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ultrafine grained structure in 201 austenitic stainless steel
Materials Science and Engineering A 530 (2011) 378–381
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of reversion annealing on the formation of nano/ultrafine grained structure in 201 austenitic stainless steel Mohammad Moallemi, Abbas Najafizadeh, Ahmad Kermanpur, Ahad Rezaee ∗ Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 15 April 2011 Received in revised form 20 August 2011 Accepted 28 September 2011 Available online 5 October 2011 Keywords: Stainless steel Nano/ultrafine grained structure Thermomechanical Martensite treatment
a b s t r a c t The formation of nano/ultrafine grain structure in a 201 austenitic stainless steel was investigated by the martensite thermomechanical treatment. Cast ingots were first homogenized, then hot-forged and solution-annealed to reduce the initial grain size. Cold rolling was then conducted down to 90% reduction in thickness, followed by reversion annealing at a temperature in the range of 1023–1173 K for 15–1800 s. The effect of reversion parameters on grain refinement was investigated. The resulting microstructures were characterized by a scanning electron microscopy equipped with X-ray energy-dispersive spectrometer, an X-ray diffractometer and a Feritscope. The hardness was measured by the Vickers method. The results show that a nano/ultrafine-grained structure formed in the initial stages of the reversion, but significant grain growth took place during the entire course of reversion. Initially lowered, the volume fraction of martensite increased again during the reversion treatment due to carbide precipitation. A fully austenitic nano grained 201 stainless steel with the average grain size of 100 nm was produced, possessing a yield strength of about 1370 MPa. Published by Elsevier B.V.
1. Introduction Austenitic stainless steels (ASSs) are conventionally used in various applications in which good corrosion resistance is required. Metastable ASSs are one of the important grades of the ASSs in which ␥-austenite can be transformed to ␣ -martensite during deformation. Hence, metastable grades exhibit higher tensile strength and better formability than those in stable ones. The metastable ASSs are employed in various structural applications, such as railway and automotive structural components, due to the necessity of weight reduction and crash safety of automobiles. However, they possess relatively low yield strength, which limits their structural applications [1,2]. The yield strength can be improved by methods such as solution hardening, precipitation hardening, and grain refinement. Grain refinement is an effective way to increase yield strength without impairing ductility [3]. Many studies have been conducted in relation to the production of ultrafine/nano grained stainless steels in order to achieve high strength and good ductility. For this purpose, several techniques have been used including severe plastic deformation such as high pressure torsion, equal-channel angular pressing and accumulative roll bonding [4]. Another route to fabricate ultrafine grained
∗ Corresponding author. Tel.: +98 9126188197; fax: +98 3113912752. E-mail address: [email protected] (A. Rezaee). 0921-5093/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.msea.2011.09.099
steel called advanced thermomechanical process includes several methods such as conventional cold rolling-annealing. The second route is more suitable from the viewpoint of industrial applications. It is usually difficult to obtain grains with sub-micrometer size by any conventional thermomechanical treatment. Recently, a thermomechanical process has been developed by reversion transformation of the strain-induced martensite (SIM) to austenite to obtain nano/submicron grain size in metastable ASSs. In this process, called martensite process, the strain-induced martensite formed by heavy cold roll is reverted to austenite during subsequent annealing, leading to considerable grain refinement [5,6]. The martensite process has been extensively studied for ASSs. Tomimura et al. [7] have developed a thermomechanical treatment to obtain austenitic structure with a grain size of about 200 nm in a non-commercial metastable ASS with the yield strength of about 0.8 GPa. Ma et al. [8], Johanssen et al. [9], Di Schino et al. [10], Rajasekhara et al. [11], Misra et al. [12], Eskandari et al. [13], and Forouzan et al. [14], have obtained nano/ultrafine grain structure in the 300 series of the commercial ASSs by using martensite treatment. According to the authors’ knowledge, less work has been performed on developing nano/ultrafine grain structure in the 200 series stainless steels. The 200 series of ASSs contain high manganese instead of nickel as the major austenite forming element in order to reduce the manufacturing costs. The aim of the present study was to investigate effect of the reversion annealing in the martensite process on the formation of nano/ultrafine grain structure in the 201 stainless steel.
