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Evaluation of the effect of grain size on chromium carbide precipitation and intergranular corrosion of 316L stainless steel
Corrosion Science 66 (2013) 211–216
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Evaluation of the effect of grain size on chromium carbide precipitation and intergranular corrosion of 316L stainless steel Shu-Xin Li ⇑, Yan-Ni He, Shu-Rong Yu, Peng-Yi Zhang School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China
a r t i c l e
i n f o
Article history: Received 27 July 2012 Accepted 18 September 2012 Available online 27 September 2012 Keywords: A. Stainless steel B. Polarization C. Intergranular corrosion
a b s t r a c t The effect of grain size on chromium carbide precipitation and sensitization of 316L stainless steel was investigated based on inspection of microstructures and electrochemical potentiokinetic reactivation test. Various grain size samples were produced by heat treating the base material at 1100 °C for different durations. The result showed that chromium carbide precipitations were much delayed in larger grains. The degree of sensitization decreases with increasing grain size. The diffusion bonded joint has good resistance to intergranular corrosion due to the coarsened grains and the increased percentage of low energy coincident site lattice, while the grain coarsening is the main contribution. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Failures due to intergranular corrosion and intergranular stress corrosion cracking in structures and components of austenitic stainless steel materials have always been a tough problem in engineering practice. Effort has been made by reducing carbon content, changing chemical composition, increasing grain size and introducing appropriate strain to shift the time–temperature–sensitization curve to the right, thus increasing the time of sensitization and desensitization process and leading to an increase in intergranular corrosion resistance. Researchers have been extensively conducted on sensitization of austenitic stainless steel in terms of testing methods [1–10], influences of strain and strain state [11–14] and grain size [15–17] on carbide precipitation, aging condition on chromium carbide precipitation [18] and models for prediction of chromium depletion [19,20]. Also, increasing coincident site lattice (CSL) in austenitic stainless steels by grain boundary engineering has been an effective way to enhance resistance to intergranular corrosion [21–27]. Among these studies with regard to the effect of grain size on sensitization of austenitic stainless steels, most of them dealt with 304 stainless steel. Only a few talks about the effect of grain size on intergranular corrosion in 316 stainless steel. It has been proved [17] that the carbide precipitation and sensitization in the deformed 304 stainless steel are different from those in the deformed 316 stainless steel because a high volume fraction of strain-induced, a0 -martensite in 304 can nucleate a 2-phase (a0 / ⇑ Corresponding author. School of Petrochemical Engineering, Lanzhou University of Technology, 287 Langongping Road, Lanzhou 730050, China. Tel.: +86 931 2973647; fax: +86 931 2973648. E-mail address: [email protected] (S.-X. Li). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.09.022
c) fine-grained microstructure, which promotes a very rapid precipitation of carbides. While this 2-phase microstructure is generally absent in 316 stainless steel. Any processing that changes the reactivity of a surface will cause a change in corrosion response of austenitic stainless steels. In diffusion bonding of 316L stainless steel, the bonded joint was kept in the furnace at 1100 °C under 10 MPa for 3 h. The long exposure at high temperature promoted severe grain coarsening in the diffusion bonded joint. The joint experienced the temperature range 900–450 °C for around 45 min during the cooling process, which could increase the sensitization susceptibility for austenitic stainless steels and thus the corrosion response would be changed correspondingly. However, the previous work [28] indicated that the diffusion bonded joint has much less susceptibility to intergranular corrosion than the base material although suffered long exposure to the sensitization temperature. No chromium carbide precipitation was observed in the joint even after 100 h0 treatment at 650 °C. It showed very low susceptibility to intergranular corrosion in electrochemical potentiokinetic reactivation tests due to coarsened grains and increased percentage of twin-induced low CSL boundaries in the joint. However, the degree of sensitization in electrochemical test is the result of combined effect of microstructural texture, grain size and electrochemical parameters. In a given electrochemical condition, how the grain size affects the electrochemical behavior of intergranular corrosion and to what extent the sensitization susceptibility changes with the grain size have not been clearly understood for 316L stainless steel. Therefore, in the present study, the effect of the grain size on susceptibility to intergranular corrosion of 316L stainless steel was studied. Samples with various grain sizes were produced by
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heat-heating the base material at 1100 °C for different durations and then sensitized at 650 °C. Double loop electrochemical potentiokinetic reactivation test (DLEPR) has been successfully employed to characterize the degree of sensitization of austenitic stainless steel [29–36]. Thus, in this study, the DLEPR test was conducted on samples of various grain sizes of 316L stainless steel and the diffusion bonded joint to evaluate their intergranular corrosion behavior. Microscopic structures were inspected after sensitization treatment and DLEPR tests.
