Such tools as cold chisels and lathe tools may be heated and laid in or on warm ashes until nearly cold, when they may be ground, hardened, and tempered. Quite frequently, if not generally, these tools are not treated in this manner, but it is no doubt the course to pursue to get the best results.
89. Hardening and Tempering.—When steel has been properly heated, forged, finished, or ground, the next two steps are hardening and tempering. These two processes are often understood as one, but they are entirely different in their results. The confusion arises because the two operations are sometimes performed with one heating of the steel as in hardening and tempering a cold chisel, or other similar tools.
As the steel has been subjected to severe strains during the heating and forging operations, its structure may have been somewhat altered. It can be restored to the proper crystalline structure by the hardening, scientifically known as refining. The hardening or refining heat is always lower than the forging heat, and should be only as high as is necessary to harden the steel to the required density by sudden cooling. Then this first operation of cooling will harden and refine the steel at the same time.
Extreme hardness is always accompanied by extreme brittleness, a quality undesirable in any cutting tool, and especially so in a tool required to withstand sudden shocks. As the hardness is reduced by subsequent heating, the toughness increases. This modification, called tempering, is accomplished by reheating the hardened portion of the tool until a sufficient toughness has been obtained, when the process is stopped by again plunging the tool into cold water. The heat for tempering may be supplied from the uncooled portion of the tool as in tempering a cold chisel, from the forge fire, from another hot piece of metal, or from a carefully heated furnace.
It has been found that the colored oxides formed on the surface of a piece of polished steel or iron represent a definite temperature in that metal. These colors have been used, therefore, to determine the desired temperature in tempering a tool. When we say “temper a tool to a light straw,” we mean that the hardened tool is to be heated again to a degree which will produce that color; namely, about 430 degrees Fahr. The colors as they appear are light straw, dark straw, bronze, bronze with purple spots, purple, dark blue. The light color appears first. Do not allow the colors to pass too quickly, as will happen if the heat applied is too intense.
Fig. 70.—Hardening a Chisel.
There are two distinct methods of hardening and tempering. The one generally followed in tempering cold chisels, lathe and various other tools, requires only one heating. The tool is heated to a proper hardening temperature at the end, where hardness is desired, and also over an excess area to supply the heat for tempering. About 2 inches of the cutting end is heated; about 1 inch of this is plunged perpendicularly into water, as shown in [Fig. 70]; it is then kept in motion perpendicularly between the places indicated at a and b, while the end is cooling. This will prevent a fixed cooling point and prevent a fracture that might possibly occur if it were held in one position while cooling. The portion between b and c should retain sufficient heat to produce the necessary temper. When the end is perfectly cold it should be removed and immediately polished with sandstone or emery cloth to remove the scale of oxide so that the different colors may be more readily seen as they move from b toward the point. The heat in the portion between b and c flows toward the point, causing the colors to appear as the heat extends. When the desired color covers the point, it should again be plunged into the water and left there until entirely cold. In this method the first cooling is the hardening, and the second the tempering. A comparative color chart is appended to this chapter for guidance in obtaining the tempers for various tools.
Fig. 71.—Hardening A Reamer.