By the second method the steel is heated as in the first method, then it is cooled off entirely by immersing the tool exactly perpendicularly, as shown in hardening a reamer in [Fig. 71]; after this it is polished. The temper is then drawn by holding the tool in contact with a piece of heated metal, cast iron preferably. In [Fig. 72] the reamer is shown inside of a heated bushing, which is a more practical way than laying it on top of a heated flat plate. The bushing will impart sufficient heat to the tool to produce the desired color, when it should be again cooled. This method is used mostly for tempering plane bits, wood chisels, milling cutters, taps, reamers, and various other tools of a like nature.
Sometimes tools having sharp protruding edges, as milling cutters, taps, reamers, etc., are very liable to crack by the sudden cooling in water; this difficulty is avoided by using oil for hardening and tempering. Any so treated are called oil-tempered tools.
Fig. 72.—Tempering a Reamer.
The above methods of tempering are such as are ordinarily used when only a common shop equipment is at hand, and the operator must depend entirely upon his judgment of the colors which represent the proper forging, annealing, hardening, and tempering heats. The degree of accuracy that has been attained in this practice is most surprising.
In large manufacturing establishments where many duplicate pieces are to be tempered, a more modern as well as scientific apparatus is employed to relieve the operator of dependence upon his discernment of colors. Here the steel is heated in a furnace, to which is attached a pyrometer that registers the exact degree of temperature. In this manner all pieces can be heated uniformly for any of the four required heats.
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| A. Natural Bar. | B. Refined. |
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| C. Too hot. | D. Burned. |
Fig. 73.—Sectional Views of Tool Steel, showing theEffects of Proper and Improper Treatment. | |
The views in [Fig. 73] were photographed from the same grade or bar of steel to show the various granular structures produced by different heat treatments. [A] shows the condition of the natural bar, which was broken to be photographed just as it was received from the steel makers. The lower left side shows where it was nicked with the cutter to be broken. [B] shows the structure when proper conditions of heating and hardening have been maintained. Notice how much finer the structure here appears to be; this effect was caused by, and previously referred to as, the refining heat of steel. A similar condition should be produced with any tool steel under correct treatment. [C] shows a much coarser structure; it was heated too hot and hardened in the same manner. If a tool were made thus, its weakness would be hardly noticeable at the time, but the structure shows that it is considerably weaker. [D] shows the condition of the stock after being burned. It has produced from a quality of steel that was valuable, a metal worthless for any kind of tool.
90. Casehardening.—Another method of hardening, called casehardening, is used for wrought iron and low carbon or soft steel parts which are to be subjected to considerable friction. Neither of these metals could be hardened by the other methods mentioned. This process adds carbon to the exterior surfaces only, and for that reason is called casehardening, as the outside is made extremely hard, while the inner portion or core remains in a condition like that produced by sudden cooling, thus providing a hard wearing surface and great strength at the same time. It is similar to the old cementation process of steel making, but is not prolonged sufficiently to allow the hardening to continue through the entire structure.
The articles to be hardened are packed in a box somewhat similar to an annealing box. This should be partly filled with charred leather, ground bone, or wood or bone charcoal, all of which are highly carbonaceous materials; then the articles are placed in and entirely surrounded with a thin coating of cyanide of potassium, especially if iron is being hardened. The remaining space in the box is filled with the leather, bone, or pieces of charcoal. The box should be provided with a lid that will drop loosely between the outer projecting rims. The outer edges of this lid should be luted with clay to keep it as air-tight as possible. If a few small holes are provided in the center of the lid, test wires can be inserted; by removing a wire and cooling it, the progress of the operation may be known. These wires should be inserted before the box is placed in the furnace. The box and its contents are then placed in a suitable furnace and kept thoroughly heated from 6 to 15 hours, depending upon the depth of hardness required. Then it is withdrawn, the lid removed, and the articles quickly plunged into a large tank of water. This will complete the hardening.