Search Results for hot cracking

Stainless steel pipe (273-mm OD x 8-mm wall thickness) used in the fabrication of large manifolds developed crack-like decohesions during a routine inductive bending procedure. The imperfections, which were found near the outside diameter, were around 3 mm in length oriented in the circumferential direction and penetrated nearly 2 mm into the pipe wall. The pipes were made of titanium-stabilized austenitic stainless steel X6CrNiMoTi17-12-2. Six hypotheses were considered during the investigation, which ultimately concluded that the failure was caused by liquation cracking due to overheating.

Liquid penetrant inspection of an ASTM A296 grade CA-15 residual heat removal pump impeller from a nuclear plant revealed a crack like indication that approximated the outer contour of the wear ring. Examination of a section containing the crack and three sections from near the main crack indication revealed that the failure was caused by hot cracking related to original weld repairs performed on the impeller casting.

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 presence of subsurface cracks in a longitudinal weld seam of an AISI type 316 stainless steel heat-exchanger shell was revealed by radiographic testing. Numerous intergranular cracks associated with the root pass of the weld, which had propagated both parallel and normal to the weld seam, were revealed by metallographic examination (hot shortness). It was indicated by energy-dispersive spectroscopy that type 316 electrode was not used for the root pass and instead a nickel-copper alloy electrode was employed. It was thus concluded that cracking was caused due to the use of an incorrect electrode for the root pass as these electrodes are crack sensitive if overheated. The weld seam was completely ground out and replaced with the correct electrode material as a corrective measure.

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.

Routine inspections of a carbon steel wood pulp digester revealed a sharply increasing number of cracks in the overlay metal on the internal surface of the digester after 1 and 2 years of service. The weld overlay was composed of type 309 stainless steel on the top fourth of the digester and of a proprietary high-nickel material on the bottom three-fourths. Examination revealed three distinct modes of deterioration. General corrosion was linked to the use of unspecified overlay metal. Cracking resulted during installation from the use of a material susceptible to hot cracking. Deep corrosion fissures then developed at hot crack sites as a result of crevice corrosion. Use of the appropriate overlay material was recommended.

A catastrophic brittle fracture occurred in a welded steel (ASTM A517 grade H) trapezoidal cross-section box girder while the concrete deck of a large bridge was being poured. The failure occurred across the full width of a 57 mm (2 1 4 in.) thick, 760 mm (30 in.) wide flange and arrested 100 mm (4 in.) down the slant web. Failure analysis revealed a major deficiency in fracture toughness. The failure occurred as a brittle fracture after the formation of a welding hot crack and approximately 40 mm (1 1 2 in.) of slow crack growth. It was recommended that bridges fabricated from this grade of steel undergo frequent inspection and that stringent test requirements be imposed as a condition of use in non-redundant main load-carrying components.

Fusing of the switch contacts of a boiler feed pump drive motor led to the failure of a turbine. After rubbing of most of the Ni-Cr steel LP wheels had occurred, due to the admission of water carried over with the steam, a copper-rich alloy from the interstage gland rings melted, penetrated the wheel material, and gave rise to radial and circumferential cracking in four of the LP wheels. It was concluded that when the rotor moved axially and the wheels came into contact with the diaphragms there was a tendency for the former to dish, with the development of both radial and circumferential tensile stresses on the side in contact with the adjacent diaphragm. In the presence of the molten copper-rich alloy, these stresses gave rise to severe hot cracking.

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.

A T-piece from a copper hot water system failed. Microscopic examination of a polished section revealed a main crack and branching transcrystalline cracks running from the outer surface of the pipe into the pipe wall. The crack appearance indicated disintegration by stress-corrosion cracking. Although copper is not susceptible in the pure state, it is prone to stress-corrosion cracking under tensile stress in the presence of other elements in a damp ammoniacal atmosphere. The material was not defective, but a phosphorus-deoxidized copper type. The residual phosphorus combined with oxygen to form phosphorus pentoxide. Hard soldering in turn prevented the formation of cuprous oxide, and hydrogen embrittlement occurred.

A full lift disk, made of die cast brass, which served as a lifting aid in a safety valve, had cracked in service at a number of locations in the vicinity of the threaded hole. During microscopic examination, agglomeration of oxide inclusions were noted in the region of the cracks. Because the die cast brass was alloyed with aluminum, these inclusions consisted predominantly of aluminum oxide. The tolerable limit in pores and oxide inclusions was greatly exceeded in the lift disk under examination. Above all, the numerous oxide skins disrupted the cohesion of the microstructure and were primarily responsible for the failure of the lift disk.