Replacement of vanadium by niobium in 86–5–2 high-speed tool steels

Abstract

The microstructure and mechanical properties of a series of experimental high-speed steels based on the S6–5–2 (M2) composition have been investigated. In these alloys the 2% vanadium component of the S6–5–2 composition has been partially or completely replaced by niobium additions of up to 3%. It has been shown that niobium-replacement steels have superior cutting properties, which are attributed to the formation of a high volume fraction of niobium-rich carbides. X-ray energy analysis indicates that NbC replaces VC as the MC phase, and niobium changes the chemical composition of the M₆C and/or M₂₃C₆ carbides. The niobium-replacement grades undergo secondary hardening on tempering, and niobium raises the liquation temperature allowing higher hardening temperatures to be used. Since niobium-rich carbides replace carbides containing the more expensive tungsten, molybdenum, and vanadium alloying elements, it is suggested that niobium could be used to reduce the amounts of these elements needed, and so to improve the performance/cost ratio of S6–5–2.