Abstract:
In this investigation, commonly used boiler steel(SA210)was friction stir processed (FSPed) using
pinless tool with plunge depth of 1 mm, transverse speed as 30 mm min−1 and tool-spindle rotational
speeds of 600 revolution per minute (rpm), 300 rpm, and 150 rpm respectively with three number of
passes in each case. Scanning Electron Microscopy (SEM) and Optical Microscopy (OM) analyses were
done in order to characterize the Friction Stir Processed specimens. Prediction of the grain size
distribution after friction stir processing under different rotational speeds was done using Electron
Back Scattered Diffraction (EBSD). The initial grain sizes of the selected boiler steel(25 μm) has been
decreased to 7.3 μm, 4.3 μm, and 1.75 μm respectively with decrease 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 2, 2.5 and 3 in comparison to base steel respectively with the
decrease 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 8%, 16% and 25% respectively in comparison
to base metal, which may be accredited to enlarge in grain boundaries which hinder the dislocation
movement. FSPed steel having grain size of 1.75 μm offered 68% resistance to hot corrosion in
comparison to base steel. The enhanced corrosion resistance might be attributed to the increased
microhardness, fine grain structures and the presence of protective oxides on the top surface layer.
