Effect of cobalt on precipitation in Fe-Cr-Co-Mo-Ni-C stainless steels

• Published in: Materials letters 2021-04, Vol.289, p.129439, Article 129439
• Main Authors: Liu, Zhen-Bao, Zhang, Bo-Ning, Sha, Gang, Jin, Shen-Bao, Yang, Zhi-Yong, Liang, Jian-Xiong, Tian, Zhi-Ling
• Format: Article
• Language: English
• Subjects: Atom probe; Chromium; Cobalt; Intermetallic alloys and compounds; Intermetallic compounds; Iron; Martensitic stainless steels; Materials science; Molybdenum; Nanoparticles; Nickel; Thermodynamic equilibrium; Ultra-high strength steel
• ISSN: 0167-577X; 1873-4979
• DOI: 10.1016/j.matlet.2021.129439

•Aging-induced precipitation hardening phases were analyzed by APT.•Microhardness increases significantly during aging of Fe-Cr-Co-Ni-Mo-C steels.•Co promotes Mo-rich and Cr-rich precipitation during a short aging time.•Co has different effects on equilibrium fractions of Mo/Cr-rich precipitations. Nano-scale precipitation is critical for martensitic steels to achieve ultra-high strengths. To date, there is a lack of in-depth understanding about tailoring Co addition to optimize strengthening precipitations in Fe-Cr-Co-Mo-Ni-C steels. This research reveals that a high addition of Co in a Fe-Cr-Co-Mo-Ni-C steel significantly promotes multiple precipitation of Mo-rich intermetallic compounds (IMCs), carbides and α’-Cr, and achieve an exceptional hardening effect during aging at 540 °C. In particular, a variation of Co content from 5.1 to 12.8 wt% enhances α’-Cr precipitation, and accelerates precipitation kinetics of Mo-rich phase during early-stage aging. However, few influences were observed on the solubility of Mo and equilibrium fraction of Mo-rich phases, according to both long-term aged microstructures and thermodynamic equilibrium calculation.