Strain gradient induced grain refinement far below the size limit in a low carbon hypoeutectoid steel (0.19 wt% C) via pipe inner surface grinding treatment

Abstract

A low carbon hypoeutectoid steel (0.19 wt% C) with proeutectoid ferrite and pearlite dual-components was subjected to surface plastic deformation via pipe inner surface grinding (PISG) at room temperature. The deformation microstructures for each component were systematically characterized along depth, and the patterns of structural evolution toward nanometer regime as well as the governing parameters were addressed. Proeutectoid ferrite grains were refined down to 17 nm, and the pattern covering a length scale of 4–5 orders of magnitude from micron- to nanometer-scale follows: formation of cellular dislocation structure (CDS), elongated dislocation structure (EDS), ultrafine lamellar structure (UFL) and finally the nanolaminated structure (NL). The pearlite experiences the deformation and refinement, and finally the transforming the ultrafine pearlite (UFP) into nanolaminated pearlite (NLP) with the ferrite lamellae as thin as 20 nm. Refinement for both UFL (UFP) and NL (NLP) can be realized via forming novel extended boundaries within ferrite lamellae. A critical lattice curvature of ∼2.8° is required for forming such extended boundary, corresponding to a minimum strain gradient of 0.25 μm−1 for a 100 nm-thick lamella. Refinement below size limit (expressed by lamellar thickness dT in nm) is correlated with the strain gradient (χ, in μm−1) by: dT=12.5χ. Refinement contributions from strain gradient caused by PISG processing and material heterogeneity were discussed.