Search Results for 347

A pressure probe assembly comprised of type 347 stainless steel housing, brazed with AMS 4772D filler metal to the pressure probe, failed due to detachment of a rectangular segment from the housing. The presence of a large brazing metal devoid region in the pressure probe-housing joint was revealed by visual examination. Fatigue marks, emanating from multiple crack origins on the inside surface of the housing at the brazed joint were revealed by further study of the fracture. A poor metallurgical bond was confirmed by the presence of large irregular voids, flux trapped braze metal and separation between braze and housing.

A number of AISI 347 stainless steel bellows intended for use in the control rod drive mechanism of a fast breeder reactor were found to be leaking before being placed in service. The bellows, which had been in storage for one year in a seacoast environment, exhibited a leak rate on the order of 1 x 10−7 cu cm/s (6 x 10−8 cu in./s). Optical metallography revealed numerous pits and cracks on the surfaces of the bellow convolutes, which had been welded to one another using an autogenous gas tungsten arc welding process. Microhardness measurements indicated that the bellows had not been adequately stress relieved. It was recommended that a complete stress-relieving treatment be applied to the formed bellows. Improvement of storage conditions to avoid direct and prolonged contact of the bellows with the humid, chloride-containing environment was also recommended.

A fused-salt electrolytic-cell pot containing a molten eutectic mixture of sodium, potassium, and lithium chlorides and operating at melt temperatures from 500 to 650 deg C (930 to 1200 deg F) exhibited excessive corrosion after two months of service. The pot was a welded cylinder with 3-mm thick type 304 stainless steel walls and was about 305 mm (12 in.) in height and diam. Analysis (visual inspection and 500x micrographs etched with CuCl2) supported the conclusions that the pot failed by intergranular corrosion because an unstabilized austenitic stainless steel containing more than 0.03% carbon had been sensitized and placed in contact in service with a corrosive medium at temperatures in the sensitizing range. Recommendations included changing material for the pot from type 304 stainless steel to Hastelloy N (70Ni-17Mo-7Cr-5Fe). Maximum corrosion resistance and ductility are developed in Hastelloy N when the alloy is solution heat treated at 1120 deg C (2050 deg F) and is either quenched in water or rapidly cooled in air. An alternative, but less suitable, material for the pot was type 347 (stabilized grade) stainless steel. After welding, the 347 should be stress relieved at 900 deg C (1650 deg F) for 2 h and rapidly cooled to minimize residual stresses.

A type 321 stainless steel bellows expansion joint on a 17-cm (6 in.) OD inlet line (347 stainless) in a gas-turbine test facility cracked during operation. The line carried high-purity nitrogen gas at 1034 kPa (150 psi) with a flow rate of 5.4 to 8.2 kg/s (12 to 18 lb/s). Cracking occurred in welded joints and in unwelded portions of the bellows. The bellows were made by forming the convolution halves from stainless steel sheet, then welding the convolutions together. Evidence from visual examination, liquid penetrant inspection chemical analysis, hardness tests, and metallographic examination of sections etched with Vilella's reagent supports the conclusions that failure of the bellows occurred by intergranular fatigue cracking. Secondary degrading effects on the piping existed as well. Recommendations included the acceptability of Type 321 stainless steel (provided open-cycle testing does not result in surface oxidation and crevices) Although type 347 stainless steel would be better, and Inconel 600 would be an even better choice. Welds would also need modified processing for reheating and annealing. Prevention of oil leakage into the system would minimize carburization of the piping and bellows.

A 321 stainless steel radar coolant-system assembly fabricated by torch brazing with AWS type 3A flux, failed at the brazed joint when subjected to mild handling before installation, after being stored for about two years. It was revealed by visual examination of the failed braze that the filler metal had not covered all mating surfaces. Lack of a metallurgical bond between the brazing alloy and stainless steel and instead mechanical bonding of the filler metal to an oxide layer on the stainless steel surface was revealed by examination of the broken joint at the cup. It was indicated by the thickness of the oxide layer that the steel surface was not protected from oxidation by the flux during torch heating. It was concluded that the failure was caused by lack of a metallurgical bond between the brazing alloy and the stainless steel. Components made of 347 stainless steel (better brazeability) brazed with a larger torch tip (wider heat distribution) and AWS type 3B flux (better filler-metal flow) were recommended for radar coolant-system assembly.

