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Study of decomposition of ferrite in a duplex stainless steel cold worked and aged at 450–500 °C
Materials Science and Engineering A 499 (2009) 489–492
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Study of decomposition of ferrite in a duplex stainless steel cold worked and aged at 450–500 ◦ C Mats Hättestrand ∗ , Petter Larsson, Guocai Chai, Jan-Olof Nilsson, Joakim Odqvist R&D Centre, Sandvik Materials Technology, SE-811 81 Sandviken, Sweden
a r t i c l e
i n f o
Article history: Received 21 May 2008 Received in revised form 30 August 2008 Accepted 7 September 2008 Keywords: Duplex stainless steel Embrittlement Spinodal decomposition Cold working
a b s t r a c t The influence of cold-deformation on ferrite decomposition in duplex stainless steel during heat treatment at 450–500 ◦ C was investigated using micro-hardness measurements and transmission electron microscopy. It was found that cold-deformation can change the mechanism of the ˛ → ˛ + ˛ phase separation in the ferrite from nucleation and growth to spinodal decomposition. This finding is discussed in terms of the influence of an increased dislocation density on coherency strains. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Duplex stainless steel (DSS) is a class of steel with a microstructure consisting of approximately equal amounts of ferrite and austenite and where the amount of chromium in both phases is sufficiently high (about 13 wt.%) to render the material stainless [1,2]. DSS has an attractive combination of corrosion resistance and good mechanical properties in the temperature range −50 to 250 ◦ C. A drawback, however, is the tendency for embrittlement at temperatures above 250 ◦ C, which sets an upper limit to the recommended service temperature of the material. The embrittlement is an inevitable consequence of a miscibility gap in the Fe–Cr system within which the ferrite (˛) may decompose into Fe-rich ˛-phase and Cr-rich ˛ -phase, as shown schematically in Fig. 1. Inside the spinodal the decomposition occurs through spinodal decomposition, whereas immediately outside the spinodal line decomposition may take place by nucleation and growth [3]. The transformation mechanism is affected by steel composition and temperature. Furthermore, as shown in this work, mechanical deformation prior to heat treatment can affect the ferrite decomposition. The present study addresses the issue of how cold-deformation of DSS affects microstructural changes during subsequent iso-thermal heat treatment in the temperature range 450–500 ◦ C.
The investigated steel was the commercial duplex stainless steel SAF 2507. The composition of this steel is given in Table 1. The material was delivered as hot extruded tube. A piece of tube with a width of 40 mm and a length of 400 mm was stretched with a total strain of 15%. Small specimens taken from deformed and undeformed material were iso-thermally heat treated at 450 ◦ C or 500 ◦ C for times up to 240 h. Micro-hardness measurements were performed to determine the hardness of the ferrite in different material conditions. Investigations using transmission electron microscopy (TEM) were performed using a JEOL 2000FX instrument. Thin foils for TEM analysis were electropolished at room temperature using a voltage of 20–25 V in a solution of 5% perchloric acid in 95% acetic acid. The material conditions selected for the TEM investigation are listed in Table 2.
∗ Corresponding author. Tel.: +46 26 263131; fax: +46 26 264410. E-mail address: [email protected] (M. Hättestrand). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.09.021
3. Results The hardness of the ferrite as a function of aging time for materials exposed to different deformation and aging treatments is shown in Fig. 2. The increase of hardness caused by aging reflects the nature of the phase transformations in the materials. Note the very small increase of hardness of the undeformed material aged at 500 ◦ C, which indicates a different phase separation mechanism for this material condition. TEM results from unaged material are shown in Figs. 3 and 4. Fig. 3 shows dislocations in a ferrite grain in undeformed material.
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M. Hättestrand et al. / Materials Science and Engineering A 499 (2009) 489–492
Table 1 Composition of duplex stainless steel SAF 2507 (wt.%, bal. Fe, guidance only).
SAF 2507
C (max)
Si (max)
P (max)
S (max)
Mn
Cr
Ni
Mo
N
0.03
0.8
0.035
0.015
1.2
25
7
4
0.3
Fig. 3. Dislocation structure in undeformed material. Note the relatively low dislocation density.
