The working fluid of a hypersonic wind tunnel is freon 14 heated in molten-metal-bath heat exchangers. The coils of the heaters have failed several times from various causes. They have been replaced each time with a stainless steel deemed more appropriate, but they continue to fail. In this case study, the history of failures is traced, the causes are analyzed, and recommendations are made for future design and maintenance. Coils fabricated from AISI 316 should provide satisfactory service life if reasonable precautionary measures are observed during maintenance and testing.

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.

Metal-induced embrittlement is a phenomenon in which the ductility or the fracture stress of a solid metal is reduced by surface contact with another metal in either the liquid or solid form. This article summarizes some of the characteristics of liquid-metal- and solid-metal-induced embrittlement. This phenomenon shares many of these characteristics with other modes of environmentally induced cracking, such as hydrogen embrittlement and stress-corrosion cracking. The discussion covers the occurrence, failure analysis, and service failures of the embrittlement. The article also briefly reviews some commercial alloy systems in which liquid-metal-induced embrittlement or solid-metal-induced embrittlement has been documented and describes some examples of cracking due to these phenomena, either in manufacturing or in service.

Several cases of embrittlement failure are analyzed, including liquid-metal embrittlement (LME) of an aluminum alloy pipe in a natural gas plant, solid metal-induced embrittlement (SMIE) of a brass valve in an aircraft engine oil cooler, LME of a cadmium-plated steel screw from a crashed helicopter, and LME of a steel gear by a copper alloy from an overheated bearing. The case histories illustrate how LME and SMIE failures can be diagnosed and distinguished from other failure modes, and shed light on the underlying causes of failure and how they might be prevented. The application of LME as a failure analysis tool is also discussed.

Severely reduced wall thickness was encountered at the liquid line of a lead-bath pan that was used in a continuous strip or wire oil-tempering unit. Replacement of the pan was necessary after six months of service. The pan, 6.9 m (22.5 ft) long, 0.6 m (2 ft) wide, and 38 cm (15 in.) deep with a 2.5-cm (1-in.) wall thickness, was a type 309 stainless steel weldment. Operating temperatures of the lead bath in the pan ranged from 805 deg C (1480 deg F) at the entry end to 845 deg C (1550 deg F) at the exit end. Analysis (visual inspection. metallographic analysis, moisture testing, and etched micrographs using Murakami's reagent) supported the conclusions that thinning of the pan walls at the surface of the molten lead resulted from using coke of high moisture content and from the low fluctuating coke level. Recommendations included reducing the supply of oxygen attacking the grain boundaries and the hydrogen that readily promoted decarburization with the use of dry (2 to 3% moisture content) coke. Maintaining a thick layer of coke over the entire surface of molten lead in the pan would exclude atmospheric oxygen from the grain boundaries.

Two new chrome-plated CDA 377 brass valves intended for inert gas service failed on initial installation. After a pickling operation to clean the metal, the outer surfaces of the valves had been flashed with copper and then plated with nickel and chromium for aesthetic purposes. One of the valves failed by dezincification. The porous copper matrix could not sustain the clamping loads imposed by tightening the pressure relief fitting. The second valve failed by shear overload of the pressure relief fitting. Overload was facilitated by a reduction of cross-sectional area caused by intergranular attack and slight dezincification of the inner bore surface of the fitting. Dezincification and intergranular attack were attributed to excessive exposure to nonoxidizing acids in the pickling bath.

Friction and wear are important when considering the operation and efficiency of components and mechanical systems. Among the different types and mechanisms of wear, adhesive wear is very serious. Adhesion results in a high coefficient of friction as well as in serious damage to the contacting surfaces. In extreme cases, it may lead to complete prevention of sliding; as such, adhesive wear represents one of the fundamental causes of failure for most metal sliding contacts, accounting for approximately 70% of typical component failures. This article discusses the mechanism and failure modes of adhesive wear including scoring, scuffing, seizure, and galling, and describes the processes involved in classic laboratory-type and standardized tests for the evaluation of adhesive wear. It includes information on standardized galling tests, twist compression, slider-on-flat-surface, load-scanning, and scratch tests. After a discussion on gear scuffing, information on the material-dependent adhesive wear and factors preventing adhesive wear is provided.

