Unraveling the Role of Carbon on the Strengthening Mechanisms of Low Mn-Si Martensitic Steels

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The role of carbon in the strengthening mechanisms of martensitic steels has been studied for decades. However, uncertainties still exist regarding how the distribution of carbon to various locations inside martensite contributes to the development of its observed microstructure and high strength. The strengthening mechanisms depend on carbon content and process parameters,but are also inter-related. The current work is an attempt to address the existing uncertainties regarding the relation between martensite dislocation density, prior austenite grain size (PAGS) and the manner in which total carbon is distributed into martensite interstitial sites and segregations near dislocations.Two steel alloys with different carbon content were selected to study, having following composition in wt.%: 0.3C-3.6Mn-1.5Si and 0.6C-3.5Mn-1.5Si. Alloys were heat treated in dilatometer to obtain martensite with different prior austenite grain sizes (PAGS). Microstructure characterization was performed using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) & Electron back-scatter diffraction (EBSD) and strength was evaluated by hardness measurements. It was found that the main factor influencing the carbon distribution in martensite is the martensite dislocation density, but the magnitude of its influence depends upon PAGS and alloy composition. In 0.3 wt.% C alloy, dislocation density decreases with increasing PAGS, as a result carbon atoms migrating towards dislocations during cooling also decrease, leaving higher number of carbon atoms at interstitial sites. However in 0.6 wt.% C alloy, such trend is not observed. On increasing the total carbon content in the alloy, very small increase in interstitial carbon content of martensite is observed if the PAGs are large. An increase in hardness was observed when the samples were introduced in liquid nitrogen, even in those cases where contribution from all strengthening mechanisms remained almost the same. This was unexpected and is most probably due to the relaxation of residual stresses and formations of carbon clusters at cryogenic temperature. In the end, an extension to already existing model is proposed to connect several microstructural features together in order to explain how they interact and evolve to give rise to observed strength of martensite.