Search Results for Overheating

Leakage from the top of a fire-extinguisher case, made of 1541 steel tubing and closed by spinning was observed during testing. Three small folds were observed on the surface by visual examination and one was sectioned. A very fine transverse fissure through the section was revealed. Streaks of ferrite were observed by metallographic examination. It was concluded that cracking of the top of the fire-extinguisher case was the result of ferrite streaks formed due to metal overheating. The temperature of the metal was recommended to be controlled so that the spinning operation is done at a lower temperature to avoid formation of ferrite streaks.

An air bottle, machined from a solid block of aluminum alloy 2219-T852, displayed liquid-penetrant crack indications after assembly welding. The air bottle was machined to rough shape, a 3.8 mm (0.15 in.) wall thickness cylindrical cup with a 19 mm (3/4 in.) wall thickness integral boss on one side. After annealing, hot spinning, annealing a second time, and tack welding a port fitting, the assembly was torch preheated to 120 to 150 deg C (250 to 300 deg F). The port fitting was then welded in place. Final full heat treatment to the T62 temper was followed by machining, testing, and inspection. The crack indications were found only on one side of the boss and on the lower portion of the hot-spun dome region. The metallographic specimens revealed triangular voids and severe intergranular cracks. The cracks displayed the glossy surfaces typical of melted and resolidified material. The localized cracks in the air bottle were from grain-boundary eutectic melting caused by local torch overheating used in preparation for assembly welding of a port fitting. A change in design was scheduled to semiautomatic welding without the use of preheating for the joining of the port fitting for the dome opening.

The center portions of two adjacent low-carbon steel boiler tubes (made to ASME SA-192 specifications) ruptured during a start-up period after seven months in service. It was indicated by reports that there had been sufficient water in the boiler two hours before start-up. The microstructure near the rupture edge was revealed by metallographic examination to consist of ferrite and acicular martensite or bainite. The microstructure and the observed lack of cold work indicated a temperature above the transformation temperature of 727 deg C had been reached. Swelling of the tubes was disclosed by the wall thickness and OD of the tubing. The tubes were concluded to have failed due to rapid overheating.

The tubes of a stationary industrial boiler, 64 mm in diam and made of 1.25Cr-0.5Mo steel (ASME SA-213, grade T-11) failed by two different types of rupture. Noticeable swelling of the tubes in the area of rupture was revealed by visual examination. The tubes with slight longitudinal splits were interpreted to have failed by stress rupture resulting from prolonged overheating at 540 to 650 deg C as the microstructure exhibited extensive spheroidization and coalescence of carbides. The larger ruptures were tensile failures that resulted from rapid overheating to 815 to 870 deg C as a completely martensitic structure was revealed at the edges of the ruptures in these tubes because of rapid quenching by escaping fluid. The prolonged-overheating failures were concluded to have been the primary ruptures and that local loss of circulation had caused rapid overheating in adjacent tubes. Poor boiler circulation and high furnace temperatures were believed to have caused the prolonged overheating.

A 3.8-cm diam 6 x 37 rope of improved plow steel wire failed in service during dumping of a ladle of hot slag. A heavy blue oxide extending 0.6 to 0.9 m back from each side of the break was revealed on examination of the rope. Tensile fractures were shown by the broken ends of the rope. Recrystallization of the steel was revealed during microscopic examination of the wires adjacent to the break which indicated that the wires had been heated in excess of 700 deg C (1292 deg F). The tensile strength of the wires in the rope that broke was 896 MPa whereas the specification required it to be 1724 MPa. Thus, a 50% loss in tensile strength of the wires was caused by overheating which lead to failure of the rope. It was recommended that prolonged exposure of wire ropes to extreme conditions should be avoided.

Cylinder fatigue can result from abnormal heating in service. Fatigue can be experienced also by piston heads, exhaust valves, and turbosupercharger housings (castings). Pistons from different engines series can sometimes fit, but because of slight design modifications, they may not function properly. Circumferential cracks and fractures near the head-to- barrel junctions have occurred on numerous cylinders of reciprocating piston engines. In most instances, cracks were caused by high cyclic pressures and high temperatures resulting most probably from detonation. At times, fractures or cracks (or both) were also caused by a combination of unfavorable temperature distribution (and possibly excessive pressures around the cylinder barrel), un-nitrided internal surfaces of cylinder barrels, and inadequate thread contours, which caused high stress concentrations at the thread roots. One example of the most common type of cylinder failure is illustrated.

Pendant-style reheater, constructed of ASME SA-213, grade T-11, steel ruptured. A set of four tubes, specified to be 64 mm OD x 3.4 mm minimum wall thickness was examined. A small quantity of loose debris was removed from the inside of one of the tubes. The major constituent was revealed by EDS analysis of the debris to be iron with traces of phosphorus, manganese, sodium, calcium, copper, zinc, potassium, silicon, chromium, and molybdenum. Thus the debris was interpreted to be the scale from ID of the tube with boiler feedwater chemicals from the attemperation spray. The likely cause of failure was concluded to be exfoliation of the scale from the ID surface of the tube. Creep failures were interpreted to be caused by localized temperatures higher than the maximum service temperature. Replacement of the affected tubes was recommended. Inspection of the tubes by radiography to find the circuits with the greatest accumulation of debris and replacing them as necessary was recommended on an annual basis.

An aftercooler was of conventional design and fitted with brass tubes through which cooling-water circulated. Air at 100 psi pressure was passed over the outsides of the tubes, entering the vessel near to the upper tubeplate on one side and leaving it by a branch adjacent to the lower tubeplate on the opposite side. After a mishap, the paint had been burned off the upper half of the shell. Internally, most of the tubes were found to be twisted or bent. The casing of the pump used to circulate the cooling water was also found to be cracked after the mishap. All the evidence pointed to the probability that a fire had occurred within the vessel. Some months before the failure, one of the tubes situated towards the center of the nest developed a leak. Owing to the difficulty of inserting a replacement tube, the defective one was scaled by means of a length of screwed rod fitted with nuts and washers at each end. This assembly became loose, thereby allowing air under pressure to enter the waterside of the cooler and expel the water, leading to overheating and ultimately to the damage described.

To forged AISI 4140 steel trailer kingpins fractured after 4 to 6 months of service. Fractographic and metallographic examination revealed that cracks were present in the spool-flange shoulder region of the defective kingpins prior to installation on the trailers. The cracks grew and coalesced during service. Consideration of the manufacturing process suggested that the cracks were the result of overheating of the kingpin blanks prior to forging, which was exacerbated during forging by deformation heating in the highly-strained region. This view was supported by results of two types of tensile tests conducted near the incipient melting temperature at the grain boundaries. All kingpins made by the supplier of the fractured ones were ultrasonically inspected and six more anticipated to fail were found. It was recommended that the heating of forging blanks be more carefully controlled, especially with respect to the accuracy of the optical pyrometer temperature readout. Also, procedures must be developed such that forging blanks that trigger the over-temperature alarm are reliably and permanently removed from the production line.