- Email: [email protected]
submicron grained AISI 301 stainless steel
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 2 2 0– 1 2 2 3
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / m a t c h a r
Effect of martensite to austenite reversion on the formation of nano/submicron grained AISI 301 stainless steel M. Karimi, A. Najafizadeh, A. Kermanpur⁎, M. Eskandari Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
AR TIC LE D ATA
ABSTR ACT
Article history:
The martensite to austenite reversion behavior of 90% cold rolled AISI 301 stainless steel
Received 18 February 2009
was investigated in order to refine the grain size. Cold rolled specimens were annealed at
Received in revised form 18 April 2009
600–900 °C, and subsequently characterized by scanning electron microscopy, X-ray
Accepted 22 April 2009
diffraction, Feritscope, and hardness measurements. The effects of annealing parameters on the formation of fully-austenitic nano/submicron grained structure and the mechanisms
Keywords:
involved were studied. It was found that annealing at 800 °C for 10 s exhibited the smallest
Martensite reversion
average austenite grain size of 240 ± 60 nm with an almost fully-austenitic structure.
Nano/submicron grain structure
© 2009 Elsevier Inc. All rights reserved.
Austenitic stainless steels Thermomechanical treatment
1.
Introduction
Austenitic stainless steels are widely used as structural materials for applications requiring excellent resistance to general corrosion in combination with good mechanical properties. These alloys show the best corrosion resistance and ductility among the stainless steels. However, they possess a relatively low strength, particularly yield strength, which limits their structural applications. Grain refining of austenitic stainless steels is an effective method to increase their strength, while keeping their toughness high. Recently, an advanced thermomechanical process has been developed to produce nano/submicron grain structures in metastable austenitic steels [1–3]. This process includes heavy cold rolling to form strain-induced martensite from austenite, followed by reversion of this martensite to austenite at relatively low annealing temperatures and times. It is established that austenite grains reverted from the deformed strain-induced martensite are very fine, while austenite grains reversed from deformed austenite are relatively coarse [1]. Very few studies have been conducted on the grain refinement of commercial austenitic stainless steels [4,5]. Some
researchers [2,4] have reported that martensite can be transformed to austenite with different shear and diffusional mechanisms depending on its chemical composition and temperature range. Johannsen et al. [4] reported that the volume fraction of martensite after annealing was increased with increasing annealing temperature above 750 °C. They showed that at these temperatures, martensite was formed during cooling from annealing temperatures. The aim of this work was to investigate the martensite to austenite reversion behavior on the formation of nano/ submicron grained AISI 301 stainless steel. The reversion of martensite and the precipitation taking place during annealing treatment were also studied.
2.
Materials and Experimental Procedures
The chemical composition of the AISI 301 stainless steel used in this investigation is given in Table 1. The hot rolled steel strips with an initial thickness of 8 mm were cold rolled to sheets of 0.8 mm thickness. The reduction of each pass was around 0.1 mm. Before each pass, the specimens were cooled
⁎ Corresponding author. Tel.: +98 311 3915738; fax: +98 311 3912752. E-mail address: [email protected] (A. Kermanpur). 1044-5803/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2009.04.014
1221
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 2 2 0–1 2 2 3
Table 1 – Chemical composition of AISI 301 used (wt.%). C
Mn
Ni
Cr
Mo
Si
Cu
Al
Co
P
S
Nb
N
0.11
0.66
6.91
16.2
0.27
0.67
0.53
0.06
0.1
0.03
< 0.03
0.003
0.005
in a solution of ethanol and ice brine (about −10 °C). The reversion of martensite to austenite for the 90% cold rolled specimens was performed by isothermal annealing at temperatures between 600 and 900 °C for different times from 1 to 100 min in the furnace. For short time annealing, (10–60 s) a salt solution consisting of 50% Na2CO3 + 50% NaCl was used. The microstructures of the specimens were revealed by optical and scanning electron microscopy (SEM Philips X230). The software Clemex was used for grain size measurements based on the intercept method [6]. Almost 100 grains were selected by the software to determine the grain size and the 95% confidence band of the results was reported. The amount of strain-induced martensite was measured by means of X-ray diffraction (XRD, Philips X' Pert with Cu Kα anode) and Feritscope (Fischer, MP30E).
