In-Situ Study of Pearlite Nucleation and Growth During Isothermal Austenite Decomposition in Nearly Eutectoid Steel S.E
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Acta Materialia 51 (2003) 3927–3938 www.actamat-journals.com In-situ study of pearlite nucleation and growth during isothermal austenite decomposition in nearly eutectoid steel S.E. Offerman a,b,∗, L.J.G.W. van Wilderen a,b, N.H. van Dijk a, J. Sietsma b, M.Th. Rekveldt a, S. van der Zwaag b, 1 a Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands b Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Received 23 January 2003; received in revised form 10 April 2003; accepted 14 April 2003 Abstract The evolution of the microstructure during the isothermal austenite/pearlite transformation in a nearly eutectoid steel was studied by the three-dimensional neutron depolarization technique, which simultaneously provides information about the pearlite fraction, the average pearlite colony size, and the spatial distribution of the pearlite colonies during the transformation. The in-situ measurements show that the pearlite nucleation rate increases linearly with time with a temperature-dependent slope. The in-situ measured average pearlite growth rate is accurately described by the Zener- Hillert theory, which assumes that volume diffusion of carbon is the rate-controlling mechanism. The measured overall transformation rate deviates from the predictions of the theory developed by Kolmogorov, Johnson, Mehl, and Avrami. 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Pearlitic steels; Phase transformation kinetics; Neutron depolarization 1. Introduction alternating plates. Pearlite that consists of fine plates is harder and stronger than pearlite that con- Pearlite is a common constituent of a wide var- sists of coarse plates. This morphology of pearlite iety of steels and provides a substantial contri- is largely determined by the evolution of the bution to the strength. A pearlite colony consists austenite/pearlite phase transformation during the of two interpenetrating single crystals of ferrite and production process. Control of the pearlite phase cementite (Fe3C), which are primarily ordered as transformation kinetics is thus of vital importance for the production of tailor-made steels. Despite the large variety of austenite/pearlite ∗ Corresponding author. Tel.: +31-15-278-5630; fax: +31- phase transformation models that have been pro- 15-278-8303. posed and the experiments that have been perfor- E-mail address: [email protected] (S.E. med to test them in the past 60 years, the kinetics Offerman). 1 Present address: Faculty of Aerospace Engineering, Delft of this transformation is still not completely under- University of Technology, Kluyverweg 1, 2629 HS Delft, stood. The reason why the pearlite nucleation The Netherlands mechanism is still not fully understood, lies in the 1359-6454/03/$30.00 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(03)00217-9 3928 S.E. Offerman et al. / Acta Materialia 51 (2003) 3927–3938 experimental difficulty to measure nucleation specimen to room temperature. At each stage the phenomena and in particular the nucleation of largest pearlite colony size is determined, which is pearlite. The nucleation mechanism of pearlite a measure for the pearlite growth rate. This method involves the formation of two crystallographic has two drawbacks. The first drawback is that it phases. In hypo-eutectoid steels the pro-eutectoid only gives an estimate of the pearlite growth rate ferrite nucleates first and continues to grow with if the largest pearlite colony can be related to the the same crystallographic orientation during the first pearlite colony that nucleated. The second pearlite formation as part of a pearlite colony [1,2]. drawback is that the method only reflects the high- In this case the cementite nucleation is the rate- est observed growth rate. limiting step in the formation of pearlite. In hyper- These drawbacks can be avoided by using the eutectoid steels the roles of ferrite and cementite three-dimensional Neutron Depolarization (3DND) are reversed and in perfectly eutectoid steel the technique [17,18], which has created the opport- pearlite nucleation is assumed to take place at the unity to study phase transformations in-situ in the austenite grain corners, edges, and boundaries. bulk of steel. The 3DND technique provides a However, the exact nature of the pearlite unique insight into the formation of the microstruc- nucleation mechanism is still under debate. ture as it probes the volume fraction of the mag- Another important and continuing subject of netic phase, the mean magnetic particle size, and debate is the rate-controlling mechanism for the the spatial distribution of the forming ferromag- growth of pearlite. There are two different theories netic phase in the paramagnetic (austenite) matrix proposed for the growth of pearlite. The Zener-Hil- [19]. The