The bottom flange of a vertical pipe coupled to an isolating valve in a steam supply line to a turbine failed. Steam pressure was 1,500 psi and the temperature 416 deg C (780 deg F). Multiple cracking occurred in the bore of the flange. A quarter-segment was cut out and examined. The cracks were located in the part of the flange that formed a continuation of the pipe bore. The majority of them originated at the end of the flange bore and extended axially along the pipe and radially across the flange face. Magnetic crack detection revealed a further number of cracks in the weld deposit. While the fracture in the weld metal was of the ductile type exhibiting a fine fibrous appearance, that in the flange material was of the cleavage type. Microscopic examination revealed that the cracks were blunt-ended fissures of the type characteristic of corrosion-fatigue. It was concluded that cracking was due to corrosion-fatigue, which arose from the combined effect of a fluctuating tensile stress in the presence of a mildly corrosive environment.

During the inspection of a boiler containing cracks at the superheater header connection, cracking also was detected within the main steam drum. This was fabricated from a Mn-Mo-V low-alloy steel. It operated with water and saturated steam at approximately 335 deg C. Cracking was detected at the nozzles connecting the tubes for the entry of steam and hot water to the drum, at the downcomers, and at the connection to the safety valve. All cracks had a similar morphology, running in a longitudinal direction along the drum from the cutouts in the shell. All the cracks had developed under the influence of the hoop stress and were associated with the locally increased stress levels relating to the cutouts at nozzle and pipe connections. At their ends the cracks were filled with corrosion products, and their surfaces were seen to be very irregular. The process of crack growth was not due to fatigue only but can most probably be attributed to corrosion fatigue. The boiler steam drum design should be reviewed to reduce the local level of stress at the shell-nozzle connections.

A nuclear steam-generator vessel constructed of 100-mm thick SA302, grade B, steel was found to have a small leak. The leak originated in the circumferential closure weld joining the transition cone to the upper shell. The welds had been fabricated from the outside by the submerged arc process with a backing strip. The backing was back gouged off, and the weld was completed from the inside with E8018-C3 electrodes by the shielded metal arc process. Striations of the type normally associated with progressive or fatigue-type failures including beach marks that allowed tracing the origin of the fracture to the pits on the inner surface of the vessel were revealed. Copper deposits with zinc were revealed by EDS examination of discolorations. Pitting was revealed to have been caused by poor oxygen control in the steam generators and release of chloride into the steam generators. It was concluded by series of controlled crack-propagation-rate stress-corrosion tests that A302, grade B, steel was susceptible to transgranular stress-corrosion attack in constant extension rate testing with as low as 1 ppm chloride present. It was recommended to maintain the coolant environment low in oxygen and chloride. Copper ions in solution should be eliminated or minimized.

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.

The results of a failure analysis of a series of Cr-Mo-V steel turbine studs which had experienced a service lifetime of some 50,000 h are described. It was observed that certain studs suffered complete fracture while others showed significant defects located at the first stress bearing thread. Crack extension was the result of marked creep embrittlement and reverse temper embrittlement (RTE). Selected approaches were examined to assess the effects of RTE on the material toughness of selected studs. It was observed that Auger electron microscopy results which indicated the extent of grain boundary phosphorus segregation exhibited a good relationship with ambient temperature Charpy data. The electrochemical polarization kinetic reactivation, EPR, approach, however, proved disappointing in that the overlapping scatter in the minimum current density, Ir, for an embrittled and a non-embrittled material was such that no clear decision of the toughness properties was possible by this approach. The initial results obtained from small punch testing showed good agreement with other reported data and could be related to the FATT. Indeed, this small punch test, combined with a miniature sample sampling method, represents an attractive approach to the toughness assessment of critical power plant components.

A crack was discovered in a cast steel (ASTM A 356, grade 6) steam turbine casing during normal overhaul of the turbine. The mechanical properties of the casting all exceeded the requirements of the specification. When the fracture surface was examined visually, an internal-porosity defect was observed adjoining a tapped hole. A second, much larger cavity was also detected. Investigation (visual inspection and 7500x SEM fractographs) supported the conclusions that failure occurred through a zone of structural weakness that was caused by internal casting defects and a tapped hole. The combination of cyclic loading (thermal fatigue), an aggressive service environment (steam), and internal defects resulted in gradual crack propagation, which was, at times, intergranular-with or without corrosive attack-and, at other times, was transgranular.

A superheater in a generator produced 80 t/h of steam at 400 deg C and 41 kPa. Failure took place at the connection from the collector to the vent line used during start up. The material of construction was carbon steel, and the unit had 240,000 h of operation at the time of failure, with 99 shutdowns. Widespread cracking on the inside was apparent, the most severe cracking being some distance from the nozzle connection in a downstream direction. Widespread cracking and pitting were observed also at the connections to the safety valve and soot blower. Pitting was most apparent on the downstream sides of the openings in the shell. In all the damaged areas the mechanism of failure involved surface pitting and subsequent SCC. This failure showed the problems that can develop where there are long lines in which condensation may occur and return periodically to a superheater or other hot component. In this particular case, control of dissolved solids in the boiler feedwater may have been inadequate.

