Friction stir welding of ultrafine grained plain low-carbon steel formed by the martensite process

The combination of friction stir welding (FSW) and a thermomechanical process for ultrafine grained steel (martensite process) was studied in order to obtain a preferable balance between the strength of the base metal and that in the stir zone while maintaining a high strength in both areas. Plain low-carbon steels with three kinds of microstructures, (lath-martensite, ultrafine lamellar dislocation cell structure and multiphased ultrafine grained structure) were prepared by the martensite process and examined for FS welding. The microstructure and hardness around/in the stir zone were significantly affected by the microstructure of the base metal. After the FSW, the cold-rolled steel had the highest strength in the stir zone with the smallest ferrite grain size of the three. Subsequent annealing of the cold-rolled and FS welded steel realized higher strength in both the base metal and the stir zone than that of the conventional plain low-carbon steel.