# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 3 Microstructural Development during USRP
### Microstructural Changes in Ferrous Based Materials Subjected to USRP  
A wide variety of ferrous-based materials have been engineered with the USRP process with different processing parameters. [PERSON] et al. [49] performed USRP on carbon steels (with carbon in the range of 0.5 wt.% to 0.7 wt.%). The authors observed extensive grain refinement at the top (0 \(\upmu\)m-2 \(\upmu\)m), wherein preotetoid ferrite grains were refined in size and cementie platelets in pearlite were fractured. Below that (depth 2 \(\upmu\)m-8 \(\upmu\)m), preoutetoid ferrite grains were deformed, whereas cementie platelets in pearlite were intact. Below 8 \(\upmu\)m depth hardly, there was any deformation. [PERSON] et al. [50] have subjected a spring steel (with 0.5 wt.% C, 1.0 wt.% Mn and 1.12 wt.% Cr) to USRP with static load up to 862 N. They observed a gradient of microstructures up to a depth of 48 \(\upmu\)m and within that acicular microstructure observed in the top 4 \(\upmu\)m. [PERSON] et al. [51] have explored microstructural change of ultra-high-strength steel (45 CrNiMoVA) subjected to USRP, and they correlated the microstructural change to surface integrity. The authors observed the formation of nano-sized structures at the surface. The top surface (up to 5 \(\upmu\)m depth)  
Figure 3: Schematic presentation of microstructure generated during the USRP process. appeared to have undergone plastic flow. They also reported the development of residual stress, which is partially linked to the deformation structure. [PERSON] et al. [22] investigated the effect of USRP on the quenched and tempered 40 Cr grade shaft steel (C: 0.4 wt.%, Cr: 1.0 wt.%). USRP is performed using a sintered carbide tool of 5 mm with 400 N static force. Their work indicated a top layer of about 150 \(\upmu\)m where the plastic flow was observed. The extent of plastic deformation was decreasing along the depth direction. The same trend was observed in the case of grain refinement. The authors observed extremely small, uniform, randomly oriented nanostructured grains of size 3 nm-7 nm at the top. Extreme grain refinement was attributed to the body center cubic (BCC) structure, which had many slip systems. During rolling, a large amount of dislocation activity took place, and as a consequence, dislocation density will increase and form a tangled network, which then rearranges to form subgrain structures. With more strain, subgrains will change into new grains with small and high-angle grain boundaries. With the generation and annihilation of dislocation reaching equilibrium, the grain size would become stable and produce extremely fine randomly oriented nanoscale grains. [PERSON] et al. [52] explored the effect of USRP on the SCC tendency of high-strength pipeline steel. They observed a top layer having an extensive plastic flow structure. Below that layer, there was a region of transition followed by the matrix region. Extensive grain refinement was observed at the surface to produce nanostructures of 40 nm-100 nm. [PERSON] et al. [21] conducted USRP on 17-4 precipitation hardened SS and studied the nanocrystallization surface. The authors revealed that the nanocrystalline structure formed after USRP had a thickness of 650 \(\upmu\)m. During surface nanocrystallization, elongated ultrafine grains were formed first, which were subsequently refined by dislocation tangles, dislocation bands, and dislocation walls through a dislocation glide. Figure 4 represents the TEM of the USRP'ed specimen. The authors observed nano-equivaxed grains with an average short-axis measuring 84.25 nm due to the SPD during USRP. [PERSON] et al. [53] performed ultrasonic severe surface rolling (USSR) on 316 LS and studied gradient heterogeneous microstructure and their mechanical properties. Although the authors have decided to use the nomenclature of USSR, it is essential to point out that their process is fundamentally the same as USRP. The TEM analysis of the SLM'ed specimen after USSR treatment revealed near-equivaxed nanograins on the top surface.