Postweld heat treatment of rotary friction welded AISI 1018 steel: microstructure, mechanical properties and corrosion resistance evaluation

The present study investigated the effect of post-heat treatment (PWHT) on the microstructure, mechanical properties and corrosion resistance of rotary friction welded AISI 1018 low-carbon steel. Metallographic analysis revealed that as-welded sample showed heterogenous microstructure consisted of fine grains of polygonal ferrite in weld zone (WZ) and coarse grains of ferrite and elongated pearlite in thermomechanical affected zone (TMAZ), while the postweld heat treated sample showed homogenous microstructure consisted of equiaxed grains ferrite and pearlite both in WZ and TMAZ. Mechanical test results showed that as-welded sample exhibited 36% tensile elongation and 82% bending elongation relative to base metal. After applying PWHT, the tensile elongation enhanced by 178% and bending elongation enhanced by 243% relative to the as-welded sample. The improved elongations after PWHT was caused by homogeneous microstructure consisted of equiaxed grains of ferrite and pearlite in weld joint. Electrochemical results revealed that the order of the corrosion rate is heat treated > base metal > as-welded in 0.5 M H₂SO₄ environment. The difference in corrosion behavior is primarily related to microstructure development where fine grain structure of as-welded sample with low volume fraction of pearlite showed lowest corrosion rate.