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Phase transformation induced by severe plastic deformation in the AISI 304L stainless steel
Materials Science and Engineering A358 (2003) 32 /36 www.elsevier.com/locate/msea
Phase transformation induced by severe plastic deformation in the AISI 304L stainless steel S.S.M. Tavares a,*, D. Gunderov b, V. Stolyarov b, J.M. Neto c a
b
UFF-Depto. de Eng. Mecaˆnica, Rua Passo da Pa´tria, 156, CEP 24210-240, Niteroi, Brazil Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa, Russia c UFRJ-Instituto de Fı´sica, Rio de Janeiro, Brazil Received 21 October 2002; received in revised form 24 February 2003
Abstract The phase transformation caused by high pressure torsion (HPT) of a cold rolled AISI 304L stainless steel was investigated. After cold rolling the steel has a microstructure of martensite a? (bcc) with a magnetization saturation of 136.0 A m 2 kg1. The HPT processing with pressures of 2 and 5 GPa promoted the a? (bcc) 0/o (hcp) partial transformation, as observed by X-ray diffraction. The magnetization saturation decreased by the increase of true deformation by HPT, which is in agreement to the a? decrease. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Metals and alloys; Severe plastic deformation; X-ray diffraction; Magnetic measurements
1. Introduction The metastable austenitic stainless steels (AISI 304, 304L, 316,. . .) are widely used in many industrial components. These steels are susceptible to martensitic transformations induced by plastic deformation at low temperatures [1 /4]. Mongonon and Thomas [1] studied the martensitic phases formed during the tensile test of a AISI 304 steel. They have found that the o martensite (hcp, paramagnetic) is formed at the beginning of deformation and reaches a peak value at about 5% of tensile strain. After that the amount of this phase decreased to almost zero at 20% of plastic strain. The a? martensite (bcc, ferromagnetic) increased continuously with strain. At higher strains the a? was the only martensite present in the steel. Similar results were reported by Seetharaman and Krishnan [2] in the AISI 316 steel deformed by rolling and tension test. The sequence of transformation g0/o0/a? was therefore proposed for metastable steels deformed by uniaxial tension and rolling.
* Corresponding author. E-mail address: [email protected] (S.S.M. Tavares).
The most common methods for detection and analysis of the martensite phases in austenitic stainless steels are X-ray diffraction and saturation magnetization measurements. The magnetic methods are used only to analyse the a? martensite. The volume fraction of the ferromagnetic phase a? of a given sample can be calculated by dividing its magnetization saturation (ms) in emu g1 by the intrinsic magnetization saturation of martensite. This value ranges from 130 to 160 emu g1 in the Ref. [1,5]. Seiki et al. [6] determined the coercive field (Hc) of a 304 steel as function of deformation by tension. They found an increase of coercive field with the amount of deformation in the range of 0 /100% of deformation, which gave 0/45% of martensite. Tavares et al. [7] found the inverse effect for higher strain degrees by cold rolling in a 304L steel. The Hc increased with the applied deformation and also with the heat treatment at 400 8C after deformation. This treatment promotes a small increase in the amount of martensite a? (up to 5%) [1,7,8]. Previous studies of the dependence of phase transformations on deformation involved moderate true strains, achieved by tension or rolling [4]. The effect of larger strains has yet to be investigated.
0921-5093/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(03)00263-6
S.S.M. Tavares et al. / Materials Science and Engineering A358 (2003) 32 /36
This paper deals with transformation induced in a AISI 304L steel strained by using high pressure torsion (HPT), a severe plastic deformation (SPD) method. Before the HPT processing the material was cold rolled to a maximum martensite a? (bcc) content.
