Martensite formation and tempering
One of the goals in the project on martensite is to develop a model on how the fraction of martensite depends on e.g. alloy composition, temperature, cooling rate and deformation and how the mechanical properties are affected. One important task is how the mechanical properties depend on the microstructure of martensite including mechanical martensite in metastable stainless steels as well as the amount of residual austenite. Another goal is therefore to find a way to characterize lath and plate martensite and understand how these two structures are affected by the above variables.
The first step for calculating the fraction of martensite formed is to be able to predict the martensite start temperature. A thermodynamic method has been developed and it is now possible to calculate Ms temperatures for lath and plate martensite in commercial steels with satisfactory accuracy. It is also possible to take previous transformations of the austenite into account when calculating the Ms temperature. Software for these calculations has been developed and distributed to the industrial partners. Since a thermodynamic method is applied it is very important to have a correct thermodynamic description at low temperatures where the martensitic transformation in steels takes place. The thermodynamic evaluations of Fe-Cr, Fe-Ni, Fe-Cr-Ni and Fe-C are therefore an important part of the martensite project. Especially the addition of Zener ordering is important. In the first version of the software it has been added separately but will later on be taken into account directly through the thermodynamic description of Fe-C. The thermodynamic evaluations have, as already mentioned, been supported by ab-intio calculations.
A large effort has been made to characterize the change in martensite structure in Fe-C going from low carbon contents to high carbon contents focusing on the transition from lath to plate martensite using LOM, SEM, EBSD and TEM. A method for characterizing the transition from lath to plate martensite has recently been developed using EBSD.
A 3D elastoplastic phase-field model has successfully been developed to simulate the martensitic microstructure evolution in steels. The model is based on the phase field microelasticity model proposed by Khachaturyan. In the present work plastic deformation kinetics as well as anisotropic elastic properties of steels have been incorporated. The model has been applied on polycrystalline materials and it has been concluded that the stress distribution and the evolution of microstructure can be reasonably well predicted.
The on-going work is currently focusing on developing a model for the evolution of the martensite fraction. Modeling as well as experiments on the tempering of martensite and the precipitation of carbides is in progress. Furthermore, work is being performed on the relation of steructure and properties for martensite.
Figure: Light optical image of plate martensite in Fe-C
Figure: Martensitic microstructure formation from a 3D phase-field simulation of martensitic transformation in a pure elastic material of Fe-0-3%C steel