Tool steel may be hardened by heating to a low red heat and plunging in some cooling medium, as water, brine, or oil.

Use Of Pyrometers. Necessity For Accurate Temperature Readings

At the present time, when so much attention is given to obtaining exact temperatures in the various processes of heat-treating steel some form of temperature gage is absolutely essential. The gage used for determining high temperatures is called a pyrometer.

A good pyrometer is a necessity, if the heating of steel is entrusted to a man who has not had a wide experience in gaging temperatures with the eye. It is also a great aid to the skilled man, as furnace conditions vary. Changing degrees of light in the hardening room may deceive even the most experienced hardener; a man's physical condition may affect his vision; or any one of a dozen things may cause him to heat steel to a temperature that will not produce the best results possible.

In a hardening room having several furnaces, it is not always necessary to provide a pyrometer for each furnace, as a number of furnaces can be connected with one instrument so that by moving a switch each furnace is connected in turn and its temperature can be read from the indicator.

As so much depends on the accuracy of the gage used in temperature readings, it is always best to have one that is known generally as a reliable instrument. The extremely high temperatures to which the fire - ends of. these gages are subjected, makes it necessary to watch very closely even the most satisfactory makes, for an instrument of this kind, unless fairly accurate, is worse than none. When using a pyrometer for gaging heats, the fire end should be located as nearly as possible at the same height as the work being heated in the furnace. The temperature in a furnace varies many times; that is, it may be much higher 18 inches above the floor of. the furnace than it is at the floor level, and if the work rested on the bottom and was but 2 or 3 inches in height, and the fire end of the gage was located 18 or 20 inches above the work, the readings might not accurately indicate the temperature to which the piece was heated.

Clay Cones

In order to make sure that the instrument is recording correctly, it is a good plan to check it occasionally with one of known accuracy; or, in the absence of a second gage, it may be checked by means of clay temperature determining cones, called by some sentinel pyrometers. The cone should be located as nearly as possible at the same height as the fire end of the pyrometer. When making the test, have the furnace at a temperature somewhat lower than the fusing point of the cone, gradually raising the temperature until the cone fuses. Notice at the fusing time the reading of the pyrometer; if the reading agrees with the predetermined fusing point of the cone, it is reasonably certain that the other readings of the gage are correct. Some hardeners, however, insist on testing at several different temperatures, say at 1350° F., 1850° F., and 2250° F., asserting that if the three readings are correct they know the gage is absolutely reliable at the time at any temperature.

Fig. 14. Electric Pyrometer Which Registers Temperature, up to 3000 Degrees Fahrenheit.

Courtesy of the Brown Instrunent Company, Philadelphia, Pennsylsania.

The cones are convenient also where there is no pyrometer, as high temperatures may be accurately gaged by their use. Clay cones are cheap, reliable, and easily obtainable in a large range of temperature determinations. Each cone has marked on it its fusing point, so that there is absolutely no need of error in its use.

Various Types

There are a number of satisfactory makes of pyrometers on the market, any one of which will show satisfactory results if given the same consideration a careful workman is supposed to give any tool or machine used for accurate gaging. Fig. 13 shows a pyrometer that may be employed in gaging the temperatures of four different furnaces. Each furnace is numbered and its fire end is joined by means of wires to the proper connections on the pyrometer. By turning the pointer to the proper number, the temperature of that furnace is shown on the dial of the gage.

Fig. 14 illustrates another style of pyrometer which gages temperatures to 3000° F. By use of the switchboard shown in Fig. 15, this gage may be used to determine the temperatures of eight furnaces.

Various Types 30013

Fig. 16. Brown Alum Pyrometer Courier of brown Instrument company Philadelphia, Pennsylvania.

Fig. 16 shows an alarm pyrometer. This can be set to have the alarm ring when the furnace temperature rises or falls beyond the desired limits. Such an instrument is especially desirable where temperatures must not exceed certain limits, as is the case with certain high-grade carbon and alloy steels, and where tools and other articles that are to resist great strains are being hardened.

