Materials Science Forum Vols. 475-479 (2005) pp. 175-178 online at http://www.scientific.net © (2005) Trans Tech Publications, Switzerland
Effect of Initial structure on Recrystallized Austenite Grain Size of Fe-32%Ni Alloy In-Jin Shon1, a, Seok-Jae Lee2, b, Young-Seob Seo2, c, Young-Kook Lee2, d, Yong-Hwan Jeong3, e and Chong-Sool Choi4, f 1
Division of Advanced Materials Engineering, Chonbuk National University, Chonbuk 561-756, Korea
2
Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea 3
Advanced Zr Alloy Development Team, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea
4
Research Institute of Iron and Steel Technology, Yonsei University, Seoul 120-749, Korea. b
[email protected] [email protected] [email protected]
Keywords: Fe-32%Ni Alloy, Recrystallized Austenite Grain Size, Martensite, Austenite, Reverse
Transformation Abstract Recrystallization behaviors have been investigated with respect to two different kinds of the initial structures, original austenite and martensite, in an Fe-32%Ni alloy. The recrystallized austenite grain size from the martensite is much smaller than that from the original austenite, and decreases linearly with increasing the initial hardness, independent of the initial structure. The recrystallization sequences are different between the two structures: only one step due to recrystallization appears in hardness-temperature curve of the original austenite, whereas two steps corresponding to reverse transformation of α’ to r’ and recrystallization are shown in that of the martensite. 1. Introduction It is well known that grain refinement improves both strength and toughness simultaneously in polycrystalline materials. Recently, the various new methods, such as strain-induced dynamic transformation [1] and equal channel angular processing [2] have been attempted to reduce the grain size of low carbon steels. In the present study, in order to study the effect of the initial structure on the recrystallized grain size, two kinds of specimens having different structures, original austenite and martensite, were prepared by heat treatment in an Fe-32%Ni alloy and were cold-rolled at different reductions of thickness. After recrystallization annealing the austenite grain sizes were investigated as a function of the reduction in thickness for the two initial structures. 2. Experimental Procedure An Fe-32%Ni alloy was melted by means of a high frequency induction vacuum furnace and was 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 the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 165.132.62.75-06/02/07,03:38:52)
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cast into a metallic mould. The ingot of 4 kg was homogenized at 1200℃ for 24 hours in a protective atmosphere, and hot-rolled to bars with 12mm in diameter. From the bars, the plate specimens with different thicknesses between 1 and 10mm were prepared by rolling and machining. Austenitic structure of the specimens was obtained by quenching them into water of room temperature from 980℃(Hereafter it is called original austenite). Martensitic structure was made by subzero-treating the original austenite specimens at liquid nitrogen (-196℃). All specimens were cold-rolled at 100℃ to prevent the formation of stress-induced martensite during rolling, and recrystallization-annealed for 1 hour at 800℃. The martensite start temperature (Ms) of the alloy was measured by observing the surface relief during subzero cooling, and it was –42℃. The specimens for optical microstructure were electro-polished in a solution of 10% HClO2 + 90% CH3COOH, and etched in a 2% nital solution. 3. Results and Discussion Fig. 1 shows the optical microstructures of martensite and original austenite of the Fe-32%Ni alloy. The martensite structure consists of numerous lenticular plates with various sizes and orientations. It was confirmed from X-ray diffraction tests that the martensite structure had no retained austenite, and that the original austenite structure is 100% r.
