Austenoferritic duplex steels possess microstructures containing comparable volume fractions of hard and soft phases which are bipercolated. They are therefore liable to develop large internal stresses during straining. The evolution and the relaxation of these stresses and the occurrence of damage are characterized using Bausehinger tests. Thermally aged and non-aged material behaviour are compared. Plastic flow in the hard phase is shown to be significantly efficient in releaving these internal stresses.