Fig. 1. The miscibility gap in the Fe–Cr system (shown schematically). Inside the spinodal (area 1) decomposition occurs through spinodal decomposition. Outside the spinodal (area 2) decomposition may take place by nucleation and growth.
Table 2 Investigated material conditions. Sample
Deformation (strain)
Heat treatment
1 2 3 4
0% 15% 15% 0%
500 ◦ C/3 h 500 ◦ C/3 h 500 ◦ C/48 h 450 ◦ C/240 h
Note the relatively low dislocation density. The dislocation density was much higher in the deformed material, as shown in Fig. 4. TEM results from undeformed and aged material are shown in Figs. 5–8. Careful alignment of the specimen with reference to the electron beam was performed to visualize the ˛ → ˛ + ˛ phase separation in aged material. Figs. 5 and 6 show a ferrite grain in sample 1 (undeformed, aged at 500 ◦ C for 3 h) with the electron beam parallel to the 0 0 1 direction and to the 1 1 1 direction, respectively. In both cases small particles of ˛ of a few nanometer in size are visible. Close to ferrite/austenite phase boundaries a zone of approximately 3 m width without any ˛ -particles could
Fig. 4. Dislocation structure in material deformed 15%. Note the high dislocation density.
Fig. 5. Ferrite grain in sample 1 aligned with the electron beam parallel to the 0 0 1 crystal direction. Note the presence of small ˛ -particles.
Fig. 2. Results from micro-hardness measurements. The graphs show the hardness of the ferrite as a function of aging time for materials exposed to different deformation and aging treatments.
be found. This is illustrated in Fig. 7. Altogether, the observations performed on undeformed material aged at 500 ◦ C indicates that in this material condition ˛ → ˛ + ˛ phase separation takes place via a nucleation and growth mechanism. Fig. 8 shows a ferrite grain in sample 4 (undeformed, aged at 450 ◦ C for 240 h) with the electron beam parallel to the 0 0 1 direction. A mottled contrast can be seen, typical of the presence of
M. Hättestrand et al. / Materials Science and Engineering A 499 (2009) 489–492
491
Fig. 6. Ferrite grain in sample 1 aligned with the electron beam parallel to the 1 1 1 crystal direction. Note the presence of small ˛ -particles.
Fig. 9. Ferrite grain in sample 2 aligned with the electron beam parallel to the 0 0 1 crystal direction. Note the absence of ˛ -particles.
spinodal decomposition [2]. The mechanism of the ˛ → ˛ + ˛ phase separation in undeformed material aged at 450 ◦ C seems to be spinodal decomposition. A different phase separation mechanism at lower temperature (450 ◦ C compared to 500 ◦ C) is not surprising, since it merely reflects the position of the miscibility gap and the spinodal line.
TEM results from deformed and aged material are shown in Figs. 9–11. Figs. 9 and 10 show ferrite grains in sample 2 (deformed 15%, aged at 500 ◦ C for 3 h) and in sample 3 (deformed 15%, aged at 500 ◦ C for 48 h), respectively. In both cases the electron beam is parallel to the 0 0 1 direction. In comparison with sample 1 (Fig. 5) the absence of ˛ -particles in deformed material is notable. In sample 3
Fig. 7. Ferrite grain in sample 1 close to a ferrite/austenite phase boundary. An approximately 3 mm wide zone at the phase boundary is free from ˛ -particles.
Fig. 10. Ferrite grain in sample 3 aligned with the electron beam parallel to the 0 0 1 crystal direction. A mottled contrast indicates the presence of spinodal decomposition.
Fig. 8. Ferrite grain in sample 4 aligned with the electron beam parallel to the 0 0 1 crystal direction. A mottled contrast indicates the presence of spinodal decomposition.
Fig. 11. A ferrite/austenite phase boundary in sample 3. Mottled contrast in the ferrite grain is present also in the immediate vicinity of the austenite grain.
