Friction stir welding of high-strength steel

The feasibility of friction stir welding of 12.7-mm-thick high-strength steel plates with the martensitic microstructure of the base material has been investigated. Microstructural investigation revealed a mixture of upper bainite and martensite microstructure in the stir zone. The thermo-mechanically affected zone exhibits a unique layered microstructure of fine equiaxed ferrite, bainite, and martensite. The martensitic structure of the base material transformed into the tempered martensite in the heat-affected zone. The mechanical response of the various microstructures at the different part of the welds has been correlated with a detailed microhardness investigation across the entire cross-section of the weld.