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Crack growth behavior of warm-rolled 316L austenitic stainless steel in high-temperature hydrogenated water
Journal of Nuclear Materials 476 (2016) 243e254
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Crack growth behavior of warm-rolled 316L austenitic stainless steel in high-temperature hydrogenated water Kyoung Joon Choi a, Seung Chang Yoo a, Hyung-Ha Jin b, Junhyun Kwon b, Min-Jae Choi b, Seong Sik Hwang b, Ji Hyun Kim a, * a
Department of Nuclear Science and Engineering, School of Mechanical and Nuclear Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea Nuclear Materials Safety Research Division, Korea Atomic Energy Research Institute (KAERI), 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea
b
h i g h l i g h t s 316L Stainless steels were used for the study of crack growth behavior in PWR water. Warm rolling was applied to simulate the irradiation hardening of stainless steels. DH concentration was changed to see the effect on crack growth and oxide structure. Warm-rolled stainless steels showed higher rates of corrosion and crack growth. Higher DH concentration also promoted the rates of corrosion and crack growth.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2015 Received in revised form 10 February 2016 Accepted 29 April 2016 Available online 30 April 2016
To investigate the effects of warm rolling on the crack growth of 316L austenitic stainless steel, the crack growth rate was measured and the oxide structure was characterized in high-temperature hydrogenated water. The warm-rolled specimens showed a higher crack growth rate compared to the as-received specimens because the slip bands and dislocations produced during warm rolling served as paths for corrosion and cracking. The crack growth rate increased with the dissolved hydrogen concentration. This may be attributed to the decrease in performance and stability of the protective oxide layer formed on the surface of stainless steel in high-temperature water. © 2016 Elsevier B.V. All rights reserved.
Keywords: 316L stainless steel Warm rolling Crack growth rate measurement TEM Crack growth Oxidation
1. Introduction Irradiation-assisted stress corrosion cracking (IASCC) of the internals in a light water reactor is a critical issue for long-term operation. IASCC typically occurs at doses between 0.5 dpa (for boiling water reactors, BWRs) and 2e3 dpa (for pressurized water reactors, PWRs) [1,2], and it can therefore be expected that many light water reactors are exposed to the risk of IASCC. Irradiation produces defects and defect clusters in grains, alters
* Corresponding author. E-mail address: [email protected] (J.H. Kim). http://dx.doi.org/10.1016/j.jnucmat.2016.04.051 0022-3115/© 2016 Elsevier B.V. All rights reserved.
dislocations and dislocation loop structures, produces defecteimpurity and defecteclustereimpurity complexes, and eventually leading to radiation-induced hardening [1e8]. It also leads to changes in the stability of second-phase precipitates and to the local alloy chemistry near grain boundaries, precipitates, and defect clusters. The grain boundary microchemistry that differs significantly from the bulk composition can be produced in association with both radiation-induced segregation and thermallydriven segregation of alloying and impurity elements. Among several radiation-related damages, radiation-induced segregation and radiation-induced hardening have been shown to make steels more susceptible to IASCC. The radiation damages can be increased by long-term operation, consequently resulting in the increase in
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Fig. 1. Schematic of (a) heat treatment at 650 C for 100 h and warm rolling processing at 250 C, and (b) orientation of 0.5T CT for the crack growth rate measurement testing.
the degradation of reactor internals, including IASCC. In fact, some cracking of the internal structures, such as guide tube support pins and baffle former bolts in operating PWR plants, has already been identified. Among various internal structures, baffle-former bolts assembling formers and baffles exhibit relatively high susceptibility to IASCC owing to the relatively high irradiation dose and tensile stress occurring after the assembly of formers and baffles. Further, for the extension of life-time of PWRs, the integrity of the internal structures needs to be studied more carefully. Regardless of those importance, however, preparing the materials which are irradiated under operating PWRs or simulating facilities is of difficulty as it accompanies high cost, long-exposure time, and radiation safety. Therefore, a warm rolling and heat treatment was carried out to simulate irradiated materials because it is a time-saving, cost-effective, and radiation-free technique. Even if the warm rolling and heat treatment represents those advantages, the process is not able to fully simulate irradiation of materials; it is suitable for the simulation of the radiation-induced segregation and radiation-induced hardening, which were known as the essential phenomena of IASCC. The warm rolling induces the formation of dislocations in the material and harden materials. The heat treatment causes the sensitization of stainless steels resulting in Cr depletion and Ni enrichment similar to radiation- induced segregation. The detail experimental procedure used in this study is described in the ‘Section 2.1. Materials’. In addition, it has been recently reported that the stress
corrosion cracking (SCC) susceptibility for Ni base alloys depends on the dissolved hydrogen (DH) concentration, and the SCC susceptibility is maximum under around 10 cm3/kg of DH concentration where the Ni/NiO phase transition exists [9,10]. In other words, the peak of stress corrosion rates occurring at the phase transition tends to decrease symmetrically as delta EcP values move away from the Ni/NiO phase transition [11,12]. To reduce the susceptibility of the alloy to SCC, there has been a tendency to increase the DH concentration of the primary section of pressurized water power plants up to approximately 50 cm3/kg. Although 50 cm3/kg of DH concentration can reduce the susceptibility of a Ni base alloy to SCC, the susceptibility of stainless steel to SCC may be aggravated [13e15]. Therefore, in this study, warm rolling and heat-treatment were applied to 316L stainless steel, in order to simulate the effect of radiation damages such as radiation-induced segregation and hardening. Further, the crack growth rate was tested under constant load with simulated primary water conditions of a PWR, which are 325 C, 17.5 MPa, dissolved oxygen concentration of less than 5 ppb, DH concentration of 25 cm3/kg, and LiOH and H3BO3 concentrations of 2 and 1200 ppm (mg/kg), respectively. Moreover, to investigate the effect of DH concentration on the crack growth of as-received stainless steel, the DH concentration was varied between 25 and 50 cm3/kg under the simulated primary water conditions of a PWR. The possible mechanism for the results of crack growth behavior
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Fig. 2. Micro-structural and micro-chemical analysis of as-received and warm-rolled 316L stainless steels used in this study. (a) OM and SEM images of the as-received 316L stainless steel, (b) OM and SEM images of the warm rolled 316L stainless steel, and (c) Dark field TEM image of grain boundary region and line EDS profile across the grain boundary in the as-received and warm-rolled 316L stainless steels.
obtained from this study is explained based on the analysis of the structure and composition of the oxide film and the fracture surfaces of the post-test specimens by using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). 2. Experimental 2.1. Materials To investigate the effects of warm rolling and DH concentration
on the crack growth rate, a plate of 316L austenitic stainless steel was used; its composition is presented in Table 1. Fig. 1 (a) illustrates the heat treatment and the warm rolling process that were used to simulate radiation damages, such as radiation-induced segregation and radiation-induced hardening. Heat treatment was performed to simulate chromium depletion in the grain boundary, which is similar to radiation induced segregation. Next, warm rolling was performed to simulate radiation induced hardening. Although other methods (such as cold working) can be used to simulate hardening, warm rolling was selected so as to avoid the formation of unfavourable microstructural phase (e.g. martensitic
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Fig. 3. Schematic of the experimental system used for the crack growth measurement testing in high-temperature and high-pressure water.
phase), which is not observed in irradiated stainless steel [16,17]. The phase change from austenitic structure into martensitic structure was actually not observed after the warm rolling, and it was confirmed through the microstructural analysis. To avoid any excessive distortion of grain or anisotropic microstructure, warm rolling was performed in multiple step and direction. By this procedure, key microstructural features such as grain size and shape were kept same after the warm rolling process. Detailed process is explained below. Heat treatment was performed at 630 C for 100 h. Then, 30% of warm rolling at 250 C was conducted in the ‘c’ direction (first step), followed by 20% of warm rolling in the ‘a’ and ‘b’ directions (second and third steps). Finally, 20% of warm rolling was conducted in the ‘c’ direction (fourth step). All the warm rolling steps were performed at 250 C, and 20%-20%-50% warm-rolled 316L stainless steel plates were prepared. The hardening and the microstructural change were confirmed through the hardness values, tensile tests, optical microscopy (OM) and scanning electron microscopy (SEM) analyses (shown in Table 2 and Fig. 2). Fig. 2 (a) and (b) are OM and SEM images, representing the microstructures of as-received and warm rolled (‘S-T’ plane) stainless steels. In the OM images, it was observed that as-received stainless steel has austenitic structure and its grain size is about 73.4 mm. And the warm-rolled stainless steel also has austenitic structure and its grain size is about 75.1 mm, representing no change of grain size or shape. However, SEM images indicate
that there exist many dislocations slipping in individual grains of warm-rolled stainless steel comparing to one of as-received stainless steel. It is considered that the slip bands caused the hardening of warm-rolled stainless steel, which was confirmed through the results of tensile test and hardness test shown in Table 2. After the warm rolling and heat treatment, the elongation was reduced, but yield stress and tensile strength was increased. And, the Cr depletion and Ni enrichment in grain boundary, simulated with heat treatment, was confirmed through the TEM analysis as shown Fig. 2 (c). 2.2. Crack growth rate measurement 2.2.1. Specimen For the crack growth measurement, the as-received and 20%20%-50% warm-rolled specimens were machined as 0.5T compact tension-type (0.5T CT) specimens having 10.6 mm of initial crack (noted crack), 25 mm of length, 12.5 mm of thickness and 5% sidegrooves according to the American Society of Testing and Materials Standard (ASTM) E399 [18,19]. The 20%-20%-50% warm-rolled specimen has S-T orientation, i.e., the crack growth direction is parallel to main rolling direction (‘c’ direction or T direction), as shown in Fig. 1. Further, the as-received and 20%-20%-50% warmrolled specimens were machined with 5% side grooves on each side and instrumented with platinum current and potential probe leads for direct current (DC) potential drop measurements of the
Fig. 4. Measured data during the crack growth measurement of as-received 316L stainless steel in PWR water. (a) Total measurement curve with as-received 316L stainless steel in PWR water, (b) Measured data of crack length and DH concentration with 25 cm3/kg of DH concentration in PWR water, and (c) Measured data with 50 cm3/kg of DH concentration in PWR water.
