Investigation on the thermostability of residual stress and microstructure in shot peened SAF 2507 duplex stainless steel

SAF 2507 duplex stainless steels were treated by shot peening (SP) and then isothermally annealed at elevated temperatures (600-750 degrees C). The thermal stability of surface residual stress and microstructure of shot peened material were investigated. Results showed that most of the stress relaxation took place within the initial 30 min, and the relaxation behavior could be described by the Zener-Wert-Avrami function. The derived activation energies of stress relaxation for gamma and alpha phase were close to the vacancy-migration-energy in iron. gamma phase exhibited higher stress and microstrain relaxation rate than alpha phase, which was ascribed to the higher stored energy in gamma phase. A heavy reduction in dislocations and dislocation cells accompanying with the growth of subgrains was observed in the recrystallized microstructure. The recrystallized grains formed in gamma phase were much finer than that in a phase. Moreover, SP accelerated the precipitation kinetics of sigma phase, a mass of sigma phase was precipitated in the deformed layer after isothermal annealing.