Evolution of carbides and its characterization in HAZ during NG-TIG welding of Alloy 617B

Abstract

The carbides evolution affected by heating will have great effect on the mechanical property of welded components. The paper focused on the microstructure characterization and carbides evolution under different thermal cycle in narrow-gap tungsten inert gas welding (NG-TIG) on Alloy 617B. The carbides were characterized to be M23C6 by TEM in 617B base metal (BM), which distribute along the grain boundaries and within the grains in roughly similar size and characteristics. However, many kinds of multi-carbides and various characteristics were found in heat affected zone (HAZ) with different distance to fusion line, which contributed to different heating peak temperatures (Tp). As the Tp reaches the solvus temperature of M23C6 carbides, M23C6 carbides show partial dissolution and M6C will form at the interface γ/M23C6 due to the segregation of element Mo. Upon the eutectic temperature, the surviving M23C6 carbides react with the surrounding γ matrix and then it leads to the formation of liquid film. Some phases with lamellar structure composed of M6C and M23C6 multi-carbides and islands of γ-austenite will occur as the temperature decreases. In addition, a large number of fine granular Cr-rich second phases like texturing morphology, scattered around each new kind of multi-carbide after the post weld heat treatment (PWHT). Basing on the analysis result, it can be seen that the evolution behavior of carbides is determined by the thermal input during NG-TIG welding process. No adverse effects, such as liquation cracking, have been brought into the welded joint due to the low thermal input, proving that NG-TIG is a suitable welding method to join Alloy 617B.