Investigation into recurring military helicopter landing gear failure

The skid landing gear of a military helicopter failed while the helicopter was moored on the ground at the airport. The rear cross tube of the landing gear assembly was found fractured into two separated pieces. The fracture occurred in the right flange of the rear cross tube, where a connection with the spring tube is established using a clamp. The initial visual inspection of the fracture zone revealed a presence of heavy corrosion and significant damage of the anti-corrosion protective layer on the outer surface of the flange of the rear cross tube. Fractographic analysis highlighted corrosion as the main cause of the failure. Evidence was found to show that the fracture was initiated from corrosion pits located on the exterior, underside of the cylindrical part of the flange. Metallographic examination discovered corrosion pits with micro cracks and multiple branched secondary cracks in the crack origin area, indicating the occurrence of stress corrosion cracking. Chemical analysis of the corrosion deposits showed the presence of sodium, chlorine, calcium and sulfur. The stress analysis of the helicopter landing gear assembly, carried out by means of finite element method, confirmed that the crack origin was located at the area with the maximum tensile stress in the flange.     

Failure investigation of 321 stainless steel pipe to flange weld joint

Failure investigation was conducted on a refinery pipe-to-flange weld joint that suffered cracking. Both the pipe and flange are made of AISI 321 stainless steel. The flange was circumferentially welded to the pipe which is seam welded. The investigation revealed that both the circumferential and seam welds were in sound conditions, namely no evidences of sensitization, lack of weld penetration, and voids or porosities. Thus, welding practices were not suspected to be the cause of failure. The failure of the weld joint was found to have started at the δ-ferrite phase in the flange material and propagated through the circumferential and seam welds. The failure mode was concluded to be chloride stress corrosion cracking synergized by the presence of H₂S. The presence of corrosive compounds in the refinery stream and the residual stresses at the weld joint triggered active anodic dissolution of the δ-ferrite precipitates, resulting in cracking of the material.     

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.     

Corrosion Induced Explosion of a High-Pressure Fire-Extinguishing Gas Cylinder

The failure of a high-pressure fire-extinguishing cylinder was investigated. Failure was induced by internal surface corrosion and stress corrosion cracking (SCC) due to condensation of carbonic acid. In internal surface, especially the area near the bottom of the exploded cylinder, severe corrosion was characterized by local pits. SCC initiated from these local corrosion pits was observed by metallurgical analysis. Microstructure of the failure cylinder near the internal surface consisted of multiple-banded structure and the banded structure could accelerate local corrosion initiation and propagation. The corrosion products built up on the fracture surface were primarily ferrous carbonate (FeCO₃). The determination of moisture in fire-extinguishing gas was also examined.     

110钢级φ88.9mm×6.45mm超级13Cr钢油管刺穿失效分析

采用宏观分析、化学成分分析、力学性能测试、金相检验及扫描电镜断口分析等手段,对某油田一根规格为φ88.9mm×6.45mm的110钢级超级13Cr马氏体不锈钢油管的刺穿失效原因进行了分析。结果表明:油管失效的实质是油管发生了氯离子应力腐蚀开裂,裂纹起源于外壁腐蚀坑底部,从外壁向内壁扩展,直到穿透壁厚,形成刺穿通道,高压流体从内向外刺出并在随后的过程中形成了刺穿孔洞。     

Fracture analysis on the 4th compressor disc of some engine

Some plane was caused fire and exploded because of the fracture of the 4th disc of the engine. In this paper, in order to give the fracture mechanism and causes of the 4th disc some observation and analysis were made, such as the visual inspection of some debris of the 4th disc, the macroscopic and microscopic observation of the fracture surface on the flange and web of the 4th disc. The corrosion pits on the web surface and at the edge of the fracture surface were analyzed by energy spectrum. The beach marks on the fracture surface were measured. The material quality of the web was also analyzed. The vibration peculiarity of the engine was tested and calculated. The fracture mechanism of the 4th compressor disc is high cycle fatigue. The microstructure, hardness, chemical composition and the web thickness of the fracture site all meet with technical requirements. The fatigue failure has no connection with the material quality. The corrosion pits at the R3 arc transition between the web and the flange induced the accident. The cracking and propagation of the 4th compressor disc are mainly related to the vibration stresses.     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

Failure Analysis: Sulfide Stress Corrosion Cracking and Hydrogen-Induced Cracking of A216-WCC Wellhead Flow Control Valve Body

The wellhead flow control valve bodies which are the focal point of this failure case study were installed in some of the upstream facilities of Khangiran’s sour gas wells. These valve bodies have been operating satisfactorily for 3 years in wet H₂S environment before some pits and cracks were detected in all of them during the periodical technical inspections. One failed valve body was investigated by chemical and microstructural analytical techniques to find out the failure cause and provide preventive measures. The valve body alloy was A216-WCC cast carbon steel. During investigation many cracks were observed on the inner surface of the valve body grown from the surface pits. The results indicate that flow control valve body failed due to combination of hydrogen-induced corrosion cracking and sulfide stress corrosion cracking. According to HIC and SSC laboratory tests and also with regard to cost of engineering materials, it was evident that the best alternative for the valve body alloy is A217-WC9 cast Cr–Mo steel.     

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.     

A metallurgical examination of failed zirconium alloy autoclaves

Ten autoclaves, made from Zircadyne 705® (a Zr-2.5wt.% Nb alloy), failed in a neutral brine environment during tests designed to probe the mineral reactions occurring in thermally enhanced oil recovery techniques. Two of the failed autoclaves were examined metallurgically to determine the cause(s) of failure. Failure of the autoclaves was shown to be by a process of delayed hydride cracking that initiated at corrosion pits. It was believed that the presence of ferric chloride and/or potassium compounds in the corrosive environment was responsible for the pitting corrosion. Suggestions were made as to alternate materials for the construction of the autoclaves.     

