During a routine inspection, cracks were discovered in several aluminum alloy (similar to either 2014 or 2017) coupling nuts on the fuel lines of a missile. The fuel lines had been exposed to a marine atmosphere for six months while the missile stood on an outdoor test stand near the seacoast. A complete check was then made, both visually and with the aid of a low-power magnifying glass, of all coupling nuts of this type on the missile. Investigation (visual inspection, spectrographic and chemical analysis, and metallographic examination) supported the conclusion that the cracking of the aluminum alloy coupling nuts was caused by stress corrosion. Contributing factors included use of a material that is susceptible to this type of failure, sustained tensile stressing in the presence of a marine (chloride-bearing) atmosphere, and an elongated grain structure transverse to the direction of stress. The elongated grain structure transverse to the direction of stress was a consequence of following the generally used procedure of machining this type of nut from bar stock. Recommendations included changing the materials specification for new coupling nuts for this application to permit use of only aluminum alloys 6061-T6 and T651 and 2024-T6, T62, and T851.

The lower receiver of the M16 rifle is an anodized forging of aluminum alloy 7075-T6. Degradation of the receivers was observed after three years of service in a hot, humid atmosphere. The affected areas were those in frequent contact with the user's hands. There was no question that the material failed as a result of exfoliation corrosion, so an investigation was undertaken, centered around the study of thermal treatments that would increase the exfoliation resistance and still develop the required 448 MPa (65 ksi) yield strength. The results of the study concluded that rolled bar stock should be preferred to extruded bar stock. Differences in grain structure of the forgings, as induced by differences in thermal-mechanical history of the forged material, can have a significant effect on susceptibility to exfoliation corrosion. Regarding thermal treatment, the results show conclusively that large changes in strength and exfoliation characteristics of 7075 forgings can be induced by changes in temperature or time of thermal treatment. With regard to the effect of quenching rate on exfoliation characteristics, a cold-water quench below 25 deg C (75 deg F) would appear to be far superior to an elevated-temperature quench to minimize exfoliation for 7075 forgings in the T6 temper.

During the operation of tractors with cantilevered body, the lateral wall of the hypoeutectic cast iron cylinder blocks cracked repeatedly. Three of the blocks were examined. The grain structure of the thick-walled part consisted of uniformly distributed graphite of medium flake size in a basic mass of pearlite with little ferrite. But the thin-walled part showed a structure of dendrites of precipitated primary solid solution grains with pearlitic-ferritic structure and a residual liquid phase with granular graphite in the ferritic matrix. The structure was formed by undercooling of the residual melt. In this case, it was promoted by fast cooling of the thin wall and had comparatively low strength. The fracture formation in the cylinder blocks was ascribed primarily to casting stresses. They could be alleviated by better filleting of the transition cross sections. The fracture was promoted by the formation of undercooled microstructure of low strength in the thin-walled part. Similar damage appeared in a cylinder head, in which case, the cracks were promoted by a supercooled structure.

In 1979, during a routine bridge inspection, a fatigue crack was discovered in the top flange plate of one tie girder in a tied arch bridge crossing the Mississippi River. Metallographic analysis indicated a banding or segregation problem in the middle of the plate, where the carbon content was twice what it should have been. Based on this and results of ultrasonic testing, which revealed that the banding occurred in 24-ft lengths, it was decided to close the bridge and replace the defective steel. The steel used in the construction of this bridge was specified as ASTM A441, commonly used in structural applications. Testing showed an increase in hardness and weight percent carbon and manganese in the banded region. Further testing revealed that the area containing the segregation and coarse grain structure had a lower than expected toughness and a transition temperature 90 deg F higher than specified by the ASTM standards. The fatigue crack growth rate through this area was much faster than expected. All of these property changes resulted from increased carbon levels, higher yield strength, and larger than normal grain size.

