Steel heating furnaces

Capacities for steel heating furnaces are based on uninterrupted operation throughout the work week. (Delays in the mill or forge shop reduce the weight of steel heated in the furnace, but do not reduce the heating capacity of the furnace.)... 

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. 

The carbonization by-products are usually refined, within the coke plant, into commodity chemicals such as elemental sulfur (qv), ammonium sulfate, benzene, toluene, xylene, and naphthalene (qv) (see also Ammonium compounds BTX processing). Subsequent processing of these chemicals produces a host of other chemicals and materials. The COG is a valuable heating fuel used mainly within steel (qv) plants for such purposes as firing blast furnace stoves, soaking furnaces for semifinished steel, annealing furnaces, and lime kilns as well as heating the coke ovens themselves. 

Indirect rotary calciner. This is a bare metal cyhnder surrounded on the outside by a fired or elec trally heated furnace. It is suitable for operation at medium temperatures up to the maximum which can be tolerated by the metal w of the cylinder, usuaUy 6.50 to 700 K for carbon steel and 800 to 1025 K for stainless steel. 

Semi-finished products are first heated in a re-heat furnace until they are red hot (around 1200 C). On all types of mill the semi-finished products go first to a roughing stand. A stand is a collection of steel rolls (or drums) on which pressure can be applied to squeeze the hot steel passing through them, and arranged so as to form the steel into the required shape. The roughing stand is the first part of the rolling mill. The large semi-finished product is often passed... 

Table C.l shows some typical stress values for mechanical design as a function of temperature. A higher safety factor is taken into account at higher temperatures compared with the nominal tensile strength. Although the corrosion resistance of S S 3 04 and 316 types is good, their thermal resistance above 600 °C is not satisfactory. Heat-resistant steels for furnaces and cracking units are discussed in Ullmann [1].

The Leeds Northrup Co. has developed a two-couple continuous curve recorder for automatically heating furnaces to definite temperatures. As applied in carburizing and tempering of steel one thermocouple is placed in the furnace chamber and the other in contact with the steel piece. The controller automatically maintains the furnace for instance at 1,000°C. until the steel has reached 900°C. and then it drops the furnace to 900 C. Such an arrangement for control can be applied to a great many processes. 

The first thermobalance was probably the instrument described by Honda (30) in 1915. This instrument, as shown in Figure 3.18, consisted of a balance with a quartz beam. The sample was placed in a porcelain or magnesia crucible, G, which was suspended in an electrically heated furnace, J. Attached to the opposite end of the balance beam was a thin steel wire helix, E, which was immersed in oil contained in a Dewar flask, H. The Dewar-flask-helix assembly was adjusted by a screw mechanism to maintain the balance beam in a null position. A rather low heating rate was employed, since it took 10-14 hours to attain a temperature of 1000°C. However. Honda used a quasi-isothermal heating cycle in that during a mass-loss transition the furnace temperature was maintained at a constant temperature until the transition was completed. This procedure alone sometimes required 1-4 hours. Convection currents were evident above 300°C, as might be expected from the furnace sample arrangement. A sample mass of about 0.6 g was normally employed. 

One of the first precise vacuum or inert-atmosphere instruments was designed and constructed by Whitehead and Breger (37). The furnace was constructed from an alundum core, 9 in. in length by 2 in. ID, wound with Chromel A resistance wire. The core was shielded by four sheet-nickel cylinders, mounted on three posts, and the entire assembly was placed inside a 12 x 24-in. Pyrex bell jar. All electrical connections were made through the bottom of the bell jar mounting base. The sample block was made in the dimensions shown from Type 446 or 309 stainless steel. The furnace heating rate was controlled by a Leeds and Northrup Micromax controller the differential temperatures were recorded on a Beckman Photocell recorder. 

The reactor is a thin-walled stainless steel vessel 1-in. i.d. X 6-in. long and is contained along with its heating furnace and insulation by a 1-liter high pressure autoclave (not shown). The reactor internal pressure is balanced by adding C02 from an external source to the autoclave. 

The coke oven gas, practically freed of tar, ammonia and light oil, is extracted with monoethanolamine to remove H2S, HCN, and SO2 to levels required for air pollution abatement. One third of the desulfuriaed gas is returned to the coke ovens as a source of heat for carbonizing the coal in the ovens. Two-thirds of the gas flows to the steel plant furnaces where it is used as the source of heat for reheating steel products. 

To illustrate this, calculated carbon profiles are plotted in Figure 5.48 for an 0.85 wt% C steel heated for 90 min at 1050 °C for various assumed values of kc, the corrosion constant.The profiles are plotted relative to the original metal surface and refer to the following values of kc 4.1 x 10 mm s , which is a realistic value for a furnace atmosphere a hypothetical value of 4.1 x 10 mm s to represent a very high scaling rate and 0 mm s representing the case where the atmosphere just fails to form a scale but still reduces the surface carbon content to very low values. 

For resistance-heated furnaces on a modest scale (e.g., a 1-5 1 reactor), a simple, trapped, diffusion-pumped apparatus constructed of stainless steel or glass capable of achieving <10 mbar is sufficient. 

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