Liquid metals austenitic steels

The carbon potentials in liquid alkali metals are important in material compatibility problems. Carbon as a minor component of several materials of technical importance strongly influences the strength and ductility of the materials. The alkali metals have the ability to wet the surfaces of metals or alloys. In this state they tend to exchange carbon until they reach the chemical equilibrium. The carbon exchange between sodium and austenitic chromium nickel steels is extensively studied. As is shown in Fig. 8, in which the chemical activities of carbon in sodium and in the Crl8-Ni9 steel are compared as functions of temperature sodium containing 0.1 wppm carbon decarburizes an austenitic steel with a carbon content of 0.05 w-% carbon at a temperature of 650 °C and carburizes the same steel at 550 °C. 

Chromium carbide is among the compounds detected as precipitating the low temperature regions of liquid metal circuits, and the system Na—Cr—C is one of the most intensively studied systems There is some evidence that the most stable chromium carbide CrjsCg is formed at temperatures between 550 and 700 °C even in stainless steels, where the chemical activity of chromium is well below unity. This reaction is the chemical process causing the carburization of austenitic CrNi steels. CrjjCs precipitates in the surface zones of the material. 

Typical thickness of passivation film in PbBi alloy is about 10 pm and 20 pm respectively for austenitic and perlitic steel, though scattering within the range fi om 10 pm to 100 pm. The film thermal conductivity is about 5 W/(mK). There is a good reason to believe that the film is not continuous because of its saturation with liquid metal. Thus, thermal resistance is rather low and temperature difference across the film is about 5K. 

The ferrite regions on the austenitic steels often exhibit a porous-like structure that may be related to surface destabilization due to selective leaching 7,II,16,17], dissolution-triggered phase separation 18], or a localized corrosion product [19]. In contrast, the ferritic/martensitic steels tend to dissolve more slowly and maintain planar surfaces in contact with the liquid metal (although such surfaces are not smooth on a microscopic basis). 

S] Tas, H., De Schutter, F., LeMaitre, P., and DeKeyser, J., "Instability of Austenitic Stainless Steels in Contact with Liquid Metals, Proceedings of Fourth International Conference on Liquid Metal Engineering and Technology, Vol. 3, Societe Fran-caise d Energie Nucleaire, 1988, pp. 523-1-523-12. 

When stainless steel alloys are exposed to specific molten metals, there are potential problems of liquid metal embrittlement (LME) and liquid metal cracking (LMC) development. Molten tin at 248°F (120°C) has induced LME in austenitic stainless steels. At 570°E (300°C), the fatigue limit was lowered. At about 785-1060°F (420-570°C), zinc slowly eroded unstressed 18-8 stainless steel. At about 1060-1380°F (570-750°C), zinc penetrated to the matrix via a Zn-Ni compound. Molten cadmium can also cause LMC of austenitic grades above 570°F (300°C). 

In lead alloys, the corrosion kinetics of austenitic steels is linear and the dissolution rate increases with temperature and seems independent of the dissolved oxygen concentration and the liquid metal or alloy velocity in a low fluid velocity range. Empirical correlations have been established to express the corrosion rate [26] ... 

A comprehensive literature review on the degradation of mechanical properties of structural materials exposed to liquid Pb and Pb-Bi is given in Ref. [16]. As reported in this reference most of the experiments have been performed in liquid Pb-Bi and the extension of these experimental findings to pure Pb cordd lead to incorrect estimations, since liquid Pb-Bi seems to be more aggressive than liquid Pb. Moreover, the main structural materials tested were the 9Cr ferritic/martensitic steel 791 and the austenitic steel AISI316. However, a generalization of the residts obtained in order to predict the behavior of other 9Cr ferritic/martensitic and austenitic steels is not feasible, since as, for instance, minor alloying elements in the steel impact the materials behavior in these liquid metals. 

The potentialities of synergetic effects between neutron irradiation and the heavy liquid metals have been investigated mainly within two irradiation campaigns, i.e., ASTIR at BR2 reactor [99] and LEXUR-H at BOR60 reactor [1(X)]. The ASTIR experimental campaign was conducted on austenitic and ferritic/martensitic steels exposed... 

The 304 and 316 austenitic steels behave quite well in liquid metal sodium environments with low oxygen content at 650°C and below. The feedback from several SFR operations is good even for long exposure times like in BOR60 or Ph6nix reactors. Even if this item is not a primary concern about SFR operation, it is necessary to ascertain that this assertion remains valid for a 60-year exposition to liquid sodium at 550°C with low oxygen content (<3 wt ppm) in normal conditions. Somehow the thickness of the affected material needs to be predicted thanks to a better understanding of the different phenomena that may occur oxidation/reduction, dissolution/diffusion. 

In situations in which liquid-metal embrittlement has occurred, it has been mainly due to the zinc embrittlement of austenitic stainless steels. Isolated failures have been attributed to welding in the presence of residues of zinc-rich paint or to the heat treating of welded pipe components that carried splatters of zinc-rich paint. However, most of the reported failures due to zinc embrittlement have involved welding or fire exposure of austenitic stainless steel in contact with galvanized steel components. 

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

Metals that remain ductile at very low temperatures are preferred for use with liquid hydrogen. Examples include aluminum, copper, Monel, Inconel, titanium, austenitic stainless steels, brass, and bronze [3.19]. 

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