Effect of Post Weld Heat Treatment on Cryogenic Mechanical Properties of Electron Beam Welded Cast Inconel 718
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Ni-Fe based superalloy Inconel 718 was originally developed for aerospace applications in the intermediate temperature range up to 923 K (650 C) to provide the combination of high strength and good weldability.[1,2] Recently, the Inconel 718 has been considered to be a combustor head for a liquid propellant rocket engine because of its high strength coupled with good castability.[3] However, the combustor head is subjected to extremely high pressure at cryogenic temperature as a result of an explosive combustion thrust of liquid oxygen and fuel. The combustor mixing head is assembled by electron beam (EB) welding of cast Inconel 718. Hence, any major concerns should be directed at the cryogenic mechanical properties of EB welded cast Inconel 718. A few studies in the literature are available that are related with EB welding of cast Inconel 718.[4–6] However, all of them are focused on the microfissuring behavior in the heatSANG-HYUN BAE, SOON-IL KWON, and JOONG-GEUN YOON, Graduate Students, and HYUN-UK HONG, Professor, are with the Department of Materials Science and Engineering, Changwon National University, Changwon, Gyeongnam 641-773, Republic of Korea. Contact e-mail: [email protected] JE-HYUN LEE, Professor, JEONG-HYEON DO, Senior Researcher, and IN-SOO KIM, BAIG-GYU CHOI, and CHANG-YONG JO, Principal Researchers, are with the High Temperature Materials Research Group, Korea Institute of Materials Science, Changwon, Gyeongnam 641-831, Republic of Korea. Manuscript submitted August 25, 2013. Article published online December 12, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
affected-zone (HAZ) of the cast alloy. Therefore, the present study aims to investigate the characterization of microstructures and cryogenic mechanical properties of EB welded cast Inconel 718 with or without post weld heat treatment (PWHT). The material used in the current investigation was cast Inconel 718 with nominal composition of Ni-19.3Fe18.9Cr-5Nb-2.9Mo-1.1Ti-0.58Al-0.064C (all in wt pct). The weld coupons with 10 mm thickness were prepared by casting in a vacuum furnace. To eliminate casting porosity and to dissolve effectively Nb-rich Laves and d phases, the coupons were hot isostatic pressed (HIP) under 100 MPa argon atmosphere at 1436 K (1163 C) for 4 hours. The heating rate to peak temperature was 6~7 C/min during HIP. After the HIP cycle, one group of coupons was solution heat-treated at 1325 K (1052 C) for 1 hour followed by air cooling, and then EB welded to make a butt-joint. The EB welds were subjected to a PWHT [aging at 991 K (718 C) for 8 hours, then furnace cooled to 894 K (621 C), and soaked for 8 hours followed by air cooling]. The other group of HIP coupons was solution heat-treated and aged under the same conditions as the one above (fully heat-treated). EB welds were made with these pre-aged coupons. No PWHT was conducted. Autogenous full penetration EB welding was carried out at 120 kV, 34 mA, and 12 mm/s in a high vacuum welding chamber (4 9 104 mbar) with an EBG30-150 K230 EB welder (Probeam AG & Co.
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