M. Moallemi et al. / Materials Science and Engineering A 530 (2011) 378–381
379
Table 1 The chemical composition of the AISI 201 steel used in this work. C
Mn
P
Si
Cr
Mo
Ni
Al
Nb
N
Fe
0.08
5.9
0.04
0.54
16.6
0.11
3.7
0.05
0.002
0.04
Bal.
2. Materials and experimental procedures The chemical composition of the 201 stainless steel used in this study is given in Table 1. The alloy was prepared using an induction melting furnace under air. The cast ingots (300 mm × 70 mm × 40 mm) were homogenized at 1473 K for 54 ks. The homogenized specimens (70 mm × 50 mm × 40 mm) were then forged at a temperature range of 1273–1473 K to the dimension of 350 mm × 20 mm × 20 mm. Several specimens with the dimension of 8 mm × 20 mm × 50 mm were then machined from the forged specimens for the subsequent solution annealing and thermomechanical processes. The plates were then solutiontreated at 1423 K for 7.2 ks, resulting in grain size of about 55 m. The thermomechanical treatment used to produce nano/ultrafine grain structure is shown in Fig. 1. The annealed specimens were cold rolled down to 90% reduction in thickness and were then subjected to the reversion transformation at temperature range of 1023–1173 K for various lengths of time between 15 s and 1.8 ks. All specimens were quenched in water after reversion. The phase characterization was conducted during cold rolling and annealing by a Feritscope (Fischer MP30) and X-ray diffractometer (XRD Philip’s X’Pert K␣ mode). The scan speed of XRD was 3◦ min−1 and the voltage and current were 40 kV and 200 mA, respectively. Before XRD, the specimens were electro-polished to remove any possible deformation-induced martensite on the surface. The electrolyte solute was 200 ml HClO4 + 800 ml ethanol at 30 V. The austenite grain boundaries and carbide precipitates were revealed by electro-etching in 65% nitric acid solution at 1 V and in 10% oxalic acid solution at 6 V, respectively. The microstructures were observed using a scanning electron microscope (SEM Philips XL30) equipped with energy dispersive spectrometer (EDS). The grain size was calculated by the Clemex image analysis software. The specimen hardness (H) was measured by the Vickers method with the indenting load of 10 kg. 3. Results and discussion The variations of volume fraction of SIM and hardness versus reduction percentage are illustrated in Fig. 2. As can be seen, the volume fraction of SIM is increased with an increase in the reduction of
Fig. 1. Schematic illustration of the thermomechanical process to obtain nano/ultrafine grain structure in 201 stainless steel.
Fig. 2. Effect of thickness reduction on the martensite fraction and hardness.
thickness and saturated at 35% reduction leading to the saturation strain (εc ) of 0.4. It is noteworthy that εc plays an important role in the formation of ultrafine grain size. In the previous works [5], εc was 0.7 for the 301 stainless steel. This can be attributed to the lower stacking fault energy (SFE) for the 201 steel compared to that of the 301 stainless steel. The reason can be ascribed to: (i) Ni partially replaced with other austenitizing elements, such as Mn and N and (ii) lower amount of carbon content, both of which promote austenite to martensite transformation. Lower εc results in more fragmentation of martensite during the subsequent rolling passes. This leads to an increase in the nucleation sites during austenite reversion. The hardness variation in Fig. 2 shows that the increasing rate of hardness is relatively higher for the strains before saturation due to the formation of SIM during cold rolling. After saturation, the hardness is further increased by increasing dislocation density and consequent strain hardening during the deformation. Fig. 3 shows the phase changes investigated by XRD at each stage of the thermomechanical treatment after annealing at 1173 K for 0.03–1.8 ks. After 90% cold rolling of the solution-treated specimen, the microstructure was changed to fully martensite. As can be observed, the martensite was reversely transformed to austenite after annealing treatment. The results show that almost all the amount of martensite was reverted to austenite before 60 s. The amount of martensite after 90% reduction in thickness followed by annealing treatment at the temperature range of 1023–1173 K for 0.015–1.8 ks is exhibited in Fig. 4. As shown, the higher the temperature, the faster the revision rate. For instance, at temperatures of 1123 and 1173 K, complete reversion took place
Fig. 3. The XRD patterns for each stage of the thermomechanical treatment: (A) after solution annealing, (B) after 90% cold rolling and reversion annealing at 1173 K for: (C) 15 s, (D) 60 s, (E) 600 s, and (F) 1200 s.
