The investigation carried out in this paper analyses the microstructure of the HCCI materials before and after the heat treatment using nanoindentation technique and scanning electron-microscopy in order to understand the effect of metallurgical factors such as morphology and volume fraction of carbides as well as mechanical properties on the further study of machinability of the materials. Present results show that changes in carbon and silicon contents as well as heat treatment strongly affect the mechanical properties and their variation in the material. The sample with higher C-Si contents has more eutectic carbides in a bainite matrix leading to a lower hardness in the as cast sample, while the sample with lower C-Si contents has higher hardness with austenite and martensite in the matrix. After annealing treatment, both sets of materials are softer than before, with the higher C- Si material being a little harder due to the presence of more eutectic carbides. Both low and high C-Si samples show similar hardness after the hardening treatment and the microstructure consists of secondary carbides and martensite surrounding the interconnecting eutectic carbides. In general, the larger volume fraction of the primary carbides associated with higher C-Si contents leads to a decrease in hardness.