Evidence of destructive pitting on the gear teeth (AMS 6263 steel) in the area of the pitchline was exhibited by an idler gear for the generator drive of an aircraft engine following test-stand engine testing. The case hardness was investigated to be lower than specified and it was suggested that it had resulted from surface defects. A decarburized surface layer and subsurface oxidation in the vicinity of pitting were revealed by metallographic examination of the 2% nital etched gear tooth sample. It was concluded that pitting had resulted as a combination of both the defects. The causes for the defects were reported based on previous investigation of heat treatment facilities. Oxide layer was caused by inadequate purging of air before carburization while decarburization was attributed to defects in the copper plating applied to the gear for its protection during austenitizing in an exothermic atmosphere. It was recommended that steps be taken during heat treatment to ensure neither of the two occurred.

Several teeth of a bevel pinion which was part of a drive unit in an edging mill failed after three months in service. Specifications required that the pinion be made from a 2317 steel forging and that the teeth be carburized and hardened to a case hardness of 56 HRC and a core hardness of 250 HRB. Two teeth were revealed by visual examination to have broken at the root and fatigue marks extending across almost the entire tooth were exhibited by the surface of the fracture. Cracking in all the tooth was showed by magnetic-particle inspection. The pinion was concluded to have failed by tooth-bending fatigue. Spalling was also noted on the pressure (drive) side of each tooth at the toe end which indicated some mechanical misalignment of the pinion with the mating gear that caused the cyclic shock load to be applied to the toe ends of the teeth.

Two intermediate impeller drive gears (made of AMS 6263 steel, gas carburized, hardened, and tempered) exhibited evidence of pitting and abnormal wear after production tests in test-stand engines. The gears were examined for hardness, case depth, and microstructure of case and core. It was found that gear 1 had a lower hardness than specified while the case hardness of gear 2 was found to be within limits. Both the pitting and the wear pattern were revealed to be more severe on gear 1 than on gear 2. Surface-contact fatigue (pitting) of gear 1 (cause of lower carbon content of the carburized case and hence lower hardness) was found to be the reason for failure. It was recommended that the depth of the carburized case on impeller drive gears be increased from 0.4 to 0.6 mm to 0.6 to 0.9 mm to improve load-carrying potential and wear resistance. A minimum case-hardness requirement was set at 81 HRA.

The failure of a spiral bevel gear from the transmission of a helicopter was discovered when the transmission was removed after an in-flight incident. Two adjacent teeth from the carburized AISI 9310 steel gear were found to have undergone fatigue failure. Internal initiation occurred in a region depleted of chromium and nickel. This condition coincides with a microstructural inhomogeneity consisting of large, soft ferrite grains. Its origin was probably contamination of the solidifying ingot during the consumable vacuum arc remelting operation.

The gear of a spiral bevel gear set broke into three pieces after about two years of service. The gear (made of 4817 steel) broke along the root of a tooth intersected by three of the six 22-mm diam holes used to mount the gear to a hub. Fatigue progression for about 6.4 mm at the acute-angle intersections of three mounting holes with the root fillets of three teeth was revealed by examination of gear. Cracks at the intersections of the remaining three mounting holes and the adjacent tooth-root fillets were revealed by magnetic-particle inspection. Through hardening at the acute-angle intersections of the mounting holes and tooth-root fillets was revealed by metallographic examination. Design of the gear and placement of the mounting holes, which resulted in through hardening, were concluded to be the contributing factors to the fatigue failure of the gear.

A tool used to stretch reinforcement wires in prestressed concrete failed. All eight individual jaws were broken. Visual examination of the fracture surfaces indicated that about half of the broken parts had a partially dendritic appearance. Further, fracture surfaces near the exteriors of the parts were clean and smooth, and there was evidence of a case. Examination of the flat surfaces of the parts revealed surface cracking where actual failure had not occurred. Chemical analysis showed the material to be a low-alloy carburizing steel. The microstructure was compatible with a steel which is cast, carburized, quenched, and tempered. The structure was generally satisfactory, except for the presence of severe shrinkage porosity. It was concluded that the presence of shrinkage porosity in critical areas was the primary cause of fracture. Extremely high hardness indicating a lack of adequate tempering was the secondary cause.

