Failure analysis of stress corrosion cracking in heat exchanger tubes during start-up operation

The cracking failure of a new heat exchanger during first start-up operation has been analyzed. Through the investigation of the operating history of the equipment, analysis of the chemical compositions of tube material and corrosion products, metallographic test of specimens with cracks, the cracking mode can be described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. This kind of cracking was induced by the chloride in high temperature steam and tensile stress. The residual tensile stress due to seal expansion has been proved by numerical calculation. The pre-heating steam which was polluted by the catalyst with chloride is the main reason for the tube cracking in this case.     

一种新型核级奥氏体不锈钢焊接接头的晶间腐蚀研究

利用"不锈钢硫酸-硫酸铜腐蚀实验"、光学显微镜和扫描电镜等测试方法分别对未敏化和经敏化处理(650℃×100 h)的800H和新型Cr18Ni30Mo2Al3Nb合金焊接接头的抗晶间腐蚀性能进行研究,结果表明,800H和Cr18Ni30Mo2Al3Nb接头焊缝组织均为单一的奥氏体基体,800H合金中TiN缺陷处易引起点蚀,而Cr18Ni30Mo2Al3Nb无明显点蚀现象;对比腐蚀失重、腐蚀深度等实验结果,未敏化和敏化态Cr18Ni30Mo2Al3Nb焊接接头的抗晶间腐蚀性能明显优于相同状态的800H。     

Investigation of stress corrosion cracking behavior of super 13Cr tubing by full-scale tubular goods corrosion test system

In this paper, a self-built device called “full-scale tubular goods corrosion test system” was used to test a 6 m length super 13Cr tubing (with coupling) to study its corrosion performance in spent acid. The specimen fractured at the tubing and was investigated by visual inspection, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and mechanical test. It was the joint function of tensile force (78.6% actual yield strength), inner pressure (70 Mpa) and spent acid that induced stress corrosion cracking (SCC) of the tubing at 120 °C. Three different areas were found on the fracture surface, including crack initiation area, crack expansion area, and final fracture area. The fracture initiated from the “X” shape corrosion cracks which were evolved from small corrosion pits. The reduction of ductility and toughness may also facilitate SCC of the tubing.     

On the stress corrosion cracking mechanisms of austenitic stainless steels

In this paper, experimental results on stress corrosion cracking in austenitic stainless steels are described. Crack growth data in sodium chloride solution for AISI 304 steel obtained for different metallurgical conditions, acoustic emission data recorded during crack growth and fractographic observations have been discussed with a view to identifying the operating mechanism. Some of the experimental observations such as crack propagation occurring in discontinuous jumps of the order of a few microns, lowering of the threshold stress intensity andJ-integral values on sensitization and cold working, typical transgranular fractographic features, transition in mode of fracture from transgranular to intergranular in sensitized conditions and activation energies of the order of 50 to 65 kJ/mol can all be accounted by hydrogen embrittlement mechanism. Hydrogen generated at the crack tip by corrosion reaction diffuses ahead of the crack tip under hydrostatic stress and influences the deformation process at the crack tip and also leads to the brittle component of the crack advance in jumps.     

加热炉不锈钢盘管泄漏失效分析

某气田再生气加热炉奥氏体不锈钢盘管在运行49h后发生泄漏,采用化学成分分析、力学性能测试、渗透检测、宏微观形貌分析、晶间腐蚀试验以及能谱分析等方法对盘管失效原因进行了分析。结果表明：由于进入加热炉盘管的介质中含有较高浓度的氯离子,导致盘管发生了氯化物应力腐蚀开裂。并提出了相应的防护措施。     

Stress corrosion cracking of AISI 321 stainless steel in acidic chloride solution

The stress corrosion cracking (SCC) of AISI 321 stainless steel in acidic chloride solution was studied by slow strain rate (SSR) technique and fracture mechanics method. The fractured surface was characterized by cleavage fracture. In order to clarify the SCC mechanism, the effects of inhibitor KI on SCC behaviour were also included in this paper. A study showed that the inhibition effects of KI on SCC were mainly attributed to the anodic reaction of the corrosion process. The results of strain distribution in front of the crack tip of the fatigue pre-cracked plate specimens in air, in the blank solution (acidic chloride solution without inhibitor KI) and in the solution added with KI measured by speckle interferometry (SPI) support the unified mechanism of SCC and corrosion fatigue cracking (CFC).     

Biologically induced corrosion and consequent fracture of a pump shaft coupling

During a routine start-up exercise of a standby service water pump, a threaded coupling that joined sections of a 41.5 ft (12.7 m) long pump shaft experienced fracture. The pump was taken out of service and examined to determine the cause of fracture. It was apparent early in the examination that the fracture involved hydrogen stress cracking. However, the nature of the corrosive attack suggested an interaction between the threaded coupling and biological organisms living in the freshwater environment of the pump shaft. The organisms had colonized on the coupling, changing the local environment and creating conditions favorable to hydrogen stress cracking. This paper describes the analysis of the fracture of the coupling and provides an example of how biologically induced corrosion can result in unexpected fracture of a relatively basic machine part.     

