A closed-loop hot water heating system at a museum in South Carolina was the subject of failure evaluation. The system consisted of plain carbon steel pipes (Schedule 40) made of ASTM A 106 or A 53 (ERW or seamless). The supply and return lines were made of the same materials. The fittings were mechanically threaded assemblies. Temperatures ranged from 150 to 155 deg F (65.6 to 68.3 deg C). Leaks in the system had reportedly initiated immediately after the building had been placed in service. The cause of corrosion inside the steel pipes was attributed to tuberculation caused by oxygen concentration cells and oxygen-pitting related corrosion. Both types of corrosion are due to the poor quality of the water and the lack of corrosion control in the water system.

A T-piece from a copper hot water system failed. Microscopic examination of a polished section revealed a main crack and branching transcrystalline cracks running from the outer surface of the pipe into the pipe wall. The crack appearance indicated disintegration by stress-corrosion cracking. Although copper is not susceptible in the pure state, it is prone to stress-corrosion cracking under tensile stress in the presence of other elements in a damp ammoniacal atmosphere. The material was not defective, but a phosphorus-deoxidized copper type. The residual phosphorus combined with oxygen to form phosphorus pentoxide. Hard soldering in turn prevented the formation of cuprous oxide, and hydrogen embrittlement occurred.

In a copper hot water system, a bent pipe was soldered into a straight pipe with twice the diameter. The neighborhood of the soldered joint was covered with corrosion product predominantly blue-green in color, presumably carbonates. When these corrosion products were scratched off it was seen that the copper beneath this layer had not suffered noticeable attack. The object of the examination was the localized deep cavities located almost symmetrically to both sides of the inserted end of the narrower tube on the internal wall of the wider tube which had in one place been eaten right through. The symmetrical location on each side of the point of insertion of the narrower pipe and the localized sharp delineation of the attack indicated erosion due to the formation of turbulence. By avoiding sharp transitions and abrupt changes in cross section it is possible to design the pipe work so that localized turbulence is obviated. Degassing and cleansing of the water also would reduce the danger of erosion particularly in the case of softened water, which takes up oxygen and carbon dioxide very readily thus becoming particularly aggressive.

Several failures occurred in 64-mm schedule 80 type 304 stainless steel (ASME SA-312, grade TP304) piping in a steam-plant heat-exchanger system near tee fittings at which cool water returning from the heat exchanger was combined with hot water from a bypass. Various portions of the piping were subjected to temperatures ranging from 29 to 288 deg C. Each of the failures were revealed to consist of transgranular cracking in and/or close to the circumferential butt weld joining the tee fitting to the downstream pipe leg, where the hot bypass water mixed with the cool return water. The transgranular cracks suggested that thermal fatigue was a more likely cause of failure than SCC. It was concluded by temperature measurements that circumferential temperature gradients, in combination with inadequate flexibility in the piping system as a whole, had caused the failures. The tee fitting was redesigned to alleviate the thermal stress pattern.

A heat-treated, cadmium-plated AISI 8740 steel bolt broke through the head-to-shank fillet while being handled during assembly. Fractographic and metallographic examination of the bolt traced the cause of failure to quench cracking, which occurred when the part was water cooled following hot heading and prior to the production run. The process chart for hot heading was changed from water quenching to air cooling following the forming operation.

A straight-tube cooler type heat exchanger had been in service for about ten years serving a coal pulverizer in Georgia. Non-potable cooling water from a local lake passed through the inner surfaces of the copper tubing and was cooling the hot oil that surrounded the outer diametral surfaces. Several of the heat exchangers used in the same application at the plant had experienced a severe reduction in efficiency in the past few years. One heat exchanger reportedly experienced some form of leakage following discovery of oil contaminating the cooling water. This heat exchanger was the subject of a failure investigation to determine the cause and location of the leaks. Corrosion products primarily contained copper oxide, as would be expected from a copper tubing. The product also exhibited the presence of a significant amount of iron oxides. Metallographic cross sectioning of the tubes and microscopic analysis revealed several large and small well rounded corrosion pits present at the inner diametral surfaces. The cause of corrosion was attributed to corrosive waters that were not only corroding the copper, but were corroding steel pipes upstream from the tubing.

