# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 3 Microstructural Development during USRP
### Microstructural Changes in Ferrous Based Materials Subjected to USRP
The authors also observed lamellar grains and high dislocation density due to SPD. The average grain size measured on the top surface was 93.9 nm. Figure 5 represents the corresponding TEM micrograph of the USSR'ed specimen, indicating grain refinement and the presence of dislocations on the top surface. The SAED pattern indicates that the formed nanograins have random crystallographic orientation. Figure 4: TEM of 17-4 precipitation hardened SS after USRP: (**a**) bright-field image and (**b**) dark-field image and the inset shows grain size distribution. Reprinted with permission from ref. [21]. Copyright 2018 Elsevier. [PERSON] et al. [46] have explored the effect of USRP on the microstructure and fatigue behavior of axle steel (C: 0.30 wt.%, Cr: 1.0 wt.%, Ni: 0.3 wt.%). They used a static load in the range of 1 to 10 kN and observed an SPD layer with refined microstructure at the top of the substrate. The direction of the plastic deformation was in line with the feeding direction of the roll. [PERSON] et al. [27] have used USRP to engineer the surface of 25 CrNi2 MoV, which is used under rolling contact fatigue conditions. In this investigation, a lubricant was used between the roll and the substrate. The microstructure of the untreated material consisted of martensite and residual austenite. After USRP, deformation features were observed, and it showed a gradient along the depth direction. They reported a maximum severity in deformation at a depth of 30 \(\upmu\)m from the surface. A significant fraction of the grains were in the scale of ultrafine grains (less than 1 \(\upmu\)m). [PERSON] et al. [25] have explored the USRP microstructure-fatigue and wear correlation of 23-8 engine valve steel (C: 0.33 wt.%, Mn: 2.5 wt.%, Ni: 8.0 wt.%, Cr: 23 wt.%). The microstructural study showed that the average grain diameter reduced from 6.77 \(\upmu\)m-1.70 \(\upmu\)m. The texture study indicated that the USRP treated material had grains oriented randomly and did not hold any preferential texture due to USRP treatment. The TEM study of the top material revealed that the microstructure consisted of nanoscale grains. [PERSON] et al. [54] investigated the effect of USRP on stainless steel (SS) 304 material to explore the effect of changes in microstructure towards resistance to cavitation erosion damage. The authors observed that the surface undergoes phase transformation to produce martensite. The amount of martensite was maximum, and austenite was minimum at the specimen surface. The extent of the martensite formation increased along with the increase in rolling duration, attributed to the increased level of plastic deformation. After a certain rolling level, the martensite retransformed to austenite, attributed to the heating effect associated with the continued rolling deformation. Below the transformed region, there was a region of considerable grain refinement. The grain size was reduced to 5 \(\upmu\)m-10 \(\upmu\)m from an initial grain size of 20 \(\upmu\)m-40 \(\upmu\)m. The refined grains were segmented further due to the formation of twin boundaries. With continued USRP, grain refinement would not continue. Instead, the twin boundaries become blurred, and defect levels continue to increase. Sometimes defect accumulation would lead to crack formation. The total observed deformation layer depth was in the range of 100 \(\upmu\)m-120 \(\upmu\)m.