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Korean Journal of Metals and Materials > Volume 55(7); 2017 > Article
Korean Journal of Metals and Materials 2017;55(7): 470-476. doi: https://doi.org/10.3365/KJMM.2017.55.7.470
Alumina-Forming 오스테나이트계 스테인리스강의 냉간가공을 통한 미세 NbC 석출 촉진과 크리프 특성 향상
박현화1, 강전연2, 하헌영2, 이태호2, 홍현욱1
1창원대학교 신소재공학부
2재료연구소 철강재료연구실
Acceleration of Nano-Sized NbC Precipitation and Improvement of Creep Resistance in Alumina-Forming Austenitic Stainless Steel via Cold Working
Hyun Hwa Park1, Jun Yun Kang2, Heon-Young Ha2, Tae-Ho Lee2, Hyun Uk Hong1
1Department of Materials Science and Engineering, Chanagwon National University, Changwon 51140, Republic of Korea
2Ferrous Alloy Departmentm, Korea Institute of Materials Science, Changwon 51508, Republic of Korea
Correspondence  Hyun Uk Hong ,Tel: +82-55-213-3699, Email: huhong@changwon.ac.kr
Received: 10 November 2016;  Accepted: 7 February 2017.  Published online: 4 July 2017.
ABSTRACT
A feasibility study on microstructure design has been carried out to improve creep rupture life in alumina-forming austenitic (AFA) Fe-14Cr-20Ni-2.3Al stainless steel with and without 2 wt% W addition. After creep tests under 780 °C/80 MPa conditions, it was observed that internal cracks initiated primarily from grain boundaries in the W-free steel while voids nucleating at primary coarse NbC carbides elongated into fractures in the W-added steel. This result indicates that the W addition shifts the fracture mode from intergranular to transgranular. Based on this observation, the fine intragranular NbC carbides, which need to precipitate as much as possible during the initial stage of creep, can be a promising metallurgical factor for enhancing the creep resistance of both the AFA steels. Cold work at 5% strain prior to creep accelerated the heterogeneous precipitation of nano-sized intragranular NbC during the early stage of creep, which was responsible for the strong hardening effect by pinning the dislocations. The creep life was found to be 3 times longer as compared to the unpre-strained AFA steels. Applying 5% cold work induces the generation of dislocations in the matrix, which can lead to the nucleation of nano-scale NbC without recrystallization. The intragranular NbC carbide showed excellent resistance to coarsening during creep: their size was as small as around 30 nm after creep rupture.
Keywords: alloys, cold working, creep, transmission eletron microscopy (TEM), AFA stainless steel
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