Failures in boilers and other equipment taking place in power plants that use steam as the working fluid are discussed in this article. The discussion is mainly concerned with failures in Rankine cycle systems that use fossil fuels as the primary heat source. The general procedure and techniques followed in failure investigation of boilers and related equipment are discussed. The article is framed with an objective to provide systematic information on various damage mechanisms leading to the failure of boiler tubes, headers, and drums, supplemented by representative case studies for a greater understanding of the respective damage mechanism.

The failure of a boiler operating at 540 °C and 9.4 MPa was investigated by examining material samples from the near-failure region and by thermodynamic analysis. A scanning Auger microprobe, SEM, and commercial thermodynamic software codes were used in the investigation. Results indicated that the boiler failure was caused by grain-boundary segregation of phosphorous, tin, and nitrogen and the in-service formation of carbide films and granules on the grain boundaries.

On attempting to manipulate or bend a boiler tube some 22 ft. long, sudden failure occurred at what appeared to be a butt weld in the tube. Externally, the weld reinforcement had been ground flush and the entire tube surface painted. Internally, the appearance and width of the heated band suggested that the weld had been made by the oxy-gas process. A lack of root fusion over most of its length was evident. Examination of the fracture faces, which were of crystalline appearance indicative of brittle behavior, indicated incomplete fusion of the weld root. Microscopic examination showed the deposit to possess a large grain size with a low carbon content disposed as carbides along the grain boundaries, a feature which would provide an explanation of the brittle behavior. Subsequent inspection showed that this tube was one of several of the batch ordered for retubing of a boiler and which had a 2 ft. length welded to one end to make up the length.

A seamless hot-drawn boiler tube NW 300 of 318 mm OD and 9 mm wall thickness made of steel 15Mo3 was bent with sand filling after preheating allegedly to 1000 deg C. In the process it had cracked repeatedly in the drawn fiber. The composition corresponded to specifications, but exceptionally high copper content was noticeable. Microstructural examination showed the damage was due to overheating and burning during preheating and bending. Furthermore, crack formation was promoted by precipitation of metallic copper that had penetrated into the austenitic grain boundaries under the influence of tensile stresses that arose during bending. This phenomenon is known as “solder brittleness.”

A weld that attached the head to the shell of a preheater containing steam at 1.4 MPa (200 psi) and was used in the manufacture of paper cracked in service. The original joint contained a 6.4 by 50 mm backing ring that had been tack welded to the inside surface of the shell in a position to project beyond the fully beveled top edge of the shell. The projecting edge of the ring fitted against a deep undercut on the inner corner of the rim of the head. The internal 90-deg angle in this undercut was sharp, with almost no fillet. A bevel from the lower edge of the undercut to the outside of the head completed the groove for the circumferential attachment weld. Investigation (visual inspection and actual size views etched in hot 50% hydrochloric acid) supported the conclusion that cracking occurred in the HAZ in the head of the original design, originating in the sharp corner of the undercut, which was an inherent stress raiser. Recommendations included revised joint design to ensure full root penetration.

After ten years of satisfactory operation, economizer-tube failures occurred in a large black liquor recovery boiler for a paper mill. The economizer contained 1320 finned tubes. Two fins ran longitudinally for most of the tube length and were attached by fillet welding on one side. The economizer tube leaks occurred at the end of the fin near the bottom of the economizer. A sample from a tube that had not failed showed heavy pitting attack on the inside of the tube, probably due to excess oxygen in the feedwater. Penetrant testing revealed numerous longitudinal cracks on the inside in the area of the fin tip. Cracking at the end of the fin-to-tube fillet weld was noted. The results indicate the failures were due to corrosion fatigue whose stresses were primarily thermally induced. A temporary solution included inspecting all tubes with shear-wave ultrasonics. Tubes with the most severe cracking were ground and repair welded. The square corners of the fins were trimmed back with a gradual taper so that expansion strains would be more gradually transferred to the tube surface. Water chemistry was closely evaluated and monitored, especially with regard to oxygen content.

