Surface Hardening of Austenitic Fe-Cr-Ni Alloys for Accident-tolerant Nuclear Fuel Cladding

Résumé du livre

Three Fe-Cr-Ni alloys (AISI-316L, AL-6XN and IN-718) were surface-engineered by infusion of interstitial solutes (C and N) for potential application as nuclear fuel cladding. However, an 'activation' process, which removes the Cr-rich oxide layer that blocks the infusion of interstitial solutes, is needed before these alloys can be successfully surface-engineered. An innovative and highly effective aqueous acid activation process was applied. The surfaces of these Fe-Cr-Ni alloys are successfully activated in 300 s to 600 s at room temperature compared to 21.6 ks at carburizing temperature with gaseous HCl activation. Moreover, the activated surface of Fe-Cr-Ni alloys can be well protected by immersing in ethanol before these specimens are transfered into the carburizing furnace. Specimens of the Fe-Cr-Ni alloy AISI-316L, IN-718 and AL6-XN were successfully surface-engineered by low-temperature carburization and low-temperature nitro-carburization. Near the alloy surface, up to 15 at% to 20 at% of carbon and 20 at% of nitrogen were observed in Fe-Cr-Ni alloys - without detectable carbides. This high carbon and nitrogen fraction results in a threefold increase of surface hardness, significantly improved wear resistance and corrosion resistance of Fe-Cr-Ni alloys. Cr depletion can be seen in all the surface-engineered Fe-Cr-Ni alloys. Low-temperature carburization and low-temperature nitro-carburization can induce ferromagnetism in the surface of IN-718 ni-based alloy. The ferromagnetism detected in low-temperature carburized and low-temperature nitro-carburized IN-718 may have interesting applications, independent of nuclear technology. Thermal stability of low-temperature carburized material was tested by post-carburization heat exposure. No observable difference of properties (i. e. no observable precipitates, no detectable diffusion of interstitial solutes and no obvious change of the shapes of X-ray diffraction peaks and Auger concentration profiles) appeared in surface-engineered Fe-Cr-Ni alloys after exposure at 620K in air in comparison with as-carburized Fe-Cr-Ni alloys. However, properties of low-temperature carburized and low-temperature nitro-carburized Fe-Cr-Ni alloys showed significant variation (i. e. metallography, X-ray diffractometry and scanning Auger microprobe profile) after exposure in the simulated loss of coolant accident environment (1070K for 3.6 ks) compared to those of as-carburized Fe-Cr-Ni alloys. Carbon depletion and infusion were observed in the low-temperature carburized and low-temperature nitro-carburized Fe-Cr-Ni alloys after exposure to 1070K for 3.6 ks. Among all these surface-engineered Fe-Cr-Ni alloys, low-temperature carburized IN-718 did not show observable oxidation after exposure to the simulated loss of coolant accident environment. Non-treated and low-temperature carburized AISI-316L have severe stress corrosion cracking on the surface after exposure in a boiling water reactor simulated environment. However, stress corrosion cracking is not observable in the carburized IN-718 specimen. Finally, exposure to 1.5MeV proton irradiation at elevated temperature (620K), as well as exposure in the simulated loss of coolant accident environment (with no radiation at 1070K for 3.6 ks), causes precipitation of nano-sized Cr-rich carbides in low-temperature carburized IN-718. These prevented the loss of Cr (as well as C) by evaporation under the loss of coolant accident. Among all the surface-engineered alloys, low-temperature carburized IN-718 has the most potential for nuclear cladding.