Precipitation Sensitivity to Alloy Composition in Fe-Cr-Mn Austenitic Steels Developed for Reduced Activation for Fusion Application

Abstract

Special austenitic steels are being designed in which alloying elements like Mo, Nb, and Ni are replaced with Mn, W, V, Ti, and/or Ta to reduce the long-term radioactivity induced by fusion reactor irradiation. However, the new steels still need to have properties otherwise similar to commercial steels like type 316. Precipitation strongly affects strength and radiation-resistance in austenitic steels during irradiation at 400-600°C, and precipitation is also usually quite sensitive to alloy composition. The initial stage of development was to define a base Fe-Cr-Mn-C composition that formed stable austenite after annealing and cold-working, and resisted recovery or excessive formation of coarse carbide and intermetallic phases during elevated temperature annealing. These studies produced a Fe-12Cr-20Mn-0.25C base alloy. The next stage was to add the minor alloying elements W, Ti, V, P, and B for more strength and radiation-resistance. One of the goals was to produce fine MC precipitation behavior similar to the Ti-modified Fe-Cr-Ni prime candidate alloy (PCA). Additions of Ti+V+P+B produced fine MC precipitation along network dislocations and recovery/recrystallization resistance in 20% cold worked material aged at 800°C for 166h, whereas W, Ti, W+Ti, or Ti+P+B additions did not. Addition of W+Ti+V+P+B also produced fine MC, but caused some a phase formation and more recrystallization as well. These new alloys, therefore, achieved several of the initial design goals. Their fine MC precipitation and recovery/recrystallization behavior during aging is similar to that of the PCA. Calculations show that the new steels have over 10 s times less long-term radioactivity than type 316.

Developing an austenitic stainless steel for improved performance in advanced fossil power facilities

“…A somewhat similar combination of minor alloying elements added to a new Fe-12Cr-20Mn-0.25C austenitic steel (being developed under the Fusion Materials Program for reduced long-term induced radioactivity) also produces fine, stable MC precipitate microstructures at high temperatures. 27 Finally, these steels have the potential for applications beyond those of interest to the U.S. Department of Energy advanced energy programs that have sponsored their development thus far, particularly applications requiring high-temperature strength. The RT-UPS steels are currently the subject of technology-transfer activity at ORNL.…”

Reduced Activation Alloy Development for Fusion

“…To reduce activation, the development of Mn-stabilized stainless steels analogous to Ni-stabilized steels is being investigated. 21 One approach followed in Europe involves the investigation of commercially available Mn-stabilized steels; 7,8 however, these steels generally contain nitrogen and small amounts of nickel, and are therefore not reduced-activation steels as denned in the United States. This alloy has tensile properties comparable to those of type 316 stainless steel.…”