Effect of local carbide formation behavior on repair weld liquation cracking susceptibility for long-term-serviced 247LC superalloy

Abstract

This study assessed the liquation cracking temperature range (LCTR) for 247LC superalloy repair welds subjected to long-term and high-temperature exposure. To achieve this, the exposed specimens underwent simulations at 1273 K for 500, 1000, 3000, and 5000 h to replicate the long-term service conditions of gas turbines. Spot-Varestraint testing was employed to quantitatively measure LCTR variations. The metallurgical mechanism of the evaluated LCTR in the exposed 247LC superalloys was elucidated by analyzing the microstructural characteristics at the liquation cracking surface and performing thermodynamic calculations. The LCTR was evaluated to be 410 K for specimens exposed for 500 h, 440 K for 1000 h, 385 K for 3000 h, and 610 K for 5000 h. This indicates that the liquation cracking susceptibility of the repaired weld heat-affected zone significantly increased during the long-term service of the 247LC superalloy. After 5000 h of exposure, the formation of fine MC, M6C, and M23C6 carbides on the liquation cracking surface was characterized using transmission electron microscopy. The welding metallurgical mechanism underlying the severe liquation cracking susceptibility can be attributed to the fractions of MC (liquation initiation temperature: 1125–1356 K) and M6C (liquation initiation temperature: 1390–1420 K), which have lower liquation initiation temperatures than the solidus of the 247LC alloy (1530 K). Notably, the area fraction of MC and M6C increased during 5000 h of exposure, which led to an increase in LCTR from 410 K to 610 K.