3.
behavior at 800 to 1000 °C for a time range similar to that of Fig. 2a. Reversion rate at these temperatures was so high that most of the martensite was reverted to austenite after around 1 min. However, at these temperatures, the amount of martensite after cooling increased with increasing annealing time up to 10 min beyond which it remained approximately constant. According to Fig. 2b, the amount of transformed martensite is lower at temperatures of 950 and 1000 °C. Increasing amounts of martensite during annealing is an unexpected phenomenon that has also been reported in previous studies [1,4,7–9]. As reported in some of these works [6–8], this increase took place at lower temperatures (about 300–400 °C). One possible mechanism proposed to account for this is the recovery and relaxation of the matrix adjacent to the martensite laths that lead to martensite
Results and Discussion
Fig. 1 shows the XRD patterns of the specimens with 90% cold rolling and annealing at 900 °C for times from 1 to 100 min. The XRD pattern of the 90% cold rolled specimen shows only martensite reflections with a preferred orientation along the (200)ά plane. After 1 min annealing at 900 °C, almost all the martensite was reverted to austenite; the intensities of (110)ά, (200)ά, and (211)ά martensite reflections compared with the cold rolled specimen drastically decreased, and (111)γ, (200)γ, (220)γ, and (311)γ austenite reflections appeared. With increasing annealing times, the patterns showed a significant increase in the intensity of martensite reflections with a preferred orientation along (211)ά. Fig. 2 presents variations in martensite volume fraction measured using a Feritscope versus annealing times at different temperatures. During annealing at 600, 650, 700, and 750 °C for times between 1 and 100 min (Fig. 2a), no significant martensite reversion was observed. Fig. 2b shows annealing
Fig. 1 – The XRD patterns for 90% cold rolled and annealed specimens at 900 °C for different annealing times.
Fig. 2 – Martensite content in the specimens annealed at a) 600, 650, 700, and 750 °C; and b) 800, 850, 900, 950, and 1000 °C as measured by Feritscope.
1222
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 2 2 0– 1 2 2 3
Fig. 5 – SEM image of the specimen annealed at 800 °C for 10 s. Fig. 3 – Microstructure of the annealed specimen at 900 °C for 100 min.
growth. Another mechanism proposed is carbide precipitation that depletes carbon from austenite matrix and leads to the increase in the Ms temperature. Johannsen et al. [4] studied annealing of a cold rolled AISI 301 stainless steel for a constant time of 30 min at various temperatures between 600 and 900 °C. The highest amount of reverted austenite was achieved for annealing at 750 °C for 30 min. They concluded that the increased volume fraction of martensite at temperatures above 750 °C was due to the formation of carbides and, consequently, due to the increasing Ms temperature. Fig. 3 shows the optical micrograph of the annealed specimen at 900 °C for 100 min illustrating the high segregation of the second phases at grain boundaries. This is a socalled “ditch structure” that normally appeared in the sensitized austenitic stainless steels [10]. The EDS analysis of the microstructure indicated that the relative amounts of Nb, Cr, and C in the grain boundaries were higher than those inside the grains. The carbon depletion in the austenitic matrix can be due to the carbide formation. Based on the equation proposed by Eichelmann and Hull [11], it is well known that the Ms temperature of steel is increased by carbon depletion of the austenitic matrix. For example, the Ms tem-
perature can be increased from − 22 to 162 °C by complete depletion of carbon in the steel used in the present investigation (with 0.11 wt.%C). Therefore, if sufficient amount of carbon is precipitated in the form of carbides in the matrix, formation of thermally-induced martensite will be expected during the cooling stage of the annealing treatment. It may hence be concluded that, as the annealing temperature increases, both the reversion and precipitation rates intensify. However, as a lower amount of carbides may form at 950 and 1000 °C, the volume fraction of thermally-induced martensite is lower at these temperatures compared to the specimens annealed at 800 and 850 °C, as shown in Fig. 2b. In order to prevent carbide precipitation and to achieve a nano-grained austenitic structure, annealing treatment was also carried out for very short times using a salt bath furnace. Fig. 4 presents the martensite content of the specimens annealed at 750, 800, and 850 °C for times less than 1 min in a salt bath. The specimens annealed at 800 and 850 °C exhibited a microstructure almost fully reverted to austenite. The finest average austenite grain size was 240 ± 60 nm at 800 °C for 10 s. The SEM microstructure of the nano/submicron grained specimen is shown in Fig. 5. This microstructure is consisted of equiaxed austenite grains with a relatively uniform grain size distribution. The present work showed that the formation of nano/ submicron grained structure in 301 stainless steel can be achieved when the 90% cold rolled specimen was annealed at 800 °C for 10 s. Work is in progress to investigate effect of repetition of the thermomechanical treatment to enhance grain refinement.
4.
Fig. 4 – Martensite content measured by Feritscope for the specimens annealed at 750, 800, and 850 °C.
