tempering Black Iron knives
Tempering Punches, Screw Drivers, and Similar Tools.-Tools like punches, screw drivers, scratch awls, etc., may be tempered in the same manner as a cold chisel, but may be made harder or softer according to the requirements of the tool. A scratch awl should be made somewhat harder than a cold chisel, a rock drill somewhat harder, a center punch just a little harder, a punch for lining up holes somewhat softer, a screw driver somewhat softer, etc.
Different grades of tool steel will have different degrees of hardness when quenched at the same color. Therefore, it may be necessary to experiment a little with the first piece of a new lot of steel in order to secure the desired degree of hardness.
Tempering Knives.-Knives and tools with delicate parts are usually hardened and tempered in a manner slightly different from that used for cold chisels, in order to avoid the danger of overheating and warping and to insure uniform hardening and tempering of the cutting edges.
After a knife is forged, it should be annealed. It is then heated slowly and uniformly to a dark red, or the critical temperature. It is then quickly cooled by dipping edgeways in clean tepid water or oil, thick edge first. This method of dipping helps to insure uniform cooling and therefore uniform hardening and freedom from warping. It is then polished and reheated by drawing it back and forth through a flame, or by laying it against a large piece of red hot iron and turning it frequently to insure uniform heating. When the desired color, usually blue, appears, it is again quickly cooled.
Another method of heating knives and similar tools for hardening and tempering is to draw them slowly back and forth inside a pipe in the forge fire. The pipe should first be uniformly heated in a big fire and then turned frequently to keep it uniformly heated on all. sides. The knife should not be allowed to touch the pipe.
source: wrought iron forge