During the pre-test inspection following the stress calculation check on a 7-ton capacity Scotch derrick crane, it was noted that threads on the back stay anchorage bolts were of unusually fine pitch (11 tpi) and that the machined faces of the nuts showed irregular pits or depressions disposed in an annular manner. When sectioned, the nuts showed a surprising method of construction. The nuts for the bolts had been made by using conventional pipe couplings inserted into sleeves made from hexagonal bar and the coupling secured to the sleeve by welding at each outer face. The ends of the sleeve bore were chamfered to form a weld preparation. After welding, the faces were machined which resulted in the removal of most of the weld metal and revealed a pronounced lack of penetration. All bolts used to anchor derrick crane back stays should be designed in accordance with the recommendations of British Standard 327:1964 (Clauses 10 and 18).

A 140 ft. (42.7 m) long boom on a dragline crane used in coal strip-mining operations failed. One of the principal load-bearing longitudinal beams or chords of the trussed boom had fractured adjacent to a bolt hole at a location about halfway along the length of the boom. Over the lifetime of the crane, several repairs had been made to the boom. At least a year before the failure, a reinforcing gusset plate had been bolted and welded to this chord at this location. Stereomicroscopy revealed microcracks in the weld metal. A fatigue crack 45 mm (1.8 in.) long was observed to emanate from this microcrack. Scanning electron microscopy showed an overload crack extended across the remaining cross section of the chord. It was concluded that the presence of the bolt hole used to attach the gusset plate to the chord created a stress riser adjacent to the hole. Repeated high tensile stresses on the chord during the lifting of enormous loads initiated a fatigue crack in the weld region adjacent to the bolt hole.

SAE grade 5 U-bolts were used to fasten auxiliary dual wheels to the axles on a farm tractor. Under typical farm usage, the bolts are expected to have infinite life. However, several U-bolts made of 29 mm diam rod broke after less than 100 h of service. The bolt legs in which the failures occurred were all in the same position relative to the direction of wheel rotation. Visual examination showed the break was a fairly flat transverse fracture in the threaded section between the washer and the nut. The appearance of the fracture surfaces was characteristic of failure by low-cycle fatigue, with a smooth matte fatigue failure region showing beach marks and generally extending over about 40 to 60% of the fracture surface, which indicated severe overload. The point of initiation of fatigue was at the root of the last thread at the edge of the nut on the side toward this washer. The U-bolts fractured in fatigue because the bolt material had poor hardenability relative to the diam of the bolts. The bolt material was changed from 1045 steel to 1527 steel, a warm-finished low-alloy steel. The diameter of the bolts was reduced to 27.2 mm and the threads were rolled rather than cut.

A portion of a 19 mm (0.75 in.) diam structural steel bolt was found on the floor of a manufacturing shop. This shop contained an overhead crane system that ran on rails supported by girders and columns. Inspection of the crane system revealed that the bolt had come from a joint in the supporting girders and could be considered one of the principal fasteners in the track system. Analysis (visual inspection, metallographic exam, and hardness testing) supported the conclusions that fatigue induced by the overhead movement of the crane produced failure of the bolt. The bolt was deficient in strength for the cyclic applied loads in this case and probably was not tightened sufficiently. Recommendations included removing the remaining bolts in the crane support assembly and replacing them with a higher-strength, more fatigue-resistant bolt, for example, SAE grade F, 104 to 108 HRB. The bolts should be tightened according to the specifications of the manufacturer, and the system should be periodically inspected for correct tightness.

Two bolts from the stressed structure of a church building that had broken during stressing were examined to establish the cause of fracture. The fracture of one of the first bolt occurred in a double-vee groove weld whose root was not completely welded. The second bolt had cracked outside of the weld seam closely under the head. Neither one had been particularly deformed before fracture. The composition of the head pieces corresponded approximately to manganese steel (Material No. 1 0845), a weldable construction steel with increased yield point and strength, while the shafts were made from Cr-Mo steel (Material No. 1.7225) according to DIN 17200. It was found that the bolts were not made from a suitable alloy steel, but were welded together from two unsuitable steels, one of which lacked sufficient strength. The austenitic weld seams showed hot tears and were not welded through to the root. Also, the pieces were not preheated before welding, so that stress cracks occurred in the transition zones. The second bolt was overstressed during the impact caused by the breaking of the first bolt.

A bolt manufacturer observed that products made from certain shipments of steel 41 Cr4 wire were prone to the formation of quench cracks in their rolled threads. The affected wire was tested and found to be highly sensitive to overheating because of the metallurgical method by which it was produced. A stronger decarburization of the case was a contributing factor that could not be prevented by working because the thread was rolled. Hardening tests conducted by the bolt manufacturer showed that quench cracks did not occur in specimens that were turned down before hardening and when notches were machined instead of beaten with a chisel.

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.

