LOW-TEMPERATURE AGING KINETICS IN CAST DUPLEX STAINLESS-STEELS - EXPERIMENTAL CHARACTERIZATION

Low-temperature aging (280 to 300 degrees C) of duplex (austenite-ferr ite) stainless steels and their resulting long-term loss of toughness is a well-known problem in the energy-producing as well as in other in dustrial sectors. In order to analyze the problem, the phenomenon was assumed to be thermally activated, accelerated aging at higher tempera tures (350 to 400 degrees C) was planned, and the activation energy wa s estimated. Assuming that the cause of brittleness is the ferrite's s pinodal decomposition into the alpha and alpha' phases, the present wo rk analyzes the aging kinetics at three temperatures (280, 350, and 40 0 degrees C) by means of microhardness measurements of the ferritic ph ase taken in three different duplex stainless steels named after their ferrite content, 12F, 18F and 22E In view of the microhardness result s, an exponential law is proposed to represent the temporal evolution for each of the steels, with three parameters, of which one varies wit h aging temperature. It should be noted that there is no unique value for the activation energy in all the 280 to 400 degrees C range. This should be divided into smaller subranges for more precision, and here it is shown that in all cases the extrapolation to the service tempera ture (280 degrees C) of the results obtained in the 350 to 400 degrees C range produces non-conservative results. These results are discusse d considering the effect of the G-phase precipitation on the spinodal decomposition kinetics. For assessment applications a mathematical law is proposed that represents the aging kinetics of these steels and th at provides reliable results for their service temperature (280 degree s C).