While undergoing vibration testing, a type 347 stainless steel inlet header for a fuel-to-air heat exchanger cracked in the header tube adjacent to the weld bead between the tube and header duct. Investigation (visual inspection and liquid penetrant inspection) supported the conclusion that the crack in the header tube was the result of a stress concentration at the toe of the weld joining a doubler collar to the tube. The stress concentration was caused by undercutting from poor welding technique and an unfavorable joint design that did not permit a good fit-up. Recommendations included manufacturing the doubler collar so that it could be placed in intimate contact with the header duct, and a revised weld procedure was recommended to result in a smaller, controlled, homogeneous weld joint with less distortion.

Handles welded to the top cover plate of a chemical-plant downcomer broke at the welds when the handles were used to lift the cover. The handles were fabricated of low-carbon steel rod; the cover was of type 502 stainless steel plate. The attachment welds were made with type 347 stainless steel filler metal to form a fillet between the handle and the cover. The structure was found to contain a zone of brittle martensite in the portion of the weld adjacent to the low-carbon steel handle; fracture had occurred in this zone. The brittle martensite layer in the weld was the result of using too large a welding rod and too much heat input, melting of the low-carbon steel handle, which diluted the austenitic stainless steel filler metal and formed martensitic steel in the weld zone. Because it was impractical to preheat and postheat the type 502 stainless steel cover plate, the low-carbon steel handle was welded to low-carbon steel plate, using low-carbon steel electrodes. This plate was then welded to the type 502 stainless steel plate with type 310 stainless steel electrodes. This design produced a large weld section over which the load was distributed.

Several fractures occurred in flange studs used for remote handling of radioactive equipment. The studs, of quenched-and-tempered type 414 stainless steel, fractured in the HAZs produced in the studs during the circumferential welding that joined the studs to the flanges. The weld deposits were of type 347 stainless steel, and the flanges were type 304 stainless steel. Metallographic examination of the failed studs revealed that the HAZs contained regions of martensite and that intergranular cracks, which initiated at the stud surfaces during welding, propagated to complete separation under subsequent loading. The studs fractured under service loads as a result of intergranular crack propagation in the HAZ. Rapid heating and cooling during attachment welding produced a martensitic structure in the HAZ of the stud, which cracked circumferentially from the combination of thermal-gradient and phase-change stresses. Joining the studs to the flanges by welding should be discontinued. They should be attached by screw threads, using a key and keyway to prevent turning in service.

Beveled weld-joint V-sections were fabricated to connect inlet and outlet sections of tubes in a type 347 stainless steel heat exchanger for a nitric acid concentrator. Each V-section was permanently marked with the tube numbers by a small electric-arc pencil. After one to two years of service, multiple leaks were observed in the heat-exchanger tubes. Investigation supported the conclusion that the corrosion occurred at two general locations: the stop point of the welds used to connect the inlet and outlet legs of the heat exchanger, and the stop points on the identifying numerals. Recommendations included replaced the material with type 304L stainless steel.

Welds in two CMo steel catalytic gas-oil desulfurizer reactors cracked under hydrogen pressure-temperature conditions that would not have been predicted by the June 1977 revision of the Nelson Curve for that material. Evidence of severe cracking was found in five weld-joint areas during examination of a naphtha desulfurizer by ultrasonic shear wave techniques. Defect indications were found in longitudinal and circumferential seam welds of the ASTM A204, grade A, steel sheet. The vessel was found to have a type 405 stainless steel liner for corrosion protection that was spot welded to the base metal and all vessel welds were found to be overlaid with type 309 stainless steel. Long longitudinal cracks in the weld metal, as well as transverse cracks were exposed after the weld overlay was ground off. A decarburized region on either side of the crack was revealed by metallurgical examination of a cross section of a longitudinal crack. It was concluded that the damage was caused by a form of hydrogen attack. Installation of a used Cr-Mo steel vessel with a type 347 stainless steel weld overlay was suggested as a corrective action.

A weld in a fuel-line tube broke after 159 h of engine testing. The 6.4-mm (0.25-in.) OD x 0.7-mm (0.028-in.) wall thickness tube and the end adapters were all of type 347 stainless steel. The butt joints between tube and end adapters were made by automated gas tungsten arc (orbital arc) welding. It was found that the tube had failed in the HAZ. Examination of a plastic replica of the fracture surface in a transmission electron microscope established that the crack origin was at the outer surface of the tube. The crack growth was by fatigue; closely spaced fatigue striations were found near the origin, and more widely spaced striations near the inner surface. The quality of the weld and the chemical composition of the tube both conformed to the specifications. However, the fuel-line assembly had vibrated excessively in service. The fuel-line fracture was caused by fatigue induced by severe vibration in service. Additional tube clamps were provided to damp the critical vibrational stresses. No further fuel-line fractures were encountered.