Cyclic deformation behaviours of 18Mn3Si2CrNiMo multiphase (martensite/bainite/retained austensite) steel

Abstract

A series of multiphase microstructures with different contents of bainite, martensite and retained austenite are obtained by the austempering treatment of 18Mn3Si2CrNiMo steel at different temperatures below Ms. And the low cycle fatigue (LCF) behaviour of the test steel at different cycle times, such as the first cycle, the maximum peak cyclic stress cycle and the half-life cycle, is also investigated. The results show that the reduction in mobile dislocation density is the main cause of very high strain hardening observed under the first cycle of cyclic tensile load. Through the analysis of fatigue life, it is found that the transformation of large block retained austenite to martensite in the sample after austempered at 315 °C accelerates the fatigue failure behaviour. The test steel with a microstructure of 69.5% VM + 23.5% VB + 7.0% Vγ at the lower total strain amplitude and with 49.9% VM + 40.6% VB + 9.5% Vγ at the higher total strain amplitude has an optimal LCF life.