Explosive cladding is a viable method for cladding different materials together, but the complicated behavior of materials under ballistic impacts raises the probability of interfacial shear failure. To better understand the relationship between impact energy and interfacial shear, investigators conducted an extensive study on the shear strength of explosively cladded Inconel 625 and plain carbon steel samples. They found that by increasing impact energy, the adhesion strength of the resulting cladding can be improved. Beyond a certain point, however, additional impact energy reduces shear strength significantly, causing the cladding process to fail. The findings reveal the decisive role of plastic strain localization and the associated development of microcracks in cladding failures. An attempt is thus made to determine the optimum cladding parameters for the materials of interest.

An alloy IN-690 (N06690) incinerator liner approximately 0.8 mm (0.031 in.) thick failed after only 250 h of service burning solid waste. Investigation supported the conclusion that the root cause of the failure was overfiring during startup and sulfidation of the nickel-base alloy. No recommendations were made.

Failure of a pilot scale test melter resulted from severe overheating of an Inconel 690 (690) jacketed molybdenum electrode. Extreme temperatures were required to melt the glass during this campaign because the feed material contained a very high waste loading. Metallurgical evaluation revealed the presence of an alloy containing nickel and molybdenum in several ingots found on the bottom of the melter and on a drip which had solidified on the electrode sheath. This indicates that a major portion of the electrode assembly was exposed to a temperature of at least 1317 deg C, the nickel/molybdenum eutectic temperature. Small regions on the end of the 690 sheath showed evidence of melting, indicating that this localized region exceeded 1345 deg C, the melting point of 690. In addition to nickel, antimony was found on the grain boundaries of the molybdenum electrode. This also contributed to the failure of the electrode. The source of the antimony was not identified but is believed to have originated from the feed material. Metallurgical evaluation also revealed that nickel had attacked the grain boundaries of the molybdenum/tungsten drain valve. This component did not fail in service; however, intergranular attack led to degradation of the mechanical properties, resulting in the fracture of the drain valve tip during disassembly. Antimony was not observed on this component.

Waspaloy (AMS 5586) fabricated inner ring of a spray-manifold assembly failed transversely through the manifold tubing at the edge of the tube and support sleeve brazed joint. The assembly was brazed with AWS BAu-4 filler metal (AMS 4787). Fatigue beach marks propagating from extremities of a granular gold-tinted surface region adjacent to the tube-to-sleeve brazed joint and extending circumferentially were revealed by microscopic examination. Embrittlement of the tube caused by molten braze metal penetration along grain boundaries was evidenced by micrographs of a granular portion of the fracture. It was revealed by the initial fracture profile that fatigue cracks begun as an intergranular separation and subsequently became transgranular. It was concluded that failure of the tube was caused by excessive alloying between the braze metal and the Waspaloy. Reduced temperatures during torch debrazing or rebrazing were recommended to minimize molten braze metal penetration.

This case study demonstrates that Alloy 601 (UNS N06601) is susceptible to strain-age cracking. The observation illustrates the potential importance of post weld heat treatment to the successful utilization of this alloy in certain applications.

At a power plant, C-276 nickel alloy welds (N10276) on a C-276 duct floor completely disappeared in less than half a year. A continuous supply of flue gas came in contact with the closed bypass duct. The unscrubbed combustion products condensed on the cold duct, then the closed damper conducted heat from the chimney and reheated the condensate. Investigation (visual inspection and welded coupon testing) supported the conclusion that the corrosion was caused by “Green Death,” a corrosive medium used to test for pitting resistance (11.9% H2SO4 + 1.3% HCl + 1% FeCl3 + 1% CuCl2) at 103 deg C (217 deg F). Such conditions exist at power plants. Recommendations included repairing the C-276 plates with a 686CPT weld alloy, and if that did not correct the situation, replacing the plates with 686 plate (N06686) welded with 686 CPT.

