Direct spheroidization of high carbon steels: effect of thermomechanical processing

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1 olid tate Phenomena Vols (20) pp Online available since 20/Jun/30 at (20) Trans Tech Publications, witzerland doi:0.4028/ Direct spheroidization of high carbon steels: effect of thermomechanical processing Matteo Caruso, a, Hector Verboomen, b and téphane Godet, c ervice Matières et Matériaux, Université ibre de Bruxelles (U..B.), 50 Avenue F.D. Roosevelt, CP 94/03, 050 Bruxelles, Belgium a mcaruso@ulb.ac.be, b hverboom@ulb.ac.be, c sgodet@ulb.ac.be Key words: high carbon steel, pearlite, divorced eutectoid transformation (DET), spheroidization, mechanical properties Abstract. The eutectoid transformation of austenite can occur cooperatively (pearlite transformation) or by means of a non-cooperative mode (Divorced Eutectoid Transformation). In the cooperative mode, ferrite and cementite grow together, leading to the typical lamellar microstructure of pearlite. In the non-cooperative mode, spheroidal cementite particles grow directly from the undissolved carbides in the austenite phase. The transformation product is a fully spheroidized microstructure. In this study, the parameters promoting the occurrence of DET in a hypereutectoid steel (austenitization temperature, cooling rate, presence of proeutectoid cementite in the initial microstructure) were investigated. It is shown that low undercooling levels and a homogenous distribution of fine carbides in the austenite promote the DET over the lamellar transformation mode. The spheroidized microstructures produced by DET lead to larger ductilities comparing to those obtained by the lamellar transformation mode. Introduction High carbon steels exhibiting a spheroidized microstructure can be successfully employed for sheet applications, involving subsequent cold rolling or machining. Indeed, the globular morphology of pearlite, consisting of ferrite and spherical particles of cementite, exhibits high toughness, good cold formability and machinability compared to the lamellar morphology. In the current industrial practice, long batch annealing treatments at temperatures below A e are needed in order to spheroidize the lamellar structure. The aim of this work is to investigate the acceleration effect of an intercritical annealing (i.e. between A e and A cm ) on the spheroidization process of a hypereutectoid steel. Previous work [2] demonstrate that the decomposition of austenite containing undissolved carbides can occur through a non cooperative transformation mode (Divorced Eutectoid Transformation) that leads directly to a spheroidized microstructure. In the present study, the influence of the processing parameters (austenitization temperature and cooling rates) and of the initial microstructure was studied. The mechanical properties of the resulting microstructures were characterized by tensile testing. Experimental Procedure The chemical composition of the hypereutectoid steel investigated is given in Table. Table. Chemical composition of the hypereutectoid steel investigated. C Mn i N P Weight %,23 0,69 0,20 0,006 0,008 The A e and A cm temperatures were calculated using Thermocalc and experimentally verified by Differential canning Calorimetry. They were found to be 75 C and 925 C, respectively. In order to evaluate the effect of the carbide distribution, two different initial microstructures were used. The first one was an equilibrium microstructure consisting of pearlite and proeutectoid cementite). It All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /07/,6:27:56)

