3.1 Effect evaluation of solution heat treatment holding time
The high intergranular corrosion resistance of austenitic stainless steel required by the nuclear power industry is greatly affected by the process parameters of solution heat treatment during the material production process that determines the solubility of precipitates near the grain boundary
1,10). Therefore, ASTM A262 Practice A and E, TEM, and DL-EPR tests have been conducted to investigate the effect of heat treatment temperature of 1,038~1,121°C, the solution heat treatment condition specified in the nuclear regulatory requirements, and the holding time of 0.5~1.0 per 2.54cm on the intergranular corrosion.
In the specimen with the thickness of 2.54cm subjected to sensitization at 675°C for 1 hour before performing solution heat treatment, ditch structure, in which the sensitization by chromium carbide continuously precipitated at the grain boundary is suspected, was observed in the ASTM A262 Practice A test, as shown in
Fig. 5. Also, as in
Fig. 6, in the ASTM A262 Practice E test, micro cracks in micro fissure form were observed in the bent areas on which the bending test was conducted. It is known that the formation of these micro fissures is due to the ditch structure caused by intergranular corrosion
11).
Fig. 5
Etch structures on the sensitized 2.54cm-thick specimen (675°C for 1 h, WC) by ASTM A262 Practice A test (×250)
Fig. 6
Bent area view of the sensitized 2.54cm-thick specimen (675°C for 1 h, WC) by ASTM A262 Practice E test (×60)
For analyzing the causes of forming the ditch structure and micro cracks in the form of micro fissures observed in the ASTM A262 practice test, TEM analysis was performed as shown in
Fig. 7, and it was possible to verify the presence of chromium carbide (Cr
23C
6) of the FCC structure clustered in the form of nanoparticles with a diameter of about 50~300 nm at the grain boundary. From this result, it is thought that the ditch structure and micro cracks identified in the surface of the sensitized specimen is caused by the precipitation of chromium carbide at the grain boundary.
Fig. 7
TEM analysis results of the sensitized 2.54cm- thick specimen (675°C for 1 h, WC). (a) dark field image (x7k), (b) mapping on ’A’ (20k), (c) EDS spectra on ’B’, (d) SAD pattern on ’B’, respectively
On the other hand, in all specimens subjected to solution heat treatment for 1 minute, 5 minutes, 10 minutes, 15 minutes and 30 minutes at 1,038°C and 1,121°C, as shown in
Fig. 8 and
Fig. 9, chromium carbide at grain boundary which was continuously precipitated in the ASTM A262 Practice A test was completely dissolved and a step structure, which is the conforming structure that can pass the test was observed. Also, as in
Fig. 10 and
Fig. 11, in the ASTM A262 Practice E test, micro cracks of micro-fissure-type were not observed in the bent areas of the bending test.
Fig. 8
Etch structures of the 2.54cm-thick specimens solution heat treated at 1,038°C for (a) 1min, (b) 5min, (c) 10min, (d) 15min, (e) 30min, respectively, by ASTM A262 Practice A test (×250)
Fig. 9
Etch structures on the 2.54cm-thick specimens solution heat treated at 1,121°C for (a) 1min, (b) 5min, (c) 10min, (d) 15min, (e) 30min, respectively, by ASTM A262 Practice A test (×250)
Fig. 10
Bent area view of the 2.54cm-thick specimens solution heat treated at 1,038°C for (a) 1min, (b) 5min, (c) 10min, (d) 15min, (e) 30min, respectively, by ASTM A262 Practice E test (×60).
Fig. 11
Bent area view of the 2.54cm-thick specimens solution heat treated at 1,121°C for (a) 1min, (b) 5min, (c) 10min, (d) 15min, (e) 30min, respectively, by ASTM A262 Practice E test (×60)
The result of the observation of step structure without micro cracks in the ASTM A262 Practice test was consistent with the TEM analysis result of
Fig. 12, in which the chromium carbide precipitated at the grain boundary was completely dissolved into the grains and disappeared through solution heat treatment at 1,038°C for 1 minute or longer.
