Rapid Solidification Microstructure and Carbide Precipitation Behavior in Electron Beam Melted High-Speed Steel
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COMPARED to conventional ingot casting and powder metallurgy, electron beam melting (EBM) has successfully demonstrated its capability to fabricate near-net-shape parts with the very fine microstructure and minimum segregation due to its intrinsic rapid solidification. EBM belongs to the group of additive
J. JIN and R. GAO are with the School of Materials Science and Engineering, Beihang University, 37 Xueyuan Road, Beijing 100191, China. H. PENG, H. GUO, and S. GONG are with the School of Materials Science and Engineering, Beihang University and also with the Key Laboratory of High-Temperature Structural Materials & Coatings Technology, Ministry of Industry and Information Technology, Beihang University, 37 Xueyuan Road, Beijing 100191, China. Contact e-mail: [email protected] B. CHEN is with the School of Engineering, University of Leicester, Leicester, LE1 7RH, UK. Contact e-mail: [email protected] Manuscript submitted June 16, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
manufacturing (AM).[1] In contrast to selective laser melting (SLM) that operates at lower temperatures, EBM can work at high temperatures of up to 1200 C. This in situ heating characteristic makes EBM particularly attractive for processing high-performance alloys that are susceptible to thermal stress-induced cracking or solidification/hot cracking. For example, EBM was applied to produce precipitation-strengthened nickelbase superalloys,[2–6] cobalt-base alloys,[7,8] as well as titanium aluminide intermetallic alloys.[9–11] However, little effort has been made regarding EBM high-speed steels that are also susceptible to cracking. Note the EBM work[12] on H13 steel that has a carbon content of 0.37 wt pct and limited degree of alloying does not represent those highly alloyed high-speed steels. Up until now, there have been few publications on high-speed steels fabricated by SLM[13–17] and blown powder AM.[18] The carbon content in those steels (including M2,[13,14,18] M50[16] and a high-strength Fe85Cr4Mo8V2C1[15]) were all below 1.0 wt pct, except for a very recent work on an SLM M3:2 steel.[17] With
the increasing carbon content, higher hardness and wear resistance can be achieved, but this poses challenges for processing due to material susceptibility to cracking. This paper focuses on S390 high-speed steel containing 1.64C-10.12W-2.00Mo-4.97V-4.90Cr-7.86Co (all in wt pct). Compared to M2 steel containing 0.90C-6.15W-4.89Mo-1.82V-3.97Cr, the S390 steel has a higher carbon content and a greater degree of alloying. For carbon content of higher than 1.3 wt pct, the primary solidification phase is austenite, as opposed to delta-ferrite. In this sense, revealing the solidification sequence and primary carbide precipitation behavior in S390 steel, a highly alloyed high-carbon steel, during the EBM process helps advancing non-equilibrium solidification theories. High-speed steels generally have good hardenability due to their high alloy contents. For such steels when processed using ingot casting, the presence of coarse primary carbide ne
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