Suppression of intergranular corrosion by surface grain boundary engineering of 304 austenitic stainless steel using laser peening plus annealing

Grain boundary engineering (GBE) has a high potential to suppress intergranular degradation and improve weld properties. Thermomechanical processing is typically used to optimize grain boundary character distributions (GBCDs) for the GBE of face-centered cubic materials with low stacking fault energy. Pre-straining plus annealing has achieved the optimal GBCD for GBE of austenitic stainless steels. Typically, cold rolling is used for pre-straining, but is not suitable for products with complex shapes. Laser peening can introduce strain locally and flexibly, even on complex parts. Therefore, this study attempted to apply laser peening as a pre-straining method during the thermomechanical processing. Several sets of process parameters during laser peening were selected to introduce suitable pre-strain into the surface of a 304 steel specimen based on previous GBE studies focusing on cold rolling plus annealing. Annealing at 1260 K for 48 h following laser peening led to an optimal GBCD with over 80 % frequency for coincidence site lattice boundaries and disconnected random boundaries. An intergranular corrosion test revealed that the laser-peened and annealed 304 steel with an optimal GBCD exhibits excellent intergranular corrosion resistance, which is far greater than that of an as-received sample and comparable to that of a cold-rolled and annealed sample.