When bulging occurred in mortar tubes made of British I steel during elevated-temperature test firing, a test program was formulated to evaluate the high-temperature properties (at 540 to 650 deg C, or 1000 to 1200 deg F) of the British I steel and of several alternative alloys including a maraging steel (18% Ni, grade 250), a vanadium-modified 4337 gun steel (4337V), H19 tool steel, and high-temperature alloys Rene 41, Inconel 718, and Udimet 630. All the alloys evaluated had been used in mortar tubes previously or were known to meet the estimated minimum yield strength. The alloys fall in this order of decreasing strengths: Udimet 630, Inconel 718, Rene 41, H19 tool steel, British I steel, 4337V gun steel, and maraging steel. When cycled between room temperature and 540 to 650 deg C (1000 to 1200 deg F), only Udimet 630, Inconel 718, and Rene 41 retained yield strengths higher than the minimum. Also, these three alloys maintained high strengths over the tested range, whereas the others decreased in yield strength as cycling progressed. Analysis showed Inconel 718 was considered best suited for 81-mm mortar tubes, and widespread industrial use ensured its availability.

A double-walled, hemispherical metal beam exit window made of alloy 718 developed a crack during service, leading to coolant leakage. The window had been exposed to radiation damage from 800 MeV protons and a cyclic stress from 600 MPa tensile to near zero induced by numerous temperature cycles calculated to be from 400 to 30 deg C (752 to 86 deg F). The window was activated to >200 Sv/h. It was determined through analysis using remote handling techniques and hot cells that the crack initiated near a spot weld used to affix thermocouples to the window surface. In addition to analysis of the crack, some of the irradiated material from the window was used to measure mechanical properties. Hot cell techniques for preparation of samples and testing were developed to determine true operating conditions of radiation, strain, and temperature.

A portion of the wall of a reactor vessel used in burning impurities from carbon particles failed by localized melting. The vessel was made of Hastelloy X (Ni-22Cr-9Mo-18Fe). Considering the service environment, melting could have been caused either by excessive carburization (which would have lowered the melting point of the alloy markedly) or by overheating. A small specimen containing melted and unmelted metal was removed from the vessel wall and examined metallographically. It was observed that the interface between the melted zone and the unaffected base metal was composed of large grains and enlarged grain boundaries. An area a short distance away from the melted zone was fine grained and relatively free of massive carbides. This evidence supported the conclusion that the vessel failed by melting that resulted from heating to about 1230 to 1260 deg C (2250 to 2300 deg F), which exceeded normal operating temperatures, and carburization was not the principal cause of failure. No recommendations were made.

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.

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.

Several of the springs, made of 1.1 mm diam Inconel X-750 wire and used for tightening the interstage packing ring in a high-pressure turbine, were found broken after approximately seven years of operation. Intergranular cracks about 1.3 mm in depth and oriented at an angle of 45 deg to the axis of the wire were revealed by metallographic examination. A light-gray phase, which had the appearance of liquid-metal corrosion, was observed to have penetrated the grains on the fracture surfaces. The spring wires were found to fracture in a brittle manner characteristic of fracture from torsional loading (along a plane 45 deg to the wire axis). Liquid-metal embrittlement was expected to have been caused by metals (Sn, Zn, Pb) which melt much below maximum service temperature of the turbine. The springs were concluded to have fractured by intergranular stress-corrosion cracking promoted by the action of liquid zinc and tin in combination with static and torsional stresses on the spring wire. As a corrective measure, Na, Sn, and Zn which were present in pigmented oil used as a lubricant during spring winding was cleaned thoroughly by the spring manufacturer before shipment to remove all contaminants.

Cracking occurred in an ASME SB166 Inconel 600 safe-end forging on a nuclear reactor coolant water recirculation nozzle while it was in service. The safe-end was welded to a stainless-steel-clad carbon steel nozzle and a type 316 stainless steel transition metal pipe segment. An Inconel 600 thermal sleeve was welded to the safe-end, and a repair weld had obviously been made on the outside surface of the safe-end to correct a machining error. Initial visual examination of the safe-end disclosed that the cracking extended over approximately 85 deg of the circular circumference of the piece. Investigation (visual inspection, on-site radiographic inspection, limited ultrasonic inspection, chemical analysis, 53x metallographic cross sections and SEM images etched in 8:1 phosphoric acid) supported the conclusion that the cracking mechanism was intergranular SCC. No recommendations were made.

Jet pumps, which have no moving parts, provide a continuous circulation path for a major portion of the core coolant flow in a boiling water reactor. Part of the pump is held in place by a beam-and-bolt assembly, wherein the beam is preloaded by the bolt. The Alloy X-750 beams had been heat treated by heating at 885 deg C (1625 deg F) for 24 h and aging at 705 deg C (1300 deg F) for 20 h. Jet pump beams were found to have failed in two nuclear reactors, and other beams were found to be cracked. Investigation (visual inspection, metallurgical examination, tension testing, and simulated service testing in oxygenated water) supported the conclusion that intergranular SCC under sustained bending loading was responsible for the failure. The location of the cracking was consistent with the results of stress analysis of the part. Recommendations included either replacing the beams, reheat treatment, or preload reduction.