A throttle arm of an aircraft engine fractured and caused loss of engine control. The broken part consisted of a 6.4-mm (1/4-in.) diam medium-carbon steel rod with a thread to fit a knurled brass nut that was inserted into the throttle knob. The threaded rod had been welded to the throttle-linkage bar by an assembly-weld deposit made on the rod adjacent to the threaded portion. The fracture surface exhibited a coarse-grain brittle texture with an initiating crack at a thread root. The throttle-arm failed by brittle fracture because of the presence of cracks at the thread roots that were within the HAZ of the adjacent weld deposit. The heat of welding had generated a coarse-grain structure with a weak grain-boundary network of ferrite that had not been corrected by postweld heat treatment. The combination of the cracks and this unfavorable microstructure provided a weakened condition that resulted in catastrophic, brittle fracture under normal applied loads. The design was altered to eliminate the weld adjacent to the threaded portion of the rod.

Following a freight train derailment, part of a fractured side frame was retained for study because a portion of its fracture surface exhibited a rock candy appearance and black scale. It was suspected of having failed, thereby precipitating the derailment. Metallography, scanning electron microscopy, EDXA, and x-ray mapping were used to study the steel in the vicinity of this part of the fracture surface. It was found to be contaminated with copper. Debye-Scherrer x-ray diffraction patterns obtained from the scale showed that it consisted of magnetite and hematite. It was concluded that some copper was accidentally left in the mold when the casting was poured. Liquid copper, carrying with it oxygen in solution, penetrated the austenite grain boundaries as the steel cooled. The oxygen reacted with the steel producing a network of scale outlining the austenite grain structure. When the casting fractured as a result of the derailment, the fracture followed the scale in the contaminated region thus creating the “rock candy” fracture.

During periodic inspection of the tubes of a reformer furnace, a soapy water leak test with the tubes pressurized with nitrogen was being carried out by site personnel in a manner contrary to the policy of the organization when one of the tubes suddenly disintegrated with explosive violence. The tube approximately 30 ft. long by 6 in. diam, was constructed of three spun cast sections butt welded together. The material specified for tubes for this service was basically a 25% chromium, 20% nickel, cast stainless steel containing 0.4% carbon to optimize creep resistance. Failure initiated in the region of the tube where the dark fracture surface and columnar grain structure were evident. These features indicated the presence of a defective zone or progressive cracking which had occurred during service. Microscopic examination of sections through the zone revealed extensive creep cracking. The cracking was intergranular and followed the interdendritic columnar structure adjacent to the outer surface.

After several years' use, an eyebolt suffered brittle fracture in the first turn of the thread. The fracture started at the notch at the root of the thread. Neither localized material defect nor an old crack were present. The investigation showed that instead of the specified steel quality St 37-2 N, a steel with about 0.5% C had been used. The microstructure with the coarse ferrite network indicated that the forged eye bolt had been normalized either at too high a temperature or not at all. In any case the anneal at 900 deg C produced a considerably more finely grained structure. In addition, the nature of the fracture and the results of the notched bar impact tests showed that in spite of the high C-content, the eye bolt had become brittle as a result of aging.

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 investigation was conducted to determine the factors responsible for the occasional formation of cracks on the parting lines of medium plain carbon and low-alloy medium-carbon steel forgings. The cracks were present on as-forged parts and grew during heat treatment. Examination revealed that areas near the parting line exhibited a large grain structure not present in the forged stock. High-temperature scale was also found in the cracks. It was concluded that the cracks were caused by material being folded over the parting line. The folding occurred because of a mismatch in the forgings and from material flow during trimming and/or material flow during forging.

A case-hardened sleeve made of C 15 (Material No. 1.0401) was flattened at two opposing sides and had cracked open at these places, the crack initiating at a face plane. The wall of the sleeve was 9 mm thick, but the flat ends were machined down to 5.5 mm from the outside. The customer had specified a 2 mm case depth and a hardness of at least HRC 55 at a depth of 1.5 mm. An etched cross section of the cracked end showed that the case layer had a depth of 2.3 mm, so that the sleeve was almost through-hardened at the flat ends. While the core material with the full wall thickness had the quench structure of low-carbon steel, the structure of the flattened area consisted of coarse acicular martensite with a small amount of pearlite (quench troostite) and ferrite. Therefore the sleeve was overheated and probably quenched directly from case. To prevent damage, it would have been necessary to have a lower case depth, carburize less deeply, and prevent overheating that causes brittleness and leads also to increased case depth, or else use a fine-grained steel of lower hardenability.

