On Carbides in Cr-Mo-V Hot-Working Tool Steels

Study on carbides in commercial special steels by electrolytic isolation-X

Abstract

Chemical and X-ray analyses were carried out on carbides, isolated electrolytically from the as-annealed, as-quenched and as-tempered structures of chromium-base hot-working tool steels.
Carbides found in the as-annealed structures are M₂₃C₆, M₆C and MC, similarly to the carbides in tungsten-base hot-working tool steels. M₇C₃ and ξ carbide expected from the equilibrium diagrams for Fe-C-Cr and Fe-C-Mo systems were not observed in the as-annealed structures. Carbide reactions during tempering are complicated, but are regarded as the combination of four sequences, which proceed separately in chromium, molybdenum, vanadium and tungsten steels; that is (i) M₃C→M₇C₃→M₂₃C₆, (ii) M₃C→Mo₂C→M₆C, (iii) M₃C→MC, and (iv) M₃C→W₂C→M₆C+M₂₃C₆.
The rate of the carbide reactions taking place towards equilibrium during tempering were very slow, owing to the low mobility of molybdenum atoms.