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.

An automobile manufacturer rejected several 1035 steel stub axles because of what appeared to be short longitudinal cracks in the surfaces of the pins. The cracks were found when six axles were examined for defects by magnetic-particle inspection. However, metallographic examination showed that these lines were not cracks but slag inclusions at and immediately below the surface. Analysis (visual inspection, metallographic examination, and 100x/500x magnetic-particle inspection) supported the conclusions that the inclusions consisted of pieces of fireclay from channel brick that were flushed into the ingot mold. Although no true cracks were present, rejection of the stub axles was nevertheless justified. Slag streaks could reduce the strength of the stub axles and lead to the formation of fatigue fractures during operation. No recommendations were made.

The heads of two AISI 8740 steel bolts severed while being installed into an Army tank recoil mechanism. Both broke into two pieces at the head-to-shank radius and the required torque value had not been attained nor exceeded prior to the failure. A total of 69 bolts from inventory and the field were tested by magnetic particle inspection. One inventory bolt failed because of a transverse crack near the head-to-shank radius. It was deduced that either a 100% magnetic particle inspection had not been conducted during bolt manufacturing, or the crack went undetected during the original inspection. Optical and electron microscopy of the broken bolts revealed topographies and the presence of black oxide consistent with quench cracking. The two bolts failed during installation due to the presence of pre-existing quench cracks. Recommendations to prevent future failures include: ensuring that 100% magnetic particle inspections are conducted after bolts are tempered; using dull cadmium plate or an alternative to the electrode position process, such as vacuum cadmium plate or ion-plate or ion-plated aluminum, to mitigate the potential for delayed failures due to hydrogen embrittlement or stress-corrosion cracking; ensuring that the radius at the shoulder/shank interface conforms to specifications; and replacing all existing bolts with new or reinspected inventory bolts.

An integral coupling and gear (Cr-Mo steel), used on a turbine-driven main boiler-feed pump, was removed from service after one year of operation because of excessive vibration. Spectrographic analysis and metallographic examination revealed the fact that gritty material in the gear teeth (found at visual inspection) was composed of the same material as the metal in the coupling. Beach marks and evidence of cold work, typical of fatigue failure, were found on the fracture surface. Chips remaining in the analysis cut were difficult to remove, indicating a strong magnetic field in the part. Evidence found supports the conclusions that failure of the coupling was by fatigue and that incomplete demagnetization of the coupling following magnetic-particle inspection caused retention of metal chips in the roots of the teeth. Improper lubrication caused gear teeth to overheat and spall, producing chips that eventually overstressed the gear, causing failure. Because the oil circulation system was not operating properly, metal chips were not removed from the coupling. Recommendations included checking the replacement coupling for residual magnetism and changing or filtering the pump oil to remove any debris.

The 1040 steel crankshaft in a reciprocating engine cracked within one year of operation. The journals of the main and crankpin bearings were inspected by the magnetic-particle method. Three to six indications of 1.5 to 9.5 mm long discontinuities were observed in at least four of the main-bearing journals. A crack along the fillet, almost entirely through the web, was observed in one of the main-bearing journals. Numerous coarse segregates, identified as sulfide inclusions, were identified by macroetching the surface during metallographic examination of a section taken through the main-bearing journal at the primary crack. Fatigue cracking with low-stress high-cycle characteristics was disclosed during macroscopic examination of the crack surface. Sulfide inclusions, which acted as stress raisers, were found to be present in the region where cracking originated. As a corrective measure, ultrasonic inspection was used in addition to magnetic-particle inspection to detect discontinuities.

A lever (machined from a casting made of AISI type 410 stainless steel, then surface hardened by nitriding) that was a component of the main fuel-control linkage of an aircraft engine fractured in flight after a service life of less than 50 h. Investigation (radiographic inspection) supported the conclusions that the lever broke at a cold shut extending through approximately 95% of the cross section. The normally applied load constituted an overload of the remainder of the lever. Recommendations included adding magnetic-particle inspection to the inspection procedures for this cast lever.

A 9310 steel gear was found to be defective after a period of engine service. A linear crack approximately was discovered by routine magnetic-particle inspection of an electron beam welded joint that attached a hollow stub shaft to the web of the gear. The welding procedure had a cosmetic weld pass on top of the initial full-penetration weld. There were no other known service failures of gears were welded by this method. One zone of the welded joint showed incomplete fusion, surrounded by two zones containing fatigue beach marks This indicated that the incomplete-fusion zone was the site at which primary fracture originated. The possible causes of incomplete-fusion include localized magnetic deflection of the electron beam, a momentary arc-out of the electron beam, and eccentricity in the small weld diam. The failure was attributed to fatigue originating at the local unfused interface of the electron beam weld, which had been the result of a deviation in the welding procedure. Examination of the possible causes of failure gave no evidence that a recurrence of the defect had ever occurred. Thus, there was no basis on which to recommend a change in design, material, or welding procedure.

A motorboat engine connecting rod forged from carbon steel fractured in two places and cracked at the small end during service. The analysis (visual inspection, 50x micrographs of sections etched with 2% nital, magnetic-particle inspection, and metallographic examination) supported the conclusion that the connecting rods were rendered susceptible to fatigue-crack initiation and propagation by the notch effect of coarse folds formed during the forging operation. One fracture was caused by fatigue resulting from operating stresses, and the other was a secondary tensile fracture. No recommendations were made.