A spiral heat exchanger made from 316L stainless steel developed a leak after eight years of service as a condenser on a distillation tower. Examination identified the leak as being located on the cooling water side in the heat affected zone (HAZ) of a weld joining two plates. Cooling water deposits were observed in a V-shaped corner formed by the weld. A metallurgical examination identified the presence of transgranular cracks in the HAZ on the cooling water side. Analysis of the cooling water revealed the presence of chlorides. Based on the metallurgical analysis and other findings, it was determined that the cracks and associated leak were the result of chloride stress-corrosion cracking.

Several type 304L (UNS S30400) stainless steel seamless tubes in a high-pressure synthesis gas cooler condensing ammonia in a fertilizer plant leaked in an unexpectedly short time. Representative samples of the tubes were subjected to chemical analysis, hardness tests, and optical microscopy examination. The tests revealed that the tubes conformed to specification. Crack morphology indicated stress-corrosion cracking by chlorides present in the cooling water. Use of a duplex stainless steel (for example, UNS S32304 S31803) as a tube material was recommended.

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 pipe in the lateral wall of a boiler powering an aircraft carrier flat-top boat failed during a test at sea. The pipe was made from ASTM 192 steel, an adequate material for the application. Microstructural analysis along with equipment operating records provided valuable insight into what caused the pipe to rupture. Although the pipe had been replaced just 50 h before the accident, the analysis revealed incrustations and corrosion pits on the inner walls and oxidation on the outer walls. Microstructural changes were also observed, indicating that the steel was exposed to high temperatures. The combined effect of pitting, incrustations, and phase transformations caused the pipe to rupture.

This article describes the characteristics of tubing of heat exchangers with respect to general corrosion, stress-corrosion cracking, selective leaching, and oxygen-cell attack, with examples. It illustrates the examination of failed parts of heat exchangers by using sample selection, visual examination, microscopic examination, chemical analysis, and mechanical tests. The article explains corrosion fatigue of tubing of heat exchangers caused by aggressive environment and cyclic stress. It also discusses the effects of design, welding practices, and elevated temperatures on the failures of heat exchangers.

A laminated-paper microwave food tray collapsed with hot food in it. Microscopic examination of the failed tray revealed no structural or material defects. Five additional trays of like construction were also tested to determine the conditions necessary to simulate the permanent deflection of the tray handles that had occurred in the failed tray. Full distortion of the handles was obtained experimentally only by dropping a full hot tray on its end onto the floor. The test results indicated that the tray had slipped from the hand of the user.

A power plant using two steam generators (vertical U-tube and shell heat exchangers, approximately 21 m (68 ft) high with a steam drum diameter of 6 m (20 ft)) experienced a steam generator tube rupture. Each steam generator contained 11,012 Inconel alloy 600 (nickel-base alloy) tubes measuring 19 mm OD, nominal wall thickness of 1.0 mm (0.042 in.), and average length of 18 m (57.75 ft). The original operating temperature of the reactor coolant was 328 deg C (621 deg F). A tube removal effort was conducted following the tube rupture event. Investigation (visual inspection, SEM fractographs, and micrographs) showed evidence of IGSCC initiating at the OD and IGA under ridgelike deposits that were analyzed and found to be slightly alkaline to very alkaline (caustic) in nature. Crack oxide analysis indicated sulfate levels in excess of expected values. The analysis supported the conclusion that that the deposits formed at locations that experienced steam blanketing or dryout at the higher levels of the steam generators. Recommendations included steam generator water-chemistry controls, chemical cleaning, and reduction of the primary reactor coolant system temperature.

