A case-hardened sleeve made of C 15 (Material No. 1.0401) was flattened at two opposing sides and had cracked open at these places, the crack initiating at a face plane. The wall of the sleeve was 9 mm thick, but the flat ends were machined down to 5.5 mm from the outside. The customer had specified a 2 mm case depth and a hardness of at least HRC 55 at a depth of 1.5 mm. An etched cross section of the cracked end showed that the case layer had a depth of 2.3 mm, so that the sleeve was almost through-hardened at the flat ends. While the core material with the full wall thickness had the quench structure of low-carbon steel, the structure of the flattened area consisted of coarse acicular martensite with a small amount of pearlite (quench troostite) and ferrite. Therefore the sleeve was overheated and probably quenched directly from case. To prevent damage, it would have been necessary to have a lower case depth, carburize less deeply, and prevent overheating that causes brittleness and leads also to increased case depth, or else use a fine-grained steel of lower hardenability.

A crane long-travel worm drive shaft was found to be chipped during unpacking after delivery. Chemical analysis showed that the steel (EN36A with a case depth of 1 mm, or 0.04 inch did not meet specifications. Magnetic particle inspection revealed a crack on the side of the shaft opposite the chip. Metallographic examination indicated that the case depth was approximately 2 mm (0.08 in.) and that a repair weld of an earlier chip had been made in the cracked area. The chipping was attributed to excessive case depth and rough handling. It was recommended that the shaft be returned to the manufacturer and a replacement requested.

Ball joints made from carburized En 353 (BS970:815A16) steel failed after several hours of being fitted into vehicles. The parts were forged, machined, and thread rolled. The threads were copper plated to prevent carburization. The heat treatment consisted of carburizing in a cyanide bath for 12 hours at 930 deg C. After tempering for 2 h at 170 to 175 deg C, the copper plate was removed by immersing in an acid bath for 45 min. The investigations found the microstructure, hardness, and chemistry all met the specification. The case depth was approximately 0.75 mm to 1.0 mm. The SEM studies showed that it was a brittle fracture and completely intergranular to a depth of about 2.5 mm. It was concluded that the failure was due to hydrogen embrittlement for the following reasons: (i) failure did not occur immediately after loading, (ii) the fracture was intergranular to a depth of two to three times the case depth, (iii) secondary cracks were observed at the surface. The hydrogen was introduced during copper plate removal by acid dipping. If the tempering operation was performed after the acid dip operation, the hydrogen would have been driven out.

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.

Component slippage in the left-side final drive train of a tracked military vehicle was detected after the vehicle had been driven 13,700 km (8500 miles) in combined highway and rough-terrain service. The slipping was traced to the mating surfaces of the final drive gear and the adjacent splined coupling sleeve. Specifications included that the gear and coupling be made from 4140 steel bar oil quenched and tempered to a hardness of 265 to 290 HB (equivalent to 27 to 31 HRC) and that the finish-machined parts be single-stage gas nitrided to produce a total case depth of 0.5 mm (0.020 in.) and a minimum surface hardness equivalent to 58 HRC. Investigation (visual inspection, low-magnification images, 500X images of polished sections etched in 2% nital, spectrographic analysis, and hardness testing) supported the conclusion that the failure occurred by crushing, or cracking, of the case as a result of several factors. Recommendations included reducing the high local stresses at the pitch line to an acceptable level with a design modification. Also suggested was specification of a core hardness of 35 to 40 HRC to provide adequate support for the case and to permit attainment of the specified surface hardness of 58 HRC.

Within about one month, several knuckle pins (AMS 6470 steel failed, and required to have a minimum case hardness of 92 h15N, a case depth of 0.4 to 0.5 mm (0.017 to 0.022 in.), and a core hardness of 285 to 341 HRB) used in engines failed over a range of 218 to 463 h in operation. Visual examination revealed beach marks typical of fatigue cracks that had nucleated at the base of the longitudinal oil hole. Micrographs of sections revealed a remelt zone and an area of untempered martensite within the region of the cracks. However, review of inspection procedures disclosed the pins had been magnetic-particle inspected by inserting a probe into the longitudinal hole. Evidence found supports the conclusions that the knuckle pins failed by fatigue fracture. The circular cracks at the longitudinal holes were the result of improper technique in magnetic-particle inspection. Thermal transformation of the metal also causes a stress concentration that may lead to fatigue failure. Recommendations included insulating the conductor to prevent arc burning at the base of the longitudinal oil hole. Also, a borescope or metal monitor could be used to inspect the hole for evidence of arc burning from magnetic-particle inspection.