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Fig. 5. Photographs of post-test fracture surfaces of as-received 316L stainless steel. (a) Macrophotograph of post-test fracture surface of as-received 316L stainless steel, and (b) High-magnification image in the region of crack growth under constant loading condition indicated in (a).
crack length as shown in Ref. [19]. The current flow through the sample was reversed about once per second, primarily to reduce measurement errors associated with thermocouple effects and amplifier offsets [20e22]. To minimizing the electrical noise from the other system (such as servo motor), Zirconia sleeves and washers were used to electrically insulate the 0.5T CT specimens from the loading pins and grips. Also, some filters (including running-average filter and low-pass filter) were applied and they could remove the temporary change but not voltage change by cracking. Running-average filter calculates the statistical average of measured data during a certain period of time, and creates the series of averages of subsets of the full CGR data set. By taking the average of the initial subset of the CGR data, the first element of the running average can be obtained. Then, the subset is shifting forward, with including the next number of the original subset in the data, and excluding the first number of the data. This process can create a new subset of data, which is averaged, and this procedure repeats over the entire CGR data. The relationship between the crack length and voltage was determined according to the method of Hicks and Pickard [18]. For the evaluation of warm rolling and dissolved hydrogen effects on corrosion or oxide structures, as-received and warm-rolled plate specimens of 10 mm 10 mm 2 mm were exposed to the high temperature and pressure water environment varing 25 and 50 cm3/kg of DH concentrations, and TEM analysis was performed.
makeup system from the laboratory water supply system. The makeup system then circulates the water through a demineralizer/ filter system to ensure cleanliness. The makeup water is supplied through a chemistry conditioning system, in which the water chemistry is adjusted, to the autoclave system. Provision has been made for injection of chemicals as well as for gas purging. In the present study, to evaluate the effects of DH concentration and warm rolling on the susceptibility to SCC growth for asreceived specimen, the crack growth rate was measured at both 25 cm3/kg and 50 cm3/kg of DH concentration. Further, for the 20%20%-50% warm-rolled specimen, the crack growth rate was measured at 25 cm3/kg DH concentration. The tests were performed in a 3.79 L stainless steel autoclave, which can maintain conditions of 1200 ppm (mg/kg) B, 2 ppm (mg/kg) Li, 25 and 50 cm3/kg of DH concentrations, dissolved oxygen < 5 ppb, 340 C, and 17.5 MPa. Proper amount of boric acid (H3BO3) and lithium hydroxide monohydrate (LiOH) were added in distilled water to simulate the primary coolant of nuclear power plant and electronic conductivity of this solution were maintain to 21e22 mS/cm. Then DH was controlled by overpressure with 99% hydrogen gas. DH was measured at room temperature and atmospheric pressure by using DH sensor installed in inlet water tank. And, copper/cuprous oxide (Cu/Cu2O) reference electrode was placed in the autoclave immediately adjacent to the 0.5T CT specimen, and used for the measurement of EcP.
2.2.2. Environmental conditions Tests were conducted in an autoclave system specially constructed for the present study, as shown schematically in Fig. 3. Careful consideration was given for ensuring extremely rigorous chemistry control, and near-theoretical water conductivity was achieved routinely. The system is fully instrumented for measuring oxygen, hydrogen, pH, and conductivity for both the inlet and outlet water. Distilled and demineralized water is supplied to the
2.2.3. Loading condition In this study, as-received specimens and 20%-20%-50% warmrolled stainless steel specimens were used to evaluate the effects of DH concentration and warm rolling on the crack growth rate. The loading conditions for the crack growth rate measurements are arranged in Table 3. Firstly, for sharpening of a machined notch, the 0.5T CT specimens was fatigue pre-cracked at a frequency of 10 Hz for load ratio (minimum load/maximum load) of 0.1 with the
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Fig. 6. Measured data during the crack growth measurement of warm-rolled 316L stainless steel in PWR water. (a) Total measurement curve with warm-rolled 316L stainless steel in PWR water, and (b) Measured data of crack length and DH concentration with 25 cm3/kg of DH concentration in PWR water.
maximum load 3500 N, determined based on ASTM E399. Next, at a frequency of 1 Hz for load ratio of 0.3, 0.5, and 0.7, the loading processes (up to step 4) were performed in air environment. Subsequently, in high temperature and pressure water environment (340 C and 17.5 MPa) which was described in the previous section 2.2.2, the following loading for the sharpening was performed at load ratio 0.7 and frequencies of 0.1 and 0.01 Hz, and finally was completed at a frequency of 0.001 Hz, for load ratio 0.7, with triangle (step 6) and trapezoidal (step 7) waveforms. Under constant loading (corresponding to about 30 MPa m1/2 of stress intensity factor KIC), the crack growth rate measurement was performed at 25 cm3/kg of DH concentration (step 8). Warm-rolled specimen was tested at only 25 cm3/kg of DH concentration, and the testing was completed at step 8. But, in case of as-received specimen, the crack growth rate measurements were performed
at both 25 and 50 cm3/kg of DH concentrations. After step 8, the crack sharpening under the fatigue loading, which is similar with the ones performed at steps 6 and 7, was performed at 50 cm3/kg of DH concentration. It could remove the previous effects of crack growth rate measurement testing at 25 cm3/kg of DH concentration. And then the crack growth rate at 50 cm3/kg of DH concentration was measured under constant loading. The stress intensity KIC at each steps were calculated by the measured load and the crack length derived by the DCPD method and corrected by post-test fractography. In the DCPD method, there are some limitations mainly associated with possible interference due to crack closure effects [20,21,23,24]. For this reason, the method is biased toward measuring the minimum crack depth. Thus, the technique will only measure an area that is then converted to an averaged value of crack length. An uneven crack front
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Fig. 7. Photographs of post-test fracture surfaces of warm-rolled 316L stainless steel. (a) Macrophotograph of post-test fracture surface of warm-rolled 316L stainless steel, and (b) High-magnification image in the region of crack growth under constant loading condition indicated in (a).
Fig. 4 illustrates the results of the crack growth measurements
for as-received 316L austenitic stainless steel under test conditions simulating the primary section in a PWR. The total elapsed time was 2325 h, and the water conditions were stabilized after precracking was performed for 420 h. At the each DH concentrations, the crack growth rates were derived by linear-fitting the several points of crack length measured with the DCPD method. At 25 cm3/kg of DH concentration (Fig. 4 (b)), the crack growth rate was found to be (4.48 ± 0.15) 109 mm/s, estimated in the region
Fig. 8. TEM image and EDS analysis results of as-received 316L stainless steel exposed to PWR water containing 25 cm3/kg of DH concentration for 1000 h. The bottom-right image in the red dotted box shows the magnified view of the top-left region of the main image. While the main structure of the outer oxide is composed of crystallite (relatively larger size indicated as ‘Outer 1’) or spinel (relatively smaller size indicated as ‘Outer 2’), the chemistry of the outer oxide is same that as shown in the table inside the TEM image. Also, note that outer 1 and outer 2 have same chemical composition, so one representative data was shown in the table. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. TEM image and EDS analysis results of as-received 316L stainless steel exposed to PWR water containing 50 cm3/kg of DH concentration for 1000 h. The bottom-right image shows the magnified view of the top-right region of the main image. While the main structure of the outer oxide is composed of crystallite (relatively larger size indicated as ‘Outer 1’) or spinel (relatively smaller size indicated as ‘Outer 2’), the chemistry of the outer oxide is same that as shown in the table inside the TEM image. Also, note that outer 1 and outer 2 have same chemical composition, so one representative data was shown in the table.