2. Experimental procedure 2.1. Diffusion bonding of 316L austenitic stainless steel As received cold-worked 316L stainless steel with the following chemical composition in wt.%: C-0.01, Si-0.41, Mn-1.41, P-0.036, S0.006, Ni-12.43, Cr-17.84, Mo-2.16, Fe-balance was used in this study. 316L stainless steel was diffusion bonded at temperature of 1100 °C under pressure of 10 MPa for holding time of 3 h in vacuum of 0.00133 Pa. The whole bonding process was conducted under vacuum and the joint was cooled down in the furnace from 1100 °C to the room temperature after 3 h holding time. The joint experienced the temperature range of 850–450 °C for around 45 min during the cooling process. The inspection of the interface and the mechanical properties of the bonded joint can be seen in the previous study [37].
3. Results and discussion 3.1. Microstructures of samples with various grain sizes Fig. 1 shows the microstructures of samples with various grain sizes. Clearly, the grain size increased as the heat-treatment duration increased from 0.5 to 3 h. The maximum and the average grain sizes were measured using an image analysis software and are listed in Table 1. The grains grew initially from 46 to 55 lm at 0.5 h and then increased dramatically after 1 h duration and the average grain size doubled at 1.5 h. As the duration increased to 3 h, the average grain size reached 173 lm, four times larger than the base material. It can also be seen from the microstructures that the longer the duration, the more unevenly distributed the grain size. The huge and the tiny grains coexist in the sample, Fig. 1(d). Also included in Table 1 is the grain size of the diffusion bonded joint. It should be noted that, different from the base material heattreated at 1100 °C for 3 h, the diffusion bonded joint experienced 1100 °C for 3 h but with 10 MPa pressure applied on the joint. Grains in the diffusion bonded joint were dramatically coarsened with the maximum grain size of 605 lm and average grain size of 220 lm, larger than the base material heat-treated for 3 h with the average grain size of 173 lm. In addition, a large number of twins were found in both heat-treated samples and the diffusion bonded joint. 3.2. Effect of the grain size on chromium carbide precipitations at grain boundaries
2.2. Preparation of various grain sizes samples Grain size variation was achieved by heat-treating the 316L stainless steel base material at 1100 °C for various durations ranging from 0.5 to 3 h, then aged at the sensitization temperature of 650 °C for durations of 2, 8, 30, 50 and 100 h. Samples were grounded and polished and then etched with chloroazotic acid to reveal the microstructure. Detailed microstructure inspection was conducted on these samples with optical microscope. 2.3. DLEPR test procedure 10 10 10 mm samples were cut from the diffusion bonded joint and the heat treated samples for DLEPR tests. Samples were mounted in epoxy resin with brass rod welded to the surface and then successively ground to 1000 grit abrasive paper. The working solution was 1.0 mol/L H2SO4 + 0.003 mol/L Na2S4O6. The electrochemical testing system consists of three electrodes (saturated calomel electrode (SCE) as reference electrode and graphite electrode as auxiliary electrode) and Powersuite software which was used to process electrochemical data. Specimen was immersed into the solution for 5–10 min to obtain open circuit potential (about 430 mV (SCE)). The specimen was started to polarize from a potential of 500 mV (SCE) and kept at this potential for 2 min, followed by anodic polarization from 500 mV (SCE) to the reverse potential 400 mV (SCE). Then the specimen was cathodically polarized to the open circuit potential. The degree of sensitization can be characterized by determining the reactivation ratio Rr (ir/ia) in polarization curve. ir is the maximum current density of the reverse scan (cathodic) and ia is the maximum current density of the forward (anodic) scan. The higher the ratio, the higher the degree of sensitization. The test was performed with the reverse potential of 400 mV (SCE), scan velocity of 1.111 mV/s, solution temperature of 40 °C and 1.0 mol/L H2SO4 + 0.003 mol/L Na2S4O6. Rr was the average of results of three samples which were tested using the same electrode.
