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.

The spindle of a helicopter-rotor blade fractured after 7383 h of flight service. At every overhaul (the spindle that failed was overhauled six times and reworked twice), any spindle that showed wear was reworked by grinding the shank to 0.1 mm (0.004 in.) under the finished diam. The spindle was then shot peened with S170 shot to an Almen intensity of 0.010 to 0.012 A. Following shot peening, the shank was nickel sulfamate plated to 0.05 mm (0.002 in.) over the finished diam, ground to finished size, and cadmium plated. Visual and stereomicroscopic exam showed faint grinding marks and circumferential grooves on the surface near the fillet at the junction of the shank and fork, which should have been peened over and covered with peening dimples. Evidence found supports the conclusions that the spindle failed in fatigue that originated near the junction of the shank and fork. The nonuniformity of the shot-peened effect on the shank and fillet portions of the spindle resulted from incomplete peeing. The fracture was of the low-stress high-cycle type, initiated by stresses well below the gross yield strength and propagated by thousands of load cycles. No recommendations were made.

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.

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.

This article describes the characteristics of tools and dies and the causes of their failures. It discusses the failure mechanisms in tool and die materials that are important to nearly all manufacturing processes, but is primarily devoted to failures of tool steels used in cold-working and hot-working applications. It reviews problems introduced during mechanical design, materials selection, machining, heat treating, finish grinding, and tool and die operation. The brittle fracture of rehardened high-speed steels is also considered. Finally, failures due to seams or laps, unconsolidated interiors, and carbide segregation and poor carbide morphology are reviewed with illustrations.

Work rolls made of indefinite chill double-poured (ICDP) iron are commonly used in the finishing trains of hot-strip mills (HSMs). In actual service, spalling, apart from other surface degeneration modes, constitutes a major mechanism of premature roll failures. Although spalling can be a culmination of roll material quality and/or mill abuse, the microstructure of a broken roll can often unveil intrinsic inadequacies in roll material quality that possibly accentuate failure. This is particularly relevant in circumstances when rolls, despite operation under similar mill environment, exhibit variations in roll life. The paper provides an insight into the microstructural characteristics of spalled ICED HSM work rolls, which underwent failure under similar mill operating environment in an integrated steel plant under the Steel Authority of India Limited. Microstructural features influencing ICDP roll quality, viz. characteristics of graphite, carbides, martensite, etc., have been extensively studied through optical microscopy, quantitative image analysis (QIA), and electron-probe microanalysis (EPMA). These are discussed in the context of spalling propensity and roll life.

Two large tension springs fractured during installation. The springs were manufactured from a grade 9254 chromium-silicon steel spring wire. The associated material specification allows wire in the cold-drawn or oil-tempered (quenched-and-tempered) condition. The specified wire tensile strength range was 1689 to 1793 MPa (245 to 260 ksi). The finished springs were to be shot peened for greater fatigue resistance. Investigation (visual inspection, 3x images, 2% nital etched 148x SEM images, chemical analysis, hardness testing, and EDS analysis) supported the conclusion that the springs failed during installation due to the presence of preexisting defects. Crack surfaces were found to be corroded and phosphate coated, indicating that the cracks occurred during manufacture. Installation, which presumably entailed some axial extension, resulted in ductile overload failure at the crack sites. Recommendations included evaluating the manufacturing steps to identify the process(es) wherein the cracking was likely occurring. It was further recommended that a suitable nondestructive method such as magnetic particle inspection be implemented.

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 case study involves two continuously cast steel crankshaft failures. Three parties performed their own failure analyses: (1) the engine manufacturer responsible for component design, specification, and application; (2) the steel supplier and forging supplier responsible for making the steel, forging the shape, and preliminary heat treatment; and (3) a supplier that provided induction hardening, finish machining, and inspection. An independent engineering firm was subsequently involved, but because each party had its own agenda, there was no agreement on the metallurgical source of the failure and thus no continued analysis to pin down and eliminate the root cause.

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 pointed ends of several stainless steel forceps split or completely fractured where split portions broke off. All the forceps were delivered in the same lot. The pointed ends of the forceps are used for probing and gripping very small objects and must be true, sound, and sharp. Analysis supported the conclusion that the failures to be the result of seams in the steel that were not joined during hot working. Recommendations included that closer inspection of the product take place at all stages of manufacturing. Inspection at the mill will minimize discrepancies at the source, and the inspection of the finished product will help detect obscure seams.

