高强管线钢冷裂敏感性的斜Y坡口实验研究

采用七种高纤维素型立向下电焊条,通过斜Y坡口裂纹实验,对高强管线钢的冷裂敏感性进行实验研究.通过理论公式计算了其冷裂敏感系数,并根据实验结果确定了避免冷裂产生的最低预热温度,为高强管线钢施工及制定焊接工艺提供了实验和理论依据,也为高强管线钢的冷裂敏感性的标准实验方法提供参考.     

Hydrogen-Assisted Cracking in Boiler Steam Drums

A statutory inspection of a boiler steam drum that had seen 10 years of service revealed a few scattered cracks on the inner surface of the drum. The drum was used in the refinery and petrochemical industry. Remnant life assessment, including inspection of other drums, showed presence of several cracks in two out of seven boiler inspected. In situ microstructural analysis revealed a variation in the microstructure in the cracked regions compared to the microstructure in uncracked regions. Additionally, a wide variation in hardness was associated with the microstructural variation. Detailed study on a sample extracted from a cracked region demonstrated that the cracking occurred in a crescent zone similar in appearance to that of a heat affected zone (HAZ) generally associated with a spot weld or other sources of hot spots in the material. Subsequent examination of more samples confirmed that repair welding was carried out at several places on the inner surface of the drums before installation and commissioning and all the cracks were around the repair welds. The failure occurred by crack initiation in the HAZ of the repair welds and the cracks then propagated progressively across the inner surface of the drums. The failure mechanism was identified to be cold cracking and the failure analysis showed that all the factors required for cold cracking, namely, vulnerable microstructure, residual stresses and hydrogen atmosphere, during welding had been present in the drum material.     

Welding failure of as-fabricated component of aluminum alloy 5052

This paper describes the failure analysis of the “tray section” made up of aluminum alloy 5052 which is used as a specimen holder in a research reactor. Fracture was observed in the central rod of alloy 5052 before it was taken for service. The fracture had occurred in a brittle mode without any gross plastic deformation at a location where the rod was welded to the stopper plate. Detailed microstructural examination was done using both optical and scanning electron microscopy. The weld fusion zone showed presence of high porosity and eutectic phases mainly along the inter-dendritic regions. These low melting temperature eutectics were rich in Si and Fe and led to weld cracking along the dendritic grains during solidification of the welds. Solidification cracking of alloy 5052 was related to pure aluminum filler wire used for welding that shifted the composition of the welds towards peak cracking sensitivity of 1.5 wt% Mg. The failure of the tray section was concluded to be due to welding defects, e.g. high porosity and solidification cracks. Recommendations to avoid this type of failure are also proposed.     

Air conditioner failure investigation-intergranular cracking in a pure copper condenser tube

A failure of a 7 1/2-year-old commercial rooftop air conditioning system was investigated. The systematic failure analysis procedure and eventual discovery of a leak in a pure copper condenser coil are discussed. Severe denting of several tubes, including the tube that leaked, was discovered in the area of the steel tube sheet. A metallurgical examination, including scanning electron microscopy, revealed intergranular cracking, a process that is common among some copper alloys suffering from stress-corrosion cracking but is rarely observed in pure copper. Research of available literature is presented, and the conditions under which pure copper may exhibit intergranular cracking are discussed. Also presented are vibration measurements of the air conditioner taken during operation, which rule out operating loads as the cause of the tube denting and eventual cracking. The cause of the denting remains unknown. However, the denting probably occurred during manufacturing, and the high stresses associated with the dents, in the presence of corrosion, resulted in intergranular cracking and eventual leaking.     