(a) original austenite (b) martensite Fig. 1 Start structures of Fe-32%Ni alloy The austenite start and finish temperatures (As, Af) were measured from the cold-rolled martensite using a differential thermal analyzer. The As increased from 300 to 440℃ with increasing the deformation from zero to 90% in thickness reduction, and the Af from 400 to 480℃. The rise in As and Af was due to the increased hardness of austenite with increase in deformation. Fig. 2 shows the hardness of martensite(α’), reversed austenite(r’) and original austenite(r) measured at room temperature as a function of reduction in thickness. In case of the reversed austenite, the hardness was measured at room temperature after heating the cold-rolled martensite for 5 minutes at a temperature 20℃ higher than the Af temperature of each specimen. It is noticeable that the hardness of the reversed austenite is higher than that of the original austenite at all deformation degrees. The reversed austenite structure(Fig. 3) is entirely different from the original austenite(Fig. 1): it is martensite-like structure, indicating that the α’ → r’ reverse transformation occurred martensitically. So, the higher hardness of reversed austenite is probably attributed to a larger density of dislocations of the reversed austenite than that of the original
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austenite, which is mainly due to the introduction of many dislocations from the martensite into the reversed austenite during the reverse transformation [3]. Fig. 4 shows the
and martensite specimens for 1hour. The hardness of original austenite shows one step associated with recrystallization, but the hardness of martensite
reveals
two steps: the first step is due to α’ → r’ reverse transformation, and the second step corresponds to the recrystallization of the reversed austenite. This indicates that the recrystallization sequences of the two different initial structures are different from each other: one is deformed martensite → reversed austenite → recrystallization of the reversed austenite, and the other is deformed original austenite → recrystallization of the austenite.
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Hardness (HR-15N)
schematic hardness-annealing temperature curves of the two initial structures after annealing cold-rolled austenite
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Fig. 2 Hardness of martensite, reversed austenite and original austenite as a function of reduction in thickness in Fe32%Ni alloy.
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Fig. 3 Reversed austenite structure of the undeformed martensite in Fe-32%Ni alloy
Fig. 4 Schematic hardness-temperature curves of the two start structures of Fe-32%Ni alloy
Fig. 5 shows a plot of the recrystallized austenite grain size versus the reduction in thickness after annealing for 1 hour at 800℃. The grain size of recrystallized austenite produced from the martensite is much smaller than that from the original austenite at any reductions of thickness. The reason is explained as follows. The driving force for recrystallization, so-called stored energy, is expected to be larger in the reversed austenite than the original austenite from the hardness in Fig. 2. So, the number of nuclei for recrystallization [4] is larger in the reversed austenite than the original austenite, resulting in the smaller recrystallized austenite grain size in the initial martensite structure. Fig. 6 shows a plot of the recrystallized austenite grain size versus the initial hardness (corresponding to Hr’ and Hr in Fig. 4) for the two structures. All data points are in accord with a straight line. This means that the recrystallized austenite grain size does not depend on the initial
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structure but the initial hardness, and the grain size decreases linearly with increasing the initial hardness, which is closely related to the stored energy acting as the driving force for recrystallization because the initial hardness of the Fe-32%Ni alloy increases with increasing the dislocation density introduced into the alloy during cold-rolling.
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original austenite martensite
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30
20
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Grain size (§-)
Grain size (§-)
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Original austenite Martensite
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Fig. 5 Relation between recrystallized austenite grain size and reduction of thickness in the two start structures of Fe32%Ni alloy.
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Hardness (HR-15N)
Fig. 6 Relation between the recrystallized austenite grain size and the initial hardness for the two structures of Fe32%Ni alloy.
4. Conclusions Effect of the initial structure on the recrystallization of Fe-32%Ni alloy has been investigated, and the results are summarized as follows. (1) The recrystallization sequences of the two different initial structures are different from each other: one is deformed martensite → reversed austenite → recrystallization of the reversed austenite, and the other is deformed original austenite → recrystallization of the austenite. (2) The recrystallized austenite grain size is much smaller in the start structure of martensite than the original austenite. (3) The recrystallized austenite grain size decreases linearly with increasing the initial hardness regardless of the initial structure. References [1] J. K. Choi, D. H. Seo, J. S. Lee, K. K. Um and W. Y. Choo: ISIJ Int. Vol. 48 (2003), p. 746 [2] D. H. Shin, J. J. Park, S. Y. Chang, Y. K. Lee and K. T. Park: ISIJ Int. Vol. 42 (2002), p. 1490 [3] C. S. Choi and K. B. Kang: J. Kor. Inst. Met. Vol. 18 (1980), p. 608 [4] P. Cotterill and P. R. Mould: Recrystallization and Grain growth in Metals (A Haisted Press book, 1976), p. 62
