A solution containing 50 to 70% calcium chloride (pH 7.5 to 8.5) was concentrated by evaporation in a brick-lined vessel by passing steam at a pressure of 15 atmospheres through a system of heating coils made of austenitic stainless steel X 10 Cr-Ni-Mo-Ti 18 12 (Material No. 1.4573). After five months one of the coils, which consisted of tubes having a wall thickness of 3.4 mm, developed a leak. Tightly closed cracks were seen on the outer surface of the tube. Further tests with color penetration process revealed multiple branched cracks. Longitudinal section showed that the cracks had started from the outside surface of the tube. Electrolytic etching further showed that they had propagated mainly across the grains. It was concluded that this was a typical case of transcrystalline stress corrosion.

A coil made of a nickel-chromium alloy (Material No. 2.4869) with approx. 80Ni and 20Cr had burned through after a brief period of operation as a heating element in a brazing furnace. The protective atmosphere consisted of an incompletely combusted coal gas. Furnace temperature reached 1150 deg C. This type of selective oxidation at which the easily oxidized chromium burns, while the nickel is not attacked, is caused by mildly oxidizing gases and is sometimes designated as green rot. Under these conditions, chromium-containing steels and alloys whose oxidation resistance is based upon formation of tight oxide layers are not stable.

A brass elbow that formed one termination of a steam heating coil failed adjacent to the brazed connection after ten years of service. Chemical analysis showed that the elbow was made from a 60-40 CuZn brass containing 3% lead and 1% tin, a typical alloy used for the manufacture of components by the hot stamping process. Microscopic examination indicated failure from dezincification. The fact that the screwed end was not affected indicated that the trouble was not caused by the condensate, which flowed through the elbow, but originated from the water heated in the vessel. The helical mode of the cracking was probably due to the torsional stresses which would be imposed on the elbow by thermally induced movements of the coil in service.

A process vessel heating coil, consisting of several 3 ft diam turns, was supplied with steam at 400 psi and a temperature of 343 deg C (650 deg F). At bi-weekly intervals well water was introduced to effect rapid cooling of the contents. After about eight months, leakage developed from a circumferential crack on the underside of the uppermost turn. Shorter cracks were found at a similar location on the bottom turn, and further leakage occurred at pinhole perforations adjacent to the crack in the top turn and near to a butt-weld in the coil. Microscopic examination revealed that the cracks were predominantly of the intergranular variety. In addition, transgranular cracks were present. Material was an austenitic stainless steel of the type specified but the absence of columbium and titanium in significant amounts showed that it was not stabilized against intergranular carbide precipitation. The transgranular cracks indicated that failure was due partly to stress-corrosion. It was concluded that the chlorides provided the main corrodent for both the stress and intercrystalline-corrosion cracking.

Although field corrosion tests had indicated that type 316L stainless steel would be a suitable material for neutralization tanks, the vessels suffered severe corrosion when placed in service. Welded coupons of type 316L had been tested along with similar Alloy 20Cb® (UNS NO8020) specimens in a lead-lined tank equipped with copper coils that had served in this function prior to construction of the new tanks. Both materials exhibited virtually no corrosion and no preferential weld attack. Type 316L was selected for the project. The subsequent corrosion was the result of the borderline passivity of type 316L in hot dilute sulfuric acid (about 0.1%). Inaccuracy of the testing was attributed to the presence of cupric ions in the lead-lined vessel fluids, which had been released by corrosion of the copper coils. Careful control of both temperature and pH was recommended to reduce the corrosion to an acceptable limit.

A root cause failure analysis was performed on a vaporizer coil removed from a horizontal forced circulation vaporizer. The carbon steel coil was wound in a right-hand helix with a coil centerline diameter of about 2 m. The vaporizer was gas fired and used Dowtherm A as the heat transfer fluid. Design conditions are based on annular fluid flow to cool the coil wall. NDE, metallographic and fractographic examinations were performed. Numerous, circumferentially oriented, OD initiating cracks were found near the crown for two coils near the non-fired end of the vaporizer. The cracking was confined to the inner diameter of the vaporizer coil at positions from 4:00 to 7:00. The cracking was characterized as transgranular and the fracture surface had beach marks. The failure mechanism was thermal fatigue. The heat transfer calculation predicted that dryout of the coil would occur for coils at the non-fired end of the vaporizer during low flow transients. Dryout results in rapid increase in the tube wall temperature. Thermal cycling of the coil is completed by liquid quenching resulting from resumption of normal flow rates and the return to annular flow. The probable root cause of failure was low flow transient operation.

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.

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.

Austenitic stainless steel (X 10 Cr-Ni-Mo-Ti 18 10, Material No. 1.4571) cooling coils were found leaking in 15 spots after eight weeks of service in an apparatus in which ammonium sulfide solution was converted into ammonium sulfate. The external temperature of the coil was approximately 175 deg C and it was cooled by water at 3 atm. Examination of two sections of the coil showed pinhead size pitting cavities at the exterior surface and partially parallel and partially angled array of fine cracks on external as well as the internal surfaces of the bend. Metallographic examination conducted on longitudinal and transverse sections showed predominantly transcrystalline cracks, originated from the pits at the external surfaces of the pipe. Their appearance suggested they were stress corrosion cracks that occur in austenitic steels under the combined effect of stresses and certain corrosion agents, especially chlorides. If chlorides were absent, hydrogen sulfide which causes similar pitting and is capable of causing cracks could be suspected. Favorable state of stresses, which could be residual or due to heat treating, bending or straightening operations, would be recommended for better behavior of the container.