The surface of a hook did not possess the smooth and shiny zinc bloom surface normally observed on hot galvanized steel parts but was matte and rough. Large cracks were observed in the zinc layer. The hook was made of silicon-killed alloy steel 41Cr4. A silicon content of 0.27% was established analytically. Silicon accelerates the reaction between iron and zinc, which should have been taken into account in the present case by reducing the dip time or a small addition of aluminum (0.1 to 0.2%) to the galvanizing bath to retard the extremely rapid growth of the zinc layer and the strong alloy formation. Even in the case of steel parts with lower silicon contents the reaction between iron and zinc can continue until the pure zinc layer has been consumed entirely if the work piece is not cooled sufficiently after withdrawal.

Mower blades manufactured from grade 1566 high-manganese carbon steel failed a standard 90 deg test. The blades had been austempered and reportedly fractured in a brittle manner during testing. The austempering treatment was intended to produce a bainitic microstructure, but investigation (visual inspection, 2% nital etched 8.9x/196x images) showed that the typical core microstructure contained alternating bands of martensite and bainite. The conclusion was that the nonuniform microstructure was likely responsible for the atypical brittle behavior of the blades, and the observed structure suggests that the austempering heat treatment was performed too close to the nominal martensite start temperature. Recommendations included raising the austempering salt-bath temperature 56 deg C (100 deg F) to account for localized compositional variation.

A brass (CDA alloy 230) pipe nipple that was part of a domestic cold water bath system failed two weeks after installation. Macrofractography, SEM examination, metallography, and chemical analyses were performed on specimens cut through the main fracture surface. The physical and background evidence obtained indicated failure due to cracking initiated by stamped markings on the pipe wall and extended by high circumferential residual stresses. It was recommended that annealed pipe be used.

This article discusses different types of mechanical fasteners, including threaded fasteners, rivets, blind fasteners, pin fasteners, special-purpose fasteners, and fasteners used with composite materials. It describes the origins and causes of fastener failures and with illustrative examples. Fatigue fracture in threaded fasteners and fretting in bolted machine parts are also discussed. The article provides a description of the different types of corrosion, such as atmospheric corrosion and liquid-immersion corrosion, in threaded fasteners. It also provides information on stress-corrosion cracking, hydrogen embrittlement, and liquid-metal embrittlement of bolts and nuts. The article explains the most commonly used protective metal coatings for ferrous metal fasteners. Zinc, cadmium, and aluminum are commonly used for such coatings. The article also illustrates the performance of the fasteners at elevated temperatures and concludes with a discussion on fastener failures in composites.

A rocket-motor case made of consumable-electrode vacuum arc remelted D-6ac alloy steel failed during hydrostatic proof-pressure testing. Close visual examination, magnetic-particle inspection, and hardness tests showed cracks that appeared to have occurred after austenitizing but before tempering. Microscopic examinations of ethereal picral etched sections indicated that the cracks appeared before or during the final tempering phase of the heat treatment and that cracking had occurred while the steel was in the as-quenched condition, before its 315 deg C (600 deg F) snap temper. Chemical analysis of the cracked metal showed a slightly higher level of carbon than in the component that did not crack. X-ray diffraction studies of material from the fractured dome showed a very low level of retained austenite, and chemical analysis showed a slightly higher content of carbon in the metal of the three cracked components. Bend tests verified the conclusion that the most likely mechanism of delayed quench cracking was isothermal transformation of retained austenite to martensite under the influence of residual quenching stresses. Recommendations included modifying the quenching portion of the heat-treating cycle and tempering in the salt pot used for quenching, immediately after quenching.

This article provides an overview of the classification of hydrogen damage. Some specific types of the damage are hydrogen embrittlement, hydrogen-induced blistering, cracking from precipitation of internal hydrogen, hydrogen attack, and cracking from hydride formation. The article focuses on the types of hydrogen embrittlement that occur in all the major commercial metal and alloy systems, including stainless steels, nickel-base alloys, aluminum and aluminum alloys, titanium and titanium alloys, copper and copper alloys, and transition and refractory metals. The specific types of hydrogen embrittlement discussed include internal reversible hydrogen embrittlement, hydrogen environment embrittlement, and hydrogen reaction embrittlement. The article describes preservice and early-service fractures of commodity-grade steel components suspected of hydrogen embrittlement. Some prevention strategies for design and manufacturing problem-induced hydrogen embrittlement are also reviewed.