HMn58-2铜制冷却器开裂失效分析

某HMn58-2铜制冷却器在库房存放4a后开裂,采用光学显微镜、力学性能试验机和布氏硬度计对开裂冷却器进行了检验和分析。结果表明:由于焊接工艺或者焊接操作不当造成的热影响区晶粒粗大和α-Cu相自β相晶界大量析出形成魏氏组织形态,降低了材料的韧性、塑性和强度;此外,冷却器铜板冷卷后没有及时进行退火处理,使得材料内部存在较大的内应力;两方面共同作用导致冷却器在存放过程中于焊缝处萌生裂纹并不断扩展最终造成开裂失效。     

Experimental investigation of the hot cracking mechanism in welds on the microscopic scale

The results of the accurate experimental observations on binary Al-Si alloys are presented, which clearly demonstrate that the solidification cracking is a result of the accumulation of macroscopic tensile displacement in aβmicroscopic intergranular liquid film of segregates at the final stage of the weld metal solidification. The reconstructed mechanism of crack initiation provides a clear phenomenological interrelation between the cracking susceptibility, parameters of the welding process and properties of the base and filler material. The correspondent numerical model takes into account the effects of displacement accumulation as well as the influence of thermo-dynamical and thermo-mechanical properties of the welded material. It is successfully applied for development of technological means for elimination of the solidification cracking during welding of aluminium alloys AA6056, such as a multi-beam welding.     

Process analysis and optimization for failure energy of spot welded titanium alloy

Titanium and its alloys have been applied in many industrial fields because of their high specific strength, good corrosion resistance and high thermal stability. Whereas, there is limited valuable references for recommendations of welding parameter selection and specific standards of the small scale resistance spot welding (SSRSW) of titanium alloy though it has been applied in many industrial production fields. Seventeen tests were designed according to the three-level three-factor Box–Behnken experimental design and the mathematical model correlating the process parameters and the failure energy was established on the basis of response surface methodology (RSM) technique. And then this model was used to analyze the effects/interactions of the welding parameters on the failure energy. The verification test results which were conducted with completely new welding parameters verified that the model presented in this paper was effective and robust. Sensitivity analysis was also carried out to explore the impact of each process parameter on the quality of welding joint. The optimal combination of process parameters for maximum failure energy of the welded joint was obtained using the model based on artificial fish swarm algorithm (AFSA).     

结构钢焊接匹配性研究进展

对结构钢焊接的高强匹配和低强匹配两种技术路线的发展进行了综述。在焊缝金属具有足够韧性的情况下,高强匹配能利用焊缝金属的高强度降低位于焊缝中裂纹的扩展驱动力、从而有利于焊接接头抗断性能的提高;随着结构钢强度的提高,焊接冷裂纹敏感性增大、焊缝韧性降低,采用焊接裂纹敏感性相对较低、焊缝金属塑韧性相对较高的低强匹配焊接技术能较好的控制焊接冷裂纹的产生并保证接头的抗断性能。     

Investigation of hydrogen embrittlement failure in a steam separator by field metallography

In this work the failure by cracking of a steam separator vessel constructed from ASTM A-516 Gr.60 steel was investigated by field metallography. The application of this non-destructive method allowed the determination of failure mechanism, which was useful to give recommendations for repair by welding and prevent future failures. The cracks identifications were made by penetrant liquid and after this; the field metallography test was performed in the diverse regions with cracks. Finally, the replicas were analyzed in laboratory by optical microscopy. The failure was caused by hydrogen embrittlement process principally in the heat affected zone (HAZ) because of the high reinforcement welding joint, residual stresses values and elevated concentrations of compounds controlling the parameters of the boiler feed water.     

12Cr1MoV联箱与支管焊缝裂纹分析

为研究某厂联箱与支管运行1.4×105h后发生开裂的原因,采用宏观渗透及光学显微镜、扫描电镜、能谱仪、显微硬度计等测试方法对失效管进行检测.研究表明:焊后存在焊接缺陷,大量的裂纹垂直于焊缝,加速裂纹的扩展,改变应力分布状态;部分夹杂物沿直线分布,使材料受力具有方向敏感性,导致材料开裂失效;断口表面主要是Fe的氧化物,焊件长期过热,外表面形成不均匀氧化层,产生应力集中,致使焊缝从管壁外侧开裂;母材硬度超标.优化焊接工艺方案,焊前预热焊后回火,消除管件外壁的氧化层及减少设备停机次数均能有效防止同类焊接结构开裂失效.     

