One of the main advantages of having a forge in the farm shop is to be able to redress and make and temper tools like cold chisels, punches, screw drivers, picks, wrecking bars, etc. Tool steel for making cold chisels arid punches and similar tools may be bought from a blacksmith or ordered through a hardware store; or it may be secured from parts of old machines, such as hay rake teeth, pitchfork tines, axles and drive shafts from old automobiles.
301. Nature of Tool Steel.—Tool steel contains more carbon than mild steel, and it is granular, while mild steel is fibrous or stringy (see Art. 328, page 242). The smaller the size of the grains or particles in tool steel, the tougher and stronger it is. When tool steel is heated above a certain temperature, called the critical temperature, the grain size increases. (The critical temperature is usually between 1300 and 1600°F., depending upon the carbon content, and for practical purposes is indicated by a dark-red color.) If the steel is heated only slightly above the critical temperature, the fine grain size may be restored by allowing it to cool slowly and then reheating it to just the critical temperature. If the steel is heated to a white heat, however, the grain size will be permanently enlarged and the steel damaged or possibly ruined. If tool steel is hammered with heavy blows while it is just above the critical temperature, the grain size will be made smaller, and the steel thereby refined and improved. It is evident, therefore, that a piece of steel may be improved or damaged or even ruined, depending upon how it is heated and forged.
302. Heating Tool Steel.—Tool steel should be heated slowly and evenly in a good, clean, deep, coke fire. Uneven heating, which is usually caused by heating in poor shallow fire or by too rapid heating, results in unequal expansion, which in turn may cause internal cracks and flaws.
Tool steel should not be healed above a bright^rcd or loio-orange heat, and to this temperature only for heavjf hammering. Heating higher is likely to ruin the grain structure. In case a piece of steel is accidentally heated a little too hot, the grain size may be restored by (1) allowing it to cool slowly and then reheating, being careful not to overheat it again, or (2) by heavy hammering at a bright-red or low-orange heat. The damage done by overheating will depend upon the temperature to which it was heated, and upon how carefully it is subsequently heated and handled.
303. Forging Tool Steel.—Since the making of a satisfactory tool depends so largely upon the proper heating and handling of the steel, the following points should be kept in mind when forging with it.
1. Tool steel has a much narrower range of forging temperatures than mild steel. Hammering below a red heat may cause cracking or splitting, while temperatures above a bright red or dark orange may damage the grain structure.
2. Tool steel should always be uniformly heated throughout before it is hammered. Otherwise the outside parts, which are hotter, may stretch away from the inside parts, which are colder, and thus cause internal flaws.
3. Very light hammering should be avoided, even when the steel is well heated, because this may likewise draw the outer surface without affecting the inner parts.
4. As much of the forging as possible should be done by heavy hammering at a bright-red or dark-orange heat.—slightly above the critical temperature—as this will make the grain size smaller and thus refine and improve the steel.
5. When a piece is l>eing finished and smoothed by moderate blows, it should not be above a dark-red heat.
304. Annealing Tool Steel.—After a tool has been forged, it is best to anneal it, or soften it, before hardening and tempering. This is to relieve any strains that may have been set up by alternate heating and cooling and by hammering. Annealing is done by heating the tool to a uniform dark-red heat and placing it somewhere out of drafts, as in dry ashes, or lime, and allowing it to cool very slowly. (Copper and brass may be softened by heating to a red heat and plunging quickly into water.)
305. Hardening and Tempering Tool Steel.—If tool steel is heated to a dark red, or the critical temperature, and then quenched (cooled quickly by dipping in water or other solution), it will be made very hard, the degree of hardness depending upon the carbon content of the steel and the rapidity of cooling. The higher the carbon content, the harder it will be; and the more rapid the cooling, the harder it will be.
A tool thus hardened is too hard and brittle and must be tempered, or softened somewhat. This is done by reheating the tool to a certain temperature (always below the hardening temperature) and quickly cooling it again. The amount of softening accomplished will depend upon the temperature to which the tool is reheated. For practical purposes in the farm shop, these temperatures are judged by the color of the oxide or scale on the steel as it is being reheated. A straw color, for example, indicates that the tool has been reheated to a comparatively low temperature, and if quenched on a straw color, it will be rather hard. A blue color, on the other hand, indicates that the tool has been reheated considerably higher, and, if quenched on a blue, it will be softer.
306. Hardening and Tempering a Cold Chisel.—After a cold chisel is forged and annealed, it may be hardened and tempered as follows:
1. Heat the end to a dark red, back 2 or 3 in. from the cutting edge.
2. Cool about half of this heated part by dipping in clean water and moving it about quickly up and down and sideways, until the end is cold enough to hold in the hands.
3. Quickly polish one side of the cutting end by rubbing with emery cloth, a piece of an old grinding wheel, a piece of brick, or an old file.
4. Carefully watch the colors pass toward the cutting end. The first color to pass down will be yellow, followed in turn by straw, brown, purple, dark blue, and light blue.
5. When the dark blue reaches the cutting edge, dip the end quickly into water and move it about rapidly. If much heat is left in the shank above the cutting edge, cool this pari slowly so as not to harden the shank and make it brittle. This is done by simply dipping only the cutting end and keeping it cool while the heat in the shank above slowly dissipates into the air.
6. When all redness has left the shank, drop the tool into the bucket or tub until it is entirely cool.
Fig- 208.—Tempering a cold chisel.
A. The end i« heated to cherry red back about 3 in. from the cutting edge. Then about half the heated portion is cooled in clean water, moving the tool about rapidly, up and down and sideways, to prevent too sharp demarcation between the hot and cold parts.
B. The end is then quickly polished by vigorous rubbing with emery cloth or other abrasive to enable the colors to be seen as they pass down. When a dark blue appears at the cutting edge, the end o( the tool—and only the end—is again dipped, working it up and down and around, and keeping it cold while any heat in the shank of the tool is slowly given up to the air.
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