The author’s previous study on the sensitization of the base material and the diffusion bonded joint of 316L stainless steel has showed [28] that chromium carbide precipitations could be seen clearly in the base material after 8 h treatment at 650 °C while almost no precipitations were observed in the diffusion bonded joint after 100 h. One of the main reasons contributed to the great improvement of intergranular corrosion resistance of the joint is the coarsened grains. Fig. 2 shows the microstructures of sensitized samples of various grain sizes. A number of chromium carbides precipitation were found in the sample of the average grain size of 55 lm for 20 h durations, Fig. 2(a), but no precipitations were observed at 8 h. As mentioned above that the base material of the average grain size of 46 lm sensitized at 8 h, having less sensitization time than the sample of 55 lm. When the grain size increased to 77 lm, chromium carbides started to precipitate after 20 h and were clearly seen at 30 h, shown in Fig. 2(b). But for the samples with the average grain sizes of 89 and 173 lm in Fig. 2 (c) and (d) respectively, only a few chromium carbide precipitates were found at 50 and 100 h durations. The grain size affects the time to start the sensitization and to reach the complete sensitization. Increasing the grain size can dramatically delay the onset of sensitization as larger grains have much wider chromium depleted-zones and lower grain boundary chromium concentrations. The study on measuring the sensitization rates and M23C6 precipitation behavior over a range of grain sizes from 15 to 150 lm in 304 stainless steel also showed [13] that the sensitization process is further accelerated as the grain size decreases. The precipitation of Cr-rich carbides and distribution of chromium concentration by cellular automaton simulation indicated [38] that the precipitation of Cr-rich carbides of the large grain microstructure is less than that of the small grain microstructure. 3.3. Effect of the grain size on the electrochemical behavior Typical DLEPR polarization curves for samples of various grain sizes were presented for 20 h in Fig. 3(a) and 50 h in Fig. 3(b).
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Fig. 1. Microstructures of samples with various grain sizes produced by heat treating the base material at 1100 °C for various durations (a) Base material (0 h) (b) 0.5 h (c) 2 h (d) 3 h.
Table 1 The maximum and the average grain sizes of samples. No.
BM
1
2
3
4
5
6
DBJ
Durations at 1100 °C (h) Max grain size Dmax (lm) Average grain size Dav (lm)
0 187 46
0.5 202 55
1.0 285 77
1.5 296 89
2.0 356 110
2.5 389 145
3.0 407 173
3.0 605 220
Note: BM represents the base material; DBJ represents the diffusion bonded joint.
Fig. 2. Microstructures of samples with various grain sizes sensitized at 650 °C for various durations of (a) 55 lm, 20 h (b) 77 lm, 30 h, (c) 89 lm, 50 h (d) 173 lm, 100 h.
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Fig. 4. The change of Rr with the average grain size for various samples sensitized at 650 °C for 100 h, 50 h, 20 h and 2 h.
Fig. 3. Comparison of DLEPR polarization curves for various grain sizes (a) Grain sizes of 77 lm and 173 lm sensitized at 650 °C for 20 h (b) Grain sizes of 55 lm and 145 lm sensitized at 650 °C for 50 h.
The reactivation ratio Rr decreased from 0.1114 to 0.0821 with increasing grain size from 77 to173 lm in Fig. 3(a), indicating that the bigger the grain size, the less the susceptibility to intergranular corrosion is. The same tendency of Rr exists in Fig. 3(b) for 50 h, the reactivation ratio Rr decreased from 0.1936 to 0.0988 with increasing grain size from 55 to145 lm. The DLEPR results are consistent with the microstructure inspections. The change of Rr with the average grain size was plotted in Fig. 4. As can be seen that the biggest Rr always occurred in the smallest grain size at all heat-treat durations. The samples sensitized at 2 and 20 h follow the trend of Rr decreasing with increasing grain size except for the grain of 110 lm at 2 h. But as the grain size increased, the scatter exits in the data and it is hard to follow this trend. This is especially true in the 100 h’ sensitized samples. The biggest and the smallest Rr happen at the grains of 55 and 77 lm respectively, while the rest of Rr scatter around 0.1530. This can be explained that as the grain size is greater than 100 lm, it is getting harder for the chromium carbide to precipitate on the grain boundaries and more time is needed for the precipitation, as shown in Fig. 2(c) and (d) in which there are only a few precipitations observed on the samples. On the other hand, the grains are becoming even more unevenly distributed as the grain size exceeds 100 lm, which leads to a big difference in grain size distribution in the electrochemical sample. Each Rr reflects the
electrochemical response of the sample, so the bigger the grains in the sample, the smaller Rr is, causing some error in testing data. An attempt was made to set up a relationship between the grain size and the reactivation ratio Rr at the given condition. But it is difficult to find as the Rr is the result of combined effect of the testing parameters (such as the exposure time, temperature and scan velocity, etc.), the grain size and the texture of the material. A small variation in these parameters may result in a big change in Rr, especially the reactivation potential and the solution temperature [28]. Also, the Rr varies with the sensitization durations as shown in Fig. 4. In fact, effort has been made to develop a relationship between the grain size and the corrosion resistance [15], but hard to follow a specific relationship. Ralston and Birbilis [39] intended to set up a Hall–Petch relationship between grain size and corrosion response but failed due to the inherent difficulty of measuring the true grain size from two dimensional images and the effect of grain refinement processing on corrosion response. Comparing the sample heat-treated at 1100 °C for 3 h (the average grain size of 173 lm) with the diffusion bonded joint of 220 lm, it can be seen that the latter has smaller Rr than the former with the exception of 100 h sensitized sample. Also a few precipitations were found in the 3 h heat-treated sample in Fig. 2(d), while almost no chromium carbides precipitated at grain boundaries in the diffusion bonded joint after 100 h, indicating that the diffusion bonded joint has less susceptibility to intergranular corrosion than the 3 h heat-treated base material at 1100 °C. Fig. 4 also tells the effect of the sensitization duration on the degree of sensitization. As the sensitization time increases, Rr increases correspondingly. This is easily understood as the longer exposure to the sensitization temperature, the more chromium carbides precipitate. 3.4. Effect of the texture on intergranular corrosion It is generally accepted that deformation has a big effect on electrochemical response of austenitic stainless steels. Although the stain in the 316L stainless steel diffusion bonded joint is no more than 0.05% [37], the texture of the joint has been changed due to the deformation in the diffusion bonding process. As illustrated above that the sample heat-treated at 1100 °C for 3 h (0% strain) has less resistance to intergranular corrosion than the diffusion bonded joint on which 10 MPa pressure was applied in diffusion bonding process. It is difficult to isolate the grain size effect
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Fig. 5. Microstructures of samples after the DLEPR test for various grain sizes (a) 55 lm (b) 77 lm (c) 110 lm (d) 173 lm.
