Enhancing the strength of structural steel through severe plastic deformation based thermomechanical treatment

Structural steel (hypoeutectoid steel) was subject to severe deformation through a novel method of thermo-mechanical processing coupled with equal channel angular pressing (ECAP). The ECAP experiment was conducted at a specific temperature for the hot ECAP (H-ECAP) process after which the sample gained a superior strength of 811 MPa with reasonable elongation. Whereas when the ECAP was conducted at room temperature (RT-ECAP), the sample strength was  659 MPa with an elongation of 16%. The electron backscattered diffraction (EBSD) analysis revealed grain refinement and grain misorientation as a result of the ECAP process. The H-ECAP process resulted in an increased pearlite volume fraction and uniform pearlite fragmentation due to the strain induced transformation generated by hot deformation. The internal structural modification of the pearlite grain was revealed in the TEM micrographs. Nanoindentation analysis further exposed carbon supersaturation in the ferrite matrix. The X-ray line profile analysis (XRDLPA) illustrated that the H-ECAP processed sample has a higher dislocation density than the RT-ECAP processed sample. Factors such as grain refinement, morphological changes in the grains and the dislocation density predominantly contribute to the increased strength of the H-ECAP processed sample. In the overall observation, H-ECAP processed sample exhibited superior mechanical properties than the RT-ECAP processed sample.