Fatigue behavior of a nial precipitation hardening medium carbon steel

Abstract

The fatigue behavior of an Fe-0.30C-4.48Ni-l.32Al steel tempered to give three different microstructures of the same ultimate tensile strength has been investigated by light and electron microscopy, low and high cycle fatigue tests, X-ray line broadening and stress relaxation measurements. The three different heat treatments produced the following structures: I) a conventional quenched and tempered microstructure with a high density of dislocations and elongated carbides, II) a microstructure of high dislocation density, coarse carbides and fine coherent NiAl precipitates and III) a highly tempered micro-structure with a recovered dislocation substructure, coarse carbides and fine coherent NiAl precipitates. In low cycle, strain controlled fatigue cyclic softening in Treatment I was accompanied by a rearrangement of the dislocation substructure and a reduction in both the internal stress and lattice microstrain. Treatment II, which remained cyclically stable during the initial portion of the fatigue life, showed little change in the internal stress and dislocation density and showed a slight increase in lattice microstrain. Treat-ment III, which initially cyclically hardened, exhibited a rise in internal stress, lattice microstrain and dislocation density. The behavior of Treatments II and III is attributed in part to the presence of the fine NiAl precipitates which appear to reduce the tendency of the transformation induced dislocation substructure to rearrange itself into a cell structure during fatigue. In high cycle, stress controlled fatigue Treatment II showed the best fatigue resistance and Treatment I the worst. Improvement in life was attributed to improved resistance to crack initiation.