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Acicular structure
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Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0048060
EISBN: 978-1-62708-224-2
... machining and chatter marks were evident on the threads. Beach marks that emanated from the thread-root locations on opposite sides of the fracture surface identified these locations to be the origins of the fracture. A medium-coarse slightly acicular structure was revealed by metallographic examination...
Abstract
The crane hook (rated for 13000 kg) failed in the threaded shank while lifting a load of 9072 kg. The metal in the hook was revealed by chemical analysis to be killed 1020 steel. It was disclosed by visual examination that the fracture had at the last thread on the shank and rough machining and chatter marks were evident on the threads. Beach marks that emanated from the thread-root locations on opposite sides of the fracture surface identified these locations to be the origins of the fracture. A medium-coarse slightly acicular structure was revealed by metallographic examination which indicated that the material was in the as-forged condition (which meant lower fatigue strength). The fracture was concluded to have occurred due to stress concentration in the root of the last thread. Normalizing of the crane hook after forging was suggested as a corrective measure. A stress-relief groove with a diam slightly smaller than the root diam was placed at the end of the thread and a large-radius fillet was machined at the change in diameter of the shank.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0048068
EISBN: 978-1-62708-224-2
... cracking started was revealed by visual examination of the fracture region. Almost 50% of the fracture surface was found to contain beach marks (indicative of fatigue failure) while the remainder contained cleavage facets. A medium-coarse acicular as-forged structure was revealed by metallographic...
Abstract
A hook on a two-leg chain (each 13 mm diam, included angle 60 deg) failed at the junction of the eye and shank while lifting a 4990 kg load. The diam of the hook at this junction was approximately 22 mm. Light intergranular oxidation at the surface on the side of the hook where cracking started was revealed by visual examination of the fracture region. Almost 50% of the fracture surface was found to contain beach marks (indicative of fatigue failure) while the remainder contained cleavage facets. A medium-coarse acicular as-forged structure was revealed by metallographic examination and the metal was showed by chemical analysis to be semikilled 1015 steel. The fatigue fracture was concluded to have initiated in the intergranular oxidation region and the failure of the hook was contributed by the poor fatigue and impact properties of the forged structure. As a corrective measure, the chain-sling hook was replaced with one made of normalized, fully killed, finegrain 1020 steel.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.machtools.c9001250
EISBN: 978-1-62708-223-5
... with 2% nital). 200 × Figure 4 illustrates the structure of a longitudinal microsection through the fracture surface in the vicinity of the cross. The structure on the left side of the picture is finely acicular and was strongly attacked by the etching reagent. Adjacent to this and extending...
Abstract
A broken cross-recessed die was examined. Examination of the unetched, polished section for impurities revealed several coarse streaks of slag. The purity did not therefore correspond to the requirements set for a high speed tool steel of the given theoretical quality DMo 5. After etching with 5% nital the polished surface exhibited a pronounced, easily-visible, fibrous structure. Microscopic examination revealed that this etch pattern was produced by marked segregation bands. The very unfavorable structure for a high speed steel tool of these dimensions and subject to such stresses together with the low purity favored the fracture of the tool.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c9001214
EISBN: 978-1-62708-235-8
... the quench structure of low-carbon steel, the structure of the flattened area consisted of coarse acicular martensite with a small amount of pearlite (quench troostite) and ferrite. Therefore the sleeve was overheated and probably quenched directly from case. To prevent damage, it would have been necessary...
