After the mixing machines were introduced into service, excessive assembly stresses and inappropriate detail design caused the premature failures of ice cream drink mixer blades shortly. The mixer blade is slightly deformed by the contact between the wavy washer at the bottom of the assembly and the bends at the bottom shoulders of the two mixer arms. Multiple fatigue crack origins on the inside radii of the bends at the bottom shoulders was revealed by analysis of the failed mixer blades. It was revealed by metallographic examination that the shoulders on the arms were cold bent, introducing tensile residual stresses on the inside radii of the shoulders and creating a localized area of fatigue susceptibility due to the inherent notch sensitivity of cold-formed 300 series stainless steel. It was established that the physical root cause was the design of the mixer blade. The addition of a stand-off washer between the wavy washer and the bottom shoulders of the blade or modification of the shape of the wavy washer to prevent contact with the blade shoulders was recommended.

A tubular heat exchanger in a refinery reformer unit leaked after one month of service. The exchanger contained 167 type 304 stainless steel U-bent integral-finned tubes. Cracks in the tube wall were revealed during examination. Hardness of the tube was found to be 30 HRC at the inside surface and up to 40 HRC at the base of the fin midway between the roots which indicated that the fins were cold formed and not subsequently annealed thus susceptible to SCC because of a high residual stress level. It was revealed by metallographic examination that the fracture was predominantly by transgranular branched cracking and had originated from the inside surface. It was concluded that the tubes failed in SCC caused by chlorides in the presence of high residual stresses. The finned tubes were ordered in the annealed condition as a corrective measure.

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.

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.

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.

Fragments of screen bars which as structural elements of a condenser had come into contact with cooling water from the mouth of a river were received. The screen bars were made of stainless austenitic Cr-Ni-Mo steel X 5 Cr-Ni-Mo18 10 (Material No. 1.4401). The bars were fractured repeatedly. The ruptures did not occur exclusively or even preferentially at the loops, but just as frequently at locations between them. The mistake made in this case was annealing the steel at a temperature in the critical region. This was probably done to relieve stresses that originated during cold-forming and led to damage by stress corrosion. This would have been the correct method for a ferritic steel, but not austenitic steel, which requires the special heat treatment indicated. When an anneal in the critical region is unavoidable and the indicated additional treatment is impossible or difficult, a type of steel has to be chosen which is resistant to intergranular corrosion.

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.

Following disruption of the austenitic stainless steel basket of a hydro-extractor used for the separation of crystals of salt (sodium chloride) from glycerin, samples of the broken parts were analyzed. Examination revealed that the fish-plates joining the reinforcing hoops had broken, the shell had split from top to bottom adjacent to the weld, the top and bottom cover plates had become loose, all the rivets having pulled out, and the shaft was also found to be bent. Fracture took place in an irregular manner and was of the shear type towards both ends; it occurred immediately adjacent to the weld or a short distance from it and on alternate sides. Microscopical examination did not reveal any intergranular carbide precipitation, such as is well known to result in the weld-decay mode of failure. It was concluded that the primary cause of failure was stress-corrosion cracking arising from the combined effect of residual stresses and the corrosive effect of the material being centrifuged. If the shell had been stress-relieved after fabrication, the failure likely would not have occurred.

The secondary cooling water system pressure boundary of Savannah River Site reactors includes expansion joints utilizing a thin-wall bellows. While successfully used for over thirty years, an occasional replacement has been required because of the development of small, circumferential fatigue cracks in a bellows convolute. One such crack was recently shown to have initiated from a weld heat-affected zone liquation microcrack. The crack, initially open to the outer surface of the rolled and seam welded cylindrical bellows section, was closed when cold forming of the convolutes placed the outer surface in residual compression. However, the bellows was placed in tension when installed, and the tensile stresses reopened the microcrack. This five to eight grain diameter microcrack was extended by ductile fatigue processes. Initial extension was by relatively rapid propagation through the large-grained weld metal, followed by slower extension through the fine-grained base metal. A significant through-wall crack was not developed until the crack extended into the base metal on both sides of the weld. Leakage of cooling water was subsequently detected and the bellows removed and a replacement installed.

Two tanks made of AISI type 304 stainless steel exhibited cracking in the heat-affected zone (HAZ) of the weld that joined the dished end and the shell. The dished ends had been produced by cold deformation. Hardness measurement and simulation tests showed that the deformation was equivalent to a 30% reduction in thickness. Residual stresses were measured at about 135 MPa (20 ksi). The HAZ was found to be sensitized. The tanks had been stored in a coastal atmosphere for about 4 years before installation. The failure was attributed to intergranular stress-corrosion cracking in a sensitized HAZ due to chloride from the environment. Use of low-carbon type AISI 304L was recommended. Minimization of fit-up stresses and covering with polyethylene sheets during storage were also suggested.