2. Experimental An AISI 304L steel sheet with 3 mm in thickness was deformed by cold rolling till 0.30 mm, corresponding to a true strain of /2.3 in the thickness. The samples B, C, D and E were then subjected to deformations by HPT processing as described in Ref. [10]. By this method very high torsion strains are imposed. Two parameters are controlled in the HPT processing: the pressure (P ) and the numbers of rotation (n). Table 1 shows the HPT parameters selected to process each sample and the corresponding true strains calculated by equation from Ref. [9]. The HPT processing was carried out at room temperature. After deformation the samples were analysed using Xray diffraction in a Siemens diffractometer model D500 using Cu Ka radiation. Magnetic measurements at room temperature were carried out in vibrating sample magnetometer EGG PAR model 4500 with external magnetic fields up to 7500 Oe. Microhardness measurements were also carried out in the deformed samples at loading 100 g and duration of 15 s.
3. Results Fig. 1 shows the X-ray diffractograms of the sample A, that was deformed by cold rolling. The high intensity peaks are from a? martensite. The magnetization saturation at this condition is 136.0 Am2 kg1. Fig. 2 show the diffractogram of sample B, which was processed using HPT with n /0.5 and P /2 GPa. In ˚ ), identified this diffractogram a small peak (d/1.94 A by an arrow, is observed. This d value is coincident with that of the plane (1 0 1) of hexagonal o phase.
33
Figs. 3 and 4 show the diffractograms of the samples processed with P /5 GPa and n/2 and 5, respectively. Two other peaks observed in these diffractograms ˚ (o1 1 0) confirm the presence of the phase o: d /1.27 A ˚ (o1 1 2). Fig. 5 shows the diffractogram and d /1.076 A of sample E, the most deformed one. The peaks of o1 1 0 and o1 1 2 are no more present, probably due to texture ˚ effects. However, an intense peak related to d/2.066 A appears in this diffractogram. This peak can be indexed as o0 0 1. Fig. 6 compares the diffractograms of samples C, D and E in the 2u range of 40/558. This comparison shows that the intensity of the o0 0 2 reflection increases with the amount of deformation, and in the more severely deformed sample (E) this reflection becomes clearly separated from the a1 1 1 peak. Table 2 shows the magnetic properties of samples A to E. Comparing samples A and B it can be seen that the deformation by the HPT with P /2 GPa and n /0.5 caused a small decrease in the magnetization saturation (ms), which is consistent with the formation of the paramagnetic phase o. Comparing now samples B and C, the increase of deformation causes a further decrease of ms and an increase of coercive field and residual induction. The ms decrease confirms the increase of phase o from sample B to C. Samples C and D show similar properties; in this case, the increase of deformation did not cause the decrease of ms nor the increase of Hc. The most deformed sample (E) presented the lower ms and the higher Hc values. At this condition the amount of phase o is maximum, and there is a high preferential orientation (texture). As found earlier [1,2], the o phase is formed in the beginning of cold rolling or tension test in metastable steels. There is some evidence that the formation of a? is precedeed by o, and so the g0/o0/a? sequence is suggested. The o martensite disappears when the samples are highly deformed by rolling or tension. Indeed, the diffractogram of the sample A (Fig. 1) does not show any peak of o. However, the shear stresses and the high pressure developed in the HPT process bring back the hcp phase o. It is worth noting that the diffractograms of all samples analysed did not contain any peak of the austenite phase (g). If present, the phase g should be less
Table 1 Samples identification and processing conditions by HPT Samplea
A B C D E
True strain by rolling
2.3 2.3 2.3 2.3 2.3 a
HPT parameters n (number of anvil rotations)
P (GPa)
Estimated true straina
/ 0.5 2 5 8
/ 2 5 5 5
/ 3.7 5.1 6 6.5
Estimated for the points located at the radius equal to 2.0 mm.
