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Korean Journal of Metals and Materials > Volume 56(4); 2018 > Article
Korean Journal of Metals and Materials 2018;56(4): 296-303. doi: https://doi.org/10.3365/KJMM.2018.56.4.296
오스테나이트계 스테인리스강 용접부 델타-페라이트의 열화에 따른 미세조직 변화가 기계적 물성 및 부식 특성에 미치는 영향
박상규1,2, 이호중1, 이종현2
1한국수력원자력㈜ 중앙연구원
2충남대학교 신소재공학과
Effects of Thermal Aging Induced Microstructure Evolution on Mechanical and Corrosion Properties of Delta-Ferrite in Austenitic Stainless Steel Weld
Sang Kyu Park1,2, Ho Jung Lee1, Jong Hyeon Lee2
1Central Research Institute, Korea Hydro&Nuclear Power, Daejeon 34101, Republic of Korea
2Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea
Correspondence  Jong Hyeon Lee ,Tel: +82-42-821-6596, Email: jonglee@cnu.ac.kr
Received: 8 January 2018;  Accepted: 9 January 2018.  Published online: 5 April 2018.
ABSTRACT
347 austenitic stainless steel weld (ASSW) was thermally aged at 343, 400 and 450 °C up to 20,000 h in this study. Effects of thermal aging induced microstructure evolution on the mechanical properties and corrosion resistance of delta-ferrite in the 347 ASSW were qualitatively and quantitatively assessed by high resolution transmission electron microscopy (0.07 nm) with energy-dispersive spectroscope (EDS) and Fast Fourier Transform (FFT). After thermal aging at 343 °C for 20,000 h and 400 °C over 5,000 h, fluctuation of major alloying elements such as Fe, Cr, and Ni was observed by spinodal decomposition in the delta-ferrite. Meanwhile, a Ni+Si-rich G phase was developed as thermal aging progressed at 400 °C for 20,000 h and 450 °C for 5,000 h in the delta-ferrite. Such microstructure evolutions tended to be accelerated with increasing aging temperature and exposure period, while the G phase was formed at a higher exposure temperature and/ or period compared to spinidal decomposition. These effects increased the tensile strength and decreased the elongation of 347 ASSW at room temperature, compared to the as-welded condition. Moreover, when spinodal decomposition and G phase were observed, the degree of sensitization values of the 347 ASSW in the double loop-electrochemical potentiodynamic reactivation tests were significantly increased, due to localized Cr depletion in the delta-ferrite.
Keywords: thermal aging, austenitic stainless steel weld, delta-ferrite, spinodal decomposition, sensitization
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