Effect of Welding and Post-weld Heat Treatment on Tensile Properties of Nimonic 263 at Room and Elevated Temperatures
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INTRODUCTION
WITH increasing firing temperature up to 1700 K (1427 C) for a land-based gas turbine generator, the materials for the combustor and transition piece are exposed to 973 K to 1073 K (700 C to 800 C) even with an advanced thermal barrier coating and cooling system.[1,2] The fabrication of such parts inevitably involves welding process, and Nimonic 263 has been developed to offer improved ductility in welded assemblies to replace Nimonic 80A.[3] This precipitationhardened Nimonic 263 is a good candidate material for a gas turbine combustor and transition piece because of its excellent creep strength and good oxidation resistance at elevated temperatures, along with the improved ductility enhancing the resistance to thermalmechanical fatigue (TMF).[3–10] For the successful application, however, proper mechanical properties must also be maintained for the welded structures, such as combustor and transition piece, at room and elevated temperatures. Currently, only a manufacturer’s report is available for the mechanical properties MINWOO JEON, Graduate Student, and SANGSHIK KIM, Professor, are with the Department of Materials Science and Engineering, Gyeongsang National University, Jinju, South Korea. Contact e-mail: [email protected] JAE-HYUN LEE, Professor, is with the Department of Materials Science and Engineering, Changwon National University, Changwon, South Korea. TA KWAN WOO, Vice President, is with Sungil Co., Ltd., Busan, South Korea. Manuscript submitted July 30, 2009. Article published online November 5, 2010 974—VOLUME 42A, APRIL 2011
of Nimonic 263 weldment.[3] The mechanical behavior of Nimonic 263 weldment may be substantially different from that of base metal because the microstructural evolution in Ni-base superalloy during fusion welding, including microsegregation in the interdendritic region and inhomogeneous distribution of second-phase precipitate particles in the weldment, tends to affect the mechanical behavior significantly.[11–14] The postweld heat treatment (PWHT) is often performed to improve the resistance to brittle fracture of welded Ni-base superalloy by relaxing residual stress.[15] Despite the possibility of having a wide range of property recommendations, the manufacturer recommends a direct aging for Nimonic 263 weldment, and no studies have been conducted on how the PWHT affects the mechanical properties.[5] In this study, the tensile behavior of as-received and as-welded Nimonic 263 specimens at room temperature and 1053 K (780 C) was examined. The microstructural changes during fusion welding and PWHT were characterized by using an optical microscope and a scanning electron microscope (SEM). The effect of varying resolutionization time from 0.5 hours to 20 hours during PWHT was examined on the tensile behavior of as-welded Nimonic 263 specimen. The change in carbide morphology with welding and different PWHT conditions was characterized by using an SEM and an energy-dispersive X-ray spectroscopy (EDS) technique to identify its effect on the tensile behavior. The fracture mod
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