# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 4 Effect of USRP on Mechanical and Tribological Properties of Engineering Materials
During USRP testing, the spindle frequency was set to 20 kHz with a working amplitude of 30 \(\upmu\)m and a normal static load of 350 N. It's important to note that the tube was spun at 200 rpm with a calculated shot density of 38,500 repetitions/mm\({}^{2}\). To study the wear behavior of the tube, a self-made impact sliding wear test apparatus was used with varying sliding velocities of 0 mm s\({}^{-1}\), 60 mm s\({}^{-1}\), 90 mm s\({}^{-1}\), and 120 mm s\({}^{-1}\). From USRP, the sheer plastic deformation from the process resulted in a greater fraction of sub-structured and surface deformed grains than the mainly recrystallized grains with the base specimen. There was a decrease in wear scar width for all sliding velocities when subjected to sliding, as shown in Figure 11. It is essential to note the more significant presence of cracking from the base material than the USRP specimen. Although each sample did exhibit some form of delamination, the accumulation of wear debris and abrasive wear for the bulk sample was substantially more than the USRP sample. This increase in wear resistance is primarily attributed to the refined grains and increased compressive stresses induced from USRP. Table 1 demonstrates the effect of USRP on a broad range of engineering materials and their corresponding enhancement in mechanical properties and influence of microstructural features. The introduction of in-depth RCS during USRP in metals and alloys due to the SPD has been reported by scholars. [PERSON] et al. [30] conducted USRP experiments on 23-8 engine valve steel revealed a high RCS of 751 MPa on the surface, which became 808 MPa at 20 \(\upmu\)m depth when the authors used a frequency of 20 kHz and static force of 900 N. When the frequency changed to 30 kHz without changing the static load the RCS on the surface became 803 MPa and 876 MPa at 20 \(\upmu\)m depth. The authors summarized that the formation of higher grain dislocations at higher vibrating frequencies induces higher RCS quantities. Similar results were reported during USRP on Ti6 Al4V [77]. Figure 11: 2D wear profiles of the (**a**) base and (**b**) USRP Inconel 690 alloy tubes from impact sliding, wear test. Reprinted with permission from ref. [64]. Copyright 2020 Elsevier. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials** & **Static Force or Pressure** & **Roller Material, Hardness and Diameter** & **Frequency and Amplitude** & **Observation** \\ \hline
33 Cr23 Ni8 Mn3 & 900 N & Carbide; Unspecified; 15 mm & 20 kHz; 30 kHz; 10 μm & Improved fatigue resistance by 38.3\% \\
[25] & 900 N & 15 mm & 10 μm & Increase in residual stress by 3.6 times \\  & & 15 mm & 10 μm & Surface roughness reduced by 75\% \\  & & & & Microhardness increases by 31.3\% \\
17-4 PH SS & 600 N & WC/Co; Unspecified; 12 mm & 28 kHz; 15 μm & Surface roughness reduced by 57.69\% \\
[26] & 600 N & 12 mm & 28 kHz; 15 μm & Microhardness increased by 20\% \\  & & 12 mm & 15 μm & Yield strength increased by 24.3\% \\  & & & & Elongation reduced by 52.6\% \\  & & & & Elastic modulus increased by 10.93\% \\
25 CrNi2 MoV Steel & 900 N & Carbide alloy; Unspecified; 12 μm & Surface roughness reduced by 76.2\% \\
[27] & 900 N & Unspecified; Unspecified & 12 μm & Increase in microhardness by 11.3\% \\  & & & Unspecified & Increase in residual stress by 3.5 times \\  & & & & Fatigue life improvement by 21 times \\
300M steel & & WC/Co; 28 kHz  
[PERSON] et al. [28] conducted USRP experiments on 300M steel and studied the surface integrity and wear behavior. In their study, the authors used a tungsten carbide ball of 14 mm diameter with a frequency of 28 kHz and three different processing passes of 1, 5, and 10. The authors revealed superior enhancement in surface hardness with an increase in the number of processing passes compared to the untreated specimen. Figure 12 shows the surface microhardness profiles of the untreated and USRP treated 300M steel with different passes.