Research of Estimation Method of Thermal Aging Embrittlement on Cast Austenitic Stainless Steel

Transcription

1 E-Journal of Advanced Maintenance Vol. 6-4 (2015) Japan Society of Maintenology Research of Estimation Method of Thermal Aging Embrittlement on Cast Austenitic Stainless Steel Teruyoshi ABE 1,*, Kimihiro NOGIWA 2, Takashi ONITSUKA 3, Takahisa NAKAMURA 1, and Yasuhide SAKAKIBARA 1 1 Fugen Decommissioning Engineering Center, Japan Atomic Energy Agency, Kizaki, Tsuruga, Fukui , Japan 2 Sumitomo Heavy Industries, Ltd., Shinagawa-ku, Osaki, Tokyo , Japan 3 University of FUKUI, 1-2-4, Kanawa-cho, Tsuruga, Fukui , Japan ABSTRACT Thermal aging embrittlement of cast austenitic stainless steel components from the decommissioned Advanced Thermal prototype Reactor (ATR) Fugen power station has been characterized. Cast stainless steel materials were obtained from recirculation pump casing. The actual time at temperature for the materials was 138,000 h at 275 o C. The Fugen serviced material show modest decrease in Charpy-impact properties and a small increase in micro-vickers hardness in ferrite phase because of thermal aging at relatively low service temperatures. The fracture toughness prediction method (H3T model) predicts slightly lower values for Charpy-impact energy obtained from the Fugen material. The results from microstructural analysis suggest that the prediction method have the potential to provide higher accuracy by considering activation energy for embrittlement at low service temperatures. KEYWORDS ATR, Fugen power station, cast austenite stainless steel, thermal aging embrittlement, serviced materials, Charpy impact test, atom probe, recirculation pump casing, spinodal decomposition, H3T model ARTICLE INFORMATION Article history: Received 19 June 2014 Accepted 17 November Introduction Metastable austenitic cast stainless steels with coexistent ferrite phase up to 25wt%, which improves castability, mechanical properties, weldability, and corrosion resistance, are widely used as the structural material such as pumps, valves and piping. However, it has been reported the deterioration in fracture toughness so called thermal aging embrittlement (TAE) after long term operation at the elevated temperature [1], [2]. It is important for the aging management of the nuclear power plant to investigate the characteristics of the TAE and predict the degree of the embrittlement. Although it takes tens of years to appear the influence of the TAE around 300 o C, many of the research projects accomplished with the accelerated test data acquired at the elevated temperature from 350 to 400 o C in order to study the mechanism of the embrittlement [3]-[5]. Based on these findings, some models have been proposed to predict degree of the TAE [4], [6], and H3T model [7] has been proposed in Japan as well. However, to adopt extrapolating prediction at lower operating temperature, investigation of degree of the TAE and validity examination of the prediction formulas by using serviced materials are necessary. The Advanced Thermal prototype Reactor (ATR) Fugen power station completed 25 year operation and is being decommissioned at present. In this study, mechanical characteristic testing and microstructure evaluation have been carried out using Fugen serviced metastable austenitic cast stainless steels as the investigation targets, in order to determine the degree of the TAE of the serviced materials actually exposed at operating temperature for long operating years and the validity examination of the prediction formulas on the basis of newly obtained findings. * Corresponding author, abe.teruyoshi@jaea.go.jp ISSN / JSM and the authors. All rights reserved. 146

2 2. Investigation method 2.1. Sample material A reactor coolant pump (RCP) casing, as shown in Fig. 1, collected from Fugen was chosen as an evaluation target. Table 1 shows chemical composition of the RCP casing conforms to JIS Grade SCS13. The serviced condition of the RCP was 138,000 h at 275 o C. Calculated volume fraction of the ferrite phase of the RCP casing was 13.2wt% from an equation of ASTM A800 [8] and chemical composition. Fig. 2 shows the relationship between thermal aging time and temperature obtained from preceding studies, the test sample material of this investigation is located at the lowest temperature and longer time condition than preceding studies. Aging parameter P will be described latter. Fig. 1 Schematic diagram of the test sample collected from Fugen. Fig. 2 Relationship between aging time and aging temperature. 147