will also introduce error in crack length as will local crack extensions such as crack tunneling. In this study, post-test fractography was conducted in order to normalize the process and to establish more accurate crack length values. 3. Results
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Fig. 10. TEM image and EDS analysis results of warm-rolled 316L stainless steel exposed to PWR water containing 25 cm3/kg of DH concentration for 1000 h. The bottom-right image shows the magnified view of the top-center region of the main image. While the main structure of the outer oxide is composed of crystallite (relatively larger size indicated as ‘Outer 1’) or spinel (relatively smaller size indicated as ‘Outer 2’), the chemistry of the outer oxide is same that as shown in the table inside the TEM image. Also, note that outer 1 and outer 2 have same chemical composition, so one representative data was shown in the table.
from 1279 h to 1750 h. Conversely, at 50 cm3/kg of DH concentration (Fig. 4 (c)), the growth rate was found to be (2.31 ± 0.01) 108 mm/s, estimated in the region from 1878 h to 2325 h; this growth rate is relatively high compared to that at 25 cm3/kg of DH concentration. The fracture mode was determined based on Fig. 5 (which show photographs of post-test fracture surfaces of as-received stainless steel). In Fig. 5 (b), which is a high-magnification image in the region of steps 8e11 (between line (2) and line (3)) indicated in Fig. 5 (a), there are fracture surfaces formed at 25 cm3/kg and 50 cm3/kg of DH concentrations under constant loading. Under both DH concentration conditions, the transgranular cracks were observed in photographs (shown in Fig. 5). Fig. 6 shows the results of the crack growth measurement performed for 20%-20%-50% warm-rolled 316L austenitic stainless steel under test conditions simulating the primary section in a PWR. The total elapsed time was 1370 h, and the water conditions were stabilized after pre-cracking was performed for 570 h. Under 25 cm3/kg of DH concentration (Fig. 6 (b)), the crack growth rate was found to be (4.70 ± 0.02) 108 mm/s, estimated in the region from 570 h to 1370 h; this crack growth rate is relatively compared to that of the as-received specimen. The fracture mode was determined based on Fig. 7 (which shows photographs of the post-test fracture surfaces of warm-rolled stainless steel). In Fig. 7 (b), which is a high-magnification image in the region of step 8 (between line (2) and line (3)) indicated in Fig. 7 (a), there is fracture surface formed at 50 cm3/kg of DH concentration under constant loading. The transgranular crack also was observed in photographs (shown in Fig. 7). Fig. 8 indicates TEM bright field images and EDS analysis results, which indicate the oxide structures formed on the surface of asreceived stainless steel exposed to 25 cm3/kg of DH concentration for 1000 h. The oxide layer is composed of an inner layer (Cr rich oxide or protective oxide; FeCr2O4) and outer layer (Fe based oxide or spinel shaped oxide; Fe3O4). The EDS results indicate that the inner oxide has relatively high concentration of chromium, like Crrich oxide, but the outer oxide has relatively high concentration of
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iron, like Fe-based oxide. While the main structure of outer oxide is composed of crystallite (relatively larger size indicated as ‘Outer 1’) or spinel (relatively smaller size indicated as ‘Outer 2’), the chemistry of the outer oxide is same as that shown in the table inside the TEM image. Unlike the outer oxide layer consisting of several components or spinel, the inner oxide layer appears to consist of one structure and looks relatively uniform. Figs. 9 and 10 also show TEM bright field images and EDS analysis results indicating the oxide chemistry and structures of the as-received stainless steel exposed to 50 cm3/kg of DH concentration for 1000 h (Fig. 9) and of the 20%-20%-50% warm-rolled stainless steel exposed to 25 cm3/kg of DH concentration for 1000 h (Fig. 10), respectively. Commonly, the oxide structures consist of two layers, including an inner layer (Cr-rich oxide or protective oxide; FeCr2O4) and outer layer (Fe-based oxide or spinel-shaped oxide; Fe3O4). EDS results also show that Cr-rich oxides have relatively high concentration of Cr content but low concentration of Fe content. Further, the outer layers have relatively low concentration of Cr content but high concentration of Fe content. As expected from the results, the structure and composition of oxide layers are not affected by the dissolved hydrogen in the solution; nevertheless, the as-received stainless steel exposed to 50 cm3/kg of DH concentration has a relatively thick inner oxide layer compared to the specimen exposed to 25 cm3/kg of DH concentration. Additionally, the as-received stainless steel exposed to 50 cm3/kg of DH concentration exhibits local dissolution, as shown in Fig. 9. Based on the results, the corrosion rate seems higher at 50 cm3/kg of DH concentration than at 25 cm3/kg of DH concentration. Fig. 10 represents the oxide structure of 20%-20%-50% warmrolled stainless steel exposed to 25 cm3/kg of DH concentration for 1000 h (Fig. 11). This specimen does not have significantly thick inner oxide layer compared to the as-received stainless steel, but shows relatively several local dissolutions, indicating that the warm-rolled stainless steel has lower resistance on corrosion compared to the as-received stainless steel (shown in Fig. 8). 4. Discussion Based on the crack growth rate measurements and the characterization of oxide structures, which were performed using asreceived and 20%-20%-50% warm-rolled 316L stainless steels in high-temperature hydrogenated water, this section mainly describes the effects of warm rolling (‘4.1’ section) and DH concentration (‘4.2’ section) on the crack growth and corrosion resistance. 4.1. Effect of warm rolling on crack growth rate Fig. 11 illustrates a summary of the crack growth rate measurement data as a function of DH concentration, in comparison with existing literature data for cold-worked stainless steels in PWR water [25]. The results of the crack growth rate measurements for the as-received and warm-rolled specimens are shown for 25 cm3/kg of DH concentration, to demonstrate the effect of warm rolling on SCC growth. As can be seen, the crack growth rates increased with heat treatment and warm rolling in the simulation of the irradiation damages (such as radiation induced segregation and radiation hardening). Initially, because the depletion of Cr in grain boundary reduces the resistance on corrosion and induces the increased susceptibility to SCC, it was expected that crack growth rate was accelerated with warm rolling [1,15,26]. In the 20%-20%50% warm-rolled stainless steel, an increase in the crack growth rate was observed through the crack growth rate measurements shown in Fig. 11. To evaluate the effect of warm rolling on the corrosion
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Fig. 11. Crack growth rate measurement data as a function of DH concentration compared with existing literature data for cold-worked 304L and 316L stainless steels in PWR water.
resistance and SCC growth, the oxide films formed on the surfaces of the as-received and 20%-20%-50% warm-rolled stainless steels at 25 cm3/kg of DH concentration were additionally characterized (as shown in Figs. 8 and 10). In the as-received and 20%-20%50% warm-rolled stainless steels, the oxide films consist of an outer layer (Fe3O4) and inner layer (FeCr2O4), as confirmed through TEM-EDS analysis. The structure and composition of oxide film was not affected by warm rolling. However, the local dissolution was observed on surface of the 20%-20%-50% warm-rolled stainless steel exposed to 25 cm3/kg of DH concentration. Slip bands, induced by warm rolling, may be attributed not only to the increase in the local dissolution owing to the presence of lower bonding energy points compared to “perfect” crystals, but also to the linear degradation of passive film stability [27e30]. Further, multiplication of dislocations induces high stress concentration and modifies the local potential. Thus, it seems that corrosion resistance is decreased by the increased number of dislocations due to potential difference between matrix and dislocation inclusion [27,31,32]. In other words, the 20%-20%-50% warm-rolled stainless steel, having slip bands and dislocations that serve as paths for corrosion and cracking, shows localized corrosion on the surface (shown in Fig. 10). As a result, the warm-rolled stainless steel has a lower corrosion resistance, causing an increase in the crack growth rate. Further, the fracture mode was transgranular, not intergranular. This can be explained with the direction of 0.5 T specimen and warm rolling: 50% warm rolling, on the upper side or in direction “L”, resulted in the formation of slip band or dislocation, which is parallel to upper side or direction “L”. It is also parallel to the crack surface of the CT specimen. In this case, although the 20%-20%-50% warm-rolled stainless steel has sensitized grain boundary and slip band, which serve as potential crack paths [1], the cracks grow more easily in path that have a similar direction with that of the crack. Thus, the slip bands, which are directed parallel to the crack growth direction of 0.5 T specimen acted as paths for cracking, and transgranular cracking was observed.
