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On the misuse of the term bainite
Scripta mater. 43 (2000) 831– 833 www.elsevier.com/locate/scriptamat
ON THE MISUSE OF THE TERM BAINITE M. Hillert Department of Materials Science and Engineering, KTH, SE-10044 Stockholm, Sweden
G.R. Purdy Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7 (Received May 5, 2000) (Accepted May 23, 2000) Keywords: Bainite; Steel; Diffusion; Phase transformation
Introduction In steels one finds two different types of eutectoid structures, i.e. mixtures of ferrite and cementite. They are called pearlite and bainite and are both formed by decomposition of the high-temperature phase austenite. There is now general agreement that pearlite forms by the edgewise growth of a stack of alternating lamellae of ferrite and cementite. An early proposal that sidewise growth by repeated nucleation of a new lamellae at the side of the stack would also be an important growth mechanism [1], was long accepted but was finally proven incorrect [2]. For bainite there has been a more long-lived controversy. Based upon microscopic observations, Hultgren [3] proposed that bainite forms by the edgewise growth of a set of parallel Widmansta¨tten plates of ferrite, followed by the transformation of the interjacent spaces to a mixture of cementite and ferrite. On the other hand, long ago it was generally believed that all the transformation products of austenite were formed by a primary reaction to martensite and, as described in a recent review [4], that idea in some way survived longer for bainite than for pearlite. In an attempt to rationalize the various transformation products of austenite, Zener [5] developed the idea in scientific terms and thus proposed without any proof and without giving any reason that bainite forms in an manner similar to martensite. Evidently, his hypothesis cannot cover the case described by Hultgren, who focused on so-called upper bainite. Of course, Zener’s hypothesis should not be dismissed without experimental test, at least for so-called lower bainite which may form with a shape reminiscent of plate martensite. However, it should immediately be mentioned that by the addition of silicon one can prevent the formation of cementite in the interjacent spaces for some time and can thus form stacks of Widmansta¨tten plates separated by plates of austenite. This austenite is often retained after cooling to room temperature, probably because the carbon content of austenite in equilibrium with ferrite is very high at low temperatures. The resulting structure of ferrite ⫹ austenite bears no resemblence to martensite. For some reason, this structure is today often called bainite although without cementite it is not a eutectoid structure. It should also be emphasized that the use of electron microscopy has now allowed the observation of ferrite as the leading phase in the growth of bainite, in the form of tips of Widmansta¨tten ferrite plates, even for many cases of lower bainite. 1359-6462/00/$–see front matter. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00484-X
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ON THE MISUSE OF THE TERM BAINITE
Vol. 43, No. 9
Discussion Support for Zener’s hypothesis came through the observation by Ko and Cottrell [6] that lower bainite gives rise to a surface relief, similar to that resulting from martensite. However, it was soon shown that Widmansta¨tten ferrite also gives a surface relief [7] and today there seems to be wide agreement [8,9,10] that a martensitic type of growth by shear cannot be proven by studying the surface relief. An argument against Zener’s hypothesis came through measurements of the edgewise growth rate of Widmansta¨tten ferrite and bainite in low alloy steels [11,12]. Information from a wide range of temperatures gave a single curve and it was possible to explain the growth rates as controlled by carbon diffusion. Hehemann [13] tried to keep Zener’s hypothesis alive by maintaining that the tip of Widmansta¨tten ferrite advances in very quick but short steps. However, in a discussion with Kinsman and Aaronson [14] he was finally forced to admit that he found it “difficult to argue against these diffusion controlled models.” For a long time after him there did not seem to be any serious proponent for Zener’s hypothesis until it was again taken up by Bhadeshia [15,16]. It is not the purpose of the present note to discuss Bhadeshia’s arguments for Zener’s hypothesis. However, his many publications on bainite, arguing for Zener’s