This paper presents the numerical and experimental results of hardening an annular zone on the flat surface of a X20Cr13 steel sample by friction stir processing (FSP) with a WC-Co hard alloy tool moving along circular and fan-shaped paths. A finite element model of the process is proposed for predicting the temperature distribution through the width and depth of the annular zone for the considered tool paths and for detecting the reverse tempering regions. The influence of the paths of a cylindrical friction stir tool with a flat end on microhardness distribution in the surface layer of the hardened zone was studied experimentally. It was shown that FSP along a fan-shaped path provides uniform hardening of the annular zone, while processing along a circular trajectory leads to softening of the material in the regions where the friction tracks overlap. The uniformity of surface hardness in the FSP-hardened annular zone of X20Cr13 steel was evaluated by calculating the CU index proposed by Campana. The hardening behavior is in full agreement with the results of finite element simulation of the FSP process. Hardness measurements and microstructural studies showed that the fan-shaped tool path provides surface layer hardening to a depth of up to 400 μm with a CU index ranging from 0.78 to 1.00. In the case of a circular path, the CU index ranges from 0.48 to 0.72 at the same depth. XRD analysis of the hardened layer revealed pronounced peaks corresponding to the (110)α, (200)α and (211)α lines, indicating the formation of martensite with different tetragonality at the given depth. The proposed research methods can be applied to evaluate the FSP efficiency when using other workpiece and tool materials.