4.2. Effect of DH concentration on crack growth Fig. 11 also illustrates the distribution of crack growth rate with DH concentration for both the present study and a previous study [25], to demonstrate the effect of DH concentration on SCC growth. The increase of DH concentration could enhance the crack growth rate of stainless steel 316L. To evaluate the effect of dissolved hydrogen on corrosion resistance and SCC growth, the oxide films formed on the surface of as-received stainless steel at 25 cm3/kg and 50 cm3/kg of DH concentrations for 1000 h were additionally characterized (as shown in Figs. 9 and 10). As described in the previous section, at both DH concentration conditions, the oxide films consist of an outer layer (Fe3O4) and inner layer (FeCr2O4), which were confirmed through TEM-EDS analysis. Because the inner layer or Cr-rich oxide has a relatively non-porous structure compared to the outer layer and is thermodynamically stable, it plays an important role as a barrier to corrosion and SCC. The structure and composition of oxide film is not affected by the change in the DH concentration in the solution; nevertheless, the thickness and the corrosion rate of the inner layer appear to increase with dissolved hydrogen. Additionally, the local dissolution was observed on surface of stainless steel exposed at 50 cm3/kg of DH concentration. Based on the results of this study and the previous study [33], which evaluated the effect of DH concentration on the corrosion resistance of stainless steel, the increases in the inner oxide layer and crack growth rate can be explained. Firstly, with the increase in dissolved hydrogen, cathodic process can be promoted and result in a higher critical and passive current. Therefore, the protective
Table 1 Chemical compositions of Type 316L stainless steel used in this study. Chemical composition (wt%) Fe Bal.
C 0.016
Si 0.49
Mn 1.38
P 0.033
S 0.002
Cr 16.6
Ni 10
Cu 0.13
N 0.09
Mo 2.02
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Table 2 Material properties of as-received and warm-rolled 316L stainless steels used in this study. Sample I.D.
As-received Warm-rolled
Heat treatment & warm rolling
Hardness (HV0.5)
Material properties (MPa or %)
1st
2nd
3rd
Avg.
Y.S.
T.S.
Elongation
e 20% (c)
e 20% (a-b)
e 30% (c)
135.48 295.34
218.5 810.0
574.0 911.5
65.5 14.5
Table 3 Loading sequence for the crack growth rate measurement testing in high-temperature water. Step
Load ratio
Waveform
Frequency (Hz)
0 1 2 3 4 5 6 7 8 9 10 11
0.1 0.3 0.5 0.7 0.7 0.7 0.7 0.7 1 0.7 0.7 1
Sine Sine Sine Sine Sine Sine Triangle Trapezoidal Constant Triangle Trapezoidal Constant
10 1 1 1 0.1 0.01 0.001 0.001 DH ¼ 25 cm3/kg 0.001 0.001 DH ¼ 50 cm3/kg
performance of the inner oxide film can drop dramatically. In other words, the increased dissolved hydrogen can reduce the stability of the inner oxide film. Next, ion diffusion is much easier, and the iron release rate may increase by reduction in the inner oxide film stability. Also, the adsorbed hydrogen atoms accelerate selfdiffusion and diffusivity of cavities. Thus, although the number of hydrogen ions may not be changed by the increased dissolved hydrogen, the reduced stability of the inner oxide causes the increase in the hydrogen adsorption, which decreases the local growth rate and the stability of the passive film. This can considerably reduce the effective solubility of hydrogen in a metal [34,35], thus accelerating the corrosion. Next, with the increase in dissolved hydrogen at high temperatures, more H2 diffuses through the porous outer layer and comes into the inner protective layer. Then, hydrogen can be adsorbed as an atom (dissociative chemisorption) because physisorption is possible only at low temperatures (less than 25 C). The hydrogen adsorption can locally decrease the growth rate or formation rate of the inner protective layer, resulting in the formation of the nonuniform stress field [36e40]. This can cause the interruption or the formation of small crack in the inner protective layer. In fact, at 50 cm3/kg of DH concentration (shown in Fig. 10), empty spaces were observed in the inner protective layer. The empty spaces may be attributed to the increase in the corrosion rate and oxide thickness. Thus, the increase in the DH concentration can accelerate the corrosion rate of stainless steel by aggravating the stability of the inner protective layer, resulting in the increase in crack growth rate.
5. Conclusion To investigate the effects of DH concentration and warm rolling on the resistance to SCC growth of 316L austenitic stainless steels, the crack growth was measured using as-received and 20%-20%50% warm-rolled stainless steels under simulated PWR primary water conditions. For simulating the radiation damages such as radiation-induced segregation and hardening, a warm rolling process was established, and the sample subjected to the warm rolling process was tested under PWR primary water conditions with 25 cm3/kg of DH
Hold (sec)
9000
9000
Kmax (MPa m1/2) Load: 3500(N) 26 28 30 30 30 30 30 30 30 30 30
concentration. The crack growth rate of warm-rolled 316L stainless steel was higher than that of as-received ones at both 25 and 50 cm3/kg of DH concentrations. It is believed that slip band and sensitized GB, which were formed during warm-rolling process and played a role as paths for more active corrosion and SCC, are relatively more effective to CGR than DH. Further, the dissolved hydrogen decreases the stability of the protective oxide layer and accelerates the corrosion rate, and the growth rate at 50 cm3/kg DH concentration may be higher than that at 25 cm3/kg DH concentration. Acknowledgement This work was financially supported by the Nuclear Power Core Technology Development Program (Nos. 20131520000140 and 2014151040004A) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade Industry and Energy (MOTIE) of Republic of Korea. References [1] K.E. Sickafus, E.A. Kotomin, B.P. Uberuaga, Radiation Effects in Solids, NATO Science Series II, Sicily, Italy, 2007. [2] P. Scott, A review of irradiation assisted stress corrosion cracking, J. Nucl. Mater. 211 (1994) 101e122. [3] H.M. Chung, W.J. Shack, Irradiation Assisted Stress Corrosion Cracking Behavior of Austenitic Stainless Steels Applicable to LWR Core Internals, NUREG/CR-6892, ANL 04/10, 2006. [4] A. Jenssen, L.G. Ljungberg, J. Walmsely, S. Fisher, Importance of molybdenum on irradiation-assisted stress corrosion cracking in austenitic stainless steels, Corr. Sci. 54 (1998) 48e60. [5] E.A. Kenik, Radiation-induced segregation in irradiated Type 304 stainless steels, J. Nucl. Mater. 187 (1992) 239e246. [6] S. Nakahigashi, M. Kodama, K. Fukuya, S. Nishimura, S. Yamamoto, K. Saito, T. Saito, Effects of neutron irradiation on corrosion and segregation behavior in austenitic stainless steels, J. Nucl. Mater. 179 (1991) 1061e1064. [7] G.S. Was, J.T. Busby, J. Gan, E.A. Kenik, A. Jenssen, S.M. Bruemmer, P.M. Scott, P.L. Andresen, Emulation of neutron irradiation effects with protons: validation of principle, J. Nucl. Mater. 300 (2002) 198e216. [8] G.R. Odette, G. Lucas, The effects of intermediate temperature irradiation on the mechanical behavior of 300-series austenitic stainless steels, J. Nucl. Mater. 179 (1991) 572e576. [9] T. Terachi, N. Totsuka, T. Yamada, T. Nakagawa, H. Deguchi, M. Horiuchi, M. Oschitani, Influence of dissolved hydrogen on structure of oxide film on alloy 600 formed primary water of pressurized water reactors, Nucl. Sci. Tech. 40 (2003) 509e516. [10] D.S. Morton, S.A. Attanasio, J.S. Fish, M.K. Schurman, Influence of Dissolved
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[25] Q. Raquer, E. Herms, F. Vaillant, T. Couvant, SCC of cold-worked austenitic stainless steels in PWR conditions, Adv. Mat. Sci. 7 (2007) 33e46. [26] T.G. Gooch, et al., Weld decay in AISI 304 stainless steel, Met. Const. Brit. Weld. J. 3 (1971) 927e932. [27] L. Peguet, B. Malki, B. Baroux, Influence of cold working on the pitting corrosion resistance of stainless steels, Corr. Sci. 49 (2007) 1933e1948. [28] N.D. Green, G.A. Saltzman, Scanning electrochemical microscopy part 13. Evaluation of the tip shapes of nanometer size microelectrodes, J. Elect. Chem. 328 (1992) 47e62. [29] M.C. Young, J.Y. Huang, R.C. Kuo, Corrosion fatigue behavior of cold-worked 304L stainless steel in a simulated BWR coolant environment, Mat. Trans. 50 (2009) 657e663. [30] H.S. Khatak, B. Raj, Corrosion of Austenitic Stainless Steels: Mechanism, Mitigation and Monitoring, Woodhead Publishing Limited, Elsevier, 2002. [31] K. Arioka, T. Yamada, T. Terachi, G. Chiba, Influence of carbide precipitation and rolling direction on intergranular stress corrosion cracking of austenitic stainless steels in hydrogenated high-temperature water, Corrosion 62 (2006) 568e575. [32] A. Miller, Y. Estrin, X.Z. Hu, Magnetic force microscopy of fatigue crack tip region in a316L stainless steel, Scr. Mater 47 (2002) 441e446. [33] T. Terachi, T. Yamada, T. Miyamoto, K. Arioka, K. Fukuya, Corrosion B behavior of stainless steels in simulated PWR primary waterdeffect of chromium content in alloys and dissolved hydrogen, J. Nucl. Sci. Tech. 45 (2008) 975e984. [34] K. Dozaki, D. Akutagawa, N. Nagata, H. Takiguchi, K. Norring, Effects of dissolved hydrogen content in PWR primary water on PWSCC initiation property, E J. Adv. Maint. 2 (2010) 65e76. [35] D.S. Morton, E.O. Connor, R.A. Etien, N. Lewis, E. Richey, SCC Initiation Testing of Nickel-based Alloys in High Temperature Water, International Cooperative Group on Environmentally Assisted Cracking, 2012. [36] T. Terachi, T. Yamada, T. Miyamoto, K. Arioka, SCC growth behaviors of austenitic stainless steels in simulated PWR primary water, J. Nucl. Mater. 426 (2012) 59e70. [37] S. Chen, M. Gao, R.P. Wei, Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels, Met. Mat. Trans. A 27A (1996) 29e40. [38] J.G. Yu, J.L. Luo, C.S. Zhang, P.R. Norton, Photoelectrochemical study of hydrogen-loaded passive film, J. Electrochem. Soc. 150 (2003) G291eG297. [39] T. Terachi, K. Fuji, K. Arioka, Microstructural characterization of SCC crack tip and oxide film for SUS 316 stainless steel in simulated PWR primary water at 320 C, J. Nucl. Sci. Tech. 42 (2005) 225e232. [40] D.D. Macdonald, The point defect model for the passive state, J. Electrochem. Soc. 139 (1992) 3434e3449.
