Effect of strong magnetic field on isothermal transformation of degenerate pearlite in an Fe-C-Mo alloy

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1 Available online at Acta Metall. Sin.(Engl. Lett.)Vol.23 No.4 pp August 2010 Effect of strong magnetic field on isothermal transformation of degenerate pearlite in an Fe-C-Mo alloy Zhenni ZHOU, Guohong ZHANG and Kaiming WU Hubei Province Key Laboratory of Systems Science in Metallurgical Processing, Institute of Advanced Steels and Welding Technology, Wuhan University of Science and Technology, Wuhan , China Manuscript received 10 December 2009; in revised form 1 April 2010 The pearlite transformation in a Mo-containing iron alloy was investigated under 12 T magnetic field. The pearlite transformation was accelerated owing to the application of a strong magnetic field. Pearlite was of degenerated morphology without the presence of a strong magnetic field; but the degeneracy of pearlite is reduced when a strong magnetic field was applied, which may be attributed to the effect of strong magnetic field on faster carbon diffusion and less molybdenum segregation caused by a strong magnetic field. KEY WORDS Steels; Pearlite; Phase transformation; Strong magnetic field 1 Introduction Owing to the scientific and industrial importance, the influence of alloying elements on the growth kinetics of proeutectoid ferrite in Fe-C-X alloys has attracted much research interest [1 3]. The alloying elements investigated include Mo, Cr, Mn, Ni, Co, Al, Si etc. Because Mo preferentially inhibits pearlite formation and exhibits a deep bay at intermediate temperatures [4 6], Fe-C-Mo alloy is regarded as one of the alloy systems in which alloy element effects are actively investigated [1 14]. Most of these studies are focused on solute drag effect of Mo, the growth kinetics of ferrite allotriomorphs and the incomplete transformation [1 12]. In recent years, degenerate ferrite transformation and carbide precipitation of an Fe-0.28C-3.0Mo (wt pct) alloy were investigated under strong magnetic fields [13,14]. It was found that the solute drag effect, caused by Mo element accumulation at ferrite/austenite boundaries below the TTT-diagram bay, is greatly reduced by the application of strong magnetic field [13]. The sequence of molybdenum carbide precipitation during isothermal holding in an Fe-C-Mo allloy was changed by applying a 12 T high magnetic field [14]. The present work aims to study the effects of strong magnetic filed on pearlite transformation of this alloy. The increasing availability of strong magnetic fields makes it possible to investigate magnetic field effects on various phase transformations in steels, including martensite [15,16], Corresponding author. Professor, PhD; Tel.: or ; Fax: address: wukaiming@wust.edu.cn; wukaiming2000@yahoo.com (Kaiming WU)

2 249 bainite [17,18], ferrite [19,20] and pearlite [21,22] transformations. Xu et al. [21] showed an increase for the number of pearlite nodules of hypereutectoid steels in a field of 10 T. Recently, the investigation for the effect of strong magnetic field on the formation of pearlite in a Fe-0.12C steel revealed that pearlite colonies elongate and align along the field direction, and that this tendency increases with increasing magnetic field strength [22]. These investigations are focused on Fe-C alloys, but few studies have been made for magnetic field effects on pearlite transformation in Mo alloyed steel. And the alloying element Mo has much strong influence on pearlite transformation [23 26]. Thus, the present work focuses the effects of strong magnetic field on pearlite transformation in a Mo alloyed steel, especially on the morphology of pearlite. 2 Experimental In order to avoid the complication of interactive effects among multiple solutes, the alloy studied was prepared by vacuum induction melting utilizing high purity electrolytic iron, graphite and molybdenum. The chemical composition (wt pct) of steel sample is C 0.28%, Si<0.01%, Mn<0.01%, P<0.01%, S<0.01%, Mo 3.0%. Ingots were hot worked and then homogenized at 1250 C for two days in a vacuum quartz capsule. Specimens of 4 mm 4 mm 18 mm were cut from the homogenized sample and austenitized at 910 C for 30 min in an argon atmosphere and then isothermally transformed in a salt bath at temperatures ranging from 610 to 700 C with and without the presence of a 12 T magnetic field. Specimens were immediately quenched after isothermal holding. The schematic illustration of heat treatment apparatus and the relative position between the specimen and magnetic field direction are shown in Fig.1. After heat treatment specimens were polished and etched with 3 vol. pct nital solution for microscopy analysis. The polished planes were perpendicular to the field direction. The mean grain size after austenitization was µm. Fig.1 Schematic illustration of heat treatment apparatus (a) and the relative position between the specimen and magnetic field direction (b).