from other micro-structural changes which are caused by the thermomechanical processing used to achieve variation in grain size [15]. Compared to the strain of ranging from 5% up to 20% which greatly changes the texture of the austenitic stainless steels [12,13,16], the small strain of 0.05% in the diffusion bonded joint can be neglected. But this small deformation which was to accelerate the diffusion of atoms did promote grain coarsening and an inP crease of twin boundaries proportion. The frequency of 3 CSL boundaries of 316L diffusion bonded joint is 59.4%, higher than P that of the base material of 36% and the 3 CSL boundaries dominated the whole joint with about 96.5% [28]. The fraction of lowCSL boundaries of 316 stainless steel for 1100 °C solution treatment is only 28.8% in Yu and Chen’s study [38], much less than that of the diffusion bonded joint as their sample was only solution treated without applying mechanical force, in which the grain size increased but with no apparent twins produced. Therefore, it can be concluded that both the grain coarsening and the increased amount of twin-induced grain boundaries are contributed to the increase of intergranular corrosion resistance of the 316L diffusion bonded joint. But compared with the increased amount of twin-induced grain boundaries, the grain coarsening is the main contribution. Many studies focused on improving the intergranular corrosion resistance of austenitic stainless steels by changing the texture of austenitic stainless steel through grain boundary engineering [21–27], such as introducing the strain to increase the percentage of low energy grain boundary and twin boundaries. However, the study in the present paper indicates that increasing grain size at an optimum level could also be a way to reduce the susceptibility to intergranular corrosion resistance for 316L stainless steel. But increasing the grain size will lead to a decrease in ductility. It is a big challenge to obtain optimized grain size for improvement of intergranular corrosion resistance without losing its good mechanical properties.
sample of the average grain size of 55 lm in Fig. 5(a), that it was almost entirely sensitized with large amount of chromium carbides precipitated at grain boundaries. As the grain size increased, the precipitations could be clearly seen but only parts of grains were sensitized in the sample of 77 lm in Fig. 5(b) and slight intergranular corrosion was observed in the sample of 110 lm, Fig. 5(c). While no intergranular corrosion occurred in the sample of 173 lm, Fig. 5(d). The microstructure analysis is consistent with the reactivation result. The DLEPR results provide the further evidence that the bigger the grain size of 316L stainless steel, the less susceptibility to the intergranular corrosion resistance.
3.5. Microstructures of samples of various grain sizes after DLEPR test
The authors are grateful for the supports provided by China Natural Science Foundation (No. 50805072) and PHD foundation of Lanzhou University of Technology (SB05200801) for which due acknowledgement is given.
Fig. 5 presents the microstructures of various grain sizes sensitized at 650 °C for 50 h after the DLEPR test. It can be seen from the
4. Conclusions The effect of the grain size on the intergranular corrosion of 316L stainless steel was investigated. Both the DLEPR tests and microstructure inspections showed that the 316L stainless steel has less susceptibility to intergranular corrosion as the grain size increased. Samples with the average grain size above 89 lm were found to have only a few chromium carbide precipitations on grain boundaries. The long duration at high temperature and the small strain caused the grain coarsening and the increase in the amount of the twin boundaries, leading to the great improvement of intergranular corrosion resistance of the 316L diffusion bonded joint. But the grain coarsening is the main contribution. The study in the present paper suggests that increasing grain size at an optimum level could be an effective way to increase the intergranular corrosion resistance of 316L stainless steel, but it is a big challenge to obtain optimized grain size for improvement of corrosion resistance without losing its good mechanical properties.
