Corrosion Failure of an In-service Economizer Tube

This investigation was primarily aimed to examine the in-service failure of an economizer tube. Apart from preliminary visual examination, the analysis for the probable cause of failure was supplemented with radiographic examination, metallographic examination, energy dispersive spectroscopy, and scanning electron microscopy. Finally, it was concluded that there was seepage of water from an unusual blowhole-like through-thickness hole in the weld region of the economizer tube. This water vaporized and reacted with the flue gases and caused localized corrosion and led to the ultimate failure of the tube.     

Failure of a Secondary Superheater Tube in a 140-MW Thermal Power Plant

This investigation was conducted to determine the cause of failure of a welded joint of a secondary superheater tube from a 140-MW thermal power plant. Chemical analyses along with detailed optical microscopic examination of a secondary superheater tube were carried out to predict the probable cause of failure. Microstructure of the secondary superheater tube welded between austenitic stainless steel to low alloy steel revealed presence of a thin layer of metal carbide along the weld interface which eventually led to intergranular cracking at austenite grain boundaries. It was concluded that the formation of brittle carbide layer was due to migration of carbon at elevated temperature led to failure of the tube.     

Structural changes of radial forging die surface during service under thermo-mechanical fatigue

Radial forging is one of the modern open die forging techniques and has a wide application in producing machine parts. During operation at high temperatures, severe temperature change associated with mechanical loads and the resultant wearing of the die surface lead to intense variation in strain on the die surface. Therefore, under this operating condition, thermo-mechanical fatigue (TMF) occurs on the surface of the radial forging die. TMF decreases the life of the die severely. In the present research, different layers were deposited on a 1.2714 steel die by SMAW and GTAW, with a weld wire of UDIMET 520. The microstructure of the radial forging die surface was investigated during welding and service using an optical microscope and scanning electron microscope. The results revealed that, after welding, the structure of the radial forging die surface includes the γ matrix with a homogeneous distribution of fine semi-spherical carbides. The weld structure consisted mostly of columnar dendrites with low grain boundaries. Also, microstructural investigation of the die surface during operation showed that the weld structure of the die surface has remained without any considerable change. Only dendrites were deformed and broken. Moreover, grain boundaries of the dendrites were revealed during service.     

Failure analysis of a natural gas pipeline

In the present work, the failure investigation of a 30 in. diameter gas transmission pipeline (API 5L X-60 grade steel) has been described. The failure was due to a longitudinal crack developing in the centerline of longitudinal weld joint. Mechanisms and morphology of crack initiation and propagation were studied through different tests including: thickness measurement, chemical composition analysis, metallographic inspection, mechanical property testing, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The experiments resulted to the conclusion that some factors associated with Sulphide Stress Cracking (SSC) and metallurgical defects cause the failure of weld joint pipe. Detailed examination revealed that these factors are inappropriate welding parameters, pitting corrosion on longitudinal weld, and hydrogen permeation to the weld metal.     

Failure analysis of an open die forging drop hammer

This paper analyses the failure of a large open die forging drop hammer. The analysis had to be semi-destructive as it was wanted to repair the equipment and to return it into service. A visual examination was performed at the forging shop and fatigue initiated at two internal defects blamed for the failure. A small sample was extracted from the fracture surface. Unfortunately, due to the desire of recovering the drop hammer this sample did not belong to those zones which were identified to be the origins of the failure but to the zone of progression of the crack by fatigue close to the overload final failure. Fractographic analysis by scanning electron microscopy revealed the presence of some ductile dimples pointing to an acceleration of the cracking process at this stage.     

Root cause analysis for 316L stainless steel tube leakages

Perforation of ASTM A270 TP316L stainless steel tube, used for transportation of ozonated high purity water in a pharmaceutical plant, was discovered after 3 months in actual service. The current investigation was conducted in order to explore the root causes of failure. Various techniques including on-site investigation, emission spectroscopy, ion chromatography, radioscopy, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and intergranular corrosion testing were implemented for failure analysis of the tube components. The results revealed that the perforation of tube was initiated from the outer wall and extended to the inner wall by pitting corrosion. The stagnant state of chloride-containing water was the main reason for inducing such corrosion attack. The weld metal was the most susceptible to corrosion attack leading to perforation of the wall thickness, although initiation sites of pitting corrosion were also observed in the base metal. The dimensions of each pit mouth are very small, but enlarged subsurface cavities were observed. The selective dissolution of material due to galvanic effects between delta-ferrite and the austenite matrix occurred in the weld zone. It is suggested that failure prevention could be achieved by controlling the quality of the insulation system. In addition, careful control of welding conditions must be implemented during fabrication.     

