Soaking Pit

a furnace for heating steel ingots before rolling. Soaking pits are installed at the front ends of blooming and slabbing mills, which are usually used for rolling ingots weighing not less than 2–3 tons. The working space in soaking pits is in the shape of a parallelepiped and has a cover that is moved by means of a ground-type crane. The ingots are loaded into the soaking pit and removed from it using a crane with a tong grip. The ingots are positioned vertically in the soaking pit. This removes the danger of displacement of the pipe when ingots with unsolidified cores are placed in the pit, and also permits rapid heating of the metal, since heat is supplied over the entire surface of the ingot. Several soaking pits are combined into a cluster with a common structure and control room.

Soaking pits are batch furnaces that are characterized by heat and temperature conditions that vary in time. Soaking pits are usually heated with blast-furnace or coke-oven blast-furnace gas. Some plants use natural blast-furnace gas. The optimum heat power of soaking pits is 200–300 megajoules per ton of charge.

Soaking pits are classified according to design features as regenerative, recuperative (with heating from the center of the bottom or by a single top burner), and electrical. The working space of regenerative soaking pits is 3.0–5.5 m long, 2.2 m wide, and 3 m deep. The ends of the working chamber are in contact with the regenerators for heating gas and air. The main disadvantages of soaking pits of this type are inadequate design of the fuel combustion system and the difficulty of automating the heating process. Regenerative soaking pits are not used in new mills.

In recuperative soaking pits with heating from the center of the bottom, the working space is in the shape of a square with a side of 4.5–5.0 m; it is 3 m deep. The ingots are placed near the walls. The burner is located in the center of the bottom and directed vertically upward. Recuperator chambers for heating air are located adjacent to the working chamber. The main advantage of recuperative soaking pits with heating from the center of the bottom is the high quality of heating of the metal. Control of the heating conditions can be automated. A disadvantage of soaking pits of this type is air loss in the recuperators. Numerous mills built in the USSR during the 1950’s are equipped with soaking pits of this type.

Recuperative soaking pits with a single top burner have working spaces 6–10 m long, 2.5–3.5 m wide, and 3.5–4.5 m deep. The pit is heated by a burner located horizontally in the upper part of one of the end walls of the chamber. The products of combustion are exhausted through ports in the lower part of the same wall, which adjoins the recuperator for heating air. Since the heated air is supplied to the burner through an injector, air losses in the recuperator are prevented. The advantage of soaking pits of this type is efficiency in the use of space in the building. Soaking pits of this type were installed in rolling mills constructed in the USSR during the 1960’s.

Electric soaking pits are used in some cases for heating ingots of high-quality steel. The heating elements in such soaking pits are carborundum troughs filled with petroleum coke breeze as the resistance material. The troughs are arranged along the entire length of the working chamber. Electric soaking pits may be of the twin type (with a separator wall), with two-sided heating, with three heaters, and with a single heater in the center. Oxidation of metal in soaking pits of this type does not exceed 0.2–0.3 percent (instead of 1 percent or more in other soaking pits). A carburizing environment may be generated in soaking pits by introducing small quantities of gas or mazut into the working chamber. Electric soaking pits are economically efficient for heating hot ingots.