Numerous cracks observed on the surface of a forged A470 Class 4 alloy steel steam turbine rotor disc from an air compressor in a nitric acid plant were found to be the result of caustic induced stress-corrosion cracking (SCC). No material defects or anomalies were observed in the disc sample that could have contributed to crack initiation or propagation or secondary crack propagation. Chlorides detected in the fracture surface deposits were likely the primary cause for the pitting observed on the disc surfaces and within the turbine blade attachment area. It was recommended that the potential for water carryover or feedwater induction into the turbine be addressed via an engineering evaluation of the plant's water treatment procedures, steam separation equipment, and start-up procedures.

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.

Within the first few months of operation of an 8 km (5 mile) long 455 mm (18 in.) diam high-pressure steam line between a coal-fired electricity-generating plant and a paper mill, several of the Inconel 600 bellows failed. The steam line operated at 6030 kPa (875 psi) and 420 deg C (790 deg F). Metallographic sections, energy-dispersive x-ray spectra, chemical analyses, tensile tests, and Auger microscope analyses showed the failed bellows met the specifications for the material. However, investigation also showed entire oxide thickness was contaminated with relatively large amounts of sodium, calcium, potassium, aluminum, and sulfur, alkali, alkali earth, and other contaminants that completely permeated even the thin oxides on the fracture surfaces. Additional investigation of the purity of the steam itself as reported by the power plant showed that corrosion and cracks were ultimately caused by the steam. While under normal operation, the steam's purity posed no problem to the material, during boiler cleaning operations, the generating plant had allowed contamination to get into the steam line.

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.

Gross wastage and embrittlement were observed in plain carbon steel desuperheaters in five new Naval power plants. The gross wastage could be duplicated in laboratory bomb tests using sodium hydroxide solutions and was concluded to be caused by free caustic concentrated by high heat flux. The embrittlement was shown to be caused by the flow of corrosion generated hydrogen which converted the cementite to methane which nucleated voids in the steel. A thermodynamic estimate indicated that a small amount of chromium would stabilize the carbides against decomposition by hydrogen in this temperature range, and laboratory tests with 2-14% Cr steel verified this.

An explosion occurred in a portion of a horizontal, U-shaped expansion loop in a steam main approximately 10-in. diam which had been operating at 400 psi for six years. Steam conditions varied from 538 deg C (450 deg F) saturated to 343 deg C (650 deg F) superheated. Fracture occurred longitudinally through the upper wall over a length of approximately 68 in. The sample received for examination was ultrasonically tested, which indicated a band of internal defects extending 1 in. in from the edge. Subsequently, the portion of the pipe embodying the other side of the rupture was obtained for examination. Transverse sections through this and the mating portion already received, followed by magnetic crack detection, revealed the presence of defective zones. Subsequent ultrasonic examination of other sections of the steam main indicated suspect areas in a number of lengths of pipe. These defects were basically laminations of a similar form to those which resulted in the failure of the portion of pipe.

A main steam pipe was found to be leaking due to a large circumferential crack in a pipe-to-fitting weld in one of two steam leads between the superheater outlet nozzles and the turbine stop valves (a line made of SA335-P22 material). The main crack surface was found to be rough, oriented about normal to the outside surface, and had a dark oxidized appearance. The cracking was found to be predominantly intergranular. Distinct shiny bands that etched slower than the remainder of the sample at the top of each individual weld bead were revealed by microscopic examination. These bands were found contain small cracks and microvoids. A mechanism of intergranular creep rupture at elevated temperature was identified as a result of a series of stress-rupture and tensile tests. It was revealed by the crack shape that cracking initiated on the pipe exterior, then propagated inward and in the circumferential direction in response to a bending moment load. It was concluded that the primary cause of failure was the occurrence of bending stresses that exceeded the stress levels predicted by design calculations and that were higher than the maximum allowable primary membrane stress.

A heat exchanger made of a pipe in which oil was heated from the outside from approximately 90 deg C to 170 deg C, by superheated steam of about 8 to 10 atmospheres had developed a leak at the rolled joint of the pipe and pipe bottom. The pipes were supposed to be made from St 35.29 steel and annealed at the rolled joint to 100 mm length. The outer pipe surface was strongly pitted by corrosion all around the rolled joint. In the vicinity of the steam chamber the pipe wall had oxidized through from the exterior to the interior at one spot. Adjoining this spot, grooves caused by erosion were noticeable. This was a typical case of crevice corrosion. The rolled joint evidently was not entirely tight, so that saturated steam condensate could penetrate into the gap.

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.

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.

During a planned shut-down in 1990 it appeared that the bottom manifold parts made of wrought Incoloy 800H had undergone diametrical expansion of up to 2% due to creep. Further, cracking at the outer diam was found. It was decided to replace these parts. Microscopical investigations showed that the cracking could not be caused by creep. It was found that the cracking was confined to a 4-mm deep coarse-grained zone (ASTM 0-1) at the outer diameter. The cracking appeared to be caused by strain-induced intergranular oxidation. When the cracks reached the fine-grained material, the oxidation-cracks stopped. To determine the residual creep life of the sound (non-cracked) bottom manifold material, iso-stress creep tests were performed. It was found that tertiary creep started at 7% strain. The time-to-rupture was greater than 100,000 h. It was concluded that the bottom manifold (and thus the furnace) could be used safely during the foreseen production period.

Microstructural examinations on transverse cross sections of a steam reformer tube, showed the presence of large macrovoids elongated in the radial direction and emanating from the internal surface of the tube. The macrovoids were located at the interdendritic regions, and were partially filled by a Mn-Fe bearing chromium oxide film. The areas adjacent to the oxide film were chemically depleted in C, Cr and Mn and rich in Fe and Ni. Associated with this depletion were a large concentration of microvoids. It was suggested that the dissolution of carbides in areas surrounding the macrovoids and the concentration of stresses at their tips, caused extensive localized plastic deformation which led to the formation of microvoids and subsequently to the spalling of the oxide film. The non-protective character of the film induced a progressive deterioration of the grain boundaries properties. Grain boundary sliding and dislocation motion were enhanced, causing a local increase in the steady state strain rate and the premature failure of the tube.

The steam tug Hercules was an ocean-going and bay tug for 55 years before being retired. It is now being restored by the National Park Service. A broken steam valve was obtained for microstructural examination. The body was gray cast iron, and the stem and seat were brass. The examination centered on corrosion of the brass components. The seat and shaft were alpha brass, with a hardness of 64 and 79 DPH, respectively. A nut held the shaft onto the seat, and was alpha-beta brass with a hardness of 197 DPH. Welded on the end of the shaft was a ring of hard (DPH 294) alpha-beta brass, which seated against the nut. The brass seat and stem show little corrosion. However, the alpha-beta brass nut and welded tip showed extensive dezincification. This process of removal of Zn and the retention of Cu began in the high Zn beta phase, but eventually both phases were attacked. The depth of penetration was consistent with dezincification rates reported in the literature for such brasses in salt water if the valve had been in service about 55 years.

An investigation of wear and failure of babbitt bushes was completed in this study. The results showed that wear at dry sliding of babbitt obtained by plasma spraying was less than that of babbitt in the as-cast state and after a deformation heat treatment. The failure of babbitt bushes was caused by a simultaneous and interrelated exhibition of fatigue and wear processes that depend considerably on cohesion strength between the bush and the bearing base and accumulation of defects on the contact surface between the bush and the shaft.

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.