The secondary cooling water system pressure boundary of Savannah River Site reactors includes expansion joints utilizing a thin-wall bellows. While successfully used for over thirty years, an occasional replacement has been required because of the development of small, circumferential fatigue cracks in a bellows convolute. One such crack was recently shown to have initiated from a weld heat-affected zone liquation microcrack. The crack, initially open to the outer surface of the rolled and seam welded cylindrical bellows section, was closed when cold forming of the convolutes placed the outer surface in residual compression. However, the bellows was placed in tension when installed, and the tensile stresses reopened the microcrack. This five to eight grain diameter microcrack was extended by ductile fatigue processes. Initial extension was by relatively rapid propagation through the large-grained weld metal, followed by slower extension through the fine-grained base metal. A significant through-wall crack was not developed until the crack extended into the base metal on both sides of the weld. Leakage of cooling water was subsequently detected and the bellows removed and a replacement installed.

A fluorescent liquid-penetrant inspection of an experimental stator vane of a first-stage axial compressor revealed the presence of a longitudinal crack over 50 mm (2 in.) long at the edge of a resistance seam weld. The vane was made of titanium alloy Ti-6Al-4V (AMS 4911). The crack was opened by fracturing the vane. The crack surface displayed fatigue beach marks emanating from the seam-weld interface. Both the leading-edge and trailing-edge seam welds exhibited weld-metal expulsions up to 3.6 mm (0.14 in.) in length. Metallographic examination confirmed that metal expulsion from the resistance welds was generally present. The stator vane failed by a fatigue crack that initiated at internal surface discontinuities caused by metal expulsion from the resistance seam weld used in fabricating the vane. Expulsion of metal from seam welds should be eliminated by a slight reduction in welding current to reduce the temperature, by an increase in the electrode force, or both.

Several AISI type 321 stainless steel welded oil tank assemblies used on helicopter engine systems began to leak in service. One failure, a fracture on the aft side of a spot weld, was submitted for analysis. SEM fractography examination revealed fatigue failure. The failure initiated at an overload fracture near the root of the weld and was followed by mode III fatigue crack propagation (tearing) around the periphery of the weld. The initial overload fracture was caused by a high external load, which produced a concentrated stress and fracture at the weld root. The subsequent fatigue fracture was caused by engine vibrations during operation of the aircraft. Fracture characteristics indicated that the fatigue would not have occurred if the initial damage had not taken place.

A welded ferritic stainless steel heat exchanger cracked prior to service. The welding filler metal was identified as an austenitic stainless steel and the joining method as gas tungsten arc welding. Investigation (visual inspection, SEM images, 5.9x images, and 8.9x/119x images etched with Vilella's reagent followed by electrolytic etching in 10% oxalic acid) supported the conclusion that the heat exchanger cracked due to weld cold cracking or postwelding brittle overload that occurred via flexure during fabrication. The brittle nature of the weld was likely due to a combination of high residual stresses, a mixed microstructure, inclusions, and gross grain coarsening. These synergistic factors resulted from extreme heat input during fillet welding. Recommendations included altering the welding variables such as current, voltage, and travel speed to substantially reduce the heat input.

Penetration by molten copper occurred in the economizer of a large water-tube boiler. A cross section through a weld and the crack in the tube revealed a crack was an intergranular fissure. Small fissures of the same type also extended from its flanks. The main fissure was filled with an oxide scale in which were embedded particles having the appearance of metallic copper. It was concluded that the cracking that occurred at the time of re-welding was due to intergranular penetration by copper present in the deposit within the tubes, which had not been completely removed prior to welding. Subsequently, it was ascertained that trouble had been experienced with the centrifugal feed pumps, resulting in scuffing of some bronze rings. The presumption is that bronze particles had been carried in mechanical suspension in the feed water and deposited in the economizer tubes.

Six cracked A36 steel gusset plates that formed part of the roof trusses of a large manufacturing facility were discovered during a routine final inspection of a new building construction. Two different-size plates from different locations in the building were removed and tested. It was determined that the gusset plates failed in the heat-affected zone via an intergranular microcracking mode due to hydrogen-assisted underbead and toe-weld cracking. Proper nondestructive testing techniques for magnetic particle and radiographic inspection of the plate-weld gusset areas were recommended.

