Ratcheting fatigue behaviour of 42CrMo4 steel under different heat treatment conditions

In the present investigation uniaxial ratcheting behaviour of the 42CrMo4 steel has been investigated at ambient temperature using different combinations of mean stresses (σm) and stress amplitudes (σa) in two different heat treatment conditions. The results indicate that the ratcheting strain increases with increase in number of cycles for all combinations of σm and σa in both annealed and normalized conditions. Further, the value is less in the normalized condition and is attributed to the rapid attainment of stable dislocation configuration with fine grained structure accompanying it. The specimens subjected to ratcheting exhibit smaller grain size than the undeformed one and it decreases with increase in stress parameter. TEM results indicate that the dislocation substructure varies from thin stripes to walls and then to cells, as the cyclic load continuously increases. The tensile strength of the ratcheted samples reduces compared to the unratcheted one owing to cyclic softening associated with the steel in both annealed and normalized conditions.