Influence of Si on the microstructure and C redistribution in martensitic steels

In martensitic steels the distribution of C is fundamental for the technological material properties. However, the effect of Si on C segregation is not known in detail. In this study, the impact of Si on the microstructure of martensitic steels is investigated using scanning electron microscopy and the martensite/austenite orientation relationships are determined using electron backscatter diffraction and transmission Kikuchi diffraction. Transmission electron microscopy and selected area diffraction pattern analysis were used to analyse the intragranular substructure of the martensitic phase. Further, the effect of Si on the redistribution of C is investigated by atom probe tomography in combination with transmission Kikuchi diffraction measurements in order to obtain the local C distribution and correlate it with the crystallography. It was found that the Si addition forms a sigmoidal distribution at the phase boundary and functions as a barrier for C segregation, hence reduces C partitioning into the austenite, while a Si free alloy shows clear C partitioning. Further it was observed that stable C cluster formation occurs along fine twin boundaries in the martensite, inhibiting carbide formation and partitioning of the C into the austenite.