Carburization of austenitic and ferritic stainless steels in liquid sodium: Comparison between experimental observations and simulations

Abstract

Three steels were exposed in carburizing sodium at 600 and 650°C. The kinetics and extent of carburization were characterized. Numerical simulations using the coupled thermodynamic-kinetic modeling software DICTRA were performed. It was proposed that the observed carbon diffusion profiles were induced by the combined diffusion of carbon in the grains and at grain boundaries coupled with the slow formation of carbides. The blocking effect of carbides on the carbon diffusion was observed to evolve as a function of time and microstructure. Acceptable agreement between experimental and simulated intragranular carbon profiles was achieved by optimizing the labyrinth factor and phases.

CSP Gen3: Liquid-Phase Pathway to SunShot

“…Carbon, if present, can drive carburization and enhance corrosion in liquid sodium. Significant levels of carburisation of several alloys, including 316, have been observed at 600°C over periods of time up to 5000 hrs [54]; higher temperatures will accelerate carburization to greater depths. Surface carburisation of alloys would be expected to reduce fatigue resistance of the material and if complete carburisation of the tubes occurs, fracture toughness will be adversely affected.…”

“…In response, relevant species (such as Cr, Mn, Ni or Si) from the bulk of the alloy diffuse to the surface, but at a rate slower than that of corrosion, leading to composition and microstructural changes in the alloy. This could ultimately result in changes in the mechanical properties of the alloy and reduce creep and fatigue life [44], [53], [54]. There is evidence that increasing the Ni content leads to accelerated corrosion and more importantly that the nickel alloys, despite higher mechanical strength at high temperatures, are less compatible with sodium than steel (Fe) alloys [55], [56], [57].…”

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

“…More details on the procedure for carburization experiments in sodium can be found in previous works. [16,17,25] B. High-Energy X-Ray Diffraction Experiment…”

“…More details on the measurement procedure are available in previous works. [16,17,25] D. Transmission Electron Microscopy and Focused Ion Beam Thin Lamellas Preparation Three thin lamellas were extracted parallel to the sample surface of AIM1 alloy carburized at 650 °C for 1000 hours at distances of 10, 100, and 300 lm (along the S 3 axis) from the surface by focus ion beam (FIB), using a scanning electron microscope-FIB FEI Helios NanoLab 600. One lamella was extracted from AIM1 alloy carburized at 600 °C for 1000 hours at distance of 100 lm.…”

“…First reports on the kinetics of microstructural and chemical changes of candidate clad stainless steels, 316L and, a titanium-stabilized austenitic stainless steel, AIM1*, exposed in carburizing liquid sodium at temperatures between 500 °C and 650 °C for exposure times up to 5000 hours were determined from several complementary analysis techniques as optical microscopy (OM), high-energy X-ray diffraction (HEXRD), electron probe microanalyses (EPMA), and transmission electronic microscopy (TEM) analysis. [16,25,26] In that studies, the observed carburization profiles were compared to the ones predicted by diffusion-controlled transformations (DICTRA) software. In this paper, that work was reproduced on new samples of AIM1 stainless steel exposed in carburizing sodium between 500 °C and 650 °C for 1000 hours.…”