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M. Hättestrand et al. / Materials Science and Engineering A 499 (2009) 489–492
a mottled contrast can be seen (Figs. 10 and 11), which is typical of the presence of spinodal decomposition. The mottled contrast arising from spinodal decomposition is visible also in the immediate vicinity of ferrite/austenite phase boundaries, which is illustrated in Fig. 11. These observations clearly indicate that the mechanism of the ˛ → ˛ + ˛ phase separation in the deformed material aged at 500 ◦ C is spinodal decomposition. 4. Discussion The TEM observations on undeformed and deformed material aged at 500 ◦ C show that cold-deformation can change the mechanism of ferrite decomposition from nucleation and growth to spinodal decomposition. This observation is also supported by the small increase in hardness during aging at 500 ◦ C of the undeformed material (Fig. 1). It is believed that the much finer structure caused by spinodal decomposition compared to nucleation and growth of ˛ -particles, gives rise to more severe embrittlement. The observed effect of cold-deformation is of practical importance since it means that deformation may render the material more susceptible to embrittlement during subsequent heat treatments or heat exposure during service of the material. The reason for the change of phase separation mechanism as a consequence of cold-deformation and increased dislocation density is not obvious, but a possible explanation is associated with the influence of coherency strains. During spinodal decomposition there is a gradual development of compositional fluctuations that eventually leads to the binary ˛ + ˛ structure. Coherency strains will act as a barrier to this process and tend to depress the temperature, at which spinodal decomposition can occur. Above this critical temperature decomposition may still occur via a nucleation and growth mechanism. Let us assume that the presence of dislocations, which themselves induce strains in the lattice, decreases the amount of coherency strains produced by spinodal decomposition. This may offer a likely explanation of the observation that deformation and an increased dislocation density raises the temperature below which spinodal decomposition can occur. The magnitude of strains induced by spinodal decomposition in the Fe–Cr system can be compared to strains induced by dislocations. The lattice parameter of pure Fe and Cr can be used to estimate coherency strains induced by spinodal decomposition in alloys rich in Fe and Cr. A reasonable estimation of coherency strains would then be: εspinodal = (˛Cr − ˛Fe )/˛Fe = (2.884 − 2.867)/2.867 = 0.0059 [4]. The strain induced by a dislocation can be estimated from basic dislocation theory as: εdislocation = b/4r, where b is the Burgers vector and r is the distance from the dislocation core. Using this
estimation (with b = 0.2 nm), the strain 10 nm from a dislocation core would be 0.002. This value is comparable with the estimated coherency strain indicating that strain fields originating from dislocations can interact significantly with the coherency strains caused by spinodal decomposition if the dislocation density is high enough. A theoretical study supporting this assumption by Thompson and Voorhees shows that elastic energy in a system caused by applied tension or compression may either increase or decrease the coherent spinodal [5]. How non-uniform internal stress fields generated by dislocations influence the coherent spinodal is not obvious, but it is not unrealistic to believe that the increased dislocation density induced by deformation will have an effect on spinodal decomposition. The observation in undeformed and aged material of a zone close to the grain boundary denuded of ˛ -particles is noteworthy. No conclusion regarding the mechanism behind this effect can be drawn from the present investigation. However, when investigating 475 ◦ C embrittlement in an Fe–30% Cr alloy, Lagneborg [6] observed denuded zones close to ferrite/ferrite grain boundaries similar to those observed here. He suggested that the depletion of vacancies close to grain boundaries be responsible for this effect. 5. Conclusions It was found that cold-deformation of duplex stainless steel can change the nature of the ˛ → ˛ + ˛ phase separation in ferrite from nucleation and growth to spinodal decomposition. A tentative explanation for this finding is given in terms of strains from dislocations interacting with coherency strain fields originating from spinodal decomposition. The observed effect means that cold-deformation may render the material more susceptible to embrittlement. Acknowledgements This paper is published by the permission from Sandvik Materials Technology. The support from Vice President Olle Wijk is gratefully acknowledged. References [1] [2] [3] [4] [5] [6]
