by J.M.Drew St. Paul Publishing Company 1918
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FORGING AND TEMPERING STEEL TOOLS
The making and tempering of simple steel tools, such as cold chisels, drills, etc., and the welding and sharpening of plow points, should be well understood by the farm blacksmith. Let us imagine that we have a bar of tool steel in the fire and are about to make a very simple tool, say a cold chisel. The first and principal thing to have in mind is that we must not overheat the steel. Steel, owing to the carbon which it contains, is much more easily burned than iron, and the beginner is almost sure to burn the first steel he attempts to work, unless he is warned in regard to it.
The fire for steel work should be clean; that is, it should consist of a body of burning coke. Fresh coal, owing to the sulphur which it contains, has a bad effect upon steel. The steel must be heated slowly and evenly, in order to be of the same temperature and consequently the same degree of softness throughout. If it is heated too quickly the outside will be softer than the center, and will be drawn out faster as we draw out the end of the chisel, and though we cannot see any defect at the time, a crack will be apt to develop when the tool is tempered. Heat to a light red or yellow color, and draw out to the shape of a wedge. Hammer it on all sides alike as nearly as possible. Do not let the sides spread out like a dove's tail, but keep them straight with the original bar.
Draw out somewhat thinner and longer than needed; then cut off three-eighths or one-half inch, to insure an edge of sound steel. Shape the head end as shown in Fig. 32. Finish the forging by giving it a good hammering. By "good hammering" I mean a hammering that shall refine the steel at the edge of the tool and correct any overheating that it may have suffered while being forged. By "overheating" is meant any heating above the heat that will give the finest grain to the steel when it is tempered. Overheating differs from burning. Burnt steel cannot be refined or "restored." Throw it away. Overheated steel may be refined or restored to a fine grain by proper hammering while it is at just the right heat. To do this to our chisel we will heat it to a very dull red and hammer it quickly on both flat sides (not on the edges), beginning with quite heavy blows and striking lighter as the steel cools; stopping altogether as the red color disappears. This hammering will probably spread the edge of the chisel wider than you want it, but do not strike it on the edge, as that will undo what you have accomplished by hammering it on the sides. The sides can be ground off or filed off afterwards.
Before trying to temper the chisel we will allow it to cool slowly and then file it to an edge. While it is cooling, try the following experiment, which will show you, more plainly than any amount of reading, the effect of heat upon steel. Take a bar of tool steel, and after heating it to a red heat notch it on the hardy so that it will be cut nearly half off every half inch of its length for three inches. Now hold and turn it in the fire in such a way that the extreme end will begin to burn and throw off sparks before the last section or notch becomes red. This will require considerable care in handling and turning the piece in the fire. The different sections should now form a gradual scale of color, from white and sparkling (burning), down through the different shades of a red to black. When it is in this condition take it from the fire and plunge it quickly into cold water and move it about so as to cool it as quickly as possible. When it is cold break each section off by holding the notch over the corner of the anvil and striking a blow with the hammer. The first two or three pieces will break off very easily, each succeeding section showing more toughness till the last, which will probably be very hard to break from the bar. After gathering up the pieces fit them together in their original positions; then turn the upper or bar end of each section towards you, and you will have an object lesson on the effect of different degrees of heat upon steel. The sections which were overheated show a coarse, hard grain, and, as we discover in breaking them, are brittle. This coarseness of grain diminishes as we go up the scale toward the place where the bar was heated to a dull red. Here we find the grain the finest; even finer than in the original bar. In the photo engraving the first two cuts show the grain of steel that has been overheated, No. 1 being much more badly burned than No. 2. No.3, which was heated to a dun red, is seen to have the finest grain. No. 4 is the end of the original bar. It is thus seen that proper heating produces a finer grain than the original steel contained.