Carbides precipitation and secondary hardening in niobium-titanium high-speed steels

Secondary hardness due to carbide particles precipitation is a characteristic feature of high-speed steels (HSSs). The most dominant types of carbides responsible for this strengthening effect are MC and M₂C ones, other species (M₆C, M₃C, M₂₃C ₆) 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 M₂₃C ₆ 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.