Microstructure evolution and strength response of ultrafine grain medium carbon steel processed by high pressure torsion

Abstract

High pressure torsion (HPT) deformation method at increased temperature of 400°C with varying strain was applied to refine microstructure of the medium carbon steel (AISI 1045). To investigate the deformation behaviour of the ferrite-pearlite two phase structure the different shear deformation and constant high hydrostatic pressure of 7 GPa were applied. The shear stress evolution during deformation and measurement of the torque were recorded and related to structure development and mechanical strength. In order to characterize microstructure development the transmission electron microscopy (TEM) was accomplished. By execution of the first turn the grain refinement was observed at deformed disc periphery. In the centre of the disc, no matter what strain was introduced, the ferrite grain structure showed moderately deformed features and pearlite colonies were preserved. With further equivalent strain increase, equilibrium between the refinement of coarse phases and new grains restoration processes led to saturation of the grain refinement process. Upon tensile properties testing, the yield strength and ultimate strength increased with increasing equivalent strain (εeq) and only short region of strain hardening period prior failure appeared, regardless the strain applied. A small drop in the hardness across the disc was measured after execution of N= 4 and 6 turns, which may be related to formation of fine grain structure and structure recovery in disc centre. © 2010 IOP Publishing Ltd.