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Superplastic Cr-Mn stainless steel related to fusion reactor application
90
Journal
LETTER
TO THE
SUPERPLASTIC REACTOR
Jyoti
of Nuclear
Materials 161 (1989) 90-02 North-Holland. Amsterdam
EDITORS Cr-Mn
STAINLESS
STEEL RELATED
TO FUSION
APPLICATION
MUKHOPADHYAY
Division of Materials CA 95616, USA
Received
Science and Engineering,
25 April 1988; accepted
2 October
Department
of Mechanical
Engineering,
Unioersity of California,
Dam,
1988
One major problem for the first wall of a thermonuclear fusion reactor is the selection of a structural material. The first wall is that part of a reactor where the major material problems arise, such as thermal fatigue, sputtering and plasma disruption [l]. Therefore, the material for the first wall should have high structural and chemical stability, low neutron activation, good mechanical properties, good weldability, availability and minimum maintenance and safety problems [1,2]. Austenitic stainless steel AISI 316 is generally envisaged in the field of structural material for the fission reactor (Fast Breeder Reactor). Accordingly, this steel is also considered as a first choice for making structural components of a fusion reactor. As 316 type contains Ni, it has some drawbacks with respect to fusion reactor application. The problem with nickel is that the radioactive transmutation elements formed from nickel (while being irradiated in a fusion reactor) would take a long time to decay, and thus, make a difficult nuclear wastedisposal problem. Apart from that, Ni has other detrimental effects, such as, a high rate of helium production, which may cause cavity nucleation and stabilization [3]. These cavities often degrade the mechanical properties of the post-formed component. Hence, efforts should be made to minimize their occurrence by replacing Ni, so that the helium production can effectively be controlled. In the process, cavitation will automatically be checked and the final component obtained through superplastic technique would probably contain no more cavities. With the addition of Ni, the stacking fault energy of the material increases which ultimately results in decreasing the work hardening effect. On the other hand, Ni is highly soluble in liquid Li, which is considered as a potential candidate material for the coolant [3,4]. 0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Hence, from nuclear point of view, Ni is a very harmful element and it can not be used for making the first wall of thermo-nuclear fusion reactors. Recently, significant interest has been generated among scientists and research workers [5-81 about the development of alloys which contain elements, other than Ni. Thus, with considerable amount of research, it has been suggested that high Cr-Mn steel can satisfactorily be used as the structural material for the first wall of a thermo-nuclear fusion reactor, where manganese would be a favorable replacement for Ni from a radioactivity point of view [9]. In fact, in high Cr-Mn stainless steels, Ni is partially or fully replaced by Mn. The advantage of the Mn addition to Cr-Mn steel is to lower the stacking fault energy of austenite, which ultimately leads to an increase in the strain hardening rate [3,10]. Mn is also responsible for increasing the low-temperature toughness of the material [ll]. As Mn is cheaper than Ni, from economic point of view, it will be wiser to replace Ni by Mn. But one should not forget while adding Mn, that the material costs are relatively a small fraction over the overall processing costs. It appears that high Cr-Mn steel combines good strength, high resistance to radiation damage, good corrosion resistance, good weldability and less amount of void swelling, which are the primary requirements of a fusion reactor [3,5,6]. Some of these high manganese steels such as AMCR-0033 and 0034 show higher strength than that of AISI 316 [l]. Similarly, the tensile strength of weld metal is increased by 10% and 30% respectively, at ambient and at 450 o C, with respect to the base metal strength. Furthermore, the weld metal presents an average energy absorption value, i.e. 73 to 108 J cm-* which is considerably lower than the value of 2543 J cm-‘, reported for the base metal. Thus. the B.V.
