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.

The 16 mm diam 6 x 37 fiber-core improved plow steel wire rope on a scrapyard crane failed after two weeks of service under normal loading conditions. This type of rope was made of 0.71 to 0.75% carbon steel wires and a tensile strength of 1696 to 1917 MPa. The rope broke when it was attached to a chain for pulling jammed scrap from the baler. The rope was heavily abraded and several of the individual wires were broken. a uniform cold-drawn microstructure, with patches of untempered martensite in regions of severe abrasion and crown wear was revealed by metallographic examination. As a result of abrasion, a hard layer of martensite was formed on the wire. The wire was made susceptible to fatigue cracking, while bending around the sheave, by this brittle surface layer. The carbon content and tensile strength of the wire was found lower than specifications. As a corrective measure, this wire rope was substituted by the more abrasion resistant 6 x 19 rope.

Catastrophic pitting corrosion occurred in type 304L stainless steel pipe flange assemblies in an industrial food processor. During regular service the pumped medium was pureed vegetables. In situ maintenance procedures included cleaning of the assemblies with a sodium hypochlorite solution. It was determined that the assemblies failed due to an austenite-martensite galvanic couple activated by a chlorine bearing electrolyte. The martensitic areas resulted from a transformation during cold-forming operations. Solution annealing after forming, revision of the design of the pipe flange assemblies to eliminate the forming operation, and removal of the source of chlorine were recommended.

A steel splice plate in a power transmission line tower cracked while in service. Metallographic analysis indicated the presence of a white hard martensite layer near the crack, which occurred in the heel of the plate. Mechanical property tests revealed localized hardening in the area of the crack, supporting the metallurgical findings. A substantial deterioration of the Charpy impact toughness of the material in the heel region was also observed which is believed to have caused the initiation and propagation of the cracks leading to the failure.

A masonry type drill bit, designed for impact drilling in rock, fractured after a short time in service. Samples of the failed bit were analyzed using optical and scanning electron microscopy, quantitative metallography, and chemical analysis. The composition was found to be that of 18CrNi3Mo steel. Investigators also found evidence of inclusions and prior austenite grain size, although it was determined that neither played a role in the failure. Rather, according to test data, the failure occurred because of stress concentration (due to geometric discontinuities along the tooth profiles) and the cumulative effect of torque and force loading (the byproduct of continuous twisting and axial impact). Cracks readily initiate under these conditions then propagate quickly through what was found to be networks of tempered martensite, thus resulting in premature failure.

An electric transport vehicle, similar to an electric trolley or subway rail car, experienced frequent breakdowns due to in-service fractures of torsion springs that support the weight of an overhead electric pickup assembly. Scanning electron microscopy and metallographic examinations determined that the fractures stemmed from electric arc damage. Intergranular quench cracks in the transformed untempered martensite on the surface of the spring provided crack initiations that propagated during operation causing fatigue fracture.

This case study describes the failure analysis of a steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low-carbon CrMoV alloy steel that was subsequently single-pass butt welded using gas tungsten arc welding. Although no cracks were found when the welds were visually inspected, X-ray radiography showed small discontinuous surface cracks adjacent to the weld bead in the heat affected zone. Further investigation, including optical microscopy, microhardness testing, and residual stress measurements, revealed that the cracks were caused primarily by the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in high transformation stresses. These transformation stresses, working in combination with thermal stresses and constraint conditions, resulted in intergranular brittle fracture.

A 76 mm (3 in.) type 304 stainless steel tube that was used as a heat shield and water nozzle support in a hydrogen gas plant quench pot failed in a brittle manner. Visual examination of a sample from the failed tube showed that one lip of the section was eroded from service failure, whereas the opposite side exhibited a planar-type fracture. Sections were removed from the eroded area and from the opposite lip for microscopic studies and chemical analysis. The eroded edges exhibited river bed ditching, indicative of thermal fatigue. Microstructural analysis showed massive carbide formations in a martensite matrix and outlining of prior-austenite grains by a network of fine, white lines. These features indicated that the material had been transformed by carburization by the impinging gas. The outer surface exhibited a heavy scale deposit and numerous cracks that originated at the surface of the tube. The cracks were covered with scale, indicating that thermal fatigue (heat cracking) had occurred. Chemical analysis confirmed that the original material was type 304 stainless steel that had been through-carburized by the formation of an endothermic gas mixture. It was recommended that plant startup and shutdown procedures be modified to reduce or eliminate the presence of the carburizing gas mixture.

