| High temperature gas-cooled reactors pebble-bed modular(HTR-PM), as an optimal technology of the fourth-generation nuclear energy system, satisfies the requirements such as security, economy and sustainability for nuclear power plants. As the first HTR-PM commercial demonstration project in China, Huaneng Shandong Shidao Bay Nuclear Power Plant was got into construction in 2007. 12Cr2Mo1 R steel was selected as the main material for the reactor internals that will normally service at 375°C but may occasionally withstand a temperature as high as 500°C in case of accidents. Therefore, the employed 12Cr2Mo1 R steel is required to possess a sufficiently high heat-resistantance and creep rupture strength. In this dissertation, 12Cr2Mo1 R steel ingots were hot rolled to 135 mm plates and normalized and tempered following the optimized technology(denoted as NT plate). Some pieces of the NT plates were further subjected to simulated post weld heat treatment(SPWHT plate). The microstructure, tensile performance, and fracture toughness at different temperature(room temperature, 375°C, and 500°C) of the two types of plates, as well as the creep rupture strength of the SPWHT plate at 500°C and the microstructure of the post-rupture specimens were examined. The main achievements are summarized as below:The microstructures of 12Cr2Mo1 R steel under different states are dominated by bainites. Calculation of the formation free energy of carbides that may exist in the steel indicates found that the order of the carbide stability from low to high at room temperature is: Fe3 C, Mo2 C, Cr7C3, Cr23C6, Fe3Mo3 C, MoC and Cr3C2. Experimental examination of the steel plates reveals that the bainites mainly consist of four types of carbides, namely M3 C, M2 C, M7C3 and M23C6, where M refers to Fe, Cr, Mo, Mn, or their combinations, whether the steel is in the NT or SPWHT steel. Comparing the mass fractions of carbides in the two types of plates, the mass fraction of M3 C in the steel reduces due to the SPWHT while those of M2 C, M7C3 and M23C6 increase. Specially, the mass fraction of M23C6 increases considerably, by about two times. That leads to an evident increase in the total mass fraction of carbides in the steel.The present NT and SPWHT 12Cr2Mo1 R steels possess excellent mechanical properties. The room-temperature tensile strength and yield strength of the NT steel reach 620.0 MPa and 482.5 MPa respectively. Even undergone the SPWHT, the two strengths of the steel are still as high as 588.5 MPa and 451.1 MPa, respectively, which are still higher than the corresponding properties of the NT 12Cr2Mo1 R steel reported previously. The 12Cr2Mo1 R steel exhibits the blue brittleness. The temperature range for the blue brittleness to occur is about 200-400°C that is still slightly higher than previously reported.Creep rupture test was performed at 500°C for the SPWHT 12Cr2Mo1 R steel. As a result, the weight percentages of M23C6 and M2 C in the post-rupture specimens increase while those of M3 C and M7C3 decrease on the whole as the rupture time increases. The deformation in the creep rupture test promotes the precipitation of carbides and decreases the average sizes of carbides.From the creep rupture test performed at 500°C for the SPWHT 12Cr2Mo1 R steel, the service life of the reactor internals made of the steel in the HTR-PM demonstration plant was estimated to be 4062.6 years under the normal working condition(375°C, 161.2 MPa). It is much longer than the designed 60 years life of the reactor. Even when the temperature is elevated up to 500°C owing to a nuclear accident, the reactor internals can still safely serve for 6.7 years.The fracture toughness J0.2BL of the NT and SPWHT 12Cr2Mo1 R steels under the working temperature(375°C) were determined by fracture toughness test to be 356 kJ/m2 and 336kJ/m2, or 270 MPa·m1/2 and 262 MPa·m1/2 in terms of KIC, respectively. As temperature is raised to 500°C, the fracture toughness increases. |
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