Effect of Austenitization Temperature on the Precipitation of Carbides in Quenched Low Carbon Boron Steel
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TECHNICAL ARTICLE
Effect of Austenitization Temperature on the Precipitation of Carbides in Quenched Low Carbon Boron Steel C. A. Suski • C. A. S. Oliveira
Received: 21 November 2012 / Revised: 13 December 2012 / Accepted: 19 December 2012 / Published online: 3 January 2013 Ó Springer Science+Business Media New York and ASM International 2013
Abstract The objective of this study was to study the effect of austenitization temperature on the formation of carbide/borocarbide, the microstructure and the mechanical properties of low carbon boron steel after quenching and its effects on the tensile properties of the steel. Three austenitization temperatures were used: 870, 1050, and 1200 °C. The steel microstructures were analyzed by light, scanning and transmission electron microscopy techniques. Microstructural analyses showed that the highest percentage of martensite was obtained at the austenitizing temperature of 1050 °C. The lower content of martensite at the austenitization temperature of 870 °C was caused by the low percentage of boron in solution or by the largest nonequilibrium segregation of boron to the grain boundaries at 1200 °C. A lower content of borocarbide (Fe23(C,B)6) was identified for the austenitization of 1050 °C, which caused an enhanced effect of boron on the reducing of the formation of carbide/borocarbide on the grain boundaries. Keywords Boron segregation Low carbon boron steel Precipitation Quenching
C. A. Suski (&) Instituto Federal de Santa Catarina, Rua 3122, n. 340 Apto 901, Balnea´rio Camboriu´, SC CEP 88330-290, Brazil e-mail: [email protected] C. A. S. Oliveira Mechanical Engineering Department, Universidade Federal de Santa Catarina, Campus Universita´rio, Post Office Box 476, Floriano´polis, SC 88040-900, Brazil e-mail: [email protected]
Introduction Quenched and tempered low carbon boron steel has been used for many different applications, such as in automotive, furniture, domestic electric appliance, construction, petroleum, agriculture, and transport industries. Small additions of boron substantially increase the hardenability of low carbon steels, due to the segregation of boron to the boundaries of austenitic grains [1]. The enrichment of solute atoms in grain boundaries is due to either equilibrium or non-equilibrium segregation phenomena. Equilibrium segregation occurs by the diffusion of solute atoms to the atomic layers of grain boundaries at the austenitization temperature [2, 3]. The adsorption of solute atoms at the grain boundary reduces the interfacial free energy. Non-equilibrium segregation is a dynamic process that occurs during cooling from high temperatures, resulting in a wider solute-enriched zone with a width that depends on the cooling rate. The enrichment is the result of mobile vacancy–solute complexes diffusing down the vacancy gradients toward vacancy sinks [3–5]. The equilibrium concentration of vacancies in the steel increases with temperature, and rapid cooling forms the vacancy gradient necessary for boron to segregate to the austenitic g
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