INDENTATION SIZE EFFECT OF CARBONITRIDED AISI 1045 STEEL

Surface engineering is essential for materials that are used for contact, thermal and corrosive applications. Through benchmarks achieved in surface engineering, the mechanisms responsible for materials' mechanical and tribological properties are well established and exploited over the years. One phenomenon that has gained attention in recent decades is indentation size effect (ISE). Mechanistic and phenomenological theories have been developed to explain the observed size dependence of strength behaviour. The dislocation microstructure evolution, deformation mechanisms and the contribution of length scale parameters responsible for this effect is not fully understood. This work explores the effects of surface carbo-nitriding on material length scale and dislocation microstructural parameters of 1045 steel. The material was carbo-nitrided at 500^oC and 900^oC, prior to, metallographic characterization and micro/nano-indentation. The statistically stored and geometrically necessary dislocation densities increased with increasing carbo-nitriding temperature. A bi-linear relationship was also established from the plot of hardness squared versus the indentation depth. Three regimes of dislocation behaviour (dislocations source–limited/starvation, discrete dislocations, and continuum dislocations) were observed in this study. The observed trends are also explained using dislocation densities that were estimated using dislocation theories. The implications of the results are also discussed for the carbonitrided steels.