J.-O. Nilsson, Mater. Sci. Technol. 8 (1992) 685. J.-O. Nilsson, P. Liu, Mater. Sci. Technol. 7 (1991) 853. J.W. Cahn, Trans. AIME 242 (1968) 166. Smithells Metals Reference Book, eighth ed., ASM International, 2004. M.E. Thompson, P.W. Voorhees, Model. Simul. Mater. Sci. Eng. 5 (1997) 223. R. Lagneborg, Trans. ASM 60 (1967) 67.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Study of decomposition of ferrite in a duplex stainless steel cold worked and aged at 450–500 ◦ C Mats Hättestrand ∗ , Petter Larsson, Guocai Chai, Jan-Olof Nilsson, Joakim Odqvist R&D Centre, Sandvik Materials Technology, SE-811 81 Sandviken, Sweden
a r t i c l e
i n f o
Article history: Received 21 May 2008 Received in revised form 30 August 2008 Accepted 7 September 2008 Keywords: Duplex stainless steel Embrittlement Spinodal decomposition Cold working
a b s t r a c t The influence of cold-deformation on ferrite decomposition in duplex stainless steel during heat treatment at 450–500 ◦ C was investigated using micro-hardness measurements and transmission electron microscopy. It was found that cold-deformation can change the mechanism of the ˛ → ˛ + ˛ phase separation in the ferrite from nucleation and growth to spinodal decomposition. This finding is discussed in terms of the influence of an increased dislocation density on coherency strains. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Duplex stainless steel (DSS) is a class of steel with a microstructure consisting of approximately equal amounts of ferrite and austenite and where the amount of chromium in both phases is sufficiently high (about 13 wt.%) to render the material stainless [1,2]. DSS has an attractive combination of corrosion resistance and good mechanical properties in the temperature range −50 to 250 ◦ C. A drawback, however, is the tendency for embrittlement at temperatures above 250 ◦ C, which sets an upper limit to the recommended service temperature of the material. The embrittlement is an inevitable consequence of a miscibility gap in the Fe–Cr system within which the ferrite (˛) may decompose into Fe-rich ˛-phase and Cr-rich ˛ -phase, as shown schematically in Fig. 1. Inside the spinodal the decomposition occurs through spinodal decomposition, whereas immediately outside the spinodal line decomposition may take place by nucleation and growth [3]. The transformation mechanism is affected by steel composition and temperature. Furthermore, as shown in this work, mechanical deformation prior to heat treatment can affect the ferrite decomposition. The present study addresses the issue of how cold-deformation of DSS affects microstructural changes during subsequent iso-thermal heat treatment in the temperature range 450–500 ◦ C.
The investigated steel was the commercial duplex stainless steel SAF 2507. The composition of this steel is given in Table 1. The material was delivered as hot extruded tube. A piece of tube with a width of 40 mm and a length of 400 mm was stretched with a total strain of 15%. Small specimens taken from deformed and undeformed material were iso-thermally heat treated at 450 ◦ C or 500 ◦ C for times up to 240 h. Micro-hardness measurements were performed to determine the hardness of the ferrite in different material conditions. Investigations using transmission electron microscopy (TEM) were performed using a JEOL 2000FX instrument. Thin foils for TEM analysis were electropolished at room temperature using a voltage of 20–25 V in a solution of 5% perchloric acid in 95% acetic acid. The material conditions selected for the TEM investigation are listed in Table 2.
∗ Corresponding author. Tel.: +46 26 263131; fax: +46 26 264410. E-mail address: [email protected] (M. Hättestrand). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.09.021
3. Results The hardness of the ferrite as a function of aging time for materials exposed to different deformation and aging treatments is shown in Fig. 2. The increase of hardness caused by aging reflects the nature of the phase transformations in the materials. Note the very small increase of hardness of the undeformed material aged at 500 ◦ C, which indicates a different phase separation mechanism for this material condition. TEM results from unaged material are shown in Figs. 3 and 4. Fig. 3 shows dislocations in a ferrite grain in undeformed material.
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M. Hättestrand et al. / Materials Science and Engineering A 499 (2009) 489–492
Table 1 Composition of duplex stainless steel SAF 2507 (wt.%, bal. Fe, guidance only).
SAF 2507
C (max)
Si (max)
P (max)
S (max)
Mn
Cr
Ni
Mo
N
0.03
0.8
0.035
0.015
1.2
25
7
4
0.3
Fig. 3. Dislocation structure in undeformed material. Note the relatively low dislocation density.