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Crack growth behavior of warm-rolled 316L austenitic stainless steel in high-temperature hydrogenated water Kyoung Joon Choi a, Seung Chang Yoo a, Hyung-Ha Jin b, Junhyun Kwon b, Min-Jae Choi b, Seong Sik Hwang b, Ji Hyun Kim a, * a
Department of Nuclear Science and Engineering, School of Mechanical and Nuclear Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea Nuclear Materials Safety Research Division, Korea Atomic Energy Research Institute (KAERI), 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea
b
h i g h l i g h t s 316L Stainless steels were used for the study of crack growth behavior in PWR water. Warm rolling was applied to simulate the irradiation hardening of stainless steels. DH concentration was changed to see the effect on crack growth and oxide structure. Warm-rolled stainless steels showed higher rates of corrosion and crack growth. Higher DH concentration also promoted the rates of corrosion and crack growth.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2015 Received in revised form 10 February 2016 Accepted 29 April 2016 Available online 30 April 2016
To investigate the effects of warm rolling on the crack growth of 316L austenitic stainless steel, the crack growth rate was measured and the oxide structure was characterized in high-temperature hydrogenated water. The warm-rolled specimens showed a higher crack growth rate compared to the as-received specimens because the slip bands and dislocations produced during warm rolling served as paths for corrosion and cracking. The crack growth rate increased with the dissolved hydrogen concentration. This may be attributed to the decrease in performance and stability of the protective oxide layer formed on the surface of stainless steel in high-temperature water. © 2016 Elsevier B.V. All rights reserved.
Keywords: 316L stainless steel Warm rolling Crack growth rate measurement TEM Crack growth Oxidation
1. Introduction Irradiation-assisted stress corrosion cracking (IASCC) of the internals in a light water reactor is a critical issue for long-term operation. IASCC typically occurs at doses between 0.5 dpa (for boiling water reactors, BWRs) and 2e3 dpa (for pressurized water reactors, PWRs) [1,2], and it can therefore be expected that many light water reactors are exposed to the risk of IASCC. Irradiation produces defects and defect clusters in grains, alters
* Corresponding author. E-mail address: [email protected] (J.H. Kim). http://dx.doi.org/10.1016/j.jnucmat.2016.04.051 0022-3115/© 2016 Elsevier B.V. All rights reserved.
dislocations and dislocation loop structures, produces defecteimpurity and defecteclustereimpurity complexes, and eventually leading to radiation-induced hardening [1e8]. It also leads to changes in the stability of second-phase precipitates and to the local alloy chemistry near grain boundaries, precipitates, and defect clusters. The grain boundary microchemistry that differs significantly from the bulk composition can be produced in association with both radiation-induced segregation and thermallydriven segregation of alloying and impurity elements. Among several radiation-related damages, radiation-induced segregation and radiation-induced hardening have been shown to make steels more susceptible to IASCC. The radiation damages can be increased by long-term operation, consequently resulting in the increase in
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Fig. 1. Schematic of (a) heat treatment at 650 C for 100 h and warm rolling processing at 250 C, and (b) orientation of 0.5T CT for the crack growth rate measurement testing.
the degradation of reactor internals, including IASCC. In fact, some cracking of the internal structures, such as guide tube support pins and baffle former bolts in operating PWR plants, has already been identified. Among various internal structures, baffle-former bolts assembling formers and baffles exhibit relatively high susceptibility to IASCC owing to the relatively high irradiation dose and tensile stress occurring after the assembly of formers and baffles. Further, for the extension of life-time of PWRs, the integrity of the internal structures needs to be studied more carefully. Regardless of those importance, however, preparing the materials which are irradiated under operating PWRs or simulating facilities is of difficulty as it accompanies high cost, long-exposure time, and radiation safety. Therefore, a warm rolling and heat treatment was carried out to simulate irradiated materials because it is a time-saving, cost-effective, and radiation-free technique. Even if the warm rolling and heat treatment represents those advantages, the process is not able to fully simulate irradiation of materials; it is suitable for the simulation of the radiation-induced segregation and radiation-induced hardening, which were known as the essential phenomena of IASCC. The warm rolling induces the formation of dislocations in the material and harden materials. The heat treatment causes the sensitization of stainless steels resulting in Cr depletion and Ni enrichment similar to radiation- induced segregation. The detail experimental procedure used in this study is described in the ‘Section 2.1. Materials’. In addition, it has been recently reported that the stress
corrosion cracking (SCC) susceptibility for Ni base alloys depends on the dissolved hydrogen (DH) concentration, and the SCC susceptibility is maximum under around 10 cm3/kg of DH concentration where the Ni/NiO phase transition exists [9,10]. In other words, the peak of stress corrosion rates occurring at the phase transition tends to decrease symmetrically as delta EcP values move away from the Ni/NiO phase transition [11,12]. To reduce the susceptibility of the alloy to SCC, there has been a tendency to increase the DH concentration of the primary section of pressurized water power plants up to approximately 50 cm3/kg. Although 50 cm3/kg of DH concentration can reduce the susceptibility of a Ni base alloy to SCC, the susceptibility of stainless steel to SCC may be aggravated [13e15]. Therefore, in this study, warm rolling and heat-treatment were applied to 316L stainless steel, in order to simulate the effect of radiation damages such as radiation-induced segregation and hardening. Further, the crack growth rate was tested under constant load with simulated primary water conditions of a PWR, which are 325 C, 17.5 MPa, dissolved oxygen concentration of less than 5 ppb, DH concentration of 25 cm3/kg, and LiOH and H3BO3 concentrations of 2 and 1200 ppm (mg/kg), respectively. Moreover, to investigate the effect of DH concentration on the crack growth of as-received stainless steel, the DH concentration was varied between 25 and 50 cm3/kg under the simulated primary water conditions of a PWR. The possible mechanism for the results of crack growth behavior
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Fig. 2. Micro-structural and micro-chemical analysis of as-received and warm-rolled 316L stainless steels used in this study. (a) OM and SEM images of the as-received 316L stainless steel, (b) OM and SEM images of the warm rolled 316L stainless steel, and (c) Dark field TEM image of grain boundary region and line EDS profile across the grain boundary in the as-received and warm-rolled 316L stainless steels.
obtained from this study is explained based on the analysis of the structure and composition of the oxide film and the fracture surfaces of the post-test specimens by using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). 2. Experimental 2.1. Materials To investigate the effects of warm rolling and DH concentration
on the crack growth rate, a plate of 316L austenitic stainless steel was used; its composition is presented in Table 1. Fig. 1 (a) illustrates the heat treatment and the warm rolling process that were used to simulate radiation damages, such as radiation-induced segregation and radiation-induced hardening. Heat treatment was performed to simulate chromium depletion in the grain boundary, which is similar to radiation induced segregation. Next, warm rolling was performed to simulate radiation induced hardening. Although other methods (such as cold working) can be used to simulate hardening, warm rolling was selected so as to avoid the formation of unfavourable microstructural phase (e.g. martensitic
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Fig. 3. Schematic of the experimental system used for the crack growth measurement testing in high-temperature and high-pressure water.
