Development Of Deformation Structures In Medium Carbon Steel (0.45 Wt.% C) By Equal Channel Angular Processing

Ultrafme-grained (UFG) materials with submicrometer-order grain size exhibit excellent mechanical properties compared with conventional fine-grained materials as well as coarse-grained materials. Several methods have been developed to obtain UFG materials through severe plastic deformation(SPD). Among them, the equal-channel angular pressing (ECAP) technique has been proved to be an effective method for the fabrication of various bulk UFG materials without residual porosity. In performing ECAP, a material is subjected to intense plastic straining by pressing a sample repeatedly through a die containing two channels, with equal cross-sections, intersecting at an angle.Equal Channel Angular Processing of the medium carbon steel (0.45 wt.% C) was successfully carried out at 500 C with route C in which the sample is rotated 180 along its longitudinal axis in this study, obtaining a total equivalent true strain ~4. The microstructure evolution of ferrite and pearlite in ECAP was investigated by means of optical microscopy (OM) and transmission electron microscopy (TEM) . The main results are as the following: (1) The microstructure of lamellae pearlite has evolved ultrafine cementite particles throughout the ferrite matrix uniformly after four passes. The ferrite matrix is homogeneous grains, and average grain size is -0.3um. (2) At the initial stage of processing, dislocation cells and even subgrains with lamellar boundaries are developed in ferrite grains on the present deformation. With further deformation, the size of dislocations and subgrains decrease. At the larger stain, equiaxed ultrafmed ferrite grains with high-angle boundaries are achieved by grians sliding and rotation. (3) Hard cementite in the present steel has a considerable ability of plastic deformation under thepresent shear deformation conditions. (4) Which deformation behavior of cementite lamellae in pearlite colonies is closely related to the stress state of cementite which significantly depends on cementite thickness and orientation, especially on cementite thickness. (5) The lamellar cementite undergoes partial dissolution on the present deformation. The thermodynamical destability of cementite is attributed to a large number of defect in cementite introduced by severe plastic deformation providing evidence for cementite dissolution.