Steel decarburization

Steel [52013-36-2] Steel additive Steel anode Steel bearings Steel cans Steel cathodes Steel, decarburized Steel drums Steel gnt... 

If the dissolved carbon concentration in sodium is higher than its activity in austenitic and ferritic steels, general carburization of the system is expected. On the other hand, in sodium with very low carbon concentrations in which the chemical activity of carbon is lower than in the steels, decarburization of the steels can occur. This process may even decompose chromium carbides and thus enhance the dissolution of chromium out of the surface layers. 

Hydrogen at elevated temperatures can also attack the carbon in steel, forming methane bubbles that can link to form cracks. Alloying materials such as molybdenum and chromium combine with the carbon in steel to prevent decarburization by hydrogen (132). 

Uses. The sinter oxide form is used as charge nickel in the manufacture of alloy steels and stainless steels (see Steel). The oxide furnishes oxygen to the melt for decarburization and slagging. In 1993, >100, 000 metric tons of nickel contained in sinter oxide was shipped to the world s steel industry. Nickel oxide sinter is charged as a granular material to an electric furnace with steel scrap and ferrochrome the mixture is melted and blown with air to remove carbon as CO2. The melt is slagged, pouted into a ladle, the composition is adjusted, and the melt is cast into appropriate shapes. A modification of the use of sinter oxide is its injection directiy into the molten metal (33). 

Two-coat—one-fire enameling processes have also reduced the need for heavy-metal etching and nickel flashing (nickel replacement) for direct-on enamel apphcation to decarburized steels. 

To avoid decarburization and Assuring of the carbon and low-alloy steels, which is cumulative with time and, for all practical purposes irreversible, the limitations of the Nelson Curves should be followed religiously, as a minimum. Suitable low-alloy plate materials include ASTM-A204-A, B, and C and A387-A, B, C, D, and E, and similarly alloyed materials for pipe, tubes, and castings, depending upon stream temperatures and hydrogen partial pressures, as indicated by the Nelson Curves. 

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

Austenitic stainless steels are generally not decarburized in hydrogen at any temperature or hydrogen pressure. 

U. Hydrodesulfurization unit, C-0.5Mo steel exchanger tubesheet decarburized, fissured, and cracked under intergranularly cracked ASTM Type 304 cladding. 

W. C-0.5Mo steel piping in ammonia plant syngas loop decarburized and fissured. 

Operating Limits for Steels in Hydrogen Service to Avoid Decarburization... 

C-0.5Mo Steel Showing Internal Decarburization and Fissuring in High... 

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... 

The broken-line curves at the top of Figure 1 represent the tendencies for surface decarburization of steels while they are in contact with hydrogen. The solid-line curves represent the tendencies for steels to decarburize internally with resultant Assuring and cracking created by methane formation. 

Surface decarburization does not produce Assures. In this respect, it is similar to decarburization created by the exposure of steel to certain other gases, such as air, oxygen, or carbon dioxide. The usual effects of surface decarburization are a slight, localized reduction in strength and hardness and an increase in ductility. Because these effects are usually small, there is often much less concern with surface decarburization than there is with internal decarburization. 

---