Lucie Pilsová

This paper describes a metallographic preparation of the heat-exchanger austenitic stainless steels, i.e., SUPER 304H, HR3C and Sanicro 25. The as-received materials delivered after the solution annealing are very resistant to the chemical etchants. Therefore, it is more efficient to optimize the parameters of the electrolytic etching in the acidic or basic electrolytes. After the materials’ operation under elevated temperatures (600–750 °C), the microstructure is easier to etch due to the reduction of corrosion resistance. For the following analyses, it is necessary to think of the etching purpose in advance. For the grain size determination, it is sufficient to use electrolytic etching in the oxalic acid solution or chemical swabbing in the combination of 15 ml HCl, 10 ml acetic acid and 5 ml HNO₃. For the image analysis of the intermetallics, the previous methods can be too invasive and give skewed results. In many cases, this leads to the overetching and thus removal of the observed particles (intermetallics, carbides, etc.). It is suitable to use selective etching methods or only polishing for a more precise evaluation, e.g., electron microscopy.

This paper describes the influence of technological treatments (i.e., bending or welding) on the structural stability of SUPER304H austenitic steel used in reheaters and superheaters in fossil fuel power plants. Although the worldwide trend is transitioning to green power sources, the lifetime of existing power plants has to be prolonged until the transition is complete. Experimental material was tested in as-received state (straight tubes), bends, and homogeneous weld joints. Part of the specimens was solution-annealed after the technological operation. Afterwards, all the samples were thermally aged in furnace (650, 675 and 700 °C) for 7560–20,000 h. For comparison, bent specimens were placed at experimental sites on an operating powerplant for 10,000+ h. The long-term aging causes the formation of Cr-based carbides on the grain boundaries along with the Fe-Cr sigma phase. Combination of elevated temperature and residual stress accelerates formation of the sigma phase. This can be prevented by solution-annealing after bending. Mechanical properties were evaluated by Vickers hardness and tensile tests. The microstructure was observed using light optical microscopy (LOM) and scanning electron microscopy (SEM) with the energy-dispersive X-ray detector (EDXS). Electron backscatter diffraction (EBSD) and X-ray powder diffraction (XRPD) were used to characterize the brittle phases.