2 olid tate Phenomena Vols will be thereafter designated by EQ. It was obtained after a 20-minute annealing step at 960 C followed by cooling at C/min. The interlamellar spacing (IP) is 500 nm (Fig..a). The second one (Fig..b) was produced by hot rolling (ε=0,25 at 850 C) followed by air cooling (cooling rate: 500 C/min approximately). The resulting microstructure was completely pearlitic and thus fully lamellar (IP: 50 nm). In what flows, this starting microstructure will be designated by AM. The subsequent thermal treatments performed on these two microstructures are illustrated in Fig..a and Fig..b. The influence of the austenitization temperature was first investigated. The samples were austenitized at different temperatures (A e +65 C; A e +00 C; A e +50 C; A e +250 C) and furnace cooled at C/min (Fig..c). In order to study the influence of the cooling rate (Fig..d), the samples were austenitized at the same temperature (A e +65 C) and then cooled down at different cooling rates: air cooling at approximately 500 C/min and furnace cooling at 3 C/min and C/min. The carbide distribution on samples water quenched from 725 C, i.e. just before the eutectoid transformation was analysed. The obtained carbide dispersion was characterized by the means of the mean distance between carbides (interparticle distance). It was calculated using the linear intercept method conducted by image analysis on EM micrographs. The microstructures were characterized after conventional mechanical polishing and etching with Picral 5%. The ferrite bock size was measured by means of EBD techniques. The mechanical properties were evaluated by tensile testing. Figure : EM micrographs of the starting microstructures: (a) Equilibrium microstructure (EQ) consisting of pearlite proeutectoid cementite obtained by austenitization at 960 C followed by cooling at C/min; (b) Fully pearlitic microstructure (AM) obtained by hot rolling followed by air cooling at 500 C/min; (c) heat treatments performed in order to study the influence of the austenitization temperature; (d) heat treatments performed in order to study the influence of the cooling rates. Results Microstructural Evolution. The microstructures produced after the different heat treatments investigated are presented in Fig. 2 and Fig. 3. The microstructures of Fig. 2 were obtained from a fully pearlitic initial microstructure (AM) whereas the microstructures of Fig. 3 were formed from an initial microstructure consisting of pearlite and proeutectoid cementite (EQ). In both cases, when the austenitization temperature is equal to or higher than 860 C, a lamellar microstructure was obtained. The interlamellar spacing is found to decrease as the cooling rate increases. If the austenitization temperature is low enough (80 C in our experiments) as to lead to a partial dissolution of the carbides, the decomposition of austenite takes place following a non cooperative mode at slow cooling rates (up to 3 C/min). The corresponding microstructures consist of ferrite and spherical particles of cementite. Fast cooling rates after intercritical annealing lead to microstructures consisting of lamellar pearlite and particles of proeutectoid cementite (Fig. 2.a and Fig. 3.a). In the case of an initial microstructure consisting of pearlite, a homogeneous and spheroidized microstructure can be obtained using cooling rates up to 3 C/min. When proeutectoid cementite is present in the initial microstructure, the microstructure is not homogeneous for cooling rates larger than C/min and it shows evidence of a mixed transformation mode. Indeed, part of the microstructure is lamellar (cooperative mode) while the rest is spheroidized (divorced mode). ower cooling rates ( C/min) are necessary to achieve an almost spheroidized microstructure.

3 924 olid-olid Phase Transformations in Inorganic Materials a) b) c) d) e) f) Figure 2: Microstructures obtained after the thermal treatments of Fig..c and Fig..d. tarting microstructure: fully pearlitic (AM). (a) Austenitization Temperature (A.T.): 775 C, cooling rate (C.R.): 500 C/min; (b) A.T: 775 C, C.R: 3 C/min; (c) A.T: 775 C, C.R: C/s; (d) A.T: 80 C, C.R: C/min; (e) A.T: 860 C, C.R: C/min; (f) A.T: 960 C, C.R: C/min. a) b) c) d) e) f) Figure 3: Microstructures obtained after the thermal treatments of Fig..c and Fig..d. tarting microstructure: pearlite and proeutectoid cementite (EQ). (a) Austenitization Temperature (A.T.): 775 C, cooling rate (C.R.): 500 C/min; (b) A.T: 775 C, C.R: 3 C/min; (c) A.T: 775 C, C.R: C/s; (d) A.T: 80 C, C.R: C/min; (e) A.T: 860 C, C.R: C/min; (f) A.T: 960 C, C.R: C/min.

