Effect of Controlled-Cooling Temperature in Online Normalizing Process on Impact Toughness of N80-1 Tube

To enhance the impact toughness of petroleum casings for deep oil and gas exploitation, the N80-1 seamless steel tubes made of 38Mn6V steel are treated by an online normalizing process. The effects of controlled-cooling temperature on the impact properties are systematically investigated through the microstructural characterization and mechanical testing. Results indicate that as the controlled-cooling temperature decreases, the microstructure of the tube is progressively refined and becomes more uniform, the boundary length per unit of high-angle grain boundaries (HAGBs) increases, that of the low-angle grain boundaries (LAGBs) decreases, the dislocation density decreases, and the dislocation distribution becomes uniform. By comparing the impact fractures of the specimens treated at 500-520 and 600-640 ℃, it is found out that the impact fracture surfaces of the specimens treated at 500-520 ℃ exhibit significant expansion of the ductile fracture region and pronounces tortuosity in crack propagation paths. Concurrently, the quantity of V(C, N) precipitates decreases while their size increases. Consequently, these microstructural evolutions synergistically promote a substantial improvement in impact toughness, albeit with a minor reduction in strength. The controlled-cooling temperature of 500-520 ℃ can optimize the impact toughness of the tube, with KV₈ of 29 J at 0 ℃, 61% increase compared to that of the non-controlled-cooled tube.