A tungsten carbide (WC) tool was used to conduct friction stir processing (FSP) on AISI 4337 steel at 200 and 300 rpm rotating speeds, 30 mm/min feed rate, and 1 mm plunge depth. Investigations were conducted into the mechanical characteristics, such as microhardness and electron backscatter diffraction (EBSD). Particles of carbide were seen in the initial martensite phase upon annealing. Strength-ductility was enhanced by the reduction in grain size and increase in grain boundaries. The AISI 4337 steel’s microstructure and characteristics were considerably changed by FSP. The base metal (BM) had a microhardness of 344 HV, but the microhardness of the FSPed-200 and FSPed-300 samples was 524 HV and 445 HV, respectively. The treated steel has grain refinement, according to EBSD analysis, with the stir zone (SZ) showing the highest degree of refinement, followed by the retreating zone (RZ) and advance zone (AZ). Following FSP, there was a drop in the percentage of high-angle grain boundaries (HGBs), with the SZ having the highest percentage of low-angle grain boundaries (LGBs). The FSPed steel’s superior mechanical characteristics over the BM were attributed to the creation of small grain size and improved reinforcement dispersion.