电流互感器铝合金法兰开裂失效分析

某福建沿海地区用于电流互感器的铝合金法兰发生多处开裂,采用宏观检验、化学成分分析、金相分析、扫描电镜以及能谱分析等方法对该法兰开裂原因进行了分析。结果表明:法兰开裂均起始于螺栓孔处,为在腐蚀介质和应力共同作用下发生的应力腐蚀开裂;法兰螺栓扭紧力高、服役环境氯离子含量高以及法兰所用铝合金材料应力腐蚀开裂敏感性高等是造成该法兰开裂的主要原因。最后针对法兰开裂原因提出了预防措施。     

矩形脉动真空灭菌器内腔开裂原因

某医院矩形脉动真空灭菌器内腔发生开裂事故,通过宏观分析、金相检验和光谱分析等方法,结合工作介质,对内腔开裂的原因进行了分析。结果表明:灭菌器内腔与加强筋不连续焊接处存在焊接残余应力,且靠近内腔弯折处存在局部应力集中现象;灭菌器内腔的工作介质中含有氯离子,而氯离子水溶液是300系列不锈钢发生应力腐蚀开裂的敏感介质。灭菌器内腔在焊接残余应力、含氯离子介质等因素的综合作用下发生起始于靠近内腔弯折的焊接起始处的应力腐蚀开裂。     

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。     

汽轮机抽汽疏水管焊缝开裂原因分析

某电厂汽轮机二级抽汽疏水管在运行过程中于焊缝处发生开裂。采用宏观分析、化学成分分析、金相检验、扫描电镜分析等方法对疏水管焊缝开裂原因进行了分析。结果表明:由于该疏水管直管和弯管对接焊后未进行去应力焊后热处理,焊缝处残余应力较高;在焊接残余应力、工作应力及氯离子的共同作用下,疏水管焊缝内壁萌生裂纹,裂纹由管内壁向外壁不断扩展,最终导致疏水管发生应力腐蚀开裂。     

DHB型补偿器波纹管开裂失效分析

用金相显微镜，扫描电镜，俄歇电子能谱对补偿器波纹管开裂失效分析，结果表明：较高的氯离子浓度，残余应力和工作应力的叠加，是造成波纹管应力腐蚀开裂的主要原因。安装补偿器时，预先经冷紧处理，可降低波纹管因膨胀造成的工作应力，延长使用寿命 。     

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。     

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。     

0Cr18Ni9不锈钢换热管应力腐蚀开裂原因分析

某管板式换热器运行不到2个月,换热管进口端即发生开裂。通过宏观分析、化学成分分析、水质分析、金相分析、断口以及能谱分析等方法,对换热管开裂属性及原因进行了分析。结果表明:该换热管开裂为应力腐蚀开裂;循环冷却水中较高含量的氯离子以及换热管进口端处于应力腐蚀敏感温度区导致了应力腐蚀的快速发展。最后,给出了进行焊后热处理和加强循环冷却水处理的建议。     

染缸热交换器焊缝开裂原因分析

某纺织公司染缸热交换器在检修过程中发现焊缝及筒体附近母材出现裂纹。通过宏观分析、化学成分分析、硬度测试、金相检验和氯离子检测等方法对热交换器焊缝开裂的原因进行了分析。结果表明:该热交换器焊缝及筒体附近母材的开裂模式为应力腐蚀开裂。由于蒸汽冷凝水中氯离子质量浓度过高,在焊接残余应力和工作应力的共同作用下,焊缝发生应力腐蚀开裂。     

外压型不锈钢波纹管膨胀节开裂分析

奥氏体不锈钢外压轴向型波纹管膨胀节在使用较短时间后发生了开裂。采用化学成分分析、金相检验和断口分析等方法对开裂的原因进行了分析。结果表明:波纹管膨胀节的工作介质过热蒸汽中含有腐蚀性氯离子,在服役环境下受到拉伸应力的作用,导致其发生了应力腐蚀开裂。     

