Studies On High-temperature Aging Of The Super304H And HR3C Heat Resistant Steels

Currently, due to the rapid development of the thermal power generationtechnology, the running parameters of the thermal power generation unit areenhanced. Therefore, some new-type austenitic heat-resistant steels, such asSuper304H and HR3C have been widely used in the supercritical （SC） andultra-supercritical （USC） units. Performance of these materials in the USC unit hasgreat effects on the running efficiency and reliability of the power generation units.It is necessary to study the changes of the microstructures and mechanicalproperties, plastic deformation behaviors and failure mechanisms of these steels athigh temperature, using the laboratorial simulation method. The results are helpfulto the running safety evaluation of the USC units.In this paper, the Super304H and HR3C heat-resistant steels were isothermallyannealed at650℃for times till3000h. Microstructures and room-temperaturemechanical properties, high-temperature deformation behaviors and fracturemechanisms of the aged steels were studied, respectively. Finally, the stresstri-axiality theory was employed to explain these phenomena above.The as-supplied Super304H steel is of single austenitic state. After aging, exceptthe austenitic phase, Cu-rich ε-phase, Nb（C, N） and M₇C₃are all formed. In theearly aging stage, lots of M₇C₃particles precipitate along the austenitic grainboundaries, leading to the increase of the tensile strength and hardness and thedecrease of the impact toughness of the steel. As the aging time prolonged, theM₇C₃particles gradually coarsen and continuously distribute along the austeniticgrain boundaries, leading to the decrease of the mechanical properties of the steel.During high-temperature aging, continuous precipitation and dispersive distributionof the Cu-rich ε-phases and Nb（C, N） are beneficial to the thermal stability of theSuper304H steel. As for the high-temperature mechanical properties of the agedsteel, in the initial ageing stage （<300h）, the strength increases, while the plasticitydecreases sharply. When the steel aged for times from300h to500h, the M₇C₃particles gradually coarsen, leading to the decrease of both strength and plasticityof the aged steel. As the ageing time prolonged, both strength and plasticity of theaged steel gradually tends to be stable. High-temperature tensile fracture of theaged steel is in a shearing fracture mode. The as-supplied HR3C steel is mainly composed of the austenitic matrix with abit spherical MX-phase precipitates embedded in them. The twin sub-structures inthe austenitic grains are apparent. After long-term aging, the coherent twinsdisappear gradually, the M23C6carbides precipitate along the grain and coherentgrain boundary, and the MX-type carbonitrides precipitate in the austenitic grains.Precipitation strengthening effect of the grain/twin-grain boundaries and thedispersion strengthening effect of the second-phase particles have great effects onthe mechanical properties and fracture mechanisms the aged steel. As the agingtime increased, the microstructures and mechanical properties of the steel tend tobe constant, indicating good thermal stability of the HR3C steel at elevatedtemperature. In the initial annealing stage （<500h）, the high-temperaturemechanical properties of the HR3C steels are nearly unchangeable. As increasingthe annealing time from500h to1000h, they increase rapidly, however, theplasticity of the aged steel decreases continuously. On the fracture surface, thereare a large amount of shear zones, having a plastic deformation orientation of45°.In the final annealing stage （>1000h）, the microstructure and mechanicalproperties of the aged steel is not changed with the annealing time. Stresstri-axiality theory is successfully used to explain the high-temperature deformationbehaviors of the aged Super304H and HR3C steels.