A desuperheater diffuser nozzle in the steam supply line failed within nine months of service in an 8.25 MN/sq m (1200 psig) steam line. The nozzle was an austenitic stainless steel casting in conformance to material. The nozzle had numerous cracks on the inside and outside surfaces, and the cracks had penetrated through the wall thickness in several areas. The fracture surfaces had distinct beach markings delineating the crack front, representative of crack propagation stages. The cracks were transgranular and, unlike classical corrosion-fatigue cracks, exhibited branching, characteristic of chloride-induced SCC in austenitic stainless steels. The failure resulted from chloride-induced SCC, possibly assisted by cyclic stress. The recommendation for alternate material for the desuperheater nozzle included nickel base alloys per ASTM B 564, Grades 600 or 800 titanium alloy per ASTM B 367, Grades C3/C4, or ferritic stainless steel alloy per ASTM 182, Grade FXM27.

Rupture occurred at a bend in a superheated steam transfer line between a header and a desuperheater of a boiler producing 230 t/h of steam at 540 deg C and 118 kPa. The boiler had operated for 77,000 h. Rupture occurred along the outer bend radius of the 168 mm diam tube, this being of 1 Cr, 0.5 Mo steel with a wall thickness of 14 mm. The design temperature of this tube was 490 deg C, but there is evidence that it was operating at a temperature much above 500 deg C. Metallographic analysis disclosed an advanced stage of creep damage accumulation in the form of local cracks, microcracks, and aligned damage centers which showed up as voids upon repeated polish-etch cycles. Because of the local nature of creep damage that can occur, any inspection that involves in situ metallography must be conducted at exactly the right or critical position or the presence of damage may not be detected.

Several articulated bellows of 10 in. ID developed leakage from the convolutions after a service life of some 18 months. One of the units received from examination showed cracking at the crown of a convolution and at the attachment weld to the pipe. Sectioning of the bellows revealed many others cracks on the internal surface which did not penetrate to the outside. Microscopical examination showed multiple intergranular, tree-like cracking typical of stress-corrosion cracking. Concentration of sodium hydroxide occurred in the bellows unit and the stress-corrosion cracking which developed was of the form known as caustic cracking. It was recommended that water for de-superheater use should be taken after the deaerator and prior to the addition of salts which may deposit or concentrate in the desuperheater.

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 75 cm OD x 33 mm thick pipe in a horizontal section of a hot steam reheat line ruptured after 15 years in service. The failed section was manufactured from rolled plate of material specification SA387, grade C. The longitudinal seam weld was a double butt-weld that was V-welded from both sides and failure was found to propagate along the longitudinal seam and its HAZ. The fracture surface near the inner wall of the pipe was found to have a bluish gray appearance, while the fracture surface near the outer wall was rust colored (oxides). The transverse-to-the-weld specimen from the longitudinal seam weld was revealed to have lower elongation and a shear type failure rather than the cup-cone failures. It was concluded that the welded longitudinal seam exhibited embrittlement. A low-ductility intergranular fracture that progressed through the weld metal was revealed by scanning electron microscopy. The cracks were revealed to be in existence for some time before the final failure which was indicated by the extent and amount of corrosion products. It was concluded that low ductility was responsible for the original initiation of cracks in the pipe.

The intermediate pressure (IP) turbine of a thermal generating station is driven by steam from the boiler's reheater. On one particular IP turbine, a thick deposit was found on the insides of the rotor blade shrouds in two instances two years apart. The source of the deposits was not known; bulk chemical analysis had simply shown that iron was a major component. Optical microscopy and electron microprobe analysis were used to identify the deposits. In the first instance, the deposit was found to be debris that was left in the reheater tubes during boiler modification and swept to the turbine by the steam. There were still some of these debris particles present when the incident two years later was investigated but generally the second deposit was found to be of two layer oxide particles which were shown to have spalled from 2-14% chromium reheater tube surfaces.

Dezincification is a particular form of corrosive attack which may occur in a variety of environments and to which some brasses are susceptible. It is favored by waters having a high oxygen, carbon dioxide, or chloride content, and is accelerated by elevated temperatures and low water velocities. In the present study, steam turbine condenser tubes had to be renewed after 25 years of service. The tubes were nominally of 70:30 brass. The appearance of a typically corroded one showed uniform dezincification attack on the bore, extending from one-half to two-thirds through the tube wall thickness.

