Metallurgical Examination Research Articles

Inspection of internal erosion-corrosion of elbow pipe in the desalination station

Due to the worldwide decrease in the quantity of potable water in the last decade, the desalination station is among one of the most important methods to treat seawater to supply the dry regions, Sahara regions and the offshore regions with purified water. During the desalination process, many problems can appear including the corrosion problems that attribute to the materials of piping systems and theirs geometrical shapes, and the chemical composition of water taking into account the conditions of transporting. Pipe elbow is one of the necessary parts to build the integrated piping system which is considered as a linking point between different parts of the transport systems. Because of its effective role and complex forms that based mainly on welding processes, the pipe elbow is vulnerable to corrosion problems considering the liquid nature of the effluent, and the surrounding pressure and temperature. The results of the mechanical characterizations tests such as metallurgical examinations using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF) allowed us to analyze the main causes of the defect in this part. The obtained results showed that many technical solutions can be employed to mitigate and relief the encountered internal corrosion in pipe elbow, which helps greatly in enhancing the energy development in desalination industry.

An investigation of corrosion and sedimentation in the air cooler tubes of benzene drying column in linear alkyl benzene production plant

In this study the corrosion, sedimentation and subsequently choking in the air cooler tubes of benzene drying section in Bistoun Petrochemical Company, at Iran were investigated. The main reasons of failure were analyzed using standard failure analysis techniques such as visual checking, chemical and mechanical characterizations, metallurgical examinations employing optical microscopy in combination with scanning electron microscopy, X-ray fluorescence and X-ray diffraction analysis. The chemical analysis was used to determine the corrosion products. The accumulation of sediment and under-deposit corrosion has resulted in localized corrosion and perforation in the inner surface of the tubes. In addition, the erosion–corrosion accelerated the destruction of tubes. The microbial corrosion can happen because of high water content. The results showed that a combination of corrosion types has occurred. A new idea was proposed for the separation mechanism of low concentration of water from benzene in the process. Finally, the suggestions have been proposed for trouble shooting, preventing corrosion and clogging in the tubes of air cooler and subsequently avoiding production stop.

Analysis of Weld Cold Cracking In Excavator Bucket Adapter

This paper presents a failure analysis on welding of bucket adapter for an excavator that made of low carbon steel with Ideal Diameter (DI) value 341 and Carbon Equivalent 0.73. The bucket component is stated OK as the final inspection, but four months later found cracked and even it has not been used yet. Adapter mounted to bucket through CO2 gas shielded Gas Metal Arc Welding (GMAW) process, filler metal MG-50 (AWS A5.18 ER70S-G), pre-heating and post-heating. Crack occurred on the boundary of weld metal and adapter casting. The metallurgical examination was conducted in the crack origin and the adapter casting. Nondestructive test (NDT) covered visual and magnetic test (MT), while destructive test (DT) covered fractography, mechanical properties, and Electron Probe Micro Analyzer (EPMA). The investigation resulted in that crack initiated from slag inclusion in the weld metal due to insufficient cleaning on weld joint. The higher hardness of martensite structure in the heat affected zone (HAZ) of adapter indicates high cooling rate that gen erates high residual stress. High stress that caused by insufficient post heating triggered severe crack propagation. By its characteristic, crack is called the cold cracking, delayed cracking, or hydrogen-induced cracking (HIC).

Failure Analysis on Leakage of Brass Condenser Tube in Thermal Power Plant

The corrosion failure analysis of the brass tube in a power plant condenser was conducted by the methods of chemical composition analysis, macro morphology inspection of the leakage point, metallurgical examination, scanning electron microscope and energy spectrum analysis etc. The results indicated that the failure of the brass tube was due to the erosion of the internal surface. The zinc content in the etching pits decreased obviously and dezincification corrosion could be classified. Some suggestions for anti-corrosion measures of the brass tubes were put forward.