3 250 3 Results 3.1 Fraction of pearlite transformed The progress of the overall transformation was measured with the standard pointcounting technique at a magnification of 500. The measured volume fraction of transformed pearlite when a 12 T magnetic field is on and off, is listed in Table 1. Table 1 Measured fraction of pearlite transformed with and without the presence of a strong magnetic field Temperature 600 s 3600 s Off On Off On 640 C 0 Very few 0.5% 1.2% 700 C 0.2% 0.4% 0.9% 2.5% 3.2 SEM microstructures of pearlite SEM micrographs of the specimens isothermally held in a magnetic field of 0 and 12 T at 700 C for 600 s and 3600 s are presented in Figs.2 and 3. It is seen that transformed products are predominantly ferrite grains and a lot of pearlite nodules. When specimens were held for 600 s, less and smaller pearlite nodules were formed. With the increase of isothermal holding, more and bigger pearlite nodules were formed. In addition, a typical lamellar structure of pearlite was not observed. The mixed structures of ferrite and cementite are clearly seen. Ferrite and cementite layers are not continuous. This is so-called Fig.2 SEM images of specimens isothermally held at 700 C for 600 s without (a) and with (b) the presence of 12 T magnetic field. Fig.3 SEM images of specimens isothermally held at 700 C for 3600 s without (a) and with (b) the presence of 12 T magnetic field.

4 251 degenerate pearlite [23]. However, some lamellar structures or continuous layers are observed in the pearlite nodules, as marked by circles in Figs.2b and 3b, when the specimen was heat treated in a strong magnetic field. Fig.4 shows SEM micrographs of pearlite transformed at lower temperature (640 C) for 3600 s. It is seen that the pearlite is of serious degenerated morphology when the specimen was heat treated without magnetic field. It is actually a mixed microstructures consisting of granular cementite and ferrite, as marked by circles in Fig.4a. However, some lamellar structures or continuous layers are observed in the pearlite nodules, as marked by circles in Fig.4b, when the specimen was heat treated in 12 T magnetic field. This indicates that the degeneracy of pearlite is reduced by the application of a strong magnetic field. Fig.4 SEM images of specimens isothermally held at 640 C for 3600 s without (a) and with (b) 12 T magnetic field. At higher transformation temperature (700 C), pearlite formed at shorter (600 s) and longer (3600 s) holding times (Figs.2 and 3). At lower transformation temperatures, pearlite formed only at a longer holding time (3600 s), no matter whether a strong magnetic field was applied or not (Table 1). Pearlite was just observable in the specimen isothermally held at 640 C for a longer holding time (3600 s), which is caused by slow diffusion of carbon, iron and molybdenum at lower temperature. No pearlite formed at 610 C even for longer holding times. 4 Discussion 4.1 Effect of magnetic field on pearlite transformation rate The effect of alloying elements on the transformation behavior of austenite in steels may be rationalized on the basis of understanding of how the alloying elements are partitioned between the various phases during the transformation. It is reported that pearlite grows under partitioning of molybdenum between ferrite and cementite [24]. The addition of molybdenum raises eutectoid temperature and partitions over the full range of pearlite formation [25]. The diffusion of Mo and its interaction with carbon influences transformation rate and product morphology. Pearlitic alloy steels, especially Mo alloyed steel, exhibit slow growth rate. Adding 0.77 wt pct Mo to eutectoid steel is observed to increase the pearlite transformation time

5 252 by approximately times, compared with the alloy without Mo addition [26]. Mo preferentially inhibits pearlite formation when Mo concentration reaches higher levels [4 6,10 12]. This is attributed to an interaction between the alloying element and carbon in the transformation interface, leading to a reduction in the diffusion coefficient of carbon and in the driving force for diffusion [27]. In the present work, the alloy contains 3.0Mo and 0.28C, which causes the slow transformation observed. Because the ferrite is ferromagnetic below about 770 C whereas the austenite in lowalloy steels is paramagnetic, it is expected that transformation under the influence of a magnetic field will increase the driving force for austenite-pearlite transformation. The phase diagram in a ternary Fe-C-Mo alloy in the presence of 12 T magnetic field was calculated using Weiss molecular field theory, which took into account phenomenologically the influence of alloying element (Mo) on the magnetic moment and the Curie temperature of iron solid solution. The full details of calculation procedure have been reported elsewhere [19,28] The relevant thermodynamic data were taken from Uhrenius compilation [29] 900. Fig.5 shows the isoplethal + +cem section of Fe-C-3Mo (wt pct) system with 800 and without a 12 T magnetic field. The upper Ae1 temperature [30] (the upper temperature limit for pearlite formation) of the present alloy was 747 C without magnetic field whereas it increased to 758 C with a 12 T magnetic field. Thus, a 12 T magnetic field raised the transformation temperature, increased the undercooling and accelerated the pearlite transformation. Temperature / o C cem + +cem Carbon content / wt pct Fig.5 Isoplethal section of Fe-C-3.0%Mo system without (solid line) and with (dashed line) a 12 T magnetic field. 4.2 Effect of magnetic field on pearlite morphology Degenerated morphology of pearlite was observed in the specimens no matter whether a strong magnetic field was applied or not. However, the degeneracy of pearlite was reduced by the presence of a 12 T magnetic field, especially at lower temperature (Fig.4). Similar to lamellar pearlite, degenerated pearlite is also formed by diffusion process and the formation of the latter is attributed to the insufficient diffusion of carbon to develop continuous lamellae [31]. It is recently reported that a strong magnetic field can reduce grain boundary segregation of substitutional solutes in iron alloy [32]. It is suggested that magnetic field can assist carbon diffusion in γ-iron [32]. It is also reported that the solute drag effect, caused by the segregation of Mo and the interaction of carbon and Mo, in a Fe-C-Mo alloy was greatly reduced by applying a 12 T magnetic field [13]. The reduction of pearlite degeneracy may thus be attributed to the effects of strong magnetic field on the faster diffusion of carbon and less segregation of molybdenum, which nevertheless needs further studies. Recently, Song et al. investigated the effect of strong magnetic field on the formation of pearlite in a Fe-0.12C steel [22]. It was reported that pearlite colonies elongated and aligned along the field direction due to the preferential nucleation of proeutectoid ferrite in

