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The grain size and special boundary dependence of corrosion resistance in 304 austenitic stainless steels
Materials Chemistry and Physics 163 (2015) 496e500
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The grain size and special boundary dependence of corrosion resistance in 304 austenitic stainless steels Lv Jinlong b, *, Luo Hongyun a, Liang tongxiang b a
Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Xueyuan Road 37, Beijing, 100191, PR China b Beijing Key Laboratory of Fine Ceramics, Institute of Nuclear and New Energy Technology, Tsinghua University, Zhongguancun Street, Haidian District, Beijing 100084, PR China
h i g h l i g h t s The cryogenic cold deformation facilitated grain refinement and reduced fraction of CSL. The grain refinement accelerated sensitization, even though the ratio of CSL increase. Higher DOS reduced corrosion resistance of 304 austenitic stainless steel.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2015 Received in revised form 20 July 2015 Accepted 1 August 2015 Available online 12 August 2015
The effect of cold rolling temperature on grain boundary characteristic of the 304 austenitic stainless steels in thermomechanical treatment was examined. Comparing to cold rolling at room temperature and annealing, the cryogenic cold deformation and annealing promoted grain refinement and reduced fraction of coincidence site lattice. Although more coincidence site lattice restrained sensitization, the grain refinement accelerated sensitization. The sensitization deteriorated corrosion resistance of 304 austenitic stainless steel obtained by thermomechanical treatment. © 2015 Elsevier B.V. All rights reserved.
Keywords: Metals Annealing Corrosion test Ageing Diffusion
1. Introduction The concept of grain boundary design and control was developed as grain boundary engineering (GBE) [1]. Grain boundary coincidence site lattice (CSL) boundaries were resistant to intergranular deterioration processes [2] and stress corrosion cracking [3]. This was mainly attributed to the lower free energy of special boundary [4]. The twin boundaries formed in the surface of mechanical attrition treated sample weren't susceptible to carbide precipitation because of their regular and coherent atomic structure and extreme low grain boundary energy compared with those of other grain boundaries [5]. However, recently, Nie et al [6] reported an unusual phenomenon in magnesium alloys and the
* Corresponding author. Tel.: þ86 10 89792737; fax: þ86 10 69771464. E-mail address: [email protected] (L. Jinlong). http://dx.doi.org/10.1016/j.matchemphys.2015.08.004 0254-0584/© 2015 Elsevier B.V. All rights reserved.
equilibrium segregation of solute atoms formed in coherent deformation twin boundaries. The grain boundary related properties were greatly enhanced by the frequency of CSL which was greatly increased by using proper thermo-mechanical treatments [7]. A notable advantage of the GBE process for the 304 stainless steel was obtained by one-step slight strain plus annealing process [8]. A very high frequency of CSL boundaries (86%) was introduced in 316 austenitic stainless steel and the resulting steel showed a remarkably high resistance to intergranular corrosion [9]. An attempt was made to correlate the degree of sensitization (DOS) with various microstructural parameters such as grain size and grain boundary nature in AISI 316LN austenitic stainless steel. No clear trend could be established between the individual parameters and DOS, while effective grain boundary energy (EGBE) showed clear trend with DOS [10]. The grain size also affects the subsequent performance of thermo-mechanical treatments. The study on measuring the sensitization rates and M23C6 precipitation behavior
