This article reviews the applied aspects of creep and stress-rupture failures. It discusses the microstructural changes and bulk mechanical behavior of classical and nonclassical creep behavior. The article provides a description of microstructural changes and damage from creep deformation, including stress-rupture fractures. It also describes metallurgical instabilities, such as aging and carbide reactions, and evaluates the complex effects of creep-fatigue interaction. The article concludes with a discussion on thermal fatigue and creep fatigue failures.

The principal types of elevated-temperature mechanical failure are creep and stress rupture, stress relaxation, low- and high-cycle fatigue, thermal fatigue, tension overload, and combinations of these, as modified by environment. This article briefly reviews the applied aspects of creep-related failures, where the mechanical strength of a material becomes limited by creep rather than by its elastic limit. The majority of information provided is applicable to metallic materials, and only general information regarding creep-related failures of polymeric materials is given. The article also reviews various factors related to creep behavior and associated failures of materials used in high-temperature applications. The complex effects of creep-fatigue interaction, microstructural changes during classical creep, and nondestructive creep damage assessment of metallic materials are also discussed. The article describes the fracture characteristics of stress rupture. Information on various metallurgical instabilities is also provided. The article presents a description of thermal-fatigue cracks, as distinguished from creep-rupture cracks.

A pressure vessel failed causing an external fire on a nine-story coke gasifier in a refinery power plant. An investigation revealed that the failure began as cracking in the gasifier internals, which led to bulging and stress rupture of the vessel shell, and the escape of hot syngas, setting off the fire. The failure mechanisms include stress relaxation cracking of a large diameter Incoloy 825 tube, stress rupture of a 4.65 in. thick chromium steel shell wall, and the oxidation of chromium steel exposed to hot syngas. The gasifier process and operating conditions that contributed to the high-temperature degradation were also analyzed and are discussed.

Airfoil-shape impingement cooling tubes were fabricated of 0.25 mm (0.010 in.) thick Hastelloy X sheet stock, then pulse-laser-beam butt welded to cast Hastelloy X base plugs. Each weldment was then inserted through the base of a hollow cast turbine blade for a jet engine. The weldments were finally secured to the bases of the turbine blades by a brazing operation. One of the laser beam attachment welds broke after a 28-h engine test run. Exposure of the fracture surface for study under the electron microscope revealed the joint had broken in stress rupture. Failure was caused by tensile overload from stress concentration at the root of the laser beam weld, which was caused by the sharp notch created by the lack of full weld penetration. Radiographic inspection of all cooling-tube weldments was made mandatory, with rejection stipulated for joints containing subsurface weld-root notches. In addition, all turbine blades containing cooling-tube weldments were reprocessed by back-brazing. Back brazed turbine blades were reinstalled in the engine and withstood the full 150-h model test run without incident.

Failure occurred in a steel superheater tube in a power plant. The tube was specified as ASTM A 213 grade T 22, and the reported operating conditions were 13 MPa (1900 psi) at 482 deg C (900 deg F). The tube carried superheated steam and was coal fired. Investigation (visual inspection, 2% nital etched 297x images, chemical analysis, and SEM fractographs) supported the conclusion that the superheater tube failed as a result of long-term overheating. Substantial creep damage reduced the strength of the tube to the point that overload failure occurred. No recommendations were made.

A carbon steel furnace tube which should have given good service for ten years ruptured after one year. The tube showed obvious swelling at the point of rupture, and the bulged surface of the tube was oxidized at a temperature far above the design temperature. There was little or no loss in wall thickness due to corrosion or scaling, and the tube wall was thinned to a knife edge at the rupture. Metallographic examination showed the condition of the material was satisfactory. The failure was mechanical in nature, typical of short time creep rupture. The localized oxidation indicated improper furnace operation or blockage of the tube. The furnace was checked and found to have a burner tip out of order. After the tip was repaired, localized overheating was minimized and further premature failures did not occur.