34
S.S.M. Tavares et al. / Materials Science and Engineering A358 (2003) 32 /36
Fig. 1. X-ray diffractogram of the cold rolled sample (sample A).
than about 5%, which is near the limit of detection by Xray diffraction. This observation leads to conclude that the increase of o phase with the deformation by the HPT was not from the austenite phase. The o phase must be formed by a reaction a?0/o. Such a bcc 0/hcp transition has already been observed in iron under ultra-high pression (/15 GPa) by Jamielson and Lawson [11]. The 304L stainless steel, however, seems to be much more susceptible to this transformation than pure iron, since it occurred under pressures as low as 2 GPa in this work. The a?0/o transition can be considered as martensitic like transformation, since it occurs by a diffusionless mechanism. This process occurs with an important volume contraction, which is consequence of the hydrostatic pressure component.
Table 3 shows the microhardness values measured in the samples A, B, D and E. Initially, the deformation by HPT promotes a strong hardening effect (sample B, 546HV). However, a softening process occurs for higher strains (samples D and E), which may be associated the increase of phase o and the decrease of martensite a?.
4. Conclusions The X-ray diffraction and the magnetic measurements showed that the SPD by HPT after cold rolling promotes the formation of the hcp o phase in the AISI 304L steel. The amount of this phase was maximum in the most deformed sample (n/8 and P /5 GPa) and
Fig. 2. X-ray diffractogram of sample B (n/0.5, P /2 GPa).
S.S.M. Tavares et al. / Materials Science and Engineering A358 (2003) 32 /36
Fig. 3. X-ray diffractogram of sample C (n /2, P /5 GPa).
Fig. 4. X-ray diffractogram of sample D (n/5, P /5 GPa).
Fig. 5. X-ray diffractogram of sample E (n/8, P /5 GPa).
35
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S.S.M. Tavares et al. / Materials Science and Engineering A358 (2003) 32 /36 Table 2 Magnetic properties of samples A, B, C, D and E Sample
ms (A m 2 kg) 1
Hc (kA m 1)
Br (T)
A B C D E
136.0 129.8 97.6 99.4 78.3
13.2 13.4 16.4 15.9 19.0
0.99 0.58 0.73 0.78 0.55
Table 3 Microhardness values of samples A, B, D and E Sample
Microhardness
A /cold rolled B /HPT /n /0.5, P /2 GPa D /HPT /n/5, P /5 GPa E /HPT /n /8, P /5 GPa
453 546 460 453
References Fig. 6. Comparison between samples B, C, D and E, showing the evolution of the peak o0 0 2 with the increase of strain degree by HPT.
this sample presented a minimum value of magnetization saturation. It is found that the o phase forms by the reaction a?0/o.
[1] [2] [3] [4] [5] [6]
Acknowledgements
[7] [8]
The authors would like to acknowledge the Brazilian research agencies CNPq (470385/01-4 NV) and FAPERJ (E-26/150.365/2002) for financial support and R.P. da Silva for the help with X-ray diffraction analysis.
[9] [10] [11]
P.L. Mongonon, G. Thomas, Met. Trans. 1 (1970) 1587. V. Seetharaman, R. Krishnan, J. Mater. Sci. 16 (1981) 523. D.C. Cook, Met. Trans. 18A (1987) 201. S.S.M. Tavares, D. Fruchart, S. Miraglia, J. Alloys Comp. 307 (2000) 311. J. Childress, S.H. Liou, C.L. Chien, J. de Physique C8 (Suppl. 12 T.49) (1988) 113. S. Takahashi, J. Echigoya, T. Ueda, X. Li, H. Hatafuku, J. Mat. Proc. Tech. 198 (2001) 213. S.S.M. Tavares, M.R. da Silva, J.M. Neto, A. Miraglia, D. Fruchart, J. Magn. Magn. Mater. 242 /245 (2002) 1391. F. Gauzzi, R. Montanari, G. Principi, A. Perin, M.E. Tata, Mat. Sci. Eng. A 273 /275 (1999) 443. R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2) (2000) 103 /184. A.G. Popov, D.V. Gunderov, V.V. Stolyarov, J. Magn. Magn. Mater. 157/158 (1996) 33. J.C. Jamielson, A.W. Lawson, J. Appl. Phys. 33 (1962) 776.
