The effect of carbon segregation and carbide precipitation on the mechanical response of martensite

The influence of carbon distribution and carbide precipitation on the mechanical properties of the as-quenched and quenched and tempered 300M martensitic steel has been investigated. The microstructure, investigated by transmission electron microscopy (TEM) and three dimensional atom probe tomography (APT) was found to be relatively homogeneous in the as-water-quenched state, but signifi cantly evolved upon tempering and variation of quench rate. This evolution included carbon segregation to dislocations and grain boundaries and carbide precipitation. A simple mean- field precipitation model assuming heterogeneous nucleation onto the dislocations proved to satisfactorily capture the evolution of precipitation upon tempering at 120C and 150C. The material was found to behave, mechanically, as a composite and in accordance, the Bauschinger stress-strain behaviour was successfully modeled using a continuous Masing model. This model, when related to the microstructure, showed that the composite behaviour arose from the mechanical contrast between the laths, this being controlled by the local dislocation density and carbon segregation and/or precipitation onto them. Carbon segregation and carbide precipitation were observed to have a direct impact on alpha in the Taylor-like equation that was shown to control the local yield stress within the laths. When applied to martensites containing various amount of carbon, the model allowed for an empirical assessment of the e ffect of the nominal carbon content on alpha , which was found to be linearly dependent on the nominal carbon content.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat