Abstract
Ultrasonic nanocrystal surface modification (UNSM) in the 20F mode gave the best surface of 45 steel among the three modes studied: instrumented hardness HIT rose from 3962 to 5319 N/mm² (+34.2%), Vickers hardness from 587 to 694 HV, Ra fell from 0.034 to 0.020 μm, and the friction coefficient dropped from 0.712 to 0.574. Three UNSM modes (10F, 20F, 30F), each defined by static load, amplitude, feed rate and pass step, were compared with the as-received state by scanning electron microscopy, X-ray diffraction, instrumented and Vickers indentation, profilometry and ball-on-disk tests. All modes formed a layer of refined grains under the surface, and its depth grew with the treatment intensity. Raising the intensity from 20F to 30F did not improve the properties: hardness fell by 2.2%, and Ra and the friction coefficient rose, which may be associated with saturation of strain hardening and the onset of surface damage. The 20F mode is the most favourable of the three modes for hardening 45 steel.