Abstract
A case-hardened sleeve made of C 15 (Material No. 1.0401) was flattened at two opposing sides and had cracked open at these places, the crack initiating at a face plane. The wall of the sleeve was 9 mm thick, but the flat ends were machined down to 5.5 mm from the outside. The customer had specified a 2 mm case depth and a hardness of at least HRC 55 at a depth of 1.5 mm. An etched cross section of the cracked end showed that the case layer had a depth of 2.3 mm, so that the sleeve was almost through-hardened at the flat ends. While the core material with the full wall thickness had the quench structure of low-carbon steel, the structure of the flattened area consisted of coarse acicular martensite with a small amount of pearlite (quench troostite) and ferrite. Therefore the sleeve was overheated and probably quenched directly from case. To prevent damage, it would have been necessary to have a lower case depth, carburize less deeply, and prevent overheating that causes brittleness and leads also to increased case depth, or else use a fine-grained steel of lower hardenability.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c9001262
EISBN: 978-1-62708-224-2
... the austenitic grain boundaries. Such fractures are characteristic results of strong overheating. The coarse-grained, coarse acicular heat-treated structure of the chain link confirmed overheating. Because temperatures in excess of 1150 deg C are required for the solution of impurities, it is more probable...
Abstract
A chain link which was part of the hoisting mechanism of a drop hammer broke after three or four months of service. It was reportedly manufactured of the heat resistant steel 30 Cr-Mo-V 9 (Material No. 1.7707). The fracture of the chain link had a conchoidal structure and ran along the austenitic grain boundaries. Such fractures are characteristic results of strong overheating. The coarse-grained, coarse acicular heat-treated structure of the chain link confirmed overheating. Because temperatures in excess of 1150 deg C are required for the solution of impurities, it is more probable that the real damage was done during the heat-up forging (drop-forging) and could not be removed during heat-treatment.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c9001209
EISBN: 978-1-62708-224-2
... an unkilled carbon-deficient steel, and were case hardened to a depth of 0.8 to 0.9 mm. The peripheral structure at the places not showing wear consisted of coarse acicular martensite with a high percentage of retained austenite. The links therefore were strongly overheated, probably directly heated during...
Abstract
Three links of a chain showing unusually strong wear were examined. Corresponding to the stress, the wear was strongest in the bends of the links, but it was especially pronounced in the bend in which the butt weld seam was located. Investigation showed the links were manufactured from an unkilled carbon-deficient steel, and were case hardened to a depth of 0.8 to 0.9 mm. The peripheral structure at the places not showing wear consisted of coarse acicular martensite with a high percentage of retained austenite. The links therefore were strongly overheated, probably directly heated during case hardening. The butt weld seams were not tight and were covered with oxide inclusions. Given that wear occurred preferentially at the welds it may be concluded that this weld defect contributed to the substantial wear. This leaves unanswered whether these chains could have withstood the high operating stress if they had been welded satisfactorily and hardened correctly, and whether it made any sense to case harden highly stressed chains of this type.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.steel.c9001159
EISBN: 978-1-62708-232-7
... 3 shows the transition from the mild steel pipe to the welding material that had frozen in position. The otherwise fine-grained ferritic-pearlitic microstructure of the unalloyed pipe ( Fig. 4 ) has become coarse-grained and acicular (Widmanstätten structure).The microstructure of the welding seam...
Abstract
A recuperator used for preheating the combustion air for a rolling mill furnace failed after a relatively short service time because of leakage of the pipes in the colder part. The 6 % chrome steel pipes used for the warmer part connected by means of welding with austenitic electrodes to the unalloyed mild steel pipe of larger diam. Visual inspection showed corrosion and deep, trench-like erosion over the entire circumference of the seam on the side of the thicker mild steel pipe. Examination using the V2-A solution for picral etch showed the microstructure of the unalloyed pipe had become coarse-grained and acicular, and the microstructure of the welding seam had become predominantly martensitic as a result of the mixing of the weld metal with the fused pipe material. The chrome steel pipe had become partially transformed to martensite or bainite at the transition to the weld. Thus, the failure occurred due to typical contact corrosion wherein the alloyed welding seam represented the less noble electrode. The martensitic structure may have contributed to the failure as well. Due to the typical nature of the failure, no recommendations were made.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.modes.c0048757
EISBN: 978-1-62708-234-1
... surface embrittlement resulted from oxygen absorption which made the surface layers susceptible to cracking under start up and shutdown. Replacement tubes made of a heat-resistant alloy (e.g., Hastelloy C-276) were recommended. Acicular structure Overheating Tubes ASTM B337 grade 2 UNS R50400...
