One tube in a watertube boiler developed leakage from a perforation. The external surface was covered with a dark deposit indicative of local fusion. Perforation resulted from the development of a crack from the internal surface. Microscopic examination revealed extensive intergranular penetration by molten copper. Particles of copper were seen in scale deposits on the bore of the tube. The tube in general showed a ferritic structure with partially spheroidized carbide. The fact that fusion of the copper had occurred indicated temperatures of 1100 deg C (2012 deg F) had been experienced locally, and the structural condition suggested that the tube in general had been heated at a lower temperature of the order of 600 deg C (1112 deg F) for some appreciable time. In this instance, overheating of the tube in the absence of the copper deposits may not have led to failure.

A kiln, 7.6 m (25 ft) long with a 1 m (3 ft) internal diameter and a 6.3 mm (0.25 in.) wall thickness, is used to regenerate spent charcoal returned by water utilities. This charcoal contains up to 0.57% S and 2.04% Cl. The kiln is made of Inconel 601 (N06601) welded using Inconel 617 (N06617) as a filler alloy. Wet charcoal is fed in at one end of the kiln and travels while being tumbled within the inclined rotating vessel. Temperatures range from 480 deg C (900 deg F) (Zone 1) to 900 deg C (1650 deg F) (Zones 2 and 3). Steam is introduced at the discharge end at 95 g/s (750 lb/h), 34 to 69 kPa (5 to 10 psi), and 125 deg C (260 deg F). The kiln developed perforations within eight months of operation. Investigation (visual inspection, metallurgical analysis, energy-dispersive spectroscopy, and 44X micrographs) supported the conclusion that the sulfur and chlorine in the charcoal attacked the Inconel 601, forming various sulfides and chlorides. Recommendations included on-site testing, and installation of test coupons of various alloys before fabricating another kiln. The suggested alloys were RA85H, 800HT, HR-120, Haynes 556, and HR-160.

The curved parts of exit pigtails made of wrought Incoloy 800H tubing used in steam reforming furnaces failed by performance after a period of service shorter than that predicted by the designers. Examination of a set of tubes consisting of both curved (perforated) and straight parts revealed that the cracks initiated at the outer surface by a combined mechanism of creep and intergranular embrittlement. A smaller grain size resulting from cold bending fabrication procedures for the curved parts was responsible for accelerating the embrittlement. It was recommended that hot bending be used for fabrication of the curved parts. A change of alloy to a low-alloy chromium-molybdenum allay to protect against heat was also suggested.

A resistance-welded carbon steel superheater tube made to ASME SA-276 specifications failed by pitting corrosion and subsequent perforation, which caused the tube to leak. The perforation was found to have occurred at a low point in a bend near the superheater outlet header. It was found that the low points of the superheater tubes could not be completely drained during idle periods. Water-level marks were noticed on the inside surface above the area of pitting. It was revealed by microscopic examination that localized pitting had resulted from oxidation. It was concluded that water contained in the tube during shutdowns had accumulated and cumulative damage due to oxygen pitting resulted in perforation of one of the tubes. Filling the system with condensate or with treated boiler water was suggested as a corrective action. Alkalinity was suggested to be maintained at a pH of 9.0 and 200 ppm of sodium sulfite should be added to the water.

A helicopter rapidly lost altitude and struck a tree, causing a fire and severe damage. The hose clamp which was the subject of this investigation was one of two used on a short length of hose between the turbocharger and the carburetion system. The purpose of this examination was to determine whether the hose failed during or before the accident. Fracture in the failed clamp was accompanied by obvious permanent deformation and evidence of local shearing at the ends of the perforation where fracture occurred, and in the adjacent perforation. The first test involved tightening the clamps to failure with a torque wrench. In no case did the band material fracture. In a second attempt to duplicate the failure, a tensile testing machine was used to pull the two fittings apart while the hose was clamped in place. When the testing machine was operated at maximum head travel (approximately 20 in. per min.), one of the hose clamps broke in the same manner as the clamp in question. The manner of failure during the tension test indicated this clamp failed at the time of the crash because of a sudden separation between the turbocharger and the remainder of the aircraft.

Tubes in heat exchangers, made of copper alloy C44300 and used for cooling air failed after 5 to six years of service. Air passed over the shell-side surface of the tubes and was cooled by water flowing through the tubes. Water vapor in the air was condensed (pH 4.5) on the tube surfaces during the cooling process. Air flow over the tubes reversed direction every 585 mm as a result of baffling placed in the heat exchangers. An uneven ridgelike thinning and perforation of the tube wall on the leeward side of the tube was revealed by visual examination. Undercut pits on the outer surface of the tube were revealed by metallographic examination of a cross section of the failed area. Impingement attack which led to perforation was revealed by both the ridgelike appearance of the damaged area and the undercut pitting. The heat exchanger was retubed with tubes made of aluminum bronze (copper alloy C61400).

