某核电站凝汽器密封结构用镍铝青铜螺栓断裂原因

某核电站凝汽器密封结构用镍铝青铜螺栓发生断裂现象,采用宏观观察、化学成分分析、扫描电镜分析、力学性能测试、金相检验等方法对螺栓断裂的原因进行分析。结果表明：螺栓在潮湿的NH₃环境下发生了应力腐蚀开裂;断裂螺栓在服役过程中受到不均匀的应力载荷,使表面应力腐蚀倾向变大,且螺栓长期处于自身应力腐蚀敏感介质中,在二者的耦合作用下,应力腐蚀裂纹萌生;裂纹沿残余应力较高的相界区域向内扩展,最终导致螺栓发生断裂。     

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。     

不锈钢U形螺栓的断裂原因

某S30408不锈钢U形螺栓使用约0.5 a(年)后就发生了断裂。通过化学成分分析、硬度测试、拉伸试验、金相检验、非金属夹杂物测试、断口分析及能谱测试等方法对该U形螺栓的断裂原因进行了分析。结果表明:该S30408不锈钢U形螺栓在冷加工硬化时产生了残余应力,后又叠加了工作应力,再加上在潮湿的腐蚀环境下工作,螺栓发生了应力腐蚀开裂,裂纹不断扩展最终导致该U形螺栓断裂。     

螺栓断裂原因与抽检结果分析

采用断口宏观形貌观察、化学成分分析、显微组织检验、力学性能测试及腐蚀产物的X射线衍射分析等手段，对断裂与部分抽检的卡瓦螺栓进行了综合试验分析。结果表明，断裂螺栓为应力腐蚀断裂。而从抽检的螺栓中发现，有的化学成分不符合要求，有的热处理工艺不合理，有的存在表面淬火裂纹等，螺栓质量的不合格率约10％。在质量分析的基础上，提出相应的改进措施。     

风力发电机叶片连接用高强度螺栓断裂原因

XE112-2000型风力发电机叶片连接用高强度螺栓在服役过程中频繁断裂,通过宏观观察、金相检验、力学性能测试及化学成分分析等方法对螺栓的断裂原因进行了分析。结果表明:螺栓断裂形式为腐蚀疲劳断裂,主要原因是螺栓的螺纹成形工艺为先调质后滚压成形,螺纹外侧表面受到挤压,导致螺纹顶部受到相反的拉应力作用,此外,又因为螺栓受到加工硬化的影响,产生较大的残余拉应力,增加了产生微裂纹和表面缺陷的可能,导致在螺纹顶部偏中心处产生了应力腐蚀裂纹。在之后的服役过程中,由于疲劳作用,裂纹处产生应力集中,导致裂纹快速扩展,最终螺纹发生腐蚀疲劳断裂。     

换热器紧固螺栓的应力腐蚀断裂分析

紧固螺栓服役2年后发生成批断裂.采用宏观观察、化学成分分析、金相检验和扫描电镜与能谱分析等方法对失效螺栓进行了分析.结果表明,由于紧固螺栓长期浸在含有H2S介质的油料中,并在所承受的各种应力下又与腐蚀环境相互作用便产生了应力腐蚀,加上螺栓材质选用不当和硬度、显微组织不合理等其他因素,最终导致螺栓断裂.     

储气塔阀门紧固螺栓的断裂分析

储气塔运行半年即发生了阀门紧固螺栓的断裂事故。采用化学成分分析、硬度测定、金相检验以及断口的宏、微分析等方法,对螺栓断裂原因进行了分析。结果表明,由于塔内腐蚀性介质的微量渗出,导致了阀门紧固螺栓的应力腐蚀断裂。     

储气球罐连接阀螺栓断裂分析

储气球罐连接阀螺栓在使用过程中发生异常断裂。通过宏观观察、扫描电镜及能谱分析、金相检验等方法对螺栓断裂的原因进行了分析。结果表明:螺栓在低应力及环境腐蚀性介质作用下发生了应力腐蚀开裂,产生的原因是晶界析出较多高铬碳化物,局部呈网络状,使晶界贫铬,导致了晶界的耐腐蚀性能下降。可以从改善环境条件及对材料进行稳定化处理等途径解决该问题。     

