Predicting recrystallized grain size in friction stir processed 304L stainless steel

Abstract

A major dilemma faced in the nuclear industry is repair of stainless steel reactor components that have been exposed to neutron irradiation. When conventional fusion welding is used for repair, intergranular cracks develop in the heat-affected zone (HAZ). Friction stir processing (FSP), which operates at much lower peak temperatures than fusion welding, was studied as a crack repair method for irradiated 304L stainless steel. A numerical simulation of the FSP process in 304L was developed to predict temperatures and recrystallized grain size in the stir zone. The model employed an Eulerian finite element approach, where flow stresses for a large range of strain rates and temperatures inherent in FSP were used as input. Temperature predictions in three locations near the stir zone were accurate to within 4%, while prediction of welding power was accurate to within 5% of experimental measurements. The predicted recrystallized grain sizes ranged from 7.6 to 10.6 μm, while the experimentally measured grains sizes in the same locations ranged from 6.0 to 7.6 μm. The maximum error in predicted recrystallized grain size was about 39%, but the associated stir zone hardness from the predicted grain sizes was only different from the experiment by about 10%.