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.

Fatigue failures may occur in components subjected to fluctuating (time-dependent) loading as a result of progressive localized permanent damage described by the stages of crack initiation, cyclic crack propagation, and subsequent final fracture after a given number of load fluctuations. This article begins with an overview of fatigue properties and design life. This is followed by a description of the two approaches to fatigue, namely infinite-life criterion and finite-life criterion, along with information on damage tolerance criterion. The article then discusses the characteristics of fatigue fractures followed by a discussion on the effects of loading and stress distribution, and material condition on the microstructure of the material. In addition, general prevention and characteristics of corrosion fatigue, contact fatigue, and thermal fatigue are also presented.

A heat transport pump in a heavy water reactor failed (exhibiting excessive vibration) during a restart following a brief interruption in coolant flow due to a faulty valve. The pump had developed a large crack across the entire length of a bearing journal. An investigation to establish the root cause of the failure included chemical and metallurgical analysis, scanning electron fractography, mechanical property testing, finite element analysis of the shrink fitted journal, and a design review of the assembly fits. The journal failure was attributed to corrosion fatigue. Corrective actions to make the journals less susceptible to future failures were implemented and the process by which they were developed is described.

An 18-MW gas turbine exploded unexpectedly after three hours of normal operation. The catastrophic failure caused extensive damage to the rotor, casing, and nearly all turbo-compressor components. Based on their initial review, investigators believed that the failure originated at the interface between two shaft sections held together by 24 marriage bolts. Visual and SEM examination of several bolts revealed extensive deterioration of the coating layer and the presence of deep corrosion pits. It was also learned that the bolts were nearing the end of their operating life, suggesting that the effects of fatigue-assisted corrosion had advanced to the point where one of the bolts fractured and broke free. The inertial unbalance produced excessive vibration, subjecting the remaining bolts to overload failure.

Metallurgical SEM analysis provides many insights into the failure of biomedical materials and devices. The results of several such investigations are reported here, including findings and conclusions from the examination a total hip prosthesis, stainless steel and titanium compression plates, and hollow spinal rods. Some of the failure mechanisms that were identified include corrosive attack, corrosion plus erosion-corrosion, inclusions and stress gaps, production impurities, design flaws, and manufacturing defects. Failure prevention and mitigation strategies are also discussed.

Fractures and a crack occurred in a length of excavator boom rope. Failure took place at regions where local corrosion was evident. Microscopic examination of longitudinal sections disclosed that the majority of the cracks were broad, these being typical of corrosion-fatigue fissures. In addition, cracking took the form more typical of a fatigue crack and appeared to have originated at a stress-raiser introduced by a corrosion pit on the surface of the wire. The tendency for corrosion-fatigue cracking or the formation of pits from which fatigue cracks can develop can he reduced, if not prevented, in wire ropes by regular attention to lubrication.

The fracture of a clevis which formed part of the derricking gear of a large crane showed well developed conchoidal markings. These were associated with a principal origin on the outer surface at about the mid-width of the section. A number of secondary origins were apparent along this same edge. Failure was initiated at the extrados which suggested that a discrepancy in the size of the pin may have contributed to failure. Microscopic examination of a section through the main origin did not reveal any material defects or the presence of weld repairs which could have led to the premature failure. Furthermore, there were no indications that corrosion had contributed to the fatigue cracking.

A 230 mm (9 in.) thick casing, fabricated from ASTM 235-55 low-carbon steel, of a 450 Mg (500 ton) extrusion press failed after 27 years of service. Initial visual examination revealed an area that exhibited multiple origins and classic beach marks radiating out approximately 75 mm (3 in.) from the origin along the wall of a hydraulic-oil bleed hole. Investigation with a SEM showed corrosion pits along the bleed hole wall, but oxidation and corrosion prevented review of microfractographic details. Vacuum epoxy encapsulation, sectioning of the bleed hole, and metallographic examination revealed a basic microstructure of pearlite and ferrite with bands of slightly finer pearlite, with a large concentration of inclusion stringers in the area of the fracture origin. Further investigation using an energy-dispersive x-ray analyzer showed high concentrations of sulfur and manganese. Thus, the failure appeared to have resulted from corrosion-assisted fatigue, and the inclusion concentration in the fracture-initiated area indicated that the chemical-composition limits for sulfur and manganese would have greatly exceeded material specifications. A higher quality steel was recommended for the replacement unit to lessen the possibility of such gross inclusion segregation and to improve the fracture toughness of the cylinder.

