Chapter XLIV.—HARDENING AND TEMPERING.
Hardening and tempering processes are performed upon steel for three purposes:
1st. To enable it to resist abrasion and wear.
2nd. To increase its elasticity.
3rd. To enable it to cut hard substances and increase the durability of the cutting edge.
Of these, the first is the simplest, because the precise degree of hardness imparted is not of vital importance.
The second is more difficult, because the quality of the steel employed for such purposes is variable, and hence the tempering process must be varied to suit the steel. The third is of the greatest importance, because the articles to be tempered are the most expensive to make, the duty obtained is of the greatest consequence to manufacturing pursuits, and the fine grade of steel employed renders it more liable to crack in the hardening process.
In those mechanical parts of machines which are hardened to resist abrasion and wear, the quality or grade of the steel is very often selected with a view to obtain strength in the parts and ease of mechanical manipulation in cutting them to the required shape, rather than to the capacity of the steel to harden. Hence, tougher and more fibrous grades of soft steel termed “Machine” steel, are employed, meaning that the steel is especially suitable for the working parts of machines. This class of steel is of a lower grade than that known as “tool” steel. It is softer, works, both on the anvil and in the lathe, more easily, and will bear heating to a higher temperature without deteriorating. It approaches more nearly to wrought iron, and is sometimes made of so low a grade as to be scarcely distinguishable therefrom.