3 Table 1 Chemical composition of RCP casing collected from Fugen (wt%) Cr Ni Si Mn S P C Fe Bal. There is no casted or solution heat treated samples, therefore a recovery heat treatment was applied to Fugen serviced material to be a reference sample to recover the microstructure deterioration caused by thermal aging history. For an hour at 550 o C was selected as the recovery heat treatment condition, considering the thermal equilibrium of the material so that the alpha prime phase, decomposed by spinodal decomposition during thermal aging, can be completely dissolved. It has been confirmed by the previous report [2] that the ductile-brittle transition temperature (DBTT) obtained by Charpy-impact tests becomes equivalent to the value as solution heat treated after the recovery heat treatment Charpy-impact tests Charpy-impact tests conform to JIS Z with V-notched 10mm 10mm 55mm test specimen were carried out with three specimens each at room temperature, using semiautomatic machine(ci-500d, Tokyo Testing Machine Inc.) Vickers hardness tests Specimens for Vickers hardness tests were cut-off from serviced material, then mirror polished and electrolytic oxalic acid etched for 30 sec at 10 volt. Vickers hardness tests were carried out on the ten ferrite phases each of the specimen with 50g load Three Dimensional Atom Probe analyses Three Dimensional Atom Probe (3DAP) analyses were carried out in order to evaluate spinodal decomposition regarded as the main cause of TAE quantitatively. Specimens for 3DAP analysis were mirror polished and electrolytic etched alike with hardness test specimens, then selectively cut-off the ferrite phase to the needle-shaped specimens applying FIB-SEM micro-sampling method. 3DAP analyses were carried out using local electrode atom probe (LEAP) system with a reflectron for energy compensation, (LEAP3000X HR, CAMECA). Analytic conditions were 250 khz (pulse frequency) and 15% (pulse ratio) with voltage pulse mode to the specimen held at 60K. In addition, the influence of the aging temperature on the rate of spinodal decomposition was investigated using the samples re-aged at 400 o C after recovery heat treatment, as well as the samples as recovery heat treated by the 3DAP analyses. 3. Results and discussions 3.1 Results of mechanical characteristics Fig. 3 (a) shows Charpy-impact test results at room temperature. Charpy-impact energy of collected serviced material comes lower than that of recovery heat treated material identical to brand new one, which indicates the sign of TAE even at lower operating temperature of Fugen. According to the previous reports, the hardening of the ferrite phase by spinodal decomposition mainly causes TAE, which coincides the fact that the hardness of the ferrite phase exhibits higher value than that of the recovery heat treated serviced material of Fugen, as shown in Fig.3 (b). 148

4 Fig.3 Comparison of the mechanical properties before and after recovery heating: (a) the room temperature Charpy-impact energy and (b) micro-vickers hardness in ferrite phase. 3.2 Results of 3DAP analyses Fig. 4 (a) shows the distribution mapping of Cr by the 3DAP analyses applied to the ferrite phase of the RCP casing. Comparing the recovery heat treated material which is considered to be equivalent to the initial state and as collected, the migration of Cr the collected served material indicates the progress. This means the progress of spinodal decomposition was observed. An index of the variation (V) [9] which exhibits a degree of modulated structure formed by Cr migration is shown at the lower right in Fig.4. This is a subtraction of the concentration distribution of analyzing elements between uniformly dispersed in the solid solution and actually measured. The value changes in the range of 0.0 to 2.0, and the numeric value became larger with the progress of modulated structure. It has been reported by a previous report [10] that the value of V is correlated to the hardness of the ferrite phase, likewise in this research, the value of V of the serviced material was greater than that of the recovery heat treated material, therefore, hardening of the ferrite phase as showed in Fig. 3 (b) seems to be caused by spinodal decomposition. Fig. 4 (b) shows the results of the materials of recovery heat treated and thermal aged at 400 o C. It has been observed that the variation of the concentration distribution of Cr was increasing with elapse of time. Fig.4 Chromium distribution mapping in ferrite phase of Fugen serviced RCP using 3DAP 149

