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Effect of dynamically recrystallized austenite on the martensite start temperature of martensitic transformation
Materials Science and Engineering A 438–440 (2006) 254–257
Effect of dynamically recrystallized austenite on the martensite start temperature of martensitic transformation Jie Huang a,b,∗ , Zhou Xu b a
Key Laboratory for High Temperature Materials and Tests of Ministry of Education, Shanghai Jiaotong University, Shanghai 200030, PR China b Cold Rolling Mill, Baoshan Iron & Steel Co., Ltd., Shanghai 201900, PR China
Received 11 April 2005; received in revised form 20 January 2006; accepted 12 February 2006
Abstract The effect of dynamically recrystallized austenite on the martensite start temperature, Ms , in hot-deformed Fe–32% Ni alloy was studied by measuring its electrical resistance. Differing from the statically recrystallized structure, the dynamically recrystallized grains have heterogeneous dynamic substructures. The growing dynamic recrystallization grain with a dislocation density in a gradient distribution contains a few mobile dislocations near the grain boundary. These mobile dislocations promote the martensitic nucleation and increase the Ms temperature. © 2006 Elsevier B.V. All rights reserved. Keywords: Hot deformed austenite; Dynamic recrystallization; Grain size; Martensitic transformation; Ms temperature
1. Introduction
2. Experimental material and procedure
As one of the important methods of controlling microstructures and obtaining the desirable properties of steels, hot deformation has been paid more attention either in academe or in practice. Study on behaviors and microstructure features of hot deformation and subsequent transformation of austenite is of great significance to microstructral control and improvement of properties. So far many scholars have investigated the effect of hot deformed austenite on its martensitic transformation in Fe alloys [1–3]. However, most of these investigations have focused on the martensitic transformation of work-hardened austenite, while the effect of dynamically recrystallized austenite on the martensitic transformation has received relatively less attention. The main thrust of this paper is principally concerned with elucidating the effect of dynamically recrystallized austenite on the martensite starting temperature for Fe–32% Ni alloy in comparison with those of the statically recrystallized, based on the study on its dynamic recrystallization (DRX) behavior.
An Fe–32% Ni alloy was used for the experiment with its chemical composition shown in Table 1. The martensite starting temperature, Ms , of the material is below room temperature so that the alloy is in the austenitic state at ambient temperature. After a subzero treatment it will be transformed into lentoid martensite. The material in the form of plates was machined into compression cylinders 12 mm in height and 8 mm in diameter. In order to reduce the friction between the faces of the sample and the platens during compression deformation, concentric grooves about 0.1 mm in depth were machined on the ends of the sample to hold powdered glass lubricant so that the sample could be deformed homogeneously at high strains. These samples were annealed in vacuum at 1343 K for 1800 s, giving an initially austenite microstructure with initial grain size of about 200 m. Then they were deformed at the temperature of 1173–1323 K and the strain rate of 2 × 10−5 to 2.7 × 10−2 s−1 to the steady state in the high temperature compressor, followed by taking out within a second and quenching to the room temperature. Identical samples were annealed in vacuum at 1073–1343 K for 1800 s for the purpose of comparison, obtaining average grain sizes of static recrystallized microstructures ranging from 30 to 200 m. Two group of samples above were then cut into 1 mm thick slices by wire cutting and removed the surface stress layer by
∗
Corresponding author. Tel.: +86 21 62932086. E-mail addresses: [email protected], [email protected] (J. Huang). 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.069
J. Huang, Z. Xu / Materials Science and Engineering A 438–440 (2006) 254–257
255
Table 1 Chemical composition of the material used for experiment in wt.% C
Si
Mn
P
S
Ni
Al
N
O
0.007
0.01
0.04
0.005
0.0006
32.4
0.022
0.00074
0.020
chemical polishing, followed by subzero treatments. The Ms temperature was identified by measuring the electrical resistance during cooling. Usually the relative electrical resistance decreases as the temperature drops before transformation. Once the martensitic transformation breaks out, a sudden change will occur in the electrical resistance, by which the Ms temperature can be easily identified, as can be seen in Fig. 1.
stant and the stress reaches to a stable one σ s . The corresponding relationship between them is given in Fig. 3. It indicates that the dynamic grain size Ds decreases as the stable stress σ s increases [5]. Therefore, dynamically recrystallized microstructures with desirable grain sizes can be obtained by matching the deformation temperatures with the strain rates to make an effect on the stable stress s . In this experimental condition, the dynamic grain sizes range from 15 to 163 m.
