The older process of producing “blister” or “cementation” steel is not generally employed now. By this method the bars of iron were put through a soaking or prolonged heating, while they were packed in charcoal. It was similar to the casehardening process, explained in section 90.
We have at present three notable processes of making steel; namely, the crucible, Bessemer, and open-hearth.
Fig. 152.—A Crucible.
Fig. 153.—Sectional View of a Four-hole Crucible Furnace.
182. The Crucible Process.—Crucible furnaces are flat structures containing from two to twenty holes, each one capable of receiving four or six crucibles. The crucibles are earthen vessels made of fire clay, mixed with refractory materials for withstanding intense heat. Each one is capable of receiving from 70 to 80 pounds of metal. (See [Fig. 152].) In this furnace the gas and air supply may be applied independently to each hole, practically making each one a separate furnace, but all of the holes are connected with one main stack or chimney. A sectional view of a four-hole furnace is shown in [Fig. 153], where the crucibles are shown in position.
This process is the most simple. It consists of melting the stock in the crucibles and pouring it, when completely fused, into molds, as shown in [Fig. 154], forming what is known as ingots or steel castings. For that reason it is very frequently called cast steel. The stock is carefully selected and weighed so as to produce the required grade. After the ingots are cooled, the piped or hollow ends caused by shrinkage are broken off and graded by the appearance of the granular structure and luster of the fractured parts. They are then marked and piled away for future use. On the ingots shown in [Fig. 155], the piped ends can be seen.
