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.

Cluster bomb tailcone assemblies each containing two aluminum die-cast components were rejected because of the poor surface condition of the die castings. Numerous heat checks were found on the surfaces of the tailcones and radiographic inspection revealed inclusions, gas holes, and shrinkage defects in the castings. Most of the components failed to meet required mechanical properties because of these casting defects.

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.

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.

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.

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.

A low-carbon steel (St35.8) tube in a phthalic anhydride reactor system failed. Visual and stereomicroscopic examination of fracture surfaces revealed heavy oxide/deposits on the outer surface of the tube, tube wall thinning in the area of the fracture, and discolorations and oxides/deposits on the inner surface. Cross sections from the fracture surface were metallographically examined, and the deposits were analyzed. It was determined that the tube had thinned from the inner surface because of a localized overheating condition (probably resulting from a runaway chemical reaction within the tube) and then fractured, which allowed molten salt to flow into the tube.

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 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.

Near-surface porosity in zinc die castings that were subsequently plated with copper, nickel, and bright chromium was causing blemishes in the plating. Identifying die casting turbulence and hot spots were keys to process modifications that subsequently allowed porosity to be greatly minimized.

Three radiant heating element tubes from an aluminum holding furnace failed after a few months of service. One side of each of the tubes had disintegrated, leaving large holes and thinned cross sections. Microstructural analysis showed that the surface of the tube had been oxidized along the grain boundaries and had extensive precipitation inside the grains. Chemical analysis indicated that the steel used for the tubes was AISI type 316 stainless steel Specifications for the tubes had called for AISI type 310S to be used. It was recommended that other tubes made from the same batch of steel sheet be checked.

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 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.

Several tin plated, low-alloy steel couplings designed to connect sections of 180 mm (7 in.) diam casing for application in a gas well fractured under normal operating conditions. The couplings were purchased to American Petroleum Institute (API) specifications for P-110 material. Chemical analysis and mechanical testing of the failed couplings showed that they had been manufactured to the API specification for Q-125, more stringent specification than P-110, and met all requirements of the application. Fractographic examination showed that the point of initiation was an embrittled region approximately 25 mm (1 in.) from the end of the coupling. The source of the embrittlement was determined to be hydrogen charging during tin plating. Changes in the plating process were recommended.

New aircraft wing panels extruded from 7075-T6 aluminum exhibited an unusual pattern of circular black interrupted lines, which could not be removed by scouring or light sanding. The panels, subsequent to profiling and machining, were required to be penetrated inspected, shot peened, H2SO4 anodized, and coated with MIL-C-27725 integral fuel tank coating on the rib side. Scanning electron microscopy and microprobe analysis (both conventional energy-dispersive and Auger analyzers) showed that the anodic coating was applied over an improperly cleaned and contaminated surface. The expanding corrosion product had cracked and, in some places, had flaked away the anodized coating. The corrodent had penetrated the base aluminum in the form of subsurface intergranular attack to a depth of 0.035 mm (0.0014 in.). It was recommended that a vapor degreaser be used during cleaning prior to anodizing. A hot inhibited alkaline cleaner was also recommended during cleaning prior to anodizing. The panels should be dichromate sealed after anodizing. The use of deionized water was also recommended during the dichromate sealing operation. In addition, the use of an epoxy primer prior to shipment of the panels was endorsed. Most importantly, surveillance of the anodizing process itself was emphasized.

During an inspection of a structure two weeks after assembly, the heads of several cadmium-plated AISI 8740 steel fasteners were found to be completely separated from their respective shanks. SEM examination of the fracture surfaces revealed a brittle, intergranular fracture mode, indicating hydrogen embrittlement. An investigation was conducted to determine the extent of hydrogen embrittlement in the various lots of cadmium-plated 8740 steel fasteners. It was found that hydrogen embrittlement was caused by the use of a bright, impervious cadmium electroplate that hindered diffusion of mobile hydrogen outward from the surface of the pin. After the cadmium layer was removed, the mobile hydrogen contained on the surface of the steel and in the electroplated deposit was released, and the embrittlement problem was alleviated. To prevent reoccurrence, the bright cadmium layer was stripped from the pins, which were then baked and repeated with a dull, porous cadmium layer that allowed outward diffusion of hydrogen. The pins were baked again after deposition of the porous cadmium layer. This eliminated the problem.

A locked coil track rope (LCTR) is essentially composed of wires (round and rail-shaped) laid helically in different layers. These wire ropes are sometimes used in conveyors carrying empty and loaded buckets in mining areas. During service, such wire ropes may fail prematurely due to disintegration/failure of individual groups of wires. To understand the genesis of LCTR wire failures, a detailed metallurgical investigation of failed rope wires was made and included visual examination, optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Two types of failed wires were investigated; one is from a 40 mm diameter locked coil track rope and the other from a 53 mm locked coil track rope. Optical microscopy of failed round wires in the 53 mm diameter rope clearly revealed fully decarburized layers at the surface and a few grain-boundary cracks. From the location of the failure, it was clear that apart from static tensile loads, the wire ropes had been subjected to bending and unbending loads near the saddle, as fully loaded or empty buckets traveled access the conveyor. The SEM studies confirmed that the fracture had been caused by initiation of fatigue cracks in the decarburized zone under conditions of repeated bending and unbending stresses superimposed on the static tensile load.

The U-0.8wt%Ti alloy is often used in weapon applications where high strength and fairly good ductility are necessary. Components are immersion quenched in water from the gamma phase to produce a martensitic structure that is amenable to aging. Undesirable conditions occur when a component occasionally cracks during the quenching process, and when tensile specimens fail prematurely during mechanical testing. These two failures prompted an investigative analysis and a series of studies to determine the causes of the cracking and erratic behavior observed in this alloy. Quench-related failures whereby components that cracked either during or immediately after the heat treatment/quenching operation were sectioned for metallographic examination of the microstructure to examine the degree of phase transformation. Examination of premature tensile specimen failures by scanning electron microscopy and X-ray imaging of fracture surfaces revealed pockets of inclusions at the crack origins. In addition, tests were conducted to evaluate the detrimental effects of internal hydrogen on ductility and crack initiation in this alloy.

A steel splice plate in a power transmission line tower cracked while in service. Metallographic analysis indicated the presence of a white hard martensite layer near the crack, which occurred in the heel of the plate. Mechanical property tests revealed localized hardening in the area of the crack, supporting the metallurgical findings. A substantial deterioration of the Charpy impact toughness of the material in the heel region was also observed which is believed to have caused the initiation and propagation of the cracks leading to the failure.