20MnTiB钢螺栓断裂失效分析

应用扫描电子显微镜、光学显微镜、显微硬度仪和电子探针X射线显微分析仪，对发射架20MnTiB高强度螺栓断裂件的金相组织、显微硬度、断口微观形貌和合金元素分布状态进行了分析。结果表明，断裂螺栓金相组织正常，力学性能符合技术要求；螺栓断裂失效是由于在螺栓根部存在因加工不当产生的初始裂纹，在初始裂纹尖端的应力集中和露天使用环境中水介质的共同作用下，螺栓发生应力腐蚀开裂。应力腐蚀开裂的方式是阳极溶解型。     

快卸卡箍螺栓断裂原因分析

通过断口形貌观察、X射线能谱分析、金相检验和硬度检测等试验方法,对某燃油供油导管快卸卡箍螺栓的断裂原因进行了分析,并与螺栓冲击断口和氢致疲劳断口进行了比较分析。结果表明:该快卸卡箍螺栓断口特征与冲击断口和氢致疲劳断口明显不同,其断裂性质为应力腐蚀断裂,裂纹起源于螺栓光杆段的侧表面;螺栓表面加工粗糙且没有防护对裂纹的萌生有一定的影响。对螺栓表面进行防腐处理可有效避免该类故障的再次发生。     

Stress-Corrosion Cracking and Galvanic Corrosion of Internal Bolts from a Multistage Water Injection Pump

Nineteen out of 26 bolts (studs) used for assembly of multistage water pump showed severe corrosion and cracking after brief service in a severe working environment that contained saline water, CO₂, and H₂S. The failed bolts and intact nuts were supposed to be made out of a special type of austenitic stainless steel as per ASTM A 193 B8S and ASTM A 194, respectively. However, the investigation showed that bolts and nuts are made from two different alloys: an austenitic stainless steel and a nickel-base alloy. The difference in the corrosion resistance of these two alloys led to severe galvanic corrosion. The galvanic coupling between bolts and nuts in addition to the severe working environment played major role in the premature failure of bolts. The mechanisms of bolt failure were galvanic corrosion for bolts that were in direct contact with the environment and stress-corrosion cracking in the bolts remote from the severe environment. The stress-corrosion cracking was influenced by a bad fit between the bolts and nuts threads. This resulted in a crevice and the development of an aggressive chemistry between the engaged bolt/nut threads. All factors required to cause stress-corrosion cracking were available, namely, stressed bolts (bolts under tensile stress), temperatures above 60 °C, and chloride ions.     

Sensitization induced stress corrosion failure of AISI 347 stainless steel fractionator furnace tubes

Austenitic stainless steel tubes are used as furnace tubes in petrochemical industries mainly because of their corrosion resistance and mechanical strength. AISI 347 grade stainless steel is used as furnace heater tubes in the fractionator of hydrocracker unit. Even though this stainless steel is stabilized with the addition of niobium thus preventing sensitization related corrosion failures, operational and maintenance errors may result in premature failures if conditions prevail. The present work reports the premature failure of AISI 347 grade fractionator furnace tubes after nearly 8 years of service. The failure occurred after shutdown. Carbonaceous deposits were found on the inner walls of the tube and circumferential cracks were found beneath the deposit. The service exposed 347 SS alloy tube was in the sensitized condition as confirmed by microstructure and double loop electrochemical potentiodynamic reactivation test. The tube material got sensitized possibly by localized overheating at the carbon layer deposited site. During shutdown of hydrocracker unit polythionic acid formation occurred possibly due to errors in shutdown procedures. Sensitized alloy 347 tube undergone polythionic acid induced intergranular stress corrosion cracking (PASCC).     

The failure of an armour faced conveyor connector

The combined use of modern metallurgical techniques for fracture examination, laboratory test data and fracture mechanics calculations allows metallurgical failures to be examined in a quantitative manner. Complex load histories and environments can result in more than one sub-critical cracking mechanism occurring in a component. Quantitative understanding of the rate determining cracking process is a necessary prerequisite to rectifying the problem. The following case study describes a connector from an armour faced conveyor which failed in service. The failure investigation involved fractography, stress analysis, material property evaluation and fracture mechanics calculations. Fractographic evidence indicated a stress corrosion failure mechanism. Calculations of critical crack sizes showed that stress corrosion cracking alone could not account for the fracture. It was concluded that the failure was due to a sequence of three cracking processes which preceded unstable ductile fracture. Firstly, frictional heating caused rubbing or quench cracks typically 0.5–1 mm deep. Secondly, corrosion fatigue cracks grew several millimetres allowing the third fracture process, stress corrosion cracking, (SCC) to initiate and grow. In the situation described here, this process was much faster than corrosion fatigue. The influence of defect size due to rubbing cracks and the influence of K_ISCC have been compared with the corrosion fatigue life of the component. An increase in K_ISCC and hence critical defect size for SCC has been shown to increase the corrosion fatigue life of the component by a large factor. A change in design would also alleviate the problem of SCC by reducing the static stress, which is the driving force for SCC.     