The 14-cm diam main hoist shaft of a mobile shovel was found to have multiple crack indications when ultrasonically inspected in the field. A crack around the entire circumference at the change in section was revealed by magnetic-particle inspection of the shaft. The crack was found to coincide with the junction of the fillet and the smaller diam at this change in section. A slight step in the continuity of the fillet and some machining marks were noted at this junction. A fine crack extending 2.5 mm from the surface and originating at the machining marks was revealed by microscopic examination. The shaft was identified by chemical analysis to be 1040 steel (hardness 170 HRB) which was concluded to have insufficient fatigue strength. The step at the base of the fillet was revealed as the point of initiation of the fatigue crack. Shaft material was changed to 4140 steel oil-quenched and tempered to a hardness of 302 to 352 HRB and all machining discontinuities were removed.

The large steel ring produced for a nuclear application from a billet of 8822 steel was inspected. The large billet was first forged into a doughnut preform in a large press, and then formed into the ring by ring rolling. A straight-beam ultrasonic inspection was instituted and calibrated using the back-surface-reflection method to determine whether adequate ultrasonic penetration was available. Areas of indications were noted at approximately midheight and adjacent to the bore area. An axial angle-beam inspection from the outside was performed, mainly in the area of indications to reveal detectable indications. The indications were not considered serious enough to reject the forgings. A few small indications in the areas tested were revealed by magnetic particle inspection. The area was conditioned by grinding and polishing to obtain an additional inspection at a greater depth from the inside surface. A much more severe condition was revealed after the test. The indications were classified as areas of chemical segregation and nonmetallic inclusions. The ring was considered unsatisfactory for the application and replacement of the defective ring from an acceptable billet was the most economical solution.

Drastic reduction in the service life of a production gearbox was observed. Within the gearbox, the axial load on a bevel gear (8620 steel, OD 9.2 cm) was taken by a thrust-type roller bearing (3.8 cm ID, 5.6 cm OD) in which a ground surface on the back of the bevel gear served as a raceway. Spalling damage on the ground bearing raceway at five equally spaced zones was disclosed by inspection of the bevel gear. The bearing raceway was checked for runout by mounting the gear on an arbor. It was found that the raceway undulated to the extent of 0.008 mm total indicator reading and a spalled area was observed at each high point. The presence of numerous cracks that resembled grinding cracks was revealed both by magnetic-particle inspection and microscopic examination. Spalling was produced by nonuniform loading in conjunction with grinding cracks. As corrective measures, the spindle of the grinding machine was reconditioned to eliminate the undulations and retained austenite was minimized by careful heat treatment.

Alloy steel forgings used as structural members of a ski chair lift grip mechanism were identified to have contained forging laps (i.e., sharp-notched discontinuities) during an annual magnetic particle inspection of all chair lift grip structural members at a mountain resort. The material was confirmed to be 34Cr-Ni-Mo6. A heavy oxide on the dark area of one of the broken-open laps was revealed by scanning electron microscopy in conjunction with EDS. A bright area that contained ductile dimple rupture was observed adjacent to the dark area. The oxidized portion of the fracture was established to be the preexisting forging lap while the bright area was created during the breaking-open process. As a corrective action all forgings showing laps were recommended to be removed from service. Critical review and revision of the forging process and revisions to the nondestructive evaluation procedures at the forging supplier was recommended.

A bridge wheel from a 272,160 kg stripper crane fractured in the web near the rim after one year of service. The wheel was forged from 1055 steel, and the tread, hub faces, and hub bore were machined. Beach marks indicative of fatigue at ten locations were revealed by macroscopic examination of the fracture surfaces. The surface of the web was heavily scaled and decarburized. A gross forging defect extending about 1.8 mm along the fracture surface was disclosed by examination of a micrograph of a section through one of the fatigue origins. Shallower forging defects were visible along the web surface. Fatigue cracking of the wheel was initiated at forging defects in the web. Replacement wheels were machined all over and were magnetic particle inspected to detect any cracks that could act as stress raisers.

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.

In a spring leg of a main landing gear, large brittle fracture zones indicated a predominately cleavage pattern with some ductile dimples, and a tiny fatigue segment disclosed fine striations. Factors influencing failure were surface decarburization, notch sensitivity of the modified SAE 6150 spring steel, Canada's cold weather which may have had an embrittling effect on the steel, and cumulative fatigue damage from severe landing loads during service life. Replacement with heavier-duty spring legs will probably not eliminate this type of failure, but their use has reduced the number of failures substantially. Precautionary measures recommended to preclude accidents include removal of decarburization, proper operation of main landing gears, and adequate magnetic particle inspection of the legs at the beginning and end of the ski season to detect any fatigue cracks that might develop in attachment holes.

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.

Hydraulic cylinder housings were being fabricated from 4140 grade seamless steel tubing. During production, magnetic-particle inspection indicated the presence of circumferential and longitudinal cracks in a large number of cylinders. Analysis (visual inspection, dye penetrant inspection, 50x/90x/400x SEM micrographs, and metallographic analysis) supports the conclusion that the cracking problem in these components was identified as quench cracks due to their brittle, intergranular nature and the characteristic temper oxide on the fracture surfaces. Although the steel met the compositional requirements of SAE 4140, the sulfur level was 0.022% and would account for the formation of the sulfide stringers observed. Apparently, the combination of the clustered, stringer-type inclusions and the quenching conditions were too severe for this component geometry. The result was a high incidence of quench cracks that rendered the parts useless. Recommendations included changing the specification, requiring the steel to have lower sulfur concentrations. Magnetic-particle cleanliness standards should be imposed that will exclude material with harmful clusters of sulfide stringers, for example, modified AMS 2301.