Acknowledgements
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Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Evaluation of the effect of grain size on chromium carbide precipitation and intergranular corrosion of 316L stainless steel Shu-Xin Li ⇑, Yan-Ni He, Shu-Rong Yu, Peng-Yi Zhang School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China
a r t i c l e
i n f o
Article history: Received 27 July 2012 Accepted 18 September 2012 Available online 27 September 2012 Keywords: A. Stainless steel B. Polarization C. Intergranular corrosion
a b s t r a c t The effect of grain size on chromium carbide precipitation and sensitization of 316L stainless steel was investigated based on inspection of microstructures and electrochemical potentiokinetic reactivation test. Various grain size samples were produced by heat treating the base material at 1100 °C for different durations. The result showed that chromium carbide precipitations were much delayed in larger grains. The degree of sensitization decreases with increasing grain size. The diffusion bonded joint has good resistance to intergranular corrosion due to the coarsened grains and the increased percentage of low energy coincident site lattice, while the grain coarsening is the main contribution. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Failures due to intergranular corrosion and intergranular stress corrosion cracking in structures and components of austenitic stainless steel materials have always been a tough problem in engineering practice. Effort has been made by reducing carbon content, changing chemical composition, increasing grain size and introducing appropriate strain to shift the time–temperature–sensitization curve to the right, thus increasing the time of sensitization and desensitization process and leading to an increase in intergranular corrosion resistance. Researchers have been extensively conducted on sensitization of austenitic stainless steel in terms of testing methods [1–10], influences of strain and strain state [11–14] and grain size [15–17] on carbide precipitation, aging condition on chromium carbide precipitation [18] and models for prediction of chromium depletion [19,20]. Also, increasing coincident site lattice (CSL) in austenitic stainless steels by grain boundary engineering has been an effective way to enhance resistance to intergranular corrosion [21–27]. Among these studies with regard to the effect of grain size on sensitization of austenitic stainless steels, most of them dealt with 304 stainless steel. Only a few talks about the effect of grain size on intergranular corrosion in 316 stainless steel. It has been proved [17] that the carbide precipitation and sensitization in the deformed 304 stainless steel are different from those in the deformed 316 stainless steel because a high volume fraction of strain-induced, a0 -martensite in 304 can nucleate a 2-phase (a0 / ⇑ Corresponding author. School of Petrochemical Engineering, Lanzhou University of Technology, 287 Langongping Road, Lanzhou 730050, China. Tel.: +86 931 2973647; fax: +86 931 2973648. E-mail address: [email protected] (S.-X. Li). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.09.022
c) fine-grained microstructure, which promotes a very rapid precipitation of carbides. While this 2-phase microstructure is generally absent in 316 stainless steel. Any processing that changes the reactivity of a surface will cause a change in corrosion response of austenitic stainless steels. In diffusion bonding of 316L stainless steel, the bonded joint was kept in the furnace at 1100 °C under 10 MPa for 3 h. The long exposure at high temperature promoted severe grain coarsening in the diffusion bonded joint. The joint experienced the temperature range 900–450 °C for around 45 min during the cooling process, which could increase the sensitization susceptibility for austenitic stainless steels and thus the corrosion response would be changed correspondingly. However, the previous work [28] indicated that the diffusion bonded joint has much less susceptibility to intergranular corrosion than the base material although suffered long exposure to the sensitization temperature. No chromium carbide precipitation was observed in the joint even after 100 h0 treatment at 650 °C. It showed very low susceptibility to intergranular corrosion in electrochemical potentiokinetic reactivation tests due to coarsened grains and increased percentage of twin-induced low CSL boundaries in the joint. However, the degree of sensitization in electrochemical test is the result of combined effect of microstructural texture, grain size and electrochemical parameters. In a given electrochemical condition, how the grain size affects the electrochemical behavior of intergranular corrosion and to what extent the sensitization susceptibility changes with the grain size have not been clearly understood for 316L stainless steel. Therefore, in the present study, the effect of the grain size on susceptibility to intergranular corrosion of 316L stainless steel was studied. Samples with various grain sizes were produced by
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S.-X. Li et al. / Corrosion Science 66 (2013) 211–216
heat-heating the base material at 1100 °C for different durations and then sensitized at 650 °C. Double loop electrochemical potentiokinetic reactivation test (DLEPR) has been successfully employed to characterize the degree of sensitization of austenitic stainless steel [29–36]. Thus, in this study, the DLEPR test was conducted on samples of various grain sizes of 316L stainless steel and the diffusion bonded joint to evaluate their intergranular corrosion behavior. Microscopic structures were inspected after sensitization treatment and DLEPR tests.
