The primary purpose of this article is to describe general root causes of failure that are associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be common sources of failure-inducing defects in the bulk working of wrought products. The article addresses the types of flaws or defects that can be introduced during the steel forging process itself, including defects originating in the ingot-casting process. Defects found in nonferrous forgings—titanium, aluminum, and copper and copper alloys—also are covered.

This article describes the general root causes of failure associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be the common sources of failure-inducing defects in bulk working of wrought products. The article discusses the types of imperfections that can be traced to the original ingot product. These include chemical segregation; ingot pipe, porosity, and centerline shrinkage; high hydrogen content; nonmetallic inclusions; unmelted electrodes and shelf; and cracks, laminations, seams, pits, blisters, and scabs. The article provides a discussion on the imperfections found in steel forgings. The problems encountered in sheet metal forming are also discussed. The article concludes with information on the causes of failure in cold formed parts.

A clamp used for securing the hot-air ducting system on fighter aircraft fractured in an area adjacent to a slot near the end of the strap after two or three years of service. The strap was 0.8 mm (0.032 in.) thick, and the V-section was 1.3 mm (0.050 in.) thick; both were made of 19-9 DL heat-resisting alloy. The operating temperature of the duct surrounded by the clamp was 425 to 540 deg C (800 to 1000 deg F). The life of the clamp was expected to equal that of the aircraft. Investigation (visual inspection, chemical analysis, hardness testing, and 540x/2700x images etched with oxalic acid) supported the conclusion that the clamp fractured by SCC because the work metal was sensitized. Sensitization occurred during long-term exposure to the service temperature; the effects of sensitization were intensified as a result of cold forming. Recommendations included using a work metal that is less susceptible to intergranular carbide precipitation.

A roll assembly consisting of a forged AISI type 440A stainless steel sleeve shrink fitted over a 4340 steel shaft and further secured with tapered keys on opposite ends was crated and shipped by air. Upon arrival, the sleeve was found to have cracked longitudinally between the keyways. A roll manufacturer had successfully used the above procedure for many years to make them. Analysis (visual inspection; 150x micrograph of sections etched with a mixture of 2 parts HNO3, 2 parts acetic acid, and 3 parts HCI; electron microscopy; and stress testing) supported the conclusion that superficial working of the metal, probably insufficient hot working, produced a microstructure in which the carbide particles were not broken up and evenly distributed. As a result, the grains were totally surrounded with brittle carbide particles. This facilitated the formation of a crack at a fillet in the keyway. Crack growth was rapid once the crack had initiated, causing brittle fracture to occur.

An integral coupling and gear (Cr-Mo steel), used on a turbine-driven main boiler-feed pump, was removed from service after one year of operation because of excessive vibration. Spectrographic analysis and metallographic examination revealed the fact that gritty material in the gear teeth (found at visual inspection) was composed of the same material as the metal in the coupling. Beach marks and evidence of cold work, typical of fatigue failure, were found on the fracture surface. Chips remaining in the analysis cut were difficult to remove, indicating a strong magnetic field in the part. Evidence found supports the conclusions that failure of the coupling was by fatigue and that incomplete demagnetization of the coupling following magnetic-particle inspection caused retention of metal chips in the roots of the teeth. Improper lubrication caused gear teeth to overheat and spall, producing chips that eventually overstressed the gear, causing failure. Because the oil circulation system was not operating properly, metal chips were not removed from the coupling. Recommendations included checking the replacement coupling for residual magnetism and changing or filtering the pump oil to remove any debris.

A cold-formed Grade TP 304 stainless steel swaged region of a reheater tube in service for about 8000 hours cracked because of sulfur-induced stress-corrosion cracking (SCC). Cracking initiated from the external surface and a high sulfur content was detected in the outer diameter and crack deposits. Comparison of the microstructure and hardness of the swaged region and unswaged Grade TP 304 stainless steel tube metal indicated that the swaged section was not annealed to reduce the effects of cold working. The high hardness created during swaging increased the stainless steel's susceptibility to sulfur-induced SCC. Because SCC requires water to be present, cracking most likely occurred during downtime or startups. To prevent future failures, the boiler should be kept dry during downtime to avoid formation of sulfur acids, and the swaged sections of the tubes should be heat treated after swaging to reduce or eliminate strain hardening of the metal.