Stainless steel is frequently used for bone fracture fixation in spite of its sensitivity to pitting and cracking in chloride containing environments (such as organic fluids) and its susceptibility to fatigue and corrosion fatigue. A 316L stainless steel plate implant used for fixation of a femoral fracture failed after only 16 days of service and before bone callus formation had occurred. The steel used for the implant met the requirements of ASTM Standard F138 but did contain a silica-alumina inclusion that served as the initiation point for a fatigue/corrosion fatigue fracture. The fracture originated as a consequence of stress intensification at the edge of a screw hole located just above the bone fracture; several fatigue cracks were also observed on the opposite side of the screw hole edge. The crack propagated in a brittle-like fashion after a limited number of cycles under unilateral bending. The bending loads were presumably a consequence of leg oscillation during assisted perambulation.

A compression hip screw is a device designed to hold fractures in the area of the femur in alignment and under compression. A side plate, which is an integral part of the device, is attached by screws to the femur, and it holds the compression screw in position. The device analyzed had broken across the eighth hole (of nine holes) from the end of the plate. The detailed metallurgical failure analysis of the device, including metallography and fractography, is reported here. It was found that the device had adequate metallurgical integrity for the application for which it was intended. It is believed that failure was caused by the lack of a screw in the ninth hole. Evidence is also presented which indicates that the device was bent prior to insertion, and the local plastic deformation may have caused structural changes leading to premature crack initiation.

After being in service for ten years, two admiralty brass heat-exchanger tubes from a cooler in a refinery catalytic reforming unit cracked circumferentially in the area of U-bends. A blunt transgranular cracking with minimal branching propagating from the inside surface of the tube was revealed by metallography which was typical of cracking by corrosion fatigue mechanism. Corrosion deposits on both the inside- and outside-diam surfaces were found in the tubes. The presence of copper, zinc, iron, and small amounts of chloride, sulfur, silicon, tin, and manganese was revealed by energy-dispersive analysis of the deposits. It was interpreted by the hardness values (higher than typical for annealed copper tubing) that the tubes may not have been annealed after the U-bends were formed and thus the role of residual stresses in the crack was revealed. It was concluded that the tubes failed by corrosion fatigue initiated by pitting at the inside-diam surface. The tubes were recommended to be annealed after bending to reduce residual stresses from the bending operation to an acceptable level.

Coiled tubing used in drilling operations failed at the halfway point of its estimated fatigue life. The failure was found to be transverse to the tubing axis. Visual examination revealed a flat fracture surface extending 13 mm with the rest of the fracture showing shear lips indicative of tensile overload. The flat portion of the fracture surface was typical of fatigue cracking. Fatigue striations were revealed by SEM. Corrosion pitting on the tubing ID from which the fatigue crack had propagated were observed on closer examination. The corrosion pitting was speculated to have occurred when the tubing was idle and fluids accumulated at the bottom of the tubing wraps. The coiled tubing was concluded to have failed prematurely due to low-cycle fatigue initiated at corrosion pitting sites. Corrosive attack on the coiled tubing was recommended to be reduced by completely removing fluids or modifying the fluids in the tubing or purging by flowing dry nitrogen to dry it out.

Visual examination, optical and scanning electron microscopy were used to determine the cause of failure in the connector groove of a marine riser coupling. The specified steel was AISI 4142 (0.40 to 0.45% C; 0.75 to 1.00% Mn; 0.20 to 0.35% Si; 0.80 to 1.10% Cr; 0.15 to 0.25% Mo) normalized from 9000C. Microscopic examination revealed the crack's initiation point and subsequent propagation. SEM examination of chemically stripped corrosion showed that corrosion fatigue and stress corrosion might have contributed to the initial slow crack growth. Impact tests revealed a fracture transition temperature in excess of 1000C. The sequence of events leading to failure was detailed. The main recommendation was to quench and temper existing couplings and to use a lower carbon quenched and tempered steel for new couplings.

Sucker-rod pumps are operating in very aggressive environments in oil well production. The combined effect of a corrosive environment and significant mechanical loads contribute to frequent cases of failure of the rod string during operation. Standards and recommendations have been developed to control and avoid those failures. This study presents various failure cases of sucker rods in different applications. The heat treatment of the steel material and the resulting microstructure are an important factor in the behavior of the sucker rod. A spheroidized microstructure presents a weaker resistance to corrosion affecting the rod life. Non-metallic inclusions are a pitting preferential site leading to fatigue crack initiation. Heterogeneous microstructure as banded martensite and ferrite/pearlite decreases the ductility of the material affecting the fatigue propagation resistance.

