The degradation of mechanical properties of SUS 304 during high temperature exposure were investigated by comparing as solution treated specimen with the specimen serviced for 120 000 h in an ethylene plant. Additionally, a restoration of mechanical properties was examined by applying the re-solution treatment to the specimen serviced for the prolonged time, and the microstructural change which affected the degradation was discussed. There was little difference in short time tensile properties among the above three different specimens, except for rupture ductility. More than half decreases in a creep resistance, a creep rupture strength and an impact property were detected in serviced specimen. All these properties, however, could substantially be restored by the application of re-solution treatment to the serviced specimen. The degradation of these mechanical properties during high temperature service were mainly caused by the intergranular precipitation of a sigma phase, and the decrease in density of carbide precipitates, accompanied with the intergranular precipitation of a sigma phase.
