# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 4 Effect of USRP on Mechanical and Tribological Properties of Engineering Materials
USRP enhanced surface microhardness by 17.2%, 25.6%, and 39.2% after 1, 5, and 10, pass as represented in Figure 12a. The variation of microhardness along the depth direction is indicated in Figure 12b. It is evident that an increase in the number of passes significantly affects microhardness along the depth direction. The enhancement in microhardness due to multiple passes can be attributed to the grain refinement, work hardening, and high RCS. [PERSON] et al. [26] conducted a study of USRP on 17-4 PH SS; the authors revealed they could induce RCS of 800 MPa on the surface and carry the RCS effect to a depth of 400 \(\upmu\)m from the surface. The introduction of RCS plays a predominant role in protecting the weld joints from stress corrosion cracking (SCC). RTS is induced in the weld fusion zone, HAZ, and base material during welding, creating SCC failure when exposed to a chloride environment. After USRP treatment, RTS can be converted entirely to RCS. A high amount of RCS can be induced on the substrate material by selecting intense USRP parameters. In this manner, the severity of the USRP process can be enhanced. [PERSON] et al. [48] studied residual stress before and after USRP treatment on T4003 ferritic SS welded joints. The authors used an X-ray diffractometer to determine the residual stress distribution at the welded junction. Figure 13 represents the residual stress distribution of the weld joint before and after USRP treatment. It is evident from the figure that the untreated weld joint \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Materials**} & \multirow{2}{*}{\begin{tabular}{c} **Static Force or** \\ **Pressure** \\ \end{tabular} } & \begin{tabular}{c} **Roller Material,** \\ **Hardness and** \\ **Diameter** \\ \end{tabular} & \begin{tabular}{c} **Frequency and** \\ **Amplitude** \\ \end{tabular} & \begin{tabular}{c} **Observation** \\ \end{tabular} \\ \hline \multirow{4}{*}{\begin{tabular}{c} AZ31B Mg alloy \\ [40] \\ \end{tabular} } & 0 MPa, & \multirow{4}{*}{\begin{tabular}{c} Carbide; Unspecified \\ 0.1 MPa, \\ 0.15 MPa; \\ 0.2 MPa, \\ 025 MPa \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} Grain refinement by 50\% \\ Surface roughness reduced by 91.8\% \\ Microhardness increased by 83.81\% \\ Yield strength increased by 24.3 times \\ \end{tabular} } \\  & 300 N, & \multirow{4}{*}{\
\begin{tabular}{c} Unspecified Material; \\ 80 HRC; 14 mm \\ 8 \end{tabular} } & 28 kHz; & \multirow{4}{*}{
\begin{tabular}{c} Surface Roughness reduced by 34.5\% \\ Enhancement in corrosion resistance \\ \end{tabular} } \\  & 900 N & & 80 HRC; 14 mm & 6 \(\upmu\)m; & \\ \cline{1-1}  & & & 8 \(\upmu\)m & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: \(Cont.\)  
Figure 12: Microhardness profiles of untreated and USRP’ed 300M steel after 1, 5, and 10 passes (**a**) on surface (**b**) along the depth. Reprinted with permission from ref. [28]. Copyright 2021 Elsevier. possesses RTS, and after USRP treatment on the weld joint, the entire RTS is converted into RCS. The presence of RCS can close the surface fatigue cracks and can prevent the extension of cracks, thereby providing superior surface integrity. Similarly, researchers analyzed the reasons for the improvement of surface corrosion resistance after USRP treatment. The corrosion behavior of metals and alloys predominantly depends on the surface roughness; hence, the improvement of corrosion resistance can be correlated with the surface roughness. For example, [PERSON] et al. [41] studied the effect of USRP on the corrosion properties of uranium metal. In their study, the authors compared the corrosion properties of a turned uranium metal bar of surface roughness of 1 \(\upmu\)m with a USRP treated uranium metal bar. The electrochemical corrosion studies were conducted using 50 ppm NaCl solution. The authors revealed from the polarization curve that the USRP treated specimen's corrosion potential is lower than the turned specimen. However, the corrosion current of both specimens remains unchanged.