Search Results for Repair welds

A fuel-nozzle-support assembly showed transverse indications after fluorescent liquid-penetrant inspection of a repair-welded area at a fillet on the front side of the support neck adjacent to the mounting flange. Visual examination disclosed an irregular crack. The crack through the neck was sectioned; examination showed that the crack had extended through the repair weld. The crack had followed an intergranular path. The crack was opened, and binocular-microscope examination of the fracture surface showed that the surface contained dendrites with discolored oxide films that were typical of exposure to air when very hot. Several additional subsurface cracks, typical of hot tears, were observed in and near the weld. There had been too much local heat input in making the repair weld. The result was localized thermal contraction and hot tearing. The cracking of the repair weld was attributed to unfavorable welding practice that accentuated thermal contraction stresses and caused hot tearing. Recommendations involved use of a small-diameter welding electrode, a lower heat input, and deposition in shallow layers that could be effectively peened between passes to minimize internal stress.

The AISI 1080 steel crankshaft of a large-capacity double-action stamping press broke in service and was repair welded. Shortly after the crankshaft was returned to service, the repair weld fractured. The repair-weld fracture was examined ultrasonically which revealed many internal reflectors, indicating the presence of slag inclusions and porosity. A low-carbon steel flux-cored filler metal was used in repair welding the crankshaft, without any preweld or postweld heating. This resulted in the formation of martensite in the HAZ. The repair weld failed by brittle fracture, which was attributed to the combination of weld porosity, many slag inclusions and the formation of brittle martensite in the HAZ. A new repair weld was made using an E312 stainless steel electrode, which provides a weld deposit that contains considerable ferrite to prevent hot cracking. Before welding, the crankshaft was preheated to a temperature above which martensite would form. After completion, the weld was covered with an asbestos blanket, and heating was continued for 24 h. During the next 24 h, the temperature was slowly lowered. The result was a crack-free weld.

An outer fan-duct assembly of titanium alloy Ti-5Al-2.5Sn (AMS 4910) for a gas-turbine fan section cracked 75 mm (3 in.) circumferentially through a repair weld in an arc weld in the front flange-duct segment. Examination of the crack with a binocular microscope revealed no evidence of fatigue. A blue etch-anodize inspection showed the presence of an alpha case along the edges of the repair weld. The alpha case, a brittle oxide-enriched layer, forms when welds are inadequately shielded from the atmosphere during deposition. The brittleness of this layer caused transgranular cracks to form and propagate in tension under the thermal stresses created by the repair-weld heat input. The crack resulted from contamination and embrittlement of a repair weld that had received inadequate gas shielding. Thermal stresses cracked the oxide-rich layer that formed. The gas-shielding accessories of the welding torch were overhauled to ensure that leak-in or entrainment of air was eliminated. Also, the purity of the shielding-gas supplies was rechecked to make certain that these had not become contaminated.

The right front spring hanger on a dual rear axle of the tractor of a tractor-trailer combination failed, causing the vehicle to roll-over. The hanger was made from malleable cast iron that had been heat treated to produce a decarburized surface layer and a pearlitic transition layer. It had been repair welded after breaking into two pieces longitudinally in a prior incident, using cast iron as weld metal. The repair weld bead on both surfaces missed the fracture over 15 to 20% of their lengths. The incomplete repair weld and brittleness of the weld metal and heat-affected zones led to the failure.

A large shackle used in operating a dragline bucket failed in service. The shackle was made of a cast low-alloy steel (similar to AISI 4320) heat treated to a hardness of 415 BN. The shackle failed by fracturing through the load-bearing region. Examination of the fracture surface revealed a fatigue crack through about one-third of the cross section. A secondary fatigue crack, perpendicular to the main fracture, was also observed. The composition of the weld deposit corresponded to a heat treatable flux-cored arc welding filler material that was known to have been used for repair welding of these products. This shackle failed because of fatigue initiating at hydrogen cracks that had occurred in the HAZ of a repair weld. The weld had been made with a heat-treatable filler material, and a full postweld heat treatment had been performed. However, a low-hydrogen filler material had not been used to make the weld. Repair welds in high-strength steel castings should always be made with low-hydrogen filler materials.

An intermediate shaft (3 in. diam), part of a camshaft drive on a large diesel engine, broke after two weeks of service. Failure occurred at the end of the taper portion adjacent to the screwed thread. The irregular saw-tooth form of fracture was characteristic of failure from torsional fatigue. A second shaft carried as spare gear was fitted and failure took place in a similar manner in about the same period of time. Examination revealed that the tapered portion of the Fe-0.6C carbon steel shaft had been built up by welding prior to final machining. A detailed check by the engine-builder established that the manufacture of these two shafts had been subcontracted. It was ascertained that the taper portions had been machined to an incorrect angle and then subsequently built-up and remachined to the correct taper. The reduction in fatigue endurance following welding was due to heat-affected zone cracking, residual stresses, the lower fatigue strength of the weld deposited metal, and weld defects.

Several elbow subassemblies comprising segments of oil-line assemblies that recycled aircraft-engine oil from pump to filter broke in service. The components of the subassemblies were made of aluminum alloy 6061-T6. Two subassemblies were returned to the laboratory to determine cause of failure. In one, the threaded boss had separated from the elbow at the weld. In the other, the failure was by fracture of the elbow near the flange. The separation of the threaded boss from the elbow was due to a poor welding procedure. Crack propagation was accelerated by fatigue caused by cyclic service stresses. The fracture of the second elbow near the flange was caused by overaging during repair welding of the boss weld. Satisfactory weld penetration was achieved by improved training of the welders plus more careful inspection. Repair welding was prohibited, to avoid recurrence of overaging from the welding heat. Additional support for the oil line was installed to reduce vibration and minimize fatigue of the elbow.

