Abstract: Objective Duplex stainless steels, with nearly equal austenite and ferrite phases, offer excellent mechanical properties and corrosion resistance but are difficult to machine due to severe work hardening, rapid tool wear, and poor surface finish. Ultrasonic rolling (USR) is a promising post?processing technique that induces severe plastic deformation through combined static and high?frequency dynamic loads, yielding surface refinement and hardening. However, the distinct deformability of the two phases in duplex structures demands careful process control to ensure deformation uniformity. This study systematically investigates the influence of key USR parameters on surface integrity of 2205 duplex stainless steel, establishes a predictive response surface model, and identifies an optimized process window for practical applications. Methods The base material was 2205 duplex stainless steel (22.4Cr–4.6Ni–3.0Mo–0.13N). Disc specimens were turned under fixed conditions (1000 r/min, 0.02 mm/r) to obtain reproducible initial surfaces. USR was performed using a Huayun HK30F device mounted on a CNC lathe with a 14 mm cemented carbide ball. Five parameters were varied: rolling passes (1–12), static pressure (0.05–0.5 MPa), ultrasonic power (25%-60%), spindle speed (100–500 r/min), and feed rate (0.01–0.20 mm/r). Single?factor experiments first determined effective ranges. Surface roughness (Ra) was measured by white light interferometry; hardness by microindentation (50 g load). Microstructure was characterized by OM, SEM?EDS, XRD, and EBSD after electrolytic polishing. For optimization, a five?factor, three?level Box?Behnken design (RSM) was employed with roughness as the response. A quadratic regression model was fitted and validated by ANOVA. Results Single?factor tests showed non?monotonic relationships: both roughness and hardness improved then deteriorated with increasing each parameter. Optimal single?factor conditions were 5 passes, 825 W, and 200 r/min for both properties; static pressure optima differed (0.20 MPa for minimum roughness vs. 0.30 MPa for maximum hardness). Improper parameters caused defects including peeling, folding marks, cracks, spallation, scratches, and non?uniform deformation. The RSM model exhibited excellent fit (R2 = 0.9724, adjusted R2 = 0.9494, predicted R2 = 0.8971) with significant model validity (p < 0.0001) and non?significant lack?of?fit (p = 0.3452). Parameter influence ranking was: rolling passes > static pressure > spindle speed > feed rate > power. Multi?objective optimization yielded the optimal combination: 5 passes, 0.2 MPa, 190 r/min, 825 W, and 0.10 mm/r, with a predicted Ra of 0.338 μm. Five validation experiments averaged 0.339 μm (0.3% error), confirming model reliability. Under optimized conditions, Ra decreased from 1.614 μm to 0.339 μm (79% reduction), hardness increased by ~14%, and a ~179 μm thick deformation layer formed. EBSD revealed dramatic grain refinement: average grain size reduced from 40.4 μm to 2.9 μm (93% decrease), with 88.95% of grains in the 0.77–8.47 μm range. Austenite fraction rose from 40% to 67%, with morphology changing from isolated islands to elongated bands along the rolling direction. Crystallographic texture shifted from ?001? to ?111?, indicating slip system activation and grain rotation. A depth?dependent deformation gradient—from severely deformed layer to transition zone and unaffected matrix—confirmed the energy dissipation mechanism of USR. Conclusions Ultrasonic rolling effectively enhances the surface finish, hardness, and microstructure of 2205 duplex stainless steel through controlled severe plastic deformation. The validated RSM model provides a reliable tool for parameter selection, enabling defect?free processing with significant property improvements. The elucidated mechanisms—phase?specific deformation response, grain refinement kinetics, and texture evolution—offer both fundamental understanding and practical guidance for industrial surface strengthening of two?phase structural materials.