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The characterisation of work-hardened austenitic stainless steel by NDT micro-magnetic techniques
NDT&E International 37 (2004) 265–269 www.elsevier.com/locate/ndteint
The characterisation of work-hardened austenitic stainless steel by NDT micro-magnetic techniques D. O’Sullivana,*, M. Cotterella, I. Meszarosb a
Department of Mechanical and Manufacturing Engineering, Cork Institute of Technology, Bishopstown, Room A243, Cork, Ireland b Department of Materials Science and Engineering, Budapest University of Technology and Economics, H-1111 Goldmann ter 3, (V2/159), Budapest, Hungary Received 15 July 2003; revised 30 September 2003; accepted 15 October 2003
Abstract It is accepted that the work-hardening of austenitic stainless steels during machining or cold-working results in two main products: the appearance of a0 -martensite and increased dislocation densities within the host material. In machining, this results in many difficulties (poor surface finish, poor machinability and high tool wear). Non-destructive sensing is essential in today’s high volume production environments because of its ease of use, speed and non-invasive sensing. Non-destructive magnetic measurement techniques have been employed to characterise the work-hardening of an austenitic stainless steel grade (SS404) due to room-temperature plastic tensile loading. These techniques include the use of magnetic Barkhausen noise, ferromagnetic phase measurement and coercivity measurement. It was found that the dislocation density, rather than the a0 -martensite phase, to be the cause of material work-hardening. It is suggested that the use of coercivity measurement is a useful quantitative and non-destructive method for characterising work-hardening of the studied alloy in relation to the amount of its plastic deformation (work-hardening). q 2003 Elsevier Ltd. All rights reserved. Keywords: Coercivity; Work-hardening; Magnetic Barkhausen noise
1. Introduction It is widely accepted that when austenitic stainless steels are deformed or strained, there is a martensite formation and the dislocation density (or crystalline defects), within the host material, increases in accordance with the amount of deformation. The gradual transformation of austenite to strain induced martensite increases the work-hardening of these steels. The fine a0 -martensite grains which appear inside the austenite grains mainly at the intersection of shear bands, make the movement of dislocations more difficult. Martensite may form in austenitic stainless steels due to the working of the material (mechanical) or due to temperature effects (thermal). Two types of martensite can form: 1-martensite, which forms on close-packed (111) planes in the austenite, has a hexagonal close-packed (hcp) * Corresponding author. Tel.: þ353-21-432-6507; fax: þ 353-21-4326627. E-mail address: [email protected] (D. O’Sullivan). 0963-8695/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2003.10.001
crystal structure; and body-centred cubic (bcc) a0 -martensite, which forms as plates with (225) habit planes in groups bounded by faulted sheets of austenite on (111) planes [1]. All austenitic stainless steels are paramagnetic in the annealed, fully austenitic condition. The hcp 1-martensite is paramagnetic in contrast to the bcc a0 -martensite which is strongly ferromagnetic (hard-magnet) and the only magnetic phase in the low carbon austenitic stainless steels [2]. Therefore, the cold worked austenitic stainless steels have detectable magnetic properties that can be eliminated by annealing. A device that would lead to the fast detection of workhardening in a workpiece material during machine operation is desirable. Thus, any machine or material changes that need to be made to combat this problem can be completed, eliminating consequential production problems. This can lead to the optimised machining of the material. An investigation into work-hardening detection techniques is presented.
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D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269
analysing device was used for processing the noise. The applied sampling frequency was 100 kHz and the maximum magnetic field induction of the excitation was 74 G. With this sampling frequency, only the relatively low frequency components of the noise (under 10 kHz) was sampled and evaluated. The Root Mean Square (RMS) value of the noise was determined and was used to characterise the microstructural changes in the material. 2.2. Coercivity measurement Fig. 1. Schematic for coercivity measurement.
