Because of the many grades of steel now made containing various alloys, it is impossible to give any set rules for hardening as compared with those used at the time when strictly straight carbon tool steel was used for all purposes. The hardening of a carbon steel is due to a change of internal structure which takes place in the steel when heated properly to a correct temperature. The two simple general facts of hardening that may be remembered are: (1) the heat from which the steel is cooled determines the grain; and (2) the rapidity of cooling determines the hardness, everything else being equal, the more rapid the cooling, the harder the steel.
There is only one heat from which the steel may be cooled and have the proper grain, and this is known as the | hardening heat. A piece of steel when cooled from this hardening heat has an extremely fine silky looking grain and is left very hard and brittle. The hardening heat varies with the amount of carbon the steel contains, the greater the percentage of carbon, the lower the hardening heat.
To determine the hardening heat, a bar 1/4 inch or 3/8 inch square is heated to a good red heat on one end, and cooled in cold water. This end is then tested; if too hard to file it has been hardened, and the heat from which it was cooled was either the proper hardening heat or some higher heat. If the end can be filed, it was cooled from some heat below the hardening heat. If the end proves to be soft, it should be rehardened by cooling from a higher heat, if hard, it should be broken off and the fracture examined. If the grain of the broken end is very fine, the steel is properly hardened, if coarse, it was heated too hot and the end should be rehardened at a lower heat. The experiment should be repeated until the operator is able to give the steel a very fine grain every time. Any variation either above or below the hardening heat will make the grain coarse. A temperature lower than the critical heat will not make the steel as coarse in structure as a temperature correspondingly higher, but there will be some difference.
Self-hardening steel is used to a large extent in modern practice for lathe tools, much being used in the shape of small square steel held in special holders, as illustrated in Fig. 183. Self-hardening steel, as its name indicates, is almost self-hardening by nature, generally the only treatment that is required to harden the steel being to heat it red hot and allow it to cool. Sometimes the steel is cooled in an air blast or is dipped in oil. It is not necessary to draw the temper. The self-hardening quality of steel is given to it by the addition of chromium, molybdenum, tungsten, or one of that group of elements, in addition to the carbon which ordinary tool steel contains.
Fig. 183. Typical Tool Using Blade of Self-Hardening Steel.
Self-hardening steel is comparatively expensive, costing from 40 cents and upward per pound, some of the more expensive grades costing $1 or so. However, when in use, self-hardening steel will stand a much higher cutting speed than the ordinary so-called carbon steel, and for this reason it is much more economical to use, although its first cost is higher.
Self-hardening steel cannot be cut with a cold chisel and must be either cut hot or nicked with an emery wheel and snapped off. Great care must be used in forging it, as the range of temperature through which it may be forged is comparatively slight, running from a good red heat to a yellow heat. Some grades of self-hardening steel may be annealed by heating the steel to a high heat in the center of a good fire and allowing the fire and the steel to cool off together. Steel which has been annealed in this way may be hardened by heating to the hardening heat and cooling in oil.
This method of treating special grades of self-hardening steel was discovered some years ago by the men after whom it is named. It was found that if a piece of self-hardening steel is heated to a very high temperature (about the welding heat) and then suddenly cooled to about a low red heat, the steel would be in a condition to stand very much harder usage and take a much heavier cut. Steel treated in this way seemed to have the cutting edge of the tools almost burned or melted off and considerable grinding was necessary to bring them into shape. When put in use the edges would almost immediately be slightly rounded or crumble off, but after this slight breaking down of the cutting edge, the steel would stand up under excessively trying conditions of high speed and heavy cut. Tools of this character are of very little or no use for fine finishing, but are of great value for heavy and roughing cuts.
High-speed steel has a much higher critical temperature than carbon steels. A temperature of about 1350° to 1600° Fahrenheit is sufficient for carbon steels in general. High-speed steels require heating from 1800° to 2300° Fahrenheit, and to be cooled in oil such as machine, fish, or linseed. Steel of this nature is close grained and should be heated slowly until red, then forcing the heat faster up to dazzling white. When it shows signs of melting down it should be quickly put in the cooling bath. The treatment varies for the different steels, and it is advisable to get directions from the steel maker as to the treatment, which varies according to the alloys of the steel. Air blast formerly was recommended by some steel makers, but at present oil is most extensively used.
A barium-chloride bath for heating highspeed steel has been used by some manufacturers for finished tools such as milling cutters, taps, and drills. To get the high heat required without oxidizing, a thin coating of the barium chloride is formed on the steel, which, in transferring from the heating bath to the oil bath, keeps the steel from coming in contact with air and thus avoids oxidation. One trouble with the barium chloride is that a thin surface about 0.008 inch thick is soft, but for cutters that have to be ground this disadvantage is of no account.
For tools with special shapes that cannot be ground - such as gear cutters, twist drills, taps, threading dies, and many other tools not permitting grinding after hardening - a muffle furnace is best to use, as in this furnace tools do not come in contact with the flame and the thin cutting points are thus more protected.