Stress corrosion cracking of a circulation water heater tubesheet

After only three years' service of a circulation water heater, tubesheet showed sever leakage and led the plant to emergency shutdown. On-site investigation revealed extensive cracking initiated at weld area and through the tubesheet holes. Samples from failed area were removed and investigated. The chemical and micro structural properties were analyzed by optical and scanning electron microscopy. Micro structural observations have shown intergranular and transgranular crack growth. Sulfur was detected in corrosion products by EDS examinations. Sulfur compounds and chromium carbides also were detected by XRD analyses. Wet carbonaceous deposits were found on surfaces of the tube sheet. Analysis on the deposits depicted presence of caustic and chlorides. Considering all aspects of failures led to this conclusion that the material was in sensitized condition possibly due to overheating at the deposit sites. The sensitized material failed due to the acronym SCC induced by polythionic acid. Presence of chlorides and caustic aid the failures.     

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.     

Failure of evaporator tubes initiated by lamellar tearing during the commissioning of a waste heat recovery boiler

Successive failures of several new evaporator tubes during commissioning and trial run of a waste heat recovery boiler has been analyzed. The evaporator tubes are cold bent into U-shape and subsequently TIG welded to a 24-mm thick wear plate on either sides of the tube immediately adjacent to the bend. The failed samples showed stepped longitudinal cracks along the rolling direction and parallel to the weld fusion boundary at the fireside surface of the inner bend of the tube, prominently at the center of the bend. It was found that the failure of the evaporator tubes at the tube bends was initiated by lamellar tearing because of inherent defects in the material (segregation leading to banded structure), improper processing of material (inclusions with high aspect ratios along the rolling direction) and accelerated by high transverse shrinkage stresses over the entire bend portion of the tube introduced by the weld. The pre-existing fine cracks in the welded evaporator tube initiated by lamellar tearing have subsequently opened up during service when the total strain in the region increased because of steam pressure resulting in catastrophic failure of the tubes. Contrary to expectation, many of the elongated inclusions were found to be iron oxide scales and only few were manganese sulfide stringers enveloped by iron oxide scales. The iron oxide inclusions were resolved from the manganese sulfide by scanning electron microscopy and EDS spot analysis of the inclusions.     

Failure of Welded Radiator Nipples Caused by Nonmetallic Inclusions

A splitting phenomenon in the weld zone of the radiator nipples was encountered after a threading operation. During the failure analysis, some oxide traces were macroscopically observed on the surfaces of unthreaded nipples. Subsequent scanning electron microscopy–x-ray spectrometer (SEM–EDS) examination through the surfaces of the unthreaded nipples and as-pickled steel sheet coupons that were used for nipple manufacturing revealed that the oxide traces were hot-rolling scale residuals. However, none of the traces of scale residuals were encountered on the cross sections of weld zone of unthreaded nipple samples. Contrarily, some cracks and inclusion residuals were observed through the interface between the protrusion and weld zone. EDS analysis and optical microscopy affirmed the presence of MnS-type inclusions. The same kind of inclusions was also observed when one of the unthreaded nipples was intentionally separated through the weld zone. In conclusion, the main cause for the failure was determined to be the adverse effect that the MnS inclusions in the steel had on the material ductility.     

Fatigue Failure of 321 Stainless Steel Superheater Tubes from a Thermal Incinerator

Several boiler superheater tubes showed circumferential cracking at weld seams after 2 years in noncontinuous service (several shutdowns). On-site inspection revealed that several tubes were cracked and leaked; while many others were cracked, however, the severity was less pronounced. Two types of superheater tubes samples were collected: one with butt-welded tubes and the other with fillet-welded sleeve. The latter was found to be out of the boiler fireplace, and the sleeve was used as tubing support to the boiler shell. Detailed investigation showed that the butt-welded tubes contained circumferential fatigue cracks that initiated from the internal surface. The cracks initiated in the heat-affected zone and propagated as a result of tube vibration. The variations in the tube internal diameter and tube wall thickness are expected to play a role in tube fatigue failure. On the other hand, tubes with fillet-weld sleeve showed circumferential cracking as a result of fatigue crack initiation from weld defects on the tube external surface. The high vibration during several unscheduled shutdowns in addition to several other factors such as variations in tube inside diameter, wall thickness, and weld defects resulted in the initiation and propagation of fatigue cracks and premature failure. White deposits, similar to those observed when boiler tubes failed by caustic exposure, were seen in the vicinity of the tube cracks. Therefore, it was difficult to confirm whether the boiler tubes failed because of the fatigue cracks or because of the caustic salts (pH control chemical).     