3. Experimental results 3.1. Hot deformed behavior of austenite The Fe–32% Ni alloy exhibits a typical dynamic recrystallization behavior during hot deformation with the stress–strain curves shown in Fig. 2. At the beginning of the deformation, the flow stress rises with an increase in strain, producing obvious work hardening. Subsequently the work-hardening rate decreases gradually with subsequent deformation till the stress reaches to the first peak value. After then, the flow stress begins to decrease and softening appears, namely DRX takes place. In the highly strained region the flow stress does not change with the strain and remains constant, and the DRX keeps a dynamic balance [4]. It also can be seen that with the increase in deformation temperature or the decrease in strain rate the peak strain decreases, which shows the DRX begins earlier than before. Furthermore, the stress–strain curve is changing gradually from a single-peak type to a multiple-peak type. In the steady state stage of the highly strained region, the average dynamically recrystallized grain diameter Ds keeps con-
Fig. 1. Identification of the Ms temperature of samples.
Fig. 2. Effect of deformation temperature and strain rate on the curves for Fe–32% Ni alloy.
Fig. 3. Relation between the stable stress σ s and the dynamic grain size Ds for Fe–32% Ni alloy.
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Fig. 6. Transmission electron micrograph of martensite in dynamically recrystallized structure for Fe–32% Ni alloy. Fig. 4. Effect of austenite grain size on the Ms temperature in dynamically recrystallized and statically recrystallized structure.
3.2. Effect of DRX on the Ms temperature The graph in Fig. 4 show the effect of austenite grain size on the Ms temperature in dynamically recrystallized and statically recrystallized structure. The Ms temperatures of both the dynamically recrystallized structure and the statically recrystallized structure increase with an increase in austenite grain size [6–9]. The fine-grain region exhibits a higher increasing rate of the Ms temperature, while the coarse-grain region a lower rate. On the whole, the Ms temperatures of the dynamically recrystallized structures are all higher than that those of the statically recrystallized structures, from which it can be illustrated that DRX can enhance the martensitic nucleation and thereby increase the Ms temperature. Different from the statically recrystallized structure, the dynamically recrystallized structure features on the heterogeneous grains, the serrated grain boundary and the high dislocation density in the interior of a grain. Martensite plates from the statically recrystallized are integral and straight. Midribs can
be seen clearly. Some martensite plates can go through a whole austenite grain. On the contrary, the martensite plates from the dynamically recrystallized are of complex substructures (see Fig. 5). The small martensite plates are integral and straight, while the big are crooked or broken and show their irregular and serrated rims. Many martensite plates isolate near the boundary and do not go through a whole austenite grain any longer. From transmission electron microscopy images (see Fig. 6), the high dislocation density at the tip or around the martensite plate, the most irregular rims of plates and the crooked midrib and twin regions within a plate can also be seen. 4. Analysis Compared with the statically recrystallized structure, the dynamically recrystallized structure has a special effect on the martensitic transformation, which is related to its heterogeneous dislocation substructures. Previous investigations [10–14] show that the highly strained dynamically recrystallized structure is very heterogeneous and can be classified into three
Fig. 5. Optical micrographs of martensite in statically recrystallized or dynamically recrystallized structure for Fe–32% Ni alloy. (a) 1273 K, 2 × 10−3 s−1 , ε = 0; (b) 1323 K, 2 × 10−5 s−1 , ε = 0.6.
