Microstructural/mechanical characterizations of electron beam welded IN738LC joint after post-weld heat treatment

Abstract

The effect of post-weld heat treatment on microstructure and mechanical properties of electron beam welded IN738LC joint has been investigated. The fusion zone consists of γ-Ni matrix, bimodal-size γʹ particles, granular MC carbides (Ti-rich) and Cr-rich M23C6 carbide chain in IN738LC joint subjected to post-weld heat treatment. The secondary γʹ particle in fusion zone precipitates and grows ceaselessly with the increased aging time. The coarsening of γʹ particle follows the Lifshiitz-Slyozov-Wagner coarsening theory with a constant coarsening rate of 1.003 × 10−27m3/s. The secondary γʹ particle in base metal shows a similar size with that of fusion zone, but a lower volume fraction (52%) than that of fusion zone (54%). M23C6 carbide is easier to precipitate along high angle grain boundaries. As aging time increases, the morphology of M23C6 carbide changes from discrete granular shape to continuous chain. The thickness of M23C6 carbide also increases dramatically. Owing to smaller grain size and higher solute concentration in fusion zone, more M23C6 carbides form in FZ than that of base metal. With the increased aging time, the tensile strength of IN738LC joint at 900°C decreases gradually, while the elongation of joint firstly increases and then decreases, which is attributed to the increased channel width of γ-Ni matrix. Due to the formation of continuous M23C6 carbide chain, the rupture life of IN738LC joint firstly increases and then decreases as aging time increases. Combined with the mechanical properties and size stability of γʹ precipitates in IN738LC joint, 1125°C/2 h + 850°C/24 h is an optimized post-weld heat treatment regime.

Keywords

• IN738LC alloy
• EBW
• Post-weld heat treatment
• Microstructure
• M23C6 carbide
• Mechanical properties