Study on Hardness and Elastic Modulus of Surface Nanostructured 304 Stainless Steel Using Two Mechanical Methods

Ultrasonic impact treatment (UIT) can be used to create a thin nanostructured surface layer that plays a significant role in enhancing the overall strength, fatigue life, and corrosion resistance of the treated material. The hardness and elastic modulus of surface nanostructured 304 stainless steel treated by UIT have been investigated by nanoindentation and microhardness measurements. The hardness of the top nanostructured surface layer and its elastic modulus are about 38% and 30% higher, respectively, than those of the bulk material in the nanohardness testing. Also, the hardness is increased by about 23% in the Vickers microhardness testing. The nanohardness of the nanostructured surface layers decreases with depth and then trends to stable values. A hardened layer is found in the impact zone and the thickness is approximately $450–500μm$⁠. All results demonstrated that the surface nanocrystallization can effectively enhance the mechanical properties of the 304 stainless steel.