The effect of cold drawing on the hardening and tempering hardness of tool steels.

Cold drawn tool steel wire frequently shows lower hardening and tempering hardness than normally hot rolled or forged bars. As for high speed tool steels, it has been confirmed that this phenomenon is due to the decrease in density caused by the increase in the number of voids within carbides or at carbide-matrix interface.Furthermore, in this study, the void formation mechanism was also studied by the direct observation of stretched specimens using several tool steels which have various types and sizes of primary carbides. The results obtained are as follows.(1) In cold drawn tool steels such as SKH 57, SKH 59, and SKD 11, voids formed at the cracked carbide show diameters larger than 5μm.(2) The decrease in density and hardness by cold drawing is proportional to the volume fraction of carbides larger than 5μm.(3) The susceptibility of void formation is governed by the size and shape of carbides but hardly by the carbide type.