Cracking of Aluminum Alloy Rivets Used in Space Application: A Metallurgical Investigation

Adaptors used in check valve inlet pressure sensors were fabricated from austenitic stainless steel. Recently, during pretest operation and during hydrogen pressurization of an engine, a leak was noticed through the adaptor. Subsequent to disassembly, the leak was attributed to a crack in the adaptor. Detailed metallurgical investigations were carried out to understand the cause of cracking. Synergistic effect of chloride ions present in marine environment, the presence of residual stresses and sensitized grains of the material led to intergranular stress corrosion cracking. This article highlights details of investigation.     

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.     

空调四通换向阀封头失效分析

采用光学和扫描电子显微镜及能谱仪等手段，对进口的铁素体不锈钢棒制造的空调四通换向阀中的封头失效进行了分析。认为棒材内部存在的大量沿轴向分布的硫化锰夹杂物导致封头开裂，从而使空调四通换向阀产生失效。     

换热器汽包封头泄漏原因分析

在检修雏护换热器汽包时发现汽包封头泄漏，在汽包封头的阀盖角焊缝处出现裂纹，裂纹平行于焊缝。采用宏观形貌分析、化学成分分析、金相检验、显微硬度测定、能谱分析以及应力分析等方法对泄漏处进行了分析。结果表明，汽包中水质含有较高浓度的碱性元素和氧、硫、氧等强腐蚀性元素，又在焊接残余应力的作用下发生应力腐蚀开裂，最后导致汽包封头产生泄漏。     

Metallurgical Analysis to Evaluate Cracking in a 316L Grade Stainless Steel Spiral Heat Exchanger

A spiral heat exchanger (SHE) constructed of Grade 316L stainless steel developed a leak after 8 years of service as a condenser on a distillation tower. Examination of the SHE identified the leak as being located on the cooling water side in the heat affected zone (HAZ) of a weld joining two plates. Cooling water deposits were observed in a V-shaped corner formed by the weld. A metallurgical examination identified the presence of transgranular cracks at the HAZ on the cooling water side. Analysis of the cooling water revealed the presence of chlorides. Based on the metallurgical analysis and the presence of chlorides, it was determined that the cracks and resulting leak were the result of chloride stress corrosion cracking.     

Experimental assessment of thermal grain boundary embrittlement after tubeplate failure in a petrochemical heat exchanger

The analysis of multiple cracks in a heat exchanger tubeplate at a petrochemical plant led to experimental replication of in-service damage. High lifting stresses created a first leak that was repaired. Extended branched intergranular cracking developed from the tubeplate surface around the repair weld after few weeks of renewed operation. Chemical analyses failed to detect Na on crack surfaces, while operative conditions also allowed discarding a typical SCC cracking mechanism. Lack of radial interference between tubes and plate, lack of penetration and lack of fusion at tube to tubeplate weld roots were also thought as contributors to cracking. Microstructural analyses revealed martensite clusters in welds and HAZ, and austenite grain boundary precipitates in the tubeplate base material. Thermal cycles were applied to ex-service samples to replicate the conditions for these brittle micro constituents, which were found to be unstable at operating temperatures. Mechanical testing also replicated grain boundary weakness. This inadequate structure was related to welding without proper thermal cycles and heat treatments.     

Metallurgical Investigation of Cracked Adaptor Welded to Cryogenic Dump Coolant Line of Engine Test Stand

In the upper stages of satellite launch vehicles, a cryogenic propulsion system is used because of its high specific impulse. Such stages were tested for qualification using ground hot test facility. During one of the hot tests, the stainless steel adaptor welded to the engine dump coolant line, and used for accommodating temperature sensors, broke away from the tube. The detailed metallurgical investigation was carried out, and the reason for cracking was attributed to improper welding. This article highlights the details of investigation carried out.     

Failure of a Dissimilar Metal Braze in an Expansion Joint

Journal of Failure Analysis and Prevention - The failure of a hot water expansion joint in the heating system of a high-rise office building resulted in a catastrophic water leak. Brittle cracking...     

Cracking of AFNOR 7020 Aluminum Alloy Adaptor Used for Aerospace Applications: A Metallurgical Investigation

Aluminum alloys are frequently preferred materials for aerospace applications due to their high-specific density, high-specific stiffness, and ease of fabrication. In one such application, adaptors used in the torroidal shaped water tank of liquid propulsion system, were made of an AFNOR 7020 (Al–4.5Zn–1.5Mg) aluminum alloy extrusion. These pressurization adaptors, in T6 temper condition, were initially shrink fitted to the openings provided in water tank main body and later circumferentially welded, manual TIG, to configure the tank. Four such adaptors were welded at different locations of a water tank. During one of the qualification tests, cracks were noticed near to the weld fusion line of one of the adaptor. Detailed metallographic investigation on the cracked adaptor revealed that cracking was due to combined effect of locked-in stresses in the material and anodic dissolution of solute rich phases present at elongated grain boundaries of the HAZ: a typical case of Stress Corrosion Cracking (SCC).     