A rupture of a thirty-year-old U-tube on a steam generator for a closed-cycle pressurized-water nuclear power plant occurred, resulting in limited release of reactor water. A typical tube bundle can be over 9 m (30 ft) tall and 3 m (10 ft) in diam with over 3,000 22-mm (7/8-in.) diam Inconel Alloy 600 tubes. Tube support plates (TSP) separate the tubes and allow flow of the heating water/steam. Inconel Alloy 600 is susceptible to intergranular stress-corrosion cracking over time, so investigation included review of operational records, maintenance history, and procedures. It also included FEA (thermal gradients, nonlinear material behavior, residual stress, changes in wall thickness during the formation of U-bends, and TSP distortions near the ruptured tube) of three-dimensional solid models of the U-tubes. The conclusion was that distortion of the TSPs and resulting “pinching” of the U-tubes, combined with the operational stresses, caused high stresses at the location where the tube cracked. The stresses were consistent with those required to initiate and propagate a longitudinal crack.

When a steam turbine was put out of service, cracks were noticed on many of the blades in the low pressure section round the stabilization bolts and perpendicular to the blade axis. The blades were made from chrome alloy steel X20-Cr13 (Material No. 1.402). When the bolts were brazed into the blades inadmissible localized overheating of the steel must have occurred, which resulted in transformation stresses and hence reduced deformability. The cracks arose as a consequence of careless brazing. Whether the cracks should be considered as stress cracks over their entire extent or partially as fatigue cracks produced by vibration in the operation of the turbine as a result of steplike growing of microcracks could not be deduced from the fracture surfaces. Microfractography showed that the cracks developed in stages.

In a main range in a power station, steam was conveyed at a pressure of 645 psi, and a temperature of 454 deg C (850 deg F). Pipe diameter was 9 in. and the joints were of the bolted type in which a thin steel ring, serrated on both sides, was inserted between plain flanges. Thin jointing material was interposed between the serrated faces and the flanges. The first intimation of trouble was the onset of a high pitched noise audible over a radius of a quarter of a mile. The noise arose from violent lateral vibration of the serrated ring, which attained an amplitude and persisted for a sufficient number of cycles to produce an extensive system of fatigue cracks that resulted in partial disintegration of the ring. Microscopic examination of the material showed it to be a mild steel of satisfactory quality. The trouble was started by slight leakage, possibly resulting from a relaxation of the interfacial pressure on the joint faces, which eroded away the joint material locally at one face of the serrated ring. This reduced interfacial pressure at the opposite face of the ring, with resultant leakage and erosion of the joint material on this side.

Fusing of the switch contacts of a boiler feed pump drive motor led to the failure of a turbine. After rubbing of most of the Ni-Cr steel LP wheels had occurred, due to the admission of water carried over with the steam, a copper-rich alloy from the interstage gland rings melted, penetrated the wheel material, and gave rise to radial and circumferential cracking in four of the LP wheels. It was concluded that when the rotor moved axially and the wheels came into contact with the diaphragms there was a tendency for the former to dish, with the development of both radial and circumferential tensile stresses on the side in contact with the adjacent diaphragm. In the presence of the molten copper-rich alloy, these stresses gave rise to severe hot cracking.

During a hydraulic test on one of the boilers in a range, leakage occurred from the lower surface of a horizontal S bend in the main steam pipe between the drum connector box and the junction valve. The pipe in question was 15 in. bore and had been in service for about 50 years. Specimens were prepared for microscopical examination to include the defective zone and a section through a circumferential crack. The defective zone was found to contain numerous inclusions of slag and oxides of globular form. Regions surrounding the inclusions were decarburized, the indications being that this region of the plate had been heated to an excessively high temperature. A corrosion-fatigue fissure was at one location, this having originated at the internal surface of the pipe and run into an inclusion in the defective zone. The failure resulted from the development of corrosion-fatigue fissures which originated at a zone containing defects introduced at the time of manufacture. These may have had their origin in the ingot from which the plate was rolled or, alternatively, be indicative of a zone which suffered overheating and local burning at the time the forge weld was made.

A spindle made of hardenable 13% chromium steel X40 Cr13 (Material No. 1.4034) that was fastened to a superheated steam push rod made of high temperature structural steel 13Cr-Mo44 (Material No. 1.7335) by means of a convex fillet weld, fractured at the first operation of the rod directly next to the weld bead. Investigation showed that the fracture of the superheated steam push rod spindle was caused by hardening and hardening crack formation in the weld seams and adjoining areas. It would have been preferable to avoid welding near the cross sectional transitions altogether in consideration of the crack sensitivity of high hardenability steels. If for some reason this was not possible, then all precautions should have been taken that are applicable to the particular steel, such as preheating, slow cooling and stress relief tempering after welding. The selection of an austenitic additive material should have been considered because it could have equalized stresses due to its high elongation. Most probably, however, a material of lower hardenability should have been selected for the spindle if high operating properties were of paramount importance.