Abstract
Brief overheating of the 89 mm OD 6.4 mm wall thickness titanium heater tubes (ASTM B337, grade 2) was caused by a flow stoppage in a leach heater. Blue-tinted areas and patches of flaky white, yellow, and brown oxide scale was revealed on visual examination. It was disclosed by subjecting the overheated tube to a flattening test that the tube no longer met ASTM B 337 specifications. Large grain size and numerous needlelike hydride particles were disclosed in the microstructure of the overheated tube. Heating to approximately 815 deg C was revealed by the presence of the flaky oxide and increased grain size. Hydrogen and oxygen absorption was revealed by the presence of hydrides and the shallow surface embrittlement and thus susceptibility to cracking at ambient temperatures was observed. It was concluded that the titanium tubes were embrittled due to overheating the tubes and the severe surface embrittlement resulted from oxygen absorption which made the surface layers susceptible to cracking under start up and shutdown. Replacement tubes made of a heat-resistant alloy (e.g., Hastelloy C-276) were recommended.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.power.c0048814
EISBN: 978-1-62708-229-7
... the shielded metal arc and submerged arc portions of the weld. The HAZ had a more acicular structure normally associated with untempered martensite. Scanning electron microscopy of the failure surface of the plug sample revealed striations of the type normally associated with progressive or fatigue-type...
Abstract
A nuclear steam-generator vessel constructed of 100-mm thick SA302, grade B, steel was found to have a small leak. The leak originated in the circumferential closure weld joining the transition cone to the upper shell. The welds had been fabricated from the outside by the submerged arc process with a backing strip. The backing was back gouged off, and the weld was completed from the inside with E8018-C3 electrodes by the shielded metal arc process. Striations of the type normally associated with progressive or fatigue-type failures including beach marks that allowed tracing the origin of the fracture to the pits on the inner surface of the vessel were revealed. Copper deposits with zinc were revealed by EDS examination of discolorations. Pitting was revealed to have been caused by poor oxygen control in the steam generators and release of chloride into the steam generators. It was concluded by series of controlled crack-propagation-rate stress-corrosion tests that A302, grade B, steel was susceptible to transgranular stress-corrosion attack in constant extension rate testing with as low as 1 ppm chloride present. It was recommended to maintain the coolant environment low in oxygen and chloride. Copper ions in solution should be eliminated or minimized.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001434
EISBN: 978-1-62708-236-5
... , it was associated with hardening cracks of the weld under-bead variety. Microscopical examination confirmed these indications and showed the hardened layer to have an acicular structure characteristic of martensite; Figure 3 includes some of the fine cracks associated with this zone. The unaffected material...
Abstract
One of the connecting rods of a vertical, four-cylinder engine with a cylinder diameter of 5 in. failed by fatigue cracking just below the gudgeon-pin boss. Failure took place in line with the lower edge of a deposit of weld metal. The fracture surface was smooth, conchoidal, and characteristic of that resulting from fatigue. The origin of the major crack was associated with a crescent-shaped area immediately below the weld deposit. This showed brittle fracture characteristics and appeared to be an initial crack that occurred at the time of welding and from which the fatigue crack subsequently developed. The rod was made from a medium carbon or low-alloy steel in the hardened and fully tempered condition. Evidence indicated that, following modification to the oil feed system, the rod that broke was returned to service with fine cracks present immediately below the weld deposit, which served as the starting points of the fatigue cracks. Following this accident, the remaining three rods (which had been modified in a similar manner) were replaced as a precautionary measure.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.mech.c0047187
EISBN: 978-1-62708-225-9
... in peripheral speed. The microstructure at the periphery of a worn tooth at its unworn side ( Fig. 1b ) consisted of coarse acicular martensite with a large percentage of retained austenite. Parts with such a structure usually have a relatively low wear resistance because of low hardness. They are also very...