Failure analysis of IEU drill pipe wash out

Many 127.0×9.19 mm IEU G105 drill pipe failures of wash out occurred after 2367 h of pure drilling time and 8726 m of penetration footage. This paper gives a detailed investigation on these failures and a systematic analysis is carried out on service and loading conditions of the drill pipes. Measurement and inspection were performed on configuration dimensions, chemical composition, mechanical performance, metallography, macro-fractography, micro-fractography, and corrosion products. Configuration stresses at the crack positions of the drill pipe were calculated by FEA. Crack extending velocity of the drill pipe material under corrosion medium was also measured. It is thought from test and analysis results that the drill pipe wash out or fracture accidents were premature corrosion fatigue failure accident. The failure courses were as the following: corrosion pits occurred first on the internal surface at the stress concentrating area of the drill pipe, and then fatigue cracks initiated in pit bottoms, and washed out or fractured subsequently as cracks penetrated through the wall thickness of the drill pipe. The reasons of drill pipe wash out were related to configuration, material quality, and load condition of the drill pipe string.     

Failure of rockbolts in underground mines in Australia

Over the last 15 years there has been an increasing incidence of failure in rockbolts used in underground mines in Australia. Failures have also been observed in the United Kingdom where Australian Technology rockbolting is also used. Most of the failures in the United Kingdom were found to be initiated by corrosion pits, but in Australia, the fractures were considered likely to be due to stress corrosion cracking (SCC). This paper reports a metallurgical study of 44 failed rockbolts from four different underground mines in Australia. The study confirmed that failure was generally due to SCC and showed that this was usually initiated by bending of the bolts that occurred due to lateral movement of the rock strata. It also showed that many of the failed bolts had very low toughness with Charpy impact values of 4–7 Joules.     

Corrosion failure analysis of pure nickel thermocouple sheath used in BaTiO3 hydrothermal synthesis reactor

This paper presents corrosion failure analysis on a commercial pure nickel thermocouple sheath used in BaTiO₃ hydrothermal synthesis reactor. Detailed investigations of the corrosion products have been performed using stereomicroscope, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. It was found that the corrosion failure of the sheath was mainly due to general corrosion and crevice corrosion of pure nickel, and the main corrosion products were Ni(OH)₂. Many cracks were observed inside the corrosion products and along the interfaces between the corrosion products and metal matrix. BaTiO₃ particles were deposited as parallel white bands along the above cracks inside the corrosion products. Chloride ions also entered the cracks or crevices, resulting in the acidification inside the crevice and aggravated the attack of nickel matrix. Some recommendations to prevent/minimize future similar corrosion failures were proposed.     

超临界“W”火焰炉氢腐蚀爆管原因分析

针对超临界“W”火焰炉氢腐蚀爆管现象,采用宏观检查和金相检验的方法对水冷壁管爆口进行了检测分析,宏观检查表明爆口附近有多处鼓包、壁厚变薄,金相检验说明了爆口内壁金相组织出现严重的脱碳和沿晶腐蚀裂纹,即高温氢腐蚀。根据水冷壁的材质、超临界水汽特性和水质情况的分析,精处理混床出水氯离子超标是引起此次爆管的一个关键因素。     

Challenges in modelling the evolution of stress corrosion cracks from pits

The evolution of stress corrosion cracks from pits is important in many industrial applications but continues to be a challenge in both measurement and prediction. Life prediction in these circumstances has to account for pit growth kinetics, the conditions for the transition from pits to cracks, and the growth rate of cracks in the short and long crack domain. An example of importance is the performance of steam turbine rotors in power generation. Although stress corrosion failures are comparatively rare, the consequences can be severe and occasionally catastrophic. Consequently, considerable effort is being focused on evaluating the effect of operational variables on pitting and crack growth and in developing an improved basis for structural integrity assessment. A preliminary mathematical model based on deterministic equations with statistically variable input parameters was developed for simulating the evolution of the pit depth distribution at different exposure times, and the transformation to stress corrosion cracks was based on the Kondo criteria. The model predicted some features of the damage well but recent novel measurements of the evolution of stress corrosion cracks from pits combined with finite element analysis of the strain distribution suggest that the transition from a pit to a crack is more complex than can be accounted for by the Kondo approach.