Two Z-shape impeller vanes failed. The vane material was 14-hard type 301 stainless steel. The vanes were of two-piece construction, with a longitudinal weld. Analyses indicated that the vanes had not been solution annealed after welding, leaving the heat-affected zone above the welds in a highly sensitized state. The sensitized material lost corrosion resistance, became embrittled along the grain boundaries, and finally failed by intergranular cracking. Use of type 410 martensitic stainless steel was recommended.

Several compressor disks in military fighter and trainer aircraft gas turbine engines cracked prematurely in the bolt hole regions. The disks were made of precipitation-hardened AM355 martensitic stainless steel. Experimental and analytical work was performed on specimens from the fifth-stage compressor disk (judged to be the most crack-prone disk in the compressor) to determine the cause of the failures. Failure was attributed to high-strain low-cycle fatigue during service. It was also determined that the cyclic engine usage assumed in the original life calculations had been under estimated, which led to low-cycle fatigue cracking earlier than expected. Fracture mechanics analysis of the disks was carried out to assess their damage tolerance and to predict safe inspection intervals.

During a work over of an oil well, the 9% Ni steel production tubing parted three times as it was being pulled from the well. The tubing had performed satisfactorily for more than 30 years in the well A representative failure, a circumferential fracture in a connection, was analyzed. Reported to be a hydril CS connection, the pin end parted near the last threads. The external surface exhibited mechanical damage marks from the fishing operation. No signs of external corrosion or damage were detected. Visual surface examination revealed shear lips at the outside pipe, indicating that the fracture initiated at the inside surface and grew across the wall. Longitudinal cross sections revealed heavy corrosion damage to the inside pipe surface. Metallographic examination indicated that the tubing failed as a result of severe weakening from internal corrosion. Gray-colored corrosion deposits, which penetrated the pipe throughout the grain boundaries of the material and concentrated in the matrix in a layer near the inside surface of the pipe, were observed. The presence of H2S in the produced fluids and the appearance of the gray deposit indicated that the tube suffered H2S corrosion. Chemical analysis of the base metal and corrosion deposits did not detect iron or nickel sulfides, however Replacement of the remaining pipe strings according to a scheduled program was recommended. Because 9% Ni steel was not available, 13% Cr martensitic stainless steel was recommended as a replacement.

Cadmium-coated type 410 martensitic stainless steel 1 4 -14 self-drilling tapping screws fractured during retorquing tests within a few weeks after installation. The screws were used to assemble structural steel frames for granite panels that formed the outer skin of a high-rise building. Fractographic and metallographic examination showed that the fractures occurred in a brittle manner from intergranular crack propagation. Laboratory and simulated environmental tests showed that an aqueous environment was necessary for the brittle fracture/cracking phenomenon. The cracks were singular and intergranular with little branching. Secondary subsurface cracks suggested possible hydrogen embrittlement. The 410 screws had been introduced to replace conventional case-hardened carbon steel screws that conform to SAE specification J78. Carbon steel screws had a proven record of acceptable performance for the intended application. It was recommended that use of the 410 screws be discontinued in preference to the case-hardened carbon steel screws.

Failed portions of a 4140 steel axle from a prototype urban transit vehicle were examined to determine the cause of failure. The testing procedures included visual examination, macrofractography, metallography, chemical analysis, and hardness and tensile testing. The analysis showed that a salvage welding repair had resulted in the formation of martensite that developed cracks, leading to fatigue failure of the axle. It was recommended that no weld buildup or repair be permitted on the axles.