phase), which is not observed in irradiated stainless steel [16,17]. The phase change from austenitic structure into martensitic structure was actually not observed after the warm rolling, and it was confirmed through the microstructural analysis. To avoid any excessive distortion of grain or anisotropic microstructure, warm rolling was performed in multiple step and direction. By this procedure, key microstructural features such as grain size and shape were kept same after the warm rolling process. Detailed process is explained below. Heat treatment was performed at 630 C for 100 h. Then, 30% of warm rolling at 250 C was conducted in the ‘c’ direction (first step), followed by 20% of warm rolling in the ‘a’ and ‘b’ directions (second and third steps). Finally, 20% of warm rolling was conducted in the ‘c’ direction (fourth step). All the warm rolling steps were performed at 250 C, and 20%-20%-50% warm-rolled 316L stainless steel plates were prepared. The hardening and the microstructural change were confirmed through the hardness values, tensile tests, optical microscopy (OM) and scanning electron microscopy (SEM) analyses (shown in Table 2 and Fig. 2). Fig. 2 (a) and (b) are OM and SEM images, representing the microstructures of as-received and warm rolled (‘S-T’ plane) stainless steels. In the OM images, it was observed that as-received stainless steel has austenitic structure and its grain size is about 73.4 mm. And the warm-rolled stainless steel also has austenitic structure and its grain size is about 75.1 mm, representing no change of grain size or shape. However, SEM images indicate
that there exist many dislocations slipping in individual grains of warm-rolled stainless steel comparing to one of as-received stainless steel. It is considered that the slip bands caused the hardening of warm-rolled stainless steel, which was confirmed through the results of tensile test and hardness test shown in Table 2. After the warm rolling and heat treatment, the elongation was reduced, but yield stress and tensile strength was increased. And, the Cr depletion and Ni enrichment in grain boundary, simulated with heat treatment, was confirmed through the TEM analysis as shown Fig. 2 (c). 2.2. Crack growth rate measurement 2.2.1. Specimen For the crack growth measurement, the as-received and 20%20%-50% warm-rolled specimens were machined as 0.5T compact tension-type (0.5T CT) specimens having 10.6 mm of initial crack (noted crack), 25 mm of length, 12.5 mm of thickness and 5% sidegrooves according to the American Society of Testing and Materials Standard (ASTM) E399 [18,19]. The 20%-20%-50% warm-rolled specimen has S-T orientation, i.e., the crack growth direction is parallel to main rolling direction (‘c’ direction or T direction), as shown in Fig. 1. Further, the as-received and 20%-20%-50% warmrolled specimens were machined with 5% side grooves on each side and instrumented with platinum current and potential probe leads for direct current (DC) potential drop measurements of the
Fig. 4. Measured data during the crack growth measurement of as-received 316L stainless steel in PWR water. (a) Total measurement curve with as-received 316L stainless steel in PWR water, (b) Measured data of crack length and DH concentration with 25 cm3/kg of DH concentration in PWR water, and (c) Measured data with 50 cm3/kg of DH concentration in PWR water.
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Fig. 5. Photographs of post-test fracture surfaces of as-received 316L stainless steel. (a) Macrophotograph of post-test fracture surface of as-received 316L stainless steel, and (b) High-magnification image in the region of crack growth under constant loading condition indicated in (a).
crack length as shown in Ref. [19]. The current flow through the sample was reversed about once per second, primarily to reduce measurement errors associated with thermocouple effects and amplifier offsets [20e22]. To minimizing the electrical noise from the other system (such as servo motor), Zirconia sleeves and washers were used to electrically insulate the 0.5T CT specimens from the loading pins and grips. Also, some filters (including running-average filter and low-pass filter) were applied and they could remove the temporary change but not voltage change by cracking. Running-average filter calculates the statistical average of measured data during a certain period of time, and creates the series of averages of subsets of the full CGR data set. By taking the average of the initial subset of the CGR data, the first element of the running average can be obtained. Then, the subset is shifting forward, with including the next number of the original subset in the data, and excluding the first number of the data. This process can create a new subset of data, which is averaged, and this procedure repeats over the entire CGR data. The relationship between the crack length and voltage was determined according to the method of Hicks and Pickard [18]. For the evaluation of warm rolling and dissolved hydrogen effects on corrosion or oxide structures, as-received and warm-rolled plate specimens of 10 mm 10 mm 2 mm were exposed to the high temperature and pressure water environment varing 25 and 50 cm3/kg of DH concentrations, and TEM analysis was performed.
makeup system from the laboratory water supply system. The makeup system then circulates the water through a demineralizer/ filter system to ensure cleanliness. The makeup water is supplied through a chemistry conditioning system, in which the water chemistry is adjusted, to the autoclave system. Provision has been made for injection of chemicals as well as for gas purging. In the present study, to evaluate the effects of DH concentration and warm rolling on the susceptibility to SCC growth for asreceived specimen, the crack growth rate was measured at both 25 cm3/kg and 50 cm3/kg of DH concentration. Further, for the 20%20%-50% warm-rolled specimen, the crack growth rate was measured at 25 cm3/kg DH concentration. The tests were performed in a 3.79 L stainless steel autoclave, which can maintain conditions of 1200 ppm (mg/kg) B, 2 ppm (mg/kg) Li, 25 and 50 cm3/kg of DH concentrations, dissolved oxygen < 5 ppb, 340 C, and 17.5 MPa. Proper amount of boric acid (H3BO3) and lithium hydroxide monohydrate (LiOH) were added in distilled water to simulate the primary coolant of nuclear power plant and electronic conductivity of this solution were maintain to 21e22 mS/cm. Then DH was controlled by overpressure with 99% hydrogen gas. DH was measured at room temperature and atmospheric pressure by using DH sensor installed in inlet water tank. And, copper/cuprous oxide (Cu/Cu2O) reference electrode was placed in the autoclave immediately adjacent to the 0.5T CT specimen, and used for the measurement of EcP.
2.2.2. Environmental conditions Tests were conducted in an autoclave system specially constructed for the present study, as shown schematically in Fig. 3. Careful consideration was given for ensuring extremely rigorous chemistry control, and near-theoretical water conductivity was achieved routinely. The system is fully instrumented for measuring oxygen, hydrogen, pH, and conductivity for both the inlet and outlet water. Distilled and demineralized water is supplied to the
2.2.3. Loading condition In this study, as-received specimens and 20%-20%-50% warmrolled stainless steel specimens were used to evaluate the effects of DH concentration and warm rolling on the crack growth rate. The loading conditions for the crack growth rate measurements are arranged in Table 3. Firstly, for sharpening of a machined notch, the 0.5T CT specimens was fatigue pre-cracked at a frequency of 10 Hz for load ratio (minimum load/maximum load) of 0.1 with the
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Fig. 6. Measured data during the crack growth measurement of warm-rolled 316L stainless steel in PWR water. (a) Total measurement curve with warm-rolled 316L stainless steel in PWR water, and (b) Measured data of crack length and DH concentration with 25 cm3/kg of DH concentration in PWR water.
maximum load 3500 N, determined based on ASTM E399. Next, at a frequency of 1 Hz for load ratio of 0.3, 0.5, and 0.7, the loading processes (up to step 4) were performed in air environment. Subsequently, in high temperature and pressure water environment (340 C and 17.5 MPa) which was described in the previous section 2.2.2, the following loading for the sharpening was performed at load ratio 0.7 and frequencies of 0.1 and 0.01 Hz, and finally was completed at a frequency of 0.001 Hz, for load ratio 0.7, with triangle (step 6) and trapezoidal (step 7) waveforms. Under constant loading (corresponding to about 30 MPa m1/2 of stress intensity factor KIC), the crack growth rate measurement was performed at 25 cm3/kg of DH concentration (step 8). Warm-rolled specimen was tested at only 25 cm3/kg of DH concentration, and the testing was completed at step 8. But, in case of as-received specimen, the crack growth rate measurements were performed
at both 25 and 50 cm3/kg of DH concentrations. After step 8, the crack sharpening under the fatigue loading, which is similar with the ones performed at steps 6 and 7, was performed at 50 cm3/kg of DH concentration. It could remove the previous effects of crack growth rate measurement testing at 25 cm3/kg of DH concentration. And then the crack growth rate at 50 cm3/kg of DH concentration was measured under constant loading. The stress intensity KIC at each steps were calculated by the measured load and the crack length derived by the DCPD method and corrected by post-test fractography. In the DCPD method, there are some limitations mainly associated with possible interference due to crack closure effects [20,21,23,24]. For this reason, the method is biased toward measuring the minimum crack depth. Thus, the technique will only measure an area that is then converted to an averaged value of crack length. An uneven crack front
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Fig. 7. Photographs of post-test fracture surfaces of warm-rolled 316L stainless steel. (a) Macrophotograph of post-test fracture surface of warm-rolled 316L stainless steel, and (b) High-magnification image in the region of crack growth under constant loading condition indicated in (a).
