Studies On Microstructure Evolution And Mechanical Properties For Ultra-supercritical Steam Turbine High/Intermediate Pressure Rotor X12CrMoWVNbN10-1-1 Steel

Ultra-supercritical(USC) high/intermediate pressure(HP/IP) rotor, the key component in 1000 MW USC steam turbine set, services at the steam temperature of/over 600℃ and pressure of/over 30 MPa. This severe working condition requires the extremely excellent properties of materials used for rotors. The material properties are directly connected with microstructure. Therefore, it is necessary and significant to investigate the evolution law of microstructure during heat treatment for establishing reasonable heat treatment process so that the required properties can be achieved in practical engineering application. In this paper, The study of X12CrMoWVNbN10-1-1(X12) steel was carried out by many characterization methods, such as physicochemical phase analyses, X-ray diffraction, electron back scattered diffraction, scanning electron microscopy and transmission electron microscopy, etc. The main conclusions were described as follows:Firstly, the transformation mechanism, characteristics and kinetics of X12 steel was studied during isothermal decomposition of over-cooled austenite at 700 ℃(nose temperature in TTT diagram). The transformed product consisted of ferrite matrix with mainly Cr-rich M23C6 and a small amount of Cr-rich M2 N and Nb-rich MN. The precipitates nucleated along the prior austenite grain boundaries and grew inward of grain as a form of cell, also with the transformation of γâ†'α. Based on the miscrostucture obtained by isothermal transformation for different time, it was austenitized again at 1080℃ for 6h, it was found that the austenite grain size could be refined more effectively with the increase of isothermal time. When the aging time reached 120 h, the austenite grain size was not refined effectively any longer. The mechanism of refining austenite grain size by isothermal transformation was also proposed. It was considered that retained austenite was the key of refining austenite grain size and the critical volume fraction of retained austenite was approximately 3.2%.Secondly, the service environment of high temperature and pressure for USC steam turbine rotor requires that the steel should have a reasonable austenite grain size. In order to understand the growth behavior of austenite grains, their growth and the dissolution of carbonitrides in X12 steel under various austenitizing conditions were investigated. Cr-rich M23C6 and Cr-rich M2 N were dissolved completely after austenitization at 1070℃ for 60 min or at 1200℃ for 15 min, only the Nb-rich MN particles left. The state of dissolution of Nb-rich MN particles was also studied in detail as a function of austenitization holding time(tA) at different temperatures. Initially, the Nb-rich MN particles dissolved with increasing t A at 1070℃. When tA reached over 180 min, the amount of Nb-rich MN remained approximately constant. On the contrary, during the process of austenitization at 1010℃, no Nb-rich MN particles dissolved into matrix. Starting from the fine and uniform grains, abnormal grain growth was observed after austenitizing at 1010℃ for 960 min or at 1070℃ for 120 min. The plot of grain size and Nb-rich MN size against t A at 1070℃ indicated that the critical mean diameter and volume fraction of Nb-rich MN particle for pinning the austenite grain boundaries effectively would be 117 nm and 3.1×10-4 respectively. Finally, it was found that the abnormal grain growth may be responsible for splitting phenomenon of martensite start temperature, instead of concentration gradient in austenite.Thirdly, the effect of the quenching rates on microstructure and impact energy of X12 steel was investigated. The samples were heated to 1080℃ for 16 h and quenched with different rates to room temperature, then tempered at 700℃ for 24 h. Experimental results indicated that the critical quenching rate of suppressing the precipitation of Cr-rich M2 N and Fe-rich M3 C were faster than 1.5℃/min and 600℃/min respectively. And the critical quenching rate for the precipitation of ferrite was in the range of 1~1.5℃/min. The impact energy of the tempered samples with water, air and stove cooling was almost the same, approximately 33 J. However, it decreased to 23.3J when the tempered sample was quenched with a rate of 1.5℃/min. The impact energy dropped further with the decrease of quenching rate. Based on the careful conducted analysis of the microstructure and fractography, the precipitation of Cr-rich M2 N caused by the too low cooling rate would contribute for the decrease of impact energy.Lastly, base on the microstructure obtained by austenitization at 1080℃ for 16 h and cooled in furnace, the influence of tempering temperature and time on the microstructure evolution and the mechanical properties of X12 steel was investigated. The precipitation of Cr-rich M2 N, Cr-rich M7C3 and Nb-rich MN was detected in the samples tempered below 550℃ for 18 h. The formation of Cr-rich M23C6 was identified after tempering at 570℃ for 10 h. The Cr-rich M7C3 carbides were replaced gradually by Cr-rich M23C6 during the tempering over the range of 570℃ and 800℃. The precipitation behavior was discussed in detail from the microchemical data. The sequence of carbides precipitation could be summarized as follows: Fe-rich M3Câ†'Cr-rich M7C3â†'Cr-rich M23C6. A correlation between microstructure and mechanical properties is established. With respect of hardness, there was little precipitates after the tempering for 18 h below 550℃, a mass of alloying elements in the matrix could act as solution strengthening, so that the tempered hardness was approximately 430HV10. Above 550℃, much alloying elements precipitated from the saturated matrix to precipitates, leading to the weakness of solution strengthening. With regard to impact energy, there was the microstress field around Cr-rich M7C3 when tempered for 18 h below 570℃, resulting in low impact energy with approximately 7.5J; In the range of 570~700℃, metastable Cr-rich M7C3 was transformed into stable Cr-rich M23C6 and the the microstress field was released, therefore, the impact energy increased slightly. Above 700℃, the impact energy increased substantially because subgrains were formed resulted from the recovery of martensite matrix.