Influence of WC–20Co–1Ni coating by HVOF on lifespan of the downhole drilling motors

ABSTRACTDownhole drilling motor is one of the equipment used for drilling and they perform the task of directional drilling. The rotor of these motors is made of 17-4 PH steel, which is generally coated with hard chromium. In this study, the feasibility of replacing hard chromium with WC–20Co–1Ni using a high velocity oxygen fuel system was examined. The result of the metallurgical and mechanical examination showed that changing of the coating to WC–20Co–1Ni improved the wear performance and increased the lifespan of the rotor. In addition, the chemical resistance to corrosion due to the influence of the corrosive gases in the hole on the rotor improved significantly. Finally, substitution of the hard chromium coating with the WC–20Co–1Ni caused an increase in the lifespan and performance of the downhole motor in extreme environmental conditions of the drilling formations, which led to economic savings in amendment and maintenance costs.

Transient Liquid Phase Bonding of Al-6063 to Steel Alloy UNS S32304

Transient liquid phase (TLP) bonding of 6063 aluminum alloy (Al-6063) and duplex alloy 2304 stainless steel (UNS S32304) was performed using copper foil as an interlayer between the base metals. A compression load was applied normal to the specimens. Metallurgical examination of the produced joints showed three distinct regions including a reaction zone, diffusion affected zone, and the base metals. The diffusion of copper into aluminum resulted in an Al–Cu eutectic structure. However, the oxide layer on the aluminum surface controlled the dissolution behavior of copper and the extent of its wettability with the base metals. Although voids and intermetallic compounds were detected at the interfaces of the processed joints, a defect free joint was produced at 570 °C. In addition, the results from corrosion tests showed that the use of copper as an interlayer decreased the corrosion resistance of the joints. However, increase in thickness of the joining reaction zone with increasing bonding temperature was observed to increase corrosion resistance.

On the damage mechanisms in a continuous casting mold: After-service material characterization and finite element simulation

Abstract A mold is a part of a continuous casting plant where the molten steel starts to solidify. The inner surface of the mold undergoes a cyclic thermal load. In fact, service conditions are characterized by a high thermal flux, which vanishes when the plant is switched off. The highest temperatures occur in the area just beneath the free level of liquid steel (meniscus). The same area is typically characterized by thermal fatigue cracks when the mold is inspected after service. This work presents the characterization of the material of a copper mold after it had been used in the plant. The aim was to understand the damage and cracking mechanism, and our analysis confirmed that a network of cracks was present on the inner mold surface in the meniscus area, which experienced the maximum temperature gradients. Metallurgical examination demonstrated the transgranular characteristics of cracks, thus suggesting that thermal fatigue was the main cause of the damage observed. The location of the thermal fatigue cracks corresponded to the area experiencing the highest levels of plastic strains, as confirmed by the results of a finite element analysis simulating the mold in-service conditions.

Correlation and capability of using site inspection data and small specimen creep testing for a service-exposed CrMoV pipe section

ABSTRACTThis paper presents for the first time an investigation on the creep damage evolution of an ex-service CrMoV pipe section through impression creep test (ICT) and metallurgical inspection data. The study emphasises the importance of correlating the operating conditions (temperature and stress) of power plant components with the results from metallurgical examinations and small specimen creep tests. The paper seeks for a correlation among micro- and macro-hardness measurements, surface replicas data and minimum creep strain rates (obtained by ICTs) of the parent material of the pipe section. Also, optical and scanning electron microscope (SEM) micrographs have been used to assess possible metallurgical differences through the thickness of the pipe section. This investigation shows how miniature creep test specimen data could be practically used in a holistic approach for the evaluation of life consumption of power plant components and concludes that the studied parent material could have been retired from service too early.

