Nodular cast iron crankshafts and their main-bearing inserts were causing premature failures in engines within the first 1600 km (1000 mi) of operation. The failures were indicated by internal noise, operation at low pressure, and total seizing. Concurrent with the incidence of engine field failures was a manufacturing problem: the inability to maintain a similar microfinish on the cope and drag sides of a cast main-bearing journal. Investigation supported the conclusion that the root cause of the failure was carbon flotation due to the crankshafts involved in the failures showing a higher-than-normal carbon content and/or carbon equivalent. Larger and more numerous cope side graphite nodules broke open, causing ferrite caps or burrs. They then became the mechanism of failure by breaking down the oil film and eroding the beating material. A byproduct was heat, which assisted the failure. Recommendations included establishing closer control of chemical composition and foundry casting practices to alleviate the carbon-flotation form of segregation. Additionally, some nonmetallurgical practices in journal-finishing techniques were suggested to ensure optimal surface finish.

This investigation characterizes five surgical stainless steel piercings and one niobium piercing that caused adverse reactions during use, culminating with the removal of the jewelry. Chemical composition shows that none of the materials are in accordance with ISO standards for surgical implant materials. Additionally, none of the stainless steel piercings passed the pitting-resistance criterion of ISO 5832-1, which implies that [%Cr + 3.3(%Mo)] > 26. Under microscopic examination, most of the jewelry revealed the intense presence of linear irregularities on the surface. The lack of resistance to pitting corrosion associated with the poor surface finishing of the stainless steel jewelry may induce localized corrosion, promoting the release of cytotoxic metallic ions (such as Cr, Ni, and Mo) in the local tissue, which can promote several types of adverse effects in the human body, including allergic reactions. The adverse reaction to the niobium jewelry could not be directly associated with the liberation of niobium ions or the residual presence of cytotoxic elements such as Co, Ni, Mo, and Cr. The poor surface finish of the niobium jewelry seems to be the only variable of the material that may promote adverse reactions.

A new supplier for aluminum die castings was being evaluated, and the castings failed to meet the durability test requirements. Specifically, the fatigue life of the castings was low. Initial inspection of the fatigue fracture surfaces revealed large-scale porosity visible to the naked eye. New castings with reduced porosity also failed the durability tests. The fatigue fracture surfaces of additional casting fragments were very rough and contained multiple ratchet marks along the inner fillet. These observations indicated the fatigue process was heavily influenced by the presence of surface imperfections. Improving the surface finish or choosing a stronger alloy, were more likely to improve part durability than reducing the porosity.

The splined shaft (1040 steel, heat treated to a hardness of 44 to 46 HRC and a tensile strength of approximately 1448 MPa, or 210 ksi) from a front-end loader used in a salt-handling area broke after being in service approximately two weeks while operating at temperatures near -18 deg C (0 deg F). During the summer, similar shafts had a service life of 5 to eight months. Examination of the fracture surface showed brittle fatigue cracks, and visual examination of the splines disclosed heavy chatter marks at the root of the spline, with burrs and tears at the fillet area. Evidence found supports the conclusion that the shaft failed as the result of stress in the sharp fillets and rough surfaces at the root of the splines. Cold weather failure occurred sooner than in hot weather because ductile-to-brittle transition temperature of the 1040 steel shaft was too high. Recommendations include redesign of the fillet radius to a minimum of 1.6 mm (0.06 in.) and a maximum surface finish in the spline area of 0.8 microns. Material for the shafts should be modified to a nickel alloy steel, heat treated to a hardness of 28 to 32 HRC before machining.

This article reviews the general characteristics of fretting wear in mechanical components with an emphasis on steel. It focuses on the effects of physical variables and the environment on fretting wear. The variables include the amplitude of slip, normal load, frequency of vibration, type of contact and vibration, impact fretting, surface finish, and residual stresses. The form, composition, and role of the debris are briefly discussed. The article also describes the measurement, mechanism, and prevention of fretting wear. It concludes with several examples of failures related to fretting wear.

