Textile-machine crankshafts forged from 4140 steel fractured transversely on one cheek during one to three years of service. The cause of failure for two forgings (one complete fractured forging and second a section that contained the shorter shaft fracture cheek) was determined. Indication of fatigue failure was revealed by visual examination of the fracture surfaces. Rough grooves from hot trimming of the flash were visible on the surface of the cheeks. The outer face of one cheek of the throw on the forging contained shallow surface folds. Slightly decarburized forged surface was identified around one of the folds and a fatigue crack initiated in the fold and propagated across the cheek. Properties representative of 4140 steel, quenched and tempered to a hardness of 20 to 22 HRC, were observed. Tempered bainite was revealed in the general microstructure. As a corrective measure, the forgings were normalized, hardened and tempered to 28 to 32 HRC before being machined to increase fatigue strength and extremely rough surfaces were removed by careful grinding.

A bridge wheel on a crane, forged from 1055 steel, fractured after one year of service. The wheel fractured in the web between the hub and the rim. A small area containing beach marks that originated in a heavily burned area on the web surface was revealed by visual examination of the fracture surface. Surface burning to a depth of approximately 0.8 mm was disclosed by metallographic examination of a section taken through the region that contained the beach marks. A forging defect was indicated by the degree of decarburization and oxide dispersion that were visible. The failure was concluded to have been caused by surface burning during the forging operation. As a preventive measure more closely controlled heating practice during forging to eliminate surface burning was recommended. The burnt region was suggested to be removed in case burning occurs.

The fan drive support shaft, specified to be made of cold-drawn 1040 to 1045 steel, fractured after 2240 miles of service. It was revealed by visual examination of the shaft that the fracture had initiated near the fillet at an abrupt change in shaft diameter. The cracks originated at two locations approximately 180 deg apart on the outer surface of the shaft and propagated toward the center. Features typical of reversed-bending fatigue were exhibited by the fracture. A tensile specimen was machined from the center of the shaft and it indicated much lower yield strength (369 MPa) than specified. It was disclosed by metallographic examination that the microstructure was predominantly equiaxed ferrite and pearlite which indicated that the material was in either the hot-worked or normalized condition. An improvement of fatigue strength of the shaft by the development of a quenched-and-tempered microstructure was recommended.

Stainless steel liners (AISI type 321) used in bellows-type expansion joints in a duct assembly installed in a low-pressure nitrogen gas system failed in service. The duct assembly consisted of two expansion joints connected by a 32 cm (12 in.) OD pipe of ASTM A106 grade B steel. Elbows made of ASTM A234 grade B steel were attached to each end of the assembly, 180 deg apart. A 1.3 mm (0.050 in.) thick liner with an OD of 29 cm (11 in.) was welded inside each joint. The upstream ends were stable, but the downstream ends of the liners remained free, allowing the components to move with the expansion and contraction of the bellows. Investigation (visual inspection, hardness testing, and 30x fractographs) supported the conclusion that the liners failed in fatigue initiated at the intersection of the longitudinal weld forming the liner and the circumferential weld by which it attached to the bellows assembly. Recommendations included increasing the thickness of the liners from 1.3 to 1.9 mm (0.050 to 0.075 in.) in order to damp some of the stress-producing vibrations.

The stub-shaft assembly which was part of the agitator shaft in a polyvinyl chloride reactor, fractured in service after a nut that retained a loose sleeve around the smaller-diam section of the shaft had been tightened several times to reduce leakage. The shaft was made of ASTM A105, grade 2 steel, and the larger-diam section was covered with a type 316 stainless steel end cap. The cap was welded to each end using type ER316 stainless steel filler metal. The forged steel shaft was revealed to have fractured at approximately 90 deg to the shaft axis in the weld metal and not in the heat-affected zone of the forged steel shaft. Microscopic investigation and chemical analysis of the steel shaft revealed presence of martensite (offered a path of easy crack propagation) around the fusion line and dilution of the weld metal by the carbon steel shaft. The microstructure was found to be martensitic as the fusion line was approached. The forged steel shaft was concluded to have failed by ductile fracture and possible reasons were discussed. Corrective measures adopted in the replacement shaft were specified.

