Modeling of kinetics of isothermal idiomorphic ferrite formation in a medium-carbon vanadium-titanium microalloyed steel
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I. INTRODUCTION
TOUGHNESS and other mechanical properties of steels are critically affected by their microstructure. It is well known that proeutectoid ferrite can nucleate at inclusions within coarse austenite grains, resulting in a very fine microstructure. Recently, this phenomenon has received great attention because of the need for toughness, especially when conventional austenite grain-refinement techniques are not enough. Idiomorphic and acicular ferrite are the microstructures that affect the strength and, in particular, the toughness of weld deposits.[1–4] There is a large amount of work on acicular ferrite formation,[5–8] but, in contrast, idiomorphic ferrite formation has rarely been studied. Some studies reported that idiomorphs nucleate at precipitates of titanium oxide (Ti2O3), manganese sulfide (MnS), and vanadium nitride (VN).[1,9,10] Those studies also analyzed the reason such precipitates act as viable sites for intragranular ferrite nucleation. However, the nucleation and growth kinetics for idiomorphic ferrite formation were not investigated. Although the kinetics of allotriomorphic ferrite,[11,12] pearlite,[13,14] and acicular ferrite formation are well established,[15] a kinetic theory for idiomorphic ferrite formation has not been adequately developed yet. This work aims to study the isothermal decomposition of austenite into idiomorphic ferrite and to analyze the influence of the prior austenite grain size (PAGS) on the nuleation and growth of idiomorphic ferrite. A mathematical model that describes the kinetics of idiomorphic ferrite formation during the isothermal decomposition of austenite is also proposed.
C. CAPDEVILA and F.G. CABALLERO, Research Associates, and C. GARCI´A DE ANDRE´S, Research Scientist, are with the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalu´rgicas (CENIM-CSIC), E-28040 Madrid, Spain. Manuscript submitted December 1, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. EXPERIMENTAL The chemical composition of the steel studied is presented in Table I. The material was supplied in the form of 50mm-square bars, obtained by conventional casting as a square ingot (2500 kg) and hot rolling to bar. The isothermal decomposition of austenite has been analyzed by means of a high-resolution dilatometer DT 1000 Adamel-Lhomargy described elsewhere.[16] Cylindrical dilatometric samples, 2 mm in diameter and 12 mm in length and machined parallel to the rolling direction of the bar, were used for these tests. The change in length of the specimen is transmitted via an amorphous silica push rod. This variation is measured by a linear variable differential transformer (LVDT) sensor in a gas-tight enclosure enabling testing under vacuum or an inert atmosphere with accuracy better than 0.1 m. The dilatometric curve (relative change in length (dL/Lo) vs time (t)) is monitored with the help of a computer-assisted electronic device. The dilatometer is equipped with a very low thermal inertia radiation furnace. The heat radiated by two tungsten filament lam
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