高强度渗碳钢制构件的断裂分析

利用化学分析、金相检验、扫描电镜以及电子探针微区分析等方法对高强度20CrNiMo渗碳钢制矿岩牙轮钻头和汽车发动机活塞销的断裂原因进行了分析。结果表明，两构件的断裂是由于锻轧温度过高，引起硫化物在晶界上的再析出所造成的。指出了要重视这类渗碳钢的缺口敏感性及对钢中非金属夹杂物的检验方法。     

Fatigue failure of a fillet welded nozzle joint

A fillet welded joint in a nozzle of a storage tank, made of AISI 304L stainless steel (SS), had failed very close to the weld fusion line during transportation. Visual examination of the weld showed deposition of excessive weld metal in the joint. Scanning electron microscope observation of the fracture surface revealed beach and ratchet marks on the fracture surfaces. The observation of in-situ metallography of the heat-affected zone (HAZ) anticipated sensitisation of the HAZ. Bending of the nozzle to 15° anticipated the final failure, which occurred due to overloading during transportation.     

Cracking of AFNOR 7020 aluminium alloy component: A metallurgical investigation

Aluminium alloy of Al–4.5Zn–1.5Mg system, belonging to age hardenable, high strength category is being used for the fabrication of various components. The Al alloy component was part of a conduit line for filling the liquid chemical to its storage tank. The component consisted of an extruded tube, TIG welded to a bulb, which was fabricated from Al alloy forgings. A crack was observed on the conduit tube at the region where tube was welded to the bulb. Detailed investigations were carried out on the cracked component, which revealed insufficient working of the ingot during forging, resulting in remnant cast structure along with a heavy network of low melting point compound throughout the material. The cracks, which were initiated under the stresses induced during thermal treatment, machining and assembly, were found propagating through the eutectic network.

This paper highlights the investigations carried out on the failed components.     

Metallography of a pulsed Nd: YAG laser weld in a RS/PM Al---8Fe---2Mo alloy

The microstructure of a pulsed Nd:YAG laser weld in a rapid solidification/powder metallurgy (RS/PM) Al-8.0 wt.% Fe-2.3 wt.% Mo (Al---8Fe---2Mo) alloy was investigated using light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. This analysis revealed significant microstructure variations across the weld fusion zone (FZ). Near the fusion boundary, a light-etching FZ microstructure was observed to contain submicron dispersoids “entrapped” in a matrix of fine-sized dendritic alpha aluminum. At the center of the FZ, the presence of relatively coarse-sized intermetallic particles that served as growth centers for coarser dendritic alpha aluminum promoted a dark-etching microstructure. In the boundary between successive melt zones, both a heat-affected zone (HAZ) containing acicular dispersoids and a fusion boundary region (FBR) containing irregular-shaped particles in a coarse-grained dendritic alpha aluminum matrix were observed. A HAZ comprised of slightly coarsened dispersoids was also observed in the base metal bounding the FZ. Knoop hardness traverses across the FZ indicated a maximum hardness of KHN 260 in the light-etching FZ and a minimum hardness of KHN 135 in the HAZ between successive melt zones.     

Analysis of local microstructure and hardness of 13 mm gauge 2024-T351 AA friction stir welds

Abstract

The friction stir welding process has been used to join 13 mm gauge 2024-T351 aluminium alloy plates together. A detailed microstructural study of the resulting weld was carried out using differential scanning calorimetry, hardness testing, scanning electron microscopy and electron backscatter diffraction. Differential scanning calorimetry was used to explain the hardness results at a number of regions across the weld in terms of co-cluster dissolution and reformation and S phase formation, coarsening and dissolution. The 'onion rings' structure found in the nugget weld was shown to be the result of a combination of the slight grain size variations and a change in nature and size of the particles present, i.e. intragranular v. intergranular. The variation in corrosion properties and hardness of the rings is discussed in terms of the local microstructure and quench sensitivities.     