To ensure no malfunctions and although there were no apparent problems, a main fuel control was returned to the factory for examination after service on a test aircraft engine that had experienced high vibrations. When the fuel control was disassembled, a lever, cast from AMS 5350 (AISI type 410) stainless steel that was through-hardened to 26 to 32 HRC and passivated, was shown to be cracked. The crack initiated at the sharp corner of the elongated milled slot and propagated across to the outer wall. The sections around the crack were spread about 30 deg apart, showing the fracture surface under investigation had beach marks initiating at the sharp corner along the milled slot. Changes in frequency or amplitude of vibration caused different rates of propagation, resulting in a change in pattern. This evidence supported the conclusion that the lever failed in fatigue as a result of excessive vibration of the fuel control on the test engine. Recommendations included redesign of the lever with a large radius in the corner where cracking originated. This would reduce the stress-concentration factor significantly, thus minimizing the susceptibility of the lever to fatigue.

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.

An articulated rod (made from 4337 steel (AMS 6412) forging, quenched and tempered to 36 to 40 HRC) used in an overhauled aircraft engine was fractured after being in operation for 138 h. Visual examination revealed that the rod was broken into two pieces 6.4 cm from the center of the piston-pin-bushing bore. The fracture was nucleated at an electroetched numeral 5 on one of the flange surfaces. A notch, caused by arc erosion during electroetching, was revealed by metallographic examination of a polished-and-etched section through the fracture origin. A remelted zone and a layer of untempered martensite constituted the microstructure of the metal at the origin. Small cracks, caused by the high temperatures developed during electro-etching, were observed in the remelted area. It was concluded that fatigue fracture of the rod was caused by the notch resulting from electroetching and thus electroetched marking of the articulated rods was discontinued as a corrective measure.

The safety valve on a steam turbogenerator was set to open when the steam pressure reaches 2400 kPa (348 psi). The pressure had not exceeded 1790 kPa (260 psi) when the safety-valve spring shattered into 12 pieces. The steam temperature in the line varied from about 330 to 400 deg C (625 to 750 deg F). Because the spring was enclosed and mounted above the valve, its temperature was probably slightly lower. The 195 mm (7 in.) OD x 305 mm (12 in.) long spring was made from a 35 mm (1 in.) diam rod of H21 hot-work tool steel. It had been in service for about four years and had been subjected to mildly fluctuating stresses. Analysis (visual inspection, 0.3x photographs, 0.7x light fractographs, and metallographic examination) supported the conclusions that the spring failed by corrosion fatigue that resulted from application of a fluctuating load in the presence of a moisture-laden atmosphere. Recommendations included replacing all safety valves in the system with new open-top valves that had shot-peened and galvanized steel springs. Alternatively, the valve springs could be made from a corrosion-resistant metal-for example, a 300 series austenitic stainless steel or a nickel-base alloy, such as Hastelloy B or C.

A 455 mm diam x 8 mm thick wall carbon steel (ASTM A 53) discharge line for a circulating-water system at a cooling tower fractured in service; a manifold section cracked where a Y-shaped connection had been welded. Investigation (visual inspection and photographs) supported the conclusion that the pipe failed by fatigue. Cracks originated at crevices and pits in the weld area that acted as stress raisers, producing high localized stresses because of the sharp-radius corner design. Abnormally high structural stresses and alternating stresses resulting from the pump vibrations contributed to the failure. Recommendations included changing the joint design to incorporate a large-radius corner and improving fitting of the components to permit full weld penetration. Backing strips were suggested to increase weld quality, and the pipe wall thickness was increased from 8 to 9.5 mm.

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.

An LNG tanker experienced a fracture of the solid tail shaft, which is a section of the main drive shaft. The tail shaft was made of a forged low-carbon steel. In spite of two ultrasonic inspections, a large defect the size of a football in the center of the shaft was missed. During heat treating following forging, it was surmised that the defect led to the propagation of an internal brittle crack, or clink. A fatigue crack propagated from this origin to the outer surface of the shaft after about a year of service. Finally a last ligament of a few square inches held the shaft together and broke, leading to the separation of the shaft. The cause of failure was fatigue crack initiation and crack growth under reverse bending cyclic stresses. There was no indication that misalignment existed because there was no indication of fretting at the bolt holes in the flange at the end of the shaft. In the case of this shaft, a solution would have been to machine the core of the shaft to remove the brittle material or to use a tubular shaft.

