Study on complex precipitation kinetics in Cr- and Cu-added nitriding steels by atom probe tomography

• Published in: Acta materialia 2019-05, Vol.169, p.88-98
• Main Authors: Takahashi, Jun, Kawakami, Kazuto, Kawasaki, Kaoru
• Format: Article
• Language: English
• Subjects: Atom probe tomography (APT); Chromium nitride; Complex precipitation; Copper; Interfacial energy; Nitriding; Particle hardening
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2019.03.002

Cr- and Cu-added steel sheets enable hardening of the surface region (the diffusion layer) by nitriding and simultaneously strengthening of the inner region due to thermally activated Cu precipitation during nitriding. Atom probe tomography (APT) analysis revealed that precipitate pairs, each composed of a Cu particle and CrN particle, were generated in the nitrided surface region. The influence of the complex precipitation of Cu and CrN on the precipitation kinetics and hardening properties was investigated by experimentally varying the initial state of Cu atoms in the steel by pre-aging before nitriding. It was found that Cu particles and CrN particles acted as heterogeneous nucleation sites for CrN and Cu, respectively, and the complex precipitation adversely affected the hardening of the nitrided surface region depending on the initial state of Cu. In addition, it was found that there were two types of precipitate pair forms (type-A/B), which were considered to be determined by the relation between the {001}-facet-size of the Cu particle and the size of the CrN nucleus in heterogeneous nucleation. [Display omitted]