Tungsten inert gas welding (TIG) of thermomechanical treated steel is especially prone to developing coarse-grained microstructure. The formed heterogeneous, dendritic microstructure in the fusion zone (FZ) and coarse grain in the heat-affected zone (HAZ) has a detrimental influence on the mechanical properties of welds. Therefore, a specific welding process is suggested here, aiming at minimizing the extensive grain growth or even promoting a fine-grained microstructure during cooling. The effects of frequent pneumatic hammering on the weld dimension, microstructure, and mechanical properties are examined on the S700MC TIG welds. The whole welding process has been named thermomechanical welding (TMW [1,2]). The pneumatic hammer was operated with 6 bar pressure and 35 Hz frequency, using a chisel tip rectangular section with dimensions of 15 mm length and 2 mm width. Two offsets, 20 mm and 30 mm, between the TIG torch needle and chisel tip are used to evaluate the effect of hammering at different cooling temperatures. The distances between the torch needle tip and the chisel tip to the welding substrate are 4 mm, and the tilt angle of the torch needle to the substrate is fixed at 80 °. The t8/5 time of 5s and 25 s are carried out on both TIG and TMW experiments at two welding currents of 125 A and 175 A. The welding substrates are tested in two distinct conditions: as-received (hot-rolled with an initial 10 mm thickness) and cold-rolled with a 10% thickness reduction of S700MC plates. The hardness is measured by Vickers hardness (HV 0.5) along the weld, and the microstructure is observed by the light optical microscope (LOM), followed by analysis using ImageJ software for the grain size evaluation. The microstructure and hardness of the weld zones of TIG and TMW welds are compared according to the microstructures shown in Figure 1 (a). Higher hardness was measured on the TMW welds (Figure 1 (b)) compared with the TIG welds. The frequent hammering induced the vibration of the weld pool [3] and plastic deformation on the weld during TMW, resulting in microstructure refinement and enlarging the weld depth. It is concluded that the proposed TMW process is a successful approach to refining the weld microstructure and hence enhancing its mechanical property.