The wires used in a wet precipitator for cleaning the gases coming off a basic oxygen furnace failed. The system consisted of six precipitators, three separate dual units, each composed of four zones. Each zone contained rows of wires (cold drawn AISI 1008 carbon steel) suspended between parallel collector plates. It was determined that the 1008 wires failed because of corrosion fatigue. It was decided to replace all of the wires in the two zones with the highest rates of failure with cold-drawn type 304 austenitic stainless steel wire. These expensive wires, however, failed after a week by transgranular SCC. Annealed type 430 ferritic stainless steel was subsequently suggested to prevent further failures.

The support bearing of a hydrofoil vessel failed after only 220 h of operation. The bearing consisted of an outer ring made of chromium-plated AISI type 416 stainless steel and an inner ring with a spherical outer surface made of AISI type 440C stainless steel, with a plastic material, bonded to the outer ring, between the two. The inner ring was found to have failed in four places. The two metallic rings were allowed to come in contact with each other by the disappearance of the plastic material. It was revealed by examination of the fracture surfaces of the inner ring that the failure was caused by fatigue initiated in corrosion pits (caused by seawater). The fracture was found to be transgranular. It was recommended that the inner and outer rings should both be made from the more corrosion resistant 17-4 PH (AISI type 630) stainless steel.

During a refit of a twenty-year-old Naval destroyer, two cracks were found on the inside of the killed carbon-manganese steel hull plate at the forward end of the boiler room. The cracks coincided with the location of the top and bottom plates of the bilge keel. Metallurgical examination of sections cut from the cracked area identified lamellar tearing as the principle cause of the cracking. This was surprising in 6 mm thick hull plates. Corrosion fatigue and general corrosion also contributed to hull plate perforation. Although it is probable that more lamellar tears exist near the bilge keel in other ships and may be a nuisance in the future, the hull integrity of the ships is not threatened and major repairs are not needed.

This paper describes the metallurgical investigation of a broken spindle used to attach an antenna to the mast of a naval vessel. Visual inspections of both failed and intact fastener assemblies were carried out both on-board ship and in the laboratory followed by metallographic and fractographic examinations. Simulations were also performed on stressed material in a suitable environment to assess the relative importance of postulated failure mechanisms. Factors contributing to this failure including assembly procedures and applied preloads, service loading and environment, and material selection and specification. The discussion considers whether this failure was an isolated incident or is likely to be a fleet-wide problem, and suggests ways to prevent reoccurrence.

A failed 25 x 32 mm (1 x 1 in.) cadmium-plated 1040 carbon steel countersunk head type nose gear door securing bolt with a common screwdriver slot was examined. Fracture originated at a thread root and propagated across the cross section. The topography of the fracture was excessively rough and more granular than would be expected from pure mechanical fatigue. This indicated an allied corrosion mechanism. Cracks other than the one leading to failure were observed. Metallographic examination of the bolt cross section showed many cracks typical of stress-corrosion damage. It was concluded that the bolt failed by a combination of SCC and fatigue. It was recommended that aerospace-quality fasteners meeting NAS 7104, NAS 7204, or NAS 7504 be used to replace the currently used fasteners.

A hollow, splined alloy steel aircraft shaft (machined from an AMS 6415 steel forging – approximately the same composition as 4340 steel – then quenched and tempered to a hardness of 44.5 to 49 HRC) cracked in service after more than 10,000 h of flight time. The inner surface of the hollow shaft was exposed to hydraulic oil at temperatures of 0 to 80 deg C (30 to 180 deg F). Analysis (visual inspection, 15-30x low magnification examination, 4x light fractograph, chemical analysis, hardness testing) supported the conclusions that the shaft cracked in a region subjected to severe static radial, cyclic torsional, and cyclic bending loads. Cracking originated at corrosion pits on the smoothly finished surface and propagated as multiple small corrosion-fatigue cracks from separate nuclei. The originally noncorrosive environment (hydraulic oil) became corrosive in service because of the introduction of water into the oil. Recommendations included taking additional precautions in operation and maintenance to prevent the use of oil containing any water through filling spouts or air vents. Also, polishing to remove pitting corrosion (but staying within specified dimensional tolerances) was recommended as a standard maintenance procedure for shafts with long service lives.