Production of Steel Ingots Without Zonal Chemical Inhomogeneity for the Billets of Parts of Power-Generating Units

We study the technological specific features of the production of steel ingots without zonal chemical inhomogeneity aimed at manufacturing the billets of various parts of power-generating units made of 15KhN3MFA and 15Kh2MFA steels. The technology of pouring in a vacuum is developed with regard for the results of the numerical analysis of the process of solidification of a steel ingot 13.56 tons in weight by using the Poligon system of automatized modeling of the pouring processes. In order to get high-quality ingots, steel was melted in an EAF-12 according to a special technology guaranteeing low contents of sulfur, phosphorus, nonmetallic inclusions, and hydrogen. The specific structural features of an ingot with a mean diameter of 922 mm and the ratio H/D = 2.05 are taken into account in the development of the technology of forging in a 3000-ton press. Forging is performed for billets 900 × 930 × 1200 mm in size with the axial channel located at the top of the billets. To avoid confusions, sulfur prints were made from the top and bottom surfaces of the billet. The investigations demonstrate that the sulfur prints are practically identical for both surfaces, and small separate sulfur inclusions are uniformly distributed. The macrostructure corresponds to the GOST-10243. The results of UT testing demonstrate that the billets of the parts contain no defects with an area greater than 5 mm2. The mechanical properties at temperatures 20°C and 350°C are practically identical and higher more than the requirements of normative documents by 15–20%. For the additional metallurgical quality examination of the metal, we manufactured a test shell with a height of 1100 mm, an outer diameter of 870 mm, and a wall thickness of 250 mm.

High temperature corrosion of martensitic steel of reheater pipes in a desalination power plant

Abstract X20CrMoV12-1 steel is one of the most common martensitic steel grades that is known for its excellent heat-resistant and superior performance against oxidation and creep. Consequently, it has been used as a replacement for the low-alloy ferritic 2.25Cr-1Mo steel reheater pipes in a desalination power plant. Fracture occurred unexpectedly in some X20 steel reheater pipes of the boiler. Visual inspection revealed clear signs of wall thinning and corrosion at the fireside of the failed tube. Assessment of the tube cross-section included metallurgical examination, indicating that the microstructure is tempered martensite with no clear indications of thermal degradation or voids formation by creep. This was consistent with the microhardness measurements showing a typical average Vickers microhardness of 240 Hv. In addition, evaluation of oxide layer was made using Energy-dispersive X-ray spectroscopy (EDS) showing formation of corrosive deposits on top of the protective oxide layer at the fireside. The deposits contain excessive amounts of sulfur and vanadium that were initially present in the firing fuel. Remedial actions should be necessarily taken to minimize high-temperature corrosion by including fuel additives such as magnesium oxide or alumina to increase the melting point of vanadium and sulfur compounds. Also, improving the combustion efficiency, e.g. use of improved atomizer, with less air should be considered to minimize corrosion attacks.

On dissimilar metal welding of AISI4140 and AISI410 by GTAW

This paper presents the results of metallurgical and mechanical examinations of Gas Tungsten Arc Welding of dissimilar steels AISI4140 and AISI410. Two different filler materials viz., ERNiCr3 and SS410 were used. The various properties of the weldments made using the fillers were compared to select the most appropriate one to get the sound joint. The ultimate tensile and yield strengths of the weldments of SS410 were greater than those of ERNiCr3. The fracture occurred at the weld in weldments made with ERNiCr3, whereas, in the base metal of AISI410 for weldments made with SS410. Microstructure of fusion zone of ERNiCr3 was fully austenitic. Microhardness values in the weld of SS410 were higher and fluctuating compared to those in the weld of ERNiCr3. From this research work, it shall be concluded that SS410 is the best filler material to weld these base materials.

Analysis of a failed pipe elbow in geothermal production facility

Failure of a pipe elbow used in geothermal production facility was reported to occur after 2 months in service. A leak was found to form at the bottom of elbow of horizontal pipe. The main cause of failure was investigated through conducting standard failure analysis including visual examination, chemical analysis, mechanical characterization, metallurgical examinations using optical microscopy in combination with scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) analysis and corrosion test using a three-electrode potential technique. Results of this investigation indicated that the failure of elbow was caused by an erosion-corrosion with the presence of wall thinning in leak area. The failure mechanism was discussed in this paper.

