Effect of surface nano-crystallization induced by supersonic fine particles bombarding on microstructure and mechanical properties of 300M steel

Supersonic fine particles bombarding (SFPB) technology opens a new territory for engineering materials towards improved performances. Owing to its merits and emerging applications, 300M steel (tensile strength >= 1800 MPa) was treated with SFPB to create surface gradient nanostructures. The time dependent SFPB process was implemented on various 300M steel surface to investigate the microstructural evolution and mechanical property. 300M steel surface grains were sufficiently refined down to nanometer scale under high energy SFPB. In the subsurface layer, acicular martensite was found to be bent and broken, resulting in the high-density dislocation. At the early stage of SFPB, the impact affected area of 300M steel surface was deepened with increasing SFPB time, and the grains were constantly refined, which further lead to higher strength and improved hardness. However, after longer treatments of more than 90 s, bombardment energy accumulated at 300M steel surface resulted in grain growths and deteriorations of hardness. In particular, the newly formed microcracks substantially reduced the tensile strength. After SFPB treatment, the dimple size of the 300M steel surface fracture decreased significantly, and a large area of cleavage plane appeared, showing typical characteristics of ductile-brittle mixed fracture.