AFM Characterization of Structural Evolution and Roughness of AISI 304 Austenitic Stainless Steel under Severe Deformation by Wavy Rolling

Abstract

Stainless steels such as ferrritic, austenitic, martensitic and duplex stainless steels are well known for their corrosion resistance to varying extents. Among these, austenitic stainless steels exhibit superior corrosion resistance and better ductility for formability. Therefore, the ability to give simple to intricate shapes in this grade of steel brings their potential for a wide range of applications. However, the meta-stable austenite in AISI 304 is known to undergo a strain induced martensitic (SIM) transformation during conventional rolling at room temperature. This strain induced martensite causes reduction in ductility and limits formability of stainless steel. Therefore, wavy rolling technique was developed to strengthen the stainless steel through microstructural refinement. In the current study, wavy rolling with 1.5 mm amplitude was conducted on 1 mm thick stainless steel sheet to different cycles ranging from 1-4. These rolled samples were characterized by optical and Atomic Force Microscopy (AFM) with resolutions down to the nanolevel. This AFM tool is in a position to bring out the details of grain refinement and topographical roughness emerging from crystalline and microstructural defects like orientation, precipitation, stacking faults, deformation bands, slip lines and shear bands with progress in rolling as referred by the number of rolling cycles here. The structural development is semi-quantitatively related to the degree of deformation and its effect on tensile properties during wavy rolling cycle. Keywords: Structural properties; Roughness; Deformation; Wavy rolling.