Investigations on core basket bolts from a VVER 440 power plant

Non-destructive examinations using ultrasonic inspections were performed on core basket bolts at a VVER 440 unit. The four M12 bolts, manufactured from solution annealed Ti-stabilized stainless steel, had been replaced due to NDE indications. Destructive investigations were performed on three of these bolts. One bolt had suffered from intergranular stress corrosion cracking, enhanced by irradiation. Irradiation induced grain boundary segregation was observed in the material. The washer of the bolt with cracking had been unintentionally spot welded to the shielding plate, restricting the free movement of the components to adapt to the differences in thermal expansion during e.g. start-up. The stress situation is modeled using finite element computation, and the location and magnitude of high stresses due to spot welding is clearly shown. The reason for the NDE indications in the other bolts were incomplete filling of the flat slot of the bolt head when the bolt was welded to the washer to prevent unscrewing of the bolt during operation.     

FRP 结构螺栓接点的预紧力与应力松弛

应用螺栓-垫圈应力理论对FRP 结构搭接点预紧力进行了研究。理论计算的预紧力与垫圈的尺寸, 在结构试验初期与试验数据相吻合, 但随时间的增长, 由于基体的松弛效应, 预紧力有明显的减小, 然后在某一数值上又趋于稳定, 但重新按原预紧力紧固螺栓后, 1. 5 年的试验结果表明, 接点的应力松驰得到非常明显的抑制, 取得了预定的效果。      

Experimental results and finite-element predictions of the effect of nut geometry, washer and Teflon tape on the fatigue life of bolts

The effect of nut geometry, curved spring washer and a sealing material (Teflon tape) on the fatigue life of M12 and M16 ISO bolts was investigated. This was accompanied by the study of the axial and bending stress distribution in threads by numerical simulation of bolt and nut connections using the finite‐element method. The experimental results showed that the highest fatigue life is achieved for a slotted tapered nut. The presence of a spring washer also increases the fatigue life, providing that the correct tightening torque is applied to produce a pretension in the bolt. The use of Teflon tape as a filling material between engaging threads of the bolt and nut is shown to significantly increase the fatigue life. On the whole, the highest fatigue life is obtained for a slotted tapered nut using washer. Four fracture mechanisms were observed for bolt–nut connections during the fatigue tests.     

Material influence on the stress corrosion cracking of rock bolts

Rock bolt stress corrosion cracking (SCC) has been investigated using the linearly increasing stress test (LIST). One series of experiments determined the threshold stress of various bolt metallurgies (900 MPa for 1355AXRC, and 800 MPa for MAC and MA840B steels). The high values of threshold stress suggest that SCC begins in rock bolts when they are sheared by moving rock strata. SCC only occurred for environmental conditions which produce hydrogen on the sample surface, leading to hydrogen embrittlement and SCC. Different threshold potentials were determined for a range of metallurgies.Cold work was shown to increase the resistance of the steel to SCC. Rock bolt rib geometry does not have a direct impact on the SCC resistance properties of the bolt, although the process by which the ribs are produced can introduce tensile stresses into the bolt which lower its resistance to SCC.     

Development of an anti-loosening fastener and comparing its performance with different other threaded fasteners

Threaded fasteners are widely used for temporary joining of different components that require occasional disassembling. However, threaded fasteners may have the problem of loosening under vibrating conditions, which may lead to decreasing clamping force and, thereby, a system failure. In the present experimental investigation, anti-loosening ability of various fastening elements, such as conventional nut, nylock nut, flat washer, spring washer, inside and outside serrated washer, is tested with a conventional M16 high-tension steel bolt. A hybrid double nut is used to clamp the plates in which a conventional nut is used to tighten first, and one nylock nut is then placed at the outer side for further tightening. An adhesive-bonded nut is also tested, where an adhesive is placed between bolt and nut threads. All these fasteners are tested in terms of their loosening characteristics. Accelerated vibrating conditions are used for the test on an indigenously made test rig. A split bolt with taper pin is introduced for the first time to reduce loosening of fasteners. The split bolt with taper pin shows considerably good anti-loosening property compared with other threaded fasteners tested in this work.     