Phase transformation induced by severe plastic deformation in the AISI 304L stainless steel S.S.M. Tavares a,*, D. Gunderov b, V. Stolyarov b, J.M. Neto c a
b
UFF-Depto. de Eng. Mecaˆnica, Rua Passo da Pa´tria, 156, CEP 24210-240, Niteroi, Brazil Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa, Russia c UFRJ-Instituto de Fı´sica, Rio de Janeiro, Brazil Received 21 October 2002; received in revised form 24 February 2003
Abstract The phase transformation caused by high pressure torsion (HPT) of a cold rolled AISI 304L stainless steel was investigated. After cold rolling the steel has a microstructure of martensite a? (bcc) with a magnetization saturation of 136.0 A m 2 kg1. The HPT processing with pressures of 2 and 5 GPa promoted the a? (bcc) 0/o (hcp) partial transformation, as observed by X-ray diffraction. The magnetization saturation decreased by the increase of true deformation by HPT, which is in agreement to the a? decrease. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Metals and alloys; Severe plastic deformation; X-ray diffraction; Magnetic measurements
1. Introduction The metastable austenitic stainless steels (AISI 304, 304L, 316,. . .) are widely used in many industrial components. These steels are susceptible to martensitic transformations induced by plastic deformation at low temperatures [1 /4]. Mongonon and Thomas [1] studied the martensitic phases formed during the tensile test of a AISI 304 steel. They have found that the o martensite (hcp, paramagnetic) is formed at the beginning of deformation and reaches a peak value at about 5% of tensile strain. After that the amount of this phase decreased to almost zero at 20% of plastic strain. The a? martensite (bcc, ferromagnetic) increased continuously with strain. At higher strains the a? was the only martensite present in the steel. Similar results were reported by Seetharaman and Krishnan [2] in the AISI 316 steel deformed by rolling and tension test. The sequence of transformation g0/o0/a? was therefore proposed for metastable steels deformed by uniaxial tension and rolling.
* Corresponding author. E-mail address: [email protected] (S.S.M. Tavares).
The most common methods for detection and analysis of the martensite phases in austenitic stainless steels are X-ray diffraction and saturation magnetization measurements. The magnetic methods are used only to analyse the a? martensite. The volume fraction of the ferromagnetic phase a? of a given sample can be calculated by dividing its magnetization saturation (ms) in emu g1 by the intrinsic magnetization saturation of martensite. This value ranges from 130 to 160 emu g1 in the Ref. [1,5]. Seiki et al. [6] determined the coercive field (Hc) of a 304 steel as function of deformation by tension. They found an increase of coercive field with the amount of deformation in the range of 0 /100% of deformation, which gave 0/45% of martensite. Tavares et al. [7] found the inverse effect for higher strain degrees by cold rolling in a 304L steel. The Hc increased with the applied deformation and also with the heat treatment at 400 8C after deformation. This treatment promotes a small increase in the amount of martensite a? (up to 5%) [1,7,8]. Previous studies of the dependence of phase transformations on deformation involved moderate true strains, achieved by tension or rolling [4]. The effect of larger strains has yet to be investigated.
0921-5093/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(03)00263-6
S.S.M. Tavares et al. / Materials Science and Engineering A358 (2003) 32 /36
This paper deals with transformation induced in a AISI 304L steel strained by using high pressure torsion (HPT), a severe plastic deformation (SPD) method. Before the HPT processing the material was cold rolled to a maximum martensite a? (bcc) content.
