Effect of Prior Athermal Martensite on the Isothermal Transformation Kinetics Below M s in a Low-C High-Si Steel
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THE industrial sector is continuously looking for new steels for a range of applications. Those efforts have led to the development of a new generation of Advanced Multiphase High Strength Steels with very good combination of properties: high tensile strength and good ductility. Thermomechanical processing of these advanced multiphase steels often includes isothermal treatments around the martensite start temperature (Ms). When isothermal treatments are applied below the Ms temperature, a predetermined fraction of athermal martensite is present prior to the application of the isothermal treatment. This prior athermal martensite has an accelerating effect on the overall isothermal transformation close to the Ms temperature, which is generally known as the ‘‘swingback’’ phenomenon.[1–4] But, how can athermal martensite contribute to this acceleration? It is well known that martensite preferentially nucleates at prior-austenite grain boundaries, i.e., at austenite-austenite (c-c) interfaces.[5] In an isothermal treatment after quenching, the athermal martensite formation is stopped before the completion of the transformation and thus martensite-austenite (a¢-c) interfaces are present in the material, which can act as
A. NAVARRO-LO´PEZ, Ph.D. Student, J. SIETSMA, Full Professor, and M.J. SANTOFIMIA, Associate Professor, are with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected] Manuscript submitted May 8, 2015. Article published online December 29, 2015 1028—VOLUME 47A, MARCH 2016
potential nucleation sites.[6] Kawata et al.[5] reported that a further acceleration of the isothermal transformation above Ms is achieved by increasing the volume fraction of prior athermal martensite and, as a consequence, the density of a¢-c interfaces. However, the mechanisms of this acceleration are not well understood and the effect of these a¢-c boundaries on later transformation kinetics remains unclear. Microstructures obtained in isothermal treatments around the Ms temperature depend on the chemical composition and the temperature of the isothermal holding. Above Ms, in hypoeutectoid steels, researchers agree about the nature of the isothermal product obtained from the decomposition of austenite. Microstructures are generally formed by a bainitic ferrite matrix with or without carbides, depending on the alloying elements and the isothermal holding time, and retained austenite, in the form of thin films or martensite-austenite (MA) islands.[7–13] The isothermally obtained phase product is generally called bainite. While the bainitic ferrite is free of carbides in upper bainite, lower bainitic ferrite can contain a fine dispersion of plate-like carbides, depending on the silicon content.[14] However, below Ms, there is discussion about which phase product is obtained from the isothermal decomposition of austenite. Different observations have shown that isothermal products formed below the Ms temperature can be
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