Abstract: The relationship between the torsional flow behaviour and the structure strength of tempered GCr15 steels was studied in this paper. Multiple n behaviour was observed in the hardening curve when used double logarithmic coordinate axis. If the temperature is lower than 250℃, the n values increase with the decrease of tempering temperature. In high temperature tempering the n values decrease with the decrease of tempering temperature. When tempered under medium temperature the n values is the lowest in all of the tempering states. Although the torsional macro-fracture possesses two type of fracture modes ——positive fracture or shear fracture, yet the microfracture appearance is always dimples. This illustrates the major effect of plastical deformation and hardening in fracture prossess. Accordingly the cause that the slow decrease of τk value is associated with the of raise tempering temperature can be overall recognized through the change of τ0.3, τk and n values.

Key words: Hardening, High-carbon steel, Quenching and tempering, Strain, Structure