2. Experimental procedure 2.1. Diffusion bonding of 316L austenitic stainless steel As received cold-worked 316L stainless steel with the following chemical composition in wt.%: C-0.01, Si-0.41, Mn-1.41, P-0.036, S0.006, Ni-12.43, Cr-17.84, Mo-2.16, Fe-balance was used in this study. 316L stainless steel was diffusion bonded at temperature of 1100 °C under pressure of 10 MPa for holding time of 3 h in vacuum of 0.00133 Pa. The whole bonding process was conducted under vacuum and the joint was cooled down in the furnace from 1100 °C to the room temperature after 3 h holding time. The joint experienced the temperature range of 850–450 °C for around 45 min during the cooling process. The inspection of the interface and the mechanical properties of the bonded joint can be seen in the previous study [37].
3. Results and discussion 3.1. Microstructures of samples with various grain sizes Fig. 1 shows the microstructures of samples with various grain sizes. Clearly, the grain size increased as the heat-treatment duration increased from 0.5 to 3 h. The maximum and the average grain sizes were measured using an image analysis software and are listed in Table 1. The grains grew initially from 46 to 55 lm at 0.5 h and then increased dramatically after 1 h duration and the average grain size doubled at 1.5 h. As the duration increased to 3 h, the average grain size reached 173 lm, four times larger than the base material. It can also be seen from the microstructures that the longer the duration, the more unevenly distributed the grain size. The huge and the tiny grains coexist in the sample, Fig. 1(d). Also included in Table 1 is the grain size of the diffusion bonded joint. It should be noted that, different from the base material heattreated at 1100 °C for 3 h, the diffusion bonded joint experienced 1100 °C for 3 h but with 10 MPa pressure applied on the joint. Grains in the diffusion bonded joint were dramatically coarsened with the maximum grain size of 605 lm and average grain size of 220 lm, larger than the base material heat-treated for 3 h with the average grain size of 173 lm. In addition, a large number of twins were found in both heat-treated samples and the diffusion bonded joint. 3.2. Effect of the grain size on chromium carbide precipitations at grain boundaries
2.2. Preparation of various grain sizes samples Grain size variation was achieved by heat-treating the 316L stainless steel base material at 1100 °C for various durations ranging from 0.5 to 3 h, then aged at the sensitization temperature of 650 °C for durations of 2, 8, 30, 50 and 100 h. Samples were grounded and polished and then etched with chloroazotic acid to reveal the microstructure. Detailed microstructure inspection was conducted on these samples with optical microscope. 2.3. DLEPR test procedure 10 10 10 mm samples were cut from the diffusion bonded joint and the heat treated samples for DLEPR tests. Samples were mounted in epoxy resin with brass rod welded to the surface and then successively ground to 1000 grit abrasive paper. The working solution was 1.0 mol/L H2SO4 + 0.003 mol/L Na2S4O6. The electrochemical testing system consists of three electrodes (saturated calomel electrode (SCE) as reference electrode and graphite electrode as auxiliary electrode) and Powersuite software which was used to process electrochemical data. Specimen was immersed into the solution for 5–10 min to obtain open circuit potential (about 430 mV (SCE)). The specimen was started to polarize from a potential of 500 mV (SCE) and kept at this potential for 2 min, followed by anodic polarization from 500 mV (SCE) to the reverse potential 400 mV (SCE). Then the specimen was cathodically polarized to the open circuit potential. The degree of sensitization can be characterized by determining the reactivation ratio Rr (ir/ia) in polarization curve. ir is the maximum current density of the reverse scan (cathodic) and ia is the maximum current density of the forward (anodic) scan. The higher the ratio, the higher the degree of sensitization. The test was performed with the reverse potential of 400 mV (SCE), scan velocity of 1.111 mV/s, solution temperature of 40 °C and 1.0 mol/L H2SO4 + 0.003 mol/L Na2S4O6. Rr was the average of results of three samples which were tested using the same electrode.
