液氨储存罐304不锈钢法兰连接螺栓断裂失效分析

某火电厂液氨储存罐上方气氨出口气动阀上的304不锈钢法兰连接螺栓在服役过程中发生断裂,采用金相显微镜、扫描电镜、显微硬度计、拉伸试验机等设备,从显微组织、断口、硬度、拉伸性能等方面分析了该304不锈钢螺栓断裂失效的原因。结果表明:螺栓失效模式为应力腐蚀开裂;螺栓材料成分不合格(高碳、低铬),导致合金的耐蚀性能大幅降低,晶间应力腐蚀倾向增加;螺栓服役环境为紧邻海岸的海洋大气,空气中氯离子含量较高,螺栓在服役过程中表面易于发生腐蚀,在预紧力、气氨出口气动阀工作过程中产生的拉应力和氯离子的共同作用下裂纹快速沿晶扩展,直至断裂失效;此外,螺栓内部存在较多铸造缺陷,会显著降低合金的力学性能,在发生腐蚀破坏的情况下,使螺栓出现过早断裂失效。     

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。     

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。     

水轮机制动器螺栓断裂原因分析

某水电站水轮机制动器闸板螺栓发生断裂,通过宏观观察、断口微观分析、化学成分分析、金相检验及硬度测试等方法对螺栓的断裂原因进行了分析。结果表明:螺栓的螺纹表面存在折叠和微裂纹等加工缺陷,应力在该处集中;且生产过程中热处理工艺控制不当,螺栓显微组织不是调质组织,且存在魏氏组织,使螺栓力学性能下降;当受到较大冲击力时,裂纹从加工缺陷处起裂并迅速扩展,导致螺栓断裂。     

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.     

主汽门阀盖螺栓断裂失效分析

某电厂在停炉检修时发现一主汽门阀盖螺栓断裂失效,采用宏观分析、化学成分分析、硬度试验、金相分析、断口以及能谱分析等方法对螺栓断裂原因进行了分析。结果表明:螺栓断裂的主要原因为螺栓显微组织粗大且不均匀,在长期高温和应力作用下,局部位置的晶界发生蠕变产生蠕变孔洞;然后在腐蚀介质的作用下,裂纹从晶界蠕变孔洞处萌生并沿晶界扩展,最终造成一次性沿晶脆性断裂。     

某天然气公交车发动机主轴承盖螺栓断裂原因

某天然气公交车发动机主轴承盖螺栓发生断裂,通过宏观观察、化学成分分析、扫描电镜及能谱分析、硬度试验和金相检验等方法,分析了螺栓断裂的原因.结果表明:螺栓的失效模式为氢脆断裂;在服役过程中,螺栓在较长时间的应力作用下,其头部圆角处产生应力集中,且存在初始裂纹,螺栓、水汽和天然气中的微量氢元素向螺栓初始裂纹附近扩散、聚集,...     

表面渗锌双头螺栓断裂原因分析

部分表面渗锌双头螺栓在安装过程中断裂。通过对断裂螺栓、同批螺栓及调质处理后待加工的同批棒料进行理化检验，发现断裂螺栓所用材料为45Cr钢，而非规定的40Cr钢；材料中硅酸盐夹杂多且呈块状集中分布；在螺栓纵、横面有多处显微缩孔，显微组织出现半网状铁素体和针状铁素体，同时材料强度和硬度值偏高，而塑性和韧性值偏低；断裂螺栓断口及高倍金相检查均发现有明显腐蚀产物及腐蚀特征。分析认为，腐蚀造成组织问结合力降低是螺栓断裂的主要原因，调质处理工艺不当，材料冶金缺陷是造成螺栓断裂的次要原因。提出了相应的改进及预防建议。     

汽轮机低压转子1Cr12Ni3Mo2VN不锈钢叶片的开裂原因分析

对某火力发电厂2号汽轮机组末级开裂叶片进行宏观检查、化学成分分析、金相组织以及断口扫描和元素成分能谱分析。结果表明：该叶片裂纹形成属于腐蚀疲劳失效,蒸汽中存在氯、硫等腐蚀介质引起叶片发生电化学腐蚀,在离心拉应力作用下发生沿晶应力腐蚀开裂,形成裂纹源。在腐蚀介质、拉应力和激振力综合作用下,腐蚀疲劳裂纹加速扩展,并最终发生瞬时断裂。     

10.9级螺栓早期疲劳断裂失效分析

某10.9级螺栓经淬火、回火处理后装配到汽车座椅上,经历颠簸疲劳试验2万次后发生断裂。从化学成分、断口形貌、硬度、显微组织方面,对螺栓断裂的原因进行了分析。结果表明:螺纹牙底过渡部位粗糙不平且有缺口,易造成应力集中产生裂纹;螺纹表层产生脱碳,增加了开裂倾向;在疲劳试验中,由于交变应力的作用形成微裂纹,造成螺栓早期疲劳断裂。     

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。     

Cracking analysis on joint lug of aluminum alloy framework of an airplane

An aluminum alloy joint lug of the framework of an airplane was found to have a crack near the bolt hole after serving for a period. To find out the failure mode and cause, macro and micro observation, microstructure examination, EDS analysis, chemical composition analysis and dimensional check were carried out. The results show that the failure mode of the joint lug is stress corrosion cracking under the co-effect of tensile stress and corrosive environment. The cracking of the surface anodic oxidation film near the bolt hole and the exposure of part of the bolt hole inner surface to air are the main causes for the stress corrosion cracking. In addition, the outer diameter of the steel sleeve inside the bolt hole exceeded the design requirement, which resulted in greater tensile stress near the bolt hole and promoted the stress corrosion cracking. The manufacturing procedure should be adjusted to avoid the cracking of anodic oxidation film, the uncovered part of the inner surface of the bolt hole should be covered, and the outer diameter of the steel inside the bolt hole should be controlled properly to meet the design requirement, so that stress corrosion cracking near the bolt hole can be avoided.     

