Blasting Away: Steel Building

There are three general uses of the term "Blast" in the making of steel.

It's hard to overstate the importance of the steel industry in the United States, especially in the late 1800's and early 1900's. This article gives a glimpse of insight into one critical area of steel making: Blasting.

First, it is the name given to air under pressure used for purposes of combustion, etc., as in the blast furnace or the Bessemer converter; in some cases for cooling purposes, as in the case of tools. If preheated, it is termed hot blast or warm blast; if not preheated, cold blast. A blast under considerable pres- sure is sometimes referred to as a cutting blast; if weak, a soft blast.

Blast Box. - Of a Bessemer converter: A shaft furnace supplied with blast for producing pig iron by smelting iron ores. It is continuous in operation, the solid raw materials being charged at the top, and the molten pig iron and slag tapped out at the bottom at intervals. Large furnaces are about 80 to 200 feet high, have an output up to 600 or 700 (commonly 300 to 600) tons per day, and employ blast at a pressure of 5 to 25 (usually 1 to 15) pounds per square inch, and at a temperature of about 900° to 1400° F. (485° to 7600 C.) The modern equipment consists essentially of the furnace proper or stack; blowing engines for compressing the air for the blast; stoves for preheating the blast; appliances for charging the raw materials; and a cast house for disposing of the molten iron.

Stack. - There are three well defined divisions, usually circular - practically never oval or square - in section. At the bottom is the hearth (well, laboratory, or crucible) of cylindrical shape; above this the walls diverge, forming an inverted truncated cone, called the bosh (also the name for the greatest diameter of the furnace), above which the walls converge to the top (throat or mouth), forming another truncated cone set upright. In a furnace with saucer bosh or belly walls, the walls above the bosh, instead of being straight, are slightly convex. The furnace is built of fire-brick (cold blast furnaces are built of stone), usually but not always encased in a general steel jacket extending part or all the way up. The section above the bosh is supported on a mantle (mantel) ring (lintel plate) and columns. Up to a certain distance from the bottom of the furnace the walls are protected from corrosion by cooling plates of bronze or other metal through which water circulates. Sometimes water is caused to flow down the outside.

Blast. - Formerly the blast was always used cold (hence the term cold blast) but this practice is now restricted to a few small furnaces, as a rule employing charcoal for fuel. Hot blast was introduced by Neilson about 1828, and affected a great economy in fuel. The blast is introduced near the bottom of the bosh through a number of pipes or tuyeres (twyers, tweers) which are protected from burning by being encased in water-cooled metal castings (tuyere blocks). The overhang of a tuyere is the amount it projects into the furnace beyond the inner wall.

Of course, modern steel making uses much more sophisticated, computer-aided technologies, but the basics, as described above, remain.