Effect of Prior Austenite Grain Size on Pearlite Transformation in a Hypoeuctectoid Fe-C-Mn Steel
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TRODUCTION
PEARLITE is a microconstituent formed during the eutectoid decomposition of austenite by the cooperative growth of ferrite (a) and cementite (h) lamellae. It was one of the first structures in metals to be described in considerable detail, and over the years, it has been the subject of many investigations. Seminal works of Zener,[1] Hillert,[2,3] Mehl,[4] and Avrami[5] established the basis of thermodynamics and rate-controlling mechanism for the growth of pearlite. Besides all the work reported, the pearlite reaction is still attracting the interest of many researchers, and nowadays, topics such as the non-steady growth (divergent pearlite)[6–8] are discussed in the literature. Moreover, there is a wide variety of research on the effects of microstructural parameters on the mechanical properties of pearlitic steels.[9–11] A well-known fact is that the active nucleus for pearlite formation can be either ferrite or cementite, depending on the temperature and composition. The nucleation sites can be grain boundaries or inclusions,[12] and once either one of the constituent phases is nucleated, the conditions surrounding the new nucleus are ripe for the nucleation of the other, and pearlite grows in a co-operative manner. Although the close relationship between grain-boundary area and potency of nucleation sites is well-known, M.M. ARANDA and R. REMENTERIA, Ph.D. Students, C. CAPDEVILA, Research Scientist, and C. GARCI´A DE ANDRE´S, Senior Scientist, are with the Materalia Group, Centro Nacional de Investigaciones Metalu´rgicas (CENIM-CSIC), Avda. Gregorio del Amo, 8, 28040 Madrid, Spain. Contact e-mail: m.m.aranda@ cenim.csic.es B. KIM, Ph.D. Student, is with the Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. Manuscript submitted June 18, 2013. Article published online September 27, 2013 1778—VOLUME 45A, APRIL 2014
the effect of austenite grain size (AGS) on pearlite transformation has not been described in detail yet. Therefore, the goal of this work is to study the effect of AGS on pearlite volume fraction and morphology parameters such as interlamellar spacing and pearlite colony size.
II.
MATERIALS AND EXPERIMENTAL TECHNIQUES
The studied alloy is a hypoeutectoid steel and the chemical composition is shown in Table I. The value of AGS was determined on cylindrical dilatometric test pieces of 2 mm diameter and 12 mm length that were heat treated with an Adamel Lhomargy DT1000 high-resolution dilatometer at 5 K/s up to 1473 K and 1173 K (1200 C and 900 C) for 180 seconds, followed by subsequent quenching to room temperature. The method of thermal etching was used for revealing the prior austenite grain[13] and the Feret diameter was measured in order to estimate the AGS by an image analyzer program. Values of 120 and 5 lm were obtained after austenitizing at 1473 K and 1173 K (1200 C and 900 C), respectively. The pearlite transformation heat treatments were designed based on an isopleth diagram which was calculated by means of ThermoCa
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