Study On Microstructure And Mechanical Properties Of A Modified HR3C Austenitic Heat Resistant Steel

Based on the development trend of 650? ultra supercritical(USC)power plants in China and the demand for further localization of HR3C steel used for superheater/reheater in USC plants,the microstructure evolution and mechanical properties of a modified HR3C austenitic heat-resistant steel,through solution treatment,long-term thermal exposure test,"Charpy"impact test and creep test,using optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM)and X-ray energy dispersive spectrometer(EDS),were studied in this paper.The mechanism of strengthening and toughening of the modified HR3C steel was discussed and the relationship between microstructure and mechanical properties was established.Then,by comparing with a commercial HR3C steel,the mechanism of Nb in the process of microstructure evolution,strengthening and toughening were studied.The main achievements are shown as below:The modified HR3C steel is mainly composed of austenite matrix and primary Nb-rich MX-phase after solution treatment.During the thermal exposure at 650?,the chain-like Z-phase and the dust-like secondary MX-phase were precipitated in intracrystalline,and grew slowly with the prolongation of the thermal exposure time.The bar-like M23C6 carbide and the irregular Laves-phase were observed in intracrystalline after exposure for 10000 h.In addition,discontinuous M23C6 carbide precipitated along grain boundaries(GBs),and grew slowly and spheroidized with the prolongation of the thermal exposure time.The modified HR3C steel shows excellent microstructural stability.The Nb inhibits the precipitation of M23C6 carbide by promoting the formation of primary MX-phase,which consumes a large amount of C.In addition,the Nb increases the nucleation rate of the fine Z-phase and the secondary MX-phase in intracrystalline.The main fracture mode of the modified HR3C steel at room temperature(RT)after long-term thermal exposure is the transgranular fracture,and this indicates that the grain boundary strength is greater than the intragranular strength during the thermal exposure process Due to the slow growth of the precipitaties,the impact toughness decreases with the prolongation of the thermal exposure time.The higher content of Nb in the modified HR3C steel inhibits the precipitation and growth of M23C6 phase,and increases the grain boundary bonding strength of steels during long-term thermal exposure.The impact toughness of the modified HR3C steel is 3 times higher than that of the commercial HR3C steel after thermal exposure for 10000 h.The plastic deformation of the modified HR3C steel is through the dislocation slip at 650?/250 MPa,and it is occurred by the dislocation climb at 750?/130 MPa.A large number of fine dispersed Z-phases and secondary MX-phases in intracrystalline and the discontinuous M23C6 carbides along the GBs all effectively improve the creep strength of the modified HR3C steel.Based on the Larson-Miller parameter model,the creep strength of the modified HR3C steel after service at 700? for 105 h is about 35 MPa higher than that of the commercial HR3C steel,which meets the construction requirements of the 650?-class power plants.