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M. Moallemi et al. / Materials Science and Engineering A 530 (2011) 378–381
Fig. 4. Martensite content in the specimens annealed at the temperature range of 1023–1173 K measured by Feritscope.
after about 60 s, but no full reversion was achieved at temperature 1023 K. The results also showed a secondary increase in the martensite content as annealing was continued up to 1.8 ks. This increase in the amount of martensite during reversion was also reported
in several previous studies [5,9,15]. A possible mechanism for the explanation of this phenomenon is an increase in the Ms temperature of the steel due to the precipitation of carbides, which leads to the formation of thermally induced martensite during quenching. It should be noted that although C, N, Ni, and Cr are generally known to be “austenite stabilizers” (with regard to transformation to ferrite), it is believed that here the austenite stability is decreased by depletion of the matrix from C, N, Ni, and Cr due to an increase in Ms temperature, resulting in an increase in driving force for martensite formation during quenching. Fig. 5a illustrates precipitation of carbides in grain boundaries in the specimen annealed at 1173 K for 600 s. The EDS analysis of this specimen in Fig. 5b shows that these precipitations can be chromium carbides. It is suggested that chromium and carbon are depleted from the austenitic matrix due to the formation of carbides; consequently the Ms temperature is increased. The Ms temperature can be estimated with the equation presented by Eichelmann and Hull [16]: M s (◦ C) = 1305 − [1667(%C + %N) + 28(%Si) + 33(%Mn) + 42(%Cr) + 61(%Ni)]
Fig. 5. (a) SEM image of the annealed specimen at 1173 K for 600 s showing carbide precipitations in grain boundary and (b) EDS analysis of the carbide precipitates.
Fig. 6. SEM images of the specimens annealed at 1123 K for: (a) 15 s, (b) 30 s and (c) 1200 s.
M. Moallemi et al. / Materials Science and Engineering A 530 (2011) 378–381 Table 2 The grain size, austenite volume fraction and estimated yield strength of the specimens annealed at different conditions. Reversion temperature (K)/time(s)
Mean grain size (m)
1073/60 1073/180 1123/15 1123/30 1123/60 1123/300 1123/1200 1173/15
0.28 0.55 0.07 0.1 0.37 2.4 4 0.14
± ± ± ± ± ± ± ±
0.05 0.09 0.01 0.015 0.07 0.2 0.6 0.02
Austenite volume fraction (%)
Estimated yield strength (MPa)
92 95 86 95 89 86 73 90
1270 1190 1560 1370 1230 880 960 1310
For the 201 steel used in this study, the above equation shows that the Ms temperature can be increased from 246 K to 378 K by complete depletion of carbon in the austenite matrix. Therefore, the formation of thermally induced martensite could take place during reversion. This phenomenon has not been reported for the stainless steels with low carbon content, such as 316L [13] or 304L [14] confirming this suggestion. Fig. 6 shows SEM micrographs of the annealed specimens at 1123 K for different lengths of time. As shown in Fig. 6a, the smallest grain size value of about 70 nm was achieved by annealing for 15 s. However, the microstructure was not fully austenitic and consisted of a mixture of 86% austenite and 14% martensite, as measured by Feritscope. We believed that this two-phase microstructure possesses low formability during sheet forming. Fig. 6b shows microstructure with the grain size of about 100 nm and more fraction of reverted austenite (95%) that was achieved by annealing for 30 s. This specimen showed the smallest grain size along with the highest reversion fraction. However grain growth is triggered upon annealing with increasing time at this temperature (Fig. 6c). The microstructural features and the yield strength estimated by the equation H = 3 y /9.8 are presented in Table 2 under different reversion conditions [17]. As can be seen, the yield strength was estimated about 1560 MPa for the specimen annealed at 1123 K for 15 s. This can be attributed to the effects of nano grain structure along with the untransformed martensite in the structure. For the specimen annealed at 1123 K for 30 s, the yield strength was estimated about 1370 MPa. Nonetheless, the obtained yield strength is a promising value. In addition, the negative effects of the retained martensite are omitted by the formation of fully austenitic structure. For the specimen annealed at 1123 K for 1200 s, the estimated yield strength was about 960 MPa. Although the mean grain size of austenite for this specimen is quite large (e.g. 4 m), the value of yield strength is still considerable. The reason is the increase in the thermally induced martensite and carbide precipitations in grain boundaries during reversion, which causes Zener pinning and improves the yield strength of the steel.