J. Huang, Z. Xu / Materials Science and Engineering A 438–440 (2006) 254–257
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Fig. 7. Three types of dislocation density distribution (A–C) developed in the DRX grain structure (ρ0 is the initial dislocation density, ρc is the critical value required for dynamic nucleation, and D is the current DRX grain size. The present state is represented by the full lines, and one or more earlier states by broken lines.)
types according to the dislocation substructure, as illustrated schematically in Fig. 7: (A) a fine grain with a low dislocation density (namely a DRX nucleus); (B) a grain with a dislocation density in a gradient distribution, in which there is a low density near the boundary and a high density in the interior (namely a growing DRX grain); (C) a DRX grain with a high dislocation density (namely a work-hardened DRX grain). These three types of highly strained structures in the steady state are distributed fairly uniformly in a proportion thereby to produce a dynamically recrystallized structure whose average grain size changes hardly with the strain and to exhibit a stable stress. As a rule, martensitic nucleation is considered as a kind of heterogeneous nucleation. Although martensite precedes to nucleate near the grain boundary of austenite, martensite can nucleate only near the special grain boundary with high energy, not near any boundary [1]. Despite more produced grain boundaries, grain refinement is of disadvantage to the nucleation of martensite due to improve the strength of the parent-martix. It means that refined austenite grains result in hindering martensitic transformation and decreasing the Ms temperature. A few mobile dislocations introduced into the austenite during deformation contribute to the martensitic nucleation and increase the Ms temperature owing to alleviate stress concentration from the martensitic nucleation. However, the introduction of plenty of deformation dislocations will lead to work hardening of the parent-martix, which hinders the martensitic nucleation and decreases the Ms temperature [1,3]. Therefore the existence of either the DRX nucleus or the work-hardened DRX grain will result in hindering the martensitic nucleation and decreasing the Ms temperature, however, that of the growing DRX grain will contribute to the martensitic nucleation and increase the Ms temperature due to mobile dislocations near the grain boundary. Furthermore, it will be of interest to note that those high-density dislocations entwisting within the growing DRX grain will hinder the continuous growth of martensite plates and cause them crooked or broken.
5. Conclusions In the light of this experimental research, it is possible to draw the following conclusions: (i) The effects of the size of the austenite grain in dynamically recrystallized and statically recrystallized structure for Fe–32% Ni alloy on the Ms temperature are same basically, namely that the Ms temperature decreases with the grain diameter decreasing. However, the Ms temperature of the former is higher than that of the latter on the condition of the same grain size. (ii) Dynamic recrystallization has a special effect on the martensitic transformation, which is attributed to its heterogeneous dislocation substructures. Acknowledgement The authors would like to acknowledge the NSFC (China) for the financial support of this work under grant number 50471017. References [1] S. Kajiwara, Metall. Trans. 17A (1986) 1693. [2] K. Tsuzakai, S. Fukasaku, Y. Tomota, T. Maki, Mater. Trans. JIM 32 (1991) 222. [3] M. Pan, T.Y. Hsu, Acta Metall. Sin. 25 (6) (1989) 389. [4] T. Sakai, J.J. Jonas, Acta Metall. 32 (1984) 189. [5] Z. Xu, F.S. Meng, X.Y. Men, Z.Q. Fu, Z.K. Yao, Iron Steel 19 (9) (1984) 50. [6] T.Y. S Hsu, Proceedings of ICOMAT-86, Key Note Lecture, Japan Institute of Metals, 1987, p. 245. [7] T.Y. Hsu, J. Mater. Sci. 20 (1985) 23. [8] P.J. Brofman, G.S. Ansell, Metall Trans. 14A (1983) 1929. [9] J. Wu, B.H. Jiang, T.Y. Hsu, Acta Matall. 36 (1988) 1521. [10] T. Sakai, M. Ohashis, K. Chiba, J.J. Jonas, Acta Metall. 36 (1988) 1781. [11] Z. Xu, T. Sakai, J. Jpn. Inst. Met. 53 (1989) 1161. [12] Z. Xu, T. Sakai, J. Jpn. Inst. Met. (JIM) 55 (1991) 1298. [13] T. Sakai, J. Mater. Process. Technol. 53 (1995) 349. [14] Z. Xu, T. Sakai, Z.K. Yao, Mater. Sci. Technol. 5 (2) (1997) 124.