2. Experimental Tensile tests were completed on SS304 samples with dimensions of 100 mm £ 10 mm £ 3 mm. These samples were deformed to various degrees of strain within the plastic range (i.e. from the yield point (0.98%) of the material to the ultimate tensile strength point (48%)). The amount of strain was determined by the percentage of plastic deformation to be completed to samples. All tests were completed at room temperature on a Hounsfield tensile tester in accordance with BS EN 10 002-1: 1990 (Tensile testing of metallic materials). A commercial magnetometer, based on an eddy current excitation in the specimen (Fischer, Feritscope), was employed in order to measure the percentage of ferromagnetic phase in the samples after tensile loading. 2.1. Magnetic Barkhausen noise measurements The magnetic Barkhausen noise (MBN) was investigated by using sinusoidal (10 Hz) excitation magnetic field produced by a function generator and a power amplifier. The applied measuring head contained a U-shaped magnetising coil and a pick-up coil, which is perpendicular to the surface of the specimen. The signal of the pick-up coil was processed by a 0.3– 38 kHz band pass filter and amplified with a gain of 50. KRENZ TRB 4000 (Krenz-Eckelmann Industrieautomation GmbH, Wiesbaden, Germany) computer controlled signal
For the coercivity measurements, a Fo¨rster coercimeter (Type 1.093) for precision coercivity measurement was used. A schematic of the apparatus is given in Fig. 1. 2.3. Microhardness testing The Vickers hardness was measured with a load of 98.1 N.
3. Results and discussion The RMS of the MBN and volume fraction of the a0 (Alpha Prime) martensite phase (also known as the %ferrite content) of the samples was measured. Fig. 2 shows the variation of the RMS value of the noise with %plastic strain during tensile deformation. It has been observed that above 30% plastic strain, as the %ferrite content of the samples increase, the RMS of the MBN also increases nearly linearly showing that the MBN can used as an effective means to determine the amount of a0 -martensite content in strained samples. Before 30% plastic strain, the MBN signals are considered to be insignificant as the signal-to-noise ratio is too small. This is attributed to the lower volume of ferrite in these specimens, which results in reduced Barkhausen activity. The relationship between the two sensing devices can be seen in Fig. 3. After approximately 5% ferrite content,
Fig. 2. Dependence of RMS of magnetic Barkhausen noise and the amount of a0 -martensite formed in relation to plastic strain% for AISI 304 samples.
D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269
267
Fig. 3. Dependence of MBN (RMS) on the %ferrite content of the AISI 304 samples.
Fig. 4. Variation in Barkhausen noise and hardness as a function of plastic strain for AISI 304 samples.
the Barkhausen signal-to-noise ratio improves and a nearly linear relationship is observed between the two sensing devices. The RMS and the Vickers hardness values were measured. Fig. 4 shows that the hardness of the sample increase linearly to about 30% plastic strain and then begins to saturate. But in contrast to that, the MBN signals increase linearly after 30% plastic strain. This would suggest that the a0 -martensite phase formed within the strained sample, does not contribute fully to the work-hardening of these specimens. This saturation in hardness level could be a consequence of a phenomena called work softening. In his papers, Mu¨llner [5 – 7] discusses this phenomenon and explains it in terms of the dislocation reaction at twin boundaries. The main physics is the splitting of a perfect 1/2k110l dislocation into two Shockley partials of type 1/6k211l. In terms of a single moving dislocation, the first (leading) partial produces a stacking fault whereas the second (trailing) partial closes the stacking fault. This discussion can be directly applied to 1 -martensite because twining
dislocations and transformation dislocations of 1-martensite are identical, namely 1/6k211l Shockley dislocations. The role of dislocation –twin interaction (dislocation – epsilon interaction) or twin –twin interaction is to produce a group
Fig. 5. Dependence of RMS of magnetic Barkhausen noise and hardness on the temperature of heat treatment of AISI 304 samples [2,3].
