E-Journal of Advanced Maintenance Vol. 6-4 (2015) 146-152
Japan Society of Maintenology

Research of Estimation Method of Thermal Aging Embrittlement on Cast
Austenitic Stainless Steel
Teruyoshi ABE1,*, Kimihiro NOGIWA2, Takashi ONITSUKA3, Takahisa NAKAMURA1,and Yasuhide SAKAKIBARA1
1Fugen Decommissioning Engineering Center, Japan Atomic Energy Agency, 65-20 Kizaki, Tsuruga,Fukui 914-8585, Japan
2
Sumitomo Heavy Industries, Ltd., Shinagawa-ku, 2-1-1 Osaki, Tokyo 141-6025, Japan
3
University of FUKUI, 1-2-4, Kanawa-cho, Tsuruga, Fukui 914-0055, JapanABSTRACT
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 oC. 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

ARTICLE INFORMATION

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 history:
Received 19 June 2014
Accepted 17 November 2014

1.

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 thedeterioration 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 thenuclear power plant to investigate the characteristics of the TAE and predict the degree of theembrittlement.
Although it takes tens of years to appear the influence of the TAE around 300 oC, many of theresearch projects accomplished with the accelerated test data acquired at the elevated temperaturefrom 350 to 400 oC 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], andH3T model [7] has been proposed in Japan as well. However, to adopt extrapolating prediction atlower operating temperature, investigation of degree of the TAE and validity examination of theprediction 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 caststainless 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 validityexamination of the prediction formulas on the basis of newly obtained findings.

*Corresponding author, E-mail: abe.teruyoshi@jaea.go.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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T. Abe, et al./ Research of Estimation Methodof Thermal Aging Embrittlement on Cast Austenitic Stainless Steel

2.

Investigation method

2.1. Sample material
A reactor coolant pump (RCP) casing, as shown in Fig. 1, collected from Fugen was chosen as anevaluation target. Table 1 shows chemical composition of the RCP casing conforms to JIS GradeSCS13. The serviced condition of the RCP was 138,000 h at 275 oC. Calculated volume fractionof the ferrite phase of the RCP casing was 13.2wt% from an equation of ASTM A800 [8] andchemical 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 describedlatter.

Fig. 1 Schematic diagram of the test sample collected from Fugen.

Fig. 2 Relationship between aging time and aging temperature.

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T. Abe, et al./ Research of Estimation Methodof Thermal Aging Embrittlement on Cast Austenitic Stainless Steel
Table 1 Chemical composition of RCP casing collected from Fugen (wt%)
CrNiSiMnS

PCFe

19.69

8.4

0.81

0.94

0.008

0.027

0.05

Bal.

There is no casted or solution heat treated samples, therefore a recovery heat treatment wasapplied to Fugen serviced material to be a reference sample to recover the microstructure deterioration
caused by thermal aging history. For an hour at 550 oC 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 therecovery heat treatment.
2.2. Charpy-impact tests
Charpy-impact tests conform to JIS Z 2242-2005 with V-notched 10mm×10mm×55mm test
specimen were carried out with three specimens each at room temperature, using semiautomaticmachine（CI-500D, Tokyo Testing Machine Inc.）.
2.3. Vickers hardness tests
Specimens for Vickers hardness tests were cut-off from serviced material, then mirror polishedand 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.
2.4. 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 forenergy compensation, (LEAP3000X HR, CAMECA). Analytic conditions were 250 kHz (pulsefrequency) 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 oC 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 ofcollected serviced material comes lower than that of recovery heat treated material identical to brandnew 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).

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T. Abe, et al./ Research of Estimation Methodof Thermal Aging Embrittlement on Cast Austenitic Stainless Steel

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 phaseof the RCP casing. Comparing the recovery heat treated material which is considered to beequivalent to the initial state and as collected, the migration of Cr the collected served materialindicates the progress. This means the progress of spinodal decomposition was observed. An indexof the variation (V) [9] which exhibits a degree of modulated structure formed by Cr migration isshown 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 valuechanges in the range of 0.0 to 2.0, and the numeric value became larger with the progress ofmodulated 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 oC. It
has been observed that the variation of the concentration distribution of Cr was increasing with elapseof time.

Fig.4 Chromium distribution mapping in ferrite phase of Fugen serviced RCP using 3DAP149

T. Abe, et al./ Research of Estimation Methodof Thermal Aging Embrittlement on Cast Austenitic Stainless Steel

3.3 Comparison with H3T model prediction
By comparing the measured value of Fugen serviced material with the calculated result, thevalidity of prediction formula has been examined. As shown in Fig. 5, the time dependence on thefracture 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 = A+B,
t +C(1)

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 dataobtained at higher temperature, fracture toughness can be predicted using chemical composition ofmaterial or volume fraction of a ferrite phase.
In addition, aging parameter P, as proposed by Trautwein et al., is used for prediction withdifferent aging time, assuming that embrittlement depends on a single thermal activated process asfollows [1]:P = log t + 0.4343

Q 11− ,
R  673 T (2)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 ofembrittlement 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 meansthat the estimation evaluates the embrittlement conservatively against the measurements.

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T. Abe, et al./ Research of Estimation Methodof Thermal Aging Embrittlement on Cast Austenitic Stainless SteelFig. 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 thevalidity 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, furtherinvestigation on the validity of the apparent activation energy used formula [2] was carried out fromthe viewpoint of changes of the microstructure.
According to the previous reports, the value of apparent activation energy in terms of theabsorbed 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 theTAE of Fugen serviced material would precede further. However, the index of the variation V ofcollected 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 oC, it seems to beimportant that the apparent activation energy should be examined along with the influence ofchemical 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 accuracyof 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 2011 - 2009", which was151

T. Abe, et al./ Research of Estimation Methodof Thermal Aging Embrittlement on Cast Austenitic Stainless Steelcommissioned by JNES.

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