Search Results for solid-state welding

The defects observed along weldings of stainless steel pipelines employed in marine environments were evidenced by metallographic and electrochemical examination. A compilation of cases on the effect of defective weldings, in addition to improper choice of stainless steel for water pipelines, lead to the conclusion that intercrystalline corrosion in steels involved precipitation of a surplus phase at grain boundaries. Intercrystalline corrosion in austenitic stainless steels due to precipitation of chromium carbides during conditions generated due to welding and ways to avoid the precipitation (including reduction of carbon content, appropriate heat treatment, cold work of steel, reduction of austenitic grain size and stabilizing elements) were described. The presence of microcracks due to highly localized heat concentrations with consequent thermal expansion and considerable shrinkages during cooling was investigated. The specimens were taken from various sources including transverse and longitudinal welding seam, sensitized areas and it was concluded appropriate material selection with respect to medium could control some corrosion processes.

Silver solid-state bonded components containing uranium failed under zero or low applied load several years after manufacture. The final operation in their manufacture was a proof loading that applied a sustained tensile stress to the bond, which all components passed. The components comprised circular cylinders fabricated by plating a thin layer of silver on each of the contact surfaces (uranium and stainless steel) and pressing the parts together at elevated temperature to solid-state bond the two silver surfaces. The manufacturing process produced a high level of residual stress at the bond. The failures appeared to be predominantly located between the silver layer and the uranium substrate. Normal fracture location of specimens taken from similar components was at the silver/silver bond interface. Laboratory testing revealed that the uranium/silver joint was susceptible to premature failure by stress-corrosion cracking under sustained loading if the atmosphere was saturated with water vapor.

A number of rotating blades in a diffuser at a sugar beet processing plant fabricated from rectangular bars cut from rolled carbon-manganese steel plate fractured brittlely. However, apparently identical blades underwent significant plastic deformation without fracture. Inspection of both fractured and bent blades revealed similar preexisting cracks at the toes of bar attachment welds. Metallographic examination of the bent and the fractured bars revealed they had been cut parallel and transverse, respectively, to the rolling direction of the steel plate. Due to the combined effects of the low fracture toughness of the plate on planes parallel in the rolling direction, the presence of the preexisting cracks, and the relatively large section thickness of the bars, the bars whose lengths were transverse to the rolling direction fractured brittlely when subjected to impact loads. Had the poor transverse properties of thick-section plate been recognized, and all the bars properly cut with respect to the rolling direction, the premature fractures would not have occurred.

Erosion of solid surfaces can be brought about solely by liquids in two ways: from damage induced by formation and subsequent collapse of voids or cavities within the liquid, and from high-velocity impacts between a solid surface and liquid droplets. The former process is called cavitation erosion and the latter is liquid-droplet erosion. This article emphasizes on manifestations of damage and ways to minimize or repair these types of liquid impact damage, with illustrations.

This article describes some of the welding discontinuities and flaws characterized by nondestructive examinations. It focuses on nondestructive inspection methods used in the welding industry. The sources of weld discontinuities and defects as they relate to service failures or rejection in new construction inspection are also discussed. The article discusses the types of base metal cracks and metallurgical weld cracking. The article discusses the processes involved in the analysis of in-service weld failures. It briefly reviews the general types of process-related discontinuities of arc welds. Mechanical and environmental failure origins related to other types of welding processes are also described. The article explains the cause and effects of process-related discontinuities including weld porosity, inclusions, incomplete fusion, and incomplete penetration. Different fitness-for-service assessment methodologies for calculating allowable or critical flaw sizes are also discussed.

Metal-induced embrittlement is a phenomenon in which the ductility or fracture stress of a solid metal is reduced by surface contact with another metal in either liquid or solid form. This article summarizes the characteristics of solid metal induced embrittlement (SMIE) and liquid metal induced embrittlement (LMIE). It describes the unique features that assist in arriving at a clear conclusion whether SMIE or LMIE is the most probable cause of the problem. The article briefly reviews some commercial alloy systems where LMIE or SMIE has been documented. It also provides some examples of cracking due to these phenomena, either in manufacturing or in service.

