Developing fuel cladding Fe-25Cr-22Ni stainless steels with high microstructural stabilities via Mo/Nb/Ti/Ta/W alloying

In order to improve the microstructural stability of high Cr/Ni austenitic stainless steels (ASSs) at high temperatures, the present work investigated the effects of minor-alloying elements (Mo, Nb, Ti, Ta, and W) on the microstructural evolution and mechanical properties of Fe-25Cr-22Ni (wt%) ASS systematically. The designed alloy ingots were hot-rolled, solid-solutioned at 1423 K for 0.5 h, stabilized at 1173 K for 0.5 h, and then aged at 973 K for different hours. It was found that these minor-alloying elements make different contributions to the microstructural stabilities of alloys. Only a small amount of harmful σ particles exists after 408 h-aging in W/Ta-modified alloys, which indicates W and Ta can improve the microstructural stability effectively. Whereas Ti should be forbidden since it accelerates the phase precipitation of Cr23C6 and σ remarkably. Excessive Mo can also promote the formation of σ phase and the transformation of Cr23C6 to σ during aging. The influences of the precipitated phases on mechanical properties of alloys were then studied. Thermal calculations were also performed to analyze the phase precipitation caused by minor-alloying. The Fe-25Cr-22Ni-0.046C-0.73Mo-0.18Nb-0.34Ta (wt%) exhibits excellent mechanical property due to its higher microstructural stability at 973 K, which has great potential for fuel cladding application.