Fig. 1. The miscibility gap in the Fe–Cr system (shown schematically). Inside the spinodal (area 1) decomposition occurs through spinodal decomposition. Outside the spinodal (area 2) decomposition may take place by nucleation and growth.
Table 2 Investigated material conditions. Sample
Deformation (strain)
Heat treatment
1 2 3 4
0% 15% 15% 0%
500 ◦ C/3 h 500 ◦ C/3 h 500 ◦ C/48 h 450 ◦ C/240 h
Note the relatively low dislocation density. The dislocation density was much higher in the deformed material, as shown in Fig. 4. TEM results from undeformed and aged material are shown in Figs. 5–8. Careful alignment of the specimen with reference to the electron beam was performed to visualize the ˛ → ˛ + ˛ phase separation in aged material. Figs. 5 and 6 show a ferrite grain in sample 1 (undeformed, aged at 500 ◦ C for 3 h) with the electron beam parallel to the 0 0 1 direction and to the 1 1 1 direction, respectively. In both cases small particles of ˛ of a few nanometer in size are visible. Close to ferrite/austenite phase boundaries a zone of approximately 3 m width without any ˛ -particles could
Fig. 4. Dislocation structure in material deformed 15%. Note the high dislocation density.
Fig. 5. Ferrite grain in sample 1 aligned with the electron beam parallel to the 0 0 1 crystal direction. Note the presence of small ˛ -particles.
Fig. 2. Results from micro-hardness measurements. The graphs show the hardness of the ferrite as a function of aging time for materials exposed to different deformation and aging treatments.
be found. This is illustrated in Fig. 7. Altogether, the observations performed on undeformed material aged at 500 ◦ C indicates that in this material condition ˛ → ˛ + ˛ phase separation takes place via a nucleation and growth mechanism. Fig. 8 shows a ferrite grain in sample 4 (undeformed, aged at 450 ◦ C for 240 h) with the electron beam parallel to the 0 0 1 direction. A mottled contrast can be seen, typical of the presence of
M. Hättestrand et al. / Materials Science and Engineering A 499 (2009) 489–492
491
Fig. 6. Ferrite grain in sample 1 aligned with the electron beam parallel to the 1 1 1 crystal direction. Note the presence of small ˛ -particles.
Fig. 9. Ferrite grain in sample 2 aligned with the electron beam parallel to the 0 0 1 crystal direction. Note the absence of ˛ -particles.
spinodal decomposition [2]. The mechanism of the ˛ → ˛ + ˛ phase separation in undeformed material aged at 450 ◦ C seems to be spinodal decomposition. A different phase separation mechanism at lower temperature (450 ◦ C compared to 500 ◦ C) is not surprising, since it merely reflects the position of the miscibility gap and the spinodal line.
TEM results from deformed and aged material are shown in Figs. 9–11. Figs. 9 and 10 show ferrite grains in sample 2 (deformed 15%, aged at 500 ◦ C for 3 h) and in sample 3 (deformed 15%, aged at 500 ◦ C for 48 h), respectively. In both cases the electron beam is parallel to the 0 0 1 direction. In comparison with sample 1 (Fig. 5) the absence of ˛ -particles in deformed material is notable. In sample 3
Fig. 7. Ferrite grain in sample 1 close to a ferrite/austenite phase boundary. An approximately 3 mm wide zone at the phase boundary is free from ˛ -particles.
Fig. 10. Ferrite grain in sample 3 aligned with the electron beam parallel to the 0 0 1 crystal direction. A mottled contrast indicates the presence of spinodal decomposition.
Fig. 8. Ferrite grain in sample 4 aligned with the electron beam parallel to the 0 0 1 crystal direction. A mottled contrast indicates the presence of spinodal decomposition.
Fig. 11. A ferrite/austenite phase boundary in sample 3. Mottled contrast in the ferrite grain is present also in the immediate vicinity of the austenite grain.