L. Jinlong et al. / Materials Chemistry and Physics 163 (2015) 496e500
over a range of grain sizes from 15 to 150 mm in 304 stainless steel also showed that the sensitization process was accelerated as the grain size decreased [11]. The cellular automaton simulation indicated that the precipitation of Cr-rich carbides in the large grain microstructure was less than that in the small grain microstructure [12]. Li et al. [13] found that chromium carbide precipitations were much delayed in larger grains in 316L stainless steel and the DOS decreased with the increasing of grain size. At present an important purpose of the research GBE is tailoring optimized grain size and grain boundary characteristic to inhibit the formation of chromium rich carbides and improve corrosion resistance of austenitic stainless steel. The objective of the present work was to study the effect of cold rolling temperature on grain boundary characteristic distribution and grain size in thermomechanical treatment and corrosion resistance of 304 austenitic stainless steels. 2. Experimental The material used in this paper is a 304 stainless steel plate with a chemical compositions (wt.%) as follows: 0.05 C, 1.44 Mn, 0.6 Si, 17.8 Cr, 9.1 Ni, 0.035 P, 0.002 S, and balance Fe. The 8 mm sheet was solid solution annealed in 1050 C for 1 h in a vacuum tube and quenched in water to achieve chemical homogeneity. Some samples were rolled to 20% thickness reduction at room temperature. Cryogenic deformation was carried out after the sample was immersed in liquid-nitrogen and sample also was rolled to 20% thickness reduction. Then cold rolled samples were annealed in 950 C for 20 min. The samples for sensitization were heat-treated at 675 C for 2 h following water quenching. The frequency of CSL boundaries and grain boundary characteristic distribution (GBCD) were in ZEISS ULTRA 55 field emission gun SEM equipped with an HKL Technology Channel 5 EBSD detector. The frequency of CSL boundaries was cited as a percentage by length on the cross-section. Brandon's criterion [14] was adopted for the critical deviation in the grain boundary characterization P [15]. In the present study, grain boundaries with 29 were classified as CSL boundaries with low energy, the others being classified as random boundaries with high energy [16]. Cu Ka (0.154056 nm) radiation at 40 kV and 40 mA at 4 /min was used for X-ray diffraction (Rigaku Ultima IV) analysis. The samples were polished with 1000, 2000 and 3000 grit silicon carbide paper and 1.5 mm alumina slurry. The polished samples were ultrasonically cleaned in acetone and ethanol. Very high density graphite and a saturated calomel electrode (SCE) were used as the counter and the reference electrodes, respectively. All the potentials referred in this work were measured with respect to the value of SCE. Before each experiment the samples were cathodically polarized at 1.2 VSCE for 300 s. The electrochemical measurements were performed using CHI660B electrochemical station (Chenhua instrument Co. Shanghai, China) controlled by computer and software. The role of KSCN was to help to break the passive film during the reactivation cycle of the test. The double loop electrochemical potentiokinetic reactivation technique (DLEPR) was conducted using 0.5 M H2SO4 þ 0.01 M KSCN solution [17]. The DLEPR experiments were started after nearly steady state open circuit potential (OCP) had been reached (about 30 min), and the potential swept in the anodic direction at 1 mV s1 until the potential of 0.3 VSCE was reached, then the scan was reversed until the OCP. Electrochemical impedance spectroscopy (EIS) measurements were carried out in 3.5% NaCl solution at 0.2 VSCE for 1 h using a frequency range of 100 kHze10 mHz and with a 5 mV amplitude of the ac signal.
