Ratcheting Behaviour of 42CrMo4 Steel under Different Heat Treatment Conditions

Abstract

The phenomenon of ratcheting occurs under the influence of non-zero mean stress during cyclic loading operation, and it deteriorates low cycle fatigue life of engineering structures. The parameters like maximum stress, mean stress, stress amplitude, stress ratio and the metallurgical features of a material significantly control the accumulation of ratcheting strain and thereby its life. The performance of machine components, pipeline installations as well as structural materials can be affected owing to ratcheting. 42CrMo4 steel is such a material which is widely used in crankshafts, gears, axles etc. where chances of deformation dew to ratcheting cannot be ruled out. In this investigation, focus was laid to examine the ratcheting behaviour of 42CrMo4 steel under various combinations of stress parameters at varying heat treatment conditions. The associated variations in substructural features were also examined by transmission electron microscopy.
Uniaxial ratcheting behaviour of the steel was investigated at ambient temperature using different combinations of mean stresses (m) and stress amplitudes (a) in three different heat treatment conditions (annealed, normalizing and quenched-tempered). The chosen stress parameters were equal percentages (max = 60 , 70 and 80%) of ultimate tensile strength (UTS) values of annealed, normalized and quenched-tempered specimens. The annealed and normalized specimens were ratcheted up to 200 cycles. A set of ratcheted annealed and normalized specimens were subjected to tensile tests and fractographic examinations were done. The quenched-tempered specimens were ratcheted up to failure. The induced dislocation densities in the ratcheted specimens were estimated by transmission electron microscopy and some alternate methods like x-ray diffraction profile analyses and hardness measurements. Finally, to predict the ratcheting life of the investigated steel, a new stress-based model was developed. The model was also implemented to predict ratcheting lives of some other materials taking data from the literature.
The obtained results indicated that accumulation of ratcheting strain increased with increasing number of cycles for all adopted combinations of m and a in all annealed, normalized and quenched-tempered conditions. This could be attributed to the increased remnant dislocation density. Post-ratcheting tensile strength of annealed and normalized samples reduced as compared to unratcheted ones happened due to prior cyclic softening. The dislocation densities of the ratcheted specimens were significantly higher than those estimated for the unratcheted steel, in both the annealed and normalized conditions. On contrary, in quenched-tempered condition the dislocation density value of ratcheted samples get reduces compared to the unratcheted one. The newly proposed stress-based model in the present investigation predicted the ratcheting fatigue life of steel efficiently in the life range of 102-104 cycles with a life factor of 1.