Metal-framed polycarbonate (PC) ophthalmic lenses shattered from acetone solvent-induced cracking. The lenses exhibited primary and secondary cracks with solvent swelling and crazing. A laboratory accident splashed acetone onto the lenses. The metal frames gripped approximately two-thirds of the lenses' periphery and introduced an unevenly distributed force on the lenses. To prevent future failures, it was recommended to protect PC from service environments with solvents, such as acetone; or from marking pens, adhesives or soaps which contain undesirable solvents; and to not apply excessive stress on ophthalmic lenses in the form of working or residual stresses.

The working fluid of a hypersonic wind tunnel is freon 14 heated in molten-metal-bath heat exchangers. The coils of the heaters have failed several times from various causes. They have been replaced each time with a stainless steel deemed more appropriate, but they continue to fail. In this case study, the history of failures is traced, the causes are analyzed, and recommendations are made for future design and maintenance. Coils fabricated from AISI 316 should provide satisfactory service life if reasonable precautionary measures are observed during maintenance and testing.

Forged austenitic steel rings used on rotor shafts in two 100,000 kW generators burst from overstressing in a region of ventilation holes. A variety of causes contributed to the brittle fractures in the ductile austenitic alloy, including stress concentration by holes, work hardened metal in the bores, and a variable pattern of residual stress.

Initially, two vertical double-acting two-stage compressors delivering chlorine gas at a pressure of 100 psi appeared to be running satisfactorily. About six months later the LP piston-rod of the No. 2 compressor failed due to burning, the compressor being worked double-acting at the time. About five months later, the HP piston rod of the No. 1 compressor failed in a similar manner. Specimens for microscopic examination were cut from the rod in the region of the failure and from the extreme end that had been situated above the piston and hence not subjected to an appreciable rise in temperature. The material was a steel in the normalized condition with a 0.35% C content. It appears probable that deficient lubrication of the gland resulted in overheating of the rod due to friction. The presence of a sprayed-metal coating was probably an additional factor in promoting failure, as it would present to the gas a surface area considerably greater than that of a homogeneous material.

A steam jacketed autoclave of orthodox design was fabricated from mild steel for a working pressure of 320 psi. The only unusual feature in its construction was a protective layer of weld metal, which was deposited on the internal surface of the upper half of the 1 in. thick shell. The first indication of latent trouble was provided by the bolts which attached the stirring paddles to the shaft and the stationary scraper blades to the shell, either failing in service or breaking off when an attempt was made to remove them. It was the practice to renew them all annually. Microscopic examination of a failed bolt showed the path of the fracture and the secondary cracking associated with it were intergranular, suggesting that failure resulted from stress corrosion. A steel of the rimming type had been used to make the bar from which the bolt was forged. Cracks which originate at the root of threads generally result from fatigue but, in this instance, their intergranular mode of progression indicated that they were due to stress-corrosion. Examination of shell material showed that the cracks in the vessel were wholly intergranular. It was apparent from this evidence that this cracking was also due to stress-corrosion.

The safety valve on a steam turbogenerator was set to open when the steam pressure reaches 2400 kPa (348 psi). The pressure had not exceeded 1790 kPa (260 psi) when the safety-valve spring shattered into 12 pieces. The steam temperature in the line varied from about 330 to 400 deg C (625 to 750 deg F). Because the spring was enclosed and mounted above the valve, its temperature was probably slightly lower. The 195 mm (7 in.) OD x 305 mm (12 in.) long spring was made from a 35 mm (1 in.) diam rod of H21 hot-work tool steel. It had been in service for about four years and had been subjected to mildly fluctuating stresses. Analysis (visual inspection, 0.3x photographs, 0.7x light fractographs, and metallographic examination) supported the conclusions that the spring failed by corrosion fatigue that resulted from application of a fluctuating load in the presence of a moisture-laden atmosphere. Recommendations included replacing all safety valves in the system with new open-top valves that had shot-peened and galvanized steel springs. Alternatively, the valve springs could be made from a corrosion-resistant metal-for example, a 300 series austenitic stainless steel or a nickel-base alloy, such as Hastelloy B or C.

