by J.M.Drew St. Paul Publishing Company 1918
- HOME -Farm Blacksmithing
The tempering of steel tools consists of two processes: First, hardening by heating, then suddenly chilling; and, second, "drawing the temper" or softening from the chilled state to the degree of hardness desired. In the case of the chisel which we were considering, we need to have only one end-the edge-hardened. To accomplish this, heat the whole chisel to a dark or cherry red color, and holding it perpendicularly over the water, dip the end in an inch or more and keep it moving up and down for a few seconds, or until the edge is cool enough so that the water will not dry on it for the space of two seconds when it is drawn out. Now polish one side quickly with a piece of brick so that the colors denoting the degree of heat may be seen. These colors will form a band which will be seen to move towards the edge or cooler part of the tool. First will be seen a pale yellow or straw color; then darker yellow, which changes to brown; then purple, then blue.
A cold chisel needs to be quite soft, so we will wait until the blue gets to the edge; then quickly dip the edge into the water again and hold it there until the remainder of the tool is cool enough so that we may dip the whole without fear of hardening it. Tools for woodwork, such as carpenter's chisels and plane irons, are tempered to a straw color, as they require a very hard edge in order to be kept sharp, and all boys know that they are not a success when used, as a cold chisel is, for cutting nails, etc. Small articles, as penknife blades and all tools which require an equal temper throughout, are first chilled by being thrown into water when hot; then are heated to the required color by being held in a gas flame or laid on a bar of hot iron, and are dropped into the water again when the right color appears. In chilling the end of a chisel, drill, or any similar tool, it should not be held still in the water, as this is apt to start a crack at the water line. Dancing the tool up and down while chilling will lessen the danger of cracking. In retempering a tool it is a good plan to hammer it lightly before chilling, as this seems to lessen the liability of cracking.
Almost every cross-roads blacksmith has some particular receipt for a tempering fluid which he considers better than anything else for tempering tools. Plair1 water is as good as anything else for ordinary use. Salt water is often used in case it is necessary to make a tool very hard. Salt water will harden steel harder than pure water, simply because it is a better conductor of heat; but very few tools need to be made harder than water will make them. Oil is better than water in cases where it is desirable not to chill the steel too suddenly, as, for instance, in the case of thin knives or any tools which are liable to warp out of shape in cooling. A layer of oil on top of the water answers as well as all oil. Steel cannot be hardened in soapy water, for the reason that the soap in the water forms a coating on the steel the instant it comes in contact with it. This coating of soap is a poor conductor of heat, and prevents the steel from cooling suddenly.
The following table shows approximately the temperature (Fahrenheit) indicated by the colors on steel:
- 1. Very pale straw color .430 deg. Stone drills for hardest stone.
- 2. Yellow 450 deg. Wood working tools, stone drills.
- 3. Dark yellow .470 deg.
- 4. Brownish yellow 490 deg. Hammers
- 5. Brown 500 deg. Lathe tools.
- 6. Brown tinged with purple 520 deg. Drills, hard
- 7. Light purple 525 deg. Drills
- 8. Dark purple tinged with blue 550 deg. Watch springs, swords, hard cold chisels.
- 9. Dark blue 565 deg. Saws, cold chisels for soft iron
- 10. Very dark blue 500 deg. Saws, screw drivers
- 11. Dark blue tinged with green 660 deg. Too soft for any tools
No great amount of dependence should be placed upon the above table as an absolute guide for practical work, for the reason that steel varies in the degree of hardness indicated by the colors for every variation of carbon content. The greater the percentage of carbon in the steel the harder will it be for each color. For example: A cold chisel made of a certain brand of steel may be just right for a certain kind of work if tempered dark purple, whereas a chisel made from a steel containing a higher percentage of carbon, in order to stand the same work without breaking would have to be let down to a blue color. The only way to be sure of the proper tempering of a tool is to try it. A tool smith should never warrant tools made from a new brand of steel until he has made and tried one from that brand. A person may tell something about the amount of carbon in an old chisel or stone drill by the way the head turns over where it is struck by the hammer. If it breaks off in small pieces, instead of turning over, it is high in carbon. If the end frays out and turns over like a sunflower, it is low in carbon. The smith thus has something to follow in tempering. In making tools of any kind it is very important that steel containing the proper amount of carbon be used. For example: Steel containing the right percentage of carbon for lathe tools would be very poor material for making cold chisels. A good spring cannot be made from an old file. Steel makers make different grades of steel to suit different uses, and the blacksmith when ordering should always state the purpose for which he wants the steel so that the steel maker or merchant will know what temper will best suit his needs.