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Measurements of the spread in pearlite spacings
243
METALLOGRAPHY 19:243-246 (1986)
SHORT COMMUNICATION Measurements of the Spread in Pearlite Spacings
J. F. TILBURY, T. D. MOTTISHAW,* AND G. D. W. SMITH
Department of Metallurgy and Science of Materials, University of Oxford. Parks Road, Oxford OXI 3PH, England
Introduction Existing data on interlamellar spacings in pearlitic steels has been reviewed by Ridley [1]. Although it is believed that a spread in interlamellar spacings occurs, even in isothermally transformed steels, few direct measurements have been made. The measurement of interlamellar spacing in bulk specimens is difficult because the colonies intersect the surface at a variety of angles. The statistical analysis of this problem was first carried out by Pellisier et al. [2]. These workers calculated a distribution function for the apparent spacings on the assumptions of a constant true spacing, and random angles of intersection of different pearlite colonies with the surface of the specimen. Careful optical metallography revealed a discrepancy between the model and experiment, which they could only explain in terms of the existence of a spread of true pearlite spacings in their samples. They estimated that the average spacing was 1.65 times the minimum, and also found that an intracolony spread in spacing existed. Using similar methods, Ridley [1] found a average-to-minimum ratio of 1.25. In an attempt to remove the statistical errors inherent in this method, Mellor and Edmonds [3] used directionally cooled steels, and also found a ratio of 1.25. However, their analysis is complicated by the fact that transformation did not occur under strict isothermal conditions; also, the lamellae were not perfectly aligned along the specimen axis. One way to eliminate statistical corrections and alignment errors altogether is to examine thin-foil specimens of pearlitic steels in the transmission electron microscope (TEM), tilting the specimens so that each * Current address: Rex, Thompson and Partners, West Street, Farnham, Surrey, England. © Elsevier Science Publishing Co. Inc., 1986 52 Vanderbilt Ave., New York, NY 10017
0026-0800/86/$03.50
244
Tilbury et al.
colony in turn is aligned exactly parallel to the beam, and then measuring the spacings directly. Work of this kind was first carried out by Vander Voort and Ro6sz [4]. However, their measurements were made on continuously cooled steels, so intercolony variations could be expected to arise from the variation in the transformation temperature of different colonies, as well as from any inherent spread in spacings at a given transformation temperature. The range of true interlamellar spacings in their samples was from 100 to 450 nm; the mean of these spacings was 250 nm--2.5 times the minimum. As part of a wider investigation of the properties of microalloyed pearlitic steels designed for use by the wire industry [5, 6], we have now carried out the first direct measurements of the true spacings of isothermally transformed pearlites. The present work used a method similar to that of Vander Voort and Ro6sz, with two independent methods of microscope calibration being used.
Experimental Method and Result Several steels were investigated, but the results reported here refer specifically to a vanadium-containing microalloyed pearlitic steel having the following composition (wt.%): 0.12 V; 0.76 Mn; 0.27 Si; 0.77 C; balance Fe. Samples were austenized at 1050°C for 300 s before being isothermally transformed in a salt bath at 640°C. Discs, 0.5 mm thick and 3 mm in diameter, were used to reduce recoalescence to a minimum. The discs were ground, jet polished, and finally ion milled to produce electron microscope specimens. They were observed in bright field at low magnification, and those colonies with lamellae approximately parallel to the beam were then observed at higher magnification. The specimen was tilted so the lamellae were exactly parallel to the beam. A micrograph was taken and the nominal magnification and objective lens current noted. Micrograph calibration was carried out by recording images of a reference carbon grid at the same magnifications and lens currents employed for the metallography. Use was also made of a method of in situ calibration, involving specimens with latex balls of known size deposited on the surface. Spacing measurements (made exactly perpendicular to the lamellae) were averaged over 15-20 interlamellar spacings at each of several places in any one colony. These in turn were averaged to give a value for the colony as a whole. The overall results were plotted as a histogram, as shown in Fig. 1. The results show that a significant spread of true spacings exists in these isothermally transformed specimens. The range of true interlamellar spacings was from 91-133 nm. The mean-to-minimum spac-
Measurements of Pearlite Spacings
245
12
l
90 -94
95 -99
100 -104
105 -109
110 -114
115 -119
120 -T24
125 -129
130 -134
Nanometres
Fro. 1. Histogram of interlamellar spacing against frequency.
