The horizontal cross-travel shaft on a derrick failed after two years of service. The shaft was required to be made of 4140 steel quenched to a hardness of 302 to 352 HRB. The shaft was found to have fractured approximately 13 mm from the change in section between the splined end and the shaft proper. The cracks were found to have propagated in the longitudinal and transverse directions until failures occurred. It was showed by a transverse section through the spline that the longitudinal cracks were initiated at the sharp corners at the roots of the spline teeth. The shaft was subjected to reverse torsional loading by the operation of the derrick and the shaft fatigue fracture was caused by this. The fillets at the roots of the spline teeth were increased in size and polished to minimize stress concentrations in these areas.

The locking collar on a machine failed suddenly when the shaft it restrained was inadvertently subjected to an axial load slightly higher than the allowable working load. The locking collar fractured abruptly, producing four large fragments. This allowed the shaft to be propelled forcefully in the direction of the load, causing substantial damage to other machinery components in the vicinity. The failed component, which was 43 cm (17 in.) in diameter, was machined from 4140 plate and heat treated to 34 to 36 HRC. Analysis (visual inspection, composite micrographs, scanning electron microscopy, and mechanical-property analysis) supported the conclusions that the alloy steel plate used in this application contained significant brittle microstructural fibering or banding. This condition produced considerable anisotropy in ductility and toughness as revealed by mechanical testing. Unfortunately, the potential effects of anisotropy were apparently neglected when this component was designed and manufactured from the plate stock, because the loading was applied in a direction that stressed the weakest planes in the material, that is, a direction normal to the fibering. No recommendations were made.

Two tubular AISI 1025 steel posts (improved design) in a carrier vehicle failed by cracking at the radius of the flange after five weeks of service. The posts were two of four that supported the chassis of the vehicle high above the wheels. The original design involved a flat flange of low-carbon low-alloy steel that was welded to an AISI 1025 steel tube, and the improved design included placing the welded joint of the flange farther away from the flange fillet. Investigation (visual inspection and chemical analysis) supported the conclusion that the failures in the flanges of improved design were attributed to fatigue cracks initiating at the aluminum oxide inclusions in the flange fillet. Recommendations included retaining the improved design of the flange with the weld approximately 50 mm (2 in.) from the fillet, but changing the metal to a forging of AISI 4140 steel, oil quenched and tempered to a hardness of 241 to 285 HRB. Preheating to 370 deg C (700 deg F) before and during welding with AISI 4130 steel wire was specified. It was also recommended that the weld be subjected to magnetic-particle inspection and then stress relieved at 595 deg C (1100 deg F), followed by final machining.

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.

After less than 30 days in service, several cadmium-plated retaining rings, made of 4140 steel tubing and heat treated to 36 to 40 HRC, broke during operation that included holding components of a segmented fitting in place under a constant load. Photographic and 100x nital-etched micrographic examination showed a microstructure of tempered martensite with low inclusion content as well as a pit or burned spot on the outer area of the ring. The defect was approximately 0.18 mm (0.007 in.) deep and 0.5 mm (0.020 in.) in diam and had a hardness of 58 to 60 HRC. The base metal adjacent to the defect had a hardness of 36 to 40 HRC. Small cracks or fissures were also evident within the defect. Thus, the rings failed in brittle fracture as the result of an arc strike (or burn) on the surface of the ring. At the site of the arc strike, a small region of hard, brittle untempered martensite was formed as the result of an arc strike during the cadmium-plating operation. Fracture occurred readily when the ring was stressed. No recommendations were made.

Fracture of a cadmium-plated accumulator ring forged from 4140 steel was discovered during inspection and disassembly of a hydraulic-accumulator system stored at a depot. The ring had broken into five small and two large segments. The small segments of the broken ring displayed very flat fracture surfaces with no apparent yielding, but the two large segments did show evidence of bending (yielding) near the fractures. In addition, some segments contained fine radial cracks. Analysis (visual inspection, optical microscopy on polished-and-etched specimens, hardness testing, and chemical analysis) supported the conclusion that the failure was caused due to brittle fatigue, as evidenced by the intergranular nature of the fracture path. Also, hydrogen penetration occurred during the plating operation and was not relieved subsequently as required.

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.

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 tie rod, nut, and bellows from a failed 610 mm (24 in.) diam tied universal expansion joint that carried tail gases consisting of N 2 + O 2 with slight traces of nitrogen oxides and water were examined. The materials were SA 193-B7 (AISI 4140), SA 194–214, and Incoloy 800H, respectively. Visual examination of the bellows revealed cracks in heavily cold-worked areas (both inside and outside) and considerable corrosion. SEM analysis showed a classical intergranular failure pattern with microcracking. The threaded tie rod microstructure contained spheroidized carbide that was more pronounced at the tie rod end of the failure. Energy-dispersive X-ray analysis of fracture surfaces from the bellows showed the presence of chlorine and sulfur. Failure of the bellows was attributed to stress-corrosion cracking, with chlorine and sulfur being the corroding agents. The rod damage was the result of failure of the bellows, which allowed escaping hot gases to impinge on the tie rods and heat them to approximately 595 deg C (1100 deg F). It was recommended that the insulation be analyzed to determine the origin of the chlorine and sulfur and that it be replaced if necessary.

