发动机尾喷管弹性片多次出现裂纹故障,利用视频显微镜、扫描电镜等设备,通过对弹性片进行外观检查、断口分析、能谱分析、金相检查以及流场和应力分析等方法,确定弹性片裂纹性质和产生原因。结果表明:故障弹性片裂纹性质为多源疲劳,起源于弹性片薄板间的焊缝位置;飞机垂直尾翼处流场异常,造成气动载荷较大,是弹性片产生裂纹的主要原因;同时,弹性片选用0.5 mm厚的TC2薄板强度储备不足,焊缝位置存在未焊合缺陷,形成较大的应力集中,降低了弹性片的抗疲劳性能,促进了疲劳裂纹的萌生。提出相应的改进建议,避免类似故障的发生。     

Stress corrosion cracking for 316 stainless steel clips in a condensate stabilizer

In one of the gas processing facilities in Abu Dhabi, UAE; a case of 316L stainless steel material failure occurred in the fractionating column due to stress cracking corrosion twice in a cycle of less than 2 years. This paper studies the stress corrosion cracking behavior of the 316L stainless steel in an accelerated corrosion environment and compares it with a higher corrosion resistant nickel alloy (Inconel 625). The experimental work was designed according to ASTM G36 standard, the samples were immersed in a boiling magnesium chloride medium which provided the accelerated corrosion environment and the tested samples were shaped into U‐bend specimens as they underwent both plastic and elastic stresses. The specimens were then tested to determine the time required for cracks to initiate. The results of the experimental work showed that the main mode of failure was stress corrosion cracking initiated by the proven presence of chlorides, hydrogen sulfide, and water at elevated temperatures. Inconel 625 samples placed in the controlled environment showed better corrosion resistance as it took them an average of 56 days to initiate cracks, whereas it took an average of 24 days to initiate cracks in the stainless steel 316L samples. The scanning electron microscopy (SEM) micrographs showed that the cracks in the stainless steel 316L samples were longer, wider, and deeper compared to the cracks of Inconel 625.     

Corrosion failure of marine steel in sea‐mud containing sulfate reducing bacteria

The corrosion failure behavior of marine steel is affected by stress, which exists in offshore structures at sea‐mud region. The sulfate reducing bacteria (SRB) in the sea‐mud made the steel more sensitive to stress corrosion cracking (SCC) and weaken the corrosion fatigue endurance. In this paper, a kind of natural sea‐mud containing SRB was collected. Both SCC tests by slow strain rate technique and corrosion fatigue tests were performed on a kind of selected steel in sea‐mud with and without SRB at corrosion and cathodic potentials. After this, the electrochemical response of static and cyclic stress of the specimen with and without cracks in sea‐mud was analyzed in order to explain the failure mechanism. Hydrogen permeation tests were also performed in the sea‐mud at corrosion and cathodic potentials. It is concluded that the effect of SRB on environment sensitive fracture maybe explained as the consequences of the acceleration of SRB on corrosion rate and hydrogen entry into the metal.