Fig. 12
TEM micrographs of the 2.54cm-thick specimens solution heat treated at 1,038°C for (a) 1min, (b) 5min, (c) 10min, (d) 15min, (e) 30min, respectively, (×20k, ×7k)
In order to analyze the correlation with ASTM A262 test according to sensitization heat treatment and solution heat treatment and to quantify DOS, a DL-EPR test was performed. As shown in
Fig. 13 and
Fig. 14, the corrosion potential of all specimens subjected to solution heat treatment was about -0.4V
SCE, regardless of the heat treatment temperature and holding time, and anodic dissolution reaction occurred from this corrosion potential to about -0.2V
SCE, the basic passivation potential. Thereafter, the potential reached about+0.2V
SCE, a passivation region accompanied by a decrease in current density. During the reverse potential scanning, two or more current peaks were observed by hydrogen reduction reaction and anodic dissolution reaction
Fig. 13
DL-EPR curves of the 2.54cm-thick specimens sensitized at 675°C for 1 h (WC) and solution heat-treated at 1,038°C for 1~30 min (WC)
Fig. 14
DL-EPR curves of the 2.54cm-thick specimens sensitized at 675°C for 1 h (WC) and solution heat-treated at 1,121°C for 1~30 min (WC)
As a result of calculating DOS (I
r/I
p×100%), as shown in
Fig. 15, sensitized specimens in which ditch structure and micro cracks in the form of micro fissures were observed showed DOS of about 10.8%, and for all specimens with solution heat treatment of 1 minute or longer, DOS was reduced to about 0.01%, indicating no occurrence of sensitization, and this was consistent with the ASTM A262 test results in which complete step structure was formed and no micro cracks of micro fissure type were observed. AS for the DOS evaluation, with the application of ISO-12732 standard, the standards were set to the occurrence of sensitization (DOS over 5%), partial sensitization (DOS 1~5%) and no sensitization (DOS less than 1%)
6).
Fig. 15
DOS (Ir/Ip×100%) of the sensitized and solution heat treated, 2.54cm-thick specimens calculated from Figs.
13 and
14
3.2 Evaluation of uniform holding time according to material thickness
As described above, an demonstration test on the sensitization with a maximum thickness of 25.4cm, which is the maximum thickness of austenitic stainless steel used in the nuclear power industry, was performed, and through the test, DOS was evaluated according to the location of each material thickness to verify the appropriateness of 0.5 hours per 2.54cm, the minimum holding time of solution heat treatment specified in nuclear regulatory requirements.
In the specimen with 25.4cm thickness artificially sensitized for 10 hours at 675°C before the solution heat treatment, as shown in
Fig. 16, ditch structure, in which sensitization by chromium carbide continuously precipitated at the grain boundary is suspected, was observed in the ASTM A262 Practice A test. Also, as shown in
Fig. 17, micro cracks with a micro fissure-type were observed in the bent areas where the bend test was conducted in the ASTM A262 Practice E test.
Fig. 16
Etch structures on the sensitized 25.4cm-thick specimen (675°C for 10 h, WC) by ASTM A262 Practice A test (×250)
Fig. 17
Bent area view of the sensitized 25.4cm-thick specimen (675°C for 10 h, WC) by ASTM A262 Practice E test (×60)
On the other hand, in the ASTM A262 Practice A test of a specimen subjected to solution heat treatment for 5 hours at 1,038°C, step structure considered as an accepted structure, which is a conforming structure with no precipitation of chromium carbide at the grain boundary was observed, but dual structure considered as an accepted structure, another conforming structure formed by the discontinuous formation of chromium carbide due to the tendency of increasing precipitation of chromium carbide getting closer to the center part, was observed as in
Fig. 18. This phenomenon is thought to have caused by longer exposure to the sensitization temperature range due to slower cooling rate at the center than the surface in the water cooling process of solution heat treatment. However, although dual structure was observed with the move from the surface to the center, no micro cracks in micro fissure type was observed in the bent areas of the bending test in ASTM A262 Practice E test, for all positions by thickness of the specimen, as shown in
Fig. 19.
Fig. 18
Etch structures of the 25.4cm-thick specimen solution heat treated at 1,038°C for 5h. (a) 0cm(surface), (b) 2.54cm, (c) 5.08cm, (d) 7.62cm, (e) 10.16cm, and (f) 12.70cm away from the surface, respectively, by ASTM A262 Practice A test (×250)
Fig. 19
Bent area view of the 25.4cm-thick specimen solution heat treated at 1,038°C for 5h. (a) 0cm(surface), (b) 2.54cm, (c) 5.08cm, (d) 7.62cm, (e) 10.16cm, and (f) 12.70cm away from the surface, respectively, by ASTM A262 Practice E test (×60)
In order to analyze the correlation with ASTM A262 test by quantifying DOS of discontinuous chromium carbide formed according to the location per thickness of the material subjected to solution heat treatment, a DL-EPR test was performed. As shown in
Fig. 20, the corrosion potential was about -0.4V
SCE, regardless of the location per thickness, and anodic dissolution reaction occurred from this corrosion potential to about -0.2V
SCE, the basic passivation potential. Thereafter, the potential reached about+0.2V
SCE, a passivation region accompanied by a decrease in current density. During the reverse potential scanning, two or more current peaks were observed.
