Effect of phase boundary and grain boundary characteristics on mechanical properties and corrosion resistance of S32750 duplex stainless steel

S32750 duplex stainless steel samples with different initial microstructures obtained by solid solution heat treatment (SHT) were cold rolled with the thickness reduction of 80 % and subsequently annealed at 1050 degrees C. The distribution of phase boundary and grain boundary character for the as-processed samples was investigated by SEM-EBSD and XRD techniques. Furthermore, the effect of microstructure on mechanical properties and intergranular corrosion resistance was analyzed by means of tensile testing, nano-indentation and double loop electrochemical potential reactivation (DL-EPR) method. The results show that the fine-grained microstructure with the areal ratio of alpha to gamma about 1, the highest fraction of phase boundaries out of the total interfaces ( grain boundaries + phase boundaries) , and the least grain clustering within the respective alpha and gamma phases was obtained in the sample previously treated by high temperature SHT and subsequent cold rolling and annealing. As a result, the sample exhibits excellent mechanical properties. After the cold-rolled and annealed samples are sensitized at 750 degrees C for 4 h, the sigma phase precipitates readily along the alpha grain boundary. The sample previously treated by high temperature SHT and subsequent cold rolling and annealing also possess better intergranular corrosion resistance due to the relatively small amount of alpha grain boundary and high population of phase boundaries meeting K-S orientation relationship between alpha and gamma. Therefore, the strength and intergranular corrosion resistance might be improved simultaneously by controlling and designing interface character distribution via appropriate thermal mechanical treatment in duplex stainless steel.