J. A4ukhopadhyuy / Superplmtic
welded joint has an excellent toughness behaviour as well as a low notch sensitivity 161. The charpy data are also comparable with those of AISI 316 welded joints. At high temperature and in sulphurous atmospheres, complex alloys based on Fe-Mn-Al show better oxidation resistance than Fe-Cr-Ni HZ]. Apart from that, it has been demonstrated that the void swelling is less in Cr-Mn type steels than in the Cr-Ni-Fe steels [8,13]. The total swelling of the AMCR steel at 500 and 600 o C is (20-50%) lower than that of 0.2% C AISI 316 and F548SS [3]_ In recent studies, it has also been observed that Cr-Mn steel can be used in cryogenic and marine structure purposes (11,141. However, there are some drawbacks in manganese addition. Manganese evaporates more rapidly due to its high vapor pressure and makes a surface layer of 10 pm thickness, while heated at 650°C for 1000 h under a vacuum of lop4 Torr. The maximum Mn depletion can take place around 1.5% of the total manganese, this may not reduce the actual Mn content of the alloy below the lower specification limit. Additionally, such loss would not destabilize the overall micro structure of the substituted alloys [lS]. Mn-containing stainless steel shows inferior oxidation resistance with respect to Cr-Ni steel [12]. This can be improved with the addition of aluminum. For aluminum addition, Al,O, is formed, which is more protective than Cr,O,. Therefore, the drawbacks of manganese addition can be counter balanced by adding aluminum [ 121. Although aluminum has remarkable effects on grain refining, stabilizing the austenite and oxidation resistance, however, its addition to Cr-Mn steel has some detrimental effects. For example, the cast steel with high aluminum content results in inferior casting behaviour. Besides that, in case of high Al, the surface of the melt gets oxidized and such an oxide may often affect the fluidity of molten steel [ll]. Hence, the aluminum content of the steel should be as low as possible, preferably around 1% [ll]. MO is considered to be the most effective in enhancing the train hardening rate of Cr-Mn steel, since it lowers the stacking fault energy [lo]. In fact, addition of 2.5-32 MO has also been shown to improve the corrosion resistance property. However, MO has a deleterious effect on Cr-Mn steel. It contributes to a long lived residual radio-activity and its addition to Cr-Mn steel leads to a catastrophic disintegration of the steel [9], Naturally, MO should be replaced by W [Is]. While designing the alloys, care should be taken about the nitrogen content, otherwise with lithium,
Cr-MI
stainless steel
91
nitrogen forms a complex compound (Li-Cr-N) which is very reactive [16]. Nitrogen is also responsible for void nucleation, thereby affecting superplastic properties. Therefore, it is preferable to have minimum nitrogen content (< 0.4%) in the steel [17]. It is believed that up to 1% silicon might be accommodated in the steel. However, in practice, it is intended to restrict’the maximum Si content around 0.2% in the steel, owing to its strong ferrite forming tendency and subsequent effect on promoting the formation of intermetallic phases [ 151. Carbon content of the steel should be low. An increased carbon content could lead to carbide precipitation and a substantial reduction in corrosion resistance [15]. As carbon content increases, the toughness of the steel decreases [ll]. Carbon is also responsible for increasing the stacking fault energy, which results in decreasing the work hardening effect [2]. However, with accurate control of carbon, good hot workability can easily be achieved in duplex stainless steel. In this context, the border line for carbon has been placed at 0.03%, otherwise with increased carbon, the edge cracking starts severely [ 181.Eventually, edge cracking can be suppressed with the addition of titanium, but it has another effect. With C and N, Ti forms Ti(C, N) cuboids (5 pm). These cuboids play an important role in nucleating cavities in a supe~lastically formed component [19]. Other than edge cracking, carbon also plays a crucial role in determining the level of cavitation in superplastitally deformed material. Cavitation is found to increase, as the carbon content of the material is raised. It appears that the metal cavitation is caused due to high CO-CO, gas pressure from the internal oxidation of carbon [20,21]. Preliminary work [22] suggests that Cr-Mn steel has the potential to become superplastic, while deformed at elevated temperature around 1123 K and at low strain rate of 1.5 X 10m4 s-l. However, the material used in that particular investigation was not pure. It contained Si, N, Ni, P and S as impurities. In fact, the acceptability limit of these elements for the structural components of a fusion reactor is as low as < 10 ppm [23]. Therefore, considering all the above factors, a high purity Cr-Mn steel (with very low amounts of carbon and aluminum content), practically free from all other elements, such as Si, S, P, Ni, MO, N, and Ti might be appropriate for studying the supe~lasticity and cavitation phenomena and subsequently be useful for the fusion reactor program.
92
J. Mukhopodhyay
/ Superplastic
References [l] G. Piatti and P. Schiller, in: Proc. Int. Conf. On Fusion Reactor Materials, Chicago, 1986, J. Nucl. Mater. 141-143 (1986) 417. [2] E. Ruedl, D.G. Rickerby and T. Sasaki, in: Proc. Conf. on 13th SOFT, Varese, Italy, 1984, p. 1029. [3] P. Fenici,D. Boerman, V. Coen, E. Lang, C. Ponti and W. Schule, NucI. Engrg. Des/Fusion 1 (1984) 167. [4] P. Fenici, V. Coen, E. Ruedl, H. Kolbe and T. Sasaki, J. Nucl. Mater. 102 & 104 (1981) 699. [5] G. Piatti and P. Schiller, J. Nucl. Mater. 141-143 (1986) 417. [6] G. Piatti and G. Musso, J. Mater. Sci. 21 (1986) 2339. [7] G. Piatti, S. Matteazi and G. Petrone, Nucl. Engrg. Des./Fusion 2 (1985) 391. [8] D.J. Mazey, J.A. Hudson and J.M. Titchmarsh, J. Nucl. Mater. 107 (1982) 2. [9] R.L. Klueh and M.L. Grossbcck, J. Nucl. Mater. 122 (1984) 294. [lo] D.J. Drobjak and J.G. Parr, Metall. Trans. 1 (1970) 759. [ll] L. Sheng Li, Da. Zhi. Yang, G. Sheng. Wei and C.M. Wayman, Metallography 15 (1982) 355. (121 Private communication from Manganese Centre, Paris.
