Liquid carburizing

Bad-kasten, m. (electrolytic) cell, -nitrieren, n. Metal.) fused-salt nitriding, -patentieren, n. Wire) patenting with cooling in a lead or salt bath, -spannung, /. bath tension, cell voltage, -Spiegel, m, bath level, bath surface, -zementieren, n. Metal.) bath cementation, liquid carburizing,... 

Specific surface area. The specific surface area is an important criterion for the sintering activity (solid state sintering), dissolution processes (liquid phase sintering), and reaction with gaseous or solid substances during carburization. Commonly, it is in the range of 0.01 m /g (coarse powders) up to 12m /g (very fine powders). 

Carburization of container metals is another problem involved in the handling of liquid metals, especially sodium and lithium. Carbon is readily transferred between two metals of different carbon contents in contact with sodium, or a monometallic system may be carburized owing to a high carbon concentration in the sodium. An example of the extent to which this carburization may proceed is shown in Figure 10, which is a photomicrograph of the wall of a stainless steel... 

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. 

Reactions of carbon in alkali metals with carbide forming metallic elements are the driving processes of the carburization of stainless steels. The direction of the carbon exchange between the molten metals and the solid metallic materials depends on the carbon potential in the liquid metals and on the free energy of formation of the metal carbides and the chemical activity of the metallic element in the solid phase. 

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. 

Vanadium also forms a very stable carbide VC, and carburization of this metal is part of the corrosion reactions of vanadium based alloys contacted with liquid lithium as well as sodium. Vanadium alloys with contents of titanium have an even higher affinity to form solid carbides by absorbing of carbon from liquid metals. In systems in which vanadium titanium alloys and stainless steels are in contact with the same lithium or sodium, carbon migrates from the steel to the refractory metal alloy, thus passing the alkali metal serving as a transport medium The free energies of formation of the alkali acetylides are compared with the values of several metal carbides in Table V. 

Low-carbon steel can be hardened in order to improve the wear resistance by carburizing. Steel is carburized by exposing it to gas, liquid, or solid, which provides a high carbon concentration at the surface. Given the percentage carbon versus depth graphs for various times at 930°C, how can the diffusion coefficient be estimated from the graphs ... 

Among the nonmetalhc elements, oxygen and carbon have a prominent role for what concerns corrosion and mechanical resistance of stmctural materials in liquid sodium. This is because of, as already mentioned, Na and oAict metaUic alloying elements, such as Fe and Cr, can form ternary oxides. On the other hand, for what concerns the impact of carbon, carburization/decarburization processes of the structural materials can occur with consequences on the mechanical properties of the material. 

As already mentioned in previous sections, corrosion by liquid metals can proceed by various ways dissolution, oxidation, carburization, formation of intermetallic compounds, etc. The nature of the corrosion process will firstly depend on the thermodynamic data of the liquid metal/sohd material system considered. These thermodynamic data give the equilibrium state of the system and thus the products susceptible to form. Of course, kinetics data are also essential to determine whether the products predicted by thermodynamics will indeed form and in that case at what rate they are going to form. These two aspects will be treated in the following paragraphs. 

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