Characterization of the Microstructures and the Cryogenic Mechanical Properties of Electron Beam Welded Inconel 718
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THE Ni-Fe-based superalloy Inconel 718 containing 5 pct Nb (in weight percent) is a precipitation-hardened alloy that was originally developed for aerospace applications in the medium temperature range of up to 923 K (650 C) to provide a combination of high strength and good weldability.[1] The alloy is strengthened by a primary strengthener, c¢¢ (Ni3Nb, DO22: ordered bct crystal structure), and a secondary strengthener, c¢ [Ni3(Al,Ti), L12: ordered fcc crystal structure] precipitates.[1–3] The Laves phase [(Fe,Cr,Ni)2(Nb,Ti), C14: hexagonal MgZn2] is an unavoidable final freezing phase in this alloy and is always present in the as-cast and as-welded microstructures.[4–6] Although Inconel 718 has a good weldability, assuring high resistance to strain-age cracking due to the sluggish precipitation of c¢¢ particles, its as-weld fusion zone is likely to be susceptible to extensive interdendritic Nb segregation and the presence of Laves phase in interdendritic regions.[6]
SOON IL KWON and SANG HYUN BAE, Graduate Students, and HYUN UK HONG, Professor, are with the Department of Materials Science and Engineering, Changwon National University, 20 Changwondaehak-ro, Changwon, Gyeongnam 641-773, Republic of Korea. Contact e-mail: [email protected] JEONG HYEON DO, Senior Researcher, and CHANG YONG JO, Principal Researcher, are with the High Temperature Materials Research Group, Korea Institute of Materials Science, 797 Changwondaero, Sungsan-gu, Changwon, Gyeongnam 641-831, Republic of Korea. Manuscript submitted December 24, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Recently, Inconel 718 has been considered a candidate material for aerospace applications due to its high strength coupled with good castability.[7] However, the components for aerospace applications are frequently subjected to extremely high pressure at cryogenic temperatures. For this alloy to be used in aerospace assemblies, electron beam (EB) and gas tungsten arc (GTA) welding are the most common fusion joining processes. EB welding using a high energy-density beam is known to achieve joints with a higher weld efficiency due to its inherent high weld metal cooling rate as well as its low distortion. Accordingly, the major concerns are regarding the cryogenic mechanical properties of EB welds between cast and forged Inconel 718 superalloys. The small amount of literature related to EB welding that is available involves only wrought Inconel 718.[5,6,8,9] For cast Inconel 718, most studies focused on the microfissuring behavior in the heat-affected-zone (HAZ) during EBW[10] or GTAW.[11–13] Few studies on the cryogenic mechanical properties of the fusion zone of this alloy are found. An important report related with EB welds of cast Inconel 718 showed that the presence of very fine Laves particles in the fusion zone aggravated cracking when c¢¢ precipitates were present after the postweld heat treatment (PWHT).[14] Therefore, further study is needed to determine how the refinement of dendrites and the Laves size and quantity influence the cryogenic
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