Cr-Mn
starnless steel
[13] I.V. Gorynin et al., in: Proc. All Union C‘Onf. on Engineering Problems in Thermonuclear Reactors. Leningrad, 1977, p. 187 (in Russian). [14] M.T. Jahn, C.M. Fan and C.M. Wan, J. Mater. Sci. 20 (1985) 2757. [15] A.H. Bott, F.B. Pickering and G.J. Butterworth, in: Proc. Int. Conf. On Fusion Reactor Materials, Chicago, 1986, J. Nucl. Mater. 141-143 (1986) 1088. [16] E. Rued1 and T. Sasaki, J. Nucl. Mater. 116 (1983) 112. [17] H. Takahasi, T. Takyama, K. Tanikawa and R. Miura, in: Proc. First Int. Conf. On Fusion Reactor Materials, Tokyo, 1984, J. Nucl. Mater. 133 & 134 (1985) 566. [18] R.C. Gibson, H.W. Hayden and J.H. Brophy, ASM Trans. 61 (1968) 85. [19] N. Ridley and D.W. Livesey. Proc. Int. Conf. On Fracture, Waterloo, 1977, p. 533. [20] D. Capalan, R.J. Hussey. G.I. Sproule and M.J. Graham. Scripta Metall. 16 (1982) 759. (211 R.H. Bricknell and D.A. Woodford, Scripta Metall. 16 (1982) 761. [22] G. Piatti, D. Boerman and H.A. Weir, Metall. Sci. Technol. 4 (1986) 8. [23] R.W. Conn et al., Nucl. Technol./Fusion 5 (1984) 291.
Journal
LETTER
TO THE
SUPERPLASTIC REACTOR
Jyoti
of Nuclear
Materials 161 (1989) 90-02 North-Holland. Amsterdam
EDITORS Cr-Mn
STAINLESS
STEEL RELATED
TO FUSION
APPLICATION
MUKHOPADHYAY
Division of Materials CA 95616, USA
Received
Science and Engineering,
25 April 1988; accepted
2 October
Department
of Mechanical
Engineering,
Unioersity of California,
Dam,
1988
One major problem for the first wall of a thermonuclear fusion reactor is the selection of a structural material. The first wall is that part of a reactor where the major material problems arise, such as thermal fatigue, sputtering and plasma disruption [l]. Therefore, the material for the first wall should have high structural and chemical stability, low neutron activation, good mechanical properties, good weldability, availability and minimum maintenance and safety problems [1,2]. Austenitic stainless steel AISI 316 is generally envisaged in the field of structural material for the fission reactor (Fast Breeder Reactor). Accordingly, this steel is also considered as a first choice for making structural components of a fusion reactor. As 316 type contains Ni, it has some drawbacks with respect to fusion reactor application. The problem with nickel is that the radioactive transmutation elements formed from nickel (while being irradiated in a fusion reactor) would take a long time to decay, and thus, make a difficult nuclear wastedisposal problem. Apart from that, Ni has other detrimental effects, such as, a high rate of helium production, which may cause cavity nucleation and stabilization [3]. These cavities often degrade the mechanical properties of the post-formed component. Hence, efforts should be made to minimize their occurrence by replacing Ni, so that the helium production can effectively be controlled. In the process, cavitation will automatically be checked and the final component obtained through superplastic technique would probably contain no more cavities. With the addition of Ni, the stacking fault energy of the material increases which ultimately results in decreasing the work hardening effect. On the other hand, Ni is highly soluble in liquid Li, which is considered as a potential candidate material for the coolant [3,4]. 0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Hence, from nuclear point of view, Ni is a very harmful element and it can not be used for making the first wall of thermo-nuclear fusion reactors. Recently, significant interest has been generated among scientists and research workers [5-81 about the development of alloys which contain elements, other than Ni. Thus, with considerable amount of research, it has been suggested that high Cr-Mn steel can satisfactorily be used as the structural material for the first wall of a thermo-nuclear fusion reactor, where manganese would be a favorable replacement for Ni from a radioactivity point of view [9]. In fact, in high Cr-Mn stainless steels, Ni is partially or fully replaced by Mn. The advantage of the Mn addition to Cr-Mn steel is to lower the stacking fault energy of austenite, which ultimately leads to an increase in the strain hardening rate [3,10]. Mn is also responsible for increasing the low-temperature toughness of the material [ll]. As Mn is cheaper than Ni, from economic point of view, it will be wiser to replace Ni by Mn. But one should not forget while adding Mn, that the material costs are relatively a small fraction over the overall processing costs. It appears that high Cr-Mn steel combines good strength, high resistance to radiation damage, good corrosion resistance, good weldability and less amount of void swelling, which are the primary requirements of a fusion reactor [3,5,6]. Some of these high manganese steels such as AMCR-0033 and 0034 show higher strength than that of AISI 316 [l]. Similarly, the tensile strength of weld metal is increased by 10% and 30% respectively, at ambient and at 450 o C, with respect to the base metal strength. Furthermore, the weld metal presents an average energy absorption value, i.e. 73 to 108 J cm-* which is considerably lower than the value of 2543 J cm-‘, reported for the base metal. Thus. the B.V.