Hardening And Tempering Crucible Tool Steel

After the determination of the proper proportion of carbon, the next important process is the hardening of the steel. This is subdivided into two main processes - heating, and subsequent cooling.


A piece of steel should never be heated more than is necessary to give the desired result. The heat required varies with the make of steel, the amount of carbon it contains, the size and shape of the piece, and the purpose for which it is to be used. Much depends on giving a piece of steel a uniform heat throughout. The edges and corners should be no hotter than the center, and the interior should be of the same temperature as the surface; if not, the piece is likely to crack in the cooling bath, on account of the uneven changes which take place in the molecular structure. While it is highly important that the steel be heated no more than is necessary, yet it is of much importance that it be heated uniformly.

If the steel is placed in an ordinary forge, be sure that the air from the blast does not strike it. For a large piece, build a big, high fire; have it well heated through before putting in the steel. Use the blast only enough to keep a lively fire, and see that the steel is well buried in the fire in order that the air may not strike it.

Steel should always be hardened at a heat that leaves the grain fine when the piece is broken. This condition can be determined by hardening and breaking a small piece from the same bar. A coarse grain denotes a heat higher than the steel should receive.

It will be found necessary, when heating some kinds of steel, to put the articles in an iron tube so that the air cannot come in contact with them; this is especially true when hardening such tools as taps or formed mills, whose outer surfaces cannot be ground, because the oxygen in the air, acting on the carbon at the surface of the piece of steel, burns it out, leaving the surface decarbonized. Better results can be obtained with any tool if it is kept from the action of the fire when heating for hardening.


When the piece is uniformly heated, it should be plunged into a suitable bath to give it the proper hardness. It must be worked rapidly up and down or around in the bath, to prevent the steam generated by the red-hot steel from forming at any point and so preventing the liquid from coming in contact with the piece, and also to bring the piece constantly in contact with the cooler parts of the bath. If the piece is long and slender, it must be worked up and down; if it is short, with teeth on the outer edge, as a milling-machine cutter, it should be worked around rapidly, so that all the teeth may be cooled uniformly. If it is flat and has a hole through it whose inner walla must be hard, it should be swung back and forth so that the bath may pass through the aperture and at the same time strike both faces.

Delicate articles and tools having long projections or teeth, should not be dipped into a bath of very cold water or brine; for such work, a tepid bath gives better results.

If the tool is not to be hardened all over, and it is necessary to heat a larger portion of it than is to be hardened, dip the piece into the bath so that a trifle more of the tool is immersed than is to be hardened, and then work it up and down a little. If this is not done, there will be a line where the piece is expanded on one side and contracted on the other. The steel is likely to crack on this line which is called a water line.

When hardening a piece having a shoulder A on the outside, as shown in Fig. 17, or inside, as shown in Fig. 18, hardening should not stop at the shoulder, as the unequal strains occasioned by the contraction of the hardened part at the shoulder are likely to cause it to crack at that point. The piece ought not to be hardened as high as the shoulder; but should it be necessary to do so, it is well to harden a little beyond. A very satisfactory method, when sharp corners with sudden changes of sizes occur, as shown in Figs. 17 and 18, consists in placing a ring of wire in the shoulder, as shown at a or b, Fig. 19. Usually, the piece will be made as at c, for the sake of strength, but when two shoulders come in line as shown at a and b, wires may be placed at both shoulders. The wires, heating with the work, will be red hot when the piece is dipped into the bath, and will prevent the water attacking the steel too suddenly in the shoulders.

Toot with Outsids Shoulder.

Fig. 17. Toot with Outsids Shoulder.

Fig. 18. Tool with Inside Shoulder.

Use of Wire Ring in Hardening Pieces with Shoulder.

Fig. 19. Use of Wire Ring in Hardening Pieces with Shoulder.

Citric Acid Bath

An excellent bath for hardening small pieces may be made by dissolving one pound of citric acid crystals in one gallon of water. The liquid should be kept tightly covered when not in use, or it will evaporate. Small tools heated to a low red heat and dipped into this solution harden more uniformly than when immersed in clear water.