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 main steam pipe was found to be leaking due to a large circumferential crack in a pipe-to-fitting weld in one of two steam leads between the superheater outlet nozzles and the turbine stop valves (a line made of SA335-P22 material). The main crack surface was found to be rough, oriented about normal to the outside surface, and had a dark oxidized appearance. The cracking was found to be predominantly intergranular. Distinct shiny bands that etched slower than the remainder of the sample at the top of each individual weld bead were revealed by microscopic examination. These bands were found contain small cracks and microvoids. A mechanism of intergranular creep rupture at elevated temperature was identified as a result of a series of stress-rupture and tensile tests. It was revealed by the crack shape that cracking initiated on the pipe exterior, then propagated inward and in the circumferential direction in response to a bending moment load. It was concluded that the primary cause of failure was the occurrence of bending stresses that exceeded the stress levels predicted by design calculations and that were higher than the maximum allowable primary membrane stress.

Leakage was detected at the welds between stiffening plates and the pipe in a transfer line carrying butane and related petrochemical compounds. The line and reinforcing rings were of AISI 316 stainless steel, the pipe being of 508 mm diam and 6.25 mm wall thickness. The design temperature and pressure were 621 deg C and 2.75 kPa, respectively, while the operating conditions were 579 deg C and 1.03 kPa. The line was insulated. Failure occurred after approximately 90,000 h of operation, shutdowns being approximately two per annum. The cracking occurred at the toe of welds between the plates and the pipe. The creep damage failure was attributed to repeated relaxation cycles of very high thermal stresses of resulting from the periodic shutdowns, temperature fluctuations during service, or both. This failure emphasized the information available from an evaluation of the operative creep mechanism, namely grain boundary sliding, relating to the periodic nature of the loading, with high residual stresses being present.

Rupture occurred at a bend in a superheated steam transfer line between a header and a desuperheater of a boiler producing 230 t/h of steam at 540 deg C and 118 kPa. The boiler had operated for 77,000 h. Rupture occurred along the outer bend radius of the 168 mm diam tube, this being of 1 Cr, 0.5 Mo steel with a wall thickness of 14 mm. The design temperature of this tube was 490 deg C, but there is evidence that it was operating at a temperature much above 500 deg C. Metallographic analysis disclosed an advanced stage of creep damage accumulation in the form of local cracks, microcracks, and aligned damage centers which showed up as voids upon repeated polish-etch cycles. Because of the local nature of creep damage that can occur, any inspection that involves in situ metallography must be conducted at exactly the right or critical position or the presence of damage may not be detected.

After some 87,000 h of operation, failure took place in the bend of a steam pipe connecting a coil of the third superheater of a steam generator to the outlet steam collector. The unit operated at 538 deg C and 135 kPa, producing 400 t/h of steam. The 2.25Cr-1Mo steel pipe in which failure took place was 50.8 mm in diam with a nominal wall thickness of 8 mm. It connected to the AISI 321 superheater tube by means of a butt weld and was one of 46 such parallel connecting tubes. The Cr-Mo tubing was situated outside the heat transfer zone of the superheater. The overall sequence of failure involved overheating of the Cr-Mo outlet tubes, heavy oxidation, oxide cracking on thermal cycling, thermal fatigue cracking plus oxidation, creep-controlled crack growth, and rapid plastic deformation and rupture. This failure was indicative of excess temperature of the steam coming from the heat transfer zone of the coil. It showed that many damage mechanisms may combine in the transition from fracture initiation to final failure. The presence of grain boundary sliding as an indication of creep damage was useful in the characterization of the stress level as high and showed that the process of creep was not operative throughout the life of the equipment.

This article describes the visual, fractographic, and metallographic evidence typically encountered when analyzing stress rupture of turbine airfoils. Stress-rupture fractures are generally heavily oxidized, tend to be rough in texture, and are primarily intergranular and/or interdendritic in appearance compared to smoother, transgranular fatigue type fractures. Often, gross plastic yielding is visible on a macroscopic scale. Commonly observed microstructural characteristics include creep voiding along grain boundaries and/or interdendritic regions. Internal voids can also nucleate at carbides and other microconstituents, especially in single crystal castings that do not possess grain boundaries.

