A 6150 flat spring was found to be failed. The face of the spring was revealed to be under tensile stress. The failure was concluded to have begun at the dark spot on the edge where roughness resulted from shearing during the blanking operation.

Two outer valve springs made from air-melted 6150 pretempered steel wire broke during production engine testing. The springs were 50 mm in OD and 64 mm in free length, had five coils and squared-and-ground ends, and were made of 5.5 mm diam wire. It was revealed that fracture was nucleated by an apparent longitudinal subsurface defect. The defect was revealed by microscopic examination to be a large pocket of nonmetallic inclusions (alumina and silicate particles) at the origin of the fracture. Partial decarburization of the steel was observed at the periphery of the pocket of inclusions. Torsional fracture was indicated by the presence of beach marks at a 45 deg angle to the wire axis. It was established that the spring fractured by fatigue nucleated at the subsurface defect.

Initial investigation showed that a landing gear failure was the result of a hard landing with no evidence of contributory factors. The objective of reexamination was to determine whether there was any evidence of metallurgical failure. The landing gear was primarily an AISI type 6150 Cr-V steel flat spring attached at the top end to the fuselage and at the bottom end to the axle. Failure occurred at the clamping point near the top end of this spring. The failure showed evidence of severe brinelling at one corner in the clamping area. The fracture surfaces were clean, fresh, and indicative of a shock type of failure pattern. Closer examination, however, showed a fatigue crack at one corner. At this point, there was definite evidence of progression and oxidation. It was concluded that the corner in question was subjected to repeated brinelling resulting from normal landing loads, probably accentuated by looseness in the clamping device. The resulting residual tensile stress lowered the effective fatigue strength at that point against drag and side loads.

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.

After being in service for ten years the ball-and-race coal pulverizer was investigated after noises were noted in it. Its lower grinding ring was attached to the 6150 normalized steel outer main shaft while the upper grinding ring was suspended by springs from a spider attached to the shaft. A circumferential crack in the main shaft at an abrupt change in shaft diam just below the upper radial bearing was revealed by visual examination. The smaller end of the shaft was found to be slightly eccentric with the remainder when the shaft was set up in a lathe to machine out the crack for repair welding. The crack was opened by striking the small end of the shaft and the shaft was broken 1.3 cm away from the crack in the process. A previous fracture that resulted from torsional loading acting along a plane of maximum shear was revealed almost perpendicular to the axis of the shaft. Faint lines parallel to the visible crack thought to be fatigue cracks were revealed on examination of the machined surface. The shaft was repaired by welding a new section and machined to required diameters and tapers to avoid abrupt changes.

A shaft can crack twice before it fails. A Detroit electric plant had this experience with one in a coal pulverizer. Because the first crack rewelded partially (by friction) in service, the pulverizer remained serviceable until the second crack developed.

A flat spring for the main landing gear of a light aircraft failed after safe execution of a hard landing. The spring material was identified by chemical analysis to be 6150 steel. The fracture was found to have occurred near the end of the spring that was inserted through a support member about 25 mm thick and attached to the fuselage by a single bolt. Brinelling (plastic flow and indentation due to excessive localized contact pressure) was observed on the upper surface of the spring where the forward and rear edges of the spring contacted the support member. It was indicated by chevron marks that brittle fracture had started beneath the brinelled area at the forward edge of the upper surface of the spring. The origin of the brittle fracture was found to be a small fatigue crack that had been present for a considerable period of time before final fracture occurred. Fracture of the landing-gear spring was concluded to have been caused by a fatigue crack that resulted from excessive brinelling at the support point. Regular visual examinations to detect evidence of brinelling and wear at the support in aircraft with this configuration of landing-gear spring were recommended.

Several torsion bars had failed in a projectile weaving machine and were analyzed to determine the cause. Specimens prepared from the damaged components were subjected to visual inspection, hardness testing, chemical analysis, and metallurgical evaluations. The failed torsion bars had been fabricated from spring steel which, according to stress calculations, did not have sufficient torsional strength. Examination of the damaged parts confirmed the finding, revealing that all fractures started at a shoulder radius in an area of high stress concentration. Based on the investigation, the shoulder radius should be increased to alleviate stress and the working torsion angle of the bar should be decreased to improve safety factors.