Studying the impact of multi-pass friction stir processing on microstructure and mechanical properties of low-carbon steel

In this study, plain carbon steel (A216-WCB) was processed using a WC tool with a plunge depth of 2 mm, an advancing speed of 80 mm min-1, and a rotational speed of 2800 rpm. Friction stir processing (FSP) was performed from one to three passes submerged under water at 5 degrees C. Field emission scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS) and optical microscope (OM), Vickers microhardness (Hv), tensile and wear tests were used to characterize the FSPed samples. The initial grain size of the base metal (BM) was reduced from 25 p.m to 3 and 5 p.m in the stirred zone (SZ) and thermomechanically affected zone (TMAZ), respectively. The microhardness increased by a factor of 4.5 and 2.5 in SZ and TMAZ compared to the BM. This may be due to phase transformation (austenite to martensite) and grain size refinement according to the Hall-Petch equation. The ultimate tensile strength was improved by 23.9 % compared to BM, which may be due to increased grain boundaries that inhibit dislocation motion. The FSPed sample had 250 % more resistance to weight loss compared to BM. The increase in wear resistance may be attributed to the increase in microhardness and fine grain structures. Increasing the number of passes had the opposite effect on hardness, tensile strength, and wear resistance because the heat generated in subsequent passes leads to partial tempering of shear structures and recrystallization.