Abstract
A gear manufacturer experienced service problems with various gears and pinions that had worn prematurely or had fractured. All gears and pinions were forged from 1.60Mn-5Cr steel and were case hardened by pack carburizing. Gear Failure: One of the gears showed severe wear on the side of the teeth that came into contact with the opposing gear during engagement. The microstructure at the periphery of a worn tooth at its unworn side consisted of coarse acicular martensite with a large percentage of retained austenite. Pinion Failure: The teeth of the pinion exhibited severe spalling; the microstructure at the surface consisted of coarse acicular martensite with retained austenite. Also, a coarse network of precipitated carbide particles showed that the carburization of the case had appreciably exceeded the most favorable carbon content. This evidence supported the following conclusions: 1) High wear rate on the gears was caused by spalling of the coarse-grain surface layer. The underlying cause of the wear was overheating during the carburization. 2) Pinion failure resulted from overheating combined with excessive case carbon content. Thus, no recommendations were made.
Image
Published: 01 January 2002
Fig. 4 A properly heat treated tool of manganese oil-hardening steel has a martensitic structure (left) containing fine tempered martensite and carbides. An overheated tool material has an acicular martensitic structure (right) that cracks easily (see Fig. 5 ). Etchant, Nital; 700X.
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Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.modes.c0048289
EISBN: 978-1-62708-234-1
.... The microstructure near the rupture edge was revealed by metallographic examination to consist of ferrite and acicular martensite or bainite. The microstructure and the observed lack of cold work indicated a temperature above the transformation temperature of 727 deg C had been reached. Swelling of the tubes...
Abstract
The center portions of two adjacent low-carbon steel boiler tubes (made to ASME SA-192 specifications) ruptured during a start-up period after seven months in service. It was indicated by reports that there had been sufficient water in the boiler two hours before start-up. The microstructure near the rupture edge was revealed by metallographic examination to consist of ferrite and acicular martensite or bainite. The microstructure and the observed lack of cold work indicated a temperature above the transformation temperature of 727 deg C had been reached. Swelling of the tubes was disclosed by the wall thickness and OD of the tubing. The tubes were concluded to have failed due to rapid overheating.
Image
in Failure Analysis and Life Assessment of Structural Components and Equipment
> Failure Analysis and Prevention
Published: 01 January 2002
(A) and the as-forged (acicular) microstructure (B). The black square shapes (C) are diamond pyramid hardness indentations.
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Image
in Failure Prevention through Life Assessment of Structural Components and Equipment
> Analysis and Prevention of Component and Equipment Failures
Published: 30 August 2021
(A) and the as-forged (acicular) microstructure (B). The black square shapes (C) are diamond pyramid hardness indentations.
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Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.steel.c9001245
EISBN: 978-1-62708-232-7
... acicular martensite with fine spherical carbide inclusions ( Fig. 6 ) up to a depth of 5–6 mm. The martensite indicated no signs of tempering treatment. Adjoining the purely martensitic peripheral zone was a transition zone with increasing amount of bainite and pearlite from the exterior to the interior...
Abstract
A working roll of 210 mm diam and 500 mm face length was examined because of shell-shaped fractures. The roll consisted of Fe-0.83C-1.6Cr steel. The chromium content was low for a roll of this diam. The crack origin was located about 10 mm under the roil face. Surface hardness (HV1) of 900 kp/sq mm was exceptionally high corresponding to the martensitic peripheral structure. An untempered piece with such a thick cross section and a hardened peripheral zone with such high hardness must have high residual stresses that culminate in the transition zone. Therefore it must be very sensitive against additional stresses, be these of a mechanical or thermal nature. This contributed to the fragmenting of the roll face.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.conag.c9001501
EISBN: 978-1-62708-221-1
... and a case hardness traverse. Microscopic Examination The core structure contains an equal admixture of low-carbon martensite and lamellar pearlite. The case structure displays a very fine acicular martensite retaining less than 5% austenite. The pitted area low on the active profile shows evidence...