381
The present work clearly shows that the hardness and yield strength of the 201 stainless steel can be significantly improved through the formation of the nano/ultrafine grain structure by using the martensite treatment. This nanostructured steel can be a suitable candidate for the manufacturing of strong lightweight structures. 4. Conclusions The principal conclusions drawn from the present work are as follows: • Reversion annealing of the cold rolled 201 stainless steel at the temperature range of 1023–1173 K show a decrease followed by an increase in the martensite volume fraction. The secondary increase in the martensite content is attributed to the formation of thermally induced martensite due to the carbide precipitation and a consequent increase in the martensite start temperature Ms . • The smallest austenite grain size of about 70 nm was achieved for the specimen annealed at 1123 K for 15 s. The microstructure consisted of both martensite and austenite. • A fully austenitic nano-grained 201 stainless steel with the average grain size of 100 nm was produced by annealing the 90% cold rolled steel at 1123 K for 30 s, possessing the yield strength of about 1370 MPa. References [1] A.F. Padilha, R.L. Plaut, P.R. Rios, ISIJ Int. 43 (2003) 135–143. [2] Y.H. Kim, K.Y. Kim, Y.D. Lee, Mater. Manuf. Processes 19 (2004) 51–59. [3] L.P. Karjalainen, T. Taulavuori, M. Sellman, A. Kyrolainen, Steel. Res. Int. 79 (2008) 404–412. [4] R. Song, D. Ponge, D. Raabe, J.G. Speer, D.K. Matlock, Mater. Sci. Eng. A 441 (2006) 1–17. [5] M. Eskandari, A. Kermanpur, A. Najafizadeh, Metall. Mater. Trans. A 40 (2009) 2241–2249. [6] A. Rezaee, A. Kermanpur, A. Najafizadeh, M. Moallemi, Mater. Sci. Eng. A 528 (2011) 025–5029. [7] K. Tomimura, S. Takaki, Y. Tokunaga, ISIJ Int. 31 (1991) 1431–1437. [8] Y. Ma, J.E. Jin, Y.K. Lee, Scr. Mater. 52 (2005) 1311–1315. [9] D.L. Johannsen, A. Kyrolainen, P.J. Ferreira, Metall. Mater. Trans. A 37 (2006) 2325–2328. [10] A. Di Schino, I. Salvatori, J.M. Kenny, J. Mater. Sci. 37 (2002) 4561–4565. [11] S. Rajasekhara, P.J. Ferreira, L.P. Karjalainen, A. Kyrolainen, Metall. Mater. Trans. A 38 (2007) 1202–1210. [12] R.D.K. Misra, B.R. Kumar, M. Somani, P. Karjalainen, Scr. Mater. 59 (2008) 9–82. [13] M. Eskandari, A. Najafizadeh, A. Kermanpur, Mater. Sci. Eng. A 519 (2009) 46–50. [14] F. Forouzan, A. Najafizadeh, A. Kermanpur, A. Hedayati, R. Surki Aaliabad, Mater. Sci. Eng. A 527 (2010) 7334–7339. [15] M. Karimi, A. Najafizadeh, A. Kermanpur, M. Eskandari, Mater. Charact. 60 (2009) 1220–1223. [16] G.H. Eichelmann, F.C. Hull, Trans. Am. Soc. Metall. 45 (1953) 77–95. [17] D. Tabor, Hardness of Metals, Oxford University Press, Oxford, United Kingdom, 2000, pp. 95–114.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of reversion annealing on the formation of nano/ultrafine grained structure in 201 austenitic stainless steel Mohammad Moallemi, Abbas Najafizadeh, Ahmad Kermanpur, Ahad Rezaee ∗ Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 15 April 2011 Received in revised form 20 August 2011 Accepted 28 September 2011 Available online 5 October 2011 Keywords: Stainless steel Nano/ultrafine grained structure Thermomechanical Martensite treatment
a b s t r a c t The formation of nano/ultrafine grain structure in a 201 austenitic stainless steel was investigated by the martensite thermomechanical treatment. Cast ingots were first homogenized, then hot-forged and solution-annealed to reduce the initial grain size. Cold rolling was then conducted down to 90% reduction in thickness, followed by reversion annealing at a temperature in the range of 1023–1173 K for 15–1800 s. The effect of reversion parameters on grain refinement was investigated. The resulting microstructures were characterized by a scanning electron microscopy equipped with X-ray