Cracking of AFNOR 7020 Aluminum Alloy Weld Joint of Water Tank for Satellite Launch Vehicle: A Metallurgical Investigation

Aluminum alloys are frequently used as the preferred material for aerospace applications owing to their high-specific strength and ease of fabrication. In one such application, a torroidal-shaped water tank made of AFNOR 7020 (Al–4.5Zn–1.5Mg) alloy sheet and rings, stores water to provide for the initial run of a turbo pump and for the cooling of engine components. During the stage leak test, one of the water tanks leaked near the interconnecting ring welded to the shell. Detailed metallurgical investigations found that leakage was due to stress-assisted cracking, which was caused due to locked-in stresses introduced in the material during welding and to an inferior microstructure of the ring parent material. This article brings out the details of the investigations and recommends actions to avoid future failures.     

Metal dusting in the low-point drains of crossover piping system in petrochemical environment

In April, 2008, a leak from a welded elbow was found on the coil inlet piping of an ethane cracking furnace in one of NOVA Chemicals plants. Subsequent examinations identified more potential failure locations, e.g. abandoned thermowells, low-point drains. All these locations were identified as containing confined spaces, which were relatively isolated from the process stream and allowed the buildup of coke. Extensive analysis revealed that metal dusting of the 304H stainless steel was responsible for the material wastage and reduction of wall thickness at these specific locations. This paper describes the analysis on the low-point drains and elaborates on the corresponding mechanism of the metal dusting in these confined spaces.Two 304H stainless steel (SS) low-point drains were analyzed to investigate the root causes of the failure. These failed low-point drains were located in the crossover piping system of an ethane cracking furnace. The failure was in the form of material loss, resulting in the thinning of the plug wall and creating a leak path. The analysis confirmed that the material loss was the result of metal dusting due to excessive carbon deposition and carburization. The metal dusting is proposed to occur in the decoking process, which is an oxidizing environment, rather than the cracking process. Excessive carbon content was detected in the void formed by metal dusting and the crevice between the threads. The coke formation was in the form of catalytic coking, promoted by the existing catalysts (iron and/or nickel) in 304H SS. The metallurgical analysis revealed carburization in the steel and local burns were present simultaneously with the metal dusting. The reduction or disappearance of the original protective layer (Cr₂O₃) allowed a diffusion path of the carbonaceous gases, thus creating carburization and metal dusting. The disappearance of the protective oxide (Cr₂O₃) layer was the result of local excessive temperature causing loss of chromium.     

Investigation and Recommendations on Bottom-Dented Petroleum Pipelines

On September 21, 2015, the National Transportation Safety Board responded to a petroleum leak from a transmission pipeline in Centreville, VA. A small through crack was found leaking at a dent on the underside of the pipe, located away from any welds. The investigation found that corrosion fatigue could initiate at small dents, typically caused by impingement. While top-side dents from excavation and servicing have well-been documented and regulated, bottom-side dents, deemed acceptable per regulations, were found to be susceptible to stress corrosion and fatigue cracking. This investigation explored multiple and fundamental aspects of cracking in steel pipe dents, including nondestructive inspection, electron microscopy, finite element modeling, and long-term cyclic loading tests to characterize the cause of this pipeline accident.     

热轧环件开裂失效分析

某型号轧环在旋压时产生批次性开裂.对开裂的轧环进行了理化检验和工艺复查,认为开裂是由锻造缺陷引起的,锻造缺陷在旋压过程中发生开裂.在锻造过程中采取了有针对性的工艺措施,在后续生产中再未发生开裂现象.     

一种氦质谱检漏用双流道喷枪的设计及实验

为解决普通喷枪在检漏时对漏孔定位能力低的问题,设计了一种钟罩式双流道喷枪,并对其开展了测试实验.结果表明:喷氦法检漏时,利用钟罩式双流道喷枪可分辨出相距15mm的相邻漏孔,对单一漏孔定位范围为Φ20 mm,较大的提高了喷氦法检漏时对漏孔的定位能力.     

A structural comparison of electroless and electroplated nickel

Nickel that has been electrolessly deposited from a low temperature hypophosphite bath undergoes severe cracking when heat treated. The cracks, which are of two different types, can weaken the nickel deposit and can lead to leak paths in composite structures. In the study reported here the characteristics of thin nickel films were studied by several methods, all of which can be applied to any crystalline thin film: an X-ray method for stress and precipitation sequences; electron microscopy; cross-sectional analysis; kinetic studies. The electroless nickel was compared with a low stress electroplated nickel which does not crack with heat treatment. The electroplated nickel was found not only to have lower stress than the electroless nickel but also to have no significant secondary phase precipitation.     

