Acta Materialia 51 (2003) 4183–4196 www.actamat-journals.com Kinetics of the martensitic transformation in low-alloy steel studied by means of acoustic emission S.M.C. van Bohemen a,b,* , J. Sietsma a , M.J.M. Hermans a , I.M. Richardson a,b a Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands b Netherlands Institute for Metals Research, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Received 11 December 2002; accepted 22 April 2003 Abstract The kinetics of the martensitic transformation in three carbon steels (C60, C70 and C80) have been studied using the acoustic emission (AE) technique. It is demonstrated that the volume fraction of martensite f as a function of time t during cooling can be derived from the measured AE power υ 2 , since υ 2 df /dt. An analysis of results shows that the fraction data as a function of temperature T can be described by the Koistinen and Marburger (KM) equation with high accuracy. This indicates that the nucleation of martensite takes place heterogeneously and that the average volume of martensite crystals is constant over the extent of the transformation. The change in kinetics with carbon content is attributed to the amount of dislocations created in the neighboring austenite, which influences the degree of autocataly- sis. Also, the acoustic emission energy generated per unit volume of martensite has been found to scale with the change in dislocation density in the formed martensite as the carbon content is varied.  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Martensitic phase transformation; Kinetics; Low-alloy steels; Heterogeneous nucleation; Acoustic emission 1. Introduction Martensitic transformations can occur in many metals and alloys provided the cooling rate is rapid enough to prevent diffusion-controlled transform- ations [1]. The transition from austenite to marten- site in steels is the best-known and most important martensitic transformation because of the techno- logical importance of hardened steel. The mar- * Corresponding author. E-mail address: s.m.c.vanbohemen@tnw.tudelft.nl (S.M.C. van Bohemen). 1359-6454/03/$30.00  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6454(03)00236-2 tensitic phase in steel can be simply described as a super-saturated solution of carbon in the ferritic phase, in which the carbon content leads to a tetra- gonal distortion of the lattice. The occurrence of a phase change is governed by thermodynamics and kinetics. The underlying thermodynamics deter- mine the available driving force for transformation; the kinetics of a martensitic transformation depend solely on nucleation, because the growth of a mar- tensitic crystal usually occurs rapidly. It is well known that the mechanism of growth is displacive. How the phase nucleates, however, is even today not completely understood. This is mainly due to the great speed of formation, which makes the mar-