抄録
The effects of the ferrite around the graphite on fracture toughness in gray cast iron were investigated. Four kinds of gray cast iron were prepared. Austempered gray cast iron (AGI) for bainite matrix was made by austempering at 653 K after heating at the austenitizing temperature of 1143 K. AGI contained about 35% residual austenite (γR). AGI-S2 subject to subzero treatment after the same austempering process as AGI showed about 5% residual austenite γR in the bainite matrix. α-AGI was made by the austempering after slow cooling from the austenitizing temperature to the range of ferrite and austenite coexistence temperature, 998 K. In this process, the ferrite thickness around the graphite was about 15 μm, and the matrix was a bainite structure. Fully pearlite matrix cast iron was used as casting.
Precipitated ferrite around the graphite in austempered gray cast iron was found to be effective for improving the fracture toughness. There was good correlation between the stress intensity factor (K) calculated by the load which appeared at high amplitude AE and other mechanical properties such as elongation, Charpy impact strength, and elastic-plastic fracture toughness (J). K calculated by AE of α-AGI was 15.0 MPa⋅m1/2, AGI was 11.5 MPa, m1/2, AGI-SZ was 8.5 MPa⋅m1/2, although K of the as cast specimen was 7.8 MPa⋅m1/2.
