A cast dragline bucket tooth failed by fracturing after a short time in service. The tooth was made of medium-carbon low-alloy steel heat treated to a hardness of 555 HRB. The fracture surface was covered with chevron marks. These converged at several sites on the surface of the tooth. A hardfacing deposit was located at each of these sites. Visual inspection of the hardfacing deposits revealed numerous transverse cracks, characteristic of many types of hardfacing. This failure was caused by cracks present in hardfacing deposits that had been applied to the ultrahigh-strength steel tooth. Given the small critical crack sizes characteristic of ultrahigh-strength materials, it is generally unwise to weld them. It is particularly inadvisable to hardface ultrahigh-strength steel parts with hard, brittle, crack-prone materials when high service stresses will be encountered. The operators of the dragline bucket were warned against further hardfacing of these teeth.

Shaft fracture of a 10 hp squirrel cage motor took place at the driving end just outside the roller bearing and not at an abrupt change of section behind the bearing where it might be expected to occur. A portion of shaft to the right of the fracture was deeply grooved. About a year prior to failure the inner race of the roller bearing became slack on the shaft and the seating was built up by the metal-spray process. The shaft was machined to form a rough thread to provide the requisite mechanical key for the sprayed-on metal. Part of this sprayed-on layer became detached after the fatigue failure occurred. The quality of the welding was poor. Slag inclusions were present adjacent to the sides of the keyway, which had been re-cut shorter than the original one after the welding repair. Failure at the unusual location was caused by the presence of the weld deposit.

The shaft of an exciter that was used with a diesel-driven electric generator broke at a fillet after ten hours of service following resurfacing of the shaft by welding. The fracture surface contained a dull off-center region of final ductile fracture surrounded by regions of fatigue that had been subjected to appreciable rubbing. The fracture appeared to be typical of rotary bending fatigue under conditions of a low nominal stress with a severe stress concentration. It appeared that the fatigue cracks initiated in the surface-weld layer. The weld deposit in the original keyway displays a lack of fusion at the bottom corner. Fatigue fracture of the shaft resulted from stresses that were created by vibration acting on a crack or cracks formed in the weld deposit because of the lack of preheating and postheating. Rebuilding of exciter shafts should be discontinued, and the support plate of the exciter should be braced to reduce the amount of transmitted vibration. Also, the fillet in the exciter shaft should be carefully machined to provide an adequate radius.

High-level radioactive wastes generated during the processing of nuclear materials are kept in large underground storage tanks made of low-carbon steel. The wastes consist primarily of concentrated solutions of sodium nitrate and sodium hydroxide. Each of the tanks is equipped with a purge ventilation system designed to continuously remove hydrogen gas and vapors without letting radionuclides escape. Several intergranular cracks were discovered in the vent pipe of one such system. The pipe, made of galvanized steel sheet, connects to an exhaust fan downstream of high-efficiency particulate air filters. The failure analysis investigation concluded that nitrate-induced stress-corrosion cracking was the cause of the failure.

A carbon steel piping cross-tee assembly which conveyed hydrogen sulfide (H7S) process gas at 150 to 275 deg C (300 to 585 deg F) with a maximum allowable operating pressure of 3 MPa (450 psig) ruptured at the toe of one of the welds at the cross after several years of service. The failure was initially thought to be the result of thermal fatigue, and the internal surfaces exhibited the “elephant hide” pattern characteristic of thermal fatigue. However metallographic failure analysis found that this pattern was the result of corrosion rather than thermal fatigue. Corrosion caused failure at this location because the weld was abnormally thin as fabricated. Thus, failure resulted from inadequate deposition of weld metal and subsequent wall thinning from internal corrosion. It was recommended that the cross-tee be replaced with a like component, with more careful attention to weld quality.

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.

The failure mode of through-wall cracking of a butt weld in a 5083-O aluminum alloy piping system in an ethylene plant was identified as mercury liquid metal embrittlement. As a result of this finding, 226 of the more than 400 butt welds in the system were ultrasonically inspected for cracking. One additional weld was found that had been degraded by mercury. A welding team experienced in repairing mercury contaminated piping was recruited to make the repairs. Corrective action included the installation of a sulfur-impregnated charcoal mercury-removal bed and replacement of the aluminum equipment that was in operation prior to the installation of the mercury-removal bed.

A steel galvanizing vat measuring 3 x 1.2 x 1.2 m (10 x 4 x 4 ft) and made of 19 mm thick carbon steel plate (ASTM A285, grade B)) at a shipbuilding and ship-repair facility failed after only three months of service. To verify suspected failure cause, two T joints were made in 12.5 mm thick ASTM A285, grade B, steel plate. One joint was welded using the semiautomatic submerged arc process with one pass on each side. A second joint was welded manually by the shielded metal arc process using E6010 welding rod and four passes on each side. The silicon content of the shielded metal arc weld was 0.54%, whereas that of the submerged arc weld was 0.86%. After being weighed, the specimens were submerged in molten zinc for 850 h. Analysis (visual inspection, chemical analysis, 100x 2% nital-etched micrographs) supported the conclusions that the vat failed due to molten-zinc corrosion along elongated ferrite bands, possibly because silicon was dissolved in the ferrite and thus made it more susceptible to attack by the molten zinc. Recommendations included rewelding the vat using the manual shielded metal arc process with at least four passes on each side.