Investigation of damage features of a turbo-disk

A turbo-disk made from GH2036 superalloy was damaged in service. Three damage modes took place on the back face of the disk: radial cracking at the rim slot; severe circumferential wear on the disk and a lot of radial short cracks normal to the wear marks; non-symmetry deformation and cracking of the edges of the bolt holes on the disk-hub. All the cracks on the disk propagated in intergranular mode. Overheating on the back face of disk resulting from the intense friction between the air-seal and the back face of disk was responsible for damage of the turbo-disk, which was reflected by the microstructure features. Most evidence points that the turbo-disk failed by a creep damage promoted by overheating.     

叠层膜片联轴器螺栓断裂原因分析

采用化学成分分析、显微组织检验、硬度测定、断口宏观和微观观察及能谱分析等方法对叠层膜片联轴器断裂螺栓进行了分析.结果表明,螺栓根部的退刀槽因加工质量差造成应力集中,加上材料内部有夹杂物,同时在叠层膜片联轴器安装上有问题及机组中其他部件(如风机叶片和齿轮箱)也有损坏等各种因素的存在,该叠层膜片联轴器螺栓组使用于频繁起动的大载荷工作状态,一旦有超过该螺栓所能承受的工作应力就会引起疲劳而发生断裂,最终导致叠层膜片联轴器损坏.给出了正确设计、制造、安装和使用叠层膜片联轴器的建议.     

Bearing failure of composite bolted joints with non-uniform bolt-to-washer clearance

This paper investigates the effect of the relative positions of the bolt and the washer on bearing failure behavior of bolted composite joints with various lateral constraints. Two extreme diametral fit positions, with a positive or negative bolt hole-to-washer clearance, were considered. Experimental results show that the relative location can affect the initial failure loads but not the ultimate failure loads. The measured failure loads were also used to validate an existing analytical model.     

Load disc from Hytorc helps to achieve more consistent bolting

A washer that can make flange assembly much more convenient by preventing the bolt from turning with the nut has been introduced by Hytorc Corp. This disc also converts torque directly to axial bolt stretch, makes the nut friction and thus the bolt load more predictable, and improves speed, safety and tool weight.This is a short news story only. Visit www.worldpumps.com for the latest pump industry news     

Characterisation of Contact Pressure Distribution in Bolted Joints

Abstract: The quantification of contact area and pressure distribution in a bolted joint is essential information, as it determines the integrity of the coupling. Current bolted joint design standards are based on analytical solutions of the pressure distribution, which, because of the inherent assumptions, frequently do not accurately represent the real conditions in a joint. This study uses a non‐intrusive ultrasonic technique to quantify the contact pressure distribution in a bolted connection. The advantage of this experimental technique is that the effect of actual contact conditions can be determined. An ultrasonic wave is focused onto the clamped interface, and the reflected sound signal recorded. In areas where the contact pressure is high, most of the ultrasound is transmitted, and the reflected sound signal is weak. Whereas, when the contact pressure is low, the vast majority of the ultrasound is reflected back. A parallel experimental calibration is then used to find the relationship between the reflected sound signal and contact pressure. In this way, the pressure distribution in a clamped interface is determined for a series of different bolt torques. Two different interfaces were investigated: the first consisted of two ground surfaces clamped together, and the second a turned profile pressed against a ground surface. The effect of a washer underneath the bolt head was also considered. The turned profile was found to cause the contact to spread; there was also a certain degree of fragmentation leading to higher peak pressures than in the ground interface case. With a washer positioned under the bolt head for the turned case, the clamping performance of the bolt was improved. Good agreement was found when comparing the ultrasonic measurements with previous studies, with respect to the spread of the contact pressure distribution. However, in this study, the peak contact pressure was found to occur away from the edge of the bolt hole, and to be influenced by the edge of the bolt head.