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Korean Journal of Metals and Materials > Volume 56(5); 2018 > Article
Korean Journal of Metals and Materials 2018;56(5): 375-383. doi: https://doi.org/10.3365/KJMM.2018.56.5.375
니켈기 Inconel 740H 초내열합금과 고Cr계 Gr.92 내열강의 이종금속 용접부의 미세조직 및 크리프 파단 특성
신경용1, 이지원1, 한정민2, 이경운2, 공병욱2, 홍현욱1
1창원대학교 신소재공학부
2두산중공업 기술연구원 소재기술개발팀
Characterization of Microstructures and Creep Fracture Characteristics of Dissimilar Metal Welds between Inconel 740H Ni-Based Superalloy and Gr.92 Ferritic Steel
Kyeong-Yong Shin1, Ji-Won Lee1, Jung-Min Han2, Kyong-Woon Lee2, Byeong-Ook Kong2, Hyun-Uk Hong1
1Department of Materials Science and Engineering, Changwon National University, Changwon 51140, Republic of Korea
2Materials Technology Development Team, Corporate R&D Institute, Doosan Heavy Industries & Construction, Changwon 51711, Republic of Korea
Correspondence  Hyun-Uk Hong ,Tel: 82-55-213-3699, Email: huhong@changwon.ac.kr
Received: 14 February 2018;  Accepted: 19 March 2018.  Published online: 8 May 2018.
ABSTRACT
Microstructure evolution and the creep fracture characteristics of dissimilar welds between Ni-based superalloy Inconel 740H and High Cr ferritic steel Gr.92 were investigated. The dissimilar welds were fabricated by shielded metal arc (SMA) welding process with Ni-based austenitic Inconel 182 superalloy. Postweld heat treatment at 760 oC for 4 hours was conducted to temper the fresh martensite formed in the heat-affected zone (HAZ) of Gr.92, and to form γ΄ strengthener in Inconel 740H. The weld interface between the Gr.92 and weld metal was characterized by a discrete line with minimal inter-diffusion, whilst the weld interface on the Inconel 740H side showed an unmixed zone having epitaxial columnar growth from the base metal. It was observed that the fresh untempered martensite formed at the Gr.92-sided coarse-grained HAZ next to the weld interface during PWHT. This may be due to higher Ni contents diffused from the Ni-based weld metal. A sharp increase in hardness was detected at the coarse-grained HAZ of Gr.92 while the hardness minimum occurred at the fine-grained and intercritical HAZ (FG/IC HAZ). Fracture after creep at 650 oC/80 MPa and 700 oC/35 MPa occurred at the FG/IC HAZ of Gr.92, commonly known as a type IV fracture. This may be attributed to the strain localization at the FG/IC HAZ due to the smaller grain size as well as the lowest hardness. The elemental migration across the weld interface was not significant. It was also found that a needle-like Nb-rich δ-phase formed at the interdendritic region of the weld metal after creep at 700 oC/35 MPa.
Keywords: Ni-based superalloys, ferritic steels, welding, microstructure, creep, transmission electron microscopy (TEM)
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