Proper stress analysis during component design is imperative for accurate life and performance prediction. The total stress on a part is comprised of the applied design stress and any residual stress that may exist due to forming or machining operations. Stress-corrosion cracking may be defined as the spontaneous failure of a metal resulting from the combined effects of a corrosive environment and the effective component of tensile stress acting on the structure. However, because of the orientation dependence in aluminum, it is the residual stress occurring in the most susceptible direction that must be considered of primary importance in material selection for design configuration. A Navy UH-1N helicopter main rotor blade grip manufactured from a 2014-T6 aluminum alloy forging failed because of a design flaw that left a high residual tensile stress along the short transverse plane; this in turn provided the necessary condition for stress corrosion to initiate. A complete failure investigation to ascertain the exact cause of the failure was conducted utilizing stereomicroscopic examination, scanning electron microscopy, metallographic inspection and interpretation, energy-dispersive chemical analysis, physical and mechanical evaluation. Stereomicroscopic examination of the opened crack fracture surface revealed one large fan-shaped region that had propagated radially through the thickness of the material from two distinct origin areas on the internal diam of the grip. Higher magnification inspection near the origin area revealed a flat, wood-like appearance. Scanning electron microscopy divulged the presence of substantial mud cracking and intergranular separation on the fracture surface. Metallographic examination revealed intergranular cracking and substantial leaf separation along the elongated grains parallel to the fracture surface. Chemical composition and hardness requirements were found to be as specified. The blade grip failed due to a stress corrosion crack which initiated on the inner diam and propagated in the short transverse direction through the thickness of the component. The high residual tensile stress in the part resulting from the forging and exposed after machining of the inner diam, combined with the presence of moisture, provided the necessary conditions to facilitate crack initiation and propagation.

A chain link which was part of the hoisting mechanism of a drop hammer broke after three or four months of service. It was reportedly manufactured of the heat resistant steel 30 Cr-Mo-V 9 (Material No. 1.7707). The fracture of the chain link had a conchoidal structure and ran along the austenitic grain boundaries. Such fractures are characteristic results of strong overheating. The coarse-grained, coarse acicular heat-treated structure of the chain link confirmed overheating. Because temperatures in excess of 1150 deg C are required for the solution of impurities, it is more probable that the real damage was done during the heat-up forging (drop-forging) and could not be removed during heat-treatment.

The bone cement failed at the distal end of the prosthesis stem of femoral head prosthesis six months after implantation. A small indentation on the lateral contour of the stem was visible where the stem had broken. The degree of loosening (gap between the lateral stem contour and the bone or cement) and implant loading was revealed by the dislocation of fragments of the prosthesis. Secondary cracks that had originated at the lateral aspect of the stem were revealed by metallographic examination of a section parallel to the stem surface and perpendicular to the fracture surface of the distal fragment. Gas pores are apparent in the grain and at the grain boundaries were revealed by a transverse section. Fine parallel line structures that run diagonally through the fractograph may be slip traces were revealed by scanning electron microscopy. One of the cracks was revealed to have propagated through a larger gas pore by a ruptured gas pore. The stresses created through the fatigue process activated glide systems which served the formation of secondary cracks along glide planes.

A section from a stop-block guide fell to the floor on a crane runway after it failed. A brittle crystalline-type break was disclosed by examination of the fracture surface. The point of initiation was in a hardened heat-affected layer that had developed during flame cutting and welding. The metal was identified to be 1020 steel. It was indicated by the coarse as-rolled structure (grain size of ASTM 00 to 4) of the base metal that the weldment (stop block and guide) had not been normalized. The brittle failure was evaluated to have been initiated at a metallurgical and mechanical notch produced by flame cutting and welding. As corrective measures, fully silicon-killed 1020 steel with a maximum grain size of ASTM 5 were used to make new stop-block weldments. The weldments were normalized at 900 deg C after flame cutting and welding to improve microstructure and impact strength. All flame-cut surfaces were ground to remove notches.