497
3. Results and discussion The microstructures shown in Fig. 1aec exhibit equiaxed grains characteristic of stainless steels after different thermo-mechanical treatments. The average sizes of solid solution annealed sample, room temperature rolling and cryogenic rolling and annealed samples are shown in Fig. 2. Deformation in lower temperature promoted a0 -martensite transformation of austenitic stainless steel [18] and grain refinement [19]. Wang et al. [20] reported that the deformation of 1Cr18Ni9Ti stainless steel at liquid-nitrogen temperature had a positive effect on developing high-angle boundaries and grain refinement. More strain induced a0 -martensites in cryogenic rolled sample in Fig. 1d promote grain refinement during reversed transformation by annealing. Fig. 2 shows that the influence of the different thermomechanical treatments on CSL. Firstly, Overall, the cold deformation and annealing significantly increase fraction of CSL. Secondly, the cryogenic cold deformation and annealing reduces fraction of CSL. Several methods can be used to improve ratio of CSL, firstly, reducing the stacking fault energy of materials. Secondly, reducing the deformation temperature, Thirdly, increasing the deformation rate. Fourthly, improving the thermal stability of deformation twins. In the present thermomechanical treatments, although low temperature rolling promotes deformation twins, high storage energy after cryogenic cold deformation facilitates movement of the twin boundary, which decreases ratio of CSL in the study. In face-centered cubic metal with high stacking fault energy, high ratio of CSL can be obtained by small deformation and high temperature annealing. As far as we know, the high proportion of CSL (above 80%) is only obtained in coarse grained metal. In the present study, the medium cold rolled level is used, which results in refined grain in the subsequent annealing process. Previous studies have suggested that grain refinement can improve the stacking fault energy, which inhibits the formation of high ratio of CSL. It is worth noting that nanotwins can only be formed in metal with the low stacking fault energy (for example, copper). However, stacking fault energy of austenitic stainless steel is medium level. Therefore, proportion of CSL is less than 80%. However, it is very significant to study the effect of the CSL ratio in the refined austenitic stainless steel on its the mechanical properties and corrosion resistance. Dependencies between current and potential are obtained on the base of the DLEPR measurements in Fig. 3a. The significant activation and reactivation peaks can be observed. The magnitude of the DOS is shown in Fig. 3b. Compared with solid solution sample, the grain size of sample by thermal mechanical process reduces by 64.6% and 79.69% for # 2 and # 3 samples, while CSL increases by 177% and 130%. The DOS increases by 36.11% and 94%, respectively. This means that grain refinement accelerates sensitization, even though the ratio of CSL increases. The EIS measurements were performed at 0.2 VSCE after sample is passivated for 1 h in 3.5% NaCl solution. The Nyquist plots in Fig. 4a are all composed of depressed semicircles. The smallest semicircle indicates deteriorated property of the passive film. A significant decrease of semicircle is observed in sample with the largest DOS. It is evident that a time constant appears on the basis of peak value of phase angle in Fig. 4b. The capacitive semiarc of # 1 sample is much larger than those of # 2 and # 3 samples. In contrast, the capacitive semiarc of # 3 sample is much smaller than that of # 2 sample. The EIS results are in good agreement with DOS, which indicates that the corrosion resistance of grain refined sample decreases in this solution. Fig. 5a shows MotteSchottky plots for the passive films formed on # 1 # 2 and # 3 samples in 3.5% NaCl solution. The passive film was formed at 0.2 VSCE for 1 h. The results show that there are
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L. Jinlong et al. / Materials Chemistry and Physics 163 (2015) 496e500
Fig. 1. The crystal orientation of EBSD maps in (a) # 1: Solid solution annealed sample (b) # 2: Cold rolling at room temperature and annealed sample, (c) # 3: Cryogenic rolling and annealed sample, respectively (d). X-ray diffraction patterns for solid solution sample, cold rolled sample at room temperature and cryogenic rolled sample.
Fig. 2. Distribution of the frequency of CSL boundaries in # 1, # 2 and # 3 samples.