Premature Failure of an Industrial Mixer Timing Gears

A systematic and practical methodology was adopted to determine the root cause(s) of the premature failure of a pelletizer mixer timing gear. The investigation activities covered all possible causes of failure and included field examination, interview of engineers and operators, lubrication analysis, metallurgical examination. Fracture surfaces and microstructure of gear material were examined, and hardness profiles were developed. Analyses of shaft misalignment and teeth profiles were performed and found to be within acceptable limits. Results clearly indicate the surface hardness deficiencies in many locations specifically at center of the driven gear, area of severe pitting. The developed hardness profile for all locations is lower than that specified by the manufacturer. Fractographic analysis revealed that failure occurred by pitting followed by crack propagation. A number of cracks are seen to branch in different directions indicating the presence of high contact stresses combined with weak surface strength. Bending fatigue and pitting fatigue stress calculations revealed that the safety factor under contact is well below the desired value.

Experimental and numerical studies on the expanding fracture behavior of an explosively driven 1045 steel cylinder

The expanding fracture of an explosively driven 1045 steel cylinder was studied. The fracture process and the expanding velocity of the cylinder were recorded by a high-speed camera and a Photonic Doppler Velocimetry (PDV) probe in real time. The fragments were recovered and analyzed by metallurgical examinations. The fracture mechanism, expanding velocity and fragment size were analyzed respectively. We found that the adiabatic shear band (ASB) plays a key role in the fracture process of an expanding 1045 steel cylinder. As the deformation of the cylinder increases, multiple ASBs firstly initiate near the inner surface of the cylinder and then propagate outward along the maximum shear stress direction. After the ASBs arrive at the outer surface of the cylinder, all of the plastic deformation is concentrated in the ASBs, and meanwhile the cracks initiate near the outer surface of the cylinder and propagate inwardly along the developed ASBs. The terminal velocity of the cylinder was accurately predicted by the Gurney model and was found to be little influenced by the fracture mode. The width and thickness of the fragments were measured and found to be controlled by the momentum diffusion. Furtherly, we modeled the expanding fracture process of the cylinder. A three dimensional simulation results suggested that the propagation of the shock wave along the radial direction can't be neglected although the cylinder is very thin. At an early stage, the material near the inner and outer surface is in a compressive and tensile state, respectively, and the material in the middle of the cross-section is alternately in a tensile and compressive state due to the propagation of the shock waves. At a late stage, the whole cylinder is in a tensile state. In a two dimensional plane strain simulation, the initiation and propagation of multiple ASBs in the expanding cylinder were successfully replicated.

Metallurgical and mechanical examinations of molybdenum/graphite joints by vacuum arc pressure brazing using Ti-Zr filler materials

The aim of this work was to join Molybdenum (Mo) and a graphite to obtain laminated structure using a high-temperature and hot press welding method with a Ti-Zr foil as the binder in a vacuum condition. The microstructure and composition changes of the interface were characterized by scanning electron microscopy and X-ray diffraction, respectively. The properties of the interface were studied using shear tests and hardness measurements. The optimal processing conditions for Mo and graphite joints involved a pressure of 0.5–1MPa, holding time of 10–30s, and brazing time of 5min with a melting current of less than 150A under a 1.3×10−3Pa vacuum condition. A transition zone with a width of approximately 0.3mm and an average micro-hardness reaching 565 HV was formed, which is much higher than that of graphite and Mo. The main reason is the formation of different carbide and solid solutions such as MoC, Mo9Ti4, and Ti2C0.06. The shear bond strength of the graphite and molybdenum connection is 15.2MPa, which is higher than that of graphite. The connection mechanism of Mo and graphite was the interlayer reacting with the Mo or graphite substrate at a high temperature to form compounds and solid solutions, respectively, covering the surface of the substrate. Therefore, an effective connection of Mo and graphite was successfully achieved. A model of the Mo and graphite connection structure was introduced.