The origins of the casting are unknown. It is included here as a classic case of intergranular corrosion. The part (apparently a pump outlet) was named the “rubber casting” because of the severity of the intergranular attack. Every grain boundary has been attacked to the extent that the casting could be twisted and stretched as through made of rubber. The chemistry of the casting was acceptable for CN-7M. The reason the part failed is a continuous film of carbide with a continuous crack running parallel to the carbides. This sensitized structure produces an area depleted in protective chromium, making it susceptible to corrosion. Two solutions to this problem are available. The simplest is to ensure correct heat treatment to dissolve grain-boundary carbide film and return the protective chromium to the depleted zone. Alternatively, a low-carbon (0.03% maximum C, for example, CF-3) grade can be specified. Procedures are given in a reference for screening castings that may be susceptible to intergranular corrosion due to processing errors.

An elbow of 70 mm OD and 10 mm wall thickness made from St 35.29, and exposed to 315 atmospheres internal pressure in an oil hydraulic shear installation, cracked lengthwise after a short operating period. Because the stress was not sufficient to explain the fracture of this elbow under this pressure, an investigation was conducted to establish whether material or processing errors had occurred. Microscopic examination showed that a ferritic-pearlitic structure in select locations was very fine-grained. Other signs of fast cooling as compared to normally formed structure of the core zone were noted. It was also possible that the pipe was resting on a cold plate during bending or that it came in touch with a cold tool. This apparently caused the strains at the transition to the cross-sectional part that had been cooled more slowly. The location of the crack at just this point gave rise to the conclusion that it was formed either by the sole or contributive effect of these stresses.

During the internal fixation, the type 316LR stainless steel cortical bone screw failed. Extensive spiral deformation was revealed by the fracture surface. Dimple structure characteristic of a ductile failure mode was observed with dimples oriented uniformly in the deformation direction. A zone of heavily deformed grains at the fracture edge was revealed by longitudinal metallographic examination. The shearing fractures of a commercially pure titanium screw and a cast cobalt-chromium-molybdenum alloy were discussed for purpose of comparison.

The welds joining the liner and shell of a fluid catalytic cracking unit failed. The shell was made of ASTM A515 carbon steel welded with E7018 filler metal. The liner was made of type 405 stainless steel and was plug welded to the shell using ER309 and ER310 stainless steel filler metal. Fine cracks starting inside the weld zone and spreading outward through the weld and toward the surface were observed during examination. Decarburization and graphitization of the carbon steel at the interface was noted. The high carbon level was found to allow martensite to form eventually. The structure was found to be austenitic in the area where the grain-boundary precipitates appeared heaviest. The composition of the precipitates was analyzed using an electron microprobe to reveal presence of sulfur. Microstructural changes in the weld alloy at the interface were interpreted to be caused by dilution of the alloy and the presence of sulfur caused hot shortness. The necessary internal stress to produce extensive cracking was produced by the differential thermal expansion of the carbon and stainless steels. Periodic careful gouging of the affected areas followed by repair welding was recommended.

A 10,890-kg coil hook torch cut from 1040 steel plate failed while lifting a load of 13,600 kg after eight years of service. The normal ironing (wear) marks were exhibited by the inner surface of the hook. It was revealed by visual examination that cracking had originated at the inside radius of the hook. Beach marks (typical of fatigue fracture) were found extending over approximately 20% of the fracture surface. Numerous cracks were revealed by macroscopic examination of the torch-cut surfaces. It was revealed by macrograph of an etched specimen that the cracks had initiated in a hardened martensitic zone at the torch-cut surface and had extended up to the coarse pearlite structure beneath the martensitic zone. The fatigue fracture was concluded to have initiated in the brittle martensitic surface while failure was contributed by the 25% overload. As a corrective measure, the coil hooks were flame cut from ASTM A242 fine-grain steel plate, ground to remove the material damaged by flame cutting and stress relieved at 620 deg C.