hypothesis, seem to have spread the idea that there are two types of acicular growth of ferrite, Widmansta¨tten growth and “bainitic” growth, which can be applied to acicular growth in other systems as well. Actually, that idea had been expressed before Bhadeshia, e.g. in attempts to distinguish between two types of acicular precipitation of the fcc phase from the bcc phase in Cu-Zn alloys [17]. However, it seems that both types are now regarded as Widmansta¨tten precipitation, and that bainite is no longer used as the name for a decomposition product of  brass. In a recent paper on the formation of austenite by heating a mixture of ferrite and cementite, Kaluba, Taillard and Foct [18] mentioned “bainitic mechanism” in the title. This was naturally interpreted by Aaronson and Nie to imply that acicular austenite could form in the same manner as bainite according to Zener’s hypothesis as advocated by Bhadeshia. In discussion [19] they questioned a number of arguments by Kaluba et al. but in a reply [20] those authors declared that they “neither tried to prove a particular mechanism of bainite formation (from austenite), nor to propose a martensitic model for the growth of austenite.” They further mentioned “that both bainitic and Widmansta¨tten terms are often confused, even with regard to ferrite, due to the apparent similarity in both morphologies and in spite of existing classifications and definitions.” However, there seems to be no room for confusion if it is remembered that bainite is a eutectoid (two-phase) structure and a Widmansta¨tten plate consists of a single phase. It is now proposed that, if there really are two types of acicular precipitation of a single phase with a change in composition already at the moment of growth, they should be called Widmansta¨tten I and II. Before introducing such names, it is necessary (1) to prove how the growth mechanisms differ, (2) to show how they can be distinguished by physical measurement techniques and (3) that they are two distinct types and not members of a continuous series. According to the present authors, it is quite conceivable that the atomic structure of the phase interface of a Widmansta¨tten precipitate changes with the temperature of formation and the driving force [21]. In that case it would not seem reasonable to single out a particular interfacial structure and relate it to a particular name. In any case, the authors of the present note propose that the term “bainite” should only be used for eutectoid (two-phase) structures. If an acicular precipitation occurs without diffusion and with a glissile interface, it should grow with a high rate and should be regarded as martensite. If there is diffusion soon after its formation, the product should be called tempered martensite.
Vol. 43, No. 9
ON THE MISUSE OF THE TERM BAINITE
833
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
R. F. Mehl and W. C. Hagel, Progr. Metal Phys. 6, 74 (1956). M. Hillert, in Decompositon of Austenite by Diffusional Processes, ed. V. F. Zackay and H. I. Aaronson, p. 197, Interscience Publishers, New York (1962). A. Hultgren, Trans. ASM. 39, 815 (1947). M. Hillert, ISIJ Int. 35, 1134 (1995). C. Zener, Trans. AIME. 167, 550 (1946). T. Ko and A. H. Cottrell, J. Iron Steel Inst. 172, 307 (1952). A. P. Miodownik, in The Mechanism of Phase Transformations in Metals, Inst. Metals Monograph and Report Series, No. 18, p. 319 (1956). W.-Z Zhang and G. C. Weatherly, Acta Mater. 46, 1837 (1998). J. P. Hirth, G. Spanos, M. G. Hall, and H. I. Aaronson, Acta Mater. 48, 1047 (1997). R. C. Pond, P. Shang, T. T. Cheng, and M. Aindow, Acta Mater. 48, 1047 (2000). M. Hillert, Report, Swedish Institute of Metal Research (1960). L. Kaufman, S. V. Radcliffe, and M. Cohen, in Decompositon of Austenite by Diffusional Processes, eds. V. F. Zackay and H. I. Aaronson, p. 313, Interscience Publishers, New York (1962). J. M. Oblak and R. F. Hehemann, Transformation and Hardenability in Steels, p. 15, Climax Molybdenum Co., Ann Arbor, MI (1967). R. F. Hehemann, K. R. Kinsman, and H. I. Aaronson, Trans. AIME. 3, 1077 (1972). H. K. D. H. Bhadeshia, J. Phys. IV Fr. 7(C5), C5–367 (1997). H. K. D. H. Bhadeshia, Bainite in Steels, Institute of Materials, London (1992). R. D. Garwood, J. Inst. Metals. 83, 64 (1954). W. J. Kaluba, R. Taillard, and J. Foct, Acta Mater. 46, 5917 (1998). H. I. Aaronson and J. F. Nie, Scripta Mater. 42, 505 (2000). W. J. Kaluba, R. Taillard, and J. Foct, Scripta Mater. 42, 509 (2000). M. Hillert and G. R. Purdy, Acta Metal. 32, 823 (1984).