A number of seamless pipe nipples of 70 mm diam and 3.5 mm wall thickness made of steel type 35.8 were oxyacetylene welded to collectors of greater wall thickness with a round bead. X-ray examination showed crack initiation in the interior of the nipples close to the root of the weld seam. The cracks only appeared where the originally deposited bead was remelted in the regions of overlap. Given the construction and welding technique used, it would have been preferable to make the nipples of a steel lower in sulfur content. However, by taking advantage of all the potential in shaping and welding technology, it should be possible to prevent crack formation with steel type 35.8 of normal composition.

The onset of leakage adjacent to two butt welds in a 2 in. bore pipe was traced to the development of fine cracks. The pipe carried 40% sodium hydroxide solution. The actual temperature was not known, but the pipeline was steam traced at a pressure of 30 psi, equivalent to a temperature of 130 deg C (266 deg F). Magnetic crack detection revealed circumferential crack-like indications situated a short distance from the butt weld. Cracking originated on the bore surfaces of the tube and was of an intergranular nature reminiscent of caustic cracking in steam boilers. The strength of the solution of caustic soda and possibly the temperature also were in the range known to produce stress-corrosion cracking of mild steels in the presence of stresses of sufficient magnitude. In this instance the location of the cracking suggested that residual stresses from welding, which approach yield point magnitude, were responsible. As all other welds were suspect, the remedy was to remove the joints and to reweld followed by local stress relief.

An HK-40 alloy tubing weld in a reformer furnace of a petrochemical plant failed by leaking after a shorter time than that predicted by design specifications. Leaking occurred because of cracks that passed through the thickness of the weldment. Analysis of the cracked tubing indicated that the sulfur and phosphorus contents of the weld metal were higher than specified, the thickness was narrower at the weld, and the mechanical resistance of the weld metal was lower than specified. Cracking initiated at the weld root by coalescence of creep cavities. Propagation and expansion was aided by internal carburization. Quality control of welding procedures and filler metal was recommended.

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.

During the construction of a large-diam pipeline, several girth welds had to be cut out as a result of radiographic interpretation. The pipeline was constructed of 910 mm (36 in.) diam x 13 mm (0.5 in.) wall thickness grade X448 (x65) line pipe. The girth welds were fabricated using standard vertical down stove pipe-welding procedures with E7010 cellulosic electrodes. The crack started partially as a result of incomplete fusion on the pipe side wall, which in turn was a result of misalignment of the two pipes. The crack was typical of hydrogen cracking. Girth welds can be made using cellulosic electrodes. For high-risk girth welds, an increase in preheat and/or a reduction in the local stress by controlling lift height or depositing the hot pass locally before lifting may be required.

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.

A 2.3 mm diam steel wire broke during cable twisting. The fracture occurred obliquely to the longitudinal axis of the wire and showed a constriction at the end. Therefore it was a ductile fracture. File mark type work defects were noticeable on the wire surface at both sides of the fracture, but they had no effect on the breakage of the wire. Away from the fracture area, the wire had a normal structure of hyperfine lamellar pearlite (sorbite) of a “patented” and cold drawn steel wire. In the vicinity of the fracture, the cementite of the pearlite was partially spheroidized, while at the fracture itself it was completely spheroidized. Therefore the wire was locally annealed at this point. It was likely that the wire cracked at this point during the last drawing and then broke during twisting due to its lower strength in the weakened cross section after prior deformation.

An aircraft fuel-nozzle-support assembly exhibited cracks along the periphery of a fusion weld that attached a support arm to a fairing in a joint that approximated a T-shape in cross section. The base metal was type 321 stainless steel. Examination showed a good-quality weld penetrating to the support arm beneath, but revealed notch configurations at the inner mating surfaces at each edge of the fairing, the result of welding a poor fit-up of the support arm to the fairing. Fractures that originated at the cracks were examined by stereomicroscope and were found to contain fatigue marks that indicated crack propagation from multiple origins at the inner surface of the weld edge. Fatigue cracking was initiated at stress concentrations created by the notches at the inner surfaces between the support arm and the fairing, enhanced by poor fit-up in preparation for welding.

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.