This chapter deals with the change of the structure and mechanical properties of the austenitic steels grades TP347HFG, Super 304H and HR3C as well as their welded joints after the exposure at temperatures corresponding to the ultra-super critical (USC) and advanced ultra-super critical (A-USC) parameters. Several tube bend radii were investigated in order to understand relation between cold work plastic deformation, mechanical properties and the structural changes with special attention to the generation of sigma phase during long-term high temperature exposure. The effect of post bend solution annealing work on the material properties and structural changes was studied, too, using small punch test (SPT) and miniaturized tensile tests taken both from straight part and extrados of bends under investigation. Creep properties of either base metal either welded joints were tested and evaluated using standard creep specimens at temperatures 650, 700 and 750°C. The obtained results confirmed that the mechanical properties and the structure were significantly influenced even after relatively short-time exposure at elevated temperature. The results of experiments were compared with other results obtained from analyses of tubes after 100,000 h of exposure in a USC block where extensive precipitation of sigma phase was also identified.

The paper deals with the change of the structure and mechanical properties of the austenitic steels after the exposure at the critical temperature. The effects of the bend radius of tubes including effect of solution annealing on the mechanical properties and the structure were studied. The mechanical properties were studied using small punch test (SPT) and miniaturized tensile tests. From the results can be concluded that the mechanical properties and the structure have been influenced due to one year exposure at the working temperature significantly. Various amounts of σ-phase were found in the pulled part of the bend side of the tubes even after only thermal exposure without any loading. The effect of the heat treatment and the bend radius on the mechanical properties and the structure was also evident. Drop in fracture energy due to the presence of the σ-phase in the structure was clearly detected from force - displacement record of the small punch test. Keywords: Austenitic steels, bends, σ-phase, small punch test

The paper deals with the change in the structure and mechanical properties of the austenitic-steel grades TP347 HFG, Super 304H and HR3C, representing the most frequently used materials for superheaters and preheaters in the ultra-supercritical power industry. The effect of plastic deformation applied during the bending of tubes with different bend radii on the mechanical properties and structure was studied on material samples with or without post-bend heat treatment (PBHT). The mechanical properties of tubes with a small wall thickness were studied using the small-punch test (SPT) and miniaturized tensile-test specimens. From the results obtained, it can be concluded that the mechanical properties and the structure were significantly influenced after a one-year exposure to the working temperature. Various amounts of the s-phase were found in the extrados parts of the bends even in the cases when the bends were only exposed to high temperatures without any loading. The effects of the PBHT and the bend radius on the mechanical properties and the structure were also evident. A drop in the fracture energy due to the presence of the s-phase in the structure was clearly detected from the force-displacement record of the SPT.Avtorji v~lanku obravnavajo strukturne in mehanske spremembe austenitnih jekel vrste TP347 HFG, Super 304H in HR3C, ki se najpogosteje uporabljajo za mo~no obremenjene dele visokotemperaturnih izmenjevalnikov toplote v zahtevni industriji proizvodnje elektri~ne energije. Raziskovali so vpliv plasti~ne deformacije zaradi hladnega krivljenja cevi razli~nih premerov na mehanske lastnosti pred krivljenjem in/ali po toplotni obdelavi (PBHT; angl.: post-bend heat treatment). Mehanske lastnosti cevi z manj{o debelino stene so ugotavljali s pomo~jo preizkusov lezenja z vtiskovanjem trna (SPT; angl.: small-punch test) in nateznim preizkusom na miniaturnih preizku{ancih. Na osnovi dobljenih rezultatov ugotavljajo, da so se mehanske lastnosti in struktura jekel mo~no spremenile`e po enoletni izpostavitvi materialov na delovno temperaturo. V preizku{ancih so na{li razli~ne vsebnosti s-faze celo v primerih, ko so bili ukrivljeni (deformirani) deli cevi neobremenjeni, a izpostavljeni visokim temperaturam. O~iten je bil prav tako vpliv PBHT in premera kolen (ukrivljenosti cevi) na mehanske lastnosti in strukturo preiskovanih jekel. Preko zapisa sila-pomik med SPT preizkusom so jasno zaznali zmanj{anje energije loma zaradi prisotnosti s-faze v strukturi jekla. Klju~ne besede: austenitna nerjavna jekla, hladno ukrivljene cevi, izlo~anje s-faze, tla~ni preizkus lezenja z majhnim trnom

Low-alloy steel 16Mo3 is often used for tubes and other components of the pressure system of a boiler operating at elevated temperature (up to 530 °C). Long-term exposure of low-alloy steel components at elevated temperatures inevitably result in some kind of structural degradation; for example, creep cavitation, carbide coarsening and/or spheroidization, and, less commonly, also graphitization. Graphitization can be detected mainly in carbon steels, but also in 16Mo3 steel where the ferritic matrix is strengthened by Mo in the solid solution. The absence of strong carbide forming elements (typically Cr, V, Nb) is the principal reason why these steels are susceptible to cementite decomposition and graphitization much more than the other creep resistant steels.The paper describes microstructure and material properties of superheater 16Mo3 tube, in which graphite was found after 70.000 hours of exposure at 445 °C. The actual material properties were compared to another tube of the same material delivered in the as-received state. The comparison was based on evaluation and testing of mechanical strength, fracture properties and microstructure. Creep resistance of the steel was studied using small punch creep tests (SPCT).

Recent increase in output of nuclear power plant has been attained by enlargement of most major components such as pressure vessels. Such large components have almost reached a size limit from the points of manufacturing capacity. In order to solve this problem, it must be beneficial to apply by use of material of higher strength, which brings reduction of pressure vessel thickness and weight A508 Grade 4N is a high nickel pressure vessel steel with relatively high yield strength and a relatively low transition temperature. A study was undertaken to evaluate conventional as well as unconventional material properties of forging made of in our institute. This paper describes results of mechanical properties, structure and fracture behavior including nil-ductility transition temperature.