Thermodynamics of inclusion formation in Fe-Cr-Ti-N alloys
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I.
INTRODUCTION
THE control of the nitrogen level in steel is important because it has many harmful and, in some cases, beneficial effects on the physical properties and surface characteristics. The addition of titanium to liquid steel has become common for stabilizing nitrogen in steels. Titanium reacts with nitrogen, carbon, and oxygen to form titanium nitride, carbide, carbonitride and oxide. These inclusions may have a deleterious effect on properties such as toughness, ductility, and weldability. The inclusions can also agglomerate and cause surface quality problems. In the Fe-Cr-Ti-N system, the main inclusion of interest is titanium nitride. In order to eliminate or control the formation of these inclusions, it is necessary to have information on the thermodynamics of the system at the temperature of interest. A few investigations have been carried out on the thermodynamics of the Fe-Cr-Ti-N system, o,2] The results from these investigations show that there is a disagreement on the effect of chromium on the activity coefficient of titanium, which is a critical parameter to predict inclusion formation. Therefore, it is the purpose of this investigation to obtain the thermodynamics of titanium in Fe-Cr alloys and the formation of titanium nitride in liquid iron-chromiumtitanium-nitrogen alloys in order to predict the condition under which it will form. II.
EXPERIMENTAL PROCEDURE
A metal-nitride-gas equilibration technique was used to study the thermodynamics of titanium in the Fe-Cr-Ti system. A schematic of the experimental apparatus is shown in Figure I. A recrystallized alumina reaction tube of 4.4 cm in diameter and 80-cm length, sealed at both BAHR/OZTURK, Research Faculty, and R.J. FRUEHAN, Professor and Director, are with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Pittsburgh, PA 15213. R. MATWAY, Research Manager, is with J&L Specialty Steel Inc., Pittsburgh, PA 15230. Manuscript submitted September 27, 1994.
METALLURGICALAND MATERIALS TRANSACTIONSB
ends, was used in a molybdenum disilicide resistor furnace. The seals were fitted with a gas inlet, a gas outlet, and a thermocouple inlet. The temperature of the hot zone is controlled and measured by a thermocouple before each experiment. High-purity nitrogen and argon were used in the experiments. The argon and nitrogen were cleaned by passing them through drierite, copper chips heated to 623 K, and magnesium perchlorate. The flow rate of nitrogen was controlled at 500 cc/min by a rotameter. Iron-chromium or iron-chromium-nickel alloys of the desired compositions were made by melting high-purity iron (99.98 pct) and adding pure chromium (99.99 pct) and nickel (99.99 pct) in a recrystallized alumina crucible using a vacuum induction furnace. The melts were quenched and cut to desired sizes. For each experiment, 25 g of alloy and a 6-g cylindrical pellet of titanium nitilde was used. The titanium-nitride pellets were prepared by cold pressing pure TiN powder (
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