620℃长期时效对汽轮机转子用COST-FB2钢组织和性能的影响

Effect of Long-Term Aging at 620°C on Microstructure and Properties Evolution of COST-FB2 Steel for Steam Turbine Rotors

利用Thermo-Calc热力学软件对COST-FB2钢平衡条件下的析出相进行了计算,结合扫描电镜、透射电镜、化学相分析等手段研究了620℃不同时效时间下超超临界电站转子用COST-FB2钢的组织、M_(23)C_(6)碳化物和Laves相的演变,并分析了其变化对性能的影响。结果表明:COST-FB2钢在620℃后室温强度和塑性变化不大,高温强度和塑性有波动,冲击性能和硬度在时效前1000 h下降幅度较大,随着时间的进一步延长有所波动下降的幅度较小。时效0~10000 h过程中COST-FB2钢中马氏体板条结构比较稳定,位错密度和小角度界面下降,M_(23)C_(6)碳化物平均粒度增加;Laves相在时效2000 h开始析出,到时效10000 h过程中其尺寸不断增加,10000 h后平均直径约410 nm,其粗化程度远大于M_(23)C_(6)碳化物;在时效2000~6000 h,Laves相的单位面积数量不断增加,6000 h以后Laves相的单位面积数量开始下降,时效8000 h以后趋于平稳。国产COST-FB2钢转子大锻件在620℃时效10000 h过程中表现出较好的组织和性能稳定性。

In this study,precipitates in COST-FB2 steel equilibrium conditions were calculated using Thermo-Calc thermodynamic software.The microstructure,M_(23)C_(6) carbides,and Laves phase evolution of COST-FB2 steel for the rotors of the supercritical power station at 620°C for different aging time were studied by scanning and transmission electron microscope and chemical phase analysis.The effects of the microstructure and precipitates on the properties of the steel were also analyzed.The results show that the room temperature strength and plasticity of COST-FB2 steel change minimally after aging,whereas the strength and plasticity at high temperature fluctuate.The impact property and hardness decrease greatly after 1000 h of aging,but with the further extension of time the fluctuations decrease slightly.The martensitic lath structure of COST-FB2 steel is stable during 0~10000 h of aging,the dislocation density and small-angle boundaries decrease,and the average particle size of M_(23)C_(6) carbides increases.Laves phase begins to precipitate at 2000 h of aging,and its size increases from 2000 h to 10000 h.After 10000 h,the average diameter is approximately 410 nm,and the coarsening rate of the Laves phase is much greater than that of the M_(23)C_(6) carbides.After aging for 2000~6000 h,the number of Laves phase in the unit area increases continuously,but after 6000 h,the number begins to decrease.After 8000 h,it tends to be in equilibrium.The rotor forgings of domestic COST-FB2 steel exhibit a stable microstructure and properties after aging at 620°C for 10000 h.