Impact Toughness of High-Strength Low-Alloy Steels Joined by Friction STIR Welding

The microstructure and impact toughness of high-strength low-alloy steels joined by friction stir welding are studied. The hardened steel samples are tempered at a temperature of 600–650°C for 1 hour, followed by warm rolling to a total strain of 1.5 at the same temperature. The resulting steels are characterized by an ultrafine-grained lamella-type microstructure. The as-processed steel plates are joined by friction stir welding. A martensitic microstructure is formed in the stir zone. The steel samples with a carbon content of 0.15% show high impact toughness of 68 J/cm² at room temperature with a gradual decrease to 16 J/cm² at a temperature of –196°C, while the steel samples with a carbon content of 0.36% exhibit an impact toughness of 10 J/cm² at room temperature. The 0.36% carbon steel joints are tempered at a temperature of 650°C for 1 hour, which increases their room-temperature impact toughness to 67 J/cm².