The effect of ultrasonic nanocrystal surface modification on low temperature nitriding of ultra-high strength steel

In this research, a surface plastic deformation layer was induced on 300M ultra-high strength steel using ultrasonic nanocrystal surface modification (UNSM). The UNSM-treated and control specimens were then nitrided at 450 °C for 6 h. After gas nitriding, the microstructure, corrosion and wear resistance of 300M steel with and without UNSM pre-treatment were investigated. After nitriding, the UNSM-treated specimens generated a dense surface nitride layer that was thicker than that of the control specimens. The high defect density and refined lath martensite induced by UNSM were found to accelerate the nitrogen diffusion and efficiency. X-ray diffraction patterns revealed that α-Fe phase occupied the largest proportion in the nitriding layer of specimens without UNSM pre-treatment. However due to the higher nitriding efficiency, the main phase of UNSM-treated specimens after nitriding was found to be ε-Fe2-3N. Compared with the specimens without pre-treatment, UNSM-treated specimens exhibited higher surface hardness as well as higher corrosion and wear resistance after nitriding. [Display omitted] •Refined and distorted lath martensite generated in UNSM-treated 300M steels resulted in much higher nitriding efficiency.•UNSM + nitriding generated a denser and deeper surface nitride layer that contributed to higher corrosion resistance.•UNSM + nitriding induced high surface hardness that contributed to higher wear resistance of 300M steel.