Microstructural and Chemical Changes of a Ti-Stabilized Austenitic Stainless Steel After Exposure to Liquid Sodium at Temperatures Between 500 °C and 650 °C

Abstract

Ti-stabilized austenitic stainless steel was carburized in sodium containing a high carbon activity at three different temperatures, 500 °C, 600 °C, and 650 °C during 1000 hours and 5000 hours. The carbon profile, the carbide volume fraction, and the lattice parameter evolution as fonction of depth were determined using high-energy X-ray diffraction and electron probe microanalysis. At 650 °C and 600 °C, the carbon precipitated as M 23 C 6 and M 7 C 3 carbides in the sample. The volume fraction of M 7 C 3 carbides was lower than predicted by thermodynamic equilibrium using Thermo-Calc software ® . At 500 °C, carbides almost did not form in the steel. Instead, high carbon supersaturation of the austenitic matrix occurred. Both results demonstrate that the carburiz.ation profile was strongly influenced by the kinetics of carbide formation at temperatures lower than 650 °C. High-energy X-ray diff raction measurements demonstrated that the austenite and carbide lattice parameters evolved along the carbon profile. Both measured lattice parameter profi les of austenite and M 23 C 6 carbide were compared to the ones predicted from chemical changes of austenite and carbides.

Determination of residual stress gradient in a Ti-stabilized austenitic stainless steel cladding candidate after carburization in liquid sodium at 500 °C and 600 °C

“…In this study, the strain-free lattice parameters of austenite and M 23 C 6 were determined from an empirical relationship giving the strain-free lattice parameters of both phases as a function of their compositions. More details will be given in the following sections and can also be found in a previous work [9].…”

“…The carbon concentration reached at any depth was much higher than the solubility limit of carbon at 500 °C given by Thermo-Calc® (0.006 wt.%). Thus, a carbon supersaturation of the austenite matrix occurred: expanded austenite formed in the carburized zone [9] . This phenomenon has been previously evidenced by different authors in highly carburizing gaseous environments on low alloyed steels and stainless steels at temperatures under 550 °C [ 33 , 34 , 43-45 ] and in carburizing sodium at 500 °C [17] .…”

“…( 1) considers only the carbon contained in carbides, this excellent agreement demonstrates that carbon which diffused within the sample was mainly trapped into carbides as expected by thermodynamic equilibrium. The formation of carbides resulted in metallic depletion of the austenite, mainly in chromium and molybdenum, two strong carbide former elements [9] .…”

“…lt is well known that the mechanical properties of steels such as the ductility or the fracture strain are reduced after carburization (1-3). The carburization of stainless steel after exposure to nuclear grade liquid sodium con taining carbon was extensively studied in the past (4,5) and more recently (6)(7)(8)(9). Ti-stabilized austenitic stainless steel AIMl, a stain less steel cladding candidate for control rods, formed a carburized zone after 1000 h of exposure in carburizing liquid sodium at tem peratures of 500 °c and 600 °c (8,9).…”

“…The carburization of stainless steel after exposure to nuclear grade liquid sodium con taining carbon was extensively studied in the past (4,5) and more recently (6)(7)(8)(9). Ti-stabilized austenitic stainless steel AIMl, a stain less steel cladding candidate for control rods, formed a carburized zone after 1000 h of exposure in carburizing liquid sodium at tem peratures of 500 °c and 600 °c (8,9). Carbon diffuses into the steel and, depending on the temperature, oversaturates the austenitic matrix and/or forms carbides.…”