Fig. 4 illustrates the results of the crack growth measurements
for as-received 316L austenitic stainless steel under test conditions simulating the primary section in a PWR. The total elapsed time was 2325 h, and the water conditions were stabilized after precracking was performed for 420 h. At the each DH concentrations, the crack growth rates were derived by linear-fitting the several points of crack length measured with the DCPD method. At 25 cm3/kg of DH concentration (Fig. 4 (b)), the crack growth rate was found to be (4.48 ± 0.15) 109 mm/s, estimated in the region
Fig. 8. TEM image and EDS analysis results of as-received 316L stainless steel exposed to PWR water containing 25 cm3/kg of DH concentration for 1000 h. The bottom-right image in the red dotted box shows the magnified view of the top-left region of the main image. While the main structure of the outer oxide is composed of crystallite (relatively larger size indicated as ‘Outer 1’) or spinel (relatively smaller size indicated as ‘Outer 2’), the chemistry of the outer oxide is same that as shown in the table inside the TEM image. Also, note that outer 1 and outer 2 have same chemical composition, so one representative data was shown in the table. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. TEM image and EDS analysis results of as-received 316L stainless steel exposed to PWR water containing 50 cm3/kg of DH concentration for 1000 h. The bottom-right image shows the magnified view of the top-right region of the main image. While the main structure of the outer oxide is composed of crystallite (relatively larger size indicated as ‘Outer 1’) or spinel (relatively smaller size indicated as ‘Outer 2’), the chemistry of the outer oxide is same that as shown in the table inside the TEM image. Also, note that outer 1 and outer 2 have same chemical composition, so one representative data was shown in the table.
will also introduce error in crack length as will local crack extensions such as crack tunneling. In this study, post-test fractography was conducted in order to normalize the process and to establish more accurate crack length values. 3. Results
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Fig. 10. TEM image and EDS analysis results of warm-rolled 316L stainless steel exposed to PWR water containing 25 cm3/kg of DH concentration for 1000 h. The bottom-right image shows the magnified view of the top-center region of the main image. While the main structure of the outer oxide is composed of crystallite (relatively larger size indicated as ‘Outer 1’) or spinel (relatively smaller size indicated as ‘Outer 2’), the chemistry of the outer oxide is same that as shown in the table inside the TEM image. Also, note that outer 1 and outer 2 have same chemical composition, so one representative data was shown in the table.
from 1279 h to 1750 h. Conversely, at 50 cm3/kg of DH concentration (Fig. 4 (c)), the growth rate was found to be (2.31 ± 0.01) 108 mm/s, estimated in the region from 1878 h to 2325 h; this growth rate is relatively high compared to that at 25 cm3/kg of DH concentration. The fracture mode was determined based on Fig. 5 (which show photographs of post-test fracture surfaces of as-received stainless steel). In Fig. 5 (b), which is a high-magnification image in the region of steps 8e11 (between line (2) and line (3)) indicated in Fig. 5 (a), there are fracture surfaces formed at 25 cm3/kg and 50 cm3/kg of DH concentrations under constant loading. Under both DH concentration conditions, the transgranular cracks were observed in photographs (shown in Fig. 5). Fig. 6 shows the results of the crack growth measurement performed for 20%-20%-50% warm-rolled 316L austenitic stainless steel under test conditions simulating the primary section in a PWR. The total elapsed time was 1370 h, and the water conditions were stabilized after pre-cracking was performed for 570 h. Under 25 cm3/kg of DH concentration (Fig. 6 (b)), the crack growth rate was found to be (4.70 ± 0.02) 108 mm/s, estimated in the region from 570 h to 1370 h; this crack growth rate is relatively compared to that of the as-received specimen. The fracture mode was determined based on Fig. 7 (which shows photographs of the post-test fracture surfaces of warm-rolled stainless steel). In Fig. 7 (b), which is a high-magnification image in the region of step 8 (between line (2) and line (3)) indicated in Fig. 7 (a), there is fracture surface formed at 50 cm3/kg of DH concentration under constant loading. The transgranular crack also was observed in photographs (shown in Fig. 7). Fig. 8 indicates TEM bright field images and EDS analysis results, which indicate the oxide structures formed on the surface of asreceived stainless steel exposed to 25 cm3/kg of DH concentration for 1000 h. The oxide layer is composed of an inner layer (Cr rich oxide or protective oxide; FeCr2O4) and outer layer (Fe based oxide or spinel shaped oxide; Fe3O4). The EDS results indicate that the inner oxide has relatively high concentration of chromium, like Crrich oxide, but the outer oxide has relatively high concentration of
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iron, like Fe-based oxide. While the main structure of outer oxide is composed of crystallite (relatively larger size indicated as ‘Outer 1’) or spinel (relatively smaller size indicated as ‘Outer 2’), the chemistry of the outer oxide is same as that shown in the table inside the TEM image. Unlike the outer oxide layer consisting of several components or spinel, the inner oxide layer appears to consist of one structure and looks relatively uniform. Figs. 9 and 10 also show TEM bright field images and EDS analysis results indicating the oxide chemistry and structures of the as-received stainless steel exposed to 50 cm3/kg of DH concentration for 1000 h (Fig. 9) and of the 20%-20%-50% warm-rolled stainless steel exposed to 25 cm3/kg of DH concentration for 1000 h (Fig. 10), respectively. Commonly, the oxide structures consist of two layers, including an inner layer (Cr-rich oxide or protective oxide; FeCr2O4) and outer layer (Fe-based oxide or spinel-shaped oxide; Fe3O4). EDS results also show that Cr-rich oxides have relatively high concentration of Cr content but low concentration of Fe content. Further, the outer layers have relatively low concentration of Cr content but high concentration of Fe content. As expected from the results, the structure and composition of oxide layers are not affected by the dissolved hydrogen in the solution; nevertheless, the as-received stainless steel exposed to 50 cm3/kg of DH concentration has a relatively thick inner oxide layer compared to the specimen exposed to 25 cm3/kg of DH concentration. Additionally, the as-received stainless steel exposed to 50 cm3/kg of DH concentration exhibits local dissolution, as shown in Fig. 9. Based on the results, the corrosion rate seems higher at 50 cm3/kg of DH concentration than at 25 cm3/kg of DH concentration. Fig. 10 represents the oxide structure of 20%-20%-50% warmrolled stainless steel exposed to 25 cm3/kg of DH concentration for 1000 h (Fig. 11). This specimen does not have significantly thick inner oxide layer compared to the as-received stainless steel, but shows relatively several local dissolutions, indicating that the warm-rolled stainless steel has lower resistance on corrosion compared to the as-received stainless steel (shown in Fig. 8). 4. Discussion Based on the crack growth rate measurements and the characterization of oxide structures, which were performed using asreceived and 20%-20%-50% warm-rolled 316L stainless steels in high-temperature hydrogenated water, this section mainly describes the effects of warm rolling (‘4.1’ section) and DH concentration (‘4.2’ section) on the crack growth and corrosion resistance. 4.1. Effect of warm rolling on crack growth rate Fig. 11 illustrates a summary of the crack growth rate measurement data as a function of DH concentration, in comparison with existing literature data for cold-worked stainless steels in PWR water [25]. The results of the crack growth rate measurements for the as-received and warm-rolled specimens are shown for 25 cm3/kg of DH concentration, to demonstrate the effect of warm rolling on SCC growth. As can be seen, the crack growth rates increased with heat treatment and warm rolling in the simulation of the irradiation damages (such as radiation induced segregation and radiation hardening). Initially, because the depletion of Cr in grain boundary reduces the resistance on corrosion and induces the increased susceptibility to SCC, it was expected that crack growth rate was accelerated with warm rolling [1,15,26]. In the 20%-20%50% warm-rolled stainless steel, an increase in the crack growth rate was observed through the crack growth rate measurements shown in Fig. 11. To evaluate the effect of warm rolling on the corrosion
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Fig. 11. Crack growth rate measurement data as a function of DH concentration compared with existing literature data for cold-worked 304L and 316L stainless steels in PWR water.
resistance and SCC growth, the oxide films formed on the surfaces of the as-received and 20%-20%-50% warm-rolled stainless steels at 25 cm3/kg of DH concentration were additionally characterized (as shown in Figs. 8 and 10). In the as-received and 20%-20%50% warm-rolled stainless steels, the oxide films consist of an outer layer (Fe3O4) and inner layer (FeCr2O4), as confirmed through TEM-EDS analysis. The structure and composition of oxide film was not affected by warm rolling. However, the local dissolution was observed on surface of the 20%-20%-50% warm-rolled stainless steel exposed to 25 cm3/kg of DH concentration. Slip bands, induced by warm rolling, may be attributed not only to the increase in the local dissolution owing to the presence of lower bonding energy points compared to “perfect” crystals, but also to the linear degradation of passive film stability [27e30]. Further, multiplication of dislocations induces high stress concentration and modifies the local potential. Thus, it seems that corrosion resistance is decreased by the increased number of dislocations due to potential difference between matrix and dislocation inclusion [27,31,32]. In other words, the 20%-20%-50% warm-rolled stainless steel, having slip bands and dislocations that serve as paths for corrosion and cracking, shows localized corrosion on the surface (shown in Fig. 10). As a result, the warm-rolled stainless steel has a lower corrosion resistance, causing an increase in the crack growth rate. Further, the fracture mode was transgranular, not intergranular. This can be explained with the direction of 0.5 T specimen and warm rolling: 50% warm rolling, on the upper side or in direction “L”, resulted in the formation of slip band or dislocation, which is parallel to upper side or direction “L”. It is also parallel to the crack surface of the CT specimen. In this case, although the 20%-20%-50% warm-rolled stainless steel has sensitized grain boundary and slip band, which serve as potential crack paths [1], the cracks grow more easily in path that have a similar direction with that of the crack. Thus, the slip bands, which are directed parallel to the crack growth direction of 0.5 T specimen acted as paths for cracking, and transgranular cracking was observed.
