Effect of Strip Micro-crakes on Cleavage Fracture Toughness of 16MnR Steel

Positive and negative bending experiment for notched specimens of low alloy hot rolled steel 16MnR with different pre-loads were carried out at room temperature to introduce strip micro-crakes with different dimensions in front of notch. And then the residual stress and work hardening were eliminated through high temperature tempering. Four points bending fracture experiment was carried out at -196 ℃. And effect of stripe micro-crake dimension on cleavage fracture toughness of notched specimens was investigated. Results show that when the pre-load ratio P0/Pgy<0.861, the pre-load had little effect on the cleavage fracture notch toughness; when P0/Pgy>0.861, the strip defect was torn and formed the micro-cracks under applied loads. With the increase of P0/Pgy, the number of strip micro-cracks increased, and the size as well as the distribution region was enlarged, too. Fracture load and fracture absorbed energy rapidly decreased with pre-load ratio increased. The increase of high stress-strain region caused by the strip micro-cracks made the distribution of cleavage initiation increase, as the result, numeric values of material notch toughness Pf and W were discreted.