| The service environment of (ultra) super critical boiler steels is harsh. A series of changes would happen to the microstructure of the above steels due to the long-term service at high temperature and pressure, and deteriorated microstructure will lead to a sharp drop in creep rupture strength and rupture ductility, etc. Once the tube explosion happened, the normal operation of power plant will be affected, bringing significant economic losses and even loss of lives. In order to ensure the long-term, stable and safe operation, the analyses of creep rupture property and phase parameter for boiler steels are very important. There are two main kinds of (ultra) super critical boiler steel: ferritic heat-resistant steel and austenitic heat resistant steel. Investigations were made on the two kinds of steel and the results are as follows:Assessment of creep rupture property Based on the fact that the non-uniqueness of C value in Larson-Miller Parameter (LMP) regarded conventionally as a constant for a certain type of material is actually the main cause of the breakdown of creep strength initiated somewhere in the stress vs.. rupture life plots for SC and USC boiler steels, this paper proposes the following concepts and methods in order to effectively reduce the property overestimation tendency caused by the conventional method. They include the C-value optimization, the multi-C region analysis and the long-term (5×103h?105h) creep rupture property prediction using short-term results (?5×103h). In addition, logarithmic function is used in single temperature creep rupture test to solve the over-estimation problem by the traditional linear function. All the data sets are provided by NIMS and other publications for the related calculations and analyses. The predicted values are in very good accordance with those observed, and also the overestimation tendency can be further reduced for long-term creep rupture strength assessment through the application mentioned above.Phase parameter analysis The effect of content of carbon and niobium on phase stability and creep rupture life and the related mechanism were investigated for TP347HFG steel. Creep rupture tests were carried out under applied stresses of230and150MPa, respectively, at650?. The rupture times showed a significant difference between samples with higher C/lower Nb contents and samples with lower C/higher Nb contents, therefore an optimized ratio of carbon content to niobium content is recommended to this steel to improve its creep rupture time. In addition, the phase parameters of Super304H after solution treatment and creep rupture tests were analyzed to research the relationship between creep rupture property and phase change (Creep rupture tests were carried out under applied stresses of180,170,160,150,130and120MPa, respectively, at700?). A failure analysis was also made on the crack of a Super304H superheater tube served in a power plant with12000h to reveal the reason for crack. EPMA-EDS+MPST (Multi-phase Separation Technology), TEM-EDS and Thermo-calc methods were applied to the above analyses to determine the amount, element composition and partitioning of precipitated phases.With the use of the phase quantification and creep-rupture property assessment methods proposed, the phase stabilities and long-term creep properties of advanced boiler steels can be accurately characterized, which will promote the localization of this class of steels. |
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