일방향응고용 247LC 초내열합금 용접열영향부의 액화균열 거동에 대한 연구

Abstract

247LC superalloy is a non-weldable material with various hot crackings, and careful control of each hot cracking phenomenon is essential for the manufacture of sound welds. In this study, we quantitatively evaluated the susceptibility to liquation cracking in the heat- affected zone (especially the PMZ) of 247LC superalloy welds, and identified the metallurgical mechanism of occurrence.Based on the results, a thorough examination of the suppression of liquation cracking in the PMZ was conducted in conjunction with various process reviews. Based on the results, a thorough examination of the suppression of liquation cracking in the PMZ was conducted in conjunction with various process reviews. The liquation crack susceptibility (LCTR) was quantitatively evaluated by linking spot-varestraint test using arc spot welding and real-time temperature measurement at the crack initiation viewpoint. Gas tungsten arc welding and single-mode fiber laser processes were used as welding processes. The welds microstructure and liquation cracking behavior were analyzed using backscattered electron diffraction (EBSD) and electron probe microanalysis (EPMA). The As-cast 247LC superalloy was evaluated to have an LCTR of approximately 620 K. This was found to be due to the constitutional liquation of MC carbides and γ-γ' eutectic during weld heating and the intergranular segregation of impurity elements. By applying an optimal pre-weld heat treatment, the harmful microstructure could be minimized, and the LCTR was reduced to about 65 K. Based on these results, arc low heat input welding and laser welding were applied to achieve a liquation-free welds. Based on these results, arc low heat input welding and laser welding were applied to achieve a liquation-free welds. It was confirmed that the arc low heat input welding process is a valid process for securing a sound welds.No constitutional liquation was observed with laser beam welding, but liquation cracking occurred. No constitutional liquation was observed with laser beam welding, but liquation cracking occurred. The cracking behavior was determined to be due to pipeline diffusion.