Failure of hydrocracker heat exchanger tubes in an oil refinery by polythionic acid-stress corrosion cracking

Heat exchanger tubes of a hydrocracker unit in an oil refinery developed cracks at the bent sections after about 48 h of operation at 400 °C following a period of downtime. The tubes were fabricated from 321 stainless steel. Detailed analysis by various electron-optical techniques and X-ray diffraction showed that the tubes were cracked by stress corrosion cracking induced by polythionic acid. Due to the presence of H₂S in the environment, the inner surface of the tubes was converted into sulfur-bearing scale during operation. This could promote the formation of polythionic acid by aqueous condensates during downtime leading to cracking in the presence of residual internal stresses.     

Interplay of microbiological corrosion and alloy microstructure in stress corrosion cracking of weldments of advanced stainless steels

This paper presents an overview of the phenomenon of stress corrosion cracking (SCC) of duplex stainless steels and their weldments in marine environments and the potential role of microbial activity in inducing SCC susceptibility. As a precursor to the topic the paper also reviews the performance of the traditional corrosion-resistant alloys and their weldments and the necessity of using duplex stainless steels (DSS), in order to alleviate corrosion problems in marine environments. Given that the performance of weldments of such steels is often unsatisfactory, this review also assesses the research needs in this area. In this context the paper also discusses the recent reports on the role of microorganisms in inducing hydrogen embrittlements and corrosion fatigue.     

Failure analysis of AISI 321 tubes of heat exchanger

The present study is about the failures of tubes from a tube shell heat exchange in one of the compression stages used to cool and purify the gas produced in an off-shore platform. Conventional hydrostatic tests did not detect leaking in the heat exchanger, but these tests were conducted at room temperature, i.e., very distant to the service temperature (163 °C). However, tightness tests with helium gas have detected leaking. Some tubes were taken off for analysis and the liquid penetrating test detected cracks in crevices formed between the tube and the tube sheet. The cracks were found to be typical stress corrosion cracks with branched pattern. A non-destructive test with Eddy current probes was developed to give high percentage of detection. The work was conclusive about the main cause of failures. Due to the increase of chloride content of the water processed to concentrations higher than 1000 mg/L over the years, the substitution of AISI 321 tubes by superduplex steel is recommended.     

某酸奶厂配料罐304不锈钢夹套底部材料的失效分析

某酸奶厂二次配料罐304不锈钢夹套底部出现开裂现象,影响了工厂的正常运行。通过观察裂纹宏观形貌、裂纹金相特征、断口形貌以及断口腐蚀产物能谱分析及应力腐蚀敏感性试验等分析了开裂原因。结果表明:配料罐夹套开裂是由奥氏体不锈钢的应力腐蚀造成的;根据事故发生的原因,提出了返修和消除缺陷的方法。     

Effects of orientation,stress and exposure time on short intergranular stress corrosion crack behaviour in sensitised type 304 austenitic stainless steel

Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels occurs at susceptible grain boundaries after sensitisation. In this study, the effects of test duration, static stress (applied and residual) and microstructure orientation on the developed populations of short crack nuclei are reported for a sensitised type 304 austenitic stainless steel in an acidified potassium tetrathionate (K₂S₄O₆) solution. The crack populations were analysed using the Gumbel distribution method, showing an increase in the characteristic crack lengths with increasing time and grain size. There is a weak, but measurable effect of stress on crack length. Tensile stress increases crack growth and compressive residual stresses introduced by surface machining are shown to be beneficial. A significant dependence on sample orientation is observed and this cannot be explained in terms of the bulk microstructure properties or characteristics, which showed no significant variations.     

2205 双相不锈钢在熔融氯化物盐膜下的腐蚀

利用热重分析考察了表面涂有ZnCl2-KCl混合盐膜的2205双相不锈钢在450℃氧化性气氛中的耐蚀性，结果表明，材料发生加速腐蚀，形成厚且疏松的表面腐蚀层。并在基体内发生晶间腐蚀及沿富Cr相的择优侵蚀，在该环境下，材料中较高的Cr含量没有提供充分的保护性，甚至恶化材料的耐蚀性能。用热力学讨论其作用机理。     

锻造、轧制变形对2519A铝板抗腐蚀性能的影响

为改善2519A-T87铝合金的腐蚀性能并探求其影响因素,采用金相显微镜、透射电镜及X射线衍射仪研究了锻造以及轧制两种变形方式对该合金板晶界无沉淀带、析出相、晶粒取向与抗腐蚀性能的影响.结果表明:锻造板中晶界无沉淀带(PFZ)较轧制板的宽,晶界析出相粒子间距大,较难形成连续腐蚀通道,其抗晶间腐蚀与剥落腐蚀性能优于轧制板;PFZ与基体,析出相与PFZ构成两对电偶发生反应,而晶界上不连续分布的、大的第二相之间的间距有利于提高合金的抗应力腐蚀性能,锻造板的应力腐蚀性能(KISCC=32.1 MPa.m1/2)比轧制板(KISCC=22.1 MPa.m1/2)高;锻造板的{100}晶粒取向密度较低;第二相的大小及分布是决定该合金腐蚀性能的关键..     

