Volume 25, Issue 2 (March 1997)
Low-Temperature Aging Kinetics in Cast Duplex Stainless Steels: Experimental Characterization
Low-temperature aging (280 to 300°C) of duplex (austenite-ferrite) stainless steels and their resulting long-term loss of toughness is a well-known problem in the energy-producing as well as in other industrial sectors. In order to analyze the problem, the phenomenon was assumed to be thermally activated, accelerated aging at higher temperatures (350 to 400°C) was planned, and the activation energy was estimated. Assuming that the cause of brittleness is the ferrite's spinodal decomposition into the α and α' phases, the present work analyzes the aging kinetics at three temperatures (280, 350, and 400°C) by means of microhardness measurements of the ferritic phase taken in three different duplex stainless steels named after their ferrite content, 12F, 18F and 22F. In view of the microhardness results, an exponential law is proposed to represent the temporal evolution for each of the steels, with three parameters, of which one varies with aging temperature. It should be noted that there is no unique value for the activation energy in all the 280 to 400°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 temperature (280°C) of the results obtained in the 350 to 400°C range produces non-conservative results. These results are discussed 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 that provides reliable results for their service temperature (280°C).