Failure analysis of TP304H tubes in the superheated steam section of a reformer furnace

The cracking failure of TP304H tubes in the superheated steam section of a reformer furnace was analyzed. Through the analysis of macro-appearance, micro-appearance of specimens with cracks, metallurgical structure of specimens from an intact pipe section and cracked pipe section, energy spectrum detection of fracture surface, residual stress measurement, and investigation of the service medium, the cracking mode was described as the stress corrosion cracking (SCC) of austenitic stainless steel. In this case, the materials in the heat affected zone were sensitized by inappropriate welding technology. This together with the higher pH value in the steam due to the failure of a gas-liquid separator led to the final cracking of the reformer furnace tube. So the inappropriate welding technology and the failure of the gas-liquid separator were the main factors in this fracture accident.     

Failure analysis of 316L stainless steel tubing of the high-pressure still condenser

Sherritt International Corporation experienced corrosion failures with the 316L stainless steel tubing in a high-pressure still condenser employed for ammonia recovery. A detailed failure analysis was conducted on the condenser tubing to determine the mode and the root cause of the failure. The analysis included both optical and scanning electron microscopy (SEM) of the inner and outer surfaces of the tube as well as characterization of the corrosion products using energy-dispersive X-ray spectroscopy (EDX). Results revealed that the corrosion attack was confined to the first ～100 mm of the tubing at the inlet where the tube was connected to the top tubesheet. The tube suffered both external stress-corrosion cracking (SCC) and crevice corrosion from the shell side (water side), and wall thinning of the inner surface (the tube side) due to erosion corrosion. It was evident that failure of one of the tubes occurred due to SCC that penetrated the whole wall thickness and resulted in a leak failure. Some prevention measures are proposed to avoid this type of corrosion attack in the future.     

柴油加氢装置高压换热器管束断裂原因与防护

对热高分气与混氢换热器断裂管束进行了失效原因分析。结果表明,氯化物应力腐蚀开裂是管束失效的主要原因,氢的存在及管板与管束在溶液环境组成的腐蚀电池加快了该腐蚀的速度。针对腐蚀原因采取了优化设计和制造、安装和操作等腐蚀防护措施,效果良好。     

Failure analysis of the wall tubes of a water-tube boiler

An industrial case history of the failure of the evaporator (water wall) tubes of a water-tube boiler is presented in this study. After two years in service, a leak was detected in one of the tubes over its bent section with pitting corrosion on its internal surface. Later on, the same symptoms were discovered in other tubes of the same boiler, and several tubes were extracted for failure analysis. On the basis of visual inspection, chemical analysis, microstructural examination, hardness measurements, and residual stress measurements via X-ray diffraction (XRD), the failure mechanism of the tubes was identified as stress corrosion cracking (SCC). In this paper, the results obtained from the experimental analysis are summarized, and finite element (FE) models are used to predict the residual stress due to the bending of the tube and the operational stress at the moment of failure. It was found that tensile residual stress from an inadequate stress relief treatment prior to service and high concentration of dissolved oxygen in the feed water were the main reasons for the premature failure of the boiler tubes by SCC.     

抗硫油管断裂原因分析

采用化学成分分析、断口分析、金相检验、扫描电镜分析等方法,对某油田抗硫油管的断裂原因进行了分析。结果表明：油管在硫化氢的腐蚀作用下,首先在管壁上产生局部腐蚀,继而在应力的作用下产生微裂纹并扩展,最终发生应力腐蚀断裂。最后,通过实验室模拟井况条件试验,提出了选择合适材料的建议。     

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。     

锅炉水冷壁爆管分析

锅炉水冷壁发生了钝边和无明显塑性变形爆管。通过对失效的锅炉水冷壁管进行宏观检查、化学分析和金相试验以及X射线衍射分析，发现该管向火侧外壁(烟气侧)发生了导致管壁严重减薄的高温硫腐蚀和内壁(水侧)垢下酸性腐蚀，而垢下腐蚀产物为氢原子渗入基体与Fe3C反应生成的CH4，由它引发沿晶微裂纹，最终由氢致裂纹引起管材失效。     

Failure analysis of Integrated Nozzle Actuating System (INAS) cooling hydraulic pipeline

During post test disassembly of a developmental aero engine, a hydraulic pipeline was found fractured from near weld joint. The pipeline was manufactured from a α–β Titanium alloy and was welded using Tungsten Inert Gas Welding (TIG) technique. The pipeline was in service for approximately 100 h. Fractographic features of the surface revealed transgranular cracking mode with presence of corrosive elements near the outer wall. Crack branching was also observed on the pipeline near to the fractured surface. Evidences of corrosion pits along with microcracks were also present near to the fractured surface. From the evidences, it was corroborated that the pipe failure was due to synergistic effect of corrosion fatigue and stress corrosion cracking.     