2. Experimental An AISI 304L steel sheet with 3 mm in thickness was deformed by cold rolling till 0.30 mm, corresponding to a true strain of /2.3 in the thickness. The samples B, C, D and E were then subjected to deformations by HPT processing as described in Ref. [10]. By this method very high torsion strains are imposed. Two parameters are controlled in the HPT processing: the pressure (P ) and the numbers of rotation (n). Table 1 shows the HPT parameters selected to process each sample and the corresponding true strains calculated by equation from Ref. [9]. The HPT processing was carried out at room temperature. After deformation the samples were analysed using Xray diffraction in a Siemens diffractometer model D500 using Cu Ka radiation. Magnetic measurements at room temperature were carried out in vibrating sample magnetometer EGG PAR model 4500 with external magnetic fields up to 7500 Oe. Microhardness measurements were also carried out in the deformed samples at loading 100 g and duration of 15 s.
3. Results Fig. 1 shows the X-ray diffractograms of the sample A, that was deformed by cold rolling. The high intensity peaks are from a? martensite. The magnetization saturation at this condition is 136.0 Am2 kg1. Fig. 2 show the diffractogram of sample B, which was processed using HPT with n /0.5 and P /2 GPa. In ˚ ), identified this diffractogram a small peak (d/1.94 A by an arrow, is observed. This d value is coincident with that of the plane (1 0 1) of hexagonal o phase.
33
Figs. 3 and 4 show the diffractograms of the samples processed with P /5 GPa and n/2 and 5, respectively. Two other peaks observed in these diffractograms ˚ (o1 1 0) confirm the presence of the phase o: d /1.27 A ˚ (o1 1 2). Fig. 5 shows the diffractogram and d /1.076 A of sample E, the most deformed one. The peaks of o1 1 0 and o1 1 2 are no more present, probably due to texture ˚ effects. However, an intense peak related to d/2.066 A appears in this diffractogram. This peak can be indexed as o0 0 1. Fig. 6 compares the diffractograms of samples C, D and E in the 2u range of 40/558. This comparison shows that the intensity of the o0 0 2 reflection increases with the amount of deformation, and in the more severely deformed sample (E) this reflection becomes clearly separated from the a1 1 1 peak. Table 2 shows the magnetic properties of samples A to E. Comparing samples A and B it can be seen that the deformation by the HPT with P /2 GPa and n /0.5 caused a small decrease in the magnetization saturation (ms), which is consistent with the formation of the paramagnetic phase o. Comparing now samples B and C, the increase of deformation causes a further decrease of ms and an increase of coercive field and residual induction. The ms decrease confirms the increase of phase o from sample B to C. Samples C and D show similar properties; in this case, the increase of deformation did not cause the decrease of ms nor the increase of Hc. The most deformed sample (E) presented the lower ms and the higher Hc values. At this condition the amount of phase o is maximum, and there is a high preferential orientation (texture). As found earlier [1,2], the o phase is formed in the beginning of cold rolling or tension test in metastable steels. There is some evidence that the formation of a? is precedeed by o, and so the g0/o0/a? sequence is suggested. The o martensite disappears when the samples are highly deformed by rolling or tension. Indeed, the diffractogram of the sample A (Fig. 1) does not show any peak of o. However, the shear stresses and the high pressure developed in the HPT process bring back the hcp phase o. It is worth noting that the diffractograms of all samples analysed did not contain any peak of the austenite phase (g). If present, the phase g should be less
Table 1 Samples identification and processing conditions by HPT Samplea
A B C D E
True strain by rolling
2.3 2.3 2.3 2.3 2.3 a
HPT parameters n (number of anvil rotations)
P (GPa)
Estimated true straina
/ 0.5 2 5 8
/ 2 5 5 5
/ 3.7 5.1 6 6.5
Estimated for the points located at the radius equal to 2.0 mm.
