Behavior of steels near the incipient melting temperature
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I.
INTRODUCTION
IT is generally accepted that there are three temperature zones where the hot ductility of steel is diminished, fl,2] In zone I, which is from about 1340 ~ to the solidus temperature, the presence of a liquid film causes embrittlement, t3j Segregation of elements such as sulfur, phosphorus, and carbon, to the interdendritic region during cooling from melting, or to the grain boundaries during heating, leads to a decrease in the solidus temperature of these areas. ~2-8~For the case of specimen heating (as opposed to cooling), the grain-boundary melting temperature (Tgb) is equivalent to the incipient melting temperature (IMT), i.e., the temperature at which steel begins to transform to liquid. Knowledge of the IMT is important both industrially and scientifically. In industrial applications, zero ductility at the IMT is responsible for hot tearing and internal cracks in continuously cast products, tl'9'l~ cracking in weld metal, tl~,12j and internal cracks in wrought products. ~131 Scientifically, detection of the IMT is a convenient method for delineating the solidus for phase diagrams and can reveal information concerning grain-boundary segregation phenomena. In the recent past, isothermal high-temperature tensile testing has been commonly used to determine the IMT. I2-4'sl Ductility is expressed in terms of reduction of area, with the zero ductility temperature (ZDT) being synonymous with the IMT. Differential thermal analysis (DTA) has also been used, 181 but is a less sensitive method and essentially reveals the bulk solidus temperature rather than the IMT. Thus, isothermal hot tensile testing is potentially the more accurate method of the two, but the rapid change in ductility over a very narrow temperature range requires numerous tests to precisely determine the IMT. In addition, the results of these tests have been used to relate ductility variations to changes in the stress vs temperature behavior, but not in any great F. HASSANI, Graduate Student, Department of Mechanical Engineering, T.M. MACCAGNO, Research Associate, J.J. JONAS, Professor, and S. YUE, Assistant Professor, are with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ H3A 2A7, Canada. Manuscript submitted August 3, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
detail. Weinberg t4] used an increasing-temperature test to determine IMT, but that technique involved applying a dead weight to the specimen and thus changes in the stress vs temperature behavior could not be followed. This article describes a simple, accurate method for IMT detection based on continuous-heating tensile testing using a single specimen, and presents results concerning the effect of carbon and microalloying elements on the very-high-temperature deformation behavior of low- and medium-carbon C-Mn steels.
II.
EXPERIMENTAL PROCEDURE
A. Experimental Apparatus Testing was carried out using a closed-loop, servohydraulic testing machine of 100-kN (22-kip) capacity, equipped with high-speed computer control and data
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