The author’s previous study on the sensitization of the base material and the diffusion bonded joint of 316L stainless steel has showed [28] that chromium carbide precipitations could be seen clearly in the base material after 8 h treatment at 650 °C while almost no precipitations were observed in the diffusion bonded joint after 100 h. One of the main reasons contributed to the great improvement of intergranular corrosion resistance of the joint is the coarsened grains. Fig. 2 shows the microstructures of sensitized samples of various grain sizes. A number of chromium carbides precipitation were found in the sample of the average grain size of 55 lm for 20 h durations, Fig. 2(a), but no precipitations were observed at 8 h. As mentioned above that the base material of the average grain size of 46 lm sensitized at 8 h, having less sensitization time than the sample of 55 lm. When the grain size increased to 77 lm, chromium carbides started to precipitate after 20 h and were clearly seen at 30 h, shown in Fig. 2(b). But for the samples with the average grain sizes of 89 and 173 lm in Fig. 2 (c) and (d) respectively, only a few chromium carbide precipitates were found at 50 and 100 h durations. The grain size affects the time to start the sensitization and to reach the complete sensitization. Increasing the grain size can dramatically delay the onset of sensitization as larger grains have much wider chromium depleted-zones and lower grain boundary chromium concentrations. The study on measuring the sensitization rates and M23C6 precipitation behavior over a range of grain sizes from 15 to 150 lm in 304 stainless steel also showed [13] that the sensitization process is further accelerated as the grain size decreases. The precipitation of Cr-rich carbides and distribution of chromium concentration by cellular automaton simulation indicated [38] that the precipitation of Cr-rich carbides of the large grain microstructure is less than that of the small grain microstructure. 3.3. Effect of the grain size on the electrochemical behavior Typical DLEPR polarization curves for samples of various grain sizes were presented for 20 h in Fig. 3(a) and 50 h in Fig. 3(b).
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Fig. 1. Microstructures of samples with various grain sizes produced by heat treating the base material at 1100 °C for various durations (a) Base material (0 h) (b) 0.5 h (c) 2 h (d) 3 h.
Table 1 The maximum and the average grain sizes of samples. No.
BM
1
2
3
4
5
6
DBJ
Durations at 1100 °C (h) Max grain size Dmax (lm) Average grain size Dav (lm)
0 187 46
0.5 202 55
1.0 285 77
1.5 296 89
2.0 356 110
2.5 389 145
3.0 407 173
3.0 605 220
Note: BM represents the base material; DBJ represents the diffusion bonded joint.
Fig. 2. Microstructures of samples with various grain sizes sensitized at 650 °C for various durations of (a) 55 lm, 20 h (b) 77 lm, 30 h, (c) 89 lm, 50 h (d) 173 lm, 100 h.
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Fig. 4. The change of Rr with the average grain size for various samples sensitized at 650 °C for 100 h, 50 h, 20 h and 2 h.
Fig. 3. Comparison of DLEPR polarization curves for various grain sizes (a) Grain sizes of 77 lm and 173 lm sensitized at 650 °C for 20 h (b) Grain sizes of 55 lm and 145 lm sensitized at 650 °C for 50 h.
The reactivation ratio Rr decreased from 0.1114 to 0.0821 with increasing grain size from 77 to173 lm in Fig. 3(a), indicating that the bigger the grain size, the less the susceptibility to intergranular corrosion is. The same tendency of Rr exists in Fig. 3(b) for 50 h, the reactivation ratio Rr decreased from 0.1936 to 0.0988 with increasing grain size from 55 to145 lm. The DLEPR results are consistent with the microstructure inspections. The change of Rr with the average grain size was plotted in Fig. 4. As can be seen that the biggest Rr always occurred in the smallest grain size at all heat-treat durations. The samples sensitized at 2 and 20 h follow the trend of Rr decreasing with increasing grain size except for the grain of 110 lm at 2 h. But as the grain size increased, the scatter exits in the data and it is hard to follow this trend. This is especially true in the 100 h’ sensitized samples. The biggest and the smallest Rr happen at the grains of 55 and 77 lm respectively, while the rest of Rr scatter around 0.1530. This can be explained that as the grain size is greater than 100 lm, it is getting harder for the chromium carbide to precipitate on the grain boundaries and more time is needed for the precipitation, as shown in Fig. 2(c) and (d) in which there are only a few precipitations observed on the samples. On the other hand, the grains are becoming even more unevenly distributed as the grain size exceeds 100 lm, which leads to a big difference in grain size distribution in the electrochemical sample. Each Rr reflects the
electrochemical response of the sample, so the bigger the grains in the sample, the smaller Rr is, causing some error in testing data. An attempt was made to set up a relationship between the grain size and the reactivation ratio Rr at the given condition. But it is difficult to find as the Rr is the result of combined effect of the testing parameters (such as the exposure time, temperature and scan velocity, etc.), the grain size and the texture of the material. A small variation in these parameters may result in a big change in Rr, especially the reactivation potential and the solution temperature [28]. Also, the Rr varies with the sensitization durations as shown in Fig. 4. In fact, effort has been made to develop a relationship between the grain size and the corrosion resistance [15], but hard to follow a specific relationship. Ralston and Birbilis [39] intended to set up a Hall–Petch relationship between grain size and corrosion response but failed due to the inherent difficulty of measuring the true grain size from two dimensional images and the effect of grain refinement processing on corrosion response. Comparing the sample heat-treated at 1100 °C for 3 h (the average grain size of 173 lm) with the diffusion bonded joint of 220 lm, it can be seen that the latter has smaller Rr than the former with the exception of 100 h sensitized sample. Also a few precipitations were found in the 3 h heat-treated sample in Fig. 2(d), while almost no chromium carbides precipitated at grain boundaries in the diffusion bonded joint after 100 h, indicating that the diffusion bonded joint has less susceptibility to intergranular corrosion than the 3 h heat-treated base material at 1100 °C. Fig. 4 also tells the effect of the sensitization duration on the degree of sensitization. As the sensitization time increases, Rr increases correspondingly. This is easily understood as the longer exposure to the sensitization temperature, the more chromium carbides precipitate. 3.4. Effect of the texture on intergranular corrosion It is generally accepted that deformation has a big effect on electrochemical response of austenitic stainless steels. Although the stain in the 316L stainless steel diffusion bonded joint is no more than 0.05% [37], the texture of the joint has been changed due to the deformation in the diffusion bonding process. As illustrated above that the sample heat-treated at 1100 °C for 3 h (0% strain) has less resistance to intergranular corrosion than the diffusion bonded joint on which 10 MPa pressure was applied in diffusion bonding process. It is difficult to isolate the grain size effect
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Fig. 5. Microstructures of samples after the DLEPR test for various grain sizes (a) 55 lm (b) 77 lm (c) 110 lm (d) 173 lm.