energy-dispersive spectrometer, an X-ray diffractometer and a Feritscope. The hardness was measured by the Vickers method. The results show that a nano/ultrafine-grained structure formed in the initial stages of the reversion, but significant grain growth took place during the entire course of reversion. Initially lowered, the volume fraction of martensite increased again during the reversion treatment due to carbide precipitation. A fully austenitic nano grained 201 stainless steel with the average grain size of 100 nm was produced, possessing a yield strength of about 1370 MPa. Published by Elsevier B.V.
1. Introduction Austenitic stainless steels (ASSs) are conventionally used in various applications in which good corrosion resistance is required. Metastable ASSs are one of the important grades of the ASSs in which ␥-austenite can be transformed to ␣ -martensite during deformation. Hence, metastable grades exhibit higher tensile strength and better formability than those in stable ones. The metastable ASSs are employed in various structural applications, such as railway and automotive structural components, due to the necessity of weight reduction and crash safety of automobiles. However, they possess relatively low yield strength, which limits their structural applications [1,2]. The yield strength can be improved by methods such as solution hardening, precipitation hardening, and grain refinement. Grain refinement is an effective way to increase yield strength without impairing ductility [3]. Many studies have been conducted in relation to the production of ultrafine/nano grained stainless steels in order to achieve high strength and good ductility. For this purpose, several techniques have been used including severe plastic deformation such as high pressure torsion, equal-channel angular pressing and accumulative roll bonding [4]. Another route to fabricate ultrafine grained
∗ Corresponding author. Tel.: +98 9126188197; fax: +98 3113912752. E-mail address: [email protected] (A. Rezaee). 0921-5093/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.msea.2011.09.099
steel called advanced thermomechanical process includes several methods such as conventional cold rolling-annealing. The second route is more suitable from the viewpoint of industrial applications. It is usually difficult to obtain grains with sub-micrometer size by any conventional thermomechanical treatment. Recently, a thermomechanical process has been developed by reversion transformation of the strain-induced martensite (SIM) to austenite to obtain nano/submicron grain size in metastable ASSs. In this process, called martensite process, the strain-induced martensite formed by heavy cold roll is reverted to austenite during subsequent annealing, leading to considerable grain refinement [5,6]. The martensite process has been extensively studied for ASSs. Tomimura et al. [7] have developed a thermomechanical treatment to obtain austenitic structure with a grain size of about 200 nm in a non-commercial metastable ASS with the yield strength of about 0.8 GPa. Ma et al. [8], Johanssen et al. [9], Di Schino et al. [10], Rajasekhara et al. [11], Misra et al. [12], Eskandari et al. [13], and Forouzan et al. [14], have obtained nano/ultrafine grain structure in the 300 series of the commercial ASSs by using martensite treatment. According to the authors’ knowledge, less work has been performed on developing nano/ultrafine grain structure in the 200 series stainless steels. The 200 series of ASSs contain high manganese instead of nickel as the major austenite forming element in order to reduce the manufacturing costs. The aim of the present study was to investigate effect of the reversion annealing in the martensite process on the formation of nano/ultrafine grain structure in the 201 stainless steel.