Conclusions 1) Annealing the 90% cold rolled 301 stainless steel at temperatures above 700 °C showed a decrease followed by an increase in the martensite volume fraction. Increase of the martensite content after annealing was related to the increasing of Ms temperature above room temperature due to carbide precipitation, causing thermally-induced martensite formation during cooling. 2) The specimen annealed at 800 °C for 10 s exhibited the smallest average austenite grain size of 240 nm with an almost fully-austenitic structure.
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 2 2 0–1 2 2 3
3) For the purposes of grain refinement, it is better to use the low carbon grades of metastable austenitic stainless steel to prevent carbide precipitation and thermally-induced martensite formation.
REFERENCES [1] Tomimura K, Takaki S, Tanimoto S, Tokunaga Y. Optimal chemical composition in Fe–Cr–Ni alloys for ultra grain refining by reversion from deformation induced martensite. ISIJ Int 1991;31:721–7. [2] Tomimura K, Takaki S, Tokunaga Y. Reversion mechanism from deformation induced martensite to austenite in metastable austenitic stainless steels. ISIJ Int 1991;31:1431–7. [3] Takaki S, Tomimura K, Ueda S. Effect of pre-cold-working on diffusional reversion of deformation induced martensite in metastable austenitic stainless steel. ISIJ Int 1994;34:522–7.
1223
[4] Johannsen DL, Kyrolainen A, Ferreira PJ. Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 austenitic stainless steels. Metal Mater Trans A 2006;37:2325–8. [5] Di Schino A, Barteri M, Kenny JM. Development of ultra fine grain structure by martensitic reversion in stainless steel. J Mater Sci Lett 2002;21:751–73. [6] ASTM Standard Specification E112-96, Standard Test Methods for Determining Average Grain Size, 2004. [7] Mangonon JRPL, Thomas G. Structure and properties of thermal-mechanically treated 304 stainless steel. Metall Trans 1970;1:1587–94. [8] Guy KB, Butler EP, West DRF. Reversion of bcc alpha prime martensite in Fe–Cr–Ni austenitic stainless steels. Metal Sci 1983;17:167–76. [9] Tavares SSM, Fruchart D, Miraglia SJ. A magnetic study of the reversion of martensite ά in a 304 stainless steel. J Alloys Comp 2000;307:311–7. [10] Fontana MG. Corrosion engineering, 3rd edition. ; 1986. p. 186. [11] G.H. Eichelmann, F.C. Hull, Trans. Am. Soc. Metal. 45(1953) 77.
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / m a t c h a r
Effect of martensite to austenite reversion on the formation of nano/submicron grained AISI 301 stainless steel M. Karimi, A. Najafizadeh, A. Kermanpur⁎, M. Eskandari Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
AR TIC LE D ATA
ABSTR ACT
Article history:
The martensite to austenite reversion behavior of 90% cold rolled AISI 301 stainless steel
Received 18 February 2009
was investigated in order to refine the grain size. Cold rolled specimens were annealed at
Received in revised form 18 April 2009
600–900 °C, and subsequently characterized by scanning electron microscopy, X-ray
Accepted 22 April 2009
diffraction, Feritscope, and hardness measurements. The effects of annealing parameters on the formation of fully-austenitic nano/submicron grained structure and the mechanisms
Keywords:
involved were studied. It was found that annealing at 800 °C for 10 s exhibited the smallest
Martensite reversion
average austenite grain size of 240 ± 60 nm with an almost fully-austenitic structure.
Nano/submicron grain structure
© 2009 Elsevier Inc. All rights reserved.
Austenitic stainless steels Thermomechanical treatment
1.
Introduction
Austenitic stainless steels are widely used as structural materials for applications requiring excellent resistance to general corrosion in combination with good mechanical properties. These alloys show the best corrosion resistance and ductility among the stainless steels. However, they possess a relatively low strength, particularly yield strength, which limits their structural applications. Grain refining of austenitic stainless steels is an effective method to increase their strength, while keeping their toughness high. Recently, an advanced thermomechanical process has been developed to produce nano/submicron grain structures in metastable austenitic steels [1–3]. This process includes heavy cold rolling to form strain-induced martensite from austenite, followed by reversion of this martensite to austenite at relatively low annealing temperatures and times. It is established that austenite grains reverted from the deformed strain-induced martensite are very fine, while austenite grains reversed from deformed austenite are relatively coarse [1]. Very few studies have been conducted on the grain refinement of commercial austenitic stainless steels [4,5]. Some
researchers [2,4] have reported that martensite can be transformed to austenite with different shear and diffusional mechanisms depending on its chemical composition and temperature range. Johannsen et al. [4] reported that the volume fraction of martensite after annealing was increased with increasing annealing temperature above 750 °C. They showed that at these temperatures, martensite was formed during cooling from annealing temperatures. The aim of this work was to investigate the martensite to austenite reversion behavior on the formation of nano/ submicron grained AISI 301 stainless steel. The reversion of martensite and the precipitation taking place during annealing treatment were also studied.
