超临界机组R26高温合金螺栓典型组织与性能

Typical microstructure and properties of R26 superalloy bolt for supercritical unit

对晶粒粗大、析出相呈带状分布、粗晶粒与细晶粒交替呈带状分布3种类型R26高温合金螺栓进行化学成分检验、金相检验、力学性能试验以及扫描电镜分析,并对螺栓硬度超标和显微组织异常问题展开研究。结果表明,3种类型螺栓冲击吸收能量、室温强度和塑性指标、600℃高温强度指标处于较高水平。析出相呈带状分布、粗晶粒与细晶粒交替呈带状分布是标准不允许的带状组织,并造成螺栓硬度高于标准要求,带状区域或细晶区域存在颗粒状碳化物(富含Ni、Cr、Co、Mo、Ti合金元素)或块状析出相(富含Ti、Mo合金元素)偏聚现象,是螺栓固溶处理不充分导致结果。粗晶粒与细晶粒交替呈带状分布的螺栓在600℃高温拉伸断后伸长率仅为5.6%,高温塑性较低。晶粒粗大螺栓600℃高温拉伸断口以沿晶断裂为主,并存在较多二次裂纹,其室温塑性和冲击吸收能量低于含带状组织螺栓。室温拉伸性能指标不能有效反映R26高温合金螺栓的服役性能,关于带状组织对R26高温合金螺栓持久强度和蠕变性能的影响有待进一步研究。

Chemical composition analysis,metallographic examination,mechanical property test and SEM analysis were carried out on three types of R26 superalloy bolts with coarse grains,zonal distribution of precipitates and alternate zonal distribution of coarse and fine grains,and the abnormal hardness and microstructure of the bolts were studied.The results show that the impact absorbed energy,strength and plasticity indices at room temperature,and high temperature strength index at 600℃of the three types of bolts are in a high level.The banded distribution of precipitates and the alternating banded distribution of coarse and fine grains are not allowed by the standard,and resulting to that the hardness of the bolts are higher than the standard.Particulate carbides(rich in Ni,Cr,Co,Mo and Ti alloying elements)or massive precipitated phases(rich in Ti and Mo alloying elements)segregate in the banded area or fine-grain area,which is the result of insufficient solution treatment of bolts.The elongation of bolt with alternately banded grains is only 5.6%after high temperature tensile fracture at 600℃,the high temperature plasticity is relatively low.The high temperature tensile fracture at 600℃of the bolts with coarse grains is mainly intergranular fracture,and there are many secondary cracks.The plasticity and impact absorbed energy at room temperature of the bolts with coarse grains are lower than that of the bolts with banded microstructure.The tensile properties at room temperature can not effectively represent the service performance of R26 superalloy bolts,and the effect of banded microstructure on the stress rupture strength and creep properties of R26 superalloy bolts needs to be studied further.