A turbine spacer made of AMS 5661 alloy (Incoloy 901; composition: Fe-43Ni-13Cr-6Mo-2.5Ti) was removed from service because of a crack in the forward side of the radial rim. The crack extended axially for a distance of 16 mm across the spacer rim; radially, it extended to a depth of 6.4 mm into the web section. Analysis (visual inspection, 5000 and 10,000x TEM fractographs, chemical analysis, and 9x metallographic examination) supported the conclusions that cracking on the forward rim of the spacer occurred in fatigue that initiated on the forward rim face and that progressed into the rim and web areas. Because there was no apparent metallurgical cause for the cracking, the problem was assigned to engineering.

A transition duct was part of a 100-MW power-generation gas turbine. The duct was fabricated from several panels of a modified nickel alloy, IN-617. After six years of operation, two such ducts failed during the next two years, causing outages. Failure was in the form of a total collapse of the duct. Carbides and carbonitrides were found in all of the transitions examined. Investigation supported the conclusion that failure was caused by oxidation, oxide penetration, and oxide spallation which caused thinning of the duct wall. It was felt that the high oxygen and nitrogen partial pressures of the gases within the duct, combined with the high temperatures, facilitated nitrogen pickup. No recommendations were made.

Turbine buckets in a 37.5-MW gas turbine made of Udimet 500 superalloy failed in service. The power plant was located 1 km (0.6 miles) from the Pacific Ocean and operated on No. 2 diesel fuel, which was supplied by tanker ship. Turbine bucket failures occurred on three units after 2500 to 6400 h of operation. Investigation (visual inspection, metallographic examination, and stress analysis) supported the conclusion that the differing microstructure of the airfoil resulted in changes in mechanical properties. Because normal operation includes cycling of loads and temperatures, the shroud tip fractured due to thermomechanical fatigue in its degraded state. Recommendations included special chromium or silicon-rich coating to minimize corrosion in gas turbines operating in a marine environment with operating temperatures in the range of type 2 corrosion (650 to 750 deg C, or 1200 to 1380 deg F). Additionally, it was suggested that fuel delivery, handling, and treatment be high quality, to maintain fuel contamination within design limits, and inlet air filtration must be designed for the coastal site. Also, changing the bucket tip by increasing its thickness and changing the casting technique would reduce the stress and make the design more tolerant of corrosion.

The case and stiffener of an inner-combustion-chamber case assembly failed by completely fracturing circumferentially around the edge of a groove arc weld joining the case and stiffener to the flange. The assembly consisted of a cylindrical stiffener inserted into a cylindrical case that were both welded to a flange. The case, stiffener, flange, and weld deposit were all of nickel-base alloy 718. It was observed that a manual arc weld repair had been made along almost the entire circumference of the original weld. Investigation (visual inspection, 0.5x macrographs, and 10x etched with 2% chromic acid plus HCl views) supported the conclusions that failure was by fatigue from multiple origins caused by welding defects. Ultimate failure was by tensile overload of the sections partly separated by the fatigue cracks. Recommendations included correct fit-up of the case, stiffener, and flange and more skillful welding techniques to avoid undercutting and unfused interfaces.

The first-stage blades in a model 501D5 gas turbine had 16 cooling holes. After 32,000 h of service, the blades exhibited cracking at the cooling holes. The blade material was wrought Udimet 520 alloy, with nominal composition of 57Ni-19Cr-12Co-6Mo-1W-2Al-3Ti-0.05C-0.005B. The cooling holes' surface was not coated. Investigation supported the conclusions that the cracking at the cooling holes was due to grain-boundary oxidation and nitridation at the cooling hole surface, embrittlement and loss of local ductility of the base alloy, temperature gradient from the airfoil surface to the cooling holes, which led to relatively high thermal stresses at the holes located at the thicker sections of the airfoil, and stress concentration of 2.5 at the cooling hole and the presence of relatively high total strain (an inelastic strain of 1.2%) at the cooling hole surface. Recommendations include applying the specially designed methods given in this case study to estimate the metal temperature and stresses in order to predict the life of turbine blades under similar operating conditions.

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.