Tempered martensitic steels were investigated in isothermal and thermomechanical fatigue conditions and general features of their cyclic behaviour are reported. A cyclic anisothermal constitutive model with internal variables was formulated to describe their behaviour; it allows the description of the Baushinger effect, the continuous cyclic softening, the strain rate dependence and the plastic strain memorisation. When compared to experimental results, isothermal and anisothermal model predictions show good coherence. This model is helpful to understand and explain some experimental results such as the increase in strain softening when strain rate decreases in isothermal testing, as well as the relations between mean stress and temperature range or cyclic softening with strain amplitude in anisothermal conditions.
Microstructural evolutions of the 55NiCrMoV7 steel during tempering were investigated by transmission electron microscopy, scanning electron microscopy and X-ray diffraction in order to describe the main mechanisms of softening. The softening resistance is strongly associated with evolution of obstacles to the movement of dislocations (prior austenitic grain boundary, lath boundary, secondary carbides, etc.). Only the average size of carbides was found to be influenced by tempering conditions. Moreover, a strong correlation observed between the hardness measured after tempering and the average size of carbides showing that this easy test could in this case partially characterize the state of the microstructure after tempering. Performing hardness measurements at the as-quenched, tempered and annealed states, a kinetic law of tempering based on the work of Johnson, Mehl and Avrami has been proposed. This law was validated in the case of complex tempering and for other steels and can well describe the evolution of hardness during tempering.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.