Abstract
A spiral bevel gear and pinion set that showed "excessive wear on the pinion teeth" was submitted for analysis. This gear set was the primary drive unit for the differential and axle shafts of an exceptionally-large front-end loader in the experimental stages of development. There was no evidence of tooth bending fatigue on either part. Several cracks were associated with the spalling surfaces on the concave sides of the 4820H NiMo alloy steel pinion teeth. The gear teeth showed no indication of fatigue. The primary mode of failure was rolling contact fatigue of the concave (drive) active tooth profile. The spalled area was a consequence of this action. The pitting low on the profile appeared to have originated after the shift of the pinion tooth away from the gear center. The shift of the pinion was most often due to a bearing displacement or malfunction. The cause of this failure was continuous high overload that may also have contributed to the bearing displacement.
Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001369
EISBN: 978-1-62708-215-0
... Interface of weld buildup and base metal. Weld is on left (white, unetched), martensite on right (black and white, acicular). A crack is visible under the weld. 2% nital etch. 190× Chemical Analysis/Identification Results of the chemical analysis of the shaft are shown in Table 1 , along...
Abstract
A crane long-travel worm drive shaft was found to be chipped during unpacking after delivery. Chemical analysis showed that the steel (EN36A with a case depth of 1 mm, or 0.04 inch did not meet specifications. Magnetic particle inspection revealed a crack on the side of the shaft opposite the chip. Metallographic examination indicated that the case depth was approximately 2 mm (0.08 in.) and that a repair weld of an earlier chip had been made in the cracked area. The chipping was attributed to excessive case depth and rough handling. It was recommended that the shaft be returned to the manufacturer and a replacement requested.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0048073
EISBN: 978-1-62708-224-2
..., a small ductile shear lip, was approximately 30° off-center, indicating low stress and rotational bending. The shaft material was 1030 steel, as specified, and had a slightly acicular fine-grain structure. Conclusion The shaft failed by fatigue fracture, which originated at a sharp corner...
Abstract
A drum pinion shaft (1030 steel) which was part of the hoisting gear of a crane (capacity 18,140-kg) operating in a blooming mill failed while lifting a 9070 kg load. Chatter marks, rough-machining marks, and sharp corner radii were revealed in the keyway which extended into a shoulder at a change in diam. A circular recess below the keyway surface was revealed at each end of the keyway. A sharp corner at the end of the keyway was revealed by examination to be the origin of fracture. Beach marks were found radiating from the origin over a large portion of the fracture surface which confirmed failure of the shaft by fatigue fracture. As a corrective measure the shaft was replaced with one made of 4140 steel, quenched and tempered to a hardness of 286 to 319 HRB. The keyway was moved away from the change in section and was machined with a 1.6-mm radius in the bottom corners and a larger-radius fillet was machined at the change in section.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.conag.c9001497
EISBN: 978-1-62708-221-1
...-carbon martensite. This indicates a well quenched and tempered structure. Case structure: Acicular martensite retaining approximately 10% austenite near the surface. Excellent structure for material and heat treatment. Case depth: (specified 0.075–0.090 in.) 0.085 in. effective depth...
Abstract
A spiral bevel gear set in the differential housing of a large front-end loader moving coal in a storage area failed in service. The machine had operated approximately 1500 h. Although the failure involved only the pinion teeth, magnetic particle inspection was performed on each part. The 4817 NiMo alloy steel pinion showed no indication of additional cracking, nor did the 4820 NiMo alloy steel gear. The mode of failure was tooth bending fatigue with the origin at the designed position: root radius at midsection of tooth. The load was well centered, and progression occurred for a long period of time. The cause of failure was a suddenly applied peak overload, which initiated a crack at the root radius. Progression continued by relatively low overstress from the crack, which was now a stress-concentration point. This was a classic tooth bending fatigue failure.
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