Strain gradient plasticity phenomenon in construction Steels: Effects of heat treatment on microstructure and stress-strain behavior

Microstructure and micro-indentation size effects are studied in plain carbon and multi-constituent steels that were examined in the as-received and the solutionized, quenched and tempered conditions, after tempering at 350°C for 2 hours, or recrystallized at 900°C for 1 hr 30 mins. Three distinct regions (case, intermediate and core) were observed across the cross-sections of the solutionized, quenched and tempered construction steels. The hardness values in the case regions were higher than those in the intermediate and core regions. Significant indentation size effects were observed at indentation depths below ∼40 μm in the as-received samples; ∼45 μm for recrystallized samples, and ∼30 for the tempered samples. The tempering led to the formation of carbides, increased pearlite content and residual strains. Mechanism-based strain gradient plasticity models and dislocation theories were used to extract the underlying dislocation densities and plasticity length scales for the different microstructural regimes. These were then incorporated into dislocation theories for the estimation of stress-strain behavior. The implications of the results are then discussed for the structural integrity and the design of robust construction steels with well controlled compositions and microstructures.