ing ratio was 1.20, in good agreement with Mellor and Edmonds [3]. The frequency distribution of spacings has a skew form, of the type first reported by Mellor and Edmonds [3]. This experimental method attempts to show the true intercolony spacing variation, that is, the number of colonies in a specimen having spacings in certain ranges. However, due to the relatively small amount of thin area in a TEM specimen, the results may be weighted by the colony size, as large colonies will have a greater probability of intersecting the electron-transparent regions. The calculated growth rate of pearlite shows a dependence on spacing very similar to that shown in Fig. 1 [7]. Thus colonies having different spacings might be expected to grow at different
246
Tilbury et al.
rates and to attain different final sizes. Therefore the true intercolony variation may be to some extent masked by colony size/spacing variations, and Fig. 1 may represent the volume fractions of pearlite having each spacing, rather than the number of colonies having each spacing. Further work is needed to establish whether or not such an effect exists.
Conclusion A real colony-to-colony variation of interlamellar spacing in isothermally transformed pearlites has been observed directly for the first time. The shape of the distribution curve may be affected by an interlamellar spacing/colony size dependence due to the small area of colony sampled in the TEM.
References 1. N. Ridley, A review of the data on the interlamellar spacing of pearlite, Met. Trans. 15A: 1019-1036 (1984). 2. G. E. Pellisier, M. E. Hawkes, W. A. Johnson, and R. F. Mehl, The interlamellar spacing of pearlite, Trans. ASM 30:1049-1086 (1942). 3. B. G. Mellor and D. V. Edmonds, Process-structure relationships and the significance of pearlite interlamellar spacing measurements, Met. Trans. 8A:763-771 (1977). 4. G. F. Vander Voort and A. Ro6sz, Measurement of the interlamellar spacing of pearlite, Metallography 17:1-17 (1984). 5. B. W. Cordon, P. Timiney, T. M. Mottishaw, and G. D. W. Smith, Proc. Conf. on Developments in the Drawing of metals, London, 1983, The Metals Society, London (1983) pp. 228-234. 6. T. D. Mottishaw and G. D. W. Smith, in Proc. Conf. on Technology and Applications of HSLA Steels, Philadelphia, 1983 (M. Korchynsky Ed.), ASM, Ohio (1984). 7. B. Sundquist, The edgewise growth of pearlite, Acta Metall. 16:1413-1427 (1986). Received August 1985; accepted October 1985.
METALLOGRAPHY 19:243-246 (1986)
SHORT COMMUNICATION Measurements of the Spread in Pearlite Spacings
J. F. TILBURY, T. D. MOTTISHAW,* AND G. D. W. SMITH
Department of Metallurgy and Science of Materials, University of Oxford. Parks Road, Oxford OXI 3PH, England
Introduction Existing data on interlamellar spacings in pearlitic steels has been reviewed by Ridley [1]. Although it is believed that a spread in interlamellar spacings occurs, even in isothermally transformed steels, few direct measurements have been made. The measurement of interlamellar spacing in bulk specimens is difficult because the colonies intersect the surface at a variety of angles. The statistical analysis of this problem was first carried out by Pellisier et al. [2]. These workers calculated a distribution function for the apparent spacings on the assumptions of a constant true spacing, and random angles of intersection of different pearlite colonies with the surface of the specimen. Careful optical metallography revealed a discrepancy between the model and experiment, which they could only explain in terms of the existence of a spread of true pearlite spacings in their samples. They estimated that the average spacing was 1.65 times the minimum, and also found that an intracolony spread in spacing existed. Using similar methods, Ridley [1] found a average-to-minimum ratio of 1.25. In an attempt to remove the statistical errors inherent in this method, Mellor and Edmonds [3] used directionally cooled steels, and also found a ratio of 1.25. However, their analysis is complicated by the fact that transformation did not occur under strict isothermal conditions; also, the lamellae were not perfectly aligned along the specimen axis. One way to eliminate statistical corrections and alignment errors altogether is to examine thin-foil specimens of pearlitic steels in the transmission electron microscope (TEM), tilting the specimens so that each * Current address: Rex, Thompson and Partners, West Street, Farnham, Surrey, England. © Elsevier Science Publishing Co. Inc., 1986 52 Vanderbilt Ave., New York, NY 10017
0026-0800/86/$03.50
244
Tilbury et al.
colony in turn is aligned exactly parallel to the beam, and then measuring the spacings directly. Work of this kind was first carried out by Vander Voort and Ro6sz [4]. However, their measurements were made on continuously cooled steels, so intercolony variations could be expected to arise from the variation in the transformation temperature of different colonies, as well as from any inherent spread in spacings at a given transformation temperature. The range of true interlamellar spacings in their samples was from 100 to 450 nm; the mean of these spacings was 250 nm--2.5 times the minimum. As part of a wider investigation of the properties of microalloyed pearlitic steels designed for use by the wire industry [5, 6], we have now carried out the first direct measurements of the true spacings of isothermally transformed pearlites. The present work used a method similar to that of Vander Voort and Ro6sz, with two independent methods of microscope calibration being used.
