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Effects of pre-strain on sensitization and interganular corrosion for 304 stainless steel
Engineering Failure Analysis 106 (2019) 104179
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Effects of pre-strain on sensitization and interganular corrosion for 304 stainless steel
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Xuewen Zhang, Jianqun Tang , Hao Liu, Jianming Gong Jiangsu Key Lab of Design and Manufacture of Extreme Pressure Equipment, School of Mechanical and Power Engineering, Nanjing Tech University, Nanjing, 211816, China
A R T IC LE I N F O
ABS TRA CT
Keywords: 304 stainless steel Pre-strain Thermal aging Sensitization Intergranular corrosion
The degree of sensitization (DOS) and the susceptibility to interganular corrosion (IGC) were investigated for pre-strained 304 stainless steel (SS) following thermally aged at 750 °C for one hour. The maximum pre-strain applied to the steel is 20%.The microstructures of 304 SS subjected to pre-strain and thermal aging were observed. The volume fraction of strain-induced α'martensite and the vickers hardness were measured, respectively. The double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were used to characterize the DOS as a function of various pre-strain levels. The tests in copper‑copper sulfate‑sulfuric acid (16%) solution were performed to evaluate the susceptibility to IGC. The morphologies of the cracks caused by IGC were observed. The results show, although pre-strained 304SS had been thermally aged at 750 °C for one hour, strain-induced α'-martensite and shear bands in steel are most retained and increase with pre-strain, causing the strengthening effect and leading to the increase in hardness. The DOS and the susceptibility to IGC also increase with pre-strain. The higher the value of the DOS is, the more serious IGC becomes. Obvious intergranular cracks as well as the dissolution of grain boundaries and the grains began to appear in the steel with 5% pre-strain, and the attack had been deteriorated with the increasing pre-strain. When the pre-strain is reached 20%, the longest crack is about 250 μm and its width is about 38 μm, indicating that the severe IGC had occurred. No obvious reversion of martensite to austenite and desensitization occurred for 304 SS withstood the maximum pre-strain up to 20% and thermally aged at 750 °C for one hour.
1. Introduction 304 stainless steel (SS) belongs to typical 18Cre8Ni austenitic steel. Due to the combination of excellent corrosion resistance, good mechanical properties, great weldability and relatively low cost, 304 SS has been widely used in chemical, petrochemical, power and nuclear industries over a large temperature range. For this type stainless steel, chromium (Cr) is the necessary alloying element. The formation of the densely Cr-contained oxide film tightly bonded to the matrix endows it with excellent resistance to uniform corrosion. However, this steel is sensitive to local corrosions [1]. Intergranular corrosion (IGC) is one of these attacks, which usually occurs on the steels having been suffered from sensitization. For austenitic SS, sensitization usually refers to the precipitation of chromium carbides in the temperature range of 450 °C–850 °C, which results in the formation of less resistant Cr-depleted zones near the grain boundaries [2]. The Cr-depleted zone
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Corresponding author. E-mail address: [email protected] (J. Tang).
https://doi.org/10.1016/j.engfailanal.2019.104179 Received 13 March 2018; Received in revised form 24 August 2019; Accepted 1 September 2019 Available online 04 September 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
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is easily suffered from attack in aggressive environments, which will result in the occurrence of IGC that cracks initiate and propagate along the grain boundaries [3], even intergranular stress corrosion cracking (IGSCC) or transgranular SCC (TRSCC) under tensile stress [4–6]. The objects for studying the degree of sensitization (DOS) and IGC, however, mostly aimed at solution-annealed 304 SS following the sensitized treatment, and without considering the strain caused by residual stress, which does not correspond to the practical state. Actually, in the industries, many components will undergo plastic deformation during the process of cold working, assembling and welding (such as the fins of plate-fin heat exchanger), without the following solution-annealed treatment. A certain amount of strain caused by residual stress will be inevitably retained in steel. As the microstructure of 304 SS is metastable, the presence of the strain will lead to the changes in microstructures and properties as well as diffusion energy compared to the original solutionannealed state [7]. Some researchers have revealed that plastic deformation (strain and strain state in steel) will influence sensitization of austenitic SS. The changes in sensitization kinetics with increasing strain have been postulated to be the activation energy of Cr-diffusivity by dislocation density with strain, the decrease in the activation energy was an indication that strain accelerates sensitization development [8]. Brian et al. [9] revealed that deformation prior to sensitization greatly accelerates the kinetics of sensitization in 304 SS. For 316 SS, the transformation of induced-strain martensite also caused rapid sensitization [10]. Carbide precipitation becomes remarkably prevalent in the grain matrix with increase in the deformation, and cold deformation enhances the DOS as high as 65 times the un-deformed 304 SS at 500 °C [11]. Ramírez et al. [12] further confirmed that the deformed 316SS showed more carbide precipitates at the grain boundaries than did the un-deformed steel by transmission electron microscopy. Because deformation will lead to the changes in microstructures-properties and sensitization is a diffusion-assisted process, sensitization after pre-strain is influenced not only by strain level, but also by aging temperature and time as well as other factors. Chowdhury et al. [13] showed that the DOS of 304 SS was successfully characterized as a function of both prior deformation and sensitized temperatures at 400 °C, 500 °C and 600 °C. When the level of strain is low, the sensitization of 304 SS is controlled by lowering Cr diffusion rate in the matrix [14]. However, when the level of strain is higher, especially at higher aging temperature, the DOS may be reduced due to the desensitization for the increase in desensitization kinetics, but the occurrence of desensitization was needed the long duration to occur [15]. Trillo et al. [16] pointed that the sensitized 304 SS could undergo desensitization with aging time due to bulk Cr diffusion