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D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269
Fig. 6. Carbide precipitation in AISI 304 at 700 8C.
of trailing (‘untwining’) dislocations, that ‘close’ the twin (or 1-martensite), hence softening. To consider this in terms of a0 -martensite is much more difficult, and the dislocation reactions at martensite habit planes would have to be considered. More research looking into the deformation structure with transmission electron microscopy needs to be completed. The dislocation density rather than the a0 -martensite phase as a cause of material work-hardening has also been shown in the past by Me´sza´ros [2,3]. From Fig. 5, the reverse transformation of the a0 -phase back to austenite occurs between 350 and 600 8C. But this does not lead to the reduced hardness of the material, and this recovery only begins after all the a0 -phase has disappeared. This can be explained by the precipitation of carbides (M23C6) [4]. Microstructural examination by this author on AISI 304 samples annealed at 700 8C shows precipitation is evident along grain boundaries, within the grains, especially along shear bands (slip lines) because
the nucleation rate is higher at these irregularities (Fig. 6). The increase in carbide particles makes movement of the dislocations more difficult, thus interfering with the normal annealing process. This phenomenon can be explained according to the Orowan hardening mechanism. The coercivity, Hc, of the deformed specimens was also determined. Fig. 7 shows that the coercivity and the hardness measurements are comparable. Both increase linearly to about 30% plastic strain and then saturate. The coercivity is seen to increase more strongly than the hardness before saturation, which can be explained by the effect of internal stresses that can modify the magnetic behaviour of a0 -martensite. Coercivity is affected by material structural change. During plastic deformation—the increase in dislocation densities and resultant structural changes lead to increased material coercivity. The suggested dislocation –twin interactions could result in more magnetic domain wall movement hence reducing the coercivity of the material at high deformation. Fig. 8 compares the RMS of MBN and the coercivity signal. Work-hardening leads to two distinct products in the affected material, the formation of a0 -martensite and increased dislocation densities. From this graph, it can be concluded that the appearance of the a0 -phase, above a certain volume fraction, can be effectively measured using MBN. In this case it was not possible to detect a0 martensite below 30% plastic strain. For increased MBN sensitivity to a0 -martensite formation, it is suggested to use a higher magnetic field strength for excitation, which would allow for increase magnetic domain activity, even at low levels of deformation. The increase in dislocations with deformation can be effectively examined using coercivity measurement at all levels of material deformation. But, for the overall nondestructive testing of work-hardened specimens,
Fig. 7. Dependence of hardness and coercivity on plastic strain% of AISI 304.
D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269
269
Fig. 8. Variation in Barkhausen noise and coercivity as a function of %plastic strain for AISI 304 samples.
coercivity measurement is suggested to be the best characterisation technique.
Acknowledgements The authors gratefully acknowledge the financial assistance provided for this project by Enterprise Ireland under the International Collaboration Programme.
4. Conclusions This study examines the effects of work-hardening in plastically deformed and strained austenitic stainless steel specimens using various non-destructive sensing techniques. † It is proved that the a0 -martensite forms continuously during plastic strain. † It is believed that increased dislocation density has a much stronger affect on material work-hardening than the appearance of a0 -martensite. † MBN is an effective tool for studying the transformation mechanisms of austenite to a0 -martensite during deformation but is not an effective means to characterise the work-hardening of austenitic stainless steel. † For non-destructive testing of work-hardening; coercivity measurement is seen as the most effective method.
References [1] Reed RP. The spontaneous martensitic transformation in 18 pct Cr, 8 pct Ni steels. Acta Metall 1962;10:865–77. [2] Me´sza´ros I, Micromagnetic testing of cold work induced martensite in austenitic stainless steel stainless steel ’99 science and market konferencia, Chia Laguna, Szardı´nia, Olaszorsza´g, 1999 Ju´nius 7-10. Proceedings, vol. 3.; 1999. p. 339–44. [3] Me´sza´ros I, Ka´ldor M, Hidasi B, Ve´ rtes A, Czako´-Nagy I. Micromagnetic and Mo¨ssbauer-spectroscopic investigation of strain induced martensite in austenitic stainless steel. J Mater Engng Perform (ASM Int) 1996;5(4):538–42. [4] Novak CJ. Structure and constitution of wrought austenitic stainless steels, p4-1. In: Peckner D, Bernstein LM, editors. Handbook of stainless steels. 1977. [5] Mu¨llner P. Disclination models for deformation twinning. Solid State Phenom 2002;87:227–38. [6] Mu¨llner P, Solenthaler C. On the effect of deformation twinning on defect densities. Mater Sci Engng 1997;A230:107–15. [7] Paulus N, Uggowitzer PJ, Mu¨llner P, Speidel MO. Cold and warm work of austenitic notrogen steels. La metallurgia italiana 1994;86(12): 603–8.