A 25.4 cm (10 in.) diam gray cast iron water main pipe was buried in the soil beneath a concrete slab. The installation was believed to have been completed in the early 20th century. A leak from the pipe resulted in flooding of a warehouse. Once removed, the pipe revealed through-wall perforations and cracking along its axis. The perforations and the crack were at the 6 o'clock position. Investigation (visual inspection, radiography, unetched macrographs, and tensile testing) supported the conclusion that the failure occurred as result of years of exposure to ground water in the soil resulting in graphitic corrosion. Soils containing sulfates are particularly aggressive. Recommendations included pipe replacement. The wall thickness had been sufficiently reduced that the pipe could no longer support the required load. Water mains are designed for more than 100 years life. Ductile iron or coated and lined steel pipe, generally not susceptible to graphitic corrosion, were suggested as suitable replacement materials, and cathodic protection was also considered as a possibility.

Type 316L stainless steel pipes carrying brine at 120 deg C (250 deg F) and at a pH of about 7, failed by perforation at or near circumferential butt-weld seams. The failure was examined optically and radiographically in the field. Specimens were removed and examined metallographically and with a SEM in the laboratory. The examinations revealed a combination of failure mechanisms. The pitting failure of the welds was attributed to localized attack of an activated surface, in which anodic pits corroded rapidly. Additionally, SCC driven by residual welding stresses occurred in the base metal adjacent to the welds. Use of highly stressed austenitic stainless steels in high-chloride environments having a temperature above 65 deg C (150 deg F) should be discouraged. Solution annealing or shot peening to reduce residual stresses may be advisable. If heat treatment is not feasible after welding, the substitution of a more corrosion-resistant alloy, such as Incoloy 800 or 825, may be necessary.

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.

Plate perforation occurred in the cylindrical section and walls of the inlet foot (2.38 mm thick Incoloy 825 plate welded using INCO welding rod 135) of an inert gas fire prevention system in an oil tanker. Cross-sectional microprobe analysis showed the corrosion product to contain sulfur, mainly from the flue gas, and calcium and chlorine, mainly from the sea water. The gray corrosion product was interspersed with rust and a black carbonaceous deposit. Corrosion pitting and poor weld penetration, with carbide precipitation and heavy etching at grain boundaries, indicated sensitization and susceptibility to aqueous intergranular corrosion. Chemical analysis showed the predominant acid radical to be sulfate (6.20% in the carbonaceous deposit and 0.60% in the corrosion product), suggesting that oxidation of SO2 in the flue gas caused the corrosion. Moisture condensation, the carbon acting as a cathode, and alloy susceptibility to intergranular corrosion contributed to the corrosion.

High-chromium steel pipes 42.25 x 3.25 mm from a blast furnace gas fired recuperator for the preheating of air were heavily oxidized and perforated in places. It was found that the blast furnace gas had a high sulfur content. Both the carburization and the formation of sulfide proved that in addition, from time to time at least, combustion was incomplete and the operation was carried out in a reducing atmosphere, with the result that oxygen deficiency prevented the formation or maintenance of a protective surface layer on the external surface of the pipes. The sulfur would probably not have damaged the nickel-free steel used here at the given temperatures if it had been present as sulfur dioxide in an oxidizing atmosphere. The damage was therefore caused primarily by an incorrectly conducted combustion process.

One of 14 vertical radiant tubes (RA 333 alloy) in a heat-treating furnace failed when a hole about 5 x 12.5 cm (2 x 5 in.) corroded completely through the tube wall. The tube measured 183 cm (72 in.) in length and 8.9 cm (3 in.) in OD and had a wall thickness of about 3 mm (0.120 in.). Failure occurred where the tube passed through the refractory hearth (floor) of the furnace. Although the furnace atmosphere was neutral with respect to the work, it had a carburizing potential relative to the radiant tubes. Analysis (visual inspection, 250x spectroscopic examination of specimens etched with mixed acids, metallographic examination, and chemical analysis) supported the conclusions that the premature failure of the tube by perforation at the hearth level resulted from (1) corrosion caused by sulfur contamination from the refractory cement in contact with the tube and (2) severe local overheating at the same location. Recommendations included replacing all tubes using a low sulfur refractory cement in installation and controlling burner positioning and regulation more closely to avoid excessive heat input at the hearth level.

Two cases of failure of centrifuge baskets were investigated. The first involved a centrifuge running at approximately 1000 rpm. The basket was constructed from a perforated sheet of stainless steel rolled into a cylinder and joined by a single vee longitudinal weld. Detailed examination showed the weld had not completely penetrated the full depth of the section. The fracture faces showed a gradually progressing fatigue crack developing from a notch, formed by the lack of penetration, at the root of the weld. Microscopic examination of the parent plate showed it was a typical titanium stabilized austenitic steel. It is probable that had the basket been subjected to a periodic inspection by a competent person, this failure would not have occurred. The second case concerned a continuous duty centrifuge operating at 2200 rpm. Fracture had occurred at the circumferential weld attaching the stainless steel skirt to the basket rim and also in the region of the vertical weld which was made when the skirt was formed into a cone. Stress-corrosion cracking of the skirt material, which contained residual stresses due to cold-rolling, had been caused by the presence of sodium chloride.