ON THE MISUSE OF THE TERM BAINITE M. Hillert Department of Materials Science and Engineering, KTH, SE-10044 Stockholm, Sweden
G.R. Purdy Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7 (Received May 5, 2000) (Accepted May 23, 2000) Keywords: Bainite; Steel; Diffusion; Phase transformation
Introduction In steels one finds two different types of eutectoid structures, i.e. mixtures of ferrite and cementite. They are called pearlite and bainite and are both formed by decomposition of the high-temperature phase austenite. There is now general agreement that pearlite forms by the edgewise growth of a stack of alternating lamellae of ferrite and cementite. An early proposal that sidewise growth by repeated nucleation of a new lamellae at the side of the stack would also be an important growth mechanism [1], was long accepted but was finally proven incorrect [2]. For bainite there has been a more long-lived controversy. Based upon microscopic observations, Hultgren [3] proposed that bainite forms by the edgewise growth of a set of parallel Widmansta¨tten plates of ferrite, followed by the transformation of the interjacent spaces to a mixture of cementite and ferrite. On the other hand, long ago it was generally believed that all the transformation products of austenite were formed by a primary reaction to martensite and, as described in a recent review [4], that idea in some way survived longer for bainite than for pearlite. In an attempt to rationalize the various transformation products of austenite, Zener [5] developed the idea in scientific terms and thus proposed without any proof and without giving any reason that bainite forms in an manner similar to martensite. Evidently, his hypothesis cannot cover the case described by Hultgren, who focused on so-called upper bainite. Of course, Zener’s hypothesis should not be dismissed without experimental test, at least for so-called lower bainite which may form with a shape reminiscent of plate martensite. However, it should immediately be mentioned that by the addition of silicon one can prevent the formation of cementite in the interjacent spaces for some time and can thus form stacks of Widmansta¨tten plates separated by plates of austenite. This austenite is often retained after cooling to room temperature, probably because the carbon content of austenite in equilibrium with ferrite is very high at low temperatures. The resulting structure of ferrite ⫹ austenite bears no resemblence to martensite. For some reason, this structure is today often called bainite although without cementite it is not a eutectoid structure. It should also be emphasized that the use of electron microscopy has now allowed the observation of ferrite as the leading phase in the growth of bainite, in the form of tips of Widmansta¨tten ferrite plates, even for many cases of lower bainite. 1359-6462/00/$–see front matter. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00484-X
832
ON THE MISUSE OF THE TERM BAINITE
Vol. 43, No. 9
Discussion Support for Zener’s hypothesis came through the observation by Ko and Cottrell [6] that lower bainite gives rise to a surface relief, similar to that resulting from martensite. However, it was soon shown that Widmansta¨tten ferrite also gives a surface relief [7] and today there seems to be wide agreement [8,9,10] that a martensitic type of growth by shear cannot be proven by studying the surface relief. An argument against Zener’s hypothesis came through measurements of the edgewise growth rate of Widmansta¨tten ferrite and bainite in low alloy steels [11,12]. Information from a wide range of temperatures gave a single curve and it was possible to explain the growth rates as controlled by carbon diffusion. Hehemann [13] tried to keep Zener’s hypothesis alive by maintaining that the tip of Widmansta¨tten ferrite advances in very quick but short steps. However, in a discussion with Kinsman and Aaronson [14] he was finally forced to admit that he found it “difficult to argue against these diffusion controlled models.” For a long time after him there did not seem to be any serious proponent for Zener’s hypothesis until it was again taken up by Bhadeshia [15,16]. It is not the purpose of the present note to discuss Bhadeshia’s arguments for Zener’s hypothesis. However, his many publications on bainite, arguing for Zener’s hypothesis, seem to have spread the idea that there are two types of acicular growth of