J. A4ukhopadhyuy / Superplmtic
welded joint has an excellent toughness behaviour as well as a low notch sensitivity 161. The charpy data are also comparable with those of AISI 316 welded joints. At high temperature and in sulphurous atmospheres, complex alloys based on Fe-Mn-Al show better oxidation resistance than Fe-Cr-Ni HZ]. Apart from that, it has been demonstrated that the void swelling is less in Cr-Mn type steels than in the Cr-Ni-Fe steels [8,13]. The total swelling of the AMCR steel at 500 and 600 o C is (20-50%) lower than that of 0.2% C AISI 316 and F548SS [3]_ In recent studies, it has also been observed that Cr-Mn steel can be used in cryogenic and marine structure purposes (11,141. However, there are some drawbacks in manganese addition. Manganese evaporates more rapidly due to its high vapor pressure and makes a surface layer of 10 pm thickness, while heated at 650°C for 1000 h under a vacuum of lop4 Torr. The maximum Mn depletion can take place around 1.5% of the total manganese, this may not reduce the actual Mn content of the alloy below the lower specification limit. Additionally, such loss would not destabilize the overall micro structure of the substituted alloys [lS]. Mn-containing stainless steel shows inferior oxidation resistance with respect to Cr-Ni steel [12]. This can be improved with the addition of aluminum. For aluminum addition, Al,O, is formed, which is more protective than Cr,O,. Therefore, the drawbacks of manganese addition can be counter balanced by adding aluminum [ 121. Although aluminum has remarkable effects on grain refining, stabilizing the austenite and oxidation resistance, however, its addition to Cr-Mn steel has some detrimental effects. For example, the cast steel with high aluminum content results in inferior casting behaviour. Besides that, in case of high Al, the surface of the melt gets oxidized and such an oxide may often affect the fluidity of molten steel [ll]. Hence, the aluminum content of the steel should be as low as possible, preferably around 1% [ll]. MO is considered to be the most effective in enhancing the train hardening rate of Cr-Mn steel, since it lowers the stacking fault energy [lo]. In fact, addition of 2.5-32 MO has also been shown to improve the corrosion resistance property. However, MO has a deleterious effect on Cr-Mn steel. It contributes to a long lived residual radio-activity and its addition to Cr-Mn steel leads to a catastrophic disintegration of the steel [9], Naturally, MO should be replaced by W [Is]. While designing the alloys, care should be taken about the nitrogen content, otherwise with lithium,
Cr-MI
stainless steel
91
nitrogen forms a complex compound (Li-Cr-N) which is very reactive [16]. Nitrogen is also responsible for void nucleation, thereby affecting superplastic properties. Therefore, it is preferable to have minimum nitrogen content (< 0.4%) in the steel [17]. It is believed that up to 1% silicon might be accommodated in the steel. However, in practice, it is intended to restrict’the maximum Si content around 0.2% in the steel, owing to its strong ferrite forming tendency and subsequent effect on promoting the formation of intermetallic phases [ 151. Carbon content of the steel should be low. An increased carbon content could lead to carbide precipitation and a substantial reduction in corrosion resistance [15]. As carbon content increases, the toughness of the steel decreases [ll]. Carbon is also responsible for increasing the stacking fault energy, which results in decreasing the work hardening effect [2]. However, with accurate control of carbon, good hot workability can easily be achieved