抄録
The effect of grain size on fracture toughness of high hardness steels was studied in air at room temperature. By appropriate heat treatments four different grain sizes for the cold working roll steel and two different grain sizes for cold working die steel were obtained.
The fracture toughness value (KQ) increased significantly with coarsening grain size when delayed crack growth was observed prior to unstable fracture. Fractographic observation of crack surface showed that delayed crack propagated along grain boundaries. The crack path was significantly distoreted because of kinking and branching especially in the case of large grain size. This kinking and branching of crack reduced the effective value of stress intensity factor at the crack tip and incresed the resistivity against delayed crack propagation. Therefore, large grain size leads to the improvement of fracture toughness of high hardness steels.