The characterisation of work-hardened austenitic stainless steel by NDT micro-magnetic techniques D. O’Sullivana,*, M. Cotterella, I. Meszarosb a
Department of Mechanical and Manufacturing Engineering, Cork Institute of Technology, Bishopstown, Room A243, Cork, Ireland b Department of Materials Science and Engineering, Budapest University of Technology and Economics, H-1111 Goldmann ter 3, (V2/159), Budapest, Hungary Received 15 July 2003; revised 30 September 2003; accepted 15 October 2003
Abstract It is accepted that the work-hardening of austenitic stainless steels during machining or cold-working results in two main products: the appearance of a0 -martensite and increased dislocation densities within the host material. In machining, this results in many difficulties (poor surface finish, poor machinability and high tool wear). Non-destructive sensing is essential in today’s high volume production environments because of its ease of use, speed and non-invasive sensing. Non-destructive magnetic measurement techniques have been employed to characterise the work-hardening of an austenitic stainless steel grade (SS404) due to room-temperature plastic tensile loading. These techniques include the use of magnetic Barkhausen noise, ferromagnetic phase measurement and coercivity measurement. It was found that the dislocation density, rather than the a0 -martensite phase, to be the cause of material work-hardening. It is suggested that the use of coercivity measurement is a useful quantitative and non-destructive method for characterising work-hardening of the studied alloy in relation to the amount of its plastic deformation (work-hardening). q 2003 Elsevier Ltd. All rights reserved. Keywords: Coercivity; Work-hardening; Magnetic Barkhausen noise
1. Introduction It is widely accepted that when austenitic stainless steels are deformed or strained, there is a martensite formation and the dislocation density (or crystalline defects), within the host material, increases in accordance with the amount of deformation. The gradual transformation of austenite to strain induced martensite increases the work-hardening of these steels. The fine a0 -martensite grains which appear inside the austenite grains mainly at the intersection of shear bands, make the movement of dislocations more difficult. Martensite may form in austenitic stainless steels due to the working of the material (mechanical) or due to temperature effects (thermal). Two types of martensite can form: 1-martensite, which forms on close-packed (111) planes in the austenite, has a hexagonal close-packed (hcp) * Corresponding author. Tel.: þ353-21-432-6507; fax: þ 353-21-4326627. E-mail address: [email protected] (D. O’Sullivan). 0963-8695/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2003.10.001
crystal structure; and body-centred cubic (bcc) a0 -martensite, which forms as plates with (225) habit planes in groups bounded by faulted sheets of austenite on (111) planes [1]. All austenitic stainless steels are paramagnetic in the annealed, fully austenitic condition. The hcp 1-martensite is paramagnetic in contrast to the bcc a0 -martensite which is strongly ferromagnetic (hard-magnet) and the only magnetic phase in the low carbon austenitic stainless steels [2]. Therefore, the cold worked austenitic stainless steels have detectable magnetic properties that can be eliminated by annealing. A device that would lead to the fast detection of workhardening in a workpiece material during machine operation is desirable. Thus, any machine or material changes that need to be made to combat this problem can be completed, eliminating consequential production problems. This can lead to the optimised machining of the material. An investigation into work-hardening detection techniques is presented.
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D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269
analysing device was used for processing the noise. The applied sampling frequency was 100 kHz and the maximum magnetic field induction of the excitation was 74 G. With this sampling frequency, only the relatively low frequency components of the noise (under 10 kHz) was sampled and evaluated. The Root Mean Square (RMS) value of the noise was determined and was used to characterise the microstructural changes in the material. 2.2. Coercivity measurement Fig. 1. Schematic for coercivity measurement.
