Enhanced microstructure and mechanical properties of boiler steel via Friction Stir Processing

Abstract

Abstract The main objective of the current investigation is to achieve strengthening of SA210 Grade A1 boiler steel using friction stir processed (FSPed) using pinless WC tool with plunge depth of 1 mm, transverse speed as 40 mm/min and tool-spindle rotational speeds of 800 revolutions per minute and 1400 rpm respectively with two numbers of passes in each case. through The microstructural refinement and mechanical properties like tensile and hardness for base steel were studied and compared with friction stir processed steel. The initial grain sizes of the selected boiler steel (25 µm) has been reduced to 7.8 µm and 5.9 µm respectively with increase in rotational speed. The microhardness and tensile strength was evaluated of all the FSPed samples. The microhardness of the FSPed samples was enhanced by factor of 1.72 and 2.38 in comparison to base steel respectively with the increase in rotational speeds; this may be due to phase transformation (austenite to martensite and ferrite) verified by Image processing software “Image-J” and refinement in grain size as per Hall-Petch equation. The ultimate tensile strength was improved by 7.2% and 15% respectively in comparison to base metal, which may be accredited to enlarge in grain boundaries which hinder the dislocation movement. The characterization was done by optical microscopy and Electron Back Scattered Diffraction (EBSD) analyses with an intend to suggest mechanisms behind grain size refinement and phase transformation.