Improvement of low-cycle fatigue resistance in AISI 4140 steel by annealing treatment

The low-cycle fatigue (LCF) behaviour of AISI 4140 steel under annealed and as-received conditions was investigated at room temperature. The annealing treatment causes a marked decrease in mechanical strength but an increase in plastic energy and ductility. The annealing treatment of AISI 4140 steel significantly increases the LCF resistance of the material. In addition, the steel exhibits a transitional behaviour from initial cyclic softening to stable cyclic hardening with increasing strain amplitude. By contrast, the as-received AISI 4140 steel undergoes progressive cyclic softening until failure. Microstructural changes induced in the annealed steel cause near-Masing-type behaviour at low and high strain amplitudes. The enhancement of the LCF resistance of the annealed steel is attributed to the high magnitude of the compressive stress, which is dependent on the applied strain amplitude. A good correlation of the total failure cycles with plastic strain and elastic strain provides a precise equation model for predicting the LCF life of annealing-treated AISI 4140 steel.