Nano structure and transformation mechanism of white layer for AISI1045 steel during impact wear

Impact wear was carried out for annealed AISI1045 steel in this study. With the impact frequency of 600 times per min, an 8.29 kN impact load was applied on the constant contact area of 540.08 mm2 at the individual attack angles 60° and 90°. The microstructure of white layer for annealed AISI1045 steel was examined by SEM, TEM and HRTEM. It was found that nano structures with ferrite and cementite have been formed in white layer after impact of AISI1045 steel. Nano crystallization mechanism for ferrite is from moving, interacting and rearranging of dislocations, resulting in production of dislocation walls. With increasing strain, the density of pinned dislocations, whose Burgers vectors are usually vertical to the wall, correspondingly increases along the dislocation wall, i.e., θ/b increases. That, finally, forms subgrain boundaries with small angle θ. In addition, the density of slippery or shear-susceptible dislocations, whose Burgers vectors are often parallel to dislocation walls, also increases at the same time. The slipping of dislocations in many places along the boundary of subgrains will result in the rotation of crystals to form boundaries with large angles. Nano crystallized mechanism for cementite is mainly due to shearing fracture, neck-like shrinkage fracture and carbides dissolving under the impact. The cementite thinning and the formation of slipping stages will increase the surface energy per volume, which causes the dissolution of cementite. The chemical potential energy difference between carbon atoms and cementite is a driver for dissolving cementite. The moving, reacting and proliferating of dislocations promote carbon atoms diffusing and migrating to distant ferrite from the boundary of cementite, which accelerates the dissolution of cementite.