The failure of a 45 Mg (50 ton) rail crane bolster was investigated. Spectrochemical analysis indicated that the material was a 0.25C-1.24Mn-0.62Cr-0.24Mo cast steel. SEM examination revealed the presence of fatigue, as well as intergranular and ductile fractures. Microstructural analysis focused on an area where an antisway device had been welded to the structure and revealed the presence of coarse, untempered martensite that had resulted from faulty weld repair techniques. It was suggested that the use of proper welding procedures, including preheating and postheating, would have prevented the failure.

Failure analysis was performed on a fractured impeller from a boiler feed pump of a fossil fuel power plant. The impeller was a 12% Cr martensitic stainless steel casting. The failure occurred near the outside diameter of the shroud in the vicinity of a section change at the shroud/vane junction. Sections cut from the impeller were examined visually and by SEM fractography. Microstructural, chemical, and surface analyses and surface hardness tests were conducted on the impeller segments. The results indicated that the impeller failed in fatigue with casting defects increasing stress and initiating fracture. In addition, the composition and hardness of the impeller did not meet specifications. Revision of the casting process and institution of quality assurance methods were recommended.

Cracks were discovered between interference-fit fasteners (MoS2-coated Ti-6Al-4V) that had been incorporated into a fighter aircraft primary structural frame (D6ac steel) to enhance structural fatigue life. Examination of sections cut from the cracked frame established that the cracks propagated by stress-corrosion cracking. The cause of cracking was twofold: use of interference-fit fasteners exposed to moisture intrusion from a marine environment and poor hole quality. Failure was intensified by dissimilar-metal contact in the presence of weak acidic electrolyte (dissociated MoS2). Control of machining parameters to prevent formation of brittle martensite, use of galvanically compatible fasteners, and use of an alternate lubricant were recommended.

A cracked, martensitic stainless steel, low-pressure turbine blade from a 623 MW turbine generator was found to exhibit fatigue cracks during a routine turbine inspection. The blade was cracked at the first notch of the fir tree and the cracks initiated at pits induced by chloride attack. Examination of the blade microstructure at the fracture origins revealed oxide-filled pits and transgranular cracks. The oxide filled cracks appeared to have originated at small surface pits and probably propagated in a fatigue or corrosion-fatigue fracture mode. It was recommended that the sources of the chlorides be eliminated and that the remaining blades be inspected at regular maintenance intervals for evidence of cracking.

A helicopter main rotor bolt failed in the black-coated region between the threads and the taper section of the shank during assembly. The torque applied was approximately 100 N·m (900 in.·lbf) when the bolt sheared. No other bolts were reported to have failed. The failed bolt material conformed to AISI E4340 steel, as specified. The microstructure was tempered martensite, with hardness ranging from 41 to 45 HRC. Failure was in the shear ductile mode. The crack initiated in the area of slag inclusions. Inspection of other bolts from the same shipment was recommended.

A coupling in a line-shaft vertical turbine pump installed in a dam foundation fractured after a very short time. The coupling material was ASTM A582 416 martensitic stainless steel. Visual, macrofractographic, and scanning electron microscopic examination of the coupling showed that the fracture was brittle and was initiated by an intergranular cracking mechanism. The mode of fracture outside the crack initiation zone was transgranular cleavage. No indication of fatigue was found. The failure was attributed to improper heat treatment during manufacture, which resulted in a brittle microstructure susceptible to corrosion. The crack initiated either by stress-corrosion or hydrogen cracking. It was recommended that the couplings in the system be examined for surface cracking and, if present, corrective measures be taken.

Two type 420 martensitic stainless steel load cell bodies, which had been installed under two of the four legs of a milk storage tank failed in service. The failure occurred near a change in section and involved fracture of the entire cross section. Examination showed a brittle fracture that was preceded by a small fatigue region. Pitting corrosion was evident at the fracture origin. The areas around the load cells had been subjected to regular washdowns using high-pressure hot water, and the pitting was attributed to crevice corrosion between the load cell and the holddown bolts. Prevention of such corrosion by the use of a flexible sealant to eliminate the crevice was recommended.