Segregation and diffusion behavior of niobium in a highly alloyed nickel-base superalloy

The segregation during solidification and high temperature diffusion during homogenization of niobium in a highly alloyed nickel-base superalloy were investigated.Niobium is seriously segregated into the interdendritic regions with segregation coefficient as high as 4.30.Various niobium-enriched phases including Laves phase,δ phase,（γ＋γ＇） eutectic,MC and M6C types of carbides precipitated in the interdendritic regions.The soluble temperature of niobium-enriched phases and the degree of dendritic segregation were determined by differential thermal analysis and homogenization treatments at varied temperatures and times.The calculation of elemental diffusion indicates that the diffusion rate of niobium increases remarkably with the homogenization temperature increasing,so that the annealing time can be effectively reduced.A three-step homogenization treatment technology without incipient melting was established to eliminate the segregation of niobium and to obtain a uniform microstructure.

07Cr18Ni11Nb不锈钢斜轧穿孔分层的原因

某批次07Cr18Ni11Nb不锈钢在穿孔过程中出现分层。采用宏观及微观检查、化学成分分析、金相检验、扫描电镜和能谱分析等方法,对其穿孔分层的原因进行了分析。结果表明:该不锈钢坯料中沿轴向分布的链状和棒状初生Nb(C,N)偏析相和穿孔过程中的非均匀变形是导致分层的主要原因;穿孔过程中,初生Nb(C,N)相与奥氏体基体变形不协调,在界面处产生应力集中使管坯局部断裂强度降低,在非均匀变形产生的附加拉应力作用下,应力集中区域产生裂纹,并在后续碾轧过程中不断扩展,导致分层;建议在冶炼过程中控制碳、铌元素含量,改善重熔凝固条件,调整穿孔工艺参数,适当降低轧辊转速,以有效防止穿孔分层的发生。