from other micro-structural changes which are caused by the thermomechanical processing used to achieve variation in grain size [15]. Compared to the strain of ranging from 5% up to 20% which greatly changes the texture of the austenitic stainless steels [12,13,16], the small strain of 0.05% in the diffusion bonded joint can be neglected. But this small deformation which was to accelerate the diffusion of atoms did promote grain coarsening and an inP crease of twin boundaries proportion. The frequency of 3 CSL boundaries of 316L diffusion bonded joint is 59.4%, higher than P that of the base material of 36% and the 3 CSL boundaries dominated the whole joint with about 96.5% [28]. The fraction of lowCSL boundaries of 316 stainless steel for 1100 °C solution treatment is only 28.8% in Yu and Chen’s study [38], much less than that of the diffusion bonded joint as their sample was only solution treated without applying mechanical force, in which the grain size increased but with no apparent twins produced. Therefore, it can be concluded that both the grain coarsening and the increased amount of twin-induced grain boundaries are contributed to the increase of intergranular corrosion resistance of the 316L diffusion bonded joint. But compared with the increased amount of twin-induced grain boundaries, the grain coarsening is the main contribution. Many studies focused on improving the intergranular corrosion resistance of austenitic stainless steels by changing the texture of austenitic stainless steel through grain boundary engineering [21–27], such as introducing the strain to increase the percentage of low energy grain boundary and twin boundaries. However, the study in the present paper indicates that increasing grain size at an optimum level could also be a way to reduce the susceptibility to intergranular corrosion resistance for 316L stainless steel. But increasing the grain size will lead to a decrease in ductility. It is a big challenge to obtain optimized grain size for improvement of intergranular corrosion resistance without losing its good mechanical properties.
sample of the average grain size of 55 lm in Fig. 5(a), that it was almost entirely sensitized with large amount of chromium carbides precipitated at grain boundaries. As the grain size increased, the precipitations could be clearly seen but only parts of grains were sensitized in the sample of 77 lm in Fig. 5(b) and slight intergranular corrosion was observed in the sample of 110 lm, Fig. 5(c). While no intergranular corrosion occurred in the sample of 173 lm, Fig. 5(d). The microstructure analysis is consistent with the reactivation result. The DLEPR results provide the further evidence that the bigger the grain size of 316L stainless steel, the less susceptibility to the intergranular corrosion resistance.
3.5. Microstructures of samples of various grain sizes after DLEPR test
The authors are grateful for the supports provided by China Natural Science Foundation (No. 50805072) and PHD foundation of Lanzhou University of Technology (SB05200801) for which due acknowledgement is given.
Fig. 5 presents the microstructures of various grain sizes sensitized at 650 °C for 50 h after the DLEPR test. It can be seen from the
4. Conclusions The effect of the grain size on the intergranular corrosion of 316L stainless steel was investigated. Both the DLEPR tests and microstructure inspections showed that the 316L stainless steel has less susceptibility to intergranular corrosion as the grain size increased. Samples with the average grain size above 89 lm were found to have only a few chromium carbide precipitations on grain boundaries. The long duration at high temperature and the small strain caused the grain coarsening and the increase in the amount of the twin boundaries, leading to the great improvement of intergranular corrosion resistance of the 316L diffusion bonded joint. But the grain coarsening is the main contribution. The study in the present paper suggests that increasing grain size at an optimum level could be an effective way to increase the intergranular corrosion resistance of 316L stainless steel, but it is a big challenge to obtain optimized grain size for improvement of corrosion resistance without losing its good mechanical properties.
Acknowledgements
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