247LC 가스터빈 블레이드 재생적층부의 액화균열 거동에 대한 연구

Abstract

Remanufacturing of non-weldable superalloy blades can be impacted by the microstructural degradation caused by long-term exposure to high temperature, the remelt solidification behavior of the weld metals, and the microstructural variation of each reheated weld metal. Moreover, these impacts can further influence variations in hot cracking susceptibilities for the repair-deposition. Therefore, to achieve crack-free repair-deposition of 247LC superalloy, the liquation cracking susceptibility of each repair welding situation must be examined individually. In this study, the liquation cracking temperature range(LCTR) of long-term exposed 247LC superalloy repair- depositions was quantitatively evaluated using Varestraint testing, especially for application in the heat-affected zone(HAZ). Furthermore, metallurgical factors for reducing liquation cracking susceptibility in repair-depositions were proposed based on microstructural analysis and thermodynamic calculation of the cracking surface. The effects of long-term exposure were simulated by over-aging heat treatment, and the liquation cracking susceptibility of the remelted and reheated weld metals was quantitatively evaluated by a modified multi-bead Varestraint test. As a result, the LCTR of the repair-deposition was increased to 610 K through over-aging and to 805 K through remelting and reheating. These results demonstrate that the 247LC superalloy face significantly serious liquation cracking susceptibility in repair- deposition HAZ. The cracking was attributed to the constitutional liquation of the MC carbides, which promoting local liquation at the interface between the γ phase at lower temperature than the equilibrium solidus(1530 K) of the 247LC. Finally, the application of a single-mode fiber laser was considered to reduce cracking by minimizing the formation of MC carbides and the heat- affected zone.