Influence of microstructure on the low and high cycle fatigue behaviour of a medium carbon microalloyed steel

Abstract

This paper reports the room temperature monotonic and cyclic stress-strain (CSS) response, the low and high cycle fatigue behaviour of a medium carbon microalloyed (MA) steel in different microstructural conditions obtained by isothermal transformation at 973, 773 and 573 K following austenitizing at 1123 K. The isothermal transformations resulted in coarse pearlite (CP), fine pearlite (FP), and acicular ferrite/bainite (AF/B) microstructures, respectively. In low cycle fatigue, the CP and FP microstructures exhibited cyclic softening at low total strain amplitudes (<0.6%) and cyclic hardening at higher total strain amplitudes. The AF/B microstructure exhibited cyclic softening in the whole range of total strain amplitudes employed. In the FP microstructure similar CSS curves were obtained in both constant total strain amplitude and multiple step tests. In the AF/B microstructure, however, the CSS curve depended on the test procedure. Fatigue life in the LCF regime was greater (135%) for the CP microstructure compared with the FP and AF/B microstructures. Fatigue crack propagation in the pearlitic microstructures was through void growth and coalescence. In the AF/B microstructure it was by transgranular cleavage. The high cycle fatigue resistance (fatigue limit) of the FP structure was greatest (400 MPa) while that of the CP microstructure was the least (245 MPa). The fatigue limit correlated well with the cyclic yield strength (σ_cys) of the material and was approximately equal to 0.7σ_cys.