changes on the slop of the MotteSchottky plots at a potential around 0.5 VSCE. This feature is evident in the case of the steel containing chromium. Based on MotteSchottky theory [21], the space charge capacitances of the n-type and p-type semiconductor are given by Eqs. (1) and (2), respectively 2 C 2 ¼ CH2 þ CSC ¼
2 kT E Efb εS ε0 qND e
(1)
2 C 2 ¼ CH2 þ CSC ¼
2 kT E Efb εS ε0 qNA e
(2)
type semiconductor, C2 versus E should be linear with a negative slope which is inversely proportional to the acceptor concentration. On the other hand, n-type semiconductor yields a positive slope which is inversely proportional to the donor concentration. The dielectric constant εs is assumed as 12 for the passive films on 304 stainless steels [21]. The 50 mV/s in the MotteSchottky test is selected and this sweeping rate is fast enough to satisfy the assumption of “frozen-in defect structure” in passive film [22]. The values of donor and acceptor concentrations in Fig. 5b indicate that the oxide layer behaves like the heavily doped semiconductors. As shown in Fig. 5b, the total of donor and acceptor concentrations in # 1 sample are much lower than those in # 2 and # 3 samples. Moreover, the donor and acceptor concentrations of # 3 sample are much more than those in # 2 sample. The passive film with a deficiency in metal ions or excess with cation vacancies generally behaves as p-type, while n-type is developed in the passive films either by cation transport through interstitial diffusion or by anion diffusion into the metal [23]. The n-type oxide films Fe2O3 and Fe(OH)3 are probably formed on the sample outer layer, while p-type oxide films Fe3O4 and Cr2O3, FeCr2O4 are probably formed on inner layer for 304 austenitic stainless steel in the solution [24,25]. The MotteSchottky results are in good agreement with EIS and DOS, which indicates that the corrosion property of 304 austenitic stainless steel decreases thermomechanical treatments in this solution. 4. Conclusions
where ε0 is the vacuum permittivity (8.854 1012 F m1), εs is the dielectric constant of the sample, e is the electron charge (1.6 1019 C), k is the Boltzmann constant (1.38 1023 J K1), ND and NA are the donor or acceptor concentrations, respectively. T is the absolute temperature and Efb is the flat-band potential. For p-
The present work investigated the effect cold rolling temperature on grain boundary characteristic distribution in thermomechanical treatment, grain size and corrosion resistance of 304 austenitic stainless steels. The main conclusions are as follows: 1. The cryogenic cold deformation and annealing facilitated grain refinement and reduced fraction of CSL for 304 austenitic stainless steel.
L. Jinlong et al. / Materials Chemistry and Physics 163 (2015) 496e500
499
Fig. 3. (a) Current changes versus activation and reactivation potential obtained on the basis of DLEPR measurements and (b) DOS for # 1, # 2 and # 3 samples. DOS ¼ (Ir/Ia) 100%.
Fig. 4. (a) Nyquist plots and (b) Bode plots of all samples after being passivated at 0.2 VSCE for 1 h.
Fig. 5. (a) The MotteSchottky plots of all samples after being passivated at 0.2 VSCE for 1 h, (b) The ND and NA of the tree samples.
2. The grain refinement accelerated sensitization of 304 austenitic stainless steel, even though the ratio of CSL increased. 3. Higher DOS reduced corrosion resistance of 304 austenitic stainless steel.
References [1] P. Lin, G. Palumbo, K.T. Aust, Scr. Mater. 36 (1997) 1145e1149. [2] M. Shimada, H. Kokawa, Z.J. Wang, Y.S. Sato, I. Karibe, Acta Mater. 50 (2002) 2331e2341.
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[3] L. Tan, T.R. Allen, J.T. Busby, J. Nucl. Mater. 441 (2013) 661e666. [4] U. Wolf, F. Emst, T. Muschik, M.W. Finnis, H.F. Fischmeister, Philos. Mag. A 66 (1992) 991e1016. [5] M. Laleh, F. Kargar, Mater. Lett. 65 (2011) 1935e1937. [6] J.F. Nie, Y.M. Zhu, J.Z. Liu, X.Y. Fang, Science 340 (2013) 957e960. [7] S. Xia, B.X. Zhou, W.J. Chen, W.G. Wang, Scr. Mater. 54 (2006) 2019e2022. [8] H.Y. Bi, H. Kokawa, Z.J. Wang, M. Shimada, Y.S. Sato, Scr. Mater. 49 (2003) 219e223. [9] M. Michiuchi, H. Kokawa, Z.J. Wang, Y.S. Sato, K. Sakai, Acta Mater. 54 (2006) 5179e5184. [10] N. Parvathavarthini, S. Mulki, R.K. Dayal, I. Samajdar, K.V. Mani, B. Raj, Corros. Sci. 51 (2009) 2144e2150. [11] E.A. Trillo, R. Beltran, J.G. Maldonado, Mater. Charact. 35 (1995) 99e112. [12] X. Yu, S. Chen, Y. Liu, F. Ren, Corros. Sci. 52 (2010) 1939e1947. [13] S.X. Li, Y.N. He, S.R. Yu, P.Y. Zhang, Corros. Sci. 66 (2013) 211e216. [14] D.G. Brandon, Acta Metall. 14 (1966) 1479e1482.
