Search Results for Carburized steel

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.

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.

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.

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.