Certain kinds of steel, used for machine tools, can be hardened by first heating and then dipping into water. The common types of straight carbon or low alloyed steels which are intended for water quenching are usually heated to about 1450 oF. and. then plunged directly into water or brine, agitated to aid. in the cooling process. If the entire piece of metal is to be hardened, it is important that the heating be uniform and thorough in order to accomplish this result. In the case of hand. tools, it is a common practice to heat only that portion of the tool which is to be hard, or sometimes to heat the whole piece but dip in the water only that portion which must be hard. If such a differential hardening is done, it is important that the design be such that no weak section is located in that portion which would be only partially hardened. It is generally preferable to make a difference in hardness of any part by tempering or drawing the portion desired soft to a much higher temperature than the hard part. The tempering operation is very important and must be performed on every piece which is hardened. Tempering should be done just as soon as the quenched part is cool enough to be comfortably handled in the bare hands. Tempering temperatures may vary from 225 oF. to as high as 800 oF., but the usual temperature is 400-800 oF. These temperatures can be judged by color, but modern practice definitely favors use of ovens or furnaces with accurate control of the temperature. The time of tempering is usually not less than 2 hours. An indication of the re1ation of heat colors to temperature is given in the tables.
As an example of hand tempering, take the case of a cold chisel. The essential operation is to heat it to a bright red, about 1 ½ inches from the point, then dip the point of the chisel in water. This must be just dipped, and moved up and down sightly to avoid a sharp line of demarcation between the hard and soft, which may, if it occurs, cause the hard end to shear off bodily then the chisel is put to use. As soon as the actual edge is quenched to cold, move the chisel rapidly to the anvil, lay the hard end across the edge to support it, and rub both sides with a stone. This brightens it sufficiently for the operator to see the temper colors as they appear, coming up in straight lines across the shank.
Laying the edge of the chisel across the sharp edge of a cold anvil acts as a check to the tempering, because the heat that would be conducted to the cool end is absorbed by the anvil. As soon, therefore, as the edges are brightened. and. one can see what is happening, lift the job from the anvil so as to allow the conducted heat from the shank to have full play. When the right temper color reaches the actual edge, dip and, quench the whole tool. As another example, one can consider the common twist drill, which is a more complex tool than a chisel or ordinary turning tool. A drill is subject to considerable stress, particularly when breaking through the underside of a surface. This is due to the fact that it has then lost the support provided by the point. Twist drills must be hardened along the whole length of their flutes, as they untwist if part is left soft.
Carbon Tool Steel
CENTIGRADE | FAHRENHEIT | |
| Very Faint Yellow | ||
| Light Yellow | ||
| Straw Yellow | ||
| Dark Yellow | ||
| Yellow Brown | ||
| Spotted Red Brown | ||
| Brown Purple | ||
| Full Purple | ||
| Dark Purple | ||
| Full Blue | ||
| Dark Blue |
In moderate diffused daylight with approximate temperatures
CENTIGRADE | FAHRENHEIT | |
| Faint Red | ||
| Blood Red | ||
| Dark Cherry | ||
| Cherry | ||
| Bright Cherry | ||
| Salmon | ||
| Dark orange | ||
| Orange | ||
| Lemon | ||
| Yellow | ||
| Light Yellow | ||
| White |
According to Simon Summers, depending on the color upon which it is quenched, the tool will be better for cutting into specific materials;
- for cutting into stone --> yellow
- for cutting into wood --> purple
- for cutting into steel --> blue
