Thermal cutting is widely applied in metal structure making. This processing technology may significantly reduce the production time, increase the actual output and reduce the manual labor. Due to application of modern machine tools with numerical control (CNC), the process of cutting blanks is significantly intensified, increasing the accuracy of manufacturing thereof, thereby enabling making parts of any complexity. Thermal cutting processes are accompanied by metal smelting from the cut cavities. There is a large temperature difference (from the melting temperature to the initial one) in a relatively narrow area adjacent to the cut surface. The structural steel 09G2S applied in welded metal structures is taken for analysis. The microhardness was analyzed by the Scientific and Educational Center for Innovative Technologies at the Architectural and Construction Institute of the Federal State Budgetary Educational Institution of Higher Education “Ufa State Petroleum Technical University”. The following results were obtained in the course of analysis of the metal structure after thermal treatment: due to thermal heating at the edge, a heat-affected area was formed, consisting of an external casted section and an internal section adjacent thereto, being a metal structure feature for an overheating area with different grain growth under decrease of the heating temperature along the base metal; under the calculation it was found that cooling is proceeding lower at less current strength rate resulting in metal overheating and grain growth; due to plasma arc affecting the cut edges, the martensite appears in the form of medium-needle clusters; when calculating the depth of the heat-affected area, the optimum mode of plasma cutting is selected for making bolted mounting connections.