Three examples of corrosion-fatigue cracking from the toes of substantial fillet welds applied to seal-leaking riveted seams in steam accumulators are described. In the first case, this practice resulted in a disastrous explosion; in the second, which involved two identical vessels at the same location, cracking in course of development was discovered during internal inspection. Microscope examination of several specimens cut to intersect a crack showed it to be typical of corrosion-fatigue; it was in the form of a broad fissure, contained oxide deposits, and the termination was blunt-ended. The two cases not only serve to illustrate the danger of applying fillet welds to seal the lap edges of riveted seams, but point to the inadvisability of employing riveted construction for vessels intended for service under conditions involving frequent pressure and thermal fluctuations, as it is extremely difficult to maintain the tightness of riveted seams under these conditions. Such vessels are now almost exclusively of all-welded construction

A water tube boiler with two headers and 15.5 atm working pressure became leaky in the lower part due to the formation of cracks in the rivet-hole edges. The boiler plate of 20 mm thickness was a rimming steel with 0.05% C, traces of Si, 0.38% Mn, 0.027% P, 0.035% S, and 0.08% Cu. The mean value of the yield point was 24 (24) kg/sq mm, the tensile strength 39 (38) kg/sq mm, the elongation at fracture, d10, 26 (24)%, the necking at fracture 71 (66)% and notch impact value 11.5 (9.4) kgm/sq cm (the values in brackets are for the transverse direction). The specimen from inside surface of the boiler was polished and etched with Fry-solution, which revealed parallel striations formed due to the cold bending of the plate. The zones of slip were concentrated around the rivet holes. The cracks were formed here. The structure examination proved that the cracks had taken an exactly intercrystalline path, which is characteristic for caustic corrosion cracks. It was recommended that the internal stresses be removed through annealing or alternatively lye-resistant steel should be used.

Following leakage which developed within the furnace of a horizontal multi-tubular type boiler, examination revealed a series of cracks adjacent to the stiffening rings in the first plain furnace ring. The fire-side surface of the sample was coated with a layer of oxide scale. Microscopical examination of sections through the cracks showed them to be filled with oxide and to be of the multi-branched type, having blunt terminations. The general nature of the cracks was characteristic of cracking from thermal or corrosion fatigue, as results from the operation of varying stresses in an oxidizing or corrosive environment. The cracking in this particular case was due principally to the inordinately large gap between the components. Additionally, several of the sealing welds of the tubes to the back tube plate were cracked in a radial manner, and it would appear that in addition, abnormal thermal conditions may well have been experienced intermittently in service.

A locomotive type boiler was fitted with a copper firebox of orthodox construction. Flanged tube- and firehole-plates were attached to a wrapper plate by means of copper rivets. Shortly after it was put into service the fireside heads of a number of rivets broke off at different parts of the seams. By the time the investigation was begun a total of fifty heads had broken off. Repairs had been effected from time to time by fitting screwed rivets, none of which gave trouble in service. Microscopic examination confirmed the fracture path to be wholly intergranular. In the region of the fracture the grain boundaries were delineated as a near-continuous network of cavities and films of oxide. It was evident that the failure of the rivets in service was attributable to intergranular weakness in the material due to gassing.

The cover of a feed cock fitted to an economic boiler suddenly blew off and the plug lifted sufficiently to permit the escape of water. It was found that all four of the studs securing the cover had fractured. In each case fracture had occurred at the end of one of the screwed portions adjacent to the shank. The fractures were of a short nature, with no evidence of progressive cracking by fatigue, nor was there any sign of stretching prior to failure. The fractured faces and the shanks of the studs were of rusted appearance. Microscopic examination of the material showed it to be an austenitic nickel-chromium stainless steel, stabilized by titanium and of the free machining type. Multiple transgranular cracking characteristic of failure from stress-corrosion cracking, was present to an extensive degree. It was considered probable that there had been slight leakage of water from the valve over a period and evaporation resulted in a solution which favored failure from stress-corrosion cracking. If corrosion resistant studs were desired, those of bronze or Monel metal are to be preferred.

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.

A polyoxymethylene gear wheel that had been in operation in a boiler room failed. Investigation (visual inspection and 305x images) supported the conclusion that failure was due to postcrystallization causing considerable shrinkage. Breakdown along the crystalline superstructure started mainly at the mechanically stressed tooth flanks. In addition, oil vapors, humidity, and other degradative agents could also have contributed to the observed failure. No recommendations were made.

