The effect of Mn and Si on the morphology and kinetics of the bainite transformation in Fe-C-Ti alloys
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I.
INTRODUCTION
INCREASING interest in the application of high-strength low-alloy steels (HSLA) has resulted in numerous investigations of the precipitation of TiC within these alloys and its effect on mechanical properties, tl,21 The formation of so-called granular bainite (GB), consisting of ferrite with a high dislocation density and austenitemartensite particles, during noncontrolled rolling of Ticontaining steels also has been studied; L21however, there have been no investigations of the effect of Ti alone or combined with Si, Mn, and Mn + Si, respectively, on the transformation kinetics and the morphology of the transformation products. In addition, there is considerable confusion over the definition and classification of GB. The purpose of this paper is to investigate the effects of Ti concentration in Fe-C-Ti alloys and additions of Si, Mn, and Si + Mn on the microstructures formed at intermediate temperatures and the kinetics of the transformation. This investigation seeks to find out whether there is any fundamental difference between the precipitation of ferrite during bainite reaction and during the formation of the so-called GB structure. It is believed that these results will contribute to the understanding of the bainite transformation mechanism as a whole. II.
EXPERIMENTAL
Alloys used in this research were melted in a vacuum furnace from commercially pure iron, ferrotitanium, and S.K. LIU, Professor, is with the Department of Materials Engineering, Southwest Jiaotong University, Chengdu Sichuan, 610031, People's Republic of China. G.Y. ZHANG, former Graduate Student, Southwest Jiaotong University, is Research Assistant, Institute of Locomotive and Vehicle Technology, Changzhou, Jiangsu, People's Republic of China. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Metals Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
high-purity (99.9 pct) silicon and manganese. The chemical compositions of the alloys are shown in Table I. Ingots were forged into bars with a cross section of 20 • 20 mm and homogenized at 1200 ~ for 30 hours. Cylindrical samples 3 mm in. diameter and 10-mm long were heated to 1200 ~ for 10 minutes in an autodilatometer (Formastor), and then quenched with a stream of He or N2 to the reaction temperature. The initiation of transformation was determined by the inflection in the expansion vs time curve, but the fraction transformation was determined by the point-counting method. Because alloys T1, T2, and T3 have short incubation times at intermediate temperatures, the quenching ability of He is insufficient to ensure isothermal transformation; hence, some samples were austenitized in a salt bath and isothermally transformed in a lead pot. Transmission Electron Microscopy (TEM) observations were carried out with a H-700H microscope using a 200 kV accelerati
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