A tube in a radiant superheater, the boiler of which is coal fired, failed by creep after 17 years of service. The failed tube was specified to be made of ASME SA-213, grade T-22. Measurable swelling of the tube diameter by about 2.4 mm and tube wastage caused by corrosion or erosion were observed. Log stress versus Larson-Miller Parameter (LMP) plots were produced to assess the remaining life of the superheater. It was revealed that the estimated operating temperature of 1060 deg F was higher than the estimated design temperature of 1000 deg F and that the tube wastage had increased the actual operating stress. Tube wastage and high operating temperatures hastened the failure. A better understanding of the material condition of this superheater was recommended to verify all the suspect hot tubes.

Some examples of equipment failures involving high temperature operation are presented. They include some steam generator superheater components and a pump shaft that should not have been at high temperature. Metallographic analysis is used to determine the causes of failure in each case.

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.

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.

Tube 3 from a utility boiler in service for 13 years under operating conditions of 540 deg C (1005 deg F), 13.7 MPa (1990 psi) and 1,189,320 kg/h (2,662,000 lb/h) incurred a longitudinal rupture near its 90 deg bend while Tube 4 from the same boiler exhibited deformation near its bend. Metallographic examination revealed creep voids near the rupture in addition to graphite nodules. Exposure of the SA209 Grade T1A steel tubing to a calculated mean operating temperature of 530 deg C (983 deg F) for the 13 years resulted in graphitization and subsequent creep failure in Tube 3. The deformation in Tube 4 was likely the result of steam washing from the Tube 3 failure. Graphitization observed remote from the rupture in Tube 3 and in Tube 4 indicated that adjacent tubing also was susceptible to creep failure. In-situ metallography identified other graphitized tubes to be replaced during a scheduled outage.

Silver solid-state bonded components containing uranium failed under zero or low applied load several years after manufacture. The final operation in their manufacture was a proof loading that applied a sustained tensile stress to the bond, which all components passed. The components comprised circular cylinders fabricated by plating a thin layer of silver on each of the contact surfaces (uranium and stainless steel) and pressing the parts together at elevated temperature to solid-state bond the two silver surfaces. The manufacturing process produced a high level of residual stress at the bond. The failures appeared to be predominantly located between the silver layer and the uranium substrate. Normal fracture location of specimens taken from similar components was at the silver/silver bond interface. Laboratory testing revealed that the uranium/silver joint was susceptible to premature failure by stress-corrosion cracking under sustained loading if the atmosphere was saturated with water vapor.

Two steam-methane reformer furnaces were subjected to short-time heat excursions because of a power outage, which resulted in creep bulging in the Incoloy 800 outlet pigtails, requiring complete replacement. Each furnace had three cells, consisting of 112 vertical tubes per cell, each filled with a nickel catalyst. The tubes were centrifugally cast from ASTM A297, grade HK-40 (Fe-25Cr-20Ni-0.40C), heat-resistant alloy. The tube was concluded after metallurgical inspection to have failed from creep rupture (i.e., stress rupture). A project for detecting midwall creep fissuring was instigated as a result of the failure. It was concluded after laboratory radiography and macroexamination that if the fissure were large enough to show on a radiograph, either with or without the catalyst, the tube could be expected to fail within one year. The set up for in-service radiograph examination was described. The tubes of the furnace were radiographed during shut down and twenty-four tubes in the first furnace and 53 in the second furnace showed significant fissuring. Although, radiography was concluded to be a practical technique to provide advance information, it was limited to detecting fissures caused by third-stage creep in tubes because of the cost involved in removing the catalysts.

As the result of a leak detected in a plate-formed header at PENELEC'S Shawville Unit No. 3, an extensive failure investigation was initiated to determine the origin of cracking visible along the longitudinal weld seam. Fabricated from SA387-D material and designed for a superheater outlet temperature of 566 deg C, the 11.4 cm thick header had operated for approximately 187,000 h at the time of the failure. Discussion focuses on the results of a metallographic examination of boat samples removed from the longitudinal seam weldment in the vicinity of the failure and at other areas of the header where peak temperatures were believed to have been reached. The long-term mechanical properties of the service-exposed base metal and creep-damaged weld metal were determined by creep testing. Based on the utility's decision to replace the header within one to three years, an isostress overtemperature lead specimen approach was taken, whereby failure of a test specimen in the laboratory would precede failures in the plant. These tests revealed approximately a 2:1 difference in life for the base metal as compared to weld metal.