4 olid tate Phenomena Vols Mechanical Properties. The tensile curves of the samples investigated are presented in Fig. 4.a and Fig. 4.b. The tensile properties of the different samples are summarized in Table 2. Table 2: Tensile properties of the samples investigated. states for lamellar microstructure, for spheroidized microstructure and / for the presence of lamellar and spheroidized cementite. tarting microstructure: Fully pearlitic Pearlite and proeutectoid cementite Austenitization Temperature [ C] Cooling rate [ C/min] Cementite morphology Yield Tensile train at necking [-] Fracture stress Cementite morphology Yield Tensile train at necking [-] Fracture strain [-] / a) b) Figure 4: Tensile stress-strain curves: (a) tarting microstructure: fully pearlitic (AM); (b) tarting microstructure: pearlite and proeutectoid cementite (EQ). The yield and ultimate tensile stress of lamellar microstructures are strongly related to the interlamellar spacing: the increases as the interlamellar spacing decreases. uch microstructures exhibit a limited tensile ductility and the fracture strain does not exceed 0, in all cases. The ductility is further reduced as the austenitization temperature increases. The fracture surface is typical of a transgranular fracture mechanism and consists of cleavage facets of about 50 µm in size (Fig. 5.b). It evidences the lack of ductility resulting from by high austenitization temperatures and the associated lamellar morphology. On the contrary, a spheroidized morphology of pearlite leads to larger uniform deformations and lower levels. The homogenous distribution of cementite contributes to en the ferritic matrix and the stress at necking is about 860 MPa in all spheroidized samples. pheroidized microstructures exhibit large uniform deformations up to 0,8. As it can be seen from Table 2 and Fig. 4.b, the presence of large cementite particles reduces the uniform elongation slightly. The fracture surfaces of spheroidized microstructures (Fig. 5.b) consist of dimples and voids, which are typical of ductile fracture. The size of the voids is quite close to that of the cementite particles, supporting the fact that void nucleation occurs at cementite particles. Figure 5: Typical fracture surfaces: (a) Coarse lamellar microstructure, sample austenitized at 960 C and cooled at C/min; (b) pheroidized microstructure, sample austenitized at 775 C, cooled at 3 C/min.

5 926 olid-olid Phase Transformations in Inorganic Materials Discussion The decomposition of austenite containing undissolved carbides can occur either by the classical pearlitic transformation or by the DET transformation, depending on the carbides distribution in the austenitic matrix and on the transformation temperature, i.e. the undercooling levels [3]. The kinetic model for the DET proposed by Verhoeven [2] assumes that, as the transformation front moves into the austenite, the carbons atoms rejected from the ferrite are simply deposited onto the pre-existing carbides particles, without nucleation of new cementite particles. This leads to the growth of the carbide by an amount corresponding to the carbon mass transport. Therefore, the velocity of the DET reaction can be calculated trough a carbon mass flux balance at a transformation interface located between two particles. It can be expressed as a function of the interparticle spacing and the undercooling, as shown by Verhoeven in [2]. The velocity of the DET reaction can be compared with that of the lamellar reaction. In this study, the growth rate of the lamellar reaction was approximated as a function of the undercooling level using the neural network model of Capdevila [4] et al., which takes into account the decrease in the growth rate due to the partitioning of Mn during the lamellar transformation. This analysis yields: v = 0,0003 T 2, 43 [µm / s ] where v is the displacement speed of the transformation front and T is the undercooling level. The comparison between the velocity of the DET and of the lamellar reaction defines a transition line for undercooling levels and interparticle spacing below which the kinetics of the DET is faster than that of the lamellar reaction. This transition line is depicted in Fig. 6. The divorced transformation mode is promoted only when the undercooling level is low and the interparticle spacing is small enough. Indeed, large particle interdistances require carbon diffusion over long distances and consequently limit the velocity of the DET front. arge undercooling levels generate large driving forces for the transformation and consequently high interface velocity. Under such conditions, the DET process would require a too high diffusion flux of carbon from the transformation front to the carbide particles. The experimental undercooling levels and the corresponding interparticle spacing measured in this study are in good agreement with the kinetic model developed using the approach of Verhoeven (Fig. 6). pheroidized microstructure (Fig. 2 and 3) are obtained when the thermal treatment consists of an austenitization step at low temperatures followed by slow cooling rates, i.e. for low level of undercooling. A fully pearlitic initial microstructure promotes the DET as the partial austenitization produces a homogenous distribution of small undissolved carbides originating from former cementite lamellae. On the contrary, the austenitization of a microstructure containing proeutectoid cementite leads to an irregular distribution of carbides in the austenite matrix, constituted by few large proeutectoid cementite particles and fragments of undissolved lamellae. Figure 6: Transition line between Divorced Eutectoid Transformation and amellar Transformation as function of undercooling and interparticle spacing. Circle and square symbols show experimental data for the spheroidized and lamellar microstructures respectively. Filled symbols indicate a fully pearlitic initial microstructure (AM). Unfilled symbols indicate a pearlite and proeutectoid cementite (EQ) initial microstructure.