The shaft-and-bearing assembly in a freon compressor was subjected to severe pounding and vibration after six years of service. After about one year of service, the compressor had been shut down to replace a bearing seal. One month before the shaft failed, a second seal failure occurred, requiring the collar, spacer sleeve, seal, roller bearing, and lock washer to be replaced. The shaft was made of 4140 steel, heat treated to a hardness of 20 to 26 HRC. The seal, bearing, and lock washer were commercial components. Investigation (visual inspection, 4.5x images, x-ray diffraction, hardness testing, and microscopic exam) supported the conclusion that shaft failure was initiated by fretting between the bearing race and the bearing surface on the shaft because of improper bearing installation. Once clearance was established between the bearing and the shaft, the shaft began pounding on the inner bearing race, causing final failure of the shaft surface. Recommendations included proper fitting of the shaft and bearing race to preventing movement of the bearing on the shaft. Also, the lock washer and locknut must be installed properly.

Deformation, surface cracking, and spalling on the raceway of the outer ring (made of 4140 steel) of a large bearing caused it to be replaced from a radar antenna. The raceway surfaces were to be flame hardened to 55 HRC minimum and 50 HRC 3.2 mm below the surface, according to specifications. Samples from both the inner and outer rings were examined. A much lower hardness (25.2 to 18.9 HRC) was indicated during a vertical traverse 4.1 cm from the outer surface of the outer ring while slightly lower hardness values (46.8 to 54.8 HRC) were seen on the hardness traverse on the inner ring raceway. The lower hardness values were attributed to improper flame hardening. It was confirmed by metallographic examination of a 3% nital etched sample that the inner ring (tempered martensite and ferrite) and the outer ring (ferrite, scattered patches of pearlite, and martensite) were not properly austenitized. Displacement of metal on the outer raceway was revealed by elongation of grain structure. It was concluded that the failure of the raceway surface was due to incomplete austenitization caused by the improper heat treatment during flame hardening process.

An ASTM A193-83a grade B7 (AISI 4140) steel turbine impeller shaft fractured after 2 months of service. Failure had initiated at three separate points around the periphery of the shaft, each associated with one of three keyways. SEM fractography, metallography, and chemical analysis indicated that the mechanism of fracture initiation was torsional fatigue. Intermittent deceleration and acceleration resulting from power surges during operation of the turbine caused torsional vibration and was considered the most probable source of the required cyclic stress. Final failure took place by torsional shear.

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.

The 4140 steel steering spindle on a tricycle agricultural field chemical applicator failed, causing the loss of the front wheel and overturn of the vehicle. The spindle was a solid 120 mm (4.75 in.) diam forging. It had been machined to 115 mm (4.5 in.) in diameter to fit tightly inside a collar at one point and to 90 mm (3.5 in.) for attachment to the steering mechanism at another. Visual examination showed that the spindle fractured at the fillet welds that attached it to the collar. Macrofractography and metallography revealed that the failure initiated at the root of a weld that bridged a wide gap. The most probable cause of failure was improper preheat during welding.

This article discusses the failure of cylinder clamping rods in single cylinder diesel engines. The AISI 4140 hardened and tempered steel clamping rods were failing after 200 to 250 h of operation. The fatigue failures initiated at the root of the last thread on the clamping rod that was engaged in a blind hole in the cylinder block. The failures were caused by loose tolerances on the threads that resulted in a non-uniform distribution of load. The load was concentrated on the last threads to engage, thus causing fatigue crack nucleation at the thread root and propagation until the rod broke by overload. Changing the tolerance on the threads virtually eliminated the fatigue problem.

During natural gas drilling in the EMS region in 1956, considerable numbers of longitudinal cracks and transverse fractures occurred in the connecting pieces of the bore rods. The connectors were screwed onto the rods by means of a fine thread and tightly joined with it by shrinkage at 530 deg C. The connectors were made of SAE 4140 Cr-Mo steel. The material for the rod pipes was Fe-0.4C-1Mn steel. Structural stresses played a role in the cracking. Iron sulfide formed on the fracture planes and flake-like stress cracks occurred in the steel. The hydrogen sulfide content of the gas was the cause of damage. Hydrogen liberated by reaction with the iron caused the formation of iron sulfide after penetration of the steel, which had an explosive effect during molecular separation under high pressure. This in turn caused the crack formation in conjunction with the external and residual stresses.

A forged 4140 steel shaft that connected two runners in a hydroturbine failed catastrophically after approximately 5900 h of service. The runner and the mating section of the broken shaft were examined and tested by various methods. The results of the analyses indicated that the shaft failed by torsional fatigue starting at subsurface crack initiation sites. The forging contained regions of crack like flaws associated with particles rich in chromium, manganese, and iron. Fracture features indicated that the fatigue cracks propagated under a relatively low stress.

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.

Important clues about the probable cause of a gun tube explosion were obtained from a fractographic and metallographic examination of the fragments. The size, distribution, and surface markings of fragments may be used to localize the explosion and deduce its intensity. Microstructural features such as voids, adiabatic shear, and structural surface alterations also indicate the explosion intensity and further allow a comparison of the tube structure near and away from the explosion zone. These, and other metallurgical characteristics, are illustrated and discussed for cases of accidental and deliberately caused explosions of large caliber gun tubes.

To forged AISI 4140 steel trailer kingpins fractured after 4 to 6 months of service. Fractographic and metallographic examination revealed that cracks were present in the spool-flange shoulder region of the defective kingpins prior to installation on the trailers. The cracks grew and coalesced during service. Consideration of the manufacturing process suggested that the cracks were the result of overheating of the kingpin blanks prior to forging, which was exacerbated during forging by deformation heating in the highly-strained region. This view was supported by results of two types of tensile tests conducted near the incipient melting temperature at the grain boundaries. All kingpins made by the supplier of the fractured ones were ultrasonically inspected and six more anticipated to fail were found. It was recommended that the heating of forging blanks be more carefully controlled, especially with respect to the accuracy of the optical pyrometer temperature readout. Also, procedures must be developed such that forging blanks that trigger the over-temperature alarm are reliably and permanently removed from the production line.