Substructure of Martensite in High Carbon and High Silicon Steel

Martensite in steels can usually be classified into three different morphologies including lath martensite, lenticular martensite and thin plate martensite. The morphological evolution relies on the martensite start (Ms) temperature related to the chemical composition of alloy. The transition among three types of martensite in Fe-C alloys can be determined by carbon compositions which affects the Ms temperature. Lath martensite, formed at the highest temperature, is associated with lattice invariant strain (LIS) occurring by dislocation slips. Thin plate martensite, formed at the lowest temperature, consists of transformation twins due to LIS. For lenticular martensite, usually formed at intermediate temperature, its fine structure brings about tons of attentions because it contains both transformation twin and dislocations.
In this study, the chemical compositions of high carbon and high silicon steels are containing three different contents of carbon and cobalt. Martensite from steels are obtained by quenching to room temperature which below its Ms temperature. By Observation, The major types of martensite in the three different alloys are lenticular and plate martensite. The fine nano/microstructure of lenticular martensite was investigated by transmission electron microscopy (TEM) to make clear the effects of carbon and cobalt on morphology of lenticular martensite. The twin width and aspect ratio decreases with the increasing of carbon content, and the interface becomes smooth by adding cobalt. Moreover, the effect of prior austenite grain size, which was controlled by austenization temperature in this study, on the formation of lenticular martensite will also be further elucidated. Optical microscope (OM) was used to observe the morphology and the aspect ratio of lenticular martensite; X-Ray diffraction spectra (XRD) was applied to measure the volume fraction of martensite. The above synergetic results coupled with TEM investigations will provide an extended knowledge in the nanostructure of lenticular martensite. Besides, the effects of austenite grain size and chemical compositions on the martensitic transformation and the corresponding nanostructure will be also further understood in this study.