34
S.S.M. Tavares et al. / Materials Science and Engineering A358 (2003) 32 /36
Fig. 1. X-ray diffractogram of the cold rolled sample (sample A).
than about 5%, which is near the limit of detection by Xray diffraction. This observation leads to conclude that the increase of o phase with the deformation by the HPT was not from the austenite phase. The o phase must be formed by a reaction a?0/o. Such a bcc 0/hcp transition has already been observed in iron under ultra-high pression (/15 GPa) by Jamielson and Lawson [11]. The 304L stainless steel, however, seems to be much more susceptible to this transformation than pure iron, since it occurred under pressures as low as 2 GPa in this work. The a?0/o transition can be considered as martensitic like transformation, since it occurs by a diffusionless mechanism. This process occurs with an important volume contraction, which is consequence of the hydrostatic pressure component.
Table 3 shows the microhardness values measured in the samples A, B, D and E. Initially, the deformation by HPT promotes a strong hardening effect (sample B, 546HV). However, a softening process occurs for higher strains (samples D and E), which may be associated the increase of phase o and the decrease of martensite a?.
4. Conclusions The X-ray diffraction and the magnetic measurements showed that the SPD by HPT after cold rolling promotes the formation of the hcp o phase in the AISI 304L steel. The amount of this phase was maximum in the most deformed sample (n/8 and P /5 GPa) and
Fig. 2. X-ray diffractogram of sample B (n/0.5, P /2 GPa).
S.S.M. Tavares et al. / Materials Science and Engineering A358 (2003) 32 /36
Fig. 3. X-ray diffractogram of sample C (n /2, P /5 GPa).
Fig. 4. X-ray diffractogram of sample D (n/5, P /5 GPa).
Fig. 5. X-ray diffractogram of sample E (n/8, P /5 GPa).
35
36
S.S.M. Tavares et al. / Materials Science and Engineering A358 (2003) 32 /36 Table 2 Magnetic properties of samples A, B, C, D and E Sample
ms (A m 2 kg) 1
Hc (kA m 1)
Br (T)
A B C D E
136.0 129.8 97.6 99.4 78.3
13.2 13.4 16.4 15.9 19.0
0.99 0.58 0.73 0.78 0.55
Table 3 Microhardness values of samples A, B, D and E Sample
Microhardness
A /cold rolled B /HPT /n /0.5, P /2 GPa D /HPT /n/5, P /5 GPa E /HPT /n /8, P /5 GPa
453 546 460 453
References Fig. 6. Comparison between samples B, C, D and E, showing the evolution of the peak o0 0 2 with the increase of strain degree by HPT.
this sample presented a minimum value of magnetization saturation. It is found that the o phase forms by the reaction a?0/o.
[1] [2] [3] [4] [5] [6]
Acknowledgements
[7] [8]
The authors would like to acknowledge the Brazilian research agencies CNPq (470385/01-4 NV) and FAPERJ (E-26/150.365/2002) for financial support and R.P. da Silva for the help with X-ray diffraction analysis.
[9] [10] [11]
P.L. Mongonon, G. Thomas, Met. Trans. 1 (1970) 1587. V. Seetharaman, R. Krishnan, J. Mater. Sci. 16 (1981) 523. D.C. Cook, Met. Trans. 18A (1987) 201. S.S.M. Tavares, D. Fruchart, S. Miraglia, J. Alloys Comp. 307 (2000) 311. J. Childress, S.H. Liou, C.L. Chien, J. de Physique C8 (Suppl. 12 T.49) (1988) 113. S. Takahashi, J. Echigoya, T. Ueda, X. Li, H. Hatafuku, J. Mat. Proc. Tech. 198 (2001) 213. S.S.M. Tavares, M.R. da Silva, J.M. Neto, A. Miraglia, D. Fruchart, J. Magn. Magn. Mater. 242 /245 (2002) 1391. F. Gauzzi, R. Montanari, G. Principi, A. Perin, M.E. Tata, Mat. Sci. Eng. A 273 /275 (1999) 443. R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2) (2000) 103 /184. A.G. Popov, D.V. Gunderov, V.V. Stolyarov, J. Magn. Magn. Mater. 157/158 (1996) 33. J.C. Jamielson, A.W. Lawson, J. Appl. Phys. 33 (1962) 776.