Effect of initial treatment on the structure and mechanical properties of medium carbon steel subjected to ECAP

The present work deals with grain refinement of medium carbon steel AISI 1045 (0.45 wt.% C) having different initial ferrite-pearlite microstructure resulted from thermal and thermomechanical (TM) treatment. The purpose of prior TM steel processing was to refine grains of ferrite phase and to modify coarse lamellae pearlite structure. The final grain refinement of steel structure was then accomplished during warm Equal Channel Angular Pressing (ECAP) deformation at 400 C. Employment of this processing route, in dependence of the applied effective strain epsilon(ef), resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. When applying higher shear stress, the mixed structure of subgrains and ultrafine grains was formed within ferrite phase, regardless the initial steel modification. In pearlite grains modification of cementite lamellae due to shearing, bending, twisting and breaking was found efficient as straining increased. Processes of dynamic polygonization and recrystallization in deformed structure also contributed to submicrocrystalline grains structure formation in intensively deformed structure. Comparing results of coarse lamellae cementite spheroidization it was then more efficient when prior TM treatment of steel was introduced. The tensile deformation results confirmed the strength increase, however, deformation behaviour and strain hardening, generally for different initial structural conditions of steel, showed diversity across deformed bars.