Low cycle fatigue behaviour of a ferritic steel strengthened with nano-meter sized precipitates

Low-cycle-fatigue behaviour (Delta epsilon(t)/2 = +/-0.002 to +/-0.01) of a ferritic-steel (tensile strength similar to 800 MPa) strengthened with nano-meter-sized-precipitates has been examined. The selected steel is a hot-rolled Ti-Mo-bearing low-carbon steel (grain size similar to 3 mu m) strengthened predominantly with Ti-Mo-C and Ti-C type precipitates of size <= 30 nm distributed randomly on the grain-boundaries and the matrix. The spread of cyclic-plasticity is negligible in the steel till Delta epsilon(t)/2 = 0.0025 leading to cyclically-stable behaviour and excellent fatigue-life (no failure till 105 loading cycles). At Delta epsilon(t)/2 = 0.003, significant initial-cyclic hardening is noticed followed by minor amount of cyclic-softening. The latter is attributed to formation of low-energy-dislocation-structures e.g. dislocation-walls and channels. At Delta epsilon(t)/2 = 0.004-0.01, predominantly cyclic hardening is noticed till failure; the intensity of hardening increases with increase in Delta epsilon(t)/2. The cyclic-hardening is attributed to increase in dislocation-density and dislocation-precipitate-interaction, formation of subgrains, microbands and dislocation-clusters within both ferrite-grains and microbands. All the above types of dislocation-substructures hinder movement of dislocations and reduce opportunity for cyclic-strain-hardening at Delta epsilon(t)/2 >= 0.004 compared to that at Delta epsilon(t)/2 = 0.002-0.003. This phenomenon leads to two stages of cyclic-hardening with two distinct values of cyclic-strain-hardening-exponents (n(1)' = 0.3 at low Delta epsilon(t)/2 and n(2) = 0.04 at Delta epsilon(t)/2 >= 0.004). Two-stage cyclic-hardening is also reflected in the bilinear Coffin-Manson-relationship and fatigue-fracture. In sharp contrast to fatigue-fracture-surface at Delta epsilon(t)/2 = 0.003, that at Delta epsilon(t)/2 a 0.004 are characterised by multiple-origins of fatigue-crack, absence of fatigue-striations and occurrence of microcleavage/river-pattern indicating faster brittle-fracture at higher Delta epsilon(t)/2.