STRUCTURE AND MECHANICAL PROPERTIES OF AUSTENITIC STEELS AFFECTED BY SIGMA-PHASE PRECIPITATION DUE TO EXPOSURE TO HIGH TEMPERATURES

摘要

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 sigma-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 sigma-phase in the structure was clearly detected from the force-displacement record of the SPT.