M. Moallemi et al. / Materials Science and Engineering A 530 (2011) 378–381
379
Table 1 The chemical composition of the AISI 201 steel used in this work. C
Mn
P
Si
Cr
Mo
Ni
Al
Nb
N
Fe
0.08
5.9
0.04
0.54
16.6
0.11
3.7
0.05
0.002
0.04
Bal.
2. Materials and experimental procedures The chemical composition of the 201 stainless steel used in this study is given in Table 1. The alloy was prepared using an induction melting furnace under air. The cast ingots (300 mm × 70 mm × 40 mm) were homogenized at 1473 K for 54 ks. The homogenized specimens (70 mm × 50 mm × 40 mm) were then forged at a temperature range of 1273–1473 K to the dimension of 350 mm × 20 mm × 20 mm. Several specimens with the dimension of 8 mm × 20 mm × 50 mm were then machined from the forged specimens for the subsequent solution annealing and thermomechanical processes. The plates were then solutiontreated at 1423 K for 7.2 ks, resulting in grain size of about 55 m. The thermomechanical treatment used to produce nano/ultrafine grain structure is shown in Fig. 1. The annealed specimens were cold rolled down to 90% reduction in thickness and were then subjected to the reversion transformation at temperature range of 1023–1173 K for various lengths of time between 15 s and 1.8 ks. All specimens were quenched in water after reversion. The phase characterization was conducted during cold rolling and annealing by a Feritscope (Fischer MP30) and X-ray diffractometer (XRD Philip’s X’Pert K␣ mode). The scan speed of XRD was 3◦ min−1 and the voltage and current were 40 kV and 200 mA, respectively. Before XRD, the specimens were electro-polished to remove any possible deformation-induced martensite on the surface. The electrolyte solute was 200 ml HClO4 + 800 ml ethanol at 30 V. The austenite grain boundaries and carbide precipitates were revealed by electro-etching in 65% nitric acid solution at 1 V and in 10% oxalic acid solution at 6 V, respectively. The microstructures were observed using a scanning electron microscope (SEM Philips XL30) equipped with energy dispersive spectrometer (EDS). The grain size was calculated by the Clemex image analysis software. The specimen hardness (H) was measured by the Vickers method with the indenting load of 10 kg. 3. Results and discussion The variations of volume fraction of SIM and hardness versus reduction percentage are illustrated in Fig. 2. As can be seen, the volume fraction of SIM is increased with an increase in the reduction of
Fig. 1. Schematic illustration of the thermomechanical process to obtain nano/ultrafine grain structure in 201 stainless steel.
Fig. 2. Effect of thickness reduction on the martensite fraction and hardness.
thickness and saturated at 35% reduction leading to the saturation strain (εc ) of 0.4. It is noteworthy that εc plays an important role in the formation of ultrafine grain size. In the previous works [5], εc was 0.7 for the 301 stainless steel. This can be attributed to the lower stacking fault energy (SFE) for the 201 steel compared to that of the 301 stainless steel. The reason can be ascribed to: (i) Ni partially replaced with other austenitizing elements, such as Mn and N and (ii) lower amount of carbon content, both of which promote austenite to martensite transformation. Lower εc results in more fragmentation of martensite during the subsequent rolling passes. This leads to an increase in the nucleation sites during austenite reversion. The hardness variation in Fig. 2 shows that the increasing rate of hardness is relatively higher for the strains before saturation due to the formation of SIM during cold rolling. After saturation, the hardness is further increased by increasing dislocation density and consequent strain hardening during the deformation. Fig. 3 shows the phase changes investigated by XRD at each stage of the thermomechanical treatment after annealing at 1173 K for 0.03–1.8 ks. After 90% cold rolling of the solution-treated specimen, the microstructure was changed to fully martensite. As can be observed, the martensite was reversely transformed to austenite after annealing treatment. The results show that almost all the amount of martensite was reverted to austenite before 60 s. The amount of martensite after 90% reduction in thickness followed by annealing treatment at the temperature range of 1023–1173 K for 0.015–1.8 ks is exhibited in Fig. 4. As shown, the higher the temperature, the faster the revision rate. For instance, at temperatures of 1123 and 1173 K, complete reversion took place
Fig. 3. The XRD patterns for each stage of the thermomechanical treatment: (A) after solution annealing, (B) after 90% cold rolling and reversion annealing at 1173 K for: (C) 15 s, (D) 60 s, (E) 600 s, and (F) 1200 s.
