Abstract
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.
Highlights
IntroductionThe operating conditions in the most exposed parts of the ultra-supercritical boilers are reaching the limits of the steel’s capabilities as a working part, even when speaking of high alloyed heat-resistant steels
The operating conditions in the most exposed parts of the ultra-supercritical boilers are reaching the limits of the steel’s capabilities as a working part, even when speaking of high alloyed heat-resistant steels. It applies to the SUPER304H steel, which belongs to the complex-alloyed austenitic steels
This steel was chosen as a material for the renovation of reheaters and superheaters in fossil fuel power plants and for the construction of new ones
Summary
The operating conditions in the most exposed parts of the ultra-supercritical boilers are reaching the limits of the steel’s capabilities as a working part, even when speaking of high alloyed heat-resistant steels. It applies to the SUPER304H steel, which belongs to the complex-alloyed austenitic steels This steel was chosen as a material for the renovation of reheaters and superheaters in fossil fuel power plants and for the construction of new ones. The arrangement of the superheater requires a lot of cold bending associated with large deformations [1] This leads to a change in the mechanical properties; it is not clear what impact will have the previous deformation on the long-term operation of boiler. In the case of welding, as reported in [2], if the correct filler material is not chosen, it can cause the formation of delta ferrite It can be a suitable nucleation site for undesirable brittle phases after long term operation [3]
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