Effects of Ultrasonic Nanocrystal Surface Modification on Mechanical Properties of AISI D2 Steel

This study examines the effects of ultrasonic nanocrystal surface modification (UNSM) on D2 cold-work tool steel materials. Specifically, the effect on surface topology and mechanical properties due to different UNSM conditions is examined. Before UNSM treatment, the substrate is processed via a quenching and tempering (QT) treatment for different values of substrate hardness. We observed surface changes due to UNSM treatments in D2 substrates and examined the correlation between topology changes, roughness, hardness, and tensile properties. The results indicated that hardness increased with the refinement of the substrate surface after UNSM treatment although significant changes based on the number or orientation corresponding to UNSM treatments did not occur. However, hardness increased significantly in non-QT substrates (which initially displayed relatively high elongation). Furthermore, in the substrates, surface roughness was degraded in specimens that underwent two passes of impacts compared with those that underwent a single pass. Conversely, in QT substrates, the double pass increased surface consolidation, and thereby improved surface roughness. Furthermore, tensile strength after UNSM treatment did not exhibit a significant difference irrespective of the number of treatments and orientations, or substrate hardness. However, in the non-QT substrates, variations occurred in the elongation based on the number of treatments and orientations. This can be owing to crack formation and surface roughness changes caused by material pile-up owing to ball tip impacts. Therefore, the UNSM conditions must be appropriately adjusted by considering the initial properties of the substrate before the treatment in conjunction with the properties that should be improved.