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M. Moallemi et al. / Materials Science and Engineering A 530 (2011) 378–381
Fig. 4. Martensite content in the specimens annealed at the temperature range of 1023–1173 K measured by Feritscope.
after about 60 s, but no full reversion was achieved at temperature 1023 K. The results also showed a secondary increase in the martensite content as annealing was continued up to 1.8 ks. This increase in the amount of martensite during reversion was also reported
in several previous studies [5,9,15]. A possible mechanism for the explanation of this phenomenon is an increase in the Ms temperature of the steel due to the precipitation of carbides, which leads to the formation of thermally induced martensite during quenching. It should be noted that although C, N, Ni, and Cr are generally known to be “austenite stabilizers” (with regard to transformation to ferrite), it is believed that here the austenite stability is decreased by depletion of the matrix from C, N, Ni, and Cr due to an increase in Ms temperature, resulting in an increase in driving force for martensite formation during quenching. Fig. 5a illustrates precipitation of carbides in grain boundaries in the specimen annealed at 1173 K for 600 s. The EDS analysis of this specimen in Fig. 5b shows that these precipitations can be chromium carbides. It is suggested that chromium and carbon are depleted from the austenitic matrix due to the formation of carbides; consequently the Ms temperature is increased. The Ms temperature can be estimated with the equation presented by Eichelmann and Hull [16]: M s (◦ C) = 1305 − [1667(%C + %N) + 28(%Si) + 33(%Mn) + 42(%Cr) + 61(%Ni)]
Fig. 5. (a) SEM image of the annealed specimen at 1173 K for 600 s showing carbide precipitations in grain boundary and (b) EDS analysis of the carbide precipitates.
Fig. 6. SEM images of the specimens annealed at 1123 K for: (a) 15 s, (b) 30 s and (c) 1200 s.
M. Moallemi et al. / Materials Science and Engineering A 530 (2011) 378–381 Table 2 The grain size, austenite volume fraction and estimated yield strength of the specimens annealed at different conditions. Reversion temperature (K)/time(s)
Mean grain size (m)
1073/60 1073/180 1123/15 1123/30 1123/60 1123/300 1123/1200 1173/15
0.28 0.55 0.07 0.1 0.37 2.4 4 0.14
± ± ± ± ± ± ± ±
0.05 0.09 0.01 0.015 0.07 0.2 0.6 0.02
Austenite volume fraction (%)
Estimated yield strength (MPa)
92 95 86 95 89 86 73 90
1270 1190 1560 1370 1230 880 960 1310
For the 201 steel used in this study, the above equation shows that the Ms temperature can be increased from 246 K to 378 K by complete depletion of carbon in the austenite matrix. Therefore, the formation of thermally induced martensite could take place during reversion. This phenomenon has not been reported for the stainless steels with low carbon content, such as 316L [13] or 304L [14] confirming this suggestion. Fig. 6 shows SEM micrographs of the annealed specimens at 1123 K for different lengths of time. As shown in Fig. 6a, the smallest grain size value of about 70 nm was achieved by annealing for 15 s. However, the microstructure was not fully austenitic and consisted of a mixture of 86% austenite and 14% martensite, as measured by Feritscope. We believed that this two-phase microstructure possesses low formability during sheet forming. Fig. 6b shows microstructure with the grain size of about 100 nm and more fraction of reverted austenite (95%) that was achieved by annealing for 30 s. This specimen showed the smallest grain size along with the highest reversion fraction. However grain growth is triggered upon annealing with increasing time at this temperature (Fig. 6c). The microstructural features and the yield strength estimated by the equation H = 3 y /9.8 are presented in Table 2 under different reversion conditions [17]. As can be seen, the yield strength was estimated about 1560 MPa for the specimen annealed at 1123 K for 15 s. This can be attributed to the effects of nano grain structure along with the untransformed martensite in the structure. For the specimen annealed at 1123 K for 30 s, the yield strength was estimated about 1370 MPa. Nonetheless, the obtained yield strength is a promising value. In addition, the negative effects of the retained martensite are omitted by the formation of fully austenitic structure. For the specimen annealed at 1123 K for 1200 s, the estimated yield strength was about 960 MPa. Although the mean grain size of austenite for this specimen is quite large (e.g. 4 m), the value of yield strength is still considerable. The reason is the increase in the thermally induced martensite and carbide precipitations in grain boundaries during reversion, which causes Zener pinning and improves the yield strength of the steel.