6 olid tate Phenomena Vols The mechanical behavior of spheroidized microstructure differs strongly from that of lamellar microstructures. It is well known that the yield of lamellar pearlite is linearly related to the reciprocal of the square root of the interlamellar spacing [5]. In the case of spheroidized microstructures, the is similarly related to the interparticle spacing that depends mainly on the carbon content of the steel [6]. According to Takashi [7], the tensile of lamellar pearlitic steels depends mainly on the size of the ferrite blocks constituting the pearlitic microstructure. This is well verified in this study, as it can be seen from Fig 7.a. The ferrite block size is controlled mainly by the austenitization temperature as it is directly related to the prior austenite grain size. Due to the large ferrite block size, samples austenitized at higher temperature undergo fracture as soon as the critical stress for transgranular fracture is reached. This critical stress limits the tensile ductility of the lamellar microstructures. a) b) Figure 7: (a) Tensile as function of the ferrite block size for the lamellar microstructures; (b) Fracture strain as function of the average coarse carbide size for the spheroidized microstructures. On the contrary, the ferrite grain size of the spheroidized microstructures produced trough the DET process is found to be close to 0 µm in all cases because of the low austenitization temperature and the pinning effect of the fine dispersion of carbides that prevents grain growth. In such cases, the parameter that governs the tensile ductility is the size of the coarse carbides present at the ferrite grain boundaries (Fig. 7.b). Indeed, the fracture mode of spheroidized microstructures involves the nucleation and the growth of voids at the cementite particles. The present experimental data confirm the work of yn et al. [7], who found a linear relation between the fracture and the reciprocal of the square root of the size of the coarse cementite particles. The refinement of the microstructures consisting of fine cementite particles into a fine ferrite matrix induced by the DET process is beneficial for both the ductility and the tensile. ummary and conclusions ow undercooling levels and homogenous distributions of fine carbides after austenitization are shown here to promote the non-cooperative mode of eutectoid transformation (DET) over the lamellar transformation. Furthermore, a starting microstructure without coarse cementite particle facilitates the occurrence of the DET. The ductility of the spheroidized microstructures produced by DET is governed by the size of the coarse carbide at the ferrite boundaries. Acknowledgements The authors acknowledge the financial support of the Walloon region (Winnomat program). References [] A. Fernandez, M.Carsi, E.Taleff, O.A.Ruano: Mat. ci. Eng. Vol. 335A (2002),p. 75. [2] J.D. Verhoeven, E.D.Gibson: Met. Mat. Trans. Vol. 29A (998), p. 8. [3] N.V. uzginova,.zaho, J.ietsma: Met. Mat. Trans. Vol. 39A (2008), p. 53. [4] C.Capdevila, F.G. Caballero, G. Garcia de Andrés: IIJ Int., Vol. 45 (2005), p.238. [5] J.P. Hugo, J.H. Woodhead: J. Iron teel Inst. Vol 86 (957); p.74. [6] C.K. yn, D.R. esuer, O.D. herby: Met. Mat. Trans. Vol. 25A (994), p. 48. [7] T.Takashi: J.Jpn. Inst. Met. Vol. 42 (978), p. 76. [8] R.ong, D. Ponge, D. Raabe: cripta Materialia. Vol. 52 (2005), p. 075.

7 olid-olid Phase Transformations in Inorganic Materials doi:0.4028/ Direct pheroidization of High Carbon teels: Effect of Thermomechanical Processing doi:0.4028/