Effect of dynamically recrystallized austenite on the martensite start temperature of martensitic transformation Jie Huang a,b,∗ , Zhou Xu b a
Key Laboratory for High Temperature Materials and Tests of Ministry of Education, Shanghai Jiaotong University, Shanghai 200030, PR China b Cold Rolling Mill, Baoshan Iron & Steel Co., Ltd., Shanghai 201900, PR China
Received 11 April 2005; received in revised form 20 January 2006; accepted 12 February 2006
Abstract The effect of dynamically recrystallized austenite on the martensite start temperature, Ms , in hot-deformed Fe–32% Ni alloy was studied by measuring its electrical resistance. Differing from the statically recrystallized structure, the dynamically recrystallized grains have heterogeneous dynamic substructures. The growing dynamic recrystallization grain with a dislocation density in a gradient distribution contains a few mobile dislocations near the grain boundary. These mobile dislocations promote the martensitic nucleation and increase the Ms temperature. © 2006 Elsevier B.V. All rights reserved. Keywords: Hot deformed austenite; Dynamic recrystallization; Grain size; Martensitic transformation; Ms temperature
1. Introduction
2. Experimental material and procedure
As one of the important methods of controlling microstructures and obtaining the desirable properties of steels, hot deformation has been paid more attention either in academe or in practice. Study on behaviors and microstructure features of hot deformation and subsequent transformation of austenite is of great significance to microstructral control and improvement of properties. So far many scholars have investigated the effect of hot deformed austenite on its martensitic transformation in Fe alloys [1–3]. However, most of these investigations have focused on the martensitic transformation of work-hardened austenite, while the effect of dynamically recrystallized austenite on the martensitic transformation has received relatively less attention. The main thrust of this paper is principally concerned with elucidating the effect of dynamically recrystallized austenite on the martensite starting temperature for Fe–32% Ni alloy in comparison with those of the statically recrystallized, based on the study on its dynamic recrystallization (DRX) behavior.
An Fe–32% Ni alloy was used for the experiment with its chemical composition shown in Table 1. The martensite starting temperature, Ms , of the material is below room temperature so that the alloy is in the austenitic state at ambient temperature. After a subzero treatment it will be transformed into lentoid martensite. The material in the form of plates was machined into compression cylinders 12 mm in height and 8 mm in diameter. In order to reduce the friction between the faces of the sample and the platens during compression deformation, concentric grooves about 0.1 mm in depth were machined on the ends of the sample to hold powdered glass lubricant so that the sample could be deformed homogeneously at high strains. These samples were annealed in vacuum at 1343 K for 1800 s, giving an initially austenite microstructure with initial grain size of about 200 m. Then they were deformed at the temperature of 1173–1323 K and the strain rate of 2 × 10−5 to 2.7 × 10−2 s−1 to the steady state in the high temperature compressor, followed by taking out within a second and quenching to the room temperature. Identical samples were annealed in vacuum at 1073–1343 K for 1800 s for the purpose of comparison, obtaining average grain sizes of static recrystallized microstructures ranging from 30 to 200 m. Two group of samples above were then cut into 1 mm thick slices by wire cutting and removed the surface stress layer by
∗
Corresponding author. Tel.: +86 21 62932086. E-mail addresses: [email protected], [email protected] (J. Huang). 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.069
J. Huang, Z. Xu / Materials Science and Engineering A 438–440 (2006) 254–257
255
Table 1 Chemical composition of the material used for experiment in wt.% C
Si
Mn
P
S
Ni
Al
N
O
0.007
0.01
0.04
0.005
0.0006
32.4
0.022
0.00074
0.020
chemical polishing, followed by subzero treatments. The Ms temperature was identified by measuring the electrical resistance during cooling. Usually the relative electrical resistance decreases as the temperature drops before transformation. Once the martensitic transformation breaks out, a sudden change will occur in the electrical resistance, by which the Ms temperature can be easily identified, as can be seen in Fig. 1.
stant and the stress reaches to a stable one σ s . The corresponding relationship between them is given in Fig. 3. It indicates that the dynamic grain size Ds decreases as the stable stress σ s increases [5]. Therefore, dynamically recrystallized microstructures with desirable grain sizes can be obtained by matching the deformation temperatures with the strain rates to make an effect on the stable stress s . In this experimental condition, the dynamic grain sizes range from 15 to 163 m.
