Optimizing the fatigue strength of 18Ni maraging steel through ageing treatment

It is confirmed that the fatigue strength of high-strength metals would not continuously increase as the tensile strength enhances. In order to find out the optimal fatigue strength of maraging steel, the tensile and cyclic pull-push (R = − 1) loading tests of 18Ni maraging steel (250 grade) aged under different conditions were carried out in the present study. Meanwhile, the fracture surface and microstructural differences were also investigated. It is detected that decreasing the ageing temperature can bring about the decrement in interparticle spacing and austenite fraction, which would further improve the tensile strength and change the tensile fracture mode. Increasing tensile strength can effectively improve the fatigue strength in the low-strength region. However, as the strength further increases, the intergranular fracture would happen, which means intense deformation inhomogeneity at grain boundaries, contrarily causing the decrement in fatigue strength. Finally, it is believed that the optimal fatigue strength can be obtained in the high-strength region where there is a transition in the fracture mode. Moreover, it is found that the specimen aged at 550°C for 5h with an ultimate tensile strength of 1838MPa exhibiting the transgranular fracture possesses the highest fatigue strength of 685MPa.