Butterfly-shaped microstructural features in tempered martensite in an otherwise clean steel suggested that overloading led to premature spalling of a coal-crushing plant taper bearing. Extensive rolling contact fatigue occurred because of the overload condition. The crusher was designed to handle soft lignite coals but had been used to crush hard deep-mined anthracite coals.

A cylindrical spiral gear, part of a locomotive axle assembly, cracked ten days after it had been press-fit onto a shaft, after which it sat in place as other repairs were made. Workers at the locomotive shop reported hearing a sound, and upon inspecting the gear, found a crack extending radially from the bore to the surface of one of the tooth flanks. The crack runs the entire width of the bore, passing through an oil hole in the hub, across the spoke plate and out to the tip of one of the teeth. Design requirements call for the gear teeth to be carburized, while the remaining surfaces, protected by an anti-carburizing coating, stay unchanged. Based on extensive testing, including metallographic examination, microstructural analysis, microhardness testing, and spectroscopy, the oil hole was not protected as required, evidenced by the presence of a case layer. This oversight combined with the observation of intergranular fracture surfaces and the presence of secondary microcracks in the case layer point to hydrogen embrittlement as the primary cause of failure. It is likely that hydrogen absorption occurred during the gas carburizing process.

This article briefly reviews the various metallurgical or environmental factors that cause a weakening of the grain boundaries and, in turn, influence the occurrence of intergranular (IG) fractures. It discusses the mechanisms of IG fractures, including the dimpled IG fracture, the IG brittle fracture, and the IG fatigue fracture. The article describes some typical embrittlement mechanisms that cause the IG fracture of steels.

This article briefly reviews the factors that influence the occurrence of intergranular (IG) fractures. Because the appearance of IG fractures is often very similar, the principal focus is placed on the various metallurgical or environmental factors that cause grain boundaries to become the preferred path of crack growth. The article describes in more detail some typical mechanisms that cause IG fracture. It discusses the causes and effects of IG brittle cracking, dimpled IG fracture, IG fatigue, hydrogen embrittlement, and IG stress-corrosion cracking. The article presents a case history on IG fracture of steam generator tubes, where a lowering of the operating temperature was proposed to reduce failures.

Gears can fail in many different ways, and except for an increase in noise level and vibration, there is often no indication of difficulty until total failure occurs. This article reviews the major types of gears and the basic principles of gear-tooth contact. It discusses the loading conditions and stresses that effect gear strength and durability. The article provides information on different gear materials, the common types and causes of gear failures, and the procedures employed to analyze them. Finally, it presents a chosen few examples to illustrate a systematic approach to the failure examination.

An extensive metallurgical investigation was carried out on samples of a failed roller bearing from the support and tilting system of a basic oxygen furnace converter used in the steel melting shop of an integrated steel plant. The converter bearing was fabricated from low-carbon, carburizing grade steel and had failed in service within a year of fitting to a repaired shaft. Microscopic observations of both the broken roller and inner-race samples revealed subsurface cracking and preponderance of brittle oxide and other macroinclusions. Electron probe microanalysis studies confirmed that the brittle oxides that formed stringers were alumina, and the other macroinclusions were complex silicates. Both the alumina and silicate inclusions were deleterious to contact-fatigue properties. Microstructurally, the carburized regions of the broken roller and of inner-race samples contained high-carbon tempered martensite. Microhardness measurements revealed that. Although the core hardness of the roller and the inner-race samples were similar, the surface hardness of the roller was approximately 8.5 HRC units harder than that of the inner-race. SEM observations of the roller fracture surface revealed striations indicative of fatigue, and EDS analyses corroborated a high incidence of silicate inclusions at crack sites. The study suggests that the failure of the bearing occurred because the hardness difference between the roller bearing and the inner-race surfaces resulted in wear of the inner-race. The wear led to shaft misalignment and play during service. The misalignment, coupled with the presence of inclusions, caused fatigue failure of the roller bearing.