3. Experimental results 3.1. Hot deformed behavior of austenite The Fe–32% Ni alloy exhibits a typical dynamic recrystallization behavior during hot deformation with the stress–strain curves shown in Fig. 2. At the beginning of the deformation, the flow stress rises with an increase in strain, producing obvious work hardening. Subsequently the work-hardening rate decreases gradually with subsequent deformation till the stress reaches to the first peak value. After then, the flow stress begins to decrease and softening appears, namely DRX takes place. In the highly strained region the flow stress does not change with the strain and remains constant, and the DRX keeps a dynamic balance [4]. It also can be seen that with the increase in deformation temperature or the decrease in strain rate the peak strain decreases, which shows the DRX begins earlier than before. Furthermore, the stress–strain curve is changing gradually from a single-peak type to a multiple-peak type. In the steady state stage of the highly strained region, the average dynamically recrystallized grain diameter Ds keeps con-
Fig. 1. Identification of the Ms temperature of samples.
Fig. 2. Effect of deformation temperature and strain rate on the curves for Fe–32% Ni alloy.
Fig. 3. Relation between the stable stress σ s and the dynamic grain size Ds for Fe–32% Ni alloy.
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Fig. 6. Transmission electron micrograph of martensite in dynamically recrystallized structure for Fe–32% Ni alloy. Fig. 4. Effect of austenite grain size on the Ms temperature in dynamically recrystallized and statically recrystallized structure.
3.2. Effect of DRX on the Ms temperature The graph in Fig. 4 show the effect of austenite grain size on the Ms temperature in dynamically recrystallized and statically recrystallized structure. The Ms temperatures of both the dynamically recrystallized structure and the statically recrystallized structure increase with an increase in austenite grain size [6–9]. The fine-grain region exhibits a higher increasing rate of the Ms temperature, while the coarse-grain region a lower rate. On the whole, the Ms temperatures of the dynamically recrystallized structures are all higher than that those of the statically recrystallized structures, from which it can be illustrated that DRX can enhance the martensitic nucleation and thereby increase the Ms temperature. Different from the statically recrystallized structure, the dynamically recrystallized structure features on the heterogeneous grains, the serrated grain boundary and the high dislocation density in the interior of a grain. Martensite plates from the statically recrystallized are integral and straight. Midribs can
be seen clearly. Some martensite plates can go through a whole austenite grain. On the contrary, the martensite plates from the dynamically recrystallized are of complex substructures (see Fig. 5). The small martensite plates are integral and straight, while the big are crooked or broken and show their irregular and serrated rims. Many martensite plates isolate near the boundary and do not go through a whole austenite grain any longer. From transmission electron microscopy images (see Fig. 6), the high dislocation density at the tip or around the martensite plate, the most irregular rims of plates and the crooked midrib and twin regions within a plate can also be seen. 4. Analysis Compared with the statically recrystallized structure, the dynamically recrystallized structure has a special effect on the martensitic transformation, which is related to its heterogeneous dislocation substructures. Previous investigations [10–14] show that the highly strained dynamically recrystallized structure is very heterogeneous and can be classified into three
Fig. 5. Optical micrographs of martensite in statically recrystallized or dynamically recrystallized structure for Fe–32% Ni alloy. (a) 1273 K, 2 × 10−3 s−1 , ε = 0; (b) 1323 K, 2 × 10−5 s−1 , ε = 0.6.