Two titanium alloy wing attachment bolts from a commercial jetliner failed during the course of a routine service operation. Failure of the bolts occurred during the re-torque process as the wing was being reattached. Metallurgical failure analysis indicated that the fracture mechanism was ductile overload and that the mechanical properties of the bolts were consistent with exemplar bolts that had been supplied. After eliminating other sources of excessive load application, the most probable cause of failure was ascribed to variances between the frictional characteristics of the bolt at the time of re-torque and at the time of initial torque application several years earlier.

The bolt in a bolt and thimble assembly used to connect a wire rope to a crane hanger bracket was worn excessively. Two worn bolts, one new bolt, and a new thimble were examined. Specifications required the bolts to be made of 4140 steel heat treated to a hardness of 277 to 321 HRB. Thimbles were to be made of cast 8625 steel, but no heat treatment or hardness were specified. Analysis (visual inspection, hardness testing, and metallographic examination) supported the conclusion that the wear was due to strikingly difference hardness measurements in the bolt and thimble. Recommendations included hardening and tempering the bolts to the hardness range of 375 to 430 HRB. The thimbles should be heat treated to a similar microstructure and the same hardness range as those of the bolt. Molybdenum disulfide lubricant can be liberally applied during the initial installation of the bolts. A maintenance lubrication program was not suggested, but galling could be reduced by periodic application of a solid lubricant.

Crane collapse due to bolt fatigue and fatigue failure of a crane support column, crane tower, overhead yard crane, hoist rope, and overhead crane drive shaft are described. The first four examples relate to the structural integrity of cranes. However, equipment such as drive and hoist-train components are often subject to severe fatigue loading and are perhaps even more prone to fatigue failure. In all instances, the presence of fatigue cracks at least contributed to the failure. In most instances, fatigue was the sole cause. Further, in each case, with regular inspection, fatigue cracks probably would have been detected well before final failure.

A detailed investigative failure analysis was conducted on an autoclave which blew apart in a furnace for no apparent reason. Bolt failure resulted in separation of the autoclave lid and subsequent destruction of the furnace. Analysis using metallography, fractography, mechanical testing and exemplar tests were performed on the bolt material. Mechanical engineering analysis and leak-before-break criteria were extensively analyzed. Results led to only one possible conclusion: that an explosion occurred within the autoclave. Suggestions for autoclave design are presented as a result of the analysis.

Six galvanized high-tensile steel bolts were used to hold the wheels of a four-wheel drive vehicle. The right hand rear wheel of this vehicle detached causing the vehicle to roll and resulting in considerable damage to the body. The wheel was detached by shearing of four of the bolts and stripping the nuts from the other two bolts, which remained unbroken. SEM fractography of the fracture surfaces of the four broken bolts indicated that the failure was due to reversed bending fatigue. Optical microscopy indicated that the bolts were heat treated to a tempered martensite structure and that the nuts were manufactured from low carbon steel. The paper discusses the influence of the microstructure on the failure process the events surrounding the nature of incident and the analysis of in-service failure of the failed components utilizing conventional metallurgical techniques.

Two blade-detachment failures in large (600 kW) wind turbine generators were investigated. In the first case, bolt failures were established as the initial failure event. A fatigue crack reached a critical length, fast fracture developed and was then arrested as the bolt unloaded. Crack growth resumed when loading increased with cracking or fracture of adjacent bolts. The problem was identified as one of insufficient preload on the bolts. In the second failure on a different unit, a retaining nut on a blade assembly split, allowing a roller bearing to slide off a shaft and a blade to separate at its attachment hub. The failure was observed to be by fatigue. It was determined that pieces of the outer retaining rib (or flange) on the bearing inner cage had fractured by fatigue and were trapped between the nut and the bearing, producing excessive cyclic loading on the nut by a wedging action as the blade pitch adjusted during a revolution. Fatigue of the rim occurred as a result of inadequate lubrication in the bearing, which led to load transfer across the rollers, onto the rim.

When a steam turbine was put out of service, cracks were noticed on many of the blades in the low pressure section round the stabilization bolts and perpendicular to the blade axis. The blades were made from chrome alloy steel X20-Cr13 (Material No. 1.402). When the bolts were brazed into the blades inadmissible localized overheating of the steel must have occurred, which resulted in transformation stresses and hence reduced deformability. The cracks arose as a consequence of careless brazing. Whether the cracks should be considered as stress cracks over their entire extent or partially as fatigue cracks produced by vibration in the operation of the turbine as a result of steplike growing of microcracks could not be deduced from the fracture surfaces. Microfractography showed that the cracks developed in stages.

