Effect of Preheating Temperature on Microstructure and Mechanical Properties of TIG Welded Joints of Low-Activity Martensitic Steel

The effects of preheating temperature on the microstructural evolution and mechanical properties of low-activity martensitic steel welded joints were investigated, with emphasis on the relationship between preheating temperature, δ-ferrite content, and impact energy. Preheating temperature ranged from 150 °C to 300 °C. Optical microscopy and TEM were used for microstructural characterization, and tensile and impact tests were conducted to evaluate mechanical properties. The results showed that preheating temperature influenced the retained δ-ferrite content in the weld metal, whereas its effect on the heat-affected-zone microstructure was comparatively limited. The δ-ferrite fraction exhibited a non-monotonic dependence on preheating temperature, reaching a minimum of 2.76% in the filler weld pass at 250 °C. As the δ-ferrite fraction increased to approximately 13.27% at 150 °C, the average impact energy of the weld metal decreased from 47.37 J at 250 °C to 34.57 J at 150 °C. Impact fracture analysis revealed preferential crack propagation along δ ferrite/martensite interfaces and through δ-ferrite grains. An increase in the δ-ferrite fraction may promote the formation of interconnected ferrite networks, thereby facilitating crack propagation and reducing the impact toughness of the weld metal.