Corrosion fatigue cracking of AISI 316L stainless steel screen mesh

This paper describes the failure of stainless steel screen mesh grade AISI 316L after being in service for only 8 months. The characterization methods included visual examination, optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and metallography. The results showed that the screen mesh failed by pitting corrosion and subsequent corrosion fatigue cracking. Pitting was initiated by the attack of chlorides from PVC powder and service environment as well as the action of excessive wear. Subsequent corrosion fatigue cracking arose from the presence of chlorides along with the residual and cyclic stress concentration at the pits. Failure prevention can be achieved by annealing after wire drawing, periodic surface cleaning, and proper material selection.     

Failure Analysis of Steam Turbine Rotor Disk

This article presents an investigation into causes of failure of rotor disk of an 8.25-MW capacity steam turbine, which failed catastrophically. Four pieces of the rotor disk detached from the tenth-stage disk of the turbine rotor during this failure. Visual inspection, chemical analysis, macro- and microscopic analysis of the failed rotor disk, analysis of the operational data, and the history of the rotor operation indicated that the failure could be attributed to stress concentrations at macropores and regions of segregation in the disk. Stress corrosion cracking (SCC) subsequently took place in the regions of stress concentration because the steam in the turbine contained chloride and potassium. The SCC produced a network of cracks associated with the macro- and microporosities. It was recommended that the disk fabrication processes ensure a high-quality microstructure and that operational monitoring of the composition of steam be initiated to ensure that the chlorides and potassium concentrations are maintained below a specified level.     

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.     

Intergranular stress corrosion cracking of acoustic-filter hooks used in car

Some acoustic-filter hooks used in car fractured in service. The failure mechanism of the hooks is intergranular stress corrosion cracking. The necessary conditions (susceptible composition of the hook materials; presence of chloride in service environment; presence of residual tensile stress and occurrence of sensitization in the fracture region) caused intergranular stress corrosion mechanism. Non-qualified chemical composition of the hooks (much higher carbon content than the specified of AISI 304 stainless steel) is mainly responsible for the failure of the hooks, which is a key inherent factor.     

The effect of pit size and density on the fatigue behaviour of a pre‐corroded martensitic stainless steel

UNS S17400 steel is used in turbines for the aerospace and utility industries. While it is generally corrosion resistant, it is susceptible to pitting when exposed to aqueous chloride environments. Effects of pitting characteristics, such as depth, width, and local density on fatigue life, have been studied in this work to better inform criteria for component replacement or repair. While pit depth correlates well with cracking, the deepest pit never initiated the crack that ultimately led to failure. The clustering of pits, or local pitting density, also correlated well with crack initiation location; however, the densest region of pitting was not always the location where cracking occurred. There is likely no single metric that directly correlates pitting with fatigue cracking, rather there is a combination of pitting characteristics that ultimately lead to cracking. The results from this work suggest that pit depth and local pitting density are among the more important metrics.     

Boiler Stack Economizer Tube Failure

A metallurgical evaluation was performed to investigate the failure of a type 304 stainless steel tube from a boiler stack economizer. The tube had three distinct degradation mechanisms: pitting corrosion, chloride stress corrosion cracking, and fatigue fracture. The primary failure mechanism for the tube was fatigue fracture, but the other mechanisms may have eventually caused a tube failure in the absence of fatigue. This paper details the visual, SEM/EDS, and metallographic examinations used to determine that these failure mechanisms were each present in the same tube.     

金属波纹管补偿器的应力腐蚀开裂分析

采用宏观及微观断口分析、金相检验与化学成分检测等方法,对某铬一镍奥氏体不锈钢波纹管补偿器的腐蚀开裂失效原因进行了分析。结果表明：由于该铬一镍奥氏体不锈钢波纹管在氯离子含量超标的环境中服役,并承受来自于波纹管自身加工变形过程中形成的残余应力、工作应力以及装配应力,最终导致波纹管补偿器发生了由表及里的应力腐蚀开裂。     

Failure analysis: Chloride stress corrosion cracking of AISI 316 stainless steel downhole pressure memory gauge cover

The downhole pressure memory gauge cover which is the focal point of this failure case study was hung in the production tubing of a sour gas well at a depth of 3200 m during a multiple rate productivity index test. Approximately 140 h after start up, the tool cover failed. The memory gauge cover experienced repeated premature failures. One failed sample was investigated by chemical and microstructural analytical techniques to find out the failure cause and provide preventive measures. The gauge cover alloy was type AISI 316 stainless steel. During investigation many branched cracks were observed on the external surface of the gauge cover grown from the pits perpendicular to the cylinder axis. The results indicate that pressure memory gauge cover failed due to chloride stress corrosion cracking (CLSCC). Also according to slow strain rate test, it was evident that the best alternative for the downhole pressure memory gauge cover alloy is Nitronic 50 and the second option is 17–4 PH stainless steel.     

Marine Atmospheric SCC of Unsensitized Stainless Steel Rock Climbing Protection

A failure investigation into the root cause of fixed austenitic stainless steel climbing anchor hardware in tropical marine climates has been presented. The incident 316L climbing anchor was fixed in a seaside limestone cliff in southern Thailand and underwent transgranular chloride stress corrosion cracking (TGSCC) after 10 years of service. Since stainless steel does not normally undergo stress corrosion cracking (SCC) at ambient temperatures, the conditions known to promote ambient temperature TGSCC of austenitic stainless steel are reviewed. A mechanism that may give rise to TGSCC in limestone climbing anchors in warm marine environments is postulated.     

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。