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M. Hättestrand et al. / Materials Science and Engineering A 499 (2009) 489–492
a mottled contrast can be seen (Figs. 10 and 11), which is typical of the presence of spinodal decomposition. The mottled contrast arising from spinodal decomposition is visible also in the immediate vicinity of ferrite/austenite phase boundaries, which is illustrated in Fig. 11. These observations clearly indicate that the mechanism of the ˛ → ˛ + ˛ phase separation in the deformed material aged at 500 ◦ C is spinodal decomposition. 4. Discussion The TEM observations on undeformed and deformed material aged at 500 ◦ C show that cold-deformation can change the mechanism of ferrite decomposition from nucleation and growth to spinodal decomposition. This observation is also supported by the small increase in hardness during aging at 500 ◦ C of the undeformed material (Fig. 1). It is believed that the much finer structure caused by spinodal decomposition compared to nucleation and growth of ˛ -particles, gives rise to more severe embrittlement. The observed effect of cold-deformation is of practical importance since it means that deformation may render the material more susceptible to embrittlement during subsequent heat treatments or heat exposure during service of the material. The reason for the change of phase separation mechanism as a consequence of cold-deformation and increased dislocation density is not obvious, but a possible explanation is associated with the influence of coherency strains. During spinodal decomposition there is a gradual development of compositional fluctuations that eventually leads to the binary ˛ + ˛ structure. Coherency strains will act as a barrier to this process and tend to depress the temperature, at which spinodal decomposition can occur. Above this critical temperature decomposition may still occur via a nucleation and growth mechanism. Let us assume that the presence of dislocations, which themselves induce strains in the lattice, decreases the amount of coherency strains produced by spinodal decomposition. This may offer a likely explanation of the observation that deformation and an increased dislocation density raises the temperature below which spinodal decomposition can occur. The magnitude of strains induced by spinodal decomposition in the Fe–Cr system can be compared to strains induced by dislocations. The lattice parameter of pure Fe and Cr can be used to estimate coherency strains induced by spinodal decomposition in alloys rich in Fe and Cr. A reasonable estimation of coherency strains would then be: εspinodal = (˛Cr − ˛Fe )/˛Fe = (2.884 − 2.867)/2.867 = 0.0059 [4]. The strain induced by a dislocation can be estimated from basic dislocation theory as: εdislocation = b/4r, where b is the Burgers vector and r is the distance from the dislocation core. Using this
estimation (with b = 0.2 nm), the strain 10 nm from a dislocation core would be 0.002. This value is comparable with the estimated coherency strain indicating that strain fields originating from dislocations can interact significantly with the coherency strains caused by spinodal decomposition if the dislocation density is high enough. A theoretical study supporting this assumption by Thompson and Voorhees shows that elastic energy in a system caused by applied tension or compression may either increase or decrease the coherent spinodal [5]. How non-uniform internal stress fields generated by dislocations influence the coherent spinodal is not obvious, but it is not unrealistic to believe that the increased dislocation density induced by deformation will have an effect on spinodal decomposition. The observation in undeformed and aged material of a zone close to the grain boundary denuded of ˛ -particles is noteworthy. No conclusion regarding the mechanism behind this effect can be drawn from the present investigation. However, when investigating 475 ◦ C embrittlement in an Fe–30% Cr alloy, Lagneborg [6] observed denuded zones close to ferrite/ferrite grain boundaries similar to those observed here. He suggested that the depletion of vacancies close to grain boundaries be responsible for this effect. 5. Conclusions It was found that cold-deformation of duplex stainless steel can change the nature of the ˛ → ˛ + ˛ phase separation in ferrite from nucleation and growth to spinodal decomposition. A tentative explanation for this finding is given in terms of strains from dislocations interacting with coherency strain fields originating from spinodal decomposition. The observed effect means that cold-deformation may render the material more susceptible to embrittlement. Acknowledgements This paper is published by the permission from Sandvik Materials Technology. The support from Vice President Olle Wijk is gratefully acknowledged. References [1] [2] [3] [4] [5] [6]
J.-O. Nilsson, Mater. Sci. Technol. 8 (1992) 685. J.-O. Nilsson, P. Liu, Mater. Sci. Technol. 7 (1991) 853. J.W. Cahn, Trans. AIME 242 (1968) 166. Smithells Metals Reference Book, eighth ed., ASM International, 2004. M.E. Thompson, P.W. Voorhees, Model. Simul. Mater. Sci. Eng. 5 (1997) 223. R. Lagneborg, Trans. ASM 60 (1967) 67.