High-Temperature Transformation of Carbides in Skeleton Eutectic and Delta-Eutectoid of Cast High-Speed Steel
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SFORMATIONS UDC 669.14.018.25
HIGH-TEMPERATURE TRANSFORMATION OF CARBIDES IN SKELETON EUTECTIC AND DELTA-EUTECTOID OF CAST HIGH-SPEED STEEL A. S. Chaus,1 M. Braèík,1 M. Sahul,2 and V. Tittel1 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 3 – 9, August, 2020. Special features of high-temperature transformation of carbides in the eutectic and d-eutectoid of a cast high-speed steel of type R6M5 are studied. The conditions of formation of a vanadium-enriched carbide during annealing at 1200°C with different holds are considered.
Key words: high-speed steel, eutectic, d-eutectoid, M6C carbide, fragmentation, decomposition. The M6C carbide is assumed to be steady a priori, which explains virtual absence of publications devoted purposefully to the effect of high temperatures on the structural transformations in M6C eutectic colonies. The only exception is the report in [3] on formation of MC carbides in a skeleton eutectic of cast high-speed steel R6M5 in addition to the lamellar and rod (hexagonal in the terminology of the authors) eutectics. However, the authors of [3] do not mention where exactly they observed nucleation and precipitation of particles of MC carbide or whether there were changes in the M6C carbide itself. In this connection, we should recall reports on decomposition of M6C carbide in nickel-base alloys (containing molybdenum and chromium) under heating in a specific temperature range; the stability of the carbide is reported to depend substantially on the content of silicon in the alloy [14 – 16]. It is known from the binary Fe – C phase diagram that crystallization in iron-carbon alloys starts with formation of d-ferrite at carbon concentration of about 0.5%. At a higher carbon content, the first to be precipitated from the melt is austenite [17]. In the case of high-speed steels, the ultimate concentration of carbon at which the d-ferrite still forms increases substantially due to the effect of the strong carbide-forming elements stabilizing the ferrite. According to the sections of the equilibrium Fe – W – Mo – Cr – V – C diagram given in many publications [18 – 21], primary d-ferrite forms in alloy Fe – 6% W – 5% Mo – 4% Cr – 2% V at a carbon content of 1.1 – 1.3%. Specifically, the in-situ differential thermal analysis and the experiments made in [22] have shown that crystallization in a high-speed steel of type
INTRODUCTION High-speed steels, which are alloys of the ledeburitic class, contain a eutectic structural component formed at the final stage of primary crystallization of the melt and precipitated over the boundaries of primary grains of the solid solution. Crystallization of the eutectic in high-speed W – Mo steels including the most widely used R6M5 yields M2C, M6C and MC eutectic carbides. These carbides are known to differ in the high-temperature stability. The M6C- and MC-type carbides are known to be stable phases, while the M2C-type carbide is known to be a metastable phase. After the solidification, the high-speed steel is subjected to a thermomechan
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