[15] [16] [17] [18] [19] [20] [21] [22]
[23] [24] [25]
H. Kokawa, T. Watanabe, S. Karashima, Scr. Metall. 21 (1987) 839e842. H. Kokawa, T. Watanabe, S. Karashima, Philos. Mag. A 44 (1981) 1239e1254. du, M.K. Aydinol, Corros. Sci. 48 (2006) 3565e3583. G.H. Aydog €nninen, Acta Mater. 55 (2007) 6108e6118. J. Talonen, H. Ha Y.S. Li, N.R. Tao, K. Lu, Acta Mater. 56 (2008) 230e241. T.S. Wang, J.G. Peng, Y.W. Gao, F.C. Zhang, T.F. Jing, Mater. Sci. Eng. A 407 (2005) 84e88. J. Amri, T. Souier, B. Malki, B. Baroux, Corros. Sci. 50 (2) (2008) 431e435. M.D.C. Belo, M.G.S. Fereira, C.A. Melendres, Electrochemical and Optical Techniques for the Study and Monitoring of Metallic Corrosion, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1991, p. 219. C. Sunseri, S. Piazza, F.D. Quarto, J. Electrochem. Soc. 137 (1990) 2411e2417. N.B. Hakiki, S. Boudin, B. Rondot, M. Da Cunha Belo, Corros. Sci. 37 (1995) 1809e1822. K. Azumi, T. Ohtsuka, N. Sato, J. Electrochem. Soc. 134 (1987) 1352e1357.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The grain size and special boundary dependence of corrosion resistance in 304 austenitic stainless steels Lv Jinlong b, *, Luo Hongyun a, Liang tongxiang b a
Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Xueyuan Road 37, Beijing, 100191, PR China b Beijing Key Laboratory of Fine Ceramics, Institute of Nuclear and New Energy Technology, Tsinghua University, Zhongguancun Street, Haidian District, Beijing 100084, PR China
h i g h l i g h t s The cryogenic cold deformation facilitated grain refinement and reduced fraction of CSL. The grain refinement accelerated sensitization, even though the ratio of CSL increase. Higher DOS reduced corrosion resistance of 304 austenitic stainless steel.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2015 Received in revised form 20 July 2015 Accepted 1 August 2015 Available online 12 August 2015
The effect of cold rolling temperature on grain boundary characteristic of the 304 austenitic stainless steels in thermomechanical treatment was examined. Comparing to cold rolling at room temperature and annealing, the cryogenic cold deformation and annealing promoted grain refinement and reduced fraction of coincidence site lattice. Although more coincidence site lattice restrained sensitization, the grain refinement accelerated sensitization. The sensitization deteriorated corrosion resistance of 304 austenitic stainless steel obtained by thermomechanical treatment. © 2015 Elsevier B.V. All rights reserved.
Keywords: Metals Annealing Corrosion test Ageing Diffusion
1. Introduction The concept of grain boundary design and control was developed as grain boundary engineering (GBE) [1]. Grain boundary coincidence site lattice (CSL) boundaries were resistant to intergranular deterioration processes [2] and stress corrosion cracking [3]. This was mainly attributed to the lower free energy of special boundary [4]. The twin boundaries formed in the surface of mechanical attrition treated sample weren't susceptible to carbide precipitation because of their regular and coherent atomic structure and extreme low grain boundary energy compared with those of other grain boundaries [5]. However, recently, Nie et al [6] reported an unusual phenomenon in magnesium alloys and the
* Corresponding author. Tel.: þ86 10 89792737; fax: þ86 10 69771464. E-mail address: [email protected] (L. Jinlong). http://dx.doi.org/10.1016/j.matchemphys.2015.08.004 0254-0584/© 2015 Elsevier B.V. All rights reserved.