天然气井钻杆悬挂器断裂分析

某油田天然气井的钻杆悬挂器（套筒）在使用过程中发生断裂失效事故,采用化学成分分析、金相检验、硬度测试、断口分析、微区成分分析等方法对断裂件进行了分析。结果表明：该1Cr13钢悬挂器淬火后未经高温回火,不符合调质热处理工艺的技术要求。基体中的淬火马氏体组织硬度过高,残余内应力大,沿晶界及亚晶界聚集分布的颗粒状未溶碳化物（Fe,Cr）23C6降低了钢的抗蚀性。该套筒内联上接头的螺纹部位薄壁存在周向拉应力,使用时在高压天然气作用下,筒壁沿圆周方向附加有横向拉应力。套筒内部环境为湿硫化氢的天然气介质,结果导致该悬挂器沿纵向产生应力腐蚀断裂。     

Failure Analysis of Reformer Tubes

This paper presents a failure analysis performed to investigate cracking observed in reformer tubes. Thorough investigation showed that the tubes failed by thermal shock. No signs of advanced creep failure could be observed near the ruptures or across the microstructure. Also, no material-related abnormalities were observed in the tube microstructure. The reformer tube material is suitable for the application, and there is no need for metallurgy upgrade. It is recommended that reformer operating, start-up and shut down procedures be followed to ensure safe and sustainable operation. The condition of un-replaced tubes that had been exposed to the extreme thermal cycles has to be frequently monitored.     

1Cr18Ni9Ti钢炉管开裂原因分析

采用金相检验、力学性能试验、铁磁相含量测定、断口及能谱分析等方法对1Cr18Ni9Ti钢炉管的开裂原因进行分析。结果表明：开裂炉管的内壁裂纹为连多硫酸应力腐蚀开裂,而外壁裂纹为硫化物促进下的氯化物应力腐蚀开裂。并提出了预防炉管开裂的建议。     

Degradation of stainless steel grids in chemically aggressive environment

The present study concerns the failure analysis of a perforated austenitic stainless steel grid, operating in a shell-and-tube heat exchanger of a petrochemical industry. Macroscopic examination of the grid indicated extensive friability and severe cracking in a direction perpendicular to its normal loading, while both grid surfaces as well as the interior of the filtration holes were covered significantly by decayed deposits. Microscopic examination of selected grid areas, after the surface deposits removal, indicated severe cracking exhibiting multiple branching, which advocates for stress corrosion cracking. Besides the extensive cracking areas, voids surrounded by twinning and slip bands were observed. Elemental microanalysis carried out in the areas around voids indicated the presence of iron and chromium at proportions that can be correlated to the formation of σ-phase. The detection of oxygen, iron and chromium within the cracks is attributed to corrosion products consisting of a mixture of iron and chromium oxides. The premature catastrophic failure of the stainless grid occurred as a synergistic effect of these distinct root-causes. Potential substitution of the currently used stainless steel with another alloy of higher resistance in stress corrosion cracking and microstructure stability at high temperatures is suggested.     

Failure Analysis and Remaining Life Assessment of Methanol Reformer Tubes

High-quality failure analysis and good engineering judgment can turn plant shutdowns resulting from methanol reformer tube failures into an opportunity to improve the future performance of the reformer furnace. The plant down time can be used to evaluate remaining tube life and provide some insight into the effect of tube operating history, especially tube metal temperature on tube performance. The results can be used to minimize potential future failures and economic losses because of reformer shutdowns. In this article, the failure mechanism of a ruptured reformer tube is determined and an assessment of the remaining life of non-ruptured tubes in the reformer is discussed. Two assessment methods are reviewed (1) metallographic examination of ex-service material to characterize microstructure and creep damage and (2) modeling of creep damage accumulation using special-purpose finite-element software (WinTUBE^TM).     

Root cause analysis of stress corrosion at tube-to-tubesheet joints of a waste heat boiler

Leakage at the tube-to-tubesheet joints occurred in a waste heat boiler. The mode and the root cause of the failure were investigated by chemical composition analysis of the tube material, metallographic structure and crack observation, and corrosion product analysis of the damaged tubes, as well as the operation condition examination of the waste heat boiler. Results revealed that failure of the tubes occurred due to the stress corrosion cracking (SCC), which was caused by tensile stress and chloride-buildup in the narrow and long gap between the tube and tubesheet hole. The gap formation was further analyzed by comparison of the minimum expansion pressure from the common formula provided by the manufacturer, with that from finite element method computations. It is found that the minimum expansion pressure used in manufacture is small and cannot eliminate the initial gap. Meanwhile, the enrichment of chloride in the gap was briefly discussed.     

An evaluation of the resistance of pipeline steels to initiation and early growth of stress corrosion cracks

Safety and reliability are basic requirements on the operation of systems of high-pressure pipelines. One of the dangerous mechanisms endangering the safe operation is stress corrosion cracking (SCC). In this work, SCC initiation in two types of pipeline steels––a carbon pipeline steel and an X60 high-strength thermo-mechanically treated steel––in carbonate solution was studied. An influence of surface microstructure, different stress/strain conditions and temperature was evaluated. The X60 steel was more sensitive to the SCC process in comparison with the carbon steel. Surface local microscopical inhomogeneities were the predominant initiation sites of microcracks.     