5 3.3 Comparison with H3T model prediction T. Abe, et al./ Research of Estimation Method By comparing the measured value of Fugen serviced material with the calculated result, the validity of prediction formula has been examined. As shown in Fig. 5, the time dependence on the fracture toughness caused by thermal aging is represented by a hyperbolic function in H3T model [7] as follows: Fig. 5 The fracture toughness prediction method H3T model [7] M B = A +, (1) t + C where M is toughness measured by Charpy-impact test et al. at an aging time, A is toughness at infinite time, and each B and C is constant related to aging temperature and aging time respectively. While those constants were calculated from multiple regression analysis based on the accelerated data obtained at higher temperature, fracture toughness can be predicted using chemical composition of material or volume fraction of a ferrite phase. In addition, aging parameter P, as proposed by Trautwein et al., is used for prediction with different aging time, assuming that embrittlement depends on a single thermal activated process as follows [1]: Q 1 1 P = log t , (2) R 673 T where t is aging time, R is gas constant, T is aging temperature and Q is apparent activation energy having 100 kj/mol as a default. The same aging parameter means that the same degree of embrittlement occurs, even with different aging temperature or aging time. Fig. 6 shows the comparison of the Charpy-impact test results obtained from Fugen serviced material and the prediction results by H3T model. There is a difference of more than 50J between the estimated value drawn by the solid line and measured value plotted by solid circle, which means that the estimation evaluates the embrittlement conservatively against the measurements. 150

6 Fig. 6 Comparison of Charpy-impact energy obtained from the Fugen material and the prediction value from the H3T model The apparent activation energy in the formula (2) is very important in order to examine the validity of predicted value. Fig. 6 shows the prediction results after the apparent activation energy adjustment by the broken line, so that the estimation result can coincide with the measured result. Apparent activation energy in this case was 180 kj/mol. Focusing spinodal decomposition which is considered to be the main cause of TAE, further investigation on the validity of the apparent activation energy used formula [2] was carried out from the viewpoint of changes of the microstructure. According to the previous reports, the value of apparent activation energy in terms of the absorbed energy of Charpy-impact test, deteriorated by TAE, varies widely in a range from 75 to 230 kj/mol. Fig. 4 shows some equivalent curves of aging parameter P when the apparent activation energy is 100 kj/mol as well. The value of aging parameter is 3.38 for Fugen operating condition of 138,000 hours at 275 C, and it is greater than the value of 3.0 for 1,000 hours at 400 C, which means that the TAE of Fugen serviced material would precede further. However, the index of the variation V of collected Fugen serviced material shows smaller value than the one of the heat treated specimen, contrary to the apparent aging parameter. On the other hand, when 180 kj/mol is applied to apparent activation energy, the value of aging parameter for Fugen operating condition becomes close to 2.0, which is equivalent to the value of the heat treated specimen for 1,000 hours at 400 C. This relationship coincides with Charpy-impact test results above mentioned. Nevertheless, the discussion of the apparent activation energy depends on the data obtained by the accelerated tests executed at elevated temperature in a range from 350 to 400 o C, it seems to be important that the apparent activation energy should be examined along with the influence of chemical composition and the other embrittlement phenomena independent of spinodal decomposition, when apparent activation energy is applied to operating condition at lower temperature. 4. Conclusion Confirmation of the occurrence status of TAE and investigation of the validity of prediction formula H3T model have been carried out using Fugen serviced material. It was revealed that H3T model provides conservative estimated value in comparison to the test results of the serviced material of Fugen RCP casing. Meanwhile, it is suggested that there is still room for consideration in order to increase accuracy of the prediction based on the examination of the embrittlement mechanism such as considering activation energy. Acknowledgement This study was conducted as an "aging research in Fukui Prefecture in ", which was 151

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