in duplex stainless steel. In this context, the border line for carbon has been placed at 0.03%, otherwise with increased carbon, the edge cracking starts severely [ 181.Eventually, edge cracking can be suppressed with the addition of titanium, but it has another effect. With C and N, Ti forms Ti(C, N) cuboids (5 pm). These cuboids play an important role in nucleating cavities in a supe~lastically formed component [19]. Other than edge cracking, carbon also plays a crucial role in determining the level of cavitation in superplastitally deformed material. Cavitation is found to increase, as the carbon content of the material is raised. It appears that the metal cavitation is caused due to high CO-CO, gas pressure from the internal oxidation of carbon [20,21]. Preliminary work [22] suggests that Cr-Mn steel has the potential to become superplastic, while deformed at elevated temperature around 1123 K and at low strain rate of 1.5 X 10m4 s-l. However, the material used in that particular investigation was not pure. It contained Si, N, Ni, P and S as impurities. In fact, the acceptability limit of these elements for the structural components of a fusion reactor is as low as < 10 ppm [23]. Therefore, considering all the above factors, a high purity Cr-Mn steel (with very low amounts of carbon and aluminum content), practically free from all other elements, such as Si, S, P, Ni, MO, N, and Ti might be appropriate for studying the supe~lasticity and cavitation phenomena and subsequently be useful for the fusion reactor program.
92
J. Mukhopodhyay
/ Superplastic
References [l] G. Piatti and P. Schiller, in: Proc. Int. Conf. On Fusion Reactor Materials, Chicago, 1986, J. Nucl. Mater. 141-143 (1986) 417. [2] E. Ruedl, D.G. Rickerby and T. Sasaki, in: Proc. Conf. on 13th SOFT, Varese, Italy, 1984, p. 1029. [3] P. Fenici,D. Boerman, V. Coen, E. Lang, C. Ponti and W. Schule, NucI. Engrg. Des/Fusion 1 (1984) 167. [4] P. Fenici, V. Coen, E. Ruedl, H. Kolbe and T. Sasaki, J. Nucl. Mater. 102 & 104 (1981) 699. [5] G. Piatti and P. Schiller, J. Nucl. Mater. 141-143 (1986) 417. [6] G. Piatti and G. Musso, J. Mater. Sci. 21 (1986) 2339. [7] G. Piatti, S. Matteazi and G. Petrone, Nucl. Engrg. Des./Fusion 2 (1985) 391. [8] D.J. Mazey, J.A. Hudson and J.M. Titchmarsh, J. Nucl. Mater. 107 (1982) 2. [9] R.L. Klueh and M.L. Grossbcck, J. Nucl. Mater. 122 (1984) 294. [lo] D.J. Drobjak and J.G. Parr, Metall. Trans. 1 (1970) 759. [ll] L. Sheng Li, Da. Zhi. Yang, G. Sheng. Wei and C.M. Wayman, Metallography 15 (1982) 355. (121 Private communication from Manganese Centre, Paris.
Cr-Mn
starnless steel
[13] I.V. Gorynin et al., in: Proc. All Union C‘Onf. on Engineering Problems in Thermonuclear Reactors. Leningrad, 1977, p. 187 (in Russian). [14] M.T. Jahn, C.M. Fan and C.M. Wan, J. Mater. Sci. 20 (1985) 2757. [15] A.H. Bott, F.B. Pickering and G.J. Butterworth, in: Proc. Int. Conf. On Fusion Reactor Materials, Chicago, 1986, J. Nucl. Mater. 141-143 (1986) 1088. [16] E. Rued1 and T. Sasaki, J. Nucl. Mater. 116 (1983) 112. [17] H. Takahasi, T. Takyama, K. Tanikawa and R. Miura, in: Proc. First Int. Conf. On Fusion Reactor Materials, Tokyo, 1984, J. Nucl. Mater. 133 & 134 (1985) 566. [18] R.C. Gibson, H.W. Hayden and J.H. Brophy, ASM Trans. 61 (1968) 85. [19] N. Ridley and D.W. Livesey. Proc. Int. Conf. On Fracture, Waterloo, 1977, p. 533. [20] D. Capalan, R.J. Hussey. G.I. Sproule and M.J. Graham. Scripta Metall. 16 (1982) 759. (211 R.H. Bricknell and D.A. Woodford, Scripta Metall. 16 (1982) 761. [22] G. Piatti, D. Boerman and H.A. Weir, Metall. Sci. Technol. 4 (1986) 8. [23] R.W. Conn et al., Nucl. Technol./Fusion 5 (1984) 291.