Flow enhanced corrosion of water injection pipelines

A pipe spool from the subsea water injection piping network for oil operations at Eastern Desert was retrieved and internal corrosion and grooving were observed at the 6 O’clock position, in one section of the pipe, and not in the other. Two cuts from the sited piping were received for analysis to establish an overview of whether the failure is related to materials aspects or operating conditions. Results of visual inspection, chemical analysis, metallographic examination, SEM/EDX analysis, and mechanical testing showed that the corrosion resistance against flow for the quenched and tempered structure of the first cut was better than that of the cold rolled structure of the second cut. This is largely due to the uniform distributed polygonal ferrite and the small volume fraction of pearlite. Continuous removal of the loose adhered scales by electrochemical dissolution and mass transfer resulted in creation of fresh surfaces for further corrosive attack. This reduced the pipe wall below the critical thickness required to support the operating pressure and resulted in ductile failure of the pipe. Such mechanism of failure is known as the flow enhanced corrosion (FEC) mechanism. Failure in such mechanism is a catastrophic one that usually results in serious damage and injuries if not detected before undergoing.     

Stress corrosion cracking of stainless steel pipes for Methyl-Methacrylate process plants

In a Methyl Methacrylate (MMA) plant, tree-like transgranular cracks were found near the weld of a pipe that had been used for transferring MMA material at 110 °C and 0.77 kg/cm². The pipe was made of ASTM A312 TP304 stainless steel.In this study, it was shown that the failure was due to the stress corrosion cracking (SCC) caused by the chloride that remained in the pipe. Corrosion pitting occurred on the inside surface of the pipe. The stress corrosion cracking started from the pits and grew out through the thickness. Concentrated chloride was found in the deposit stuck to the pipe in addition to the pre-process MMA materials. Many work-hardened grains were observed in the area of SCC, providing the evidence of high residual stress due to welding, which could serve as the driving force for the SCC. Recommendations are made for preventing further failure due to SCC in such cases.     

321不锈钢焊接波纹管失效分析

应用光学显微镜、扫描电镜和能谱仪等手段对321不锈钢焊接波纹管表面腐蚀物的化学成分、焊缝区域的显微组织、腐蚀裂纹的形态特征等方面进行检验和分析。结果表明，波纹管局部电镀端内壁腐蚀导致漏气失效，即加工应力、焊接残余应力与含Cl^-或含Cl^-和溶解氧的腐蚀性介质的联合作用使波纹管局部电镀端产生了点蚀和应力腐蚀开裂；退火工艺设计的不合理和钎焊工艺控制不恰当使焊缝热影响区敏化而出现了晶间腐蚀。据此提出了降低和避免321不锈钢焊接波纹管因腐蚀而漏气的措施。     

Stress corrosion failure of large diameter pressure pipelines of prestressed concrete

The failure of a 1.5 m diameter prestressed concrete line for water supply was examined. The water pressure opened a hole of 0.5 m² in the pipe wall by breaking the concrete into fragments and by tensile severing of a number of coils of the wire winding. Flexural and tensile testing of samples of the broken materials showed no damage to the concrete, but showed significant losses of strength and ductility in the prestressing steel wire. The SEM analysis of the external and fracture surfaces of the circumferential wires revealed shallow cracking and corroded areas as expected from a stress corrosion cracking process. The failure analysis presented in this paper shows that such a process was able to exhaust the damage tolerance of the affected tube until the pipeline burst under the work pressures.     

Stress corrosion cracking fracture mechanisms in rock bolts

Rock bolts have failed by Stress Corrosion Cracking (SCC). This paper presents a detailed examination of the fracture surfaces in an attempt to understand the SCC fracture mechanism. The SCC fracture surfaces, studied using Scanning Electron Microscopy (SEM), contained the following different surfaces: Tearing Topography Surface (TTS), Corrugated Irregular Surface (CIS) and Micro Void Coalescence (MVC). TTS was characterised by a ridge pattern independent of the pearlite microstructure, but having a spacing only slightly coarser than the pearlite spacing. CIS was characterised as porous irregular corrugated surfaces joined by rough slopes. MVC found in the studied rock bolts was different to that in samples failed in a pure ductile manner. The MVC observed in rock bolts was more flat and regular than the pure MVC, being attributed to hydrogen embrittling the ductile material near the crack tip.The interface between the different fracture surfaces revealed no evidence of a third mechanism involved in the transition between fracture mechanisms.The microstructure had no effect on the diffusion of hydrogen nor on the fracture mechanisms.The following SCC mechanism is consistent with the fracture surfaces. Hydrogen diffused into the material, reaching a critical concentration level. The thus embrittled material allowed a crack to propagate through the brittle region. The crack was arrested once it propagated outside the brittle region. Once the new crack was formed, corrosion reactions started producing hydrogen that diffused into the material once again.