2. Experimental Tensile tests were completed on SS304 samples with dimensions of 100 mm £ 10 mm £ 3 mm. These samples were deformed to various degrees of strain within the plastic range (i.e. from the yield point (0.98%) of the material to the ultimate tensile strength point (48%)). The amount of strain was determined by the percentage of plastic deformation to be completed to samples. All tests were completed at room temperature on a Hounsfield tensile tester in accordance with BS EN 10 002-1: 1990 (Tensile testing of metallic materials). A commercial magnetometer, based on an eddy current excitation in the specimen (Fischer, Feritscope), was employed in order to measure the percentage of ferromagnetic phase in the samples after tensile loading. 2.1. Magnetic Barkhausen noise measurements The magnetic Barkhausen noise (MBN) was investigated by using sinusoidal (10 Hz) excitation magnetic field produced by a function generator and a power amplifier. The applied measuring head contained a U-shaped magnetising coil and a pick-up coil, which is perpendicular to the surface of the specimen. The signal of the pick-up coil was processed by a 0.3– 38 kHz band pass filter and amplified with a gain of 50. KRENZ TRB 4000 (Krenz-Eckelmann Industrieautomation GmbH, Wiesbaden, Germany) computer controlled signal
For the coercivity measurements, a Fo¨rster coercimeter (Type 1.093) for precision coercivity measurement was used. A schematic of the apparatus is given in Fig. 1. 2.3. Microhardness testing The Vickers hardness was measured with a load of 98.1 N.
3. Results and discussion The RMS of the MBN and volume fraction of the a0 (Alpha Prime) martensite phase (also known as the %ferrite content) of the samples was measured. Fig. 2 shows the variation of the RMS value of the noise with %plastic strain during tensile deformation. It has been observed that above 30% plastic strain, as the %ferrite content of the samples increase, the RMS of the MBN also increases nearly linearly showing that the MBN can used as an effective means to determine the amount of a0 -martensite content in strained samples. Before 30% plastic strain, the MBN signals are considered to be insignificant as the signal-to-noise ratio is too small. This is attributed to the lower volume of ferrite in these specimens, which results in reduced Barkhausen activity. The relationship between the two sensing devices can be seen in Fig. 3. After approximately 5% ferrite content,
Fig. 2. Dependence of RMS of magnetic Barkhausen noise and the amount of a0 -martensite formed in relation to plastic strain% for AISI 304 samples.
D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269
267
Fig. 3. Dependence of MBN (RMS) on the %ferrite content of the AISI 304 samples.
Fig. 4. Variation in Barkhausen noise and hardness as a function of plastic strain for AISI 304 samples.
the Barkhausen signal-to-noise ratio improves and a nearly linear relationship is observed between the two sensing devices. The RMS and the Vickers hardness values were measured. Fig. 4 shows that the hardness of the sample increase linearly to about 30% plastic strain and then begins to saturate. But in contrast to that, the MBN signals increase linearly after 30% plastic strain. This would suggest that the a0 -martensite phase formed within the strained sample, does not contribute fully to the work-hardening of these specimens. This saturation in hardness level could be a consequence of a phenomena called work softening. In his papers, Mu¨llner [5 – 7] discusses this phenomenon and explains it in terms of the dislocation reaction at twin boundaries. The main physics is the splitting of a perfect 1/2k110l dislocation into two Shockley partials of type 1/6k211l. In terms of a single moving dislocation, the first (leading) partial produces a stacking fault whereas the second (trailing) partial closes the stacking fault. This discussion can be directly applied to 1 -martensite because twining
dislocations and transformation dislocations of 1-martensite are identical, namely 1/6k211l Shockley dislocations. The role of dislocation –twin interaction (dislocation – epsilon interaction) or twin –twin interaction is to produce a group
Fig. 5. Dependence of RMS of magnetic Barkhausen noise and hardness on the temperature of heat treatment of AISI 304 samples [2,3].
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D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269
Fig. 6. Carbide precipitation in AISI 304 at 700 8C.
