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.

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.

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.

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.

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.

Brief overheating of the 89 mm OD 6.4 mm wall thickness titanium heater tubes (ASTM B337, grade 2) was caused by a flow stoppage in a leach heater. Blue-tinted areas and patches of flaky white, yellow, and brown oxide scale was revealed on visual examination. It was disclosed by subjecting the overheated tube to a flattening test that the tube no longer met ASTM B 337 specifications. Large grain size and numerous needlelike hydride particles were disclosed in the microstructure of the overheated tube. Heating to approximately 815 deg C was revealed by the presence of the flaky oxide and increased grain size. Hydrogen and oxygen absorption was revealed by the presence of hydrides and the shallow surface embrittlement and thus susceptibility to cracking at ambient temperatures was observed. It was concluded that the titanium tubes were embrittled due to overheating the tubes and the severe surface embrittlement resulted from oxygen absorption which made the surface layers susceptible to cracking under start up and shutdown. Replacement tubes made of a heat-resistant alloy (e.g., Hastelloy C-276) were recommended.

A ring clamp (8740 (AMS 6322), steel forged and cadmium plated) used for attaching ducts to an aircraft engine became loose after three hours of service. When the clamp was removed from the engine, the hinge tabs on one clamp half were found to be broken. Analysis (visual inspection and microscopic and metallographic examination) supported the conclusion that both hinge tabs on the clamp half fractured in a brittle manner as the result of gross overheating, or burning, during forging. The mechanical properties of the metal, especially toughness and ductility, were greatly reduced by burning. Evidence that burning was confined to the hinge end of the clamp indicated that the metal was overheated before or during the upset forging operation. Recommendations included notifying the supplier of the burned condition on the end of the clamp. The clamps should be macroetched before cadmium plating to detect overheating. The clamps in stock should be inspected to ensure that the metal had not been weakened by overheating during the upset forging operation.

Three samples from a ruptured 316 stainless steel tube were examined. The tube, 114 mm OD, wall thickness 8.00 mm, with 13 mm thick 321 stainless steel fins welded to the outer surface of the tube, was part of a heater through which sour gas, containing methane plus H2S and CO, passed at 1150 psig. The sour gas was heated to 600 deg F by burners playing on the outside of the tube burning “sweet” gas plus air. The inner and outer surfaces of all samples showed evidence of corrosive attack. Electron probe microanalysis showed the corrosion products contained sulfur with iron, together with nickel to a lesser extent. Local thinning, cavitation, and ductile deformation markings associated with the unmatched sample taken from the center of the fire showed the tube ruptured as a result of overheating. Overheating while the temperature recorder was off the chart caused severe loss of tube strength, resulting in ductile rupture. The minimum overheating temperature could be deduced at around 1200 deg F due to the presence of a eutectic observed metallographically within the surface corrosion products.

Several ruptures took place in the front wall tubes of a water tube boiler. Some rupture samples showed ductile failure while others showed brittle failure. Specimens taken from the rupture where a thick edge had been produced, i.e., with little evidence of prior plastic deformation, showed a coarse microstructure indicative of gross overheating. The examination indicated that failure in the main resulted from gross overheating arising from water starvation as could have been due to a number of causes. The ruptures in some tubes were of the type commonly found in overheated tubes, the material being drawn out to a feather edge at the time of rupture. Other ruptures in the same and other tubes were of a more brittle type, this being associated with penetration of material by molten copper derived from scale.

A chain link which was part of the hoisting mechanism of a drop hammer broke after three or four months of service. It was reportedly manufactured of the heat resistant steel 30 Cr-Mo-V 9 (Material No. 1.7707). The fracture of the chain link had a conchoidal structure and ran along the austenitic grain boundaries. Such fractures are characteristic results of strong overheating. The coarse-grained, coarse acicular heat-treated structure of the chain link confirmed overheating. Because temperatures in excess of 1150 deg C are required for the solution of impurities, it is more probable that the real damage was done during the heat-up forging (drop-forging) and could not be removed during heat-treatment.