插销法研究自保护焊U75V钢轨的冷裂敏感性

采用插销试验法研究了JDHS-38#自保护药芯焊丝焊接U75V钢轨的冷裂敏感性,评估了预热温度、线能量、焊道数3个因素对U75V钢轨临界断裂应力的影响.结果表明,现场对钢轨施焊时,采取250℃的预热温度,14 kJ/cm左右的线能量,双层焊等措施可以使钢轨接头热影响区少产生甚至不产生焊接冷裂纹,使其临界断裂应力达646 MPa,与抗拉强度相当.若采用其他工艺参数,临界断裂应力较低,且当拘束拉伸应力高于抗拉强度时,于热影响区发生氢致断裂.焊接接头过热区组织主要为马氏体、贝氏体,断口显微形貌主要为河流花样、泥状花样及爪状花样.     

Failure Analysis of HAZ Cracking in Low C–CrMoV Steel Weldment

This case study describes the failure analysis of steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low C–CrMoV alloy steel that was subsequently single pass butt welded using gas tungsten arc welding. No cracks were found in visual inspection of the welds; however, X-ray radiography showed small discontinuous cracks on the surface in the area adjacent to weld bead, i.e. heat affected zone. The welding of nozzle parts made of same material was a routine process and this type of cracking did not occur in the past. Therefore, it became essential to determine the root cause of the failure. A detailed investigation including visual examination, non-destructive testing, optical microscopy, microhardness measurements and residual stress measurements were carried out to find out the primary cause of failure and to identify actions required to avoid its reoccurrence in future. Results of the investigation revealed that the principal cause of failure was the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in the high transformation stresses. These transformation stresses combined with the thermal stresses and the constraint conditions to cause intergranular brittle fracture.     

Single pass laser joining of Inconel 718 superalloy with filler

Abstract

Microstructure and mechanical property of CO₂ laser beam welded IN 718 superalloy were studied by electron microscopy and hardness testing. The use of a welding filler wire produced a sound fusion zone with no cracking but grain boundary microfissuring occurred in the heat affected zone (HAZ) and was observed to be significantly influenced by pre-weld heat treatment and laser welding speed. Crack-free weld was produced by a pre-weld heat treatment that minimised non-equilibirum grain boundary boron segregation and inhibited grain growth. While post-weld heat treatment (PWHT) reduced the difference between the hardness values of the base alloy, HAZ and the fusion zone, it resulted in increased HAZ cracking, which was likely aided by pre-existing cracks. The PWHT cracking was, however, avoided by subjecting pre-weld material to the heat treatment condition that produces crack-free weld during welding process.     

Stress-corrosion cracking in copper refrigerant tubing

A metallurgical failure analysis was conducted to determine the cause of cracking in several sections of copper refrigeration tubing. The tubing in question was part of a new mechanical design, implemented to mitigate fatigue failures of solder joints that had occurred in tubing systems fabricated under the previous design. A comprehensive metallurgical evaluation revealed intergranular fracture of the copper in a region of the tubing that had been significantly cold worked during manufacture. On discovery of a source of moist ammonia in the system, associated with the location of failure, intergranular stress-corrosion cracking (SCC) was identified as the failure mechanism. A modified design, incorporating annealing of the formed copper tube section, was recommended to avoid future failures.     

环氧乙烷精制塔开裂原因分析

通过金相检验、断口分析以及腐蚀产物能谱分析等方法,对某石化公司环氧乙烷精制塔开裂的原因进行了分析。结果表明:材料在焊接过程中,焊接热影响区的组织受到敏化影响,铬的碳化物沿晶界析出,造成该区域贫铬,从而在应力和腐蚀介质的共同作用下,发生了晶间腐蚀开裂。最后提出了相应的预防措施。     

Solving some key failure analysis problems using advanced methods for materials testing

A dilemma encountered in engineering practice is a proliferation of newly designed (mostly high-strength and/or corrosion-resistant) steels and alloys that are unusable in industry as they are highly susceptible to failure under operating conditions including environmentally assisted cracking. The problem of materials failure has several sources, the most significant of which is how engineers select which material to use in which industry. As a rule, selection is based solely on assessing the mechanical properties of materials with little or no consideration of how these mechanical properties will interface with specific operating parameters found within different industries. The functional design, selection and use of materials aimed at preventing in-service failures depends, therefore, on finding testing methods, standards and approaches appropriate to real operating conditions guided by analysis of material performance under those conditions.     

