Study of the effect of heat treatment on low cycle fatigue in AISI 4340 steel by acoustic emission

The effect of prior heat treatment on the mechanical properties, namely the characteristics of plastic deformation, crack initiation and crack propagation, in the low cycle fatigue of AISI 4340 steel was investigated. Also studied was the effect of the changes in microstructure due to heat treating on the fatigue properties using electron microscopy. Simultaneously, during fatigue tests, acoustic emission was used to determine the extent of fatigue damage in this steel as well as the onset of crack initiation. The results show that, for this material, softening takes place in the first few cycles for all values of hardness. This softening is correlated to the acoustic emission activity detected as well as to the changes in hardness due to the different heat treatments. Mechanisms for explaining the effect of heat treatment on fatigue crack initiation as well as acoustic emission are discussed.