which eliminates the Cr-depleted condition, and the increasing strain decreases the aging time. Kina et al. [17] revealed that desensitization would occur at 750°C for sensitized 304 SS for 48 h and for solution-annealed steel for 175 h. So, aimed at the facts that 304 SS sheet is widely used in the industries (typical example is the fins of plate-fin heat exchanger) and 304 SS components are easily suffered from sensitization as well as the low residual strain during cold working, assembling and welding process, 304 SS sheetspecimens withstood residual strain up to 20% and following thermally aged at 750 °C for one hour are chose to study the effects of pre-strain on the sensitization and the susceptibility to IGC, which simulates the practical states that the 304 SS components may be suffered. In the present research, the residual strain is introduced into the steel by the means of pre-strain through uniaxial tensile test. By double loop electrochemical potentiokinetic reactivation (DL-EPR) tests and the tests in copper‑copper sulfate-sulfuric acid (16%) solution, the effects of pre-strain on the DOS and IGC of 304 SS were investigated. 2. Material and experimental procedures 2.1. Material and uniaxial tensile test The material investigated is commercial AISI 304 SS sheet with 0.8 mm thickness. The chemical composition of the steel was (in wt%): C 0.035, Mn 1.19, Ni 7.8, Si 0.412, P 0.028, S 0.057, Cr 18.98 and Fe balance. The as-received state of 304 SS was solution annealed at 1050 °C for one hour following water quenching. Flat tensile specimens (the length is 90 mm) with a gauge size of 30 mm × 10 mm × 0.8 mm were strained to 0%, 1%, 2%, 5%, 10% and 20% engineering strain at the strain rate of 1 × 10−3 s−1 using tensile testing machine (Instron5869). The pre-strained specimens were thermally aged at 750 °C for one hour following air cooling. The specimens of aged steel were electrolytically etched in 10% oxalic acid solution and the microstructures were observed by optical microscope (OM) (Zeiss AXIO Imager. A1m). 2.2. Characterization of pre-strained and aged steel Ferromagnetic volume change in austenitic SS due to pre-strain can be used to reflect the volume fraction change in straininduced martensite. So, the volume fraction of strained-induced martensite (ferromagnetic) was determined by a calibrated commercial Ferritscope (Diverse MF300F+), which is based on magnetic induction. The values of vickers hardness were measured by hardness tester (HXD-1000™).When volume fraction of martensite and the values of hardness were measured, the test points were as close as possible to the center of the gauge section. Four points are taken for each pre-strained specimen, and then the average value was taken. 2.3. Double loop electrochemical potentiokinetic reactivation (DL-EPR) test The DL-EPR test is the effective method to determine the DOS and is sensitive enough to detect the microstructural change in materials [18–20]. In the present research, the DL-EPR test was carried out in 0.5 mol/L H2SO4 + 0.01 mol/L KSCN solution with 2
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Fig. 1. Effect of pre-strain on the volume fraction of α'-martensite and vickers hardness.
Fig. 2. The microstructures of the specimens pre-strained by (a) 0%; (b) 1%; (c) 2%; (d) 10%; (e) 20%.
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Fig. 3. The microstructures in Fig. 2 at larger magnification for specimens pre-strained by (a) 1%; (b) 2%; (c) 10%; (d) 20%.
traditional three-electrode cell using Solatron 1280B electrochemical workstation. A piece of platinum was used as the counter electrode and a saturated calomel electrode (SCE) was chosen as the reference electrode. The working electrode was cut from the gauge section of the tensile specimen. Non-working surface was sealed by resistant epoxy resin, only leaving a working area of 1cm2 to be exposed in the testing solution. Prior to each test, the working electrode was finely polished with SiC paper from 150# to 1200# and ultrasonically degreased in alcohol solution. The DL-EPR test was initiated after a nearly steady-state open circuit potential (EOCP) had developed (about 10 min). The potential sweep began in the anodic direction at a scan rate of 1.66 mV·s−1 from the Ecorr till the potential of 0.3Vvs.SCE was reached. Then, the sweep was reversed in the cathodic direction until the EOCP. The electrode potential vs. current density was automatically recorded with the help of computer. From the DL-EPR curve (that is the variation of current density with electrode potential), the peak current density during reactivation (Ir) and the peak current density during activation (Ia) can be obtained. The ratio of Ir/Ia is called the DOS [18]. 2.4. Test in copper-copper sulfate-sulfuric acid (16%) solution The susceptibility to IGC was evaluated by the tests in copper‑copper sulfate‑sulfuric acid (16%) solution according to ASTM A262 practice E test (modified) [21]. The test solution was prepared by this way: 100 g analytical CuSO4·5H2O was dissolved in 700 mL distilled water and 100 mL sulfuric acid was added, the solution was diluted to 1000 mL with distilled water. A layer of copper chips with purity not less than 99.5% was laid at the bottom of the flask. The tensile specimens with pre-strain following thermal aging were placed in the flask filled with the prepared testing solution for 16 h, keeping the testing solution in the slight boiling state. After the experiments were finished, the specimens were taken out, washed, dried, and bended through 180° as the center of gauge section. The bended section was cut, and the bended surface was carefully observed with naked eyes and/or OM under small magnification. Small specimen located at the non-bended section was finely polished and observed by OM without etching and with electrolytically etching in 10% oxalic acid solution. 3. Results and discussion 3.1. Strain-induced α'-martensite and shear band as well as vickers hardness During the deformation process for austenitic SS, ε-martensite often formed in the lower deformation temperature [22] and is easily weakened after sensitization [23]. So, within the pre-strain range studied, there is non-existence of ε-martensite (nonmagnetic) [24], and strain-induced -martensite is mainly α'-martensite instead of ε-martensite under the pre-strain conditions in this research and the measured magnetic phase refers to strain-induced α'-martensite. Therefore, in pre-strained 304SS, except strain-induced α'martensite (ferromagnetic), there is no other magnetic phase formed during pre-strain or/and aged process for 304 SS, the ferrite 4
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Fig. 4. Results of DL-EPR tests for (a) as-received specimen and specimen pre-strained by (b) 0%; (c) 1%; (d) 2%; (e) 5%; (f) 10%; (g) 15%; (h) 20%.
