Study of microstructure evolution, mechanical properties and hot corrosion behavior of friction stir processed boiler steel

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 mu m) has been decreased to 7.3 mu m, 4.3 mu m, and 1.75 mu m 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 mu m 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.