technique is capable of determining these lert theory assumes that the volume diffusion of three parameters in-situ and simultaneously, which carbon in the austenite, ahead of the advancing makes it a powerful tool for the study of phase pearlite, is the rate-controlling mechanism [3,4]. transformations in the bulk of ferromagnetic The Hillert theory on the other hand assumes that materials. grain boundary diffusion of the carbon atoms is the The aim of this research is to measure in-situ rate-controlling mechanism [5]. Several experi- the pearlite fraction, the average pearlite colony mental studies were performed over the last dec- size, and the spatial distribution as a function of ades in order to measure the pearlite growth rate the isothermal transformation time. This gives and determine the rate-controlling mechanism. information about the nucleation rate, the rate-con- Both volume [6–9] and grain boundary diffusion trolling mechanism for the growth of pearlite, and [10] were claimed to be the rate-controlling mech- the validity of the KJMA theory for the prediction anism, as well as more complex mechanisms [11]. of the overall austenite/pearlite transformation rate. The theory developed by Kolmogorov, Johnson, The present paper is related to two previous papers Mehl, and Avrami [12–16], also known as the on the austenite/pearlite transformation in the same KJMA theory, predicts the overall transformation steel [20,21], which reported on the relation rate on the basis of nucleation and growth rates. between the magnetic domain structure and the The KJMA-theory is one of the oldest and most microstructure, and gave a comparison with the widely used models to describe the pearlite phase results of additional dilatometry experiments. transformation kinetics. This concept still forms the basis of many of the current phase transform- ation models. 2. Pearlite transformation kinetics The pearlite nucleation and growth rates have so far been determined from ex-situ optical and elec- 2.1. Nucleation tron microscopy measurements, in which a series of steel specimens is annealed for increasing times The nucleation rate of the newly formed phase at a particular transformation temperature. The during a phase transformation is described by the high-temperature microstructure is frozen in at sev- classical nucleation theory [22], in which the time eral stages of the transformation by quenching the dependent nucleation rate N˙ is expressed as S.E. Offerman et al. / Acta Materialia 51 (2003) 3927–3938 3929 ⌬ ∗ τ g ∗ G volume diffusion mechanism, DC,V is the volume ˙ ϭ ͩϪ ͪ ͩϪ ͪ a N Nnb Zexp exp , (1) diffusion coefficient of carbon in austenite, and l kBT t and lq are the lamella thickness of the ferrite and where Nn is the number of potential nucleation cementite, respectively. The pearlite lamellar spac- = a + q ga gq sites, Z is the Zeldovich non-equilibrium factor, ing amounts to l l l . Ceq and Ceq are the = × Ϫ23 kB 1.38 10 J/K is the Boltzmann constant, equilibrium carbon concentrations in the austenite and T the absolute temperature. The rate at which in contact with ferrite and cementite, respectively. a q the iron atoms are added onto the critical nucleus Ceq and Ceq are the equilibrium carbon concen- is taken into account by the frequency factor b∗. trations in ferrite and cementite, respectively. The The time t represents the incubation time and t is critical (theoretical minimum) spacing lc is given the isothermal transformation time. The time-inde- by pendent part of this equation is the steady state 2gaqT V nucleation rate. The energy barrier that has to be ϭ A1 m lc ⌬ ⌬ , (4) overcome in order to form a critical nucleus is T Hm referred to as the activation energy for nucleation where gaq = 0.94 J/m2 [23] is the interfacial free ⌬G∗, which can in general be written as energy of the ferrite/cementite interface in the ∗ pearlite, T (= 995 K for the studied steel) is the ⌬G ϭ⌿/⌬g2 . (2) A1 V austenite/pearlite equilibrium transition tempera- ⌬ = Ϫ ⌬ = The driving force for nucleation is the decrease in ture, T TA1 T is the undercooling, Hm Gibbs free energy per unit volume of the system 4.3 kJ/mol [23] is the change in molar enthalpy, ⌬ = × Ϫ6 3 gV during the phase transformation, which and Vm 7.1 10 m /mol the molar volume. depends on the chemical composition and the tem- The volume diffusion coefficient of carbon in aus- g perature. The creation of a new nucleus requires tenite DC,V depends on the temperature and the energy due to the formation of an interface nominal carbon concentration and can be described between the nucleus and the original phase. How- by [24] ever, in the case that the nucleus is formed at a 8339.9 Dg ϭ 4.53 ϫ 10Ϫ7ͩ1 ϩ Y (1ϪY ) ͪ (5) grain boundary the removal of incoherent C,V C C T austenite/austenite grain boundaries releases energy that can be used for the creation of a new 1 ϫ expͭϪͩ Ϫ 2.221 ϫ 10Ϫ4ͪ(17767Ϫ26436Y )ͮ, interface.