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The present work clearly shows that the hardness and yield strength of the 201 stainless steel can be significantly improved through the formation of the nano/ultrafine grain structure by using the martensite treatment. This nanostructured steel can be a suitable candidate for the manufacturing of strong lightweight structures. 4. Conclusions The principal conclusions drawn from the present work are as follows: • Reversion annealing of the cold rolled 201 stainless steel at the temperature range of 1023–1173 K show a decrease followed by an increase in the martensite volume fraction. The secondary increase in the martensite content is attributed to the formation of thermally induced martensite due to the carbide precipitation and a consequent increase in the martensite start temperature Ms . • The smallest austenite grain size of about 70 nm was achieved for the specimen annealed at 1123 K for 15 s. The microstructure consisted of both martensite and austenite. • A fully austenitic nano-grained 201 stainless steel with the average grain size of 100 nm was produced by annealing the 90% cold rolled steel at 1123 K for 30 s, possessing the yield strength of about 1370 MPa. References [1] A.F. Padilha, R.L. Plaut, P.R. Rios, ISIJ Int. 43 (2003) 135–143. [2] Y.H. Kim, K.Y. Kim, Y.D. Lee, Mater. Manuf. Processes 19 (2004) 51–59. [3] L.P. Karjalainen, T. Taulavuori, M. Sellman, A. Kyrolainen, Steel. Res. Int. 79 (2008) 404–412. [4] R. Song, D. Ponge, D. Raabe, J.G. Speer, D.K. Matlock, Mater. Sci. Eng. A 441 (2006) 1–17. [5] M. Eskandari, A. Kermanpur, A. Najafizadeh, Metall. Mater. Trans. A 40 (2009) 2241–2249. [6] A. Rezaee, A. Kermanpur, A. Najafizadeh, M. Moallemi, Mater. Sci. Eng. A 528 (2011) 025–5029. [7] K. Tomimura, S. Takaki, Y. Tokunaga, ISIJ Int. 31 (1991) 1431–1437. [8] Y. Ma, J.E. Jin, Y.K. Lee, Scr. Mater. 52 (2005) 1311–1315. [9] D.L. Johannsen, A. Kyrolainen, P.J. Ferreira, Metall. Mater. Trans. A 37 (2006) 2325–2328. [10] A. Di Schino, I. Salvatori, J.M. Kenny, J. Mater. Sci. 37 (2002) 4561–4565. [11] S. Rajasekhara, P.J. Ferreira, L.P. Karjalainen, A. Kyrolainen, Metall. Mater. Trans. A 38 (2007) 1202–1210. [12] R.D.K. Misra, B.R. Kumar, M. Somani, P. Karjalainen, Scr. Mater. 59 (2008) 9–82. [13] M. Eskandari, A. Najafizadeh, A. Kermanpur, Mater. Sci. Eng. A 519 (2009) 46–50. [14] F. Forouzan, A. Najafizadeh, A. Kermanpur, A. Hedayati, R. Surki Aaliabad, Mater. Sci. Eng. A 527 (2010) 7334–7339. [15] M. Karimi, A. Najafizadeh, A. Kermanpur, M. Eskandari, Mater. Charact. 60 (2009) 1220–1223. [16] G.H. Eichelmann, F.C. Hull, Trans. Am. Soc. Metall. 45 (1953) 77–95. [17] D. Tabor, Hardness of Metals, Oxford University Press, Oxford, United Kingdom, 2000, pp. 95–114.