6 253 the late stage of proeutectoid transformation. The pearlite colonies elongated and aligned along the field direction were not observed. In the present work, the polishing planes of field-treated specimens were perpendicular to the field direction. 4.3 Effect of austenitizing temperature on pearlite transformation Pearlite transformation is preferentially inhibited by the addition of Mo [4 6]. It is reported that pearlite did not form even for a longer time (10000 s) when specimens were austenitized at 1250 C or higher [4 6,10 12]. In these conditions the grain size after austenitization reached µm. It is worthy of noting that the present alloy was austenitized at a much lower temperature (910 C), pearlite formation was observed in the specimens isothermally held for even shorter times (600 s). In the present study, the grain size after austenitization is µm and the average is 14.6 µm. The pearlite transformation in the present study is also promoted by much smaller austenite grain size. 5 Conclusions The isothermal transformation of degenerate pearlite in an Fe-0.28C-3.0Mo (wt pct) steel was investigated under 12 T magnetic field utilizing scanning electron microscopy (SEM). Degenerate pearlite was both observed in the temperature range from 640 to 700 C without and with applying a strong magnetic field. Pearlite was of degenerated morphology when magnetic field was not applied whereas some lamellar structures formed when a strong magnetic field was applied. The reduction of pearlite degenerency is probably attributed to the assistance of carbon diffusion and reduction of molybdenum segregation caused by a strong magnetic field. Acknowledgements The authors express their thanks to Professor M. Enomoto, Ibaraki University, Japan, for providing alloy specimen and conducting magnetic heat treatment in his laboratory. The authors gratefully acknowledge the financial support for this research from State Ministry of Education (No.NCET ) and from Natural Science Foundation of Hubei Province (No.2006ABB037). REFERENCES [1] J.R. Bradley and H.I. Aaronson, Metall Trans 12A (1981) [2] K.R. Kinsman and H.I. Aaronson, Transformation and Hardenability in Steels (Climax Molybdenum Co., Michigan, 1967). [3] C. Atkinson, H.B. Aaron, K.R. Kinsman and H.I. Aaronson, Metall Trans 4 (1973) 783. [4] H. Tsubakino and H.I. Aaronson, Metall Trans 18A (1987) [5] G.J. Shiflet and H.I. Aaronson, Metall Trans 21A (1990) [6] W.T. Reynolds, F.Z. Li, C.K. Shui and H.I. Aaronson, Metall Trans 21A (1990) [7] H.A. Fletcher, A.J. Garratt-Reed, H.I. Aaronson, G.R. Purdy, W.T. Reynolds and G.D.W. Smith, Scr Mater 45 (2001) 561. [8] H.I. Aaronson, W.T. Reynolds and G.R. Purdy, Metall Mater Trans 35A (2004) [9] E.S. Humphreys, H.A. Fletcher, J.D. Hutchins, A.J. Garratt-reed, W.T. Reynolds, H.I. Aaronson, G.R. Purdy and G.D.W. Smith, Metall Mater Trans 35A (2004) [10] K.M. Wu and M. Enomoto, Scr Mater 46 (2002) 569. [11] K.M. Wu, M. Kagayama and M. Enomoto, Mater Sci Eng A343 (2003) 143. [12] M. Enomoto, N. Maruyama, K.M. Wu and T. Tarui, Mater Sci Eng 343A (2003) 151. [13] M. Enomoto, K.M. Wu and M. Kagayama, CAMP-ISIJ 17 (2004) [14] Z.N. Zhou and K.M. Wu, Scr Mater 61 (2009) 670.

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