4.2. Effect of DH concentration on crack growth Fig. 11 also illustrates the distribution of crack growth rate with DH concentration for both the present study and a previous study [25], to demonstrate the effect of DH concentration on SCC growth. The increase of DH concentration could enhance the crack growth rate of stainless steel 316L. To evaluate the effect of dissolved hydrogen on corrosion resistance and SCC growth, the oxide films formed on the surface of as-received stainless steel at 25 cm3/kg and 50 cm3/kg of DH concentrations for 1000 h were additionally characterized (as shown in Figs. 9 and 10). As described in the previous section, at both DH concentration conditions, the oxide films consist of an outer layer (Fe3O4) and inner layer (FeCr2O4), which were confirmed through TEM-EDS analysis. Because the inner layer or Cr-rich oxide has a relatively non-porous structure compared to the outer layer and is thermodynamically stable, it plays an important role as a barrier to corrosion and SCC. The structure and composition of oxide film is not affected by the change in the DH concentration in the solution; nevertheless, the thickness and the corrosion rate of the inner layer appear to increase with dissolved hydrogen. Additionally, the local dissolution was observed on surface of stainless steel exposed at 50 cm3/kg of DH concentration. Based on the results of this study and the previous study [33], which evaluated the effect of DH concentration on the corrosion resistance of stainless steel, the increases in the inner oxide layer and crack growth rate can be explained. Firstly, with the increase in dissolved hydrogen, cathodic process can be promoted and result in a higher critical and passive current. Therefore, the protective
Table 1 Chemical compositions of Type 316L stainless steel used in this study. Chemical composition (wt%) Fe Bal.
C 0.016
Si 0.49
Mn 1.38
P 0.033
S 0.002
Cr 16.6
Ni 10
Cu 0.13
N 0.09
Mo 2.02
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253
Table 2 Material properties of as-received and warm-rolled 316L stainless steels used in this study. Sample I.D.
As-received Warm-rolled
Heat treatment & warm rolling
Hardness (HV0.5)
Material properties (MPa or %)
1st
2nd
3rd
Avg.
Y.S.
T.S.
Elongation
e 20% (c)
e 20% (a-b)
e 30% (c)
135.48 295.34
218.5 810.0
574.0 911.5
65.5 14.5
Table 3 Loading sequence for the crack growth rate measurement testing in high-temperature water. Step
Load ratio
Waveform
Frequency (Hz)
0 1 2 3 4 5 6 7 8 9 10 11
0.1 0.3 0.5 0.7 0.7 0.7 0.7 0.7 1 0.7 0.7 1
Sine Sine Sine Sine Sine Sine Triangle Trapezoidal Constant Triangle Trapezoidal Constant
10 1 1 1 0.1 0.01 0.001 0.001 DH ¼ 25 cm3/kg 0.001 0.001 DH ¼ 50 cm3/kg
performance of the inner oxide film can drop dramatically. In other words, the increased dissolved hydrogen can reduce the stability of the inner oxide film. Next, ion diffusion is much easier, and the iron release rate may increase by reduction in the inner oxide film stability. Also, the adsorbed hydrogen atoms accelerate selfdiffusion and diffusivity of cavities. Thus, although the number of hydrogen ions may not be changed by the increased dissolved hydrogen, the reduced stability of the inner oxide causes the increase in the hydrogen adsorption, which decreases the local growth rate and the stability of the passive film. This can considerably reduce the effective solubility of hydrogen in a metal [34,35], thus accelerating the corrosion. Next, with the increase in dissolved hydrogen at high temperatures, more H2 diffuses through the porous outer layer and comes into the inner protective layer. Then, hydrogen can be adsorbed as an atom (dissociative chemisorption) because physisorption is possible only at low temperatures (less than 25 C). The hydrogen adsorption can locally decrease the growth rate or formation rate of the inner protective layer, resulting in the formation of the nonuniform stress field [36e40]. This can cause the interruption or the formation of small crack in the inner protective layer. In fact, at 50 cm3/kg of DH concentration (shown in Fig. 10), empty spaces were observed in the inner protective layer. The empty spaces may be attributed to the increase in the corrosion rate and oxide thickness. Thus, the increase in the DH concentration can accelerate the corrosion rate of stainless steel by aggravating the stability of the inner protective layer, resulting in the increase in crack growth rate.
5. Conclusion To investigate the effects of DH concentration and warm rolling on the resistance to SCC growth of 316L austenitic stainless steels, the crack growth was measured using as-received and 20%-20%50% warm-rolled stainless steels under simulated PWR primary water conditions. For simulating the radiation damages such as radiation-induced segregation and hardening, a warm rolling process was established, and the sample subjected to the warm rolling process was tested under PWR primary water conditions with 25 cm3/kg of DH
Hold (sec)
9000
9000
Kmax (MPa m1/2) Load: 3500(N) 26 28 30 30 30 30 30 30 30 30 30
concentration. The crack growth rate of warm-rolled 316L stainless steel was higher than that of as-received ones at both 25 and 50 cm3/kg of DH concentrations. It is believed that slip band and sensitized GB, which were formed during warm-rolling process and played a role as paths for more active corrosion and SCC, are relatively more effective to CGR than DH. Further, the dissolved hydrogen decreases the stability of the protective oxide layer and accelerates the corrosion rate, and the growth rate at 50 cm3/kg DH concentration may be higher than that at 25 cm3/kg DH concentration. Acknowledgement This work was financially supported by the Nuclear Power Core Technology Development Program (Nos. 20131520000140 and 2014151040004A) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade Industry and Energy (MOTIE) of Republic of Korea. References [1] K.E. Sickafus, E.A. Kotomin, B.P. Uberuaga, Radiation Effects in Solids, NATO Science Series II, Sicily, Italy, 2007. [2] P. Scott, A review of irradiation assisted stress corrosion cracking, J. Nucl. Mater. 211 (1994) 101e122. [3] H.M. Chung, W.J. Shack, Irradiation Assisted Stress Corrosion Cracking Behavior of Austenitic Stainless Steels Applicable to LWR Core Internals, NUREG/CR-6892, ANL 04/10, 2006. [4] A. Jenssen, L.G. Ljungberg, J. Walmsely, S. Fisher, Importance of molybdenum on irradiation-assisted stress corrosion cracking in austenitic stainless steels, Corr. Sci. 54 (1998) 48e60. [5] E.A. Kenik, Radiation-induced segregation in irradiated Type 304 stainless steels, J. Nucl. Mater. 187 (1992) 239e246. [6] S. Nakahigashi, M. Kodama, K. Fukuya, S. Nishimura, S. Yamamoto, K. Saito, T. Saito, Effects of neutron irradiation on corrosion and segregation behavior in austenitic stainless steels, J. Nucl. Mater. 179 (1991) 1061e1064. [7] G.S. Was, J.T. Busby, J. Gan, E.A. Kenik, A. Jenssen, S.M. Bruemmer, P.M. Scott, P.L. Andresen, Emulation of neutron irradiation effects with protons: validation of principle, J. Nucl. Mater. 300 (2002) 198e216. [8] G.R. Odette, G. Lucas, The effects of intermediate temperature irradiation on the mechanical behavior of 300-series austenitic stainless steels, J. Nucl. Mater. 179 (1991) 572e576. [9] T. Terachi, N. Totsuka, T. Yamada, T. Nakagawa, H. Deguchi, M. Horiuchi, M. Oschitani, Influence of dissolved hydrogen on structure of oxide film on alloy 600 formed primary water of pressurized water reactors, Nucl. Sci. Tech. 40 (2003) 509e516. [10] D.S. Morton, S.A. Attanasio, J.S. Fish, M.K. Schurman, Influence of Dissolved
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