Mechanical Property Changes during Accelerated Thermal Aging Embrittlement Heat Treatment of Cast Stainless Steel, CF-8A KhingMay Wann, Joo Suk Lee, and Changheui Jang* Department of Nuclear and Quantum Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea, chjang@kaist.ac.kr 1. Introduction Cast stainless steels are widely used in the primary coolant piping of pressurized water reactors (PWRs). This cast stainless steel contains a ferrite phase of about 10~25% in an austenite matrix. These materials suffer thermal aging embrittlement when exposed to the operating temperature of nuclear power reactor over a long period. The results of the embrittlement are loss of ductility and fracture toughness while the strength is increased. To assure the integrity of the components fabricated of cast duplex stainless steels for their lifetime or an extended lifetime, it is very important to evaluate the degree of thermal aging embrittlement[1]. It is common that the specimens are aged at higher temperature than the normal operating temperature to accelerate the thermal aging embrittlement phenomena. The purpose of this study is to provide the information on the changes of the mechanical properties of cast stainless steels during the accelerated testing. The changes in microhardness are measured and the changes in tensile properties at room temperature. 2. Experimental An experimental heat of cast stainless steel (CF8A), whose composition is shown in Table 1 was used in the study. The ferrite content of the specimen was calculated as about 20%. The specimens were exposed at 400℃ for prolonged time in the laboratory furnace to accelerate the thermal aging embrittlement phenomena in plant service environment. The accelerated aging conditions are approximately equivalent to 30, 40, and 60 years of operation at 320℃. The materials heat was originally produced to be used in. The specimens used in this study were taken from the broken pieces of fracture toughness test specimens which were conducted as part of environmental fatigue evaluation of cast stainless steels[2]. Table 1. Chemical composition of test specimen element C Mn Si Cr Ni Mo S P Co wt.% 0.045 0.60 1.17 20.08 8.38 - 0.018 0.024 0.10 Microhardness measurements and tensile test were carried out. For microhardness measurements, each specimen was mechanically polished with diamond paste up to 6 µm and was etched in a solution of 50 g NaOH in 100 ml water at 2 V for 5 seconds to distinguish between ferrite and austenite phases. Ten measurements were made for each sample and for austenite and ferrite grains, respectively and the results were averaged. Tensile tests were conducted at room temperature. To confirm these test results we have planned to perform tensile test at the high temperature of 320ºC, the operation temperature of nuclear power plant. 3. Result and Analysis 3.1 Microhardness test results and analysis The microhardness values of ferrite and austenite phases of all specimens are listed in Table 2. The data show that there is no change in the microhardness of austenite phase with aging time, having a little scattering. On the other hand, as can be clearly seen in this table, the microhardness of ferrite phase was found to increase with aging time. This result indicates that the embrittlement phenomenon is accompanied by an increase in hardness[3,4,5]. On the other hand, the optical micrographs did not show any clear changes in microstructures during the accelerated aging of the materials. Table 2. Change of microhardness of ferrite and austenite phases during accelerated aging Specimens Ferrite phase Austenite phase As received 234.9 195.9 J5 (400C+3943 hr) 280.3 210.2 J8 (400C+5270 hr) 334.5 192.5 J10 (400C+8000 hr) 360.3 201.8 3.2 Tensile test results and analysis The changes of tensile properties during the accelerated aging are shown in Table 3. Yield strength increased with aging time up to 30 years, and it decreased on further aging for 40 years. But it increased again for 60 years aged materials. An identical trend was seen for the variation of UTS with aging time also. Contrary to current study, decrease of strength in CF- 8A with similar composition was reported in both room temperature and operating temperature after accelerated aging[6]. In that study, decrease in Brinell hardness was also observed. On the other hand, similar variations of tensile strength and UTS have been reported in other Transactions of the Korean Nuclear Society Autumn Meeting Busan, Korea, October 27-28, 2005