Microstructural and failure analysis of welded primary reformer furnace tube made of HP-Nb micro alloyed heat resistant steel

In this paper, the failure of the reformer furnace tube, made of HP-Nb micro alloyed heat resistant steel is investigated. The failure was due to a longitudinal crack developing in the circumferential weld joint close to the outlet end of the tube, where the mean wall temperature had reached the highest value during service. According to the temperature records during the period of 7 years of being in service, the tube had experienced temperatures higher than the tubes designed temperature. This situation besides the occurrence of several sudden shutdowns caused the mechanical properties degradation.Macroscopic and microscopic examinations revealed that the failure had occurred due to the extensive fissuring initiated and developed from the heat affected zone (HAZ) adjacent to the root weld. The results of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) analysis, and the crack growth rate calculations indicated that the crack propagation rate was controlled by the overheating and the secondary thermal stress due to the sudden shutdowns.Moreover, the ductility at the tip of the crack had dwindled due to the nitrogen uptake at high temperature, which promoted the crack growth rate. Quantitative microstructural analysis performed at the uncracked and cracked sides of the weld joint showed that the area fraction of HAZ creep cavities were 0.3% and 0.7%, respectively, which were interpreted in turn as the accumulated creep damage level D and E. Finally, it was concluded that the creep cavities alignment and fissuring that took place at HAZ are the main causes of the cracking and failure of the tube.     

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

Abstract

A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrite. The volume fraction of ferrite and its contiguity ratio in the rolled microstructure were greater than in the forged grade. The rolled microstructure exhibited a better combination of strength and toughness than that of the forged material. The rolled steel work hardened more than the forged variety owing to its fine, uniform (bainite–martensite and ferrite) microstructure. Retained austenite present in these steels underwent a strain induced transformation to martensite during tensile deformation. The J–C analysis of the work hardening rates revealed typical three stage behaviour in both varieties during tensile deformation.     

Effect of weld morphology on mechanical response and failure of friction stir welds in a naturally aged aluminium alloy

Friction stir butt welds in 6063-T4 aluminium alloy were obtained using square and two tapered tool pin profiles. Tensile tests at 0°, 45°, and 90° to the weld line, hardness contours in the weld cross-section, temperatures in the heat affected zones, cross-sectional macrographs, transmission electron micrographs, and X-ray diffraction studies were used to characterize the welds. In transverse weld specimen, tunnel defects appearing at higher weld speeds for tapered pin profiles, were found to result in mechanical instabilities, i.e. sharp drops in load–displacement curves, much before macroscopic necking occured. Further, in comparison to the base metal, a marked reduction in ductility was observed even in transverse specimen with defect free welds. Hardness contours in the weld cross-section suggest that loss in ductility is due to significant softening in heat affected zone on the retreating side. Transmission electron microscopy images demonstrate that while recovery and overaging are responsible for softening in the heat affected zone, grain size refinement from dynamic recrystallization is responsible for strengthening of the weld nugget zone. X-ray diffraction studies in the three weld zones: weld nugget zone, heat affected zone, and the base metal corroborate these findings. A weld zone model, for use in forming simulations on friction stir welded plates of naturally aged aluminium alloys, was proposed based on mechanical characterization tests. The model was validated using finite element analysis.     

Failure Analysis of AISI-304 Stainless Steel Styrene Storage Tank

This paper presents the failure analysis of AISI-304 stainless steel tank that was fabricated by welding and used for the storage of styrene monomers. After about 13 years of satisfactory operation, significant cracking was observed adjacent to the weld joints and in base plate near tank foundation. Weld repair was by shielded gas arc welding using AISI 308 stainless steel filler wire. The failed base plate was replaced with the new AISI 304 base plate of same thickness. After a short period of time, seepage was observed along the weld bead. Upon nondestructive testing cracks were found in the heat-affected zone and in the base plate. The failure investigation was carried out on welded and base plate samples using spectroscopy, optical and scanning electron microscopy, fractography, SEM–EDS analysis, microhardness measurements, tensile and impact testing. The results revealed transgranular cracks in the HAZ and base plate, and the failure was attributed due to stress corrosion cracking. Cracks initiated as a result of combined action of stresses developed during welding and the presence of a chloride containing environment due to seawater. It was further observed that improper welding parameters were employed for weld repair which resulted in sensitization of the structure and postweld heat treatment to remove weld sensitization and minimize the residual stresses was not done.