Enhanced Refinement of Cr 23 C 6 by Heterogeneous Nucleation in Annealed Nitrogen-Alloyed 4Cr5Mo2V Die Steel
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r5Mo2V die steel is widely applied in forging, hot-extrusion, and die-casting manufacturing owing to its good heat resistance, toughness, and hardenability.[1] It is well known that a uniformly distributed spheroidized microstructure can effectively homogenize a tempered microstructure and improve the mechanical properties of the steel.[2] The shape, distribution, and size of the carbides in annealed steel play a key role in deciding its comprehensive performance. Furthermore, the refinement of M23C6 and M7C3, which are easy to develop and coarsen, is an important precondition of a uniform microstructure.[3] Therefore, significant improvement of strength and toughness can be assisted by modifying the morphologies of M23C6 and M7C3.[4] It is well known that nitrogen plays an important role in refining carbide and effectively altering its appearance, especially in austenite stainless steel.[5–9] Nitrogen atoms are mainly involved in the carbide nucleation process, and strongly promote the carbide nucleation, inducing the production of more small carbides.[10,11] However,
JINBO GU, JINGYUAN LI, and RUIJIN CHANG are with the School of Materials Science and Engineering, University of Science & Technology, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 24, 2018. Article published online November 21, 2018 518—VOLUME 50A, FEBRUARY 2019
few reports have been published regarding carbide evolution in nitrogen-alloyed die steel with alloying elements different from that of stainless steel. Furthermore, the refinement mechanism of M23C6 by nitrogen is less clarified, especially on the heterogeneous nucleus of M23C6. As Gu et al.[12] have reported that nitrogen increases the stability of retained austenite, retaining the solid-soluted Cr, and thus delaying the precipitation of M23C6. Liu et al. have reported that TiC is effective in significantly refining the primary M7C3. Escobedo et al. have shown that nitrogen promotes the precipitation of M6C instead of coarsening the M23C6.[13] Ding et al.[14] have reported that nitrogen refines M7C3 carbides by increasing the Ti(C, N) precipitates that can assist the heterogeneous nucleation of M7C3. In a previous study, the authors successfully developed a nitrogen-alloyed 4Cr5Mo2V die steel with high hardness and toughness,[12] where the spherodized annealed structure was effectively refined by nitrogen. Especially, the M23C6 carbides that account for half of the annealed carbides were obviously refined. However, the refining mechanism was less clarified. Therefore, this paper aims to investigate the refinement of Cr23C6 in annealed nitrogen-alloyed steel and further analyze the refining mechanism. The enhanced refinement of Cr23C6 by nitrogen is discussed herein. The material herein was based on 4Cr5Mo2V hot-working die steel prepared with a vacuum induction furnace (ZGJL0.05-100-2.5D, Shenyang Institute of Vacuum Technology, China), whose chemical composition was 0.39C, 0.5Si, 0.42Mn, 5.3Cr, 2.4Mo, 0.68V, and 0.042N. The ingot was homogenized at 1250 C for 3 h, m
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