Inconel 617B合金焊接接头750℃持久微观组织演变对性能的影响

Effect of microstructure evolution on properties of Inconel 617B alloy welded joint creep tested at 750 ℃

采用扫描电镜和透射电镜观察Inconel 617B合金焊接接头750℃持久试验前后微观组织,并探讨了微观组织演变对硬度的影响和持久断裂机制。结果表明:Inconel 617B合金焊接接头持久后,主要析出M_(23)C_6相和γ'相。晶界上M_(23)C_6相呈块状,晶内M_(23)C_6相为薄板状或针状;球形γ'相细小,弥散在晶内。随持久时间延长,晶界M_(23)C_6相长大并连接;晶内γ'相也有长大的趋势,但长时间持久尺寸仍十分细小;晶内除析出的M_(23)C_6相继续长大成薄板状外,还析出新的尺寸较小的针状M_(23)C_6相。持久初期,M_(23)C_6和γ'的析出使得焊接接头的硬度明显升高。随着持久时间延长,由于新析出的M_(23)C_6产生的强化增强效果不足以弥补因析出相粗化和固溶度降低造成的弱化效果,硬度有下降趋势。Inconel 617B合金焊接接头持久试样断裂在焊缝区,晶界和枝晶间M_(23)C_6相粗化严重,更易萌生空洞和裂纹,是引起沿晶断裂的主要原因。

Microstructure of Inconel 617B alloy welded joint before and after creep test at 750 ℃ was investigated by means of SEM and TEM, and the effect of precipitates on hardness and fracture mechanism was discussed. The results show that the main precipitates are M23 C6 and γ＇ after creep test. Cubeid M23C6 particles precipitate at grain boundaries, whereas needle-like or plate-like M23Cs particles precipitate in the grain interior. Besides, spherical γ＇ particles are fine and dispersed in the matrix. With the increase of creep time, M23C6 at the grain boundaries coarsens and arrays in the form of chains. γ＇ still keeps fine after long-term creep even though it tends to coarsen. Accompanied by the change of M23C6 from needles to plates, some needle-like M23 Ca particles precipitate. The precipitation of M23C6 and γ＇ result in the obvious increase of hardness in the early stage of creep. The precipitation hardening of the newly-formed M23C6 cannot compensate the decrease of hardness due to the coarsening of precipitates and the decrease of solid solubility. Therefore, the hardness of welded joint declines with further creep. As for the creep rupture specimens of Inconel 617B alloy welded joints, most of the fracture takes place in the welded zone. The serious coarsening of M23 C6 at the grain boundaries and interdendritic area accelerates the formation of cavities and cracks, which becomes the main reason for intergranular fracture.