equilibrium segregation of solute atoms formed in coherent deformation twin boundaries. The grain boundary related properties were greatly enhanced by the frequency of CSL which was greatly increased by using proper thermo-mechanical treatments [7]. A notable advantage of the GBE process for the 304 stainless steel was obtained by one-step slight strain plus annealing process [8]. A very high frequency of CSL boundaries (86%) was introduced in 316 austenitic stainless steel and the resulting steel showed a remarkably high resistance to intergranular corrosion [9]. An attempt was made to correlate the degree of sensitization (DOS) with various microstructural parameters such as grain size and grain boundary nature in AISI 316LN austenitic stainless steel. No clear trend could be established between the individual parameters and DOS, while effective grain boundary energy (EGBE) showed clear trend with DOS [10]. The grain size also affects the subsequent performance of thermo-mechanical treatments. The study on measuring the sensitization rates and M23C6 precipitation behavior
L. Jinlong et al. / Materials Chemistry and Physics 163 (2015) 496e500
over a range of grain sizes from 15 to 150 mm in 304 stainless steel also showed that the sensitization process was accelerated as the grain size decreased [11]. The cellular automaton simulation indicated that the precipitation of Cr-rich carbides in the large grain microstructure was less than that in the small grain microstructure [12]. Li et al. [13] found that chromium carbide precipitations were much delayed in larger grains in 316L stainless steel and the DOS decreased with the increasing of grain size. At present an important purpose of the research GBE is tailoring optimized grain size and grain boundary characteristic to inhibit the formation of chromium rich carbides and improve corrosion resistance of austenitic stainless steel. The objective of the present work was to study the effect of cold rolling temperature on grain boundary characteristic distribution and grain size in thermomechanical treatment and corrosion resistance of 304 austenitic stainless steels. 2. Experimental The material used in this paper is a 304 stainless steel plate with a chemical compositions (wt.%) as follows: 0.05 C, 1.44 Mn, 0.6 Si, 17.8 Cr, 9.1 Ni, 0.035 P, 0.002 S, and balance Fe. The 8 mm sheet was solid solution annealed in 1050 C for 1 h in a vacuum tube and quenched in water to achieve chemical homogeneity. Some samples were rolled to 20% thickness reduction at room temperature. Cryogenic deformation was carried out after the sample was immersed in liquid-nitrogen and sample also was rolled to 20% thickness reduction. Then cold rolled samples were annealed in 950 C for 20 min. The samples for sensitization were heat-treated at 675 C for 2 h following water quenching. The frequency of CSL boundaries and grain boundary characteristic distribution (GBCD) were in ZEISS ULTRA 55 field emission gun SEM equipped with an HKL Technology Channel 5 EBSD detector. The frequency of CSL boundaries was cited as a percentage by length on the cross-section. Brandon's criterion [14] was adopted for the critical deviation in the grain boundary characterization P [15]. In the present study, grain boundaries with 29 were classified as CSL boundaries with low energy, the others being classified as random boundaries with high energy [16]. Cu Ka (0.154056 nm) radiation at 40 kV and 40 mA at 4 /min was used for X-ray diffraction (Rigaku Ultima IV) analysis. The samples were polished with 1000, 2000 and 3000 grit silicon carbide paper and 1.5 mm alumina slurry. The polished samples were ultrasonically cleaned in acetone and ethanol. Very high density graphite and a saturated calomel electrode (SCE) were used as the counter and the reference electrodes, respectively. All the potentials referred in this work were measured with respect to the value of SCE. Before each experiment the samples were cathodically polarized at 1.2 VSCE for 300 s. The electrochemical measurements were performed using CHI660B electrochemical station (Chenhua instrument Co. Shanghai, China) controlled by computer and software. The role of KSCN was to help to break the passive film during the reactivation cycle of the test. The double loop electrochemical potentiokinetic reactivation technique (DLEPR) was conducted using 0.5 M H2SO4 þ 0.01 M KSCN solution [17]. The DLEPR experiments were started after nearly steady state open circuit potential (OCP) had been reached (about 30 min), and the potential swept in the anodic direction at 1 mV s1 until the potential of 0.3 VSCE was reached, then the scan was reversed until the OCP. Electrochemical impedance spectroscopy (EIS) measurements were carried out in 3.5% NaCl solution at 0.2 VSCE for 1 h using a frequency range of 100 kHze10 mHz and with a 5 mV amplitude of the ac signal.