304奥氏体不锈钢晶间腐蚀敏感性的非线性超声表征

利用非线性超声检测技术并辅助于XRD与微观组织分析,探讨了在650℃经过2,6,10h敏化处理的304奥氏体不锈钢样品非线性超声特征参数的变化规律。结果表明:随着敏化时间的延长,归一化非线性系数单调增大;相比于固溶试样,经2,6,10h敏化处理后样品的归一化非线性系数分别增加28%,32%,43%,意味着以非线性系数表征304不锈钢的敏化度是可行的。分析认为:晶界析出碳化物(Cr23C6)与奥氏体基体产生的错配引发了局部应变场,干扰了超声波的传播;此外,随敏化时间延长,析出相的增加进一步加剧了超声波的畸变。     

The effect of TiN inclusions and deformation-induced martensite on the corrosion properties of AISI 321 stainless steel

Stainless steel of type 321 is commonly used for the production of exhaust systems because of its temperature resistance and welding properties, which are better than those of AISI 304 or similar steels. AISI 321 is a titanium stabilized austenitic stainless steel, where this element is added to form carbides in order to avoid chromium impoverishment due to chromium carbide formation. Cold shaping can, in the case of austenitic stainless steel, cause the formation of deformation induced martensite, which can improve its mechanical properties, but unfortunately can also spoil its good resistance to corrosion. Titanium nitride inclusions are cathodic with respect to steels, and therefore cause their anodic dissolution. Martensite is, however, more susceptible to the corrosion than austenite in steels. The main aim of this study was to analyze the pitting corrosion and stress corrosion cracking which is initiated on prototype cold-formed outer exhaust sleeves during the testing of different cleaning procedures before chromium plating. Various microscopic methods were used to identify the initiation of corrosion and its propagation.     

304不锈钢晶间腐蚀敏感性的评定

采用共聚焦显微镜测量了经不同敏化处理的304不锈钢晶间腐蚀后的晶间裂纹深度,通过统计晶间裂纹深度值的分布建立了304不锈钢晶间腐蚀敏感性的评价方法。如果晶间裂纹深度值大于0．5μm,则可认为所测试的304不锈钢发生了晶间腐蚀现象。     

球罐应力腐蚀开裂研究与安全性分析

对球罐应力腐蚀开裂的原因和主要影响因素进行了分析.针对16MnR和SPV50Q球罐用钢,在分析湿硫化氢环境下应力腐蚀开裂形式的基础上,通过改进的WOL预裂纹试样的应力腐蚀开裂试验,对不同球罐用钢、不同硫化氢浓度、不同焊接状态条件下的应力腐蚀开裂进行了研究,并对设备的安全性能进行了分析,进而提出了防止应力腐蚀开裂的对策.     

Al对含P,Mo奥氏体不锈钢应力腐蚀性能的影响

用点焊接试验方法评价了合金元素Ａｌ对含Ｐ和Ｍｏ奥氏体不锈钢应力腐蚀破裂性能的影响。结果表明，单独添加Ａｌ没有改善应力腐蚀破裂性能，而复合添加Ａｌ和Ｃｕ有效地提高了抗应力腐蚀性能。１４Ｃｒ－１６Ｎｉ－２Ｃｕ－３Ａｌ钢中添加Ｍｏ，虽然降低应力腐蚀性能，但影响较小。含０．３％Ｍｏ时应力腐蚀破裂临界温度可达１２０℃。ＡＥＳ分析表明，Ａｌ和Ｃｕ元素在表面膜中富集。Ａｌ和ｃｕ的联合作用使钢表面膜的钝化能力得到加强，从而提高了抗应力腐蚀性能。     

不锈钢硫酸-硫酸铜晶间腐蚀试验结果评定方法

本文采用金相显微镜(OM)、扫描电镜(SEM)以及透射电镜(TEM)对不锈钢焊接件、不锈钢铸件晶间腐蚀试样弯曲裂纹进行分析,并与电化学动电位再活化法(EPR)试验结果进行对比,研究出晶间腐蚀所致弯曲裂纹不同于因机械破坏所致弯曲裂纹微观特征,进而总结出区分二者的方法,并结合GB 4334方法 E《不锈钢硫酸-硫酸铜晶间腐蚀试验方法》,对完善不锈钢焊接件和不锈钢铸件的晶间腐蚀结果评定方法进行了改善。