Assessment of the possibility of resistance butt welding of pipes made of heat-resistant steel 0.15C-5Cr-Mo

Introduction. Cr-Mo steels are used in high-temperature and high-pressure applications, including critical components of modern supercritical and ultra-supercritical thermal power plants. Due to its unique ability to withstand high temperatures and pressures, these steels are also used in critical components of fast breeder nuclear reactors. The heterogeneity of the microstructure and mechanical properties throughout the welded joint is a decisive factor leading to a decrease in its performance and premature failure. Post-weld heat treatment is the main method for improving mechanical properties. However, the mechanism for the evolution of mechanical properties associated with heterogeneous microstructure after heat treatment remains unclear, which complicates the design of the heat treatment process and a comprehensive assessment of its effect. The purpose of the work is to assess the possibility of the resistance butt welding method of welding pipes made of 0.15C-5Cr-Mo steel, to select technological parameters for resistance butt welding of pipes to obtain high mechanical properties. Research methods. The experiments were carried out on a resistance butt welding machine MSO-201N. Mechanical tests for static tension, chemical composition analysis and metallographic studies were carried out. Results and discussion. Technological parameters of resistance butt welding of pipes, changed in the course of our research, show that upsetting pressure and spark allowance affect the final strength properties of the welded joint. Based on the results of metallographic studies, the following features of the evolution of the microstructure can be noted. A noticeable decrease in the content of primary coarsened ferrite is observed in the structure of the weld after tempering. The use of post-weld heat treatment made it possible to reduce the hardness in the welded joint to the level of regulatory requirements. Results presented. The effect of heat treatment on mechanical properties is analyzed based on a comparison of heat treatment modes: stress relief annealing and normalization + tempering in terms of improving mechanical properties during tensile tests. The results show that after tempering, the evolution of mechanical properties in each sub-zone of the welded joint is sequential, i.e., hardness and tensile strength decrease and toughness increases. It is noteworthy that the most significant increase in toughness is observed in the weld zone, primarily due to a significant decrease in the presence of hypoeutectoid ferrite.