A DC-10 in transit from Denver to Chicago experienced failure of the center engine. The titanium compressor disk burst and severed the hydraulics of the plane. Investigation supports the conclusion that the cause of the disk rupture was the presence of a large fatigue crack near the bore emanating from a hard alpha (HA) defect. Such defects can result from occasional upsets during the vacuum melting of titanium. These nitrogen-rich alpha titanium anomalies are brittle and often have associated microcracks and microvoids. A probabilistic damage tolerance approach was recommended to address the anomalies, with the objective of enhancing rotor life management practices. The ongoing work involves the use of fracture mechanics and software (called DARWIN.) optimized for damage tolerant design and analysis of metallic structural components.

In a continuously cast aluminum press stud, two small foreign metal slivers were found that had caused difficulties with the cable sheathing press. Spectroscopic examination revealed the slivers consisted of a chromium-molybdenum-vanadium steel with minor (unintentional) additions of copper, nickel, and cobalt. A steel of similar composition, X38Cr-MoV5 1 (W-No. 2343) was used for hot working tools. The sliver originated from a damaged press tool.

During dismantling of an eccentric camshaft of 340 mm diam that had worked for a total of 450,000 load reversals, it was found that it had cracked on both sides of the eccentric cam. The shaft was made of chromium-molybdenum alloy steel 34 Cr-Mo4 (Material No. 1.7220) according to DIN 17200. Microstructural examination showed the shaft had ran hot, and there were no material defects. The shaft probably was overstressed by torsion forces. The presence of surface checks on both sides of the cam lobe that were filled with bearing metal proved that overstressing occurred through galling of the end faces of the bearing liners.

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.

Superelastic nitinol wires that fractured under various conditions were examined under a scanning electron microscope in order to characterize the fracture surfaces, produce reference data, and compare the findings with prior published work. The study revealed that nitinol fracture modes and morphologies are generally consistent with those of ductile metals, such as austenitic stainless steel, with one exception: Nitinol exhibits a unique damage mechanism under high bending strain, where damage occurs at the compression side of tight bends or kinks while the tensile side is unaffected. The damage begins as slip line formation due to plastic deformation, which progresses to cracking at high strain levels. The cracks appear to initiate from slip lines and extend in shear (mode II) manner.

Flow forming technology has emerged as a promising, economical metal forming technology due to its ability to provide high strength, high precision, thin walled tubes with excellent surface finish. This paper presents experimental observations of defects developed during flow forming of high strength SAE 4130 steel tubes. The major defects observed are fish scaling, premature burst, diametral growth, microcracks, and macrocracks. This paper analyzes the defects and arrives at the causative factors contributing to the various failure modes.

Sheet forming failures divert resources from normal business activities and have significant bottom-line impact. This article focuses on the formation, causes, and limitations of four primary categories of sheet forming failures, namely necks, fractures/splits/cracks, wrinkles/loose metal, and springback/dimensional. It discusses the processes involved in analytical tools that aid in characterizing the state of a formed part. In addition, information on draw panel analysis and troubleshooting of sheet forming failures is also provided.

This article discusses failure mechanisms in tool and die materials that are very important to nearly all manufacturing processes. It is primarily devoted to failures of tool steels used in cold working and hot working applications. The processes involved in the analysis of tool and die failures are also covered. In addition, the article focuses on a number of factors that are responsible for tool and die failures, including mechanical design, grade selection, steel quality, machining processes, heat treatment operation, and tool and die setup.