Abstract: The changing mechanism of impact properties of three Cr5 steels (H13, H11 and DVA) with unnotched, U-notched and V-notched at different tempering temperatures was studied. The fracture morphologies and microstructure were observed by using body microscopy and scanning electron microscope(SEM) and the difference of notch type in characterizing toughness was discussed. The results show that the V(U)-notched impact energy acts more on the crack extension, reflecting the ability of the material to resist crack propagation, which can be used to measure the crack sensitivity of the material, and the V-notch is more suitable for characterizing the crack sensitivity of the material due to the smaller radius of curvature on the notch tip. The unnotched impact energy acts on crack initiation and propagation, which can reflect comprehensive toughness and metallurgical defects due to the cracks are easily formed at crystal defect. Through comparison, it's that in the common tempering temperature range, 600 ℃ V-notch impact absorbed energy reaches its maximum value, indicating that the heat treatment state of the material resistance to crack expansion ability of the best; notchless impact absorbed energy of the H13, DVA, H11 steel at 560, 580, 560 ℃ is higher, and the hardness decreased less, respectively, 53.1, 50.6, 52.2 HRC, the comprehensive strength and toughness of the best.

Key words: Cr5-series steels, tempering process, impact properties, tempering brittleness