After two weeks of operation, a poppet used in a check valve to control fluid flow and with a maximum operating pressure of 24 MPa (3.5 ksi) failed during operation. Specifications required that the part be made of 1213 or 1215 rephosphorized and resulfurized steel. The poppet was specified to be case hardened to 55 to 60 HRC, with a case depth of 0.6 to 0.9 mm (0.025 to 0.035 in.); the hardness of the mating valve seat was 40 HRC. Analysis showed that the fracture occurred through two 8 mm (0.313 in.) diam holes at the narrowest section of the poppet. The valve continued to operate after it broke, which resulted in extensive loss of metal between the holes. 80x micrograph and 4x macrograph of a 5% nital etched longitudinal section, and chemical analyses showed the poppet did fit 1213 or 1215 specs. However, hardness measurements showed surface hardness was excessive-61 to 65 HRC instead of the specified 55 to 60 HRC. Thus, the poppet failed by brittle fracture, and cracking occurred across nonmetallic inclusions. Recommendation was to redesign the valve with the poppet material changed to 4140 steel, hardened, and tempered to 50 to 55 HRC.

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.

The electroplated tappet adjusting screws used in diesel engines failed during initial bend testing. The analysis of the failure showed that the fracture was nucleated from the subsurface of the screw. The fracture surface was intergranular at the ID and OD region and microvoid coalescence in the center. The improper baking after electroplating of the screw led to H2-induced blistering/cracking. The high strength of the threaded region of the adjusting screw increased the failure propensity.

A bull gear from a coal pulverizer at a utility failed by rolling-contact fatigue as the result of continual overloading of the gear and a nonuniform, case-hardened surface of the gear teeth. The gear consisted of an AISI 4140 Cr-Mo steel gear ring that was shrunk fit and pinned onto a cast iron hub. The wear and pitting pattern in the addendum area of the gear teeth indicated that either the gear or pinion was out of alignment. Beach marks observed on the fractured surface of the gear indicated that fatigue was the cause of the gear failure. Similar gears should be inspected carefully for signs of cracking or misalignment. Ultrasonic testing is recommended for detection of subsurface cracks, while magnetic particle testing will detect surface cracking. Visual inspection can be used to determine the teeth contact pattern.

Milling machine arbors were inserted with satellite spindles having a maximum speed of 1500 rpm, and broke out between the groove and the flange. The appearance of the fracture surface was the same on both arbors. The pronounced scan lines characterized the fractures as fatigue fractures. The appearance of the fracture in the arbors indicated ductile fatigue fracture which had its origin in the radii between groove and flange. These radii of 0.15 and 0.2 mm were too small for the load on the milling machine. In addition there were grooves at the base of the radii which had an unfavorable effect on the life of the component by acting as notches with their resulting stress concentration. Considering the great hardness of the case, the small radii would have been critical even without grooves. Measures were taken so that the critical radius of the milling machine was increased and the surface roughness measured more precisely.

Six wrist pins in a high-performance six-cylinder automotive engine failed after 4800 km (3000 mi) of normal operation. The pins were made of low-carbon steel that had been carburized both inside and outside. Two failed pins were examined. One had fractured into three pieces. The other had not fractured, but exhibited circumferential cracks on the surface of the central zone. Visual surface examination and metallographic and chemical analyses were performed on the specimens. Cracking was attributed primarily to poor heat treatment, resulting in a brittle grain-boundary network of cementite, and to a design that had a raised central section of the inner diameter whose fillets were locations of high stress concentration. Rough machining of the inner diameter and an excessively deep case also contributed to failure. A double type of heat treatment after carburizing and change of the design to eliminate the raised central section were recommended.

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.

A bearing cup in a drive shaft assembly on an automobile was found to have failed. A detailed analysis was conducted using the QC story approach, which begins by proposing several possible failure scenarios then following them to determine the main root cause. A number of alternative solutions were identified and then validated based on chemical analysis, endurance and hardness tests, and microstructural examination. The investigation revealed that carbonitriding can effectively eliminate the type of failure encountered because it prevents through hardening of the bearing cup assembly.

A spiral bevel gear set in the differential housing of a large front-end loader moving coal in a storage area failed in service. The machine had operated approximately 1500 h. Although the failure involved only the pinion teeth, magnetic particle inspection was performed on each part. The 4817 NiMo alloy steel pinion showed no indication of additional cracking, nor did the 4820 NiMo alloy steel gear. The mode of failure was tooth bending fatigue with the origin at the designed position: root radius at midsection of tooth. The load was well centered, and progression occurred for a long period of time. The cause of failure was a suddenly applied peak overload, which initiated a crack at the root radius. Progression continued by relatively low overstress from the crack, which was now a stress-concentration point. This was a classic tooth bending fatigue failure.