Study of Prolonged Service Degradation of Superheater Tubes and its Effects on Remaining Life Predictions

This study investigates the degradation of superheater tubes that has been in service for very long time. It also discusses the degradation effects on tube remaining life predictions. The tubes, utilized in this study, belong to an industrial boiler that has been in service for 232 000 h in a petrochemical plant and generate steam at 47 barg and 410 °C. Outcomes of this study will contribute to better understanding and development of scientific procedures to make reasonable estimate of tube remaining life considering the tubes aging while in service. This is beneficial in preventing failures and forced plant shutdown when life is consumed. It also helps industry to avoid capital expenditures on premature replacements of boiler tubes that can still serve longer time. In this study, comparison is made between short-term life predictions and actual properties found after about 26.5 years of service. The tubes condition is assessed by metallographic, mechanical and stress rupture testing. According to the results, it is found that prolonged service degradation has strong effects on remaining life predictions. Both effects that would lead to overestimating or underestimating tube remaining life were found if existing procedures are used without consideration for material degradation during service.

High-Temperature Failures

This chapter compares and contrasts the high-temperature behaviors of metals and composites. It describes the use of creep curves and stress-rupture testing along with the underlying mechanisms in creep deformation and elevated-temperature fracture. It also discusses creep-life prediction and related design methods and some of the factors involved in high-temperature fatigue, including creep-fatigue interaction and thermomechanical damage.

Microstructural and Mechanical Effects of Thermo-Mechanical Processing on ATI 718Plus® Contoured Rings

ATI 718Plus® is a nickel-base superalloy designed to promote resistance and thermal stability at elevated temperatures. Beside these properties, this material presents superior formability during forging operations, making ATI 718Plus® a suitable material for the manufacture of non-rotating and rotating components for jet engine and land-based turbines. Present contribution summarizes main results when several contoured rings were produced by ring-rolling processes considering selected parameters as temperature and deformation ratio. Effect of solution and precipitation heat treatments on ATI 718Plus® microstructure and mechanical properties are also reported. These results include tensile testing at elevated temperature and stress-rupture testing. Microstructural evaluations performed by optical microscopy and electronic microscopy, complement reported results.

Stress-Rupture Behavior of Alloys 230 and 617 for High Temperature Applications

Austenitic Alloys 230 and 617 have been identified to be the two most suitable structural materials for heat exchanger application within the purview of the next generation nuclear plant (NGNP) program. The NGNP program is aimed at developing electricity and hydrogen using heat from very-high-temperature-reactor (VHTR). A maximum operating temperature of 950 °C has been recommended to achieve the highest possible efficiency in both electricity and hydrogen generation. The identification of Alloys 617 and 230 as heat exchanger materials was based on their excellent resistance to high-temperature degradations including creep, stress-rupture, fatigue and tensile deformation. Extensive efforts have been made to evaluate the creep and stress-rupture behavior of both alloys at temperatures relevant to the NGNP application. This paper presents the results of stress-rupture testing involving these alloys as functions of applied stress and temperature. The time to rupture was gradually reduced with simultaneous increase in stress and temperature, leading to a gradual reduction in the Larson-Miller-Parameter (LMP) indicating enhanced rupture tendency.