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Fig. 5. The relationship between pre-strain and DOS (including as-received state).
content measured by Ferritscope can be taken as α'-martensite [25]. The changes in volume fraction of α'-martensite and vickers hardness with pre-strain are given in Fig. 1. The number of α'martensites is very little when the pre-strain is below 5%. Subsequently, the volume fraction of α'-martensite gradually increases. When the pre-strain is reached 20%, the volume fraction of α'-martensite is up to about 15%, which is 50 times of that in 1% prestrained specimen. Similarly, The hardness increases with pre-strain. The harness value is 207 HV for the specimen without pre-strain (0%), and it increases monotonously from 215 to 304 with 1% to 20% pre-strain. The variation in hardness has the similar tendency within the studied pre-train range as compared to the results measured by Milad, et al. [26]. The microstructures for the specimens with pre-strain following thermal aging are partially presented in Fig. 2. It is clearly observed that the microstrucures of 304 SS are greatly sensitive to the levels of pre-strain. For the specimen without pre-strain, the only phase is austenite (as shown in Fig. 2a). For the 1% pre-strained specimen, besides many austenite, there has small numbers of shear bands and strain-induced α'-martensite (dark). With the increase in pre-strain level, the shear bands and α'-martensite become remarkably apparent (as shown from Fig. 2b to e), particularly for 10% and 20% pre-strained specimen, as shown in Fig. 2d and e. Fig. 3 gives the microstructures of the pre-strained following thermally aged specimens at larger magnification, it can be clearly observed that the shear bands in one grain were initially parallel and appeared only in few grains. With pre-strain, the number of shear bands in one grain increased and more shear bands appeared on more grains. It is noted that intersecting shear bands began to appear on the grains of 5% pre-strained specimen and the number of intersecting shear bands became more and more with pre-strain. It had been proved that the intersections of shear bands can act as the nucleation sites for α'-martensite. The presence of the shear bands always precedes the onset of the formation of α'-martensite. So, the formation of the shear bands is a necessary precursor for the strain-induced α'-martensite [22], and they are directly interlinked. As mentioned above, it is believed that the formation of α'-martensite and shear band is strong dependent on the pre-strain. Besides α'-martensite and shear band, dislocation density which would increase with pre-strain [27–28], will also provide a strengthening effect, resulting in the increasing hardness with pre-strain. Though experiencing the thermally aged at 750 °C for one hour, the sizes of austenite for specimens with different pre-strain levels are the same with that of 0% pre-strained specimen (as shown in Fig. 2), α'-martensite and shear band are retained in the pre-strained specimens. So, it can be concluded that the thermal aging process at 750 °C for one hour for the pre-strained 304 SS didn't cause the obviously the reversion of α'-martensite to austenite. The mechanism of the reversion of α'-martensite to austenite may occur through shear of diffusion, but it is closely related to strain level, aging temperature and time. In this research, the aging temperature is 750 °C, so, the reversed mechanism of α'-martensite to austenite may be mainly diffusion, but the maximum strain is 20%, not enough to cause the obvious reversion, so the reversed amount can be ignore, which makes the deformed microstructures kept and the strengthening effect retained in steel. 3.2. DL-EPR test DL-EPR test can effectively quantify the DOS of austenitic SS and evaluate its tendency towards IGC. As shown in Fig. 4, it can be observed that the significant active peaks can be observed for all the specimens, but reactive peak can't be observed for as-received specimen (solution-annealed state). For specimens with pre-strain below 5%, reactive peaks are very small. Subsequently, the reactive peaks gradually increase with pre-strain. The relationship between pre-strain and the values of DOS is given in Fig. 5. The ratio of Ir/Ia for the as-received (solution-annealed state) specimen is close to zero due to the non-detectable Ir, which implies that the solution- annealed 304 SS is resistance to IGC. But, for the aged specimen without pre-strain, Ir can be detected and the ratio of Ir/Ia is about 4.7%, which indicates that thermal aging at 750 °C for one hour can cause the occurrence of sensitization. As the pre-strain was applied, the ratio of Ir/Ia is gradually increased. When the pre-strain is increased up to 5%, the ratio of Ir/Ia is 16.3%, which has an 6
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Fig. 6. Morphologies of cracks in the bended sections of (a) as-received specimen and specimens pre-strained by (b) 0%; (c) 1%; (d) 2%; (e) 5%; (f) 10%; (g) 15%; (h)20%.
