Effect of Welding Heat Input on Microstructure and Texture of Inconel 625 Weld Overlay Studied Using the Electron Backsc
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INCONEL alloys are Ni-based alloys that have numerous applications, e.g., in fabricating aircraft engines and steam engines of nuclear power reactors, because of their outstanding mechanical properties at high temperatures, high corrosion resistance, and good weldability and welding properties. Thus, Inconel alloys are essential materials for use in industries and nuclear power plants where mainly super heat-resistant steel is used. The weld overlay for use is more cost effective. Overlay welding of Inconel alloys onto an inexpensive base material can solve this problem, thus economically realizing the required structural properties. For the overlay welding of Inconel alloys, carbon steel is generally used as the base material. Carbon steel is widely used in the industries because of its good formability, strength, and low price. These properties of carbon steel compensate for the low yield strength of the Ni alloys. Moreover, since the two steel varieties have similar thermal expansion coefficients, crack formation at high temperatures due to the difference in the thermal expansion coefficients can be prevented. Thus, Inconel alloys can be used as structural materials that exhibit the essential physical and corrosion-resistant properties via the overlay welding of the two steels.[1-16] However, the microstructural changes due to the heat
JOON-SUK KIM, Master Student, and HAE-WOO LEE, Professor, are with the Department of Metallurgical Engineering, College of Engineering, Dong-A University, Busan, R. Korea, 49315. Contact e-mail: [email protected]. Manuscript submitted April 27, 2016. Article published online October 13, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
generated during welding are an important consideration while using Inconel alloys. Electroslag welding (ESW) is mainly used as the welding method for the fabrication of overlay pipes by using Inconel alloys. Even though ESW has the advantage of a higher welding speed due to the higher heat input, compared to other welding methods, microstructural changes are expected owing to the high heat input. In real industrial sites, the welding heat input is considered to be the most important factor during welding, and multilayer welding is performed rather than single-layer welding in order to realize the physical properties and dilution of the welding metal. Although several studies on the microstructure and the texture of pure Ni alloys have been conducted, there are not many reports on the analysis of the welding metal. In order to observe the microstructure of the welding metal, the optical microscope has been widely used. The microstructure of the specimen can be observed by corroding the grain boundary through chemical etching after mechanical polishing. Various techniques, such as X-ray diffraction, convergent beam electron diffraction (CBED) of transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) of scanning electron microscopy (SEM), can be used to examine the texture and the structural properties of the metal. The pole
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