2.
Materials and Experimental Procedures
The chemical composition of the AISI 301 stainless steel used in this investigation is given in Table 1. The hot rolled steel strips with an initial thickness of 8 mm were cold rolled to sheets of 0.8 mm thickness. The reduction of each pass was around 0.1 mm. Before each pass, the specimens were cooled
⁎ Corresponding author. Tel.: +98 311 3915738; fax: +98 311 3912752. E-mail address: [email protected] (A. Kermanpur). 1044-5803/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2009.04.014
1221
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 2 2 0–1 2 2 3
Table 1 – Chemical composition of AISI 301 used (wt.%). C
Mn
Ni
Cr
Mo
Si
Cu
Al
Co
P
S
Nb
N
0.11
0.66
6.91
16.2
0.27
0.67
0.53
0.06
0.1
0.03
< 0.03
0.003
0.005
in a solution of ethanol and ice brine (about −10 °C). The reversion of martensite to austenite for the 90% cold rolled specimens was performed by isothermal annealing at temperatures between 600 and 900 °C for different times from 1 to 100 min in the furnace. For short time annealing, (10–60 s) a salt solution consisting of 50% Na2CO3 + 50% NaCl was used. The microstructures of the specimens were revealed by optical and scanning electron microscopy (SEM Philips X230). The software Clemex was used for grain size measurements based on the intercept method [6]. Almost 100 grains were selected by the software to determine the grain size and the 95% confidence band of the results was reported. The amount of strain-induced martensite was measured by means of X-ray diffraction (XRD, Philips X' Pert with Cu Kα anode) and Feritscope (Fischer, MP30E).
3.
behavior at 800 to 1000 °C for a time range similar to that of Fig. 2a. Reversion rate at these temperatures was so high that most of the martensite was reverted to austenite after around 1 min. However, at these temperatures, the amount of martensite after cooling increased with increasing annealing time up to 10 min beyond which it remained approximately constant. According to Fig. 2b, the amount of transformed martensite is lower at temperatures of 950 and 1000 °C. Increasing amounts of martensite during annealing is an unexpected phenomenon that has also been reported in previous studies [1,4,7–9]. As reported in some of these works [6–8], this increase took place at lower temperatures (about 300–400 °C). One possible mechanism proposed to account for this is the recovery and relaxation of the matrix adjacent to the martensite laths that lead to martensite
Results and Discussion
Fig. 1 shows the XRD patterns of the specimens with 90% cold rolling and annealing at 900 °C for times from 1 to 100 min. The XRD pattern of the 90% cold rolled specimen shows only martensite reflections with a preferred orientation along the (200)ά plane. After 1 min annealing at 900 °C, almost all the martensite was reverted to austenite; the intensities of (110)ά, (200)ά, and (211)ά martensite reflections compared with the cold rolled specimen drastically decreased, and (111)γ, (200)γ, (220)γ, and (311)γ austenite reflections appeared. With increasing annealing times, the patterns showed a significant increase in the intensity of martensite reflections with a preferred orientation along (211)ά. Fig. 2 presents variations in martensite volume fraction measured using a Feritscope versus annealing times at different temperatures. During annealing at 600, 650, 700, and 750 °C for times between 1 and 100 min (Fig. 2a), no significant martensite reversion was observed. Fig. 2b shows annealing
Fig. 1 – The XRD patterns for 90% cold rolled and annealed specimens at 900 °C for different annealing times.
Fig. 2 – Martensite content in the specimens annealed at a) 600, 650, 700, and 750 °C; and b) 800, 850, 900, 950, and 1000 °C as measured by Feritscope.
1222
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 2 2 0– 1 2 2 3
Fig. 5 – SEM image of the specimen annealed at 800 °C for 10 s. Fig. 3 – Microstructure of the annealed specimen at 900 °C for 100 min.