obvious increase as compared to only aged specimen without pre-strain (Ir/Ia is about 4.7%). For 20% pre-strained specimen, the ratio of Ir/Ia is high up to 33.3%. The increasing Ir/Ia values correspond to the increase in DOS, which indicates that IGC tendency of 304 SS had been remarkably increased. As mentioned above [27–28], dislocation rate is enhanced with pre-strain, which results in more formation of dislocation and increases the dislocation density. For the pre-strained 304 SS following thermally aged at 750 °C for one hour, the presence of straininduced α'-martensite and shear bands as well as the increasing dislocation density will promote the nucleation and growth of carbides, leading to the precipitation of carbides along the grain boundaries and the formation of Cr-depleted zone. Since the diffusion rate of C is far higher than that of Cr, the precipitation process is controlled by the diffusion rate of Cr. Although the value of
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Fig. 7. Morphologies of cracks in the sections not-affected by bending stress for the specimens pre-strained by (a) 0%; (b) 1%; (c) 2%; (d) 5%; (e) 10%; (f) 15%; (g) 20%; (h)20% (more magnification).
diffusion coefficient for Cr is higher in martensite than in austenite [29], the sensitization of pre-strained 304 SS is actually controlled by lowering Cr diffusion rate in austenitic matrix due to the low level of the pre-strain (up to 20%) [14]. In spite of the relatively high aging temperature in the present research, but the aging time is only one hour and is not enough to cause bulk Cr diffusion to eliminate the Cr-depleted condition that is called desensitization. Studies had shown that an increasing in aging time at 750 °C promoted the healing (desensitization) of sensitized 304 SS, however, the desensitized condition was achieved after 48 h of exposure in specimens aged without previous solution treatment and after 175 h in solution-annealed specimens [17]. 316 SS specimens deformed 50% in tension and heat treated at 625 °C for up to 100 h showed an increasing degree of sensitization, a period of time longer than this would be required to initiate the desensitization, and the desensitization occurred for the same level of deformed
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specimen heat treated at 670 °C for more than 10 h [12]. Therefore, the aging treatment at the temperature of 750 °C for one hour for the specimens pre-strained up to 20% didn't lead to the occurrence of desensitization. On the contrary, the values of DOS increase with pre-strain. 3.3. Test in copper-copper sulfate-sulfuric acid (16%) solution After completing the tests in copper-copper sulfate-sulfuric acid (16%) solution, the specimens were taken out, cleaned, dried, and bended through 180° as the center of the gauge section. The morphologies of the bended section are given in Fig. 6. No any cracks are observed on the bended surface of the as-received specimen (solution-annealed state), which again confirms that the solutionannealed 304 SS is free to IGC and is consistent to the result that its ratio of Ir/Ia is close to zero (shown in Figs. 4a and 5). For the aged specimen without pre-strain, cracks can easily be detected, which shows that aged 304 SS is susceptible to IGC. Moreover, with the increasing pre-strain, the number of the cracks becomes more and more, and the cracks are much deeper and wider. For the specimen with 20% pre-strain, it has the deepest and widest cracks. In order to furthermore observe the micro-morphologies of specimens that had been tested in copper‑copper sulfate‑sulfuric acid (16%) solution. The section of the specimen without affected by stress during bending process was cut from the specimens far away from the bended sections. The microstructures are shown in Fig. 7. For only aged specimen without pre-strain, it can be clearly seen in Fig. 7a, only one or two tiny cracks can be observed, and most austenitic grains are intact. For the specimens with 1% pre-strain, the attack began to take place preferentially along the grain boundaries, and the cracks became wider and deeper, as shown in Fig. 7b. For the specimens with 2% pre-strain, partially grain boundaries were seriously attacked, which results in the complete dissolution of some grains, as shown in Fig. 7c. With the increase in pre-strain level, the more grain boundaries were preferentially attacked, the cracks continually propagated towards the interior of the steel, much wider and longer intergranular cracks can be obviously observed, more grains were completely dissolved, the features of IGC can be easily differentiated, as shown in Fig. 7d–f. Carefully observed, it can be noted that IGC mainly occurred on the deformed zone, especially for 15% and 20% pre-strained specimens, as shown in Fig. 7f–h (Fig. 7h is larger magnification in Fig. 7g). As shown in Fig. 7h, the longest crack had passed through eight grains, its length is about 250 μm and its width is about 38 μm, indicating that the severe IGC had occurred in the specimen with 20% pre-strain. The above results given in Figs. 6 and 7 suggest that the attacked degree for pre-strained following aged 304 SS in copper-copper sulfate-sulfuric acid (16%) solution corresponds to the DOS by DL-EPR tests. It is believed that pre-strain has a significant effect on DOS and IGC. In the studied pre-strain ranged from 0% to 20% and the aging temperature (750 °C)-time (one hour), DOS and the susceptibility to IGC increase with pre-strain. 4. Conclusions In the present research, the effect of pre-strain on DOS and susceptibility to IGC had been performed. The main conclusions are as follows: (1) For 304 SS withstood the maximum pre-strain up to 20% and thermally aged at 750 °C for one hour, the strain-induced α'martensite and shear band are most retained and their numbers gradually increase with the pre-strain, which remarkably affects the nucleation and precipitation of carbides. The aged process didn't cause the obvious reversion of α'-martensite to austenite and desensitization. (2) For as-received 304 SS, its DOS is zero and no cracks are founded on the bended surface, which indicates that it is free to IGC. (3) Pre-strain has a significant effect on DOS and IGC for 304SS and will reduce its corrosion resistance to IGC. For only aged specimen without pre-strain, the value of DOS is very small and the attack by IGC is relatively slight. But with the increase in prestrain, the values of DOS and the susceptibility to IGC gradually increase. The higher the value of the DOS is, the more serious IGC becomes. The most serious IGC occurred on 20% pre-strained following aged specimen, the longest intergranular crack is about 250 μm long and its width is about 38 μm. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this conflicts of interest to this manuscript. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. References [1] [2] [3] [4] [5] [6]