Flow forming technology has emerged as a promising, economical metal forming technology due to its ability to provide high strength, high precision, thin walled tubes with excellent surface finish. This paper presents experimental observations of defects developed during flow forming of high strength SAE 4130 steel tubes. The major defects observed are fish scaling, premature burst, diametral growth, microcracks, and macrocracks. This paper analyzes the defects and arrives at the causative factors contributing to the various failure modes.

One of the two AISI 4340 steel pitch horn bolts from the main rotor hub assembly failed while in service. Optical microscope revealed evidence of corrosion pitting in regions adjacent to the fracture. Fractographic examination utilizing a scanning electron microscope revealed multiple crack origins which assumed a “thumbnail” shape and displayed surface morphologies which resulted from intergranular decohesion. Many of the crack sites initiated from corrosion pits. Energy dispersive spectroscope performed on areas within the crack initiation site showed the presence of chlorides. The failure was attributed to stress-corrosion cracking. Short- and long-term recommendations to prevent future failures are given.

Sudden and unexplained bearing cap bolt fractures were experienced with reduced-shank design bolts fabricated from 42 CrMo 4 steel, quenched and tempered to a nominal hardness of 38 to 40 HRC. Fractographic analysis provided evidence favoring stress-corrosion cracking as the operating transgranular fracture failure mechanism. Water containing H7S was subsequently identified as the aggressive environment that precipitated the fractures in the presence of high tensile stress. This environment was generated by the chemical breakdown of the engine oil additive and moisture ingress into the normally sealed bearing cap chamber surrounding the bolt shank. A complete absence of fractures in bolts from one of the two vendors was attributed primarily to surface residual compressive stresses produced on the bolt shank by a finish machining operation after heat treatment. Shot cleaning, with fine cast shot, produced a surface residual compressive stress, which eliminated stress-corrosion fractures under severe laboratory conditions.

Breech bolt assemblies from the Gatling guns used on fighter aircraft failed during firing tests. Metallography of the failed components revealed considerable decarburization which resulted in a loss of surface hardness. It was also determined that the maraging steel components were not in the nitrided condition as was required. This resulted in lower wear and fatigue resistance. These components also had a silicon content nearly double of that specified. The high silicon content lowered the notch tensile strength and toughness of the components.

Cracks were discovered between interference-fit fasteners (MoS2-coated Ti-6Al-4V) that had been incorporated into a fighter aircraft primary structural frame (D6ac steel) to enhance structural fatigue life. Examination of sections cut from the cracked frame established that the cracks propagated by stress-corrosion cracking. The cause of cracking was twofold: use of interference-fit fasteners exposed to moisture intrusion from a marine environment and poor hole quality. Failure was intensified by dissimilar-metal contact in the presence of weak acidic electrolyte (dissociated MoS2). Control of machining parameters to prevent formation of brittle martensite, use of galvanically compatible fasteners, and use of an alternate lubricant were recommended.

A recurring piston shaft failure problem on the billet-loading tray of an extrusion press was investigated. Two shafts fractured within a period of 10 days. The shaft was machined from normalized EN3 (AISI C1022) steel stock without further treatment. Visual, microstructural, chemical, and mechanical (hardness and tensile properties) analyses of failed shaft specimens were conducted. The examinations showed that the shafts had failed by fatigue. It was recommended that a low-alloy steel (e.g., 3% Ni-Cr) in the hardened and tempered condition and subjected to shot-peening surface-hardening treatment be used. The provision of a stop to reduce bending stresses was also recommended.

The purpose of this investigation was to determine the root cause of the differences noted in the fatigue test data of main rotor spindle assembly retaining rods fabricated from three different vendors, as part of a Second Source evaluation process. ARL performed dimensional verification, accessed overall workmanship, and measured the respective surface roughness of the rods in an effort to identify any discrepancies. Next, mechanical testing was performed, followed by optical and electron microscopy, and chemical analysis. Finally, ARL performed laboratory heat treatments at the required aging temperature and follow-up mechanical testing.