Carbide morphology and ferrite grain size after accelerated carbide spheroidisation and refinement (ASR) of C45 steel

Abstract

The pearlite spheroidisation and grain refinement of the C45 steel was investigated. The ASR (accelerated carbide spheroidisation and refinement) process allows us to achieve a microstructure made of fine ferrite grains and globular carbides in seconds or minutes. This paper deals with the ASR process realization via the thermomechanical processing of controlled rolling. Hot rolling is a common part of structural steel processing. A conventional structure after the hot rolling of the C45 steel consists of lamellar pearlite and ferrite. During controlled rolling with a deformation at temperatures around critical temperature A1, it is possible to achieve a microstructure of a fine ferritic matrix and globular carbides. The deformation causes a recrystallization of the ferritic matrix - a grain refinement. Furthermore, a high dislocation density during the deformation enhances the diffusion and promotes carbide spheroidisation. The aim of the experimental program was to achieve a microstructure of the C45 steel consisting of fine ferritic grains and homogeneously dispersed cementitic globular particles by processing bars in a laboratory rolling mill. The morphological changes of the cementite after the thermomechanical treatment in comparison with the conventional hot rolling were investigated as well as the ferritic grain refinement. An image analysis was performed to determine the spheroidisation level of the cementite (the aspect ratio of cementite particles) and the ferritic grain size. A proper examination of a cementite morphology can be carried out only by observing the entire cementite particle. A common metallographic analysis relies on a 2D section observation. Thus, deep etching and cementite extraction were performed to study the cementite-particle shape formed during the ASR process.