A gear manufacturer experienced service problems with various gears and pinions that had worn prematurely or had fractured. All gears and pinions were forged from 1.60Mn-5Cr steel and were case hardened by pack carburizing. Gear Failure: One of the gears showed severe wear on the side of the teeth that came into contact with the opposing gear during engagement. The microstructure at the periphery of a worn tooth at its unworn side consisted of coarse acicular martensite with a large percentage of retained austenite. Pinion Failure: The teeth of the pinion exhibited severe spalling; the microstructure at the surface consisted of coarse acicular martensite with retained austenite. Also, a coarse network of precipitated carbide particles showed that the carburization of the case had appreciably exceeded the most favorable carbon content. This evidence supported the following conclusions: 1) High wear rate on the gears was caused by spalling of the coarse-grain surface layer. The underlying cause of the wear was overheating during the carburization. 2) Pinion failure resulted from overheating combined with excessive case carbon content. Thus, no recommendations were made.

A case-hardened sleeve made of C 15 (Material No. 1.0401) was flattened at two opposing sides and had cracked open at these places, the crack initiating at a face plane. The wall of the sleeve was 9 mm thick, but the flat ends were machined down to 5.5 mm from the outside. The customer had specified a 2 mm case depth and a hardness of at least HRC 55 at a depth of 1.5 mm. An etched cross section of the cracked end showed that the case layer had a depth of 2.3 mm, so that the sleeve was almost through-hardened at the flat ends. While the core material with the full wall thickness had the quench structure of low-carbon steel, the structure of the flattened area consisted of coarse acicular martensite with a small amount of pearlite (quench troostite) and ferrite. Therefore the sleeve was overheated and probably quenched directly from case. To prevent damage, it would have been necessary to have a lower case depth, carburize less deeply, and prevent overheating that causes brittleness and leads also to increased case depth, or else use a fine-grained steel of lower hardenability.

Two superheater tubes from a 6.2 MPa (900 psig) boiler failed in service because of creep rupture. One tube was carbon steel and the other was carbon steel welded to ASTM A213 Grade T22 (2.25Cr-1.0Mo) tubing. The failure in the welded tube occurred in the carbon steel section. Portions of the superheater were retubed five years previously with Grade 722 material. The failures indicated that tubes were exposed to long-term overheating conditions. While the carbon steel tube did not experience temperatures above the lower transformation temperature 727 deg C (1340 deg F), the welded tube did experience a temperature peak in excess of 727 deg C (1340 deg F). The long-term overheating conditions could have been the result of excessive heat flux and /or inadequate steam flow. In addition, the entire superheater bank should have been upgraded to Grade 722 material at the time of retubing.

A carbon-molybdenum (ASTM A209 Grade T1) steel superheater tube section in an 8.6 MPa (1250 psig) boiler cracked because of long-term overheating damage that resulted from prolonged exposure to metal temperatures between 482 deg C (900 deg F) and 551 deg C (1025 deg F). The outer diameter of the tube exhibited a crack (fissure) oriented approximately 45 deg to the longitudinal axis and 3.8 cm (1.5 in.) long. The inner diameter surface showed a fissure in the same location and orientation. Microstructure at the failure near the outer diameter surface exhibited evidence of creep cracking and creep void formation at the fissure. A nearly continuous band of graphite nodules was observed on the surface of the fissure. In addition to the graphite band formation, the microstructure near the failure exhibited carbide spheroidization from long-term overheating in all the tube regions examined. It was concluded that preferential nucleations of graphite nodules in a series of bands weakened the steel locally, producing preferred fracture paths. Formation of these graphite bands probably expedited the creep failure of the tube. Future failures may be avoided by using low-alloy steels with chromium additions such as ASTM A213 Grade T11 or T22, which are resistant to graphitization damage.

Two identical “D” tube package boilers, installed at separate plants, experienced a number of tube ruptures after relatively short operating times. The tubes, which are joined by membranes, experienced localized bulging and circumferential cracking along the fireside crown as a result of overheating and thermal fatigue. It was recommended that recent alterations to the steam-drum baffling be remodified to improve circulation in the boiler and prevent further overheating. Several thermocouples were attached to tubes in problem areas of the boiler to monitor the effects of the steam-drum modifications on tube wall temperatures.

This article provides a background of friction-bearing failures due to overheating. The failures of locomotive axles caused by overheated traction-motor support bearings are discussed. The article also describes liquid-metal embrittlement (LME) in steel. It examines the results of various axle studies, with illustrations and concludes with information on the simulation of the LME mechanism.