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Effects of pre-strain on sensitization and interganular corrosion for 304 stainless steel
T
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Xuewen Zhang, Jianqun Tang , Hao Liu, Jianming Gong Jiangsu Key Lab of Design and Manufacture of Extreme Pressure Equipment, School of Mechanical and Power Engineering, Nanjing Tech University, Nanjing, 211816, China
A R T IC LE I N F O
ABS TRA CT
Keywords: 304 stainless steel Pre-strain Thermal aging Sensitization Intergranular corrosion
The degree of sensitization (DOS) and the susceptibility to interganular corrosion (IGC) were investigated for pre-strained 304 stainless steel (SS) following thermally aged at 750 °C for one hour. The maximum pre-strain applied to the steel is 20%.The microstructures of 304 SS subjected to pre-strain and thermal aging were observed. The volume fraction of strain-induced α'martensite and the vickers hardness were measured, respectively. The double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were used to characterize the DOS as a function of various pre-strain levels. The tests in copper‑copper sulfate‑sulfuric acid (16%) solution were performed to evaluate the susceptibility to IGC. The morphologies of the cracks caused by IGC were observed. The results show, although pre-strained 304SS had been thermally aged at 750 °C for one hour, strain-induced α'-martensite and shear bands in steel are most retained and increase with pre-strain, causing the strengthening effect and leading to the increase in hardness. The DOS and the susceptibility to IGC also increase with pre-strain. The higher the value of the DOS is, the more serious IGC becomes. Obvious intergranular cracks as well as the dissolution of grain boundaries and the grains began to appear in the steel with 5% pre-strain, and the attack had been deteriorated with the increasing pre-strain. When the pre-strain is reached 20%, the longest crack is about 250 μm and its width is about 38 μm, indicating that the severe IGC had occurred. No obvious reversion of martensite to austenite and desensitization occurred for 304 SS withstood the maximum pre-strain up to 20% and thermally aged at 750 °C for one hour.
1. Introduction 304 stainless steel (SS) belongs to typical 18Cre8Ni austenitic steel. Due to the combination of excellent corrosion resistance, good mechanical properties, great weldability and relatively low cost, 304 SS has been widely used in chemical, petrochemical, power and nuclear industries over a large temperature range. For this type stainless steel, chromium (Cr) is the necessary alloying element. The formation of the densely Cr-contained oxide film tightly bonded to the matrix endows it with excellent resistance to uniform corrosion. However, this steel is sensitive to local corrosions [1]. Intergranular corrosion (IGC) is one of these attacks, which usually occurs on the steels having been suffered from sensitization. For austenitic SS, sensitization usually refers to the precipitation of chromium carbides in the temperature range of 450 °C–850 °C, which results in the formation of less resistant Cr-depleted zones near the grain boundaries [2]. The Cr-depleted zone
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Corresponding author. E-mail address: [email protected] (J. Tang).
https://doi.org/10.1016/j.engfailanal.2019.104179 Received 13 March 2018; Received in revised form 24 August 2019; Accepted 1 September 2019 Available online 04 September 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
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is easily suffered from attack in aggressive environments, which will result in the occurrence of IGC that cracks initiate and propagate along the grain boundaries [3], even intergranular stress corrosion cracking (IGSCC) or transgranular SCC (TRSCC) under tensile stress [4–6]. The objects for studying the degree of sensitization (DOS) and IGC, however, mostly aimed at solution-annealed 304 SS following the sensitized treatment, and without considering the strain caused by residual stress, which does not correspond to the practical state. Actually, in the industries, many components will undergo plastic deformation during the process of cold working, assembling and welding (such as the fins of plate-fin heat exchanger), without the following solution-annealed treatment. A certain amount of strain caused by residual stress will be inevitably retained in steel. As the microstructure of 304 SS is metastable, the presence of the strain will lead to the changes in microstructures and properties as well as diffusion energy compared to the original solutionannealed state [7]. Some researchers have revealed that plastic deformation (strain and strain state in steel) will influence sensitization of austenitic SS. The changes in sensitization kinetics with increasing strain have been postulated to be the activation energy of Cr-diffusivity by dislocation density with strain, the decrease in the activation energy was an indication that strain accelerates sensitization development [8]. Brian et al. [9] revealed that deformation prior to sensitization greatly accelerates the kinetics of sensitization in 304 SS. For 316 SS, the transformation of induced-strain martensite also caused rapid sensitization [10]. Carbide precipitation becomes remarkably prevalent in the grain matrix with increase in the deformation, and cold deformation enhances the DOS as high as 65 times the un-deformed 304 SS at 500 °C [11]. Ramírez et al. [12] further confirmed that the deformed 316SS showed more carbide precipitates at the grain boundaries than did the un-deformed steel by transmission electron microscopy. Because deformation will lead to the changes in microstructures-properties and sensitization is a diffusion-assisted process, sensitization after pre-strain is influenced not only by strain level, but also by aging temperature and time as well as other factors. Chowdhury et al. [13] showed that the DOS of 304 SS was successfully characterized as a function of both prior deformation and sensitized temperatures at 400 °C, 500 °C and 600 °C. When the level of strain is low, the sensitization of 304 SS is controlled by lowering Cr diffusion rate in the matrix [14]. However, when the level of strain is higher, especially at higher aging temperature, the DOS may be reduced due to the desensitization for the increase in desensitization kinetics, but the occurrence of desensitization was needed the long duration to occur [15]. Trillo et al. [16] pointed that the sensitized 304 SS could undergo desensitization with aging time due to bulk Cr diffusion which eliminates the Cr-depleted