Effect of grain size evolution in an Au-wire ball on debonding failure with the Al-pad in semiconductor devices

This article details how a 99.99% Au-wire ball with an appropriate grain size prevents void formation at the interface region with its underlying Al-pad, which is responsible for wire debonding failure in lead-frame packages during thermal-cycling. Metallurgical examinations showed that an Au-wire ball with a grain size smaller than 3μm in diameter allows the faster diffusion of Al atoms into it, leaving troublesome voids at the interface region with the Al-pad region. Meanwhile, the investigation revealed that an Au-wire ball with a grain size larger than 8μm effectively suppresses the void formation due to the slower diffusion of Al atoms from the Al-pad to it. However, an Au-wire ball with such a large grain size (bigger than 8μm) did not allow the formation of a tough intermetallic compound such as AuAl2, which is favorable for the reliable adhesion with the underlying Al-pad. Instead, the Au-balls with such a larger grain size were more favorable for the formation of a brittle intermetallic compound such as Au4Al at the interface region due to the lower density of active Al atoms, resulting in bonding reliability degradation. Consequently, in order to ensure the reliability of Au-wire bonded on an Al-pad in lead-frame packages during thermal-cycling, the wire ball should have a grain size of between 3μm and 8μm in diameter.

Impact of welding on the strength of high performance steel T-stub joints

In this study, experiments were carried out to investigate the influence of welding on the strength of the reheated, quenched and tempered (RQT) steel S690 and the thermo-mechanically controlled processed (TMCP) steel S385. Firstly, a specially designed welding procedure was applied to produce RQT and TMCP steel plates that were fully affected by the welding heat input for the whole plate thickness. These fully weld-affected plates were then fabricated into specimens for tensile test and metallurgical examination including microstructure observation and Vickers hardness measurement. The results showed that welding softened the heat affected zone of the RQT-S690 steel significantly but its effect on the TMCP-S385 steel was insignificant. Secondly, T-stub joints were fabricated by using the RQT and TMCP steel plates and tested in tension until failure. The design plastic resistances of the joints are then compared with the prediction by EC3 equations. It is found that the EC3 equations could produce unsafe prediction for the RQT joint. Based on the test results, a reduction factor is proposed to modify the EC3 equations to produce reasonable conservative prediction of the RQT joint design plastic resistance.

Two step friction stir spot welding between aluminium and carbon steel sheets

A two step friction stir spot welding has been developed to join a 2 mm thick low carbon steel sheet and a 3 mm thick aluminium sheet. In the first step, a consumable aluminium tool is used to develop frictional heat and plasticise the aluminium layer from the tool which fills the cavity in the aluminium sheet, progressively. In the subsequent step, the plasticised layer is squeezed by a non-consumable forming tool made of high carbon steel with an inverted semi-hemispherical profile. To understand the efficacy of the contact area of the plasticised metal, five different tool rotational speeds were used. The integrity of the joint was evaluated by different mechanical and metallurgical examinations.

Failure Analysis of Internal Fixation Medical Devices: Overview and Case Studies

Internal fixation devices, and specifically bone plate and screw assemblies, are among the most common implantable medical devices that a failure analyst may encounter. These devices most often fracture by fatigue crack initiation and growth. This article discusses potential root causes of internal fixation device fractures and describes the process of metallurgical and nondestructive examination of these assemblies. Two fractured internal fixation device case studies are reviewed.

Investigating rock anchorage failure at major road cut in Wales

Rock anchorage failures occurring only 6 years after installation to support a major cutting on the A5 trunk road west of Corwen in North Wales were exhaustively investigated, ultimately resulting in their removal and partial replacement. The failures in Glyn Bends rock cutting were detected when head assembly components were found on the ground during a routine inspection. The investigation of the 120-year design life double-corrosion-protected anchorages included the innovative use of endoscopy, selectively supported by subsequent physical dismantling and metallurgical examination. On the basis of the investigation and the subsequent recovery of some of the installed anchorages, it was confirmed that at least 24% of the anchorages originally installed had their factory corrosion protection fatally compromised by partial removal. This was aggravated by the omission of the caps, which allowed the protective grease to escape, and placing many anchorages in recesses which aggravated surface corrosion. Metallurgical investigation emphasised that corrosion protection is critical to very-high-yield steels. No records of the alterations made were ever produced. The trunk road had to be closed at short notice for 8 months on public safety grounds while the main cutting face was reprofiled to a safe unsupported angle on the north side and re-anchored on the south side.