497
3. Results and discussion The microstructures shown in Fig. 1aec exhibit equiaxed grains characteristic of stainless steels after different thermo-mechanical treatments. The average sizes of solid solution annealed sample, room temperature rolling and cryogenic rolling and annealed samples are shown in Fig. 2. Deformation in lower temperature promoted a0 -martensite transformation of austenitic stainless steel [18] and grain refinement [19]. Wang et al. [20] reported that the deformation of 1Cr18Ni9Ti stainless steel at liquid-nitrogen temperature had a positive effect on developing high-angle boundaries and grain refinement. More strain induced a0 -martensites in cryogenic rolled sample in Fig. 1d promote grain refinement during reversed transformation by annealing. Fig. 2 shows that the influence of the different thermomechanical treatments on CSL. Firstly, Overall, the cold deformation and annealing significantly increase fraction of CSL. Secondly, the cryogenic cold deformation and annealing reduces fraction of CSL. Several methods can be used to improve ratio of CSL, firstly, reducing the stacking fault energy of materials. Secondly, reducing the deformation temperature, Thirdly, increasing the deformation rate. Fourthly, improving the thermal stability of deformation twins. In the present thermomechanical treatments, although low temperature rolling promotes deformation twins, high storage energy after cryogenic cold deformation facilitates movement of the twin boundary, which decreases ratio of CSL in the study. In face-centered cubic metal with high stacking fault energy, high ratio of CSL can be obtained by small deformation and high temperature annealing. As far as we know, the high proportion of CSL (above 80%) is only obtained in coarse grained metal. In the present study, the medium cold rolled level is used, which results in refined grain in the subsequent annealing process. Previous studies have suggested that grain refinement can improve the stacking fault energy, which inhibits the formation of high ratio of CSL. It is worth noting that nanotwins can only be formed in metal with the low stacking fault energy (for example, copper). However, stacking fault energy of austenitic stainless steel is medium level. Therefore, proportion of CSL is less than 80%. However, it is very significant to study the effect of the CSL ratio in the refined austenitic stainless steel on its the mechanical properties and corrosion resistance. Dependencies between current and potential are obtained on the base of the DLEPR measurements in Fig. 3a. The significant activation and reactivation peaks can be observed. The magnitude of the DOS is shown in Fig. 3b. Compared with solid solution sample, the grain size of sample by thermal mechanical process reduces by 64.6% and 79.69% for # 2 and # 3 samples, while CSL increases by 177% and 130%. The DOS increases by 36.11% and 94%, respectively. This means that grain refinement accelerates sensitization, even though the ratio of CSL increases. The EIS measurements were performed at 0.2 VSCE after sample is passivated for 1 h in 3.5% NaCl solution. The Nyquist plots in Fig. 4a are all composed of depressed semicircles. The smallest semicircle indicates deteriorated property of the passive film. A significant decrease of semicircle is observed in sample with the largest DOS. It is evident that a time constant appears on the basis of peak value of phase angle in Fig. 4b. The capacitive semiarc of # 1 sample is much larger than those of # 2 and # 3 samples. In contrast, the capacitive semiarc of # 3 sample is much smaller than that of # 2 sample. The EIS results are in good agreement with DOS, which indicates that the corrosion resistance of grain refined sample decreases in this solution. Fig. 5a shows MotteSchottky plots for the passive films formed on # 1 # 2 and # 3 samples in 3.5% NaCl solution. The passive film was formed at 0.2 VSCE for 1 h. The results show that there are
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L. Jinlong et al. / Materials Chemistry and Physics 163 (2015) 496e500
Fig. 1. The crystal orientation of EBSD maps in (a) # 1: Solid solution annealed sample (b) # 2: Cold rolling at room temperature and annealed sample, (c) # 3: Cryogenic rolling and annealed sample, respectively (d). X-ray diffraction patterns for solid solution sample, cold rolled sample at room temperature and cryogenic rolled sample.