乙二醇不锈钢蒸发器开裂原因分析

采用金相分析、断口微观分析以及腐蚀产物能谱分析等方法，对某石化公司乙二醇不锈钢蒸发器开裂原因进行了分析和研究。结果表明，由于材料在焊接过程中，焊接热影响区的组织受到敏化，铬的碳化物沿晶界呈网状析出，造成该区域贫铬，从而在应力与腐蚀介质的共同作用下，导致设备发生了晶问应力腐蚀开裂。     

Evaluation of Stress Relaxation Cracking on 800HT Pigtails of the Steam Reformer

Stress relief cracking occurs when susceptible alloys are subjected to thermal stress after welding to reduce residual stresses and improve toughness. Cracks were observed on the external surface of the outlet pigtails of a steam reformer. The failures (cracks) occurred at the toe of the socket weld connecting the pigtail to the catalyst tube. Design parameters are 900 °C and 22 barg; the actual operating conditions are 875 °C and 20 barg. The material of construction is Alloy 800HT, which is an iron–nickel–chromium alloy. The outlet pigtail was used to transfer the process fluid of the steam reformer which is connected to a manifold header. It was requested to conduct a failure analysis on damage in the outlet pigtail tubes that occurred in the form of cracks. The failure analysis study is described in detail in this paper, and the mechanism of the crack is identified and proper recommendation is given to avoid such issues in future operations.     

An unusual failure of a nickel-aluminium bronze (NAB) hydraulic valve

Investigation of a hydraulic leak from a nickel-aluminium bronze (NAB) valve in a naval vessel revealed that two corners of a square flange had fractured in a macroscopically brittle fashion such that the valve was in three pieces – an unusual occurrence for a ductile material. Moreover, fracture surfaces were almost entirely intergranular, and there were no signs of dimpled fracture surfaces or shear lips typical of fast fracture in NAB. A few macroscopic progression markings were evident and, at high magnifications, intergranular facets exhibited slip lines and (probably) fatigue striations. Fracture surfaces produced by fatigue of specimens cut from the valve were similar to the in-service fractures for fatigue at very low (near-threshold) crack-growth rates, but not for fatigue at higher crack-growth rates. Data in the literature for NAB, especially in regard to previous failure analyses of NAB hydraulic couplings, indicated that environmentally assisted cracking modes such as stress-corrosion cracking were not involved. All the above (and other) observations indicated that failure of the valve occurred entirely by high-cycle fatigue, probably in the gigacycle regime, at very low stress amplitudes. The cyclic stresses are most probably associated with high-frequency vibration/resonance, but the source of the excitation causing this problem was not identified. High mean stresses, due to torquing bolts to specified levels on the square flange (which is free to deflect) probably facilitated fatigue crack initiation from machining grooves. Various recommendations were made in regard to preventing further failures.     

Stress corrosion cracking of a 60 MW steam turbine rotor

The paper presents a root cause analysis of a steam turbine rotor blade groove cracking. The scope of analyses included material testing and mechanical integrity calculations. In scope of material testing, fracture microstructure was assessed and basic mechanical property characteristics of the rotor discs were determined. In scope of integrity analyses, the stress fields in the blade grooves were calculated and the possibility of cracking due to different mechanisms was assessed. Both calculations and material tests confirmed the stress corrosion cracking to be the root cause of the rotor failure. This was a basis for proposing the rotor discs repair by overlay welding with a lower strength material and modifications to the groove geometry.