A type 316L stainless steel “Jewett nail” hip implant failed after 2 months of service. Fracture occurred through the first of five screw holes in the plate section. Microscopic examination of mating fracture surfaces showed that failure had initiated at the outside (convex) surface of the plate and proceeded through its thickness. The fracture morphology was characteristic of fatigue. A beveled area on the inside surface of the plate indicated that the implant had been fractured for some time prior to removal. Metallographic examination of samples cut from the plate section revealed a series of hidden repair welds on the inside surface of the plate in the vicinity of the fracture. Comparison of the microstructure in the area of the fracture with that in an area away from the weld indicated that the repair welding had resulted in the creation of an annealed, softened zone. Manufacturers should never attempt to salvage this type of critical device by welding or any other procedure that might compromise its integrity.

An investigation was conducted to determine the cause of numerous cracks and other defects on the surface of a cast ASTM A743 grade CA-15 stainless steel main boiler feed pump impeller. The surface was examined using a stereomicroscope, and macrofractography was conducted on several cross sections removed from the impeller body. Areas that appeared to have the most severe surface damage were sectioned, fractured open, and examined using SEM. The chemistry of the impeller and an apparent repair weld were also analyzed. The examination indicated that the cracks were shrinkage voids from the original casting process. Surface repair welds had been used to fill in or cover over larger shrinkage cavities. It was recommended that more stringent visual and nondestructive examination criteria be established for the castings.

Persistent cracking in a forged 1080 steel turntable rail in a wind tunnel test section was investigated. All cracks were oriented transverse to the axis of the rail, and some had propagated through the flange into the web. Through-flange cracks had been repair welded. A section of the flange containing one through-flange crack was examined using various methods. Results indicated that the cracks had initiated from intergranular quench cracks caused by the use of water as the quenching medium. Brittle propagation of the cracks was promoted by high residual stresses acting in conjunction with applied loads. Repair welding was discontinued to prevent the introduction of additional residual stress., Finite-element analysis was used to show that the rail could tolerate existing cracks. Periodic inspection to monitor the degree of cracking was recommended.

The dished ends of a heavy water/helium storage tank manufactured from 8 mm (0.3 in.) thick type 304 stainless plate leaked during hydrotesting. Repeated attempts at repair welding did not alleviate the problem. Examination of samples from one dished end revealed that the cracking was confined to the heat affected zone (HAZ) surrounding circumferential welds and, to a lesser extent, radial welds that were part of the original construction. Most of the cracks initiated and propagated from the inside surface of the dished ends. Microstructures of the base metal, HAZ, and weld metal indicated severe sensitization in the HAZ due to high heat input during welding. An intergranular corrosion test confirmed the observations. The severe sensitization was coupled with residual stresses and exposure of the assembly to a coastal atmosphere during storage prior to installation. This combination of factors resulted in failure by stress-corrosion cracking. Implementation of a new repair procedure was recommended. Repairs were successfully made using the new procedure, and all cracks in the weld repair zones were eliminated.

Two examples involved brittle fracture promoted by small fatigue cracks owing to welding deficiencies. Other parts involving inadequate welding were a ski-wheel axle flange, ski fitting (brackets), and undercarriage shock strut stub assembly. In an attach fitting for an engine mount, weld cracks (severe stress concentrations) formed during repair welding. Cracks were severely oxidized. The main cause was incorrect repair and inadequate inspection of the fitting. In a cast CrNi alloy ski wheel axle, brittle fatigue failure emanated from welding cracks (notches). These welding cracks formed during the fabrication of the axle mounting plate. So-called all-purpose electrodes were used. Thus, the main cause for failure was a manufacturing deficiency-fatigue failure developed because of improper welding during fabrication of the axle. The proper electrode should have been used.

During 5.7 years of service, dye penetrant inspection of Inconel 800H pigtail connections regularly showed cracks at weld toes. Weld repairs were not able to prevent reoccurrence but often aggravated the condition. Samples containing small, but detectable, reducer-to-pigtail cracks showed intergranular cracks originating at weld toes and filled with oxidation product, which precluded determination of the cracking mechanism. All weldments exhibited high degrees of secondary precipitates, with original fabrication welds exhibiting higher apparent levels than repair welds. SEM/EDS analysis showed base metal grain boundary precipitates to be primarily chromium carbides, but some titanium carbides were also observed. Failure was believed to result from the synergism of thermally driven tube distortion, which resulted in over-stress, and from the intergranular oxidation products and intergranular carbides which contributed to cracking. It was recommended that stresses be reduced and /or that materials and components be changed. Refinements in welding procedures and implementation of preweld/postweld heat treatments were recommended also.

The source of cracking in the circumferential weld seam in a JIS-SM50B carbon-manganese steel pipe used in a CO2 absorber was investigated, the absorber had been in service for 18 years. The seam had been weld-repaired twice, and the repair welds had been locally stress relieved. Longitudinal seams in the same vessel, which had been stress relieved in a furnace, showed no tendency toward cracking. The solution passing through the vessel contained CO2-CO-H20, KHCO, and Cl− ions. Nondestructive testing revealed that the cracks originated in the heat-affected zone and propagated into the base metal and weld. Severe branching of the cracks characteristic of stress-corrosion cracking was observed. Microexamination revealed that crack propagation was transgranular further supporting the possibility of stress-corrosion cracking. Simulation tests carried out in the vessel confirmed this mode of cracking. It was recommended that weld seams be furnace heat treated at a temperature of 600 to 640 deg C (1110 to 1180 deg F) for a minimum of 1 h per inch of section thickness.