Fig. 20
DL-EPR curves of the 25.4cm-thick specimen solution heat treated at 1,038°C for 5h with the distance from the surface
As a result of DL-EPR test, as shown in
Fig. 21, the DOS on the surface was less than 0.01%, but it was confirmed that the DOS increased with getting closer to the center, converging to about 0.6% at the depth of 12.7cm. This is considered to be the result of being exposed to the sensitization temperature range for a longer time because the cooling rate in the central part is slower than that in the surface, similar to the results of the ASTM A262 Practice A test. However, DOS less than 1% falls within the range of no occurrence of sensitization in the ISO-12732 standard, and given that there was no micro cracks observed in the ASTM A262 Practice E test, the same results that although ditch structure was observed getting closer to the center in the ASTM A262 Practice A test, the observed structure was considered as accepted structure were confirmed in the DL-EPR test.
Fig. 21
DOS (I
r/I
p×100%) of the 25.4cm-thick specimen solution heat-treated at 1,038°C for 5h with the distance from the surface calculated from
Fig. 20
In addition, as shown in
Fig. 22, the thermocouple is separately installed on the surface and the central part. The measurement results show that, during the cooling process, the surface is briefly exposed for 1.7 minutes to the sensitization temperature range of 427~816°C, while the central part is exposed for a long time of 15.3 minutes, support the test results, in which discontinuous dual structure was formed with the increase in the precipitation amount of the chromium carbide with a move closer to the center part and DOS was increased with its value converging to about 0.6%.
Fig. 22
Schematic diagram of the 25.4cm-thick specimen solution heat-treated at 1,038°C for 5h measured by thermocouples
However, considering that on the surface, the step structure with no precipitation of chromium carbide at all was observed and DOS of less than 0.01% was measured, and that the part directly affected by intergranular corrosion is the surface of the material, the results in the center part with the formation of dual structure and DOS of less than 0.01% are thought to have no effect on the for the nuclear facility components in the environment of those operation.
As for the results of finite element analysis on the uniform holding time for 12 analytical models according to the thickness and width of the material,
Fig. 23 shows an example of the analytical model RM3 and the analysis results are presented in
Fig. 24. In this case, the uniform holding time (ⓐ in
Fig. 22) refers to the time required for the central part to reach 1,038°C after the surface has reached 1,038°C divided by 2.54cm per material thickness. The arbitrarily specified thickness or width of the material was not significant considering the heat transfer occurs by the shortest distance, and as the thinner of the thickness and width increased from 1.27cm to 38.1cm, the uniform holding time increased from 0 minutes to 3.74 minutes per 2.54cm. It was confirmed that the rate of increase in uniform holding time increased as the thickness of the material increased. In this case, the uniform holding time for 25.4cm, the maximum material thickness used in the nuclear power industry, was evaluated to be 1.83 minutes per 2.54cm. This result was confirmed to be similar to the result in the thermocouple measurement test shown in
Fig. 22 in which the center part reaches the temperature 1,038°C slower than the surface by 1.78 minutes (ⓐ) per 2.54cm.
Fig. 23
FEM results of analytical model No. RM3
Fig. 24
Schemaic diagram for FEM results of 12 evaluation analytical models of uniform holding time during solution heat treatment at 1,038°C for 5h
Based on these results, as a result of confirming the holding time (ⓒ in
Fig. 22) for solution heat treatment to the center of the material of 25.4cm, which is the maximum material thickness used in the nuclear power industry, after the center reaches 1,038°C for 18.3 minutes considering the uniform holding time (ⓐ in
Fig. 22) of 1.83 minutes per 2.54cm, considering 1 minute, which is the effective holding time (ⓑ in
Fig. 22) of solution heat treatment during which the chromium carbide precipitated at the grain boundary is completely dissolved into the grains at 1,038°C in the above effect evaluation of solution heat treatment holding time, the holding time for complete solution heat treatment to the center is calculated as about 19.3 minutes by addition. Therefore, from conservative viewpoint, the holding time for complete solution heat treatment to the center of the material was determined to be up to 2 minutes per 2.54cm of the material thickness.