After some 87,000 h of operation, failure took place in the bend of a steam pipe connecting a coil of the third superheater of a steam generator to the outlet steam collector. The unit operated at 538 deg C and 135 kPa, producing 400 t/h of steam. The 2.25Cr-1Mo steel pipe in which failure took place was 50.8 mm in diam with a nominal wall thickness of 8 mm. It connected to the AISI 321 superheater tube by means of a butt weld and was one of 46 such parallel connecting tubes. The Cr-Mo tubing was situated outside the heat transfer zone of the superheater. The overall sequence of failure involved overheating of the Cr-Mo outlet tubes, heavy oxidation, oxide cracking on thermal cycling, thermal fatigue cracking plus oxidation, creep-controlled crack growth, and rapid plastic deformation and rupture. This failure was indicative of excess temperature of the steam coming from the heat transfer zone of the coil. It showed that many damage mechanisms may combine in the transition from fracture initiation to final failure. The presence of grain boundary sliding as an indication of creep damage was useful in the characterization of the stress level as high and showed that the process of creep was not operative throughout the life of the equipment.

Copper shortening has been found to occur in the rotor windings of turbo alternators and takes the form of a progressive reduction in the length of the coils leading to distortion of the end windings. The trouble results from the high loading which develops between successive layers of the strip conductor due to centrifugal force. This leads to a high frictional binding force between turns and prevents axial expansion under normal heating in service. Rotor trouble which proved to be due to copper shortening was found in a set rated at 27.5 MW. It was manufactured in 1934 at which time silver-bearing copper was not available. The use of hard-drawn silver-bearing copper for a rewind, in conjunction with special attention to blocking up the end windings, is confidently expected to effect a complete cure.

An electronic sensor coil failed continuity testing, indicating that a break was present in the polymer-coated wire. An area of the wire showed a green discoloration and the break in the wire was located in this same region. The discoloration was suspected to be an indicator of what caused the failure. SEM/EDS and FTIR results showed the break in the coil wire was associated with corrosion. The corrosion debris contained relatively high levels of sodium and chlorine, which were likely in the form of salt. Some salt deposits were noted also in other areas along the wire surface. The findings suggested salt or salt water had leaked into the sensor and caused localized corrosion to the wire, possibly at an area where preexisting damage was present in the coating. Separation occurred in the wire when the current density at the reduced cross section caused excessive localized heating, which led to melting of the wire.

The self-powered flux detectors used in some nuclear reactors are Pt or V-cored co-axial cables with MgO as an insulator and Inconel 600 as the outer sheath material. The detectors are designed to operate in a He atmosphere; to maximize the conduction of heat (generated from the interaction with gamma radiation) and to prevent corrosion. A number of failures have occurred over the years because of a loss of the He cover gas in the assembly. This has resulted in either acid attack on the Inconel 600 sheath in a wet environment or gaseous corrosion in a dry environment. In the latter case, nitriding and embrittlement occurred at temperatures as low as 300 to 400 deg C (determined from an examination of the oxidation of the Zircaloy-2 carrier rod on which the detectors were mounted). Recent results are described and discussed in terms of the oxidation and nitriding kinetics of Zircaloy-2 and Inconel 600, respectively.

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.

The metallurgical condition of a cylindrical induction melter (CIM) vessel was evaluated after approximately 375 h of operation over a two-year span at temperatures between 1400 to 1500 deg C. Wall thinning and significant grain growth was observed in the lower portion of the conical section and the drain tube. No through-wall penetrations were found in the cylindrical and conical sections of the CIM vessel and only one leak site was identified in the drain tube. Failure of the drain tube was associated with localized overheating and creep. The observed degradation resulted from cumulative service at elevated temperature. A recommendation was made to implement a support for the conical section of the CIM and to increase the wall thickness of the drain tube. Thus, the possibility of drain tube misalignment in the induction coils and localized over heating will be minimized. In addition, the use of grain stabilized Pt/Rh alloy should be evaluated as a method to prevent grain growth.

An investigation was conducted to determine why 16 out of 139 pipe bends cracked during hot induction bending. The pipe conformed to API 5L X65 PSL2 line pipe standards and measured 1016 mm (40 in.) in diam with a wall thickness of 18.5 mm. A metallurgical cross section was removed along a crack on the extrados to document the crack morphology using optical microscopy. In addition to cracking, golden-yellow streaks were visible at the extrados, and the composition was examined using scanning electron microscopy with energy dispersive spectroscopy. Based on the results, investigators concluded the pipe was contaminated with copper at the mill were it was produced.

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.

Cracking occurred in type 304L stainless steel sheaths on nichrome wire heaters that had been in storage for about 5 years in a coastal atmosphere. The cracks were discovered when the heater coils were removed from storage in their original polyethylene packing materials and straightened for use. Fractography established that fracture occurred by stress-corrosion cracking. The cracks originated at rusted areas on the cladding that occurred under iron particles left on the surface during manufacture. High hardness values indicated that solution annealing following cold working had not been carried out as specified. It was recommended that the sheathing material be fully annealed and that the outer surface be pickled and passivated.

Two hot water reheat coil valves from a heating/ventilating/air-conditioning system failed in service. The values, a 353 copper alloy 19 mm (3/4 in.) valve and a 360 copper alloy 13 mm (1/2 in.) valve, had been failing at an increasing rate. The failures were confined to the stems and seats. Visual examination revealed severe localized metal loss in the form of deep grooves with smooth and wavy surfaces. Metallographic analysis of the grooved areas revealed uniform metal loss. No evidence of intergranular or selective attack indicating erosion-corrosion was observed, Recommendations included use of a higher-copper brass, cupronickel, or Monel for the valve seats and stems and operation of the valves in either the fully opened or closed position.