Oxidation, Scale, Slag, Dross

Oxidation of any product—steel, aluminum, copper, brass, or bronze—can be minimized by close control of air/fuel ratio to a minimum of about 5% excess air. Less than that may result in presence of pic, which can cause hydrogen absorption and other defects, pollution, and hazards. 

Steel scale begins to soften at 2320 F 50°F (1271 C 28°C), depending on its composition. It melts near 2500 F (1371 C), but that also depends on its composition. 

If thick steel (which stays in the furnace for a long time) is heated in a hot furnace, the scale becomes mushy, if not liquid. Semimolten scale has caused many erroneous temperature measurements in steel heating furnaces. Scale is an insulator. Its conductance is lower in its solid form, but the high reflectivity of the molten form causes it to act as an insulator. If the scale is not shiny or glossy, optical pyrometers and radiation pyrometers measure scale temperature, but not steel temperature pyrometers indicate a temperature somewhere between furnace ceiling temperature and scale temperature, but not steel temperature. Shiny scale (semimolten) reflects radiation nearly eliminating heat transfer to the load. 

Time at Temperature. If the time is doubled, the scale formed may increase by 40%. (See fig. 8.15.) 

Furnace Atmosphere. If there is a reducing condition (a shortage of air for fuel combustion), the quantity of scale formed will be only about 20% as much as with a slight excess of air (oxidizing atmosphere). With only 50% of the air necessary to burn the fuel, almost no scale will be formed. If the combustion air were increased to just a little above the minimum to burn all the fuel, the scale formed per hour would increase about five times. If the combustion air were further increased, very little additional scale would be formed. (See fig. 8.16.) 

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