High-carbon steels, decarburization

Examples of nonuniform heating-control problems above 10(X) F (538 C) are (1) nonuniform scale formation with carbon steels, (2) questionable completion of the combustion reaction (pic contact the load surface), (3) sticky scale with resultant rolled-in scale, (4) spotty decarburization of high carbon steels, (5) some stainless steels may not tolerate contact with the reducing atmosphere within the flames, and (6) using impingement heating for steel pieces of heavy cross section could cause formation of reflective scale with resultant reduction of heat transfer. 

Medium- and high-carbon steels Oxidizing atmospheres, hydrogen at high temperatures Carbon (decarburization)... 

Additional high temperature changes cause decarburization, wherein carbon in the ferrite phase of carbon steel can be oxidized to carbon dioxide. 

Alloys other than those shown in Figure 1 are also suitable for resisting high temperature hydrogen attack. These include modifled carbon steels and low alloy steels to which carbide stabilizing elements (molybdenum, chromium, vanadium, titanium, or niobium) have been added. European alloys and heat-treating practices have been summarized by Class.11 Austenitic stainless steels are resistant to decarburization even at temperatures above 1000°F (538°C).12... 

Whether decarburization will be an issue for internal combustion engines burning H2 is difficult to predict from existing information. Low-alloy carbon steels begin to decarburize at temperatures around the operating temperature of exhaust valves, but exhaust valves and valve seats are made from high-alloy steels, austenitic alloys, and superalloys where the carbon is much more stable than low-alloy carbon steels. The hardenable martensitic valve stems of exhaust valves may experience decarburization over extended periods, and this would lead to accelerated wear because of the softened surface that results from decarburization. 

At elevated temperatures, molecular hydrogen dissociates into the atomic form, which can readily enter and diffuse through the steel. Under these conditions, the diffusion of hydrogen in steel is more rapid. As discussed in Section 4, Forms of High Temperature Hydrogen Attack, hydrogen may react with the carbon in the steel to cause either surface decarburization or... 

This is a high temperature process relevant to equipment in petroleum refineries and petrochemical plants containing hydrogen. It is associated with the formation of methane by the reaction of hydrogen and carbon inside the steel. This results in decarburization, and the methane precipitates at the non-metallic inclusions to produce cracking in a similar way to HIC. 

By argon-oxygen decarburization (ADD) the carbon content of commercial heats of 304L can now be reduced consistently to 0.01 %. AOD and vacuum induction melting are used to produce the new high-purity Fe-Cr-Mo ferritic stainless steels in Table 2. 

The hardness and strength of a steel depends on its carbon content. A loss of carbon (decarburization) lowers the tensile strength of steels and can be caused by moist hydrogen at high pressures and temperatures. Figure 3.6 shows the Nelson diagram that depicts the limit of service conditions for carbon and alloy steels in hydrogen services. 

In strongly carbon-reducing atmospheres (e.g., carbon monoxide) at high temperatures, carburization of stainless steel takes place. In oxidizing atmospheres, such as steam or carbon dioxide, carbon may be selectively removed (decarburization). Usually, complex gas mixtures are involved and the net result of the H2/H2O and CO/CO2 is critical. Under some conditions of environment and temperature, a pitting-t) e phenomenon called "metal dusting" occurs. 

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