Damage mechanisms of a steam inter-heater output tube after 200,000 hours of exploitation

Heat-resistant steel grade 12H1MF has been used for many years in domestic thermal power plants for the production of elements within the thermal energy system. Steel 12H1MF is used for manufacturing pressure vessels, which are exposed to the simultaneous effect of elevated temperature and internal and external pressure. In this study, an experimental investigation was conducted, including visual and dimensional control, hardness testing, chemical analysis, and microstructural characterization of a U-bend tube made of 12H1MF steel. The U-bend tube was formed by cold bending a steam inter-heater output tube after 200,000 hours of exploitation at a temperature of 540 °C and a maximum working pressure of 4.6 MPa. Based on the obtained experimental results, it can be confirmed that the U-bend tube was damaged due to the effect of gas corrosion, as well as that there was decarburization of the surface layer of the material on the outer surface of the tube exposed to the impact of flue gases, while the degradation of the material did not occur despite long-term exploitation. The examination determined that there was no material creep, which confirms that this tube's integrity was not damaged.