of trailing (‘untwining’) dislocations, that ‘close’ the twin (or 1-martensite), hence softening. To consider this in terms of a0 -martensite is much more difficult, and the dislocation reactions at martensite habit planes would have to be considered. More research looking into the deformation structure with transmission electron microscopy needs to be completed. The dislocation density rather than the a0 -martensite phase as a cause of material work-hardening has also been shown in the past by Me´sza´ros [2,3]. From Fig. 5, the reverse transformation of the a0 -phase back to austenite occurs between 350 and 600 8C. But this does not lead to the reduced hardness of the material, and this recovery only begins after all the a0 -phase has disappeared. This can be explained by the precipitation of carbides (M23C6) [4]. Microstructural examination by this author on AISI 304 samples annealed at 700 8C shows precipitation is evident along grain boundaries, within the grains, especially along shear bands (slip lines) because
the nucleation rate is higher at these irregularities (Fig. 6). The increase in carbide particles makes movement of the dislocations more difficult, thus interfering with the normal annealing process. This phenomenon can be explained according to the Orowan hardening mechanism. The coercivity, Hc, of the deformed specimens was also determined. Fig. 7 shows that the coercivity and the hardness measurements are comparable. Both increase linearly to about 30% plastic strain and then saturate. The coercivity is seen to increase more strongly than the hardness before saturation, which can be explained by the effect of internal stresses that can modify the magnetic behaviour of a0 -martensite. Coercivity is affected by material structural change. During plastic deformation—the increase in dislocation densities and resultant structural changes lead to increased material coercivity. The suggested dislocation –twin interactions could result in more magnetic domain wall movement hence reducing the coercivity of the material at high deformation. Fig. 8 compares the RMS of MBN and the coercivity signal. Work-hardening leads to two distinct products in the affected material, the formation of a0 -martensite and increased dislocation densities. From this graph, it can be concluded that the appearance of the a0 -phase, above a certain volume fraction, can be effectively measured using MBN. In this case it was not possible to detect a0 martensite below 30% plastic strain. For increased MBN sensitivity to a0 -martensite formation, it is suggested to use a higher magnetic field strength for excitation, which would allow for increase magnetic domain activity, even at low levels of deformation. The increase in dislocations with deformation can be effectively examined using coercivity measurement at all levels of material deformation. But, for the overall nondestructive testing of work-hardened specimens,
Fig. 7. Dependence of hardness and coercivity on plastic strain% of AISI 304.
D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269
269
Fig. 8. Variation in Barkhausen noise and coercivity as a function of %plastic strain for AISI 304 samples.
coercivity measurement is suggested to be the best characterisation technique.
Acknowledgements The authors gratefully acknowledge the financial assistance provided for this project by Enterprise Ireland under the International Collaboration Programme.
4. Conclusions This study examines the effects of work-hardening in plastically deformed and strained austenitic stainless steel specimens using various non-destructive sensing techniques. † It is proved that the a0 -martensite forms continuously during plastic strain. † It is believed that increased dislocation density has a much stronger affect on material work-hardening than the appearance of a0 -martensite. † MBN is an effective tool for studying the transformation mechanisms of austenite to a0 -martensite during deformation but is not an effective means to characterise the work-hardening of austenitic stainless steel. † For non-destructive testing of work-hardening; coercivity measurement is seen as the most effective method.
References [1] Reed RP. The spontaneous martensitic transformation in 18 pct Cr, 8 pct Ni steels. Acta Metall 1962;10:865–77. [2] Me´sza´ros I, Micromagnetic testing of cold work induced martensite in austenitic stainless steel stainless steel ’99 science and market konferencia, Chia Laguna, Szardı´nia, Olaszorsza´g, 1999 Ju´nius 7-10. Proceedings, vol. 3.; 1999. p. 339–44. [3] Me´sza´ros I, Ka´ldor M, Hidasi B, Ve´ rtes A, Czako´-Nagy I. Micromagnetic and Mo¨ssbauer-spectroscopic investigation of strain induced martensite in austenitic stainless steel. J Mater Engng Perform (ASM Int) 1996;5(4):538–42. [4] Novak CJ. Structure and constitution of wrought austenitic stainless steels, p4-1. In: Peckner D, Bernstein LM, editors. Handbook of stainless steels. 1977. [5] Mu¨llner P. Disclination models for deformation twinning. Solid State Phenom 2002;87:227–38. [6] Mu¨llner P, Solenthaler C. On the effect of deformation twinning on defect densities. Mater Sci Engng 1997;A230:107–15. [7] Paulus N, Uggowitzer PJ, Mu¨llner P, Speidel MO. Cold and warm work of austenitic notrogen steels. La metallurgia italiana 1994;86(12): 603–8.