Residual stress remaining in machined parts can be detrimental. Previous experimental evidence shows that hardness has a significant effect on its formation. Yet, no satisfactory explanation is available for the causes of such a phenomenon. This work seeks to understand the mechanism of residual stress formation and explain the effect of hardness on it. The analysis is based on the existence of several measurable factors that influence the stress field in the work-material during the cutting process. The sensitivity of these factors to hardness allows establishment of relationships between the hardness and the material loading cycle. The results of the analysis indicate that the residual stress pattern is correlated most strongly to the orientation of the primary deformation zone in metal cutting. This correlation provides a good explanation for the role of the material hardness on the residual stress formation.