Hydrogen damage is a term used to designate a number of processes in metals by which the load-carrying capacity of the metal is reduced due to the presence of hydrogen. This article introduces the general forms of hydrogen damage and provides an overview of the different types of hydrogen damage in all the major commercial alloy systems. It covers the broader topic of hydrogen damage, which can be quite complex and technical in nature. The article focuses on failure analysis where hydrogen embrittlement of a steel component is suspected. It provides practical advice for the failure analysis practitioner or for someone who is contemplating procurement of a cost-effective failure analysis of commodity-grade components suspected of hydrogen embrittlement. Some prevention strategies for design and manufacturing problem-induced hydrogen embrittlement are also provided.

The operator of an electric transit system purchased a large number of tin-plated copper connectors, putting some in service and others in reserve. Later, when some of the reserve connectors were inspected, the metal surfaces were covered with spots consisting of an ash-like powder and the plating material had separated from the substrate in many areas. Several connectors, including some that had been in service, were examined to determine what caused the change. The order stated that the connectors were to be coated with a layer of tin-bismuth (2% Bi) to guard against tin pest, a type of degradation that occurs at low temperatures. Based on the results of the investigation, which included SEM/EDS analysis, inductively coupled plasma spectroscopy, and x-ray diffraction, the metal surfaces contained less than 0.1% Bi and thus were not adequately protected against tin pest, which was confirmed as the failure mechanism in the investigation.

This article provides an overview of the electrochemical nature of corrosion and analyzes corrosion-related failures. It describes corrosion failure analysis and discusses corrective and preventive approaches to mitigate corrosion-related failures of metals. These include: change in the environment; change in the alloy or heat treatment; change in design; use of galvanic protection; use of inhibitors; use of nonmetallic coatings and liners; application of metallic coatings; use of surface treatments, thermal spray, or other surface modifications; corrosion monitoring; and preventive maintenance.

After less than 30 days in service, several cadmium-plated retaining rings, made of 4140 steel tubing and heat treated to 36 to 40 HRC, broke during operation that included holding components of a segmented fitting in place under a constant load. Photographic and 100x nital-etched micrographic examination showed a microstructure of tempered martensite with low inclusion content as well as a pit or burned spot on the outer area of the ring. The defect was approximately 0.18 mm (0.007 in.) deep and 0.5 mm (0.020 in.) in diam and had a hardness of 58 to 60 HRC. The base metal adjacent to the defect had a hardness of 36 to 40 HRC. Small cracks or fissures were also evident within the defect. Thus, the rings failed in brittle fracture as the result of an arc strike (or burn) on the surface of the ring. At the site of the arc strike, a small region of hard, brittle untempered martensite was formed as the result of an arc strike during the cadmium-plating operation. Fracture occurred readily when the ring was stressed. No recommendations were made.

An exhaust diffuser assembly failed prematurely in service. The failure occurred near the intake end of the assembly and involved fracture in the diffuser cone (Corten), diffuser in take flange (type 310 stainless steel), diffuser exit flange (type 405 stainless steel), expansion bellows (Inconel 600), and bellows intake flange (Corten). Individual segments of the failed subassemblies were examined using various methods. The analysis indicated that the weld joint in the diffuser intake flange (type 310 stainless steel to Corten steel) contained diffusion-zone solidification cracks. The joints had been produced using the mechanized gas-metal arc welding process. Cracking was attributed to improper control of welding parameters, and failure was attributed to weld defects.

This article presents a failure analysis of 37.5 mW gas turbine third stage buckets made of Udimet 500 superalloy. The buckets experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis was carried out on elements thought to have influenced the failure process: a) the stress increase from the loss of a load bearing cross-sectional area of the bucket tip shroud by the conversion of metal to the corrosion product (scale), b) influence of the tip shroud microstructure (e.g., a presence of equiaxed and columnar grains, their distribution and orientation), c) evidence of the transgranular initiation, and d) intergranular creep mechanism propagation. The most probable cause of the bucket damage was the combination of increased stresses due to corrosion-induced thinning of the tip shroud and unfavorable microstructures in the tip shroud region.

Visual examination, using the unaided eye or a low-power optical magnifier, is typically one of the first steps in a failure investigation. This article presents the guidelines for selecting samples for scanning electron microscope examination and optical metallography and for cleaning fracture surfaces. It discusses damage characterization of metals, covering various factors that influence the damage, namely stress, aggressive environment, temperature, and discontinuities.