A tie rod, nut, and bellows from a failed 610 mm (24 in.) diam tied universal expansion joint that carried tail gases consisting of N 2 + O 2 with slight traces of nitrogen oxides and water were examined. The materials were SA 193-B7 (AISI 4140), SA 194–214, and Incoloy 800H, respectively. Visual examination of the bellows revealed cracks in heavily cold-worked areas (both inside and outside) and considerable corrosion. SEM analysis showed a classical intergranular failure pattern with microcracking. The threaded tie rod microstructure contained spheroidized carbide that was more pronounced at the tie rod end of the failure. Energy-dispersive X-ray analysis of fracture surfaces from the bellows showed the presence of chlorine and sulfur. Failure of the bellows was attributed to stress-corrosion cracking, with chlorine and sulfur being the corroding agents. The rod damage was the result of failure of the bellows, which allowed escaping hot gases to impinge on the tie rods and heat them to approximately 595 deg C (1100 deg F). It was recommended that the insulation be analyzed to determine the origin of the chlorine and sulfur and that it be replaced if necessary.

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.

A valve stem made of 17-4 PH (AISI type 630) stainless steel, which was used for operating a gate valve in a steam power plant, failed after approximately four months of service, during which it had been exposed to high-purity water at approximately 175 deg C (350 deg F) and 11 MPa (1600 psi). The valve stem was reported to have been solution heat treated at 1040 +/-14 deg C (1900 +/-25 deg F) for 30 min and either air quenched or oil quenched to room temperature. The stem was then reportedly aged at 550 to 595 deg C (1025 to 1100 deg F) for four hours. Investigation (visual inspection, 0.7x/50x images, hardness testing, reheat treatment, and metallographic examination) supported the conclusion that failure was by progressive SCC that originated at a stress concentration. Also, the solution heat treatment had been either omitted or performed at too high of a temperature, and the aging treatment had been at too low of a temperature. Recommendations included the following heat treatments: after forging, solution heat treat at 1040 deg C (1900 deg F) for one hour, then oil quench; to avoid susceptibility to SCC, age at 595 deg C (1100 deg F) for four hours, then air cool.

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.

A double-walled, hemispherical metal beam exit window made of alloy 718 developed a crack during service, leading to coolant leakage. The window had been exposed to radiation damage from 800 MeV protons and a cyclic stress from 600 MPa tensile to near zero induced by numerous temperature cycles calculated to be from 400 to 30 deg C (752 to 86 deg F). The window was activated to >200 Sv/h. It was determined through analysis using remote handling techniques and hot cells that the crack initiated near a spot weld used to affix thermocouples to the window surface. In addition to analysis of the crack, some of the irradiated material from the window was used to measure mechanical properties. Hot cell techniques for preparation of samples and testing were developed to determine true operating conditions of radiation, strain, and temperature.

Tube failures occurred in quick succession in two boiler units from a bank of six boilers in a refinery. The failures were confined to the SAE 192 carbon steel horizontal support tubes of the superheater pack. In both cases, the failure was by perforation adjacent to the welded fin on the crown of the top tubes and located in an area near the upward bend of the tube. The inside of all the tubes were covered with a loosely adherent, black, alkaline, powdery deposit comprised mainly of magnetite. The corroded areas, however, had relatively less deposit. The morphology of the corrosion damage was typical of alkaline corrosion and confirmed that the boiler tubes failed as a result of steam blanketing that concentrated phosphate salts. The severe alkaline conditions developed most probably because of the decomposition of trisodium phosphate, which was used as a water treatment chemical for the boiler feed water.

Heat exchangers are devices used to transfer thermal energy between two or more fluids, between a solid surface and a fluid, or between a solid particulate and a fluid at different temperatures. This article first addresses the causes of failures in heat exchangers. It then provides a description of heat-transfer surface area, discussing the design of the tubular heat exchanger. Next, the article discusses the processes involved in the examination of failed parts. Finally, it describes the most important types of corrosion, including uniform, galvanic, pitting, stress, and erosion corrosion.