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Carbides Precipitation and Secondary Hardening in Niobium-Titanium High-Speed Steels

Abstract Secondary hardness due to carbide particles precipitation is a characteristic feature of highspeed steels (HSSs). The most dominant types of carbides responsible for this strengthening effect are MC and M2C ones, other species (M6C, M3C, M23C6) also being reported. They are typically within 5–30 nm range of size, of elongated shape and reach 3 % volume fraction. In the investigated niobium-titanium HSSs it was found that relatively large M23C6 type carbide particles provide a secondary hardness twice the value encountered in commercial grades produced conventionally or by powder metallurgy. The higher effectiveness of the M23C6 carbides was attributed to their higher volume fraction (approx. 5 %) and favourable plate-like shape. The role of the carbide forming elements in the experimental steels' crystallization and tempering was discussed.