Failure analysis of stress corrosion cracking in aircraft bolts

This research was conducted on the failure analysis of the failed clamp bolt from a helicopter engine in the RoKAF. Through the fractography, metallography, and stress analysis of the failed part, it was found that the clamp bolt was fractured by stress corrosion cracking due to the interaction of tensile residual stress and corrosive environment. The stress corrosion crack is a phenomenon that occurs in susceptible alloys and is caused by the conjoint action of a surface tensile stress and the presence of a specific corrosive environment. Therefore, it is recommended that the material of the clamp bolt should be changed to prevent similar failures.     

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。     

Analysis of Slide Gate Mounting Bolt Failure in Steel Ladle Due to Fatigue

The mounting bolt failed during slide gate operation of a steel ladle was investigated. Metallurgical analysis confirms the bolt as IS 1367 10.9 grade high-tensile 42CrMo₄ steel. On comparative study with a good un-fractured sample, a striation of fine banded ferrite in pearlite matrix was revealed in failed bolt under optical microscopy. The bolts failed due to fatigue and crack initiated from the surface of machined bolt threads. Comparatively lower hardness, low UTS associated with lower %Cr and %Mo content found to aggravate premature failure of bolts during ladle operation. Microstructure of un-fractured sample found with tempered bainite phase. The fatigue failure of bolts occurred due to repetitive nature of shear force development during steel pouring through slide gate system. Preventive measures to reduce fatigue failure of the mounting bolts are proposed.     

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel in 3.5‐wt% NaCl solution

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel are investigated in simulated seawater. Fractography is performed by using scanning electron microscopy (SEM). The macroscale fracture surface and microstructure of the failed specimen are acquired including the crack initiation, crack propagation, and pitting evolution. The maximum cyclic stress (S) versus number of cycles to failure (N) curves is derived by three‐parameter fatigue curve method. Fatigue life is predominantly controlled by the corrosion pitting‐induced crack initiation when tested in simulated seawater at lower stress levels. As the maximum cyclic stress is less than 185 MPa, the chloride ion erosion is the main influence factor, which affects the fatigue failure of the G20Mn5QT cast steel in simulated seawater.     

Failure analysis of girth weld cracking of mechanically lined pipe used in gasfield gathering system

Girth weld cracking of mechanically lined pipe was occurred after 75 days operation in a gasfield gathering system in China. Failure causes were analyzed based on operation histories, field documents, and laboratory tests. Results showed that the girth weld failure was mainly due to two aspects, girth weld martensite microstructure and external stress. The crack was initiated from sealing pass zone and filling pass zone, which is a hard and brittle martensite structure with hardness of HV 350–450. The failed pipe area had undergone the heavy rain for 2 days, pulling stress, bending stress, and shear stress generated by soil movement resulted in high stress concentration at girth weld. The girth weld cracking failure was initiated from outer carbon steel, and propagated along the weld-fusion line in intergranular mode, which is a typical stress corrosion cracking failure.     

双卡簧片失效分析

采用光学金相、扫描电镜、电子探针成分分析和显微硬度测定等方法分析了双卡簧片非正常断裂的原因。在部分双卡簧片折弯处发现有尖锐凹角并有明显微裂纹,裂纹表面有镍和铜,说明裂纹是在成形加工过程中形成。双卡簧片盛开有时弯角较尖锐并有明显裂纹是导致失效的原因。     

安全阀配对法兰焊口裂纹产生原因分析

某集气装置原料气放空安全阀配对法兰焊口处出现裂纹,通过宏观以及微观检验、化学成分分析、力学性能测试等方法对裂纹产生的原因进行了分析。结果表明：该裂纹为硫化物应力腐蚀开裂所致,焊接工艺不当或焊后冷却速度过快导致焊接接头区域出现硫化物应力腐蚀开裂敏感性组织是该焊口产生裂纹的主要原因。     

Stress corrosion cracking by anodic dissolution and hydrogen permeation in pipeline steels

The phenomenon of transgranular stress corrosion cracking is observed in buried pipelines operating in contact with diluted solutions under disbonded coatings. It was proposed that this kind of cracking is explained by the evolution and permeation of hydrogen at the crack tip, as observed in nuclear pressurevessel steels and other low-alloy steels. We discuss the mechanism of transgranular stress corrosion cracking. The combined effect of hydrogen permeation and anodic dissolution is experimentally investigated by imposing different levels of cathodic protection: from the free corrosion potential [−750 mV (SCE)] to −1300 mV (SCE). The comparison of the experimental and literature data shows that both anodic dissolution and hydrogen permeation are active and their (synergic) combination gives rise to this particular kind of cracking. Anodic dissolution is active in the crack enclave. At the same time, the cathodic discharge of protons produces hydrogen which enters steel at the crack tip. This phenomenon is intensified for more electronegative potentials. In addition, inside the cracks, the environment becomes more acid than in the bulk and the rates of both anodic and cathodic reactions increase. Hydrogen penetrating into steel increases its brittleness, which depends on the microstructure of the metal and the stress-strain field. This effect was corroborated in our experiments by monitoring the variations of the hardness (brittleness) of steel, observing the formation of internal cracks by optical microscopy, and examining fracture surfaces by scanning electron microscopy. Enricerche Spa, San Donato Milanese, Italy. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 4, pp. 17–24, July–August, 1997.