Inconel 625熔敷金属中δ相的形核与粗化机理

NUCLEATION AND COARSENING MECHANISM OF δ PHASE IN INCONEL 625 DEPOSITED METAL

利用扫描电镜(SEM)和透射电镜(TEM)研究了Inconel 625熔敷金属中δ相的形成机理,并揭示了焊后热处理(PWHT)过程中δ相粗化的内在规律.结果表明,经850℃焊后热处理的Inconel 625熔敷金属中大量析出的针条状δ相呈网格状分布在基体γ相中,且其附近出现了贫γ'相区域.δ相的形成是一个类贝氏体转变的固态相变过程,其晶核是在γ'相密排面的层错上通过切变方式形成的.同时,δ相的粗化行为是扩散控制的长大过程.保温时间较短时,实际δ相颗粒的平均尺寸符合LSW理论,随着保温时间的延长,δ相的高密度、不同向析出特征使得其实际尺寸偏离了经典LSW理论的预测值.

Using Inconel 625 wire to weld high yield strength steels or stainless steels that commonly used in nuclear power plant components and gas turbines can significantly improve high temperature mechanical properties and corrosion resistance of weld structure. However, toughness, fatigue strength and creep rupture strength of weld would decline obviously because of the precipitation of δ phase during service at elevated temperatures for a long time. This work aims to investigate nucleation mechanism of δ phase in Inconel 625 deposited metal by means of SEM and TEM. Meanwhile, coarsening inherent law of δ phase during post-weld heat treatment （PWHT） at 850 ℃ for 2, 4 and 8 h respectively was revealed. The results indicate that a large number of needle-like δ phase precipitates in Inconel 625 deposited metal after PWHT at 850℃. These 6 phases appear a grid-like distribution in y-matrix, and there are some poor y＂ phase regions appearing near δ phase. Formation process of δ phase is a solid phase transformation process which is like bainite transformation in steels. Crystal nucleus of δ phase form in the close-packed plane of y＂phase by shear mode, and coarsening behavior of δ phase is a diffusion-controlled growth process. When PWHT holding time is shorter, actual average size of δ phase is in line with LSW theory. With PWHT holding time extending, its actual average size deviates from the predicted value of classical LSW theory,because of the high-density and non-directional precipitation characteristics of δ phase.