In a main range in a power station, steam was conveyed at a pressure of 645 psi, and a temperature of 454 deg C (850 deg F). Pipe diameter was 9 in. and the joints were of the bolted type in which a thin steel ring, serrated on both sides, was inserted between plain flanges. Thin jointing material was interposed between the serrated faces and the flanges. The first intimation of trouble was the onset of a high pitched noise audible over a radius of a quarter of a mile. The noise arose from violent lateral vibration of the serrated ring, which attained an amplitude and persisted for a sufficient number of cycles to produce an extensive system of fatigue cracks that resulted in partial disintegration of the ring. Microscopic examination of the material showed it to be a mild steel of satisfactory quality. The trouble was started by slight leakage, possibly resulting from a relaxation of the interfacial pressure on the joint faces, which eroded away the joint material locally at one face of the serrated ring. This reduced interfacial pressure at the opposite face of the ring, with resultant leakage and erosion of the joint material on this side.

Hydrogen-assisted stress-corrosion cracking failure occurred in four AISI 4137 chromium molybdenum steel bolts having a hardness of 42 HRC. The normal service temperature (400 deg C, or 750 deg F) was too high for hydrogen embrittlement but, the bolts were subjected also to extended shutdown periods at ambient temperatures. The corrosive environment contained trace hydrogen chloride and acetic acid vapors as well as calcium chloride if leaks occurred. The exact service life was unknown. The bolt surfaces showed extensive corrosion deposits. Cracks had initiated at both the thread roots and the fillet under the bolt head. Multiple, branched cracking was present in a longitudinal section through the failed end of one bolt, typical of hydrogen-assisted SCC in hardened steels. Chlorides were detected within the cracks and on the fracture surface. The failed bolts were replaced with 17-4 PH stainless steel bolts (Condition H 1150M) having a hardness of 22 HRC.

Cadmium-plated high-strength steel bolts were used to facilitate quick disassembly of a vehicle. One bolt was found fractured across the root of a thread after being torqued in place for one week. The bolts were made of 8735 steel heat treated to a tensile strength of 1241 to 1379 MPa (180 to 200 ksi) with a hardness of 39 to 43 HRC, followed by cadmium plating. The bolt that failed and several that did not were examined. It was found that failure of the bolts was the result of time-dependent hydrogen embrittlement. Had the remaining bolts been torqued to the normal stress levels, all would have failed within two weeks. The bolts were baked, as specified by ASTM B 242, at 205 deg C (400 deg F) for 30 min. No further failures occurred. Baking for 30 min is the minimum baking time; however, baking times up to 24 h are recommended for greater safety.

Type 410 stainless steel bolts were used to hold together galvanized gray cast iron splice case halves. Before installation, the bolts were treated with molybdenum disulfide (MoS 2 ) antiseize compound. Several failures of splice case bolts were discovered in flooded manholes after they were in service for three to four months. Laboratory experiments were conducted to determine if the failure mode was hydrogen-stress cracking, if sulfides accelerate the failure, if heat treatment can improve the resistance against this failure mode, and if the type 305 austenitic stainless steel would serve as a replacement material. Based on test results, the solution to the hydrogen-stress cracking problem consisted of changing the bolt from type 410 to 305 stainless steel, eliminating use of MoS2, and limiting the torque to 60 N·m (540 in.·lb).

Stainless steel bolts broke after short-term exposure in boiler feed-pump applications. Specifications required that the bolts be made of a 12% Cr high-strength steel with a composition conforming to that of AISI type 410 stainless steel. Several bolts from three different installations were examined. It was found that fracture of the bolts was by intergranular stress corrosion. A metallic copper-containing antiseizure compound on the bolts in a corrosive medium set up an electro-chemical cell that produced trenchlike fissures or pits for fracture initiation. Because the bolts were not subjected to cyclic loading, fatigue or corrosion fatigue was not possible. To prevent reoccurrence, bolts were required to conform to the specified chemical composition. The hardness range for the bolts was changed from 35 to 45 HRC to 18 to 24 HRC. Petroleum jelly was used as an antiseizure lubricant in place of the copper-containing compound. As a result of these changes, bolt life was increased to more than three years.

Two nuts were used to secure the water-supply pipes to the threaded connections on hot-water and cold-water taps. The nut used on the cold-water tap fractured about one week after installation. Examination of the fracture surfaces of the coldwater nut did not reveal any obvious defects to account for the fracture, but there were indications of excessive porosity in the nut. The fracture had occurred through the root of the first thread that was adjacent to the flange of the tap. It was found that the nut from the cold-water tap failed by SCC. Apparently, sufficient stress was developed in the nut to promote this type of failure by normal installation because there was no evidence of excessive tightening of the nut. Corrosion testing of the nuts indicated that the fractured nut was highly susceptible to intergranular corrosion because of either a deficiency in magnesium content or excessive impurities, such as lead, tin, or cadmium. This composition problem with zinc alloys was recognized many years ago, and particular attention has been directed toward ensuring that high-purity zinc is used. This corrective measure reportedly resulted in virtual elimination of this type of defect.