Through the experiments about the mode Ⅱ fracture toughness K Ⅱc of bearing steel under different heat treating conditions
it is shown that the K Ⅱc values variate in a range of (17 ̄35) MPa·m 1/2
the crack propagation initiation angle θ 0 is 55° ̄62° and the rations K Ⅱc / K Ⅰc is 1.1 ̄1.6. When the content of the lower bainite equals to 31% of the combined microstructure of martensite and lower bainite (B low /M)
higher fracture toughness and optimal strength toughness matching were obtained.