Investigation on the Fatigue Crack Growth Behaviors of AISI 4340 Steel with Different Strength-Toughness Combinations

The strength, toughness, and fatigue crack growth (FCG) resistance of high-strength steels are the key performance indexes concerned in the material selection of engineering components. It is considered that the FCG resistance of high-strength steels can be optimized by simultaneous strengthening and toughening. In order to make clear whether the above view is correct or not, the strength-toughness combination and FCG resistance of AISI 4340 steel under different heat treatments are investigated and compared with those of 18Ni maraging steel in the present study. Meanwhile, the major fatigue damage mechanism of the AISI 4340 steel is analyzed and discussed. It is found that the FCG resistance of AISI 4340 steel enhances rapidly at first and then remains approximately the same with the decrement of strength (the improvement of toughness). Moreover, it is detected that simultaneously upgrading the strength and toughness of high-strength steels cannot improve the FCG resistance. Finally, it is believed that the microscopic fatigue damage mechanism is the major factor determining the FCG resistance rather than the macroscopic strength-toughness combination, and the factors affecting the FCG resistance of AISI 4340 steel are mainly the pre-existing dislocations and carbide precipitates. It has been found that the tempered AISI 4340 steel with different strength-toughness combinations reveals the similar fatigue crack growth (FCG) life, and simultaneously upgrading the strength and fracture toughness of high-strength steels cannot improve the FCG life. The factors affecting the fatigue damage of AISI 4340 steel are mainly the pre-existing dislocations and carbide precipitates.image (c) 2023 WILEY-VCH GmbH