This article focuses on the mechanisms of microbially induced or influenced corrosion (MIC) of metallic materials as an introduction to the recognition, management, and prevention of microbiological corrosion failures in piping, tanks, heat exchangers, and cooling towers. It discusses the degradation of various protective systems, such as corrosion inhibitors and lubricants. The article describes the failure analysis of steel, iron, copper, aluminum, and their alloys. It also discusses the probes available to monitor conditions relevant to MIC in industrial systems and the sampling and analysis of conditions usually achieved by the installation of removable coupons in the target system. The article also explains the prevention and control strategies of MIC in industrial systems.

This article focuses on common failures of the components associated with the flow path of industrial gas turbines. Examples of steam turbine blade failures are also discussed, because these components share some similarities with gas turbine blading. Some of the analytical methods used in the laboratory portion of the failure investigation are mentioned in the failure examples. The topics covered are creep, localized overheating, thermal-mechanical fatigue, high-cycle fatigue, fretting wear, erosive wear, high-temperature oxidation, hot corrosion, liquid metal embrittlement, and manufacturing and repair deficiencies.

A type 304 stainless steel tube that failed in a boiler stack economizer was analyzed to determine the cause. The investigation consisted of visual, SEM/EDS, and metallographic analysis. Several degradation mechanisms appeared to be at work, including pitting corrosion, chloride stress corrosion cracking, and fatigue fracture. Investigators concluded that the primary failure mechanism was fatigue fracture, although either of the other mechanisms may have eventually caused the tube to fail in the absence of fatigue.

A backwell tube situated in the combustion chamber of a 100 atm boiler, which had been in service for many years, failed. The temperature of the saturated steam was about 300 deg C. Two pipe sections with attacked areas in the circumferential welding joint were examined for cause of failure. First section showed strong pit or trench-like attack in the welding seam on the inner surface. A bluish-black corrosion product adhered to the pits. The second section showed small blisters at the welding seam. The metallographic examination of the first section showed welding seam was strongly reduced in bulk from the inside and covered with a thick crumbling layer of magnetic iron oxide (Fe3-O4). This was a corrosion product resulting from the operation of the boiler. In addition, it was decarburized from the inside, and interspersed with grain boundary cracks. This form of attack is typical for the decarburization of steel by high-pressure hydrogen. Hence, the defects in the pipe sections were the result of scaling during the operation of the steam boiler. It was recommended to avoid unnecessary overheating during the welding of materials for high-pressure steam boiler operations.

Erosion of solid surfaces can be brought about solely by liquids in two ways: from damage induced by formation and subsequent collapse of voids or cavities within the liquid, and from high-velocity impacts between a solid surface and liquid droplets. The former process is called cavitation erosion and the latter is liquid-droplet erosion. This article emphasizes on manifestations of damage and ways to minimize or repair these types of liquid impact damage, with illustrations.

This article discusses the failure analysis of several steel transmission pipeline failures, describes the causes and characteristics of specific pipeline failure modes, and introduces pipeline failure prevention and integrity management practices and methodologies. In addition, it covers the use of transmission pipeline in North America, discusses the procedures in pipeline failure analysis investigation, and provides a brief background on the most commonly observed pipeline flaws and degradation mechanisms. A case study related to hydrogen cracking and a hard spot is also presented.

This article addresses the forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. It describes the mechanisms of corrosive attack for specific forms of corrosion such as galvanic corrosion, uniform corrosion, pitting and crevice corrosion, intergranular corrosion, and velocity-affected corrosion. The article contains a table that lists combinations of alloys and environments subjected to selective leaching and the elements removed by leaching.

Following leakage which developed within the furnace of a horizontal multi-tubular type boiler, examination revealed a series of cracks adjacent to the stiffening rings in the first plain furnace ring. The fire-side surface of the sample was coated with a layer of oxide scale. Microscopical examination of sections through the cracks showed them to be filled with oxide and to be of the multi-branched type, having blunt terminations. The general nature of the cracks was characteristic of cracking from thermal or corrosion fatigue, as results from the operation of varying stresses in an oxidizing or corrosive environment. The cracking in this particular case was due principally to the inordinately large gap between the components. Additionally, several of the sealing welds of the tubes to the back tube plate were cracked in a radial manner, and it would appear that in addition, abnormal thermal conditions may well have been experienced intermittently in service.

This article explains the main types and characteristic causes of failures in boilers and other equipment in stationary and marine power plants that use steam as the working fluid with examples. It focuses on the distinctive features of each type that enable the failure analyst to determine the cause and suggest corrective action. The causes of failures include tube rupture, corrosion or scaling, fatigue, erosion, and stress-corrosion cracking. The article also describes the procedures for conducting a failure analysis.

Corrosion is the electrochemical reaction of a material and its environment. This article addresses those forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. Various forms of corrosion covered are galvanic corrosion, uniform corrosion, pitting, crevice corrosion, intergranular corrosion, selective leaching, and velocity-affected corrosion. In particular, mechanisms of corrosive attack for specific forms of corrosion, as well as evaluation and factors contributing to these forms, are described. These reviews of corrosion forms and mechanisms are intended to assist the reader in developing an understanding of the underlying principles of corrosion; acquiring such an understanding is the first step in recognizing and analyzing corrosion-related failures and in formulating preventive measures.