There are several ways in which these processes may be carried out, the result being that there are several descriptions of steel in the market. Of these, however, only a few of the most important can here be described.
Blister Steel is produced by placing bars of the purest wrought iron in a furnace between layers of charcoal powder, and subjecting them to a high temperature for a period varying from five to fourteen days, according to the quality of steel required.
This .process is called cementation.
Swedish iron is generally used for the purpose, that marked, from the Dannemora mines, being the best.
The steel differs greatly from the bar iron from which it was produced.
Its distinctive name is derived from the appearance of its surface, which is covered with blisters due to the evolution of carbonic oxide.
The bars are now brittle, the fracture is of a reddish or yellowish tinge, with but little lustre.
1 Percy's Metallurgy.
"When the blisters are small and tolerably regularly distributed the steel is of good quality; but when large, and only occurring along particular lines, they may be considered as indicative of defective composition, or want of homogeneity in the iron employed."1
Blister steel is full of fissures and cavities, which render it unfit for forging except for a few rough purposes. It is used for welding to iron for certain parts of machines, for facing hammers and 6teeling masons' points, etc, but not for edge tools. Most of the blister steel made is used for conversion into other descriptions of steel.
Spring Steel is blister steel heated to an orange red colour, and rolled or hammered.