Search Results for Surface martensite

Locked coil wire ropes, by virtue of their unique design and construction, have specialized applications in aerial ropeways, mine hoist installations, suspension bridge cables, and so forth. In such specialty ropes, the outer layer is constructed of Z-profile wires that provide not only effective interlocking but also a continuous working surface for withstanding in-service wear. The compact construction and fill-factor of locked coil wire ropes make them relatively impervious to the ingress of moisture and render them less vulnerable to corrosion. However, such ropes are comparatively more rigid than conventional wire ropes with fiber cores and therefore are more susceptible to the adverse effects of bending stresses. The reasons for premature in-service wire rope failures are rather complex but frequently may be attributed to inappropriate wire quality and/or abusive operating environment. In either case, a systematic investigation to diagnose precisely the genesis of failure is desirable. This article provides a microstructural insight into the causes of wire breakages on the outer layer of a 40 mm diam locked coil wire rope during service. The study reveals that the breakages of Z-profile wires on the outer rope layer were abrasion induced and accentuated by arrays of fine transverse cracks that developed on a surface martensite layer.

A steel splice plate in a power transmission line tower cracked while in service. Metallographic analysis indicated the presence of a white hard martensite layer near the crack, which occurred in the heel of the plate. Mechanical property tests revealed localized hardening in the area of the crack, supporting the metallurgical findings. A substantial deterioration of the Charpy impact toughness of the material in the heel region was also observed which is believed to have caused the initiation and propagation of the cracks leading to the failure.

A low-carbon steel (St35.8) tube in a phthalic anhydride reactor system failed. Visual and stereomicroscopic examination of fracture surfaces revealed heavy oxide/deposits on the outer surface of the tube, tube wall thinning in the area of the fracture, and discolorations and oxides/deposits on the inner surface. Cross sections from the fracture surface were metallographically examined, and the deposits were analyzed. It was determined that the tube had thinned from the inner surface because of a localized overheating condition (probably resulting from a runaway chemical reaction within the tube) and then fractured, which allowed molten salt to flow into the tube.

A 10,890-kg coil hook torch cut from 1040 steel plate failed while lifting a load of 13,600 kg after eight years of service. The normal ironing (wear) marks were exhibited by the inner surface of the hook. It was revealed by visual examination that cracking had originated at the inside radius of the hook. Beach marks (typical of fatigue fracture) were found extending over approximately 20% of the fracture surface. Numerous cracks were revealed by macroscopic examination of the torch-cut surfaces. It was revealed by macrograph of an etched specimen that the cracks had initiated in a hardened martensitic zone at the torch-cut surface and had extended up to the coarse pearlite structure beneath the martensitic zone. The fatigue fracture was concluded to have initiated in the brittle martensitic surface while failure was contributed by the 25% overload. As a corrective measure, the coil hooks were flame cut from ASTM A242 fine-grain steel plate, ground to remove the material damaged by flame cutting and stress relieved at 620 deg C.

Failed portions of a 4140 steel axle from a prototype urban transit vehicle were examined to determine the cause of failure. The testing procedures included visual examination, macrofractography, metallography, chemical analysis, and hardness and tensile testing. The analysis showed that a salvage welding repair had resulted in the formation of martensite that developed cracks, leading to fatigue failure of the axle. It was recommended that no weld buildup or repair be permitted on the axles.

Deformation, surface cracking, and spalling on the raceway of the outer ring (made of 4140 steel) of a large bearing caused it to be replaced from a radar antenna. The raceway surfaces were to be flame hardened to 55 HRC minimum and 50 HRC 3.2 mm below the surface, according to specifications. Samples from both the inner and outer rings were examined. A much lower hardness (25.2 to 18.9 HRC) was indicated during a vertical traverse 4.1 cm from the outer surface of the outer ring while slightly lower hardness values (46.8 to 54.8 HRC) were seen on the hardness traverse on the inner ring raceway. The lower hardness values were attributed to improper flame hardening. It was confirmed by metallographic examination of a 3% nital etched sample that the inner ring (tempered martensite and ferrite) and the outer ring (ferrite, scattered patches of pearlite, and martensite) were not properly austenitized. Displacement of metal on the outer raceway was revealed by elongation of grain structure. It was concluded that the failure of the raceway surface was due to incomplete austenitization caused by the improper heat treatment during flame hardening process.

The failure of a 45 Mg (50 ton) rail crane bolster was investigated. Spectrochemical analysis indicated that the material was a 0.25C-1.24Mn-0.62Cr-0.24Mo cast steel. SEM examination revealed the presence of fatigue, as well as intergranular and ductile fractures. Microstructural analysis focused on an area where an antisway device had been welded to the structure and revealed the presence of coarse, untempered martensite that had resulted from faulty weld repair techniques. It was suggested that the use of proper welding procedures, including preheating and postheating, would have prevented the failure.

The A2 tool steel mandrel, part of a rolling tool used for mechanically joining two tubes was fractured after making five rolled joints. A 6.4 mm diam hole was drilled by EDM through the square end of the hardened mandrel due to difficulty was experienced in withdrawing the tool. The fracture progressed into the threaded section and formed a pyramid-shape fragment after it was initiated at approximately 45 deg through the hole in the square end. An irregular zone of untempered martensite with cracks radiating from the surface of the hole (result of melting around hole) was revealed by metallographic examination. A microstructure of fine tempered martensite containing some carbide particles was exhibited by the core material away from the hole. Brittle fracture characteristics with beach marks were exhibited by the fracture surfaces which is characteristic of a torsional fatigue fracture. As a corrective measure, the hole through the square end of the mandrel was incorporated into the design of the tool and was drilled and reamed before heat treatment and specified hardness of the threaded portion and square end of the mandrel was reduced.

The specially designed sand-cast low-alloy steel jaws that were implemented to stretch the wire used in prestressed concrete beams fractured. The fractures were found to be macroscale brittle and exhibited very little evidence of deformation. The surface of the jaws was disclosed by metallographic examination to be case carburized. The case was found to be martensite with small spheroidal carbides while the core consisted of martensite plus some ferrite. The fracture was revealed to be related to shrinkage porosity. Tempering was revealed to be probably limited to about 150 deg C by the hardness values (close to the maximum hardness values attainable) for the core. It was interpreted that the low tempering temperature used may have contributed to the brittleness. The procedures used for casting the jaws were recommended to be revised to eliminate the internal shrinkage porosity. Tempering at a slightly higher temperature to reduce surface and core hardness was recommended.

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.

After less than 30 days in service, several cadmium-plated retaining rings, made of 4140 steel tubing and heat treated to 36 to 40 HRC, broke during operation that included holding components of a segmented fitting in place under a constant load. Photographic and 100x nital-etched micrographic examination showed a microstructure of tempered martensite with low inclusion content as well as a pit or burned spot on the outer area of the ring. The defect was approximately 0.18 mm (0.007 in.) deep and 0.5 mm (0.020 in.) in diam and had a hardness of 58 to 60 HRC. The base metal adjacent to the defect had a hardness of 36 to 40 HRC. Small cracks or fissures were also evident within the defect. Thus, the rings failed in brittle fracture as the result of an arc strike (or burn) on the surface of the ring. At the site of the arc strike, a small region of hard, brittle untempered martensite was formed as the result of an arc strike during the cadmium-plating operation. Fracture occurred readily when the ring was stressed. No recommendations were made.