A suspension bushing separated from the disk on the rear wheel suspension of a racing vehicle while under operation on an express highway, causing the wheel to detach from the car. Visual inspection showed fresh turning grooves at four built-up fillet welds on the torus of the outside of the disk. The welding heat and rapid cooling caused the base material of the disk – already with a martensitic structure – to harden throughout the torus, both of which caused cracking in the inner and outer fillets at the transition from bushing to disk. Visual inspection of the other rear wheel showed similar stress cracks in the hardened base material of the transition region as well as the same four welds. Rough finishing and the sharp-edged formation of the cross section transition may have also contributed to the failure. The results of the investigation suggested that machine shops neither execute nor permit repair-welds on highly stressed machine parts and especially vehicle components.

A hollow, splined alloy steel aircraft shaft (machined from an AMS 6415 steel forging – approximately the same composition as 4340 steel – then quenched and tempered to a hardness of 44.5 to 49 HRC) cracked in service after more than 10,000 h of flight time. The inner surface of the hollow shaft was exposed to hydraulic oil at temperatures of 0 to 80 deg C (30 to 180 deg F). Analysis (visual inspection, 15-30x low magnification examination, 4x light fractograph, chemical analysis, hardness testing) supported the conclusions that the shaft cracked in a region subjected to severe static radial, cyclic torsional, and cyclic bending loads. Cracking originated at corrosion pits on the smoothly finished surface and propagated as multiple small corrosion-fatigue cracks from separate nuclei. The originally noncorrosive environment (hydraulic oil) became corrosive in service because of the introduction of water into the oil. Recommendations included taking additional precautions in operation and maintenance to prevent the use of oil containing any water through filling spouts or air vents. Also, polishing to remove pitting corrosion (but staying within specified dimensional tolerances) was recommended as a standard maintenance procedure for shafts with long service lives.

This report covers case histories of failures in fixed-wing light aeroplane and helicopter components. A crankshaft of AISI 4340 Ni-Cr-Mo alloy steel, heat treated and nitrided all over, failed in bending fatigue. The nitrided layer was ground too rapidly causing excessive heat generation which induced grinding cracks and grinding burn. Tensional stresses resulting from grinding developed in a thin surface layer. On another crankshaft, chromium plating introduced undesirable residual tensile stresses. Such plating is an unsatisfactory finish for crankshafts of aircraft engines. Aircraft engine manufacturers and aeronautical standards require magnetic particle inspection to detect grinding cracks after reconditioning. Renitriding after any grinding is needed also, regardless of the amount of undersize as it introduces beneficial residual compressive stresses.

A steel eyebolt which attached a rear lift strut to the right wing of a helicopter failed by fatigue. As a contributing factor, thread cutting produced sharp notches at thread roots, reducing fatigue life. Also, design fatigue life may have been exceeded as the part was in use about 10,000 h. Cumulative damage resulting from a previous accident could have occurred too. Because of this accident, inspectors were instructed to examine threaded zones of eyebolts by magnetic particle inspection after every 100 h in service. A maraging steel drive shaft of a helicopter also failed because of corrosion (pits), and continuous abnormal misalignment as well. Corrosion probably developed from moisture and water droplets on shaft diaphragm profiles. Improved diaphragm pack seals and coatings made by an electron-coat process (such as a Sermetal finish) are now used in new shafts.

The connecting end of two forged medium-carbon steel rods used in an application in which they were subjected to severe low-frequency loading failed in service. The fractures extended completely through the connecting end. The surface hardness of the rods was found to be lower than specifications. The fractures were revealed to be in areas of the transition regions that had been rough ground to remove flash along the parting line. The presence of beach marks, indicating fatigue failure, was revealed by examination. The fracture origin was confirmed by the location and curvature of beach marks to be the rough ground surface. An incipient crack 9.5 mm along with several other cracks on one of the fractured rods was revealed by liquid penetration examination. Metallographic examination of the fractured rods indicated a banded structure consisting of zones of ferrite and pearlite. It was established that the incipient cracks found in liquid-penetrant inspection had originated at the surface in the banded region, in areas of ferrite where this constituent had been visibly deformed by grinding. Closer control on the microstructure, hardness of the forgings and smooth finish in critical area was recommended.