condition, and the increasing strain decreases the aging time. Kina et al. [17] revealed that desensitization would occur at 750°C for sensitized 304 SS for 48 h and for solution-annealed steel for 175 h. So, aimed at the facts that 304 SS sheet is widely used in the industries (typical example is the fins of plate-fin heat exchanger) and 304 SS components are easily suffered from sensitization as well as the low residual strain during cold working, assembling and welding process, 304 SS sheetspecimens withstood residual strain up to 20% and following thermally aged at 750 °C for one hour are chose to study the effects of pre-strain on the sensitization and the susceptibility to IGC, which simulates the practical states that the 304 SS components may be suffered. In the present research, the residual strain is introduced into the steel by the means of pre-strain through uniaxial tensile test. By double loop electrochemical potentiokinetic reactivation (DL-EPR) tests and the tests in copper‑copper sulfate-sulfuric acid (16%) solution, the effects of pre-strain on the DOS and IGC of 304 SS were investigated. 2. Material and experimental procedures 2.1. Material and uniaxial tensile test The material investigated is commercial AISI 304 SS sheet with 0.8 mm thickness. The chemical composition of the steel was (in wt%): C 0.035, Mn 1.19, Ni 7.8, Si 0.412, P 0.028, S 0.057, Cr 18.98 and Fe balance. The as-received state of 304 SS was solution annealed at 1050 °C for one hour following water quenching. Flat tensile specimens (the length is 90 mm) with a gauge size of 30 mm × 10 mm × 0.8 mm were strained to 0%, 1%, 2%, 5%, 10% and 20% engineering strain at the strain rate of 1 × 10−3 s−1 using tensile testing machine (Instron5869). The pre-strained specimens were thermally aged at 750 °C for one hour following air cooling. The specimens of aged steel were electrolytically etched in 10% oxalic acid solution and the microstructures were observed by optical microscope (OM) (Zeiss AXIO Imager. A1m). 2.2. Characterization of pre-strained and aged steel Ferromagnetic volume change in austenitic SS due to pre-strain can be used to reflect the volume fraction change in straininduced martensite. So, the volume fraction of strained-induced martensite (ferromagnetic) was determined by a calibrated commercial Ferritscope (Diverse MF300F+), which is based on magnetic induction. The values of vickers hardness were measured by hardness tester (HXD-1000™).When volume fraction of martensite and the values of hardness were measured, the test points were as close as possible to the center of the gauge section. Four points are taken for each pre-strained specimen, and then the average value was taken. 2.3. Double loop electrochemical potentiokinetic reactivation (DL-EPR) test The DL-EPR test is the effective method to determine the DOS and is sensitive enough to detect the microstructural change in materials [18–20]. In the present research, the DL-EPR test was carried out in 0.5 mol/L H2SO4 + 0.01 mol/L KSCN solution with 2
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Fig. 1. Effect of pre-strain on the volume fraction of α'-martensite and vickers hardness.
Fig. 2. The microstructures of the specimens pre-strained by (a) 0%; (b) 1%; (c) 2%; (d) 10%; (e) 20%.
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Fig. 3. The microstructures in Fig. 2 at larger magnification for specimens pre-strained by (a) 1%; (b) 2%; (c) 10%; (d) 20%.
traditional three-electrode cell using Solatron 1280B electrochemical workstation. A piece of platinum was used as the counter electrode and a saturated calomel electrode (SCE) was chosen as the reference electrode. The working electrode was cut from the gauge section of the tensile specimen. Non-working surface was sealed by resistant epoxy resin, only leaving a working area of 1cm2 to be exposed in the testing solution. Prior to each test, the working electrode was finely polished with SiC paper from 150# to 1200# and ultrasonically degreased in alcohol solution. The DL-EPR test was initiated after a nearly steady-state open circuit potential (EOCP) had developed (about 10 min). The potential sweep began in the anodic direction at a scan rate of 1.66 mV·s−1 from the Ecorr till the potential of 0.3Vvs.SCE was reached. Then, the sweep was reversed in the cathodic direction until the EOCP. The electrode potential vs. current density was automatically recorded with the help of computer. From the DL-EPR curve (that is the variation of current density with electrode potential), the peak current density during reactivation (Ir) and the peak current density during activation (Ia) can be obtained. The ratio of Ir/Ia is called the DOS [18]. 2.4. Test in copper-copper sulfate-sulfuric acid (16%) solution The susceptibility to IGC was evaluated by the tests in copper‑copper sulfate‑sulfuric acid (16%) solution according to ASTM A262 practice E test (modified) [21]. The test solution was prepared by this way: 100 g analytical CuSO4·5H2O was dissolved in 700 mL distilled water and 100 mL sulfuric acid was added, the solution was diluted to 1000 mL with distilled water. A layer of copper chips with purity not less than 99.5% was laid at the bottom of the flask. The tensile specimens with pre-strain following thermal aging were placed in the flask filled with the prepared testing solution for 16 h, keeping the testing solution in the slight boiling state. After the experiments were finished, the specimens were taken out, washed, dried, and bended through 180° as the center of gauge section. The bended section was cut, and the bended surface was carefully observed with naked eyes and/or OM under small magnification. Small specimen located at the non-bended section was finely polished and observed by OM without etching and with electrolytically etching in 10% oxalic acid solution. 3. Results and discussion 3.1. Strain-induced α'-martensite and shear band as well as vickers hardness During the deformation process for austenitic SS, ε-martensite often formed in the lower deformation temperature [22] and is easily weakened after sensitization [23]. So, within the pre-strain range studied, there is non-existence of ε-martensite (nonmagnetic) [24], and strain-induced -martensite is mainly α'-martensite instead of ε-martensite under the pre-strain conditions in this research and the measured magnetic phase refers to strain-induced α'-martensite. Therefore, in pre-strained 304SS, except strain-induced α'martensite (ferromagnetic), there is no other magnetic phase formed during pre-strain or/and aged process for 304 SS, the ferrite 4
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Fig. 4. Results of DL-EPR tests for (a) as-received specimen and specimen pre-strained by (b) 0%; (c) 1%; (d) 2%; (e) 5%; (f) 10%; (g) 15%; (h) 20%.
