Abstract: The mechanical properties and structure of bainite and martensite after tempering m 6-5-4-2 high speed steel have been investigated.It was found that the secondary hardening effect and resistance to heat softening as well as impact toughness are higher in the bainite than in the martensite.TEM observations show that the(Fe,M)_3C precipitated in the martensite after tempering at temperature near the peak of hardness-tempering temperature curve.However, no(Fe,M)_3C but the dispersive MC and M_2C precipitated in the ferrite and austenite of bainite after tempering at the same temperature.This indicates that the diffusion of alloy ele- ments forming carbide might exist during the process of bainite growing.

Key words: high speed steel, bainite, secondary hardening