A cast countershaft pinion on a car puller for a blast furnace broke after one month of service; expected life was 12 months. The pinion was specified to be made of 1045 steel heat treated to a hardness of 245 HRB. The pinion steel was analyzed and was a satisfactory alternative to 1045 steel. The pinion was annealed before flame or induction hardening of the teeth to a surface hardness of 363 HRB and a core hardness of 197 HRB. The broken pinion had a tooth which had failed by fatigue fracture through the tooth root because of the low strength from incomplete surface hardening of the tooth surfaces. Contributing factors included uneven loading because of misalignment and stress concentrations in the tooth roots caused by tool marks. Greater strength was provided by oil quenching and tempering the replacement pinions to a hardness of 255 to 302 HRB. Machining of the tooth roots was revised to eliminate all tool marks. Surface hardening was applied to all tooth surfaces, including the root. Proper alignment of the pinion was ensured by carefully checking the meshing of the teeth at startup.

The power-type counterbalance spring, formed from hardened-and-tempered carbon steel strip and subsequently subjected to phosphating treatment, fractured at the two locations during fatigue testing. A rust colored dark band at the inside edge of the fracture surface was disclosed during investigation. Etch pits were revealed by the cleaned surface which were never observed on properly phosphated coating. It was interpreted that the spring had been subjected to an abnormal acid attack in pickling or phosphating which had resulted in considerable absorption of hydrogen by the metal and hence embrittlement. The part was concluded to have cracked during phosphating or excessive acid pickling before phosphating.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

A cast steel bracket manufactured in accordance with ASTM A 148 grade 135/125 steel failed in railroad maintenance service. Ancillary property requirements included a 285 to 331 HB hardness range and minimum impact energy of 27 J (20 ft·lbf) at -40 deg C (-40 deg F). The conditions at the time of failure were characterized as relatively cold. Investigation (visual inspection, chemical analysis, and unetched 119x and 2% nital etched 119x SEM images) supported the conclusion that the bracket failed through brittle overload fracture due to a number of synergistic factors. The quenched-and-tempered microstructure contained solidification shrinkage, inherently poor ductility, and type II Mn-S inclusions that are known to reduce ductility. The macro and microscale fracture features confirmed that the casting was likely in low-temperature service at the time of failure. The composition and mechanical properties of the casting did not satisfy the design requirements. Recommendations included exerting better composition control, primarily with regard to melting, deoxidation, and nitrogen control. Better deoxidation practice was recommended to generate the more desirable Mn-S inclusion morphology, and reevaluation of the casting design was suggested to minimize shrinkage.

Grease-wiper springs for cams formed from stampings of 0.25-mm thick carbon spring steel (0.65 to 0.80% C) fractured at the 0.025 mm radius on the stamped 135 deg corner at a 90 deg bend after 5,000,000 cycles. Tool marks 2 to 2.3 mm from the center of the stamped bend were disclosed by visual examination. Fatigue striations originating from cracks at the 0.025 mm radius inside corner at the bend were revealed by SEM of the fractured surface. The maximum stress at the bend, in stock of maximum thickness and as a function of the radius of the 135 deg corner, was indicated by stress calculations to be very close to the maximum allowable fluctuating stress for the material. The wiper springs were concluded to be fractured in fatigue and the cyclic loading resulted from cam rotation. The maximum applied stress approached the allowable limit due to high stress-concentration factor in the spring (caused by the very small inside radius). The corner radius was increased to 0.76 mm and the tools were re-polished to avoid tool marks.

The connecting end of two forged medium-carbon steel rods used in an application in which they were subjected to severe low-frequency loading failed in service. The fractures extended completely through the connecting end. The surface hardness of the rods was found to be lower than specifications. The fractures were revealed to be in areas of the transition regions that had been rough ground to remove flash along the parting line. The presence of beach marks, indicating fatigue failure, was revealed by examination. The fracture origin was confirmed by the location and curvature of beach marks to be the rough ground surface. An incipient crack 9.5 mm along with several other cracks on one of the fractured rods was revealed by liquid penetration examination. Metallographic examination of the fractured rods indicated a banded structure consisting of zones of ferrite and pearlite. It was established that the incipient cracks found in liquid-penetrant inspection had originated at the surface in the banded region, in areas of ferrite where this constituent had been visibly deformed by grinding. Closer control on the microstructure, hardness of the forgings and smooth finish in critical area was recommended.

A pressure probe assembly comprised of type 347 stainless steel housing, brazed with AMS 4772D filler metal to the pressure probe, failed due to detachment of a rectangular segment from the housing. The presence of a large brazing metal devoid region in the pressure probe-housing joint was revealed by visual examination. Fatigue marks, emanating from multiple crack origins on the inside surface of the housing at the brazed joint were revealed by further study of the fracture. A poor metallurgical bond was confirmed by the presence of large irregular voids, flux trapped braze metal and separation between braze and housing.

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.