ferrite, Widmansta¨tten growth and “bainitic” growth, which can be applied to acicular growth in other systems as well. Actually, that idea had been expressed before Bhadeshia, e.g. in attempts to distinguish between two types of acicular precipitation of the fcc phase from the bcc phase in Cu-Zn alloys [17]. However, it seems that both types are now regarded as Widmansta¨tten precipitation, and that bainite is no longer used as the name for a decomposition product of  brass. In a recent paper on the formation of austenite by heating a mixture of ferrite and cementite, Kaluba, Taillard and Foct [18] mentioned “bainitic mechanism” in the title. This was naturally interpreted by Aaronson and Nie to imply that acicular austenite could form in the same manner as bainite according to Zener’s hypothesis as advocated by Bhadeshia. In discussion [19] they questioned a number of arguments by Kaluba et al. but in a reply [20] those authors declared that they “neither tried to prove a particular mechanism of bainite formation (from austenite), nor to propose a martensitic model for the growth of austenite.” They further mentioned “that both bainitic and Widmansta¨tten terms are often confused, even with regard to ferrite, due to the apparent similarity in both morphologies and in spite of existing classifications and definitions.” However, there seems to be no room for confusion if it is remembered that bainite is a eutectoid (two-phase) structure and a Widmansta¨tten plate consists of a single phase. It is now proposed that, if there really are two types of acicular precipitation of a single phase with a change in composition already at the moment of growth, they should be called Widmansta¨tten I and II. Before introducing such names, it is necessary (1) to prove how the growth mechanisms differ, (2) to show how they can be distinguished by physical measurement techniques and (3) that they are two distinct types and not members of a continuous series. According to the present authors, it is quite conceivable that the atomic structure of the phase interface of a Widmansta¨tten precipitate changes with the temperature of formation and the driving force [21]. In that case it would not seem reasonable to single out a particular interfacial structure and relate it to a particular name. In any case, the authors of the present note propose that the term “bainite” should only be used for eutectoid (two-phase) structures. If an acicular precipitation occurs without diffusion and with a glissile interface, it should grow with a high rate and should be regarded as martensite. If there is diffusion soon after its formation, the product should be called tempered martensite.
Vol. 43, No. 9
ON THE MISUSE OF THE TERM BAINITE
833
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
R. F. Mehl and W. C. Hagel, Progr. Metal Phys. 6, 74 (1956). M. Hillert, in Decompositon of Austenite by Diffusional Processes, ed. V. F. Zackay and H. I. Aaronson, p. 197, Interscience Publishers, New York (1962). A. Hultgren, Trans. ASM. 39, 815 (1947). M. Hillert, ISIJ Int. 35, 1134 (1995). C. Zener, Trans. AIME. 167, 550 (1946). T. Ko and A. H. Cottrell, J. Iron Steel Inst. 172, 307 (1952). A. P. Miodownik, in The Mechanism of Phase Transformations in Metals, Inst. Metals Monograph and Report Series, No. 18, p. 319 (1956). W.-Z Zhang and G. C. Weatherly, Acta Mater. 46, 1837 (1998). J. P. Hirth, G. Spanos, M. G. Hall, and H. I. Aaronson, Acta Mater. 48, 1047 (1997). R. C. Pond, P. Shang, T. T. Cheng, and M. Aindow, Acta Mater. 48, 1047 (2000). M. Hillert, Report, Swedish Institute of Metal Research (1960). L. Kaufman, S. V. Radcliffe, and M. Cohen, in Decompositon of Austenite by Diffusional Processes, eds. V. F. Zackay and H. I. Aaronson, p. 313, Interscience Publishers, New York (1962). J. M. Oblak and R. F. Hehemann, Transformation and Hardenability in Steels, p. 15, Climax Molybdenum Co., Ann Arbor, MI (1967). R. F. Hehemann, K. R. Kinsman, and H. I. Aaronson, Trans. AIME. 3, 1077 (1972). H. K. D. H. Bhadeshia, J. Phys. IV Fr. 7(C5), C5–367 (1997). H. K. D. H. Bhadeshia, Bainite in Steels, Institute of Materials, London (1992). R. D. Garwood, J. Inst. Metals. 83, 64 (1954). W. J. Kaluba, R. Taillard, and J. Foct, Acta Mater. 46, 5917 (1998). H. I. Aaronson and J. F. Nie, Scripta Mater. 42, 505 (2000). W. J. Kaluba, R. Taillard, and J. Foct, Scripta Mater. 42, 509 (2000). M. Hillert and G. R. Purdy, Acta Metal. 32, 823 (1984).