Low alloyed High carbon steels (HCS)s with duplex structure of martensite and retained austenite (RA) have considerable potential for industrial application in high abrasion environments due to their hardness, strength and low cost. Using standard compression testing, XRD, TEM, EBSD, EPMA, nano-indentation and micro-indentation measurements, we determined the mechanical stability of RA in HCS under compressive stress and recognized the phase transformation mechanism, from the macro to the nano level. We found at the initial stage of plastic deformation HCP martensite formation dominates, whereas higher compression loads trigger BCT martensite formation. The combination of this phase transformation and strain hardening increased the hardness ~30%. We also investigated the effect of Cr on the microstructure, transformation behaviour and mechanical stability of RA, with varying Cr contents. Increased Cr% altered the morphology of the RA and increased its stability, consequently, increased the critical pressure for martensitic transformation.