On the importance of local equilibria in alloy design criteria for bulk nano-pearlitic steels and ensuing mechanical properties

Abstract

A critical review of the dependence of yield strength of pearlite on interlamellar spacing unveils the contribution of solid solution strengthening of ferrite. In order to understand the evolution of solute content within pearlitic ferrite, importance of various local equilibria modes (assumed to persist during transformation) has been realized and revisited in present work. With this understanding, an alloy composition and corresponding two stage cooling process have been designed to have nano-pearlitic microstructure (interlamellar spacing <100 nm). Characterization of elemental partitioning across the growing pearlite-austenite interface using scanning transmission electron microscopy coupled with energy dispersive x-ray spectroscopy (STEM-EDS) reveals no-partitioning growth mode of pearlite. Further, mechanical properties have been assessed for samples interrupted during cooling post pearlite formation (at 635 °C, S635) and for those cooled till room temperature (SRT). Reduction in ferrite width from ∼70 nm to ∼65 nm along with increase in post transformation enrichment of pearlitic ferrite with Si from S635 and SRT, has been shown to lead to an increase in the yield strength from ∼680 MPa to ∼775 MPa in the deigned alloy.