The governor on an aircraft engine failed and upon disassembly of the unit, it was discovered that the retainer for the flyweight pivot pins was broken. The channel-shaped retainer was made of 0.8 mm (0.030 in.) thick 1018 or 1020 steel. The part was plated with copper, which acted as a stop-off during carburizing of the offset, circular thrust-bearing surface surrounding the 16-mm (0.637-in.) diam hole. The bearing surface was case hardened to a depth of 0.05 to 0.1 mm (0.002 to 0.005 in.), then austempered to obtain a minimum hardness of 600 Knoop (1-kg, or 2.2-lb, load). Considerable vibration was created in the installation because of the design of the mechanical device used to transmit power to the governor. The pins were permitted to slide axially a small distance. Analysis (visual inspection, microscopic examination, and ductility measurements) supported the conclusion that failure of the retainer was the result of fatigue caused by vibration in the flyweight assembly. Impact of the pivot pins on the retainer also contributed to failure. Recommendations included redesign of the flyweight assembly, and replacement of the channel-shaped retainer with a spring-clip type of pin retainer.

Cold-drawn type 303 stainless steel wire sections, 6.4 mm (0.25 in.) in diameter, failed during a forming operation. All of the wires failed at a gradual 90 deg bend. Investigation (visual inspection and 5.3x/71x/1187x SEM views) supported the conclusion that the wires cracked due to ductile overload. The forming stresses were sufficient to initiate surface ruptures, suggestive of having exceeded the forming limit. Recommendations included examining the forming process, including lubrication and workpiece fixturing.

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.

A gas-fired, ASTM A-106 Grade B carbon steel vaporizer failed on three different occasions during attempts to bring the vaporizer on line. Dye penetrant examination indicated the presence of multiple packets of ductile cracks on the inside of the coil radius at the bottom of the horizontal axis coils. Visual examination of the inside of the tubing indicated the presence of a carbonaceous deposit resulting from decomposition of the heat-exchanging fluid. Subsequent metallographic examination and microhardness testing indicated that the steel was heated to a temperature above the allowable operating temperature for the fluid. The probable cause for failure is thermal fatigue due to the localized overheating. Flow conditions inside the tubing should be reexamined to ensure suitable conditions for annular fluid flow.

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.

Electrical contact-finger retainers blanked and formed from annealed copper alloy C65500 (high-silicon bronze A) failed prematurely by cracking while in service in switchgear aboard seagoing vessels. In this service they were sheltered from the weather but subject to indirect exposure to the sea air. About 50% of the contact-finger retainers failed after five to eight months of service aboard ship. Investigation (visual inspection, 250x images etched with equal parts NH4OH and H2O2, emission spectrographic analysis, and stereoscopic views) supported the conclusion that the cracking was produced by stress corrosion as the combined result of: residual forming and service stresses; the concentration of tensile stress at outer square corners of the pierced slots; and preferential corrosive attack along the grain boundaries as a result of high humidity and occasional condensation of moisture containing a fairly high concentration of chlorides (seawater typically contains about 19,000 ppm of dissolved chlorides) and traces of ammonia. Recommendations included redesign of the slots, shot-blasting the formed retainers, and changing the material to a different type of silicon bronze-copper alloy C64700.

Several surgical tool failures were analyzed to understand why they occur and how to prevent them. The study included drills, catheters, and needles subjected to the rigors of biomedical applications such as corrosive environments, high stresses, sterilization, and improper cleaning procedures. Given the extreme conditions to which surgical tools can be exposed, and the potential for misuse, failures are inevitable and systematic methods for analyzing them are necessary to keep them in check.

Manufacturing process selection is a critical step in plastic product design. The article provides an overview of the functional requirements that a part must fulfil before process selection is attempted. A brief discussion on the effects of individual thermoplastic and thermosetting processes on plastic parts and the material properties is presented. The article presents process effects on molecular orientation. It also illustrates the thinking that goes into the selection of processes for size, shape, and design factors. Finally, the article describes how various processes handle reinforcement.

A water tube boiler with two headers and 15.5 atm working pressure became leaky in the lower part due to the formation of cracks in the rivet-hole edges. The boiler plate of 20 mm thickness was a rimming steel with 0.05% C, traces of Si, 0.38% Mn, 0.027% P, 0.035% S, and 0.08% Cu. The mean value of the yield point was 24 (24) kg/sq mm, the tensile strength 39 (38) kg/sq mm, the elongation at fracture, d10, 26 (24)%, the necking at fracture 71 (66)% and notch impact value 11.5 (9.4) kgm/sq cm (the values in brackets are for the transverse direction). The specimen from inside surface of the boiler was polished and etched with Fry-solution, which revealed parallel striations formed due to the cold bending of the plate. The zones of slip were concentrated around the rivet holes. The cracks were formed here. The structure examination proved that the cracks had taken an exactly intercrystalline path, which is characteristic for caustic corrosion cracks. It was recommended that the internal stresses be removed through annealing or alternatively lye-resistant steel should be used.