Thermodynamics of titanium and nitrogen in an Fe-Ni melt
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I. INTRODUCTION
IN steelmaking processes, a strong nitride-forming element such as titanium is often added to stabilize nitrogen and improve the mechanical properties of steel via the grain refinement during hot rolling. Recently, the formation of titanium nitride as a secondary inclusion precipitated during solidification has been of great interest, because it is known to help the formation of an equiaxed cast structure.[1] On the other hand, titanium nitride formed in liquid steel can agglomerate and cause a nozzle-clogging problem during continuous casting and surface defects in final products. In spite of the importance of inclusion control in Ti-bearing stainless steels and high-nickel specialty steels, the thermodynamic data of titanium in Fe-Ni melts are not well known, and no information is available at temperatures other than 1873 K. Therefore, it is the purpose of this study to obtain reliable thermodynamic data of titanium and nitrogen in Fe-Ni melts as a function of temperature.
using an induction furnace. Desired portions of master alloys were charged to make an intermediate melt composition for each experiment. Fifteen grams of the alloy and 1 g of titanium nitride pellets were placed in a high-purity alumina crucible (o.d.: 22 mm, i.d.: 18 mm, and height: 25 mm). A detailed description of the experimental procedure is given elsewhere.[2] It was found that the time required for the metal-nitride-gas equilibrium was longer than 80 hours. Therefore, for each set of experiment, the alloy melts with different initial titanium contents (0 and 0.2 mass pct Ti) were equilibrated, and, hence, the equilibrium could be approached from high and low titanium contents with respect to the equilibrium values. After each experiment, the metal samples were analyzed for Ni and Ti by the inductively coupled plasma–Auger electron spectroscopy (ICP-AES) technique (SPECTRO apparatus, manufactured by Spectro Flame), and for N and O by the inert gas fusion-infrared absorptiometry technique (LECO* RO-600 apparatus). *LECO is a trademark of the LECO Corporation, St. Joseph, MI.
II. EXPERIMENTAL PROCEDURES The metal-nitride-gas equilibration experiments were carried out to determine the levels of soluble titanium and nitrogen in Fe-Ni melts in the presence of pure solid TiN under various nitrogen pressures. The experiment was carried out in a graphite resistance furnace in the temperature range of 1843 to 1923 K. A detailed description of the furnace is given in the author’s recent work.[2] Master alloys of Fe-30 mass pct Ni and Fe-4 mass pct Ti were made by melting high-purity iron (99.99 pct purity) and adding nickel (99.6 pct purity) and titanium (99.5 pct purity) in a recrystallized magnesia crucible in an Ar atmosphere JONG-JIN PAK, Professor, is with the Division of Materials and Chemical Engineering, Hanyang University, Ansan 426-791, Korea. YONG-SOO JEONG, formerly Graduate Student, Division of Materials and Chemical Engineering, Hanyang University, is a Mettalurgical Engineer with Changwon Specialty Steel Co. Ltd.,
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