Cracking of aluminium alloy valve body used in attitude control of satellite launch vehicle – A metallurgical investigation

Aluminium alloys emerged as preferred material for aerospace application, due to their high specific strength, high specific stiffness and ease for fabrication, Al–4.5Zn–1.5Mg aluminium alloy was used for the fabrication of a valve body for an attitude control system of satellite launch vehicles. During one of the functional qualification test, while charging the gaseous medium, a pressure drop was noticed. This was attributed to a leak through a crack opening of the valve body. Detailed metallurgical analysis indicated that the failure was due to stress corrosion cracking (SCC).This paper brings out the metallurgical analysis carried out and remedial measures suggested.     

Investigation of a Boeing 747 wing main landing gear trunnion failure

A loud noise was heard from the vicinity of the port wing landing gear during pushback of a Boeing 747-300 from the terminal at Sydney (Australia) airport. Inspection showed that one of the wing landing gear trunnion fork assemblies had failed. Detailed investigation revealed that the trunnion had failed by fatigue cracking. Deep machining grooves were found at the root of an internal radius that had not been shot-peened as required, and a chemical surface process during manufacture had resulted in shallow intergranular attack at the bottom of these grooves. It is probable that the critical cracking started from some of these grooves. In addition, the wall thickness at the failure location was significantly less than the minimum required in the drawings.Since the deep machining grooves, the lack of peening and the intergranular attack were all consequences of manufacturing, the fatigue cracking probably started shortly after the component entered service. This implies that fatigue cracking was present during all the trunnion overhauls, but was not detected by non-destructive inspections during the overhauls. Quantitative fractography was used to produce a crack growth curve based on fracture surface markings thought to represent the overhaul timings. The crack growth curve suggested that the fatigue cracking was large enough to be detected by inspection during the last overhaul, if not the one before. However, it was probably not easy to detect the cracking. This investigation therefore highlights the difficulties that can be encountered when inspection is the last (or only) line of defence against failure owing to unexpected manufacturing deficiencies.     

Microstructural and thermo-mechanical analysis of quench cracking during the production of bainitic–martensitic railway wheels

Quench cracking during the production of newly developed low carbon bainitic–martensitic (LCBM) rail wheels was investigated using a microstructural and thermo-mechanical Finite Element (FE) model. The stresses associated with quench cracking during martensite phase transformation were predicted under various quenching conditions for two different grades of LCBM steels with different kinetics of martensite phase transformation. The FE analyses showed that the likelihood of quench cracking can be reduced by using a low coolant spray intensity since the internal stresses generated during the martensitic phase transformation were found to be below the steel’s flow stress. The internal stresses were predicted to be even lower with a low carbon grade LCBM steel. The microstructural and thermo-mechanical model has been used to determine favourable quenching conditions that have the potential to reduce the propensity of quench cracking during the production of LCBM railway wheels.     

Quench crack behavior of nickel-base disk superalloys

There is a need to increase the temperature capability of superalloy turbine disks to allow higher operating temperatures in advanced aircraft engines. When modifying processing and chemistry of disk alloys to achieve this capability, it is important to preserve the ability to use rapid cooling during supersolvus heat treatments to achieve coarse grain, fineγ′ microstructures. An important step in this effort is an understanding of the key variables controlling the cracking tendencies of nickel-base disk alloys during quenching from supersolvus heat treatments. The objective of this study was to investigate the quench cracking tendencies of several advanced disk superalloys during simulated heat treatments. Miniature disk specimens were rapidly quenched after solution heat treatments. The responses and failure modes were compared and related to the quench cracking tendencies of actual disk forgings. Cracking along grain boundaries was generally observed to be operative. For the alloys examined in this study, the solution temperature, not alloy chemistry, was found to be the primary factor controlling quench cracking. Alloys with high solvus temperatures show greater tendency for quench cracking.     

Stress Assisted Corrosion of Waterwall Tubes in Recovery Boiler Tubes: Failure Analysis

Waterside cracking of carbon-steel boiler tubes is one of the major safety and efficiency concerns in kraft recovery boilers in the pulp and paper industry, because any water leak into the furnace could cause a smelt-water explosion in the boiler. Failed carbon-steel boiler tubes from different kraft recovery boilers were examined to understand the role of carbon-steel microstructure on crack initiation and crack morphology. A number of carbon-steel tubes showed a deep decarburized layer on the inner surface (water-touched) and also an unusually large grain size at the inner tube surface. In some boiler tubes, cracks were found to initiate in areas with large-grained-decarburized microstructure. However, tubes without such microstructure were also found to have stress assisted corrosion (SAC) cracks. It was found that the decarburization and large grained microstructure may facilitate initiation and growth, but it is not necessary for SAC of carbon-steel boiler tubes.