An investigation was conducted to better understand the time-dependent degradation of thermal barrier coated superalloy components in gas turbines. First-stage vanes are normally subjected to the highest gas velocities and temperatures during operation, and were thus the focus of the study. The samples that were analyzed had been operating at 1350 °C in a gas turbine at a combined-cycle generating plant. They were regenerated once, then used for different lengths of time. The investigation included chemical analysis, scanning electron microscopy, SEM/energy dispersive spectroscopy, and x-ray diffraction. It was shown that degradation is driven by chemical and mechanical differences, oxide growth, depletion, and recrystallization, the combined effect of which results in exfoliation, spallation, and mechanical thinning.

In a HyL III heat exchanger's radiant pipes, metal dusting reduced the pipe thickness from 8.5 to 3 mm in just nine months, leaving craters on the inner surface. The pipes are fabricated from HK 40 alloy. The heated gas (400 to 800 deg C) consisted of CO, CO2, and H2, with a 4:1 CO/CO2 ratio. Metallographic investigations revealed that the surface of the attacked pipes consisted of (Cr, Fe) carbide. The metal dusting was the result of a decomposition process (CO to CO2 + C) that deposited C on the pipe surface. Because of the high temperature, the C subsequently diffused through the surface oxide layer (Cr2O3), triggering a succession of reactions that led to pitting and the formation of craters.

Cold cracking of structural steel weldments is a well-documented failure mechanism, and extensive work has been done to recognize welding and materials selection parameters associated with it. These efforts, however, have not fully eliminated the occurrence of such failures. This article examines a case of cold cracking failure in the construction industry. Fortunately, the failure was identified prior to final erection of the structural members and the weld was successfully reworked. The article explains how various welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure. Human factors and fabrication practices that contributed to the problem are covered as well.

This article provides a background of friction-bearing failures due to overheating. The failures of locomotive axles caused by overheated traction-motor support bearings are discussed. The article also describes liquid-metal embrittlement (LME) in steel. It examines the results of various axle studies, with illustrations and concludes with information on the simulation of the LME mechanism.

The failure mode of through-wall cracking of a butt weld in a 5083-O aluminum alloy piping system in an ethylene plant was identified as mercury liquid metal embrittlement. As a result of this finding, 226 of the more than 400 butt welds in the system were ultrasonically inspected for cracking. One additional weld was found that had been degraded by mercury. A welding team experienced in repairing mercury contaminated piping was recruited to make the repairs. Corrective action included the installation of a sulfur-impregnated charcoal mercury-removal bed and replacement of the aluminum equipment that was in operation prior to the installation of the mercury-removal bed.

Explosive cladding is a viable method for cladding different materials together, but the complicated behavior of materials under ballistic impacts raises the probability of interfacial shear failure. To better understand the relationship between impact energy and interfacial shear, investigators conducted an extensive study on the shear strength of explosively cladded Inconel 625 and plain carbon steel samples. They found that by increasing impact energy, the adhesion strength of the resulting cladding can be improved. Beyond a certain point, however, additional impact energy reduces shear strength significantly, causing the cladding process to fail. The findings reveal the decisive role of plastic strain localization and the associated development of microcracks in cladding failures. An attempt is thus made to determine the optimum cladding parameters for the materials of interest.

