Friction stir processing (FSP) is an effective technique for enhancing the mechanical properties and microstructural characteristics of AISI 4337 steel. This chapter investigates the effects of FSP on the microstructure, hardness, tensile strength, and bending behavior of AISI 4337 steel. Optical microscopy and electron backscatter diffraction were employed to analyze the microstructural changes induced by FSP. The results revealed significant grain refinement and the formation of a dual-phase martensite-ferrite structure in the processed zone. FSP led to a remarkable increase in microhardness, with values reaching up to 548 HV compared to 344 HV in the base material. The yield strength also improved from 550 MPa in the base material to 720 MPa in the FSPed steel. However, a slight reduction in elongation was observed. The enhanced mechanical properties were attributed to the combined effects of grain refinement, phase transformation, and increased dislocation density. The chapter highlights the potential of FSP as a surface modification technique for improving the performance of AISI 4337 steel in automotive and other engineering applications.