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Stress relieving
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Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.conag.c0051294
EISBN: 978-1-62708-221-1
Abstract
An agricultural tine, which is a relatively large double torsion spring with outer legs that are used to sweep through hay or other crops and turn them over, had failed. It was made hard-drawn carbon steel. Bending fatigue was revealed by visual examination to be almost certainly the cause of failure. The fatigue fracture origin was found on the inside surface of the legs at the point where they joined the coiled body of the spring. It was established that the tines after being wound up by loading with hay, sprung back through the neutral unloaded position and into the unwind direction. This movement into the unwind direction was concluded to be happening often enough to initiate fatigue. The stress relieving temperature was recommended to be increased to reduce the residual stresses from coiling and hence improve fatigue performance.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.conag.c9001198
EISBN: 978-1-62708-221-1
Abstract
Cast iron bearing caps in tractor engines fractured repeatedly after only short operating periods. The fracture originated in a cast-in groove and ran approximately radially to the shaft axis. The smallest cross section was at the point of fracture. The core structure of the caps consisted of graphite in pearlitic-ferritic matrix. Casting stresses did not play a decisive role because of the simple shape of the pieces that were without substantial cross sectional variations. Two factors exerted an unfavorable effect in addition to comparatively low strength. First, the operating stress was raised locally by the sharp-edged groove, and second, the fracture resistance of the cast iron was lowered at this critical point by the existence of a ferritic bright border. To avoid such damage in the future it was recommended to observe one or more of the following precautions: 1) Eliminate the grooves; 2) Remove the ferritic bright border; 3) Avoid undercooling in the mold and therefore the formation of granular graphite; 4) Inoculate with finely powdered ferrosilicon into the melt for the same purpose; and, 5) Anneal at lower temperature or eliminate subsequent treatment in consideration of the uncomplicated shape of the castings.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.marine.c9001429
EISBN: 978-1-62708-227-3
Abstract
Two diesel engine crankshafts of similar dimensions, the journal diam being approximately 7 in., failed due to cracking originating in the fillet at the junction between the crankpin and the web nearest to the flywheel. The cracks were discovered before rupture occurred. Several small cracks originated in the fillet, ran together and developed as two main crack fronts that ultimately merged into one, a typical example of a fatigue failure. Electromagnetic crack detection revealed the presence of a number of discontinuities which were located at a position that would correspond to the vertical axis of the original ingot. The crankshaft had not been stress-relieved after a welding operation had been carried out. The only satisfactory course to follow when dealing with a highly stressed part in which defects of the type in question are revealed during machining is to scrap the forging.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.power.c0048777
EISBN: 978-1-62708-229-7
Abstract
A 150 cm ID boiler drum made form ASTM A515, grade 70, steel failed during final hydrotesting at a pressure of approximately 26 MPa. Brittle fractures were revealed in between two SA-106C nozzles and remainder was found to involve tearing. Short, flat segments of fracture area, indicative of pre-existing cracks, were revealed by examination of the fracture surface at the drain grooves arc gouged at the nozzle sites. A thin layer of material with a dendritic structure was observed at the groove surface. The dendritic layer was revealed by qualitative microprobe analysis to contain over 1% C, higher than the carbon content of the base metal. The cracks in the drain groove surface could have occurred after arc gouging, during subsequent stress-relieving, or during the hydrostatic test. Flame cutting is not recommended for the type of steel used in the boiler drum because it can lead to local embrittlement and stress raisers, potentially initiating major failures.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c0091009
EISBN: 978-1-62708-235-8
Abstract
A thick-walled tube that was weld fabricated for use as a pressure vessel exhibited cracks. Similar cracking was apparent at the weld toes after postweld stress relief or quench-and-temper heat treatment. The cracks were not detectable by nondestructive examination after welding, immediately prior to heat treatment. Multiple-pass arc welds secured the carbon-steel flanges to the Ni-Cr-Mo-V alloy steel tubes. Investigation (visual inspection, metallographic analysis, and evaluation of the fabrication history and the analysis data) supported the conclusion that the tube failed as a result of stress-relief cracking. Very high residual stresses often result from welding thick sections of hardenable steels, even when preheating is employed. Quenched-and-tempered steels containing vanadium, as well as HSLA steels with a vanadium addition, have been shown to be susceptible to this embrittlement. Manufacturers of susceptible steels recommend use of these materials in the as-welded condition.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c9001400
EISBN: 978-1-62708-235-8
Abstract
Following the discovery numerous cracks at many of the welded seams of a mild steel CO2 absorber vessel, a sample for examination was removed from the worst affected area where repairs had been effected. A 12 in. long circumferential crack was visible. Specimens were taken to cover the several locations of cracking which, in all cases, were found to be similar and of the intergranular type filled with oxide or corrosion product. The association of the cracks with the weld seams indicated that contraction stresses from welding were primarily responsible. Failure of the absorber vessel was found to be due to stress corrosion. Although the active agent present was not positively identified, the aqueous solution of monoethanolamine was thought to be the most probable. The origin of the stresses was not elucidated but the association of the cracks with the welded seams indicated inherent residual stresses as being primarily responsible. Tests carried out tend to suggest that stress relief was not carried out. For the replacement plant, consideration of stress relieving or the use of an alternative material was advised.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.modes.c0048791
EISBN: 978-1-62708-234-1
Abstract
