Aging Damages And Remaining Life Predictions Of The As-serviced Heat-resistant Steels

With rapid development of thermal power generation technology and increase ofrunning parameters of thermal power generation units, new-type austenitic and ferriticheat resistant steels have been widely employed. Performances of these materials havegreat influences on running efficiencies and reliabilities of these power generationunits. Generally, structural damages and mechanical property degradations ofheat-resistant steel tubes in ultra-supercritical (USC) units are simulatively investigatedin laboratory through an artificial aging method. However, studies on similarity of thestructural damage of the as-serviced steel tube and that of the as-aged one with anidentical Larson-Miller (P) parameter are still limited. Therefore, comparative studieson these two kinds of steel tubes with the same P parameter are necessary. What’s more,based on a large number of experimental data, relationships between hardness and Pparameters of heat resistant steels can be developed. The study is valuable to therunning state evaluation of the as-serviced heat resistant steels in the USC powergeneration units.Microstructures and mechanical properties of the HR3C, Super304H steel tubesserviced for20000h at590℃in the USC units and those of the same steel tubes withthe identical P parameter were investigated after artificial aging at650℃for1100h inorder to identify whether the structural damages of the as-serviced HR3C, Super304Hsteel tubes and those of the as-aged HR3C, Super304H steel tubes are similar or not.Artificial aging experiments of the HR3C and T92steels, two typical heat resistantsteels of the USC units, were carried out at temperature range from650℃to700℃toinvestigate microstructure evolutions and hardness changes of the steels during aging,respectively. Moreover, relationship between the hardness of the each steel and the Pparameter was mathematically modeled and the reliabilities of the math models wereverified.Microstructures and mechanical properties of the inner sides of the HR3C,Super304H steels are approximately equal to those of the as-aged tubes; however, thestructural damages of the outer sides are severer than those of the as-aged tubes,leading to the reduction of their mechanical properties. Microstructure and mechanicalproperty mismatches between the inner and outer sides of the as-serviced steel tubesresult from their different running environments. Therefore, the artificial aging methodcan be conditionally used to simulate the structural damage of the steel tube used in the USC units, based on the same P parameter.The hardness values of the HR3C and the T92steels both decrease dramatically withthe aging time. At a given aging temperature, the hardness of HR3C increases and thendecreases, but that of T92decreases monotonously. Finally, the hardness values ofHR3C and T92both turn to be constant, as the aging time prolonged. Disappearance ofthe twin grain boundaries and precipitation and growth of the second phases along theaustenitic grain boundaries and in the austenitic grains are two main reasons of thechange of the hardness values of the T92and HR3C steels during the high-temperatureaging. Mathematical models of the relationships between the hardness values of T92and HR3C and the P parameter were obtained via a linear fitting method, respectively.Under the condition of the same P parameter, the calculated hardness values and thetested ones are coincided with each other very well. These mathematical models arelikely employed to predict the service lifes of those heat resistant steel tubes.