Gradient nanostructure and residual stresses induced by Ultrasonic Nano-crystal Surface Modification in 304 austenitic stainless steel for high strength and high ductility

Abstract

In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructure changes and mechanical properties of austenitic stainless steel 304 were investigated. The dynamic impacts induced by UNSM leads to surface nanocrystallization, martensite formation, and the generation of high magnitude of surface compressive residual stresses (-1400 MPa) and hardening. Highly dense deformation twins were generated in material subsurface to a depth of 100 µm. These deformation twins significantly improve material work-hardening capacity by acting both as dislocation blockers and dislocation emission sources. Furthermore, the gradually changing martensite volume fraction ensures strong interfacial strength between the ductile interior and the two nanocrystalline surface layers and thus prevents early necking. The microstructure with two strong surface layers and a compliant interior embedded with dense nanoscale deformation

Intergranular Corrosion of 316L Stainless Steel by Aging and UNSM (Ultrasonic Nano-crystal Surface Modification) treatment

“…또한 잔류응력을 개선시키는 방안은 초기 용접 시에 유발되었던 인장잔류응력을 표면처리를 통해 압 축잔류응력으로 변화시켜 주는 것이다. 압축잔류응력을 만 드는 표면처리 기술의 종류로는 shot peening [2][3][4][5][6][7] , laser shock peening [8][9][10][11] , water jet peening 12) , ultrasonic peening 13) , Ultrasonic Nanocrystal Surface Modification (UNSM) 14,15) 등이 있다.…”

Intergranular Corrosion Mechanism of Slightly-sensitized and UNSM-treated 316L Stainless Steel

“…The UNSM effects and their anticipated benefits are that the uniformed micro dimple pattern on the surface and nanocrystalline structure induce compressive residual stress in the subsurface layer. This change has the potential to simultaneously improve the static and fatigue strength, as well as the surface hardness and surface roughness of the work piece 13,14,16) . The nano-crystalline surface could increase the density of diffusion paths available for alloying elements to migrate and rapidly form a protective passive film 17,18) .…”

Enhanced Pitting Corrosion Resistance of 304 SS in 3.5 wt% NaCl by Ultrasonic Nanocrystal Surface Modification

“…The deformation-induced formation of nanoscale crystallites has been achieved through SPD induced by several techniques. 4,7,9,40 This grain refinement on the surface can be attributed to dislocation activities, deformation twining, and/or martensitic phase transformation. 6,9 Figure 6 presents the XRD patterns from the surfaces of a control 304 SS sample and a UNSM-treated sample.…”

“…1 The mechanical techniques for obtaining nanocrystalline surfaces include hydrostatic extrusion (HE), 3 laser shock peening (LSP), 4,5 surface mechanical attrition treatment (SMAT) [6][7][8] and ultrasonic nanocrystal surface modification (UNSM). 9,10 The SMAT technique involves mechanically inducing surface modifications through severe plastic deformation (SPD) using high-velocity (1-20 m/s) steel balls. 11 These balls are accelerated by collision between the balls and a vibrating chamber.…”