Several mercury diffusion pump stages in the Tritium Purification process at the Savannah River Site (SRS) have been removed from service for scheduled preventive maintenance. These stages have been examined to determine if failure has occurred. Evidence of fatigue around the flange portion of the pump has been seen. In addition, erosion and cavitation inside the throat of the venturi tube and corrosion on the other surface of the venturi tube has been observed. Several measures are being examined in an attempt to improve the performance of these pumps. These measures, as well as the noted observations, are described. Six stages [two machined (MP) and four electron beam (EB) welded] from the mercury diffusion pumps operating in the Tritium Purification process at SRS have been analyzed to determine their condition after nine months of usage. Several cracks were found around the necked region of the two MP stages. The EB welded stages, however, seemed to perform better in service; only two of four stages showed cracking. The cracking is caused by fatigue that has been enhanced by high stresses and tritium in the flange area. The EB welded stage appears to be a step in the right direction. Since the EB weld is a shrink fit, the surface is in compression, thereby eliminating crack propagation. In addition, shot peening has been employed to produce a compressive material surface since fatigue usually originates at the surface. Pitting was observed down the throat of the venturi. This pitting was caused by cavitation and erosion along the length of the venturi tube. Corrosion and pitting was seen on the exterior walls of the diffuser tubes. Stress-corrosion cracks were observed emanating from these corrosion pits. The corrosion likely occurred from the chloride ions present in the process cooling water. Shot peening is now being used in an attempt to place the outside of the diffuser tube in compression to eliminate the stress-corrosion cracking.

Heat exchangers are devices used to transfer thermal energy between two or more fluids, between a solid surface and a fluid, or between a solid particulate and a fluid at different temperatures. This article first addresses the causes of failures in heat exchangers. It then provides a description of heat-transfer surface area, discussing the design of the tubular heat exchanger. Next, the article discusses the processes involved in the examination of failed parts. Finally, it describes the most important types of corrosion, including uniform, galvanic, pitting, stress, and erosion corrosion.

Overload failures refer to the ductile or brittle fracture of a material when stresses exceed the load-bearing capacity of a material. This article reviews some mechanistic aspects of ductile and brittle crack propagation, including a discussion on mixed-mode cracking, which may also occur when an overload failure is caused by a combination of ductile and brittle cracking mechanisms. It describes the general aspects of fracture modes and mechanisms. The article discusses some of the material, mechanical, and environmental factors that may be involved in determining the root cause of an overload failure. It also presents examples of thermally and environmentally induced embrittlement effects that can alter the overload fracture behavior of metals.

This article aims to identify and illustrate the types of overload failures, which are categorized as failures due to insufficient material strength and underdesign, failures due to stress concentration and material defects, and failures due to material alteration. It describes the general aspects of fracture modes and mechanisms. The article briefly reviews some mechanistic aspects of ductile and brittle crack propagation, including discussion on mixed-mode cracking. Factors associated with overload failures are discussed, and, where appropriate, preventive steps for reducing the likelihood of overload fractures are included. The article focuses primarily on the contribution of embrittlement to overload failure. The embrittling phenomena are described and differentiated by their causes, effects, and remedial methods, so that failure characteristics can be directly compared during practical failure investigation. The article describes the effects of mechanical loading on a part in service and provides information on laboratory fracture examination.

During the installation of power transmission lines across a major interstate highway, a temporary anchor stabilizing one of the poles failed, resulting in the loss of the pole and the associated power lines. It also contributed to a single vehicle incident on the adjacent roadway. Post-failure analysis revealed that the fracture was precipitated by a preexisting weld-related crack. Closed form and numerical stress analyses were also conducted, with the results indicating that the anchor was installed properly within the parameters intended by the manufacturer.

X-ray diffraction (XRD) residual-stress analysis is an essential tool for failure analysis. This article focuses primarily on what the analyst should know about applying XRD residual-stress measurement techniques to failure analysis. Discussions are extended to the description of ways in which XRD can be applied to the characterization of residual stresses in a component or assembly and to the subsequent evaluation of corrective actions that alter the residual-stress state of a component for the purposes of preventing, minimizing, or eradicating the contribution of residual stress to premature failures. The article presents a practical approach to sample selection and specimen preparation, measurement location selection, and measurement depth selection; measurement validation is outlined as well. A number of case studies and examples are cited. The article also briefly summarizes the theory of XRD analysis and describes advances in equipment capability.