Linear indications on the outer surface of a cross in a piping system were revealed by dye-penetrant examination. The cross was specified to be SA403 type WP 304 stainless steel. The cross had been subjected to induction-heating stress improvement. The linear indications on the cross were located in wide bands running circumferentially below the cross-to-cap weld and above the cap-to-discharge-pipe weld. The material was found to conform to the requirements both in terms of hardness and strength. Intergranular cracks filled with oxide were observed on metallographic analysis of a sectioned and oxalic acid etched sample. The grain size was found to exceed the ASTM standard. No indications of sensitization were observed during testing with practice A of ASTM A 262. Definitive evidence of contaminants to support SCC as the failure mechanism was not disclosed during analysis. It was concluded that overheating or burning of the forging, which classically results in large grain size, intergranular fractures, and fine oxide particles dispersed throughout the grains was the possible reason for the failure.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0089766
EISBN: 978-1-62708-224-2
Abstract
Two tubular AISI 1025 steel posts (improved design) in a carrier vehicle failed by cracking at the radius of the flange after five weeks of service. The posts were two of four that supported the chassis of the vehicle high above the wheels. The original design involved a flat flange of low-carbon low-alloy steel that was welded to an AISI 1025 steel tube, and the improved design included placing the welded joint of the flange farther away from the flange fillet. Investigation (visual inspection and chemical analysis) supported the conclusion that the failures in the flanges of improved design were attributed to fatigue cracks initiating at the aluminum oxide inclusions in the flange fillet. Recommendations included retaining the improved design of the flange with the weld approximately 50 mm (2 in.) from the fillet, but changing the metal to a forging of AISI 4140 steel, oil quenched and tempered to a hardness of 241 to 285 HRB. Preheating to 370 deg C (700 deg F) before and during welding with AISI 4130 steel wire was specified. It was also recommended that the weld be subjected to magnetic-particle inspection and then stress relieved at 595 deg C (1100 deg F), followed by final machining.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.chem.c9001401
EISBN: 978-1-62708-220-4
Abstract
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.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.chem.c0048795
EISBN: 978-1-62708-220-4
Abstract
A large pressure vessel designed for use in an ammonia plant failed during hydrostatic testing. It was fabricated from ten Mn-Cr-Ni-Mo-V steel plates which were rolled and welded to form ten cylindrical shell sections and three forgings of similar composition. The fracture surfaces were metallographically examined to be typical for brittle steel fracture and associated with the circumferential weld that joined the flange forging to the first shell section. Featureless facets in the HAZ were observed and were revealed to be the fracture-initiation sites. Pronounced banding in the structure of the flange forging was revealed by examination. A greater susceptibility to cracking was interpreted from the higher hardenability found within the bands. Stress relief was concluded to have not been performed at the specified temperature level (by hardness and impact tests) which caused the formation of hard spots. The mode of crack propagation was established by microstructural examination to be transgranular cleavage. It was concluded that failure of the pressure vessel stemmed from the formation of transverse fabrication cracks in the HAZ fostered by the presence of hard spots. It was recommended that normalizing and tempering temperatures be modified and a revised forging practice explored.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.chem.c0048840
EISBN: 978-1-62708-220-4
Abstract
A spherical carbon steel fixed-catalyst bed reactor, fabricated from French steel A42C-3S, approximately equivalent to ASTM A201 grade B, failed after 20 years of service while in a standby condition. The unit was found to contain primarily hydrogen at the time of failure. The vessel had a type 304 stainless steel shroud around the catalyst bed as protection against the overheating that was possible if the gas bypassed the bed through the refractory material. The failure was observed to have begun at the toe of the shroud-support ring weld. The ring was found to have a number of small cracks at the root of the weld. The cleavage mode of fracture was confirmed by SEM. The presence of extensive secondary cracking and twinning (Neumann bands) where the fracture followed the line of the shroud-support ring was revealed by metallography. It was revealed by refinery maintenance records that the ring had been removed for hydrotest and welded without any postweld heat treatment. The final cause of failure was concluded to be cracking that developed during the installation of the new shroud ring. Stress-relief heat treatments were recommended to be performed to reduce residual-stress levels after welding.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.chem.c0091694
EISBN: 978-1-62708-220-4
Abstract
Tube sheets (found to be copper alloy C46400, or naval brass, and 5 cm (2 in.) thick) of an air compressor aftercooler were found to be cracked and leaking approximately 12 to 14 months after they had been retubed. Most of the tube sheets had been retubed several times previously because of unrelated tube failures. Sanitary (chlorinated) well water was generally used in the system, although filtered process make-up water (river water) containing ammonia was occasionally used. Investigation (visual inspection, chemical analysis, mercurous nitrate testing, unetched 5X micrographs, and 250X micrographs etched in 10% ammonium persulfate solution) supported the conclusion that the tube sheets failed by SCC as a result of the combined action of internal stresses and a corrosive environment. The internal stresses had been induced by retubing operations, and the environment had become corrosive when ammonia was introduced into the system by the occasional use of process make-up water. Recommendations included making a standard procedure to stress relieve tube sheets before each retubing operation. The stress relieving should be done by heating at 275 deg C (525 deg F) for 30 min and slowly cooling for 3 h to room temperature.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.chem.c9001407
EISBN: 978-1-62708-220-4
Abstract
The onset of leakage adjacent to two butt welds in a 2 in. bore pipe was traced to the development of fine cracks. The pipe carried 40% sodium hydroxide solution. The actual temperature was not known, but the pipeline was steam traced at a pressure of 30 psi, equivalent to a temperature of 130 deg C (266 deg F). Magnetic crack detection revealed circumferential crack-like indications situated a short distance from the butt weld. Cracking originated on the bore surfaces of the tube and was of an intergranular nature reminiscent of caustic cracking in steam boilers. The strength of the solution of caustic soda and possibly the temperature also were in the range known to produce stress-corrosion cracking of mild steels in the presence of stresses of sufficient magnitude. In this instance the location of the cracking suggested that residual stresses from welding, which approach yield point magnitude, were responsible. As all other welds were suspect, the remedy was to remove the joints and to reweld followed by local stress relief.