growth. Another mechanism proposed is carbide precipitation that depletes carbon from austenite matrix and leads to the increase in the Ms temperature. Johannsen et al. [4] studied annealing of a cold rolled AISI 301 stainless steel for a constant time of 30 min at various temperatures between 600 and 900 °C. The highest amount of reverted austenite was achieved for annealing at 750 °C for 30 min. They concluded that the increased volume fraction of martensite at temperatures above 750 °C was due to the formation of carbides and, consequently, due to the increasing Ms temperature. Fig. 3 shows the optical micrograph of the annealed specimen at 900 °C for 100 min illustrating the high segregation of the second phases at grain boundaries. This is a socalled “ditch structure” that normally appeared in the sensitized austenitic stainless steels [10]. The EDS analysis of the microstructure indicated that the relative amounts of Nb, Cr, and C in the grain boundaries were higher than those inside the grains. The carbon depletion in the austenitic matrix can be due to the carbide formation. Based on the equation proposed by Eichelmann and Hull [11], it is well known that the Ms temperature of steel is increased by carbon depletion of the austenitic matrix. For example, the Ms tem-
perature can be increased from − 22 to 162 °C by complete depletion of carbon in the steel used in the present investigation (with 0.11 wt.%C). Therefore, if sufficient amount of carbon is precipitated in the form of carbides in the matrix, formation of thermally-induced martensite will be expected during the cooling stage of the annealing treatment. It may hence be concluded that, as the annealing temperature increases, both the reversion and precipitation rates intensify. However, as a lower amount of carbides may form at 950 and 1000 °C, the volume fraction of thermally-induced martensite is lower at these temperatures compared to the specimens annealed at 800 and 850 °C, as shown in Fig. 2b. In order to prevent carbide precipitation and to achieve a nano-grained austenitic structure, annealing treatment was also carried out for very short times using a salt bath furnace. Fig. 4 presents the martensite content of the specimens annealed at 750, 800, and 850 °C for times less than 1 min in a salt bath. The specimens annealed at 800 and 850 °C exhibited a microstructure almost fully reverted to austenite. The finest average austenite grain size was 240 ± 60 nm at 800 °C for 10 s. The SEM microstructure of the nano/submicron grained specimen is shown in Fig. 5. This microstructure is consisted of equiaxed austenite grains with a relatively uniform grain size distribution. The present work showed that the formation of nano/ submicron grained structure in 301 stainless steel can be achieved when the 90% cold rolled specimen was annealed at 800 °C for 10 s. Work is in progress to investigate effect of repetition of the thermomechanical treatment to enhance grain refinement.
4.
Fig. 4 – Martensite content measured by Feritscope for the specimens annealed at 750, 800, and 850 °C.
Conclusions 1) Annealing the 90% cold rolled 301 stainless steel at temperatures above 700 °C showed a decrease followed by an increase in the martensite volume fraction. Increase of the martensite content after annealing was related to the increasing of Ms temperature above room temperature due to carbide precipitation, causing thermally-induced martensite formation during cooling. 2) The specimen annealed at 800 °C for 10 s exhibited the smallest average austenite grain size of 240 nm with an almost fully-austenitic structure.
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 2 2 0–1 2 2 3
3) For the purposes of grain refinement, it is better to use the low carbon grades of metastable austenitic stainless steel to prevent carbide precipitation and thermally-induced martensite formation.
REFERENCES [1] Tomimura K, Takaki S, Tanimoto S, Tokunaga Y. Optimal chemical composition in Fe–Cr–Ni alloys for ultra grain refining by reversion from deformation induced martensite. ISIJ Int 1991;31:721–7. [2] Tomimura K, Takaki S, Tokunaga Y. Reversion mechanism from deformation induced martensite to austenite in metastable austenitic stainless steels. ISIJ Int 1991;31:1431–7. [3] Takaki S, Tomimura K, Ueda S. Effect of pre-cold-working on diffusional reversion of deformation induced martensite in metastable austenitic stainless steel. ISIJ Int 1994;34:522–7.
1223
[4] Johannsen DL, Kyrolainen A, Ferreira PJ. Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 austenitic stainless steels. Metal Mater Trans A 2006;37:2325–8. [5] Di Schino A, Barteri M, Kenny JM. Development of ultra fine grain structure by martensitic reversion in stainless steel. J Mater Sci Lett 2002;21:751–73. [6] ASTM Standard Specification E112-96, Standard Test Methods for Determining Average Grain Size, 2004. [7] Mangonon JRPL, Thomas G. Structure and properties of thermal-mechanically treated 304 stainless steel. Metall Trans 1970;1:1587–94. [8] Guy KB, Butler EP, West DRF. Reversion of bcc alpha prime martensite in Fe–Cr–Ni austenitic stainless steels. Metal Sci 1983;17:167–76. [9] Tavares SSM, Fruchart D, Miraglia SJ. A magnetic study of the reversion of martensite ά in a 304 stainless steel. J Alloys Comp 2000;307:311–7. [10] Fontana MG. Corrosion engineering, 3rd edition. ; 1986. p. 186. [11] G.H. Eichelmann, F.C. Hull, Trans. Am. Soc. Metal. 45(1953) 77.