J. Huang, Z. Xu / Materials Science and Engineering A 438–440 (2006) 254–257
257
Fig. 7. Three types of dislocation density distribution (A–C) developed in the DRX grain structure (ρ0 is the initial dislocation density, ρc is the critical value required for dynamic nucleation, and D is the current DRX grain size. The present state is represented by the full lines, and one or more earlier states by broken lines.)
types according to the dislocation substructure, as illustrated schematically in Fig. 7: (A) a fine grain with a low dislocation density (namely a DRX nucleus); (B) a grain with a dislocation density in a gradient distribution, in which there is a low density near the boundary and a high density in the interior (namely a growing DRX grain); (C) a DRX grain with a high dislocation density (namely a work-hardened DRX grain). These three types of highly strained structures in the steady state are distributed fairly uniformly in a proportion thereby to produce a dynamically recrystallized structure whose average grain size changes hardly with the strain and to exhibit a stable stress. As a rule, martensitic nucleation is considered as a kind of heterogeneous nucleation. Although martensite precedes to nucleate near the grain boundary of austenite, martensite can nucleate only near the special grain boundary with high energy, not near any boundary [1]. Despite more produced grain boundaries, grain refinement is of disadvantage to the nucleation of martensite due to improve the strength of the parent-martix. It means that refined austenite grains result in hindering martensitic transformation and decreasing the Ms temperature. A few mobile dislocations introduced into the austenite during deformation contribute to the martensitic nucleation and increase the Ms temperature owing to alleviate stress concentration from the martensitic nucleation. However, the introduction of plenty of deformation dislocations will lead to work hardening of the parent-martix, which hinders the martensitic nucleation and decreases the Ms temperature [1,3]. Therefore the existence of either the DRX nucleus or the work-hardened DRX grain will result in hindering the martensitic nucleation and decreasing the Ms temperature, however, that of the growing DRX grain will contribute to the martensitic nucleation and increase the Ms temperature due to mobile dislocations near the grain boundary. Furthermore, it will be of interest to note that those high-density dislocations entwisting within the growing DRX grain will hinder the continuous growth of martensite plates and cause them crooked or broken.
5. Conclusions In the light of this experimental research, it is possible to draw the following conclusions: (i) The effects of the size of the austenite grain in dynamically recrystallized and statically recrystallized structure for Fe–32% Ni alloy on the Ms temperature are same basically, namely that the Ms temperature decreases with the grain diameter decreasing. However, the Ms temperature of the former is higher than that of the latter on the condition of the same grain size. (ii) Dynamic recrystallization has a special effect on the martensitic transformation, which is attributed to its heterogeneous dislocation substructures. Acknowledgement The authors would like to acknowledge the NSFC (China) for the financial support of this work under grant number 50471017. References [1] S. Kajiwara, Metall. Trans. 17A (1986) 1693. [2] K. Tsuzakai, S. Fukasaku, Y. Tomota, T. Maki, Mater. Trans. JIM 32 (1991) 222. [3] M. Pan, T.Y. Hsu, Acta Metall. Sin. 25 (6) (1989) 389. [4] T. Sakai, J.J. Jonas, Acta Metall. 32 (1984) 189. [5] Z. Xu, F.S. Meng, X.Y. Men, Z.Q. Fu, Z.K. Yao, Iron Steel 19 (9) (1984) 50. [6] T.Y. S Hsu, Proceedings of ICOMAT-86, Key Note Lecture, Japan Institute of Metals, 1987, p. 245. [7] T.Y. Hsu, J. Mater. Sci. 20 (1985) 23. [8] P.J. Brofman, G.S. Ansell, Metall Trans. 14A (1983) 1929. [9] J. Wu, B.H. Jiang, T.Y. Hsu, Acta Matall. 36 (1988) 1521. [10] T. Sakai, M. Ohashis, K. Chiba, J.J. Jonas, Acta Metall. 36 (1988) 1781. [11] Z. Xu, T. Sakai, J. Jpn. Inst. Met. 53 (1989) 1161. [12] Z. Xu, T. Sakai, J. Jpn. Inst. Met. (JIM) 55 (1991) 1298. [13] T. Sakai, J. Mater. Process. Technol. 53 (1995) 349. [14] Z. Xu, T. Sakai, Z.K. Yao, Mater. Sci. Technol. 5 (2) (1997) 124.