Effect of abnormal grain growth on microstructure and mechanical properties of friction stir welded SPCC steel plates

Friction stir welded SPCC low carbon steel plates were subjected to isothermal annealing heat treatments at 600, 650, 675 and 700 °C for various holding times. All the welds showed abnormal grain growth (AGG) in both the stir zone and the thermal-mechanically affected zone after annealing for a critical time, regardless of the welding condition and annealing temperature. As a comparison, normal grain growth occurred in the base metal. The AGG developed very quickly and finally very coarse grain structure was formed in the joints especially that the extremely large grains propagated from the thermal-mechanically affected zone into the base metal. The steel joints showed a stable D2 {112¯}<111> simple shear texture in the stir zone and the texture did not change during the entire grain growth period, no matter before or after the occurrence of AGG. Due to the AGG, the hardness suddenly decreased in the stir zone and became lower than that of the base metal. The tensile tests also revealed a lower strength in the stir zone of the annealed sample. However, the specimen showed different fracture locations depending on the annealing time.