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Fig. 5. The relationship between pre-strain and DOS (including as-received state).
content measured by Ferritscope can be taken as α'-martensite [25]. The changes in volume fraction of α'-martensite and vickers hardness with pre-strain are given in Fig. 1. The number of α'martensites is very little when the pre-strain is below 5%. Subsequently, the volume fraction of α'-martensite gradually increases. When the pre-strain is reached 20%, the volume fraction of α'-martensite is up to about 15%, which is 50 times of that in 1% prestrained specimen. Similarly, The hardness increases with pre-strain. The harness value is 207 HV for the specimen without pre-strain (0%), and it increases monotonously from 215 to 304 with 1% to 20% pre-strain. The variation in hardness has the similar tendency within the studied pre-train range as compared to the results measured by Milad, et al. [26]. The microstructures for the specimens with pre-strain following thermal aging are partially presented in Fig. 2. It is clearly observed that the microstrucures of 304 SS are greatly sensitive to the levels of pre-strain. For the specimen without pre-strain, the only phase is austenite (as shown in Fig. 2a). For the 1% pre-strained specimen, besides many austenite, there has small numbers of shear bands and strain-induced α'-martensite (dark). With the increase in pre-strain level, the shear bands and α'-martensite become remarkably apparent (as shown from Fig. 2b to e), particularly for 10% and 20% pre-strained specimen, as shown in Fig. 2d and e. Fig. 3 gives the microstructures of the pre-strained following thermally aged specimens at larger magnification, it can be clearly observed that the shear bands in one grain were initially parallel and appeared only in few grains. With pre-strain, the number of shear bands in one grain increased and more shear bands appeared on more grains. It is noted that intersecting shear bands began to appear on the grains of 5% pre-strained specimen and the number of intersecting shear bands became more and more with pre-strain. It had been proved that the intersections of shear bands can act as the nucleation sites for α'-martensite. The presence of the shear bands always precedes the onset of the formation of α'-martensite. So, the formation of the shear bands is a necessary precursor for the strain-induced α'-martensite [22], and they are directly interlinked. As mentioned above, it is believed that the formation of α'-martensite and shear band is strong dependent on the pre-strain. Besides α'-martensite and shear band, dislocation density which would increase with pre-strain [27–28], will also provide a strengthening effect, resulting in the increasing hardness with pre-strain. Though experiencing the thermally aged at 750 °C for one hour, the sizes of austenite for specimens with different pre-strain levels are the same with that of 0% pre-strained specimen (as shown in Fig. 2), α'-martensite and shear band are retained in the pre-strained specimens. So, it can be concluded that the thermal aging process at 750 °C for one hour for the pre-strained 304 SS didn't cause the obviously the reversion of α'-martensite to austenite. The mechanism of the reversion of α'-martensite to austenite may occur through shear of diffusion, but it is closely related to strain level, aging temperature and time. In this research, the aging temperature is 750 °C, so, the reversed mechanism of α'-martensite to austenite may be mainly diffusion, but the maximum strain is 20%, not enough to cause the obvious reversion, so the reversed amount can be ignore, which makes the deformed microstructures kept and the strengthening effect retained in steel. 3.2. DL-EPR test DL-EPR test can effectively quantify the DOS of austenitic SS and evaluate its tendency towards IGC. As shown in Fig. 4, it can be observed that the significant active peaks can be observed for all the specimens, but reactive peak can't be observed for as-received specimen (solution-annealed state). For specimens with pre-strain below 5%, reactive peaks are very small. Subsequently, the reactive peaks gradually increase with pre-strain. The relationship between pre-strain and the values of DOS is given in Fig. 5. The ratio of Ir/Ia for the as-received (solution-annealed state) specimen is close to zero due to the non-detectable Ir, which implies that the solution- annealed 304 SS is resistance to IGC. But, for the aged specimen without pre-strain, Ir can be detected and the ratio of Ir/Ia is about 4.7%, which indicates that thermal aging at 750 °C for one hour can cause the occurrence of sensitization. As the pre-strain was applied, the ratio of Ir/Ia is gradually increased. When the pre-strain is increased up to 5%, the ratio of Ir/Ia is 16.3%, which has an 6
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Fig. 6. Morphologies of cracks in the bended sections of (a) as-received specimen and specimens pre-strained by (b) 0%; (c) 1%; (d) 2%; (e) 5%; (f) 10%; (g) 15%; (h)20%.