Fig. 2. Distribution of the frequency of CSL boundaries in # 1, # 2 and # 3 samples.
changes on the slop of the MotteSchottky plots at a potential around 0.5 VSCE. This feature is evident in the case of the steel containing chromium. Based on MotteSchottky theory [21], the space charge capacitances of the n-type and p-type semiconductor are given by Eqs. (1) and (2), respectively 2 C 2 ¼ CH2 þ CSC ¼
2 kT E Efb εS ε0 qND e
(1)
2 C 2 ¼ CH2 þ CSC ¼
2 kT E Efb εS ε0 qNA e
(2)
type semiconductor, C2 versus E should be linear with a negative slope which is inversely proportional to the acceptor concentration. On the other hand, n-type semiconductor yields a positive slope which is inversely proportional to the donor concentration. The dielectric constant εs is assumed as 12 for the passive films on 304 stainless steels [21]. The 50 mV/s in the MotteSchottky test is selected and this sweeping rate is fast enough to satisfy the assumption of “frozen-in defect structure” in passive film [22]. The values of donor and acceptor concentrations in Fig. 5b indicate that the oxide layer behaves like the heavily doped semiconductors. As shown in Fig. 5b, the total of donor and acceptor concentrations in # 1 sample are much lower than those in # 2 and # 3 samples. Moreover, the donor and acceptor concentrations of # 3 sample are much more than those in # 2 sample. The passive film with a deficiency in metal ions or excess with cation vacancies generally behaves as p-type, while n-type is developed in the passive films either by cation transport through interstitial diffusion or by anion diffusion into the metal [23]. The n-type oxide films Fe2O3 and Fe(OH)3 are probably formed on the sample outer layer, while p-type oxide films Fe3O4 and Cr2O3, FeCr2O4 are probably formed on inner layer for 304 austenitic stainless steel in the solution [24,25]. The MotteSchottky results are in good agreement with EIS and DOS, which indicates that the corrosion property of 304 austenitic stainless steel decreases thermomechanical treatments in this solution. 4. Conclusions
where ε0 is the vacuum permittivity (8.854 1012 F m1), εs is the dielectric constant of the sample, e is the electron charge (1.6 1019 C), k is the Boltzmann constant (1.38 1023 J K1), ND and NA are the donor or acceptor concentrations, respectively. T is the absolute temperature and Efb is the flat-band potential. For p-
The present work investigated the effect cold rolling temperature on grain boundary characteristic distribution in thermomechanical treatment, grain size and corrosion resistance of 304 austenitic stainless steels. The main conclusions are as follows: 1. The cryogenic cold deformation and annealing facilitated grain refinement and reduced fraction of CSL for 304 austenitic stainless steel.
L. Jinlong et al. / Materials Chemistry and Physics 163 (2015) 496e500
499
Fig. 3. (a) Current changes versus activation and reactivation potential obtained on the basis of DLEPR measurements and (b) DOS for # 1, # 2 and # 3 samples. DOS ¼ (Ir/Ia) 100%.
Fig. 4. (a) Nyquist plots and (b) Bode plots of all samples after being passivated at 0.2 VSCE for 1 h.
Fig. 5. (a) The MotteSchottky plots of all samples after being passivated at 0.2 VSCE for 1 h, (b) The ND and NA of the tree samples.
2. The grain refinement accelerated sensitization of 304 austenitic stainless steel, even though the ratio of CSL increased. 3. Higher DOS reduced corrosion resistance of 304 austenitic stainless steel.
References [1] P. Lin, G. Palumbo, K.T. Aust, Scr. Mater. 36 (1997) 1145e1149. [2] M. Shimada, H. Kokawa, Z.J. Wang, Y.S. Sato, I. Karibe, Acta Mater. 50 (2002) 2331e2341.
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