淬火温度对超超临界火电站用COST-FB2钢显微组织和室温强度的影响

Effect of Quenching Temperature on Microstructures and Room Temperature Strength of COST-FB2 Steel Used in Ultra-supercritical Coal-fired Power Plants

COST-FB2钢因具有优越的高温持久性能而被广泛应用在620℃超超临界火电站机汽轮机转子上。本工作对取自国内首支自主研发制备的20 t级COST-FB2钢转子大锻件本体的试样进行了不同温度淬火和两次回火的热处理,并测试了其力学性能,研究了淬火温度对COST-FB2钢微观组织的影响。结果表明:在1000~1150℃内,随着淬火温度升高,COST-FB2钢的强度和硬度不断增加,塑性整体变化不大,冲击功由10 J增大至23.5 J后又开始减小;COST-FB2钢的平均晶粒尺寸由65.5μm增加到212.1μm,马氏体板条块宽度由4.75μm增加至6.06μm,位错密度由4.8×10^(9) cm^(-2)增加至11.8×10^(9) cm^(-2)。通过强化增量计算推断析出强化和位错强化的增量是COST-FB2钢回火后室温屈服强度增加的主要原因;将晶界强化和马氏体板条块界强化分别与基体强度、析出强化和位错强化增量之和叠加,得出马氏体板条块界强化增量叠加值更接近试验屈服强度值。该研究结果能够为COST-FB2钢大锻件整体热处理提供参考,助力大锻件的国产化。

COST-FB2 steel is commonly used in the turbine rotors of ultra-supercritical power plants owing to its excellent high temperature creep strength.The samples used in this work were cut from the body area of a 20-ton of COST-FB2 steel forging rotor that China independently developed.Quenching at different temperatures and two-step tempering were realized for the heat treatment of COST-FB2 steel.Then,the mechanical properties of the steel were tested,and the effect of quenching temperature on the microstructure of COST-FB2 steel was characterized.The results demonstrate that when the quenching temperature increased from 1000℃to 1150℃,the hardness and the strength of COST-FB2 steel increased continuously while the plasticity varied slightly and the impact energy first increased from 10 J to 23.5 J but then decreased.When the quenching temperature increased from 1000℃to 1150℃,the prior austenite grain(PAG)size of COST-FB2 steel increased from 65.5μm to 212.1μm,the block width increased from 4.75μm to 6.04μm,and the dislocation density increased from 4.8×10^(9) cm^(-2) to 11.8×10^(9) cm^(-2).Strengthening calculations helped determine that strengthening dispersion and dislocation are the primarily contributors for improving the yield strength of tempered COST-FB2 steel.Grain boundary and block boundary strengthening are superimposed with the sum of matrix strengthening,dispersion strengthening and dislocation strengthening increments.In summary,the superimposed block boundary strengthening increment corresponded to the experimentally obtained yield strength.This study can help improve the overall heat treatment of large forging rotors and domesticate them.