obvious increase as compared to only aged specimen without pre-strain (Ir/Ia is about 4.7%). For 20% pre-strained specimen, the ratio of Ir/Ia is high up to 33.3%. The increasing Ir/Ia values correspond to the increase in DOS, which indicates that IGC tendency of 304 SS had been remarkably increased. As mentioned above [27–28], dislocation rate is enhanced with pre-strain, which results in more formation of dislocation and increases the dislocation density. For the pre-strained 304 SS following thermally aged at 750 °C for one hour, the presence of straininduced α'-martensite and shear bands as well as the increasing dislocation density will promote the nucleation and growth of carbides, leading to the precipitation of carbides along the grain boundaries and the formation of Cr-depleted zone. Since the diffusion rate of C is far higher than that of Cr, the precipitation process is controlled by the diffusion rate of Cr. Although the value of
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Fig. 7. Morphologies of cracks in the sections not-affected by bending stress for the specimens pre-strained by (a) 0%; (b) 1%; (c) 2%; (d) 5%; (e) 10%; (f) 15%; (g) 20%; (h)20% (more magnification).
diffusion coefficient for Cr is higher in martensite than in austenite [29], the sensitization of pre-strained 304 SS is actually controlled by lowering Cr diffusion rate in austenitic matrix due to the low level of the pre-strain (up to 20%) [14]. In spite of the relatively high aging temperature in the present research, but the aging time is only one hour and is not enough to cause bulk Cr diffusion to eliminate the Cr-depleted condition that is called desensitization. Studies had shown that an increasing in aging time at 750 °C promoted the healing (desensitization) of sensitized 304 SS, however, the desensitized condition was achieved after 48 h of exposure in specimens aged without previous solution treatment and after 175 h in solution-annealed specimens [17]. 316 SS specimens deformed 50% in tension and heat treated at 625 °C for up to 100 h showed an increasing degree of sensitization, a period of time longer than this would be required to initiate the desensitization, and the desensitization occurred for the same level of deformed
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specimen heat treated at 670 °C for more than 10 h [12]. Therefore, the aging treatment at the temperature of 750 °C for one hour for the specimens pre-strained up to 20% didn't lead to the occurrence of desensitization. On the contrary, the values of DOS increase with pre-strain. 3.3. Test in copper-copper sulfate-sulfuric acid (16%) solution After completing the tests in copper-copper sulfate-sulfuric acid (16%) solution, the specimens were taken out, cleaned, dried, and bended through 180° as the center of the gauge section. The morphologies of the bended section are given in Fig. 6. No any cracks are observed on the bended surface of the as-received specimen (solution-annealed state), which again confirms that the solutionannealed 304 SS is free to IGC and is consistent to the result that its ratio of Ir/Ia is close to zero (shown in Figs. 4a and 5). For the aged specimen without pre-strain, cracks can easily be detected, which shows that aged 304 SS is susceptible to IGC. Moreover, with the increasing pre-strain, the number of the cracks becomes more and more, and the cracks are much deeper and wider. For the specimen with 20% pre-strain, it has the deepest and widest cracks. In order to furthermore observe the micro-morphologies of specimens that had been tested in copper‑copper sulfate‑sulfuric acid (16%) solution. The section of the specimen without affected by stress during bending process was cut from the specimens far away from the bended sections. The microstructures are shown in Fig. 7. For only aged specimen without pre-strain, it can be clearly seen in Fig. 7a, only one or two tiny cracks can be observed, and most austenitic grains are intact. For the specimens with 1% pre-strain, the attack began to take place preferentially along the grain boundaries, and the cracks became wider and deeper, as shown in Fig. 7b. For the specimens with 2% pre-strain, partially grain boundaries were seriously attacked, which results in the complete dissolution of some grains, as shown in Fig. 7c. With the increase in pre-strain level, the more grain boundaries were preferentially attacked, the cracks continually propagated towards the interior of the steel, much wider and longer intergranular cracks can be obviously observed, more grains were completely dissolved, the features of IGC can be easily differentiated, as shown in Fig. 7d–f. Carefully observed, it can be noted that IGC mainly occurred on the deformed zone, especially for 15% and 20% pre-strained specimens, as shown in Fig. 7f–h (Fig. 7h is larger magnification in Fig. 7g). As shown in Fig. 7h, the longest crack had passed through eight grains, its length is about 250 μm and its width is about 38 μm, indicating that the severe IGC had occurred in the specimen with 20% pre-strain. The above results given in Figs. 6 and 7 suggest that the attacked degree for pre-strained following aged 304 SS in copper-copper sulfate-sulfuric acid (16%) solution corresponds to the DOS by DL-EPR tests. It is believed that pre-strain has a significant effect on DOS and IGC. In the studied pre-strain ranged from 0% to 20% and the aging temperature (750 °C)-time (one hour), DOS and the susceptibility to IGC increase with pre-strain. 4. Conclusions In the present research, the effect of pre-strain on DOS and susceptibility to IGC had been performed. The main conclusions are as follows: (1) For 304 SS withstood the maximum pre-strain up to 20% and thermally aged at 750 °C for one hour, the strain-induced α'martensite and shear band are most retained and their numbers gradually increase with the pre-strain, which remarkably affects the nucleation and precipitation of carbides. The aged process didn't cause the obvious reversion of α'-martensite to austenite and desensitization. (2) For as-received 304 SS, its DOS is zero and no cracks are founded on the bended surface, which indicates that it is free to IGC. (3) Pre-strain has a significant effect on DOS and IGC for 304SS and will reduce its corrosion resistance to IGC. For only aged specimen without pre-strain, the value of DOS is very small and the attack by IGC is relatively slight. But with the increase in prestrain, the values of DOS and the susceptibility to IGC gradually increase. The higher the value of the DOS is, the more serious IGC becomes. The most serious IGC occurred on 20% pre-strained following aged specimen, the longest intergranular crack is about 250 μm long and its width is about 38 μm. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this conflicts of interest to this manuscript. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. References [1] [2] [3] [4] [5] [6]
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