Experimental Method and Result Several steels were investigated, but the results reported here refer specifically to a vanadium-containing microalloyed pearlitic steel having the following composition (wt.%): 0.12 V; 0.76 Mn; 0.27 Si; 0.77 C; balance Fe. Samples were austenized at 1050°C for 300 s before being isothermally transformed in a salt bath at 640°C. Discs, 0.5 mm thick and 3 mm in diameter, were used to reduce recoalescence to a minimum. The discs were ground, jet polished, and finally ion milled to produce electron microscope specimens. They were observed in bright field at low magnification, and those colonies with lamellae approximately parallel to the beam were then observed at higher magnification. The specimen was tilted so the lamellae were exactly parallel to the beam. A micrograph was taken and the nominal magnification and objective lens current noted. Micrograph calibration was carried out by recording images of a reference carbon grid at the same magnifications and lens currents employed for the metallography. Use was also made of a method of in situ calibration, involving specimens with latex balls of known size deposited on the surface. Spacing measurements (made exactly perpendicular to the lamellae) were averaged over 15-20 interlamellar spacings at each of several places in any one colony. These in turn were averaged to give a value for the colony as a whole. The overall results were plotted as a histogram, as shown in Fig. 1. The results show that a significant spread of true spacings exists in these isothermally transformed specimens. The range of true interlamellar spacings was from 91-133 nm. The mean-to-minimum spac-
Measurements of Pearlite Spacings
245
12
l
90 -94
95 -99
100 -104
105 -109
110 -114
115 -119
120 -T24
125 -129
130 -134
Nanometres
Fro. 1. Histogram of interlamellar spacing against frequency.
ing ratio was 1.20, in good agreement with Mellor and Edmonds [3]. The frequency distribution of spacings has a skew form, of the type first reported by Mellor and Edmonds [3]. This experimental method attempts to show the true intercolony spacing variation, that is, the number of colonies in a specimen having spacings in certain ranges. However, due to the relatively small amount of thin area in a TEM specimen, the results may be weighted by the colony size, as large colonies will have a greater probability of intersecting the electron-transparent regions. The calculated growth rate of pearlite shows a dependence on spacing very similar to that shown in Fig. 1 [7]. Thus colonies having different spacings might be expected to grow at different
246
Tilbury et al.
rates and to attain different final sizes. Therefore the true intercolony variation may be to some extent masked by colony size/spacing variations, and Fig. 1 may represent the volume fractions of pearlite having each spacing, rather than the number of colonies having each spacing. Further work is needed to establish whether or not such an effect exists.
Conclusion A real colony-to-colony variation of interlamellar spacing in isothermally transformed pearlites has been observed directly for the first time. The shape of the distribution curve may be affected by an interlamellar spacing/colony size dependence due to the small area of colony sampled in the TEM.
References 1. N. Ridley, A review of the data on the interlamellar spacing of pearlite, Met. Trans. 15A: 1019-1036 (1984). 2. G. E. Pellisier, M. E. Hawkes, W. A. Johnson, and R. F. Mehl, The interlamellar spacing of pearlite, Trans. ASM 30:1049-1086 (1942). 3. B. G. Mellor and D. V. Edmonds, Process-structure relationships and the significance of pearlite interlamellar spacing measurements, Met. Trans. 8A:763-771 (1977). 4. G. F. Vander Voort and A. Ro6sz, Measurement of the interlamellar spacing of pearlite, Metallography 17:1-17 (1984). 5. B. W. Cordon, P. Timiney, T. M. Mottishaw, and G. D. W. Smith, Proc. Conf. on Developments in the Drawing of metals, London, 1983, The Metals Society, London (1983) pp. 228-234. 6. T. D. Mottishaw and G. D. W. Smith, in Proc. Conf. on Technology and Applications of HSLA Steels, Philadelphia, 1983 (M. Korchynsky Ed.), ASM, Ohio (1984). 7. B. Sundquist, The edgewise growth of pearlite, Acta Metall. 16:1413-1427 (1986). Received August 1985; accepted October 1985.