Distribution Behavior of Aluminum and Titanium Between Nickel-Based Alloys and Molten Slags in the Electro Slag Remeltin
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SUPERALLOYS exhibit several key characteristics such as excellent mechanical strength, high resistance to thermal creep deformation, good surface stability, and resistance to high temperature corrosion or oxidation. They are classified into three groups: Ni-based, Fe-based, and Co-based alloys.[1] In particular, Ni-based superalloys contain significant amounts of chromium, cobalt, aluminum, and titanium, as well as small amounts of boron and/or zirconium.[2] Electro slag remelting (ESR), which is a method of refining metals using a molten slag that is electrically heated by resistance, has been employed as a refining process for the production of ultra-clean Ni alloys.[2] The advantages of the ESR process include the ability to obtain a low inclusion content (high cleanliness), homogeneity of the solidified ingot structure, and low porosity levels.[2–5] JUN GIL YANG and JOO HYUN PARK are with the Department of Materials Engineering, Hanyang University, Ansan 426-791, Korea. Contact e-mail: [email protected] Manuscript submitted September 10, 2016. Article published online May 15, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
The design of ESR slag with an appropriate melting temperature and viscosity is critical since it determines the reactivity with molten alloy droplets during the process.[2] ESR slags are normally based on a CaF2-containing system to improve the fluidity with the addition of CaO, Al2O3, MgO, etc.[2,6] Furthermore, suitable amounts of TiO2 should be added into the slag to achieve steady state, which is close to the equilibrium state at the early stage of electrode remelting.[5] The CaO-Al2O3-CaF2 ternary phase diagram, calculated using FactSageTM 7.0 (ESM Software, Hamilton, OH), is shown in Figure 1. This is a commercial thermochemical computing program and has been successfully used for computing the gas–slag–metal multiphase reaction equilibria in ferrous and non-ferrous metallurgical systems.[7–13] The calculated phase diagram is very close to that reported by Chatterjee and Zhmoidin,[14] which was recommend by Mills and Keene,[15,16] and Zaitsev et al.[17,18] It is noticeable that the melting point of ESR slag should not be higher than that of Ni-based superalloys, which ranges from about 1573 K to 1653 K (1300 °C to 1380 °C). As the slag temperature rises above the melting temperature of the alloy, the tip of the electrode melts and a film of molten metal collects into VOLUME 48B, AUGUST 2017—2147
droplets, being refined by contact with the molten slag.[1,4,8,19] However, easily oxidative elements in an electrode such as Al and Ti react with O2 in air as well as with molten slag during the ESR process, resulting in a difference of the composition between the electrode and solidified ingot, as shown in Figure 2. In order to avoid the oxidation reaction with O2 in air, ESR systems should be designed to operate under a fully enclosed inert gas atmosphere. This purging system using an inert gas can provide a low level of oxygen in the alloy melt. In addition, losses of Al and Ti in t
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