An analysis of the microindentation data obtained from a thermomechanically processed multiphase microalloyed steel

Abstract

The indentation experiments using Vickers micro- and macrohardness tester at various loads ranging from 50 mN to 98 N were performed on a multiphase (ferrite-bainite-martensite) microalloyed steel. The hardness response could be classified into three stages. At loads higher than 10 N a load independent hardness (4.74 GPa) was obtained. In the load range of 10 N to 1 N an indentation size effect (ISE) i.e., an increase in the hardness with the decrease in the load was observed, whereas, in the load range of 1 N to 50 mN a reverse ISE i.e., a decrease in the hardness with the decrease in load was observed. The experimental data were analysed using the energy balance and strain gradient plasticity (SGP) models. The energy balance model was able to predict the load independent hardness as well as both the ISE and the reverse ISE. On the other hand, the SGP model could predict only the ISE but not the reverse ISE. A modified SGP model was proposed to explain the ISE and the reverse ISE. The hardness calculated based on the proposed model compares well with the experimental data. The mathematical equivalence between the modified SGP and the energy balance models has been established.