XXIV
HARDENING AND TEMPERING: TOOL MAKING
EFFECTS OF CARBON ON STEEL
What is commonly known as carbon steel is a metal composed of iron containing varying amounts of carbon. Steels containing much carbon are called tool steels to distinguish them from the low carbon steels. Tool steel, when heated red hot and plunged into cold water, will harden, while low carbon steel treated in the same way will not do so. This is an excellent way of testing two bars of steel for carbon when one is not able to distinguish grades of steel accurately. It is carbon that gives the hardening quality. When steel is heated it becomes red at 1000° F. At 1300° F. it passes a point at which it absorbs considerable heat without any increase in the temperature, showing that some change in the structure of the metal must be taking place. If the steel is heated above this point and allowed to cool slowly, a brightening of the colour may be noticed as it passes this point, known as the point of recalescence. The brightening is due to a liberation of the heat previously absorbed.
Method of heating steel in forge fires: All steel work, including tool dressing, hardening, and tempering, was formerly done in an ordinary forge fire. Now we have special furnaces for that purpose. However, in using a forge fire, care must be taken to insure good work. The fire must be very deep—that is, a large body of coke must be put between the tuyere, and the tool, so as to prevent the blast reducing the carbon on the surface of the steel. Sulphur will injure the quality of any steel tool. Hence, a fuel low in sulphur should be used. Charcoal is the best for this purpose, but the cost and the difficulty in maintaining the heat prevent its general use in blacksmith shops. If the coal does contain sulphur a great deal of it can be extracted or reduced by one making his own coke. This is done by burning the green coal to a coke and in this way driving off much of the sulphur. Gas furnaces or oil furnaces are used. This is much better than coal, for a uniform heat can be kept and an oxidation of the steel prevented. Whether natural or artificial gas is used, all that is necessary is to adjust the supply of gas in such a way that there will be a very slight excess of gas present beyond the proper amount for combustion. The presence of this gas excludes all air from the steel and therefore prevents its oxidizing the surface of the metal.
Heating in lead: In order to prevent oxidation molten lead makes a most satisfactory bath. The lead is melted in a cast-iron pot and heated in the forge. The steel must be left in the bath until it has all been heated to the required temperature. As the steel will float in the molten lead it must be weighted down to keep it submerged.
Hardening solutions: In many cases clear cold water is used in hardening steel. Some use soft or rain water. The temperature of the water for general work should not fall below the temperature of the shop; otherwise it would extract the heat too quickly from the steel and cause cracks or breaks in the work.
Salt solution: Salt is often added to water which is to be used as a hardening solution. (1) It increases the rate at which the bath will extract the heat from the steel; (2) it prevents the formation of steam on the surface of the water. Put as much salt in rain water as it will dissolve. This is considered one of the best and easiest made solutions for hardening steel.
Oil solution: Linseed oil, lard, cotton seed oil, whale oil, and melted tallow make good hardening solutions. They are used mostly for fine work. The oil prevents the sudden chilling of the steel and lessens the chances of cracking and breaking. Springs are mostly tempered in oil.
Metallic hardening baths: For very delicate tools mercury is sometimes used. It has a greater heat conductivity than any solution mentioned. However, the fumes given off are poisonous and for this reason it is not extensively used.