During microscopic examination of a number of cases of caustic cracking, a certain feature has been recognized that appeared to be associated only with caustic cracking. This was a preferential attack on the carbide envelopes and lamellae of the pearlite grains. Evidence suggests that the intergranular path of caustic cracks in steam boilers may be due largely to the presence, probably on a submicroscopic scale, of carbides at the grain boundaries, thus rendering these regions susceptible to preferential attack. It is known that some steels are more liable to develop caustic cracks than others, although their microstructures may not show any significant differences, and it seems probable that this behavior may be related to the amount and continuity of the grain-boundary carbides.

Carbon steel tubes from a boiler feedwater heater feeding a deaerator were treated to control scale formation, but the treatment instead produced more iron oxide. The additional iron oxide reduced the tubing to a totally corroded condition. Investigation showed that the chelate injected to control the scaling was added ahead of the preheater, where the boiler water still contained oxygen. As the chelate removed iron oxide, the O2 in the water continued to form more. Recommendations included moving the chelate addition to a point after the deaerator to stop the corrosion.

Tubes in a marine boiler on a new ship failed after brief service lives. Circumferential brittle cracking was found to occur in the carbon-molybdenum steel tubes near the points where the tubes were attached to the steam drum. Fatigue striations were revealed by examination of fracture surfaces by electron microscopy at high magnification. Fatigue failures were concluded to be caused by vibrations resulting from normal steam flow at high steam demand. Too rigid support near the steam drum resulted in concentration of vibratory strain in the regions of failure. The method of supporting the tubes was changed to reduce the amount of restraint and the strain concentration.

The maximum life of base-loaded headers and piping is not possible to be predicted until they develop microcracking. The typical elements of a periodic inspection program after the occurrence of the crack was described extensively. Cracks caused by creep swelling in the stub-to-header welds in the secondary superheater outlet headers (constructed of SA335-P11 material) of a major boiler were described as an example. The OD of the header was measured to detect the amount of swelling and found to have increased 1.6% since its installation. Ligament cracks extending from tube seat to tube seat were revealed by surface inspection. Cracks were found to originate from inside the header, extend axially in the tube penetrations and radially from those holes into the ligaments. Cracks in 94 locations, ranging from small radial cracks to full 360Ý cracks were revealed by dye-penetrant inspection. The unit was operated under reduced-temperature conditions and with less load cycling than previously until a redesigned SA335-P22 header was installed.

Cracks on the outer surface near a hanger lug were revealed by visual inspection of a type 316 stainless steel main steam line of a major utility boiler system. Cracking was found to have initiated at the outside of the pipe wall or immediately beneath the surface. The microstructure of the failed pipe was found to consist of a matrix precipitate array (M23C6) and large s-phase particles in the grain boundaries. A portable grinding tool was used to prepare the surface and followed by swab etching. All material upstream of the boiler stop valve was revealed to have oriented the cracking normally or nearly so to the main hoop stress direction. Residual-stress measurements were made using a hole-drilling technique and strain gage rosettes. Large tensile axial residual stresses were measured at nearly every location investigated with a large residual hoop stress was found for locations before the stop valve. It was concluded using thermal stress analysis done using numerical methods and software identified as CREPLACYL that one or more severe thermal downshocks might cause the damage pattern that was found. The root cause of the failure was identified to be thermal fatigue, with associated creep relaxation.

A 75 mm OD x 7.4 mm wall thickness carbon steel boiler tube ruptured. A substantial degree of corrosion on the water-side surface leaving a rough area in the immediate vicinity of the rupture was revealed by visual examination. Decarburization and extensive discontinuous intergranular cracking was revealed by microscopic examination of a cross section through the tube wall at the fracture. It was concluded that the rupture occurred because of hydrogen damage involving the formation of methane by the reaction of dissolved hydrogen with carbon in the steel. Hydrogen was produced by the chemical reaction that corroded the internal tube surface. Steel embrittled by hydrogen can be restored only if grain boundary cracking or decarburization had not occurred but since the material embrittled in this manner, its replacement was recommended.