The ultimate object is to secure a fine, uniform grain throughout the piece and this can be secured by uniform heating and by thoroughly rolling it or working it at a temperature just down to its critical point. If this is correctly done the fracture will be fine and silky. Steel which has been overheated slightly and the forging stopped at too high a temperature will show a "granular" fracture. A badly overheated or "burned" steel will have iridescent colors on a fresh fracture, it will be brittle both hot and cold, and absolutely ruined.
Steel Can be Worked Cold.—As noted above, steel can be worked cold, as in the case of cold-rolled steel. Heat treatment of cold-worked steel is a very delicate operation. Cold working hardens and strengthens steel. It also introduces internal stresses. Heat-treatments are designed to eliminate the stresses without losing the hardness and strength. This is done by tempering at a low heat. Avoid the "blue" range (350 to 750°C.). Tempering for a considerable time just under the critical is liable to cause great brittleness. Annealing (reheating through the critical) destroys the effect of cold work.
FORGING
High-speed Steel.—Heat very slowly and carefully to from 1,800 to 2,000°F. and forge thoroughly and uniformly. If the forging operation is prolonged do not continue forging the tool when the steel begins to stiffen under the hammer. Do not forge below 1,700°F. (a dark lemon or orange color). Reheat frequently rather than prolong the hammering at the low heats.
After finishing the forging allow the tool to cool as slowly as possible in lime or dry ashes; avoid placing the tool on the damp ground or in a draught of air. Use a good clean fire for heating. Do not allow the tool to soak at the forging heat. Do not heat any more of the tool than is necessary in order to forge it to the desired shape.
Carbon Tool Steel.—Heat to a bright red, about 1,500 to 1,550°F. Do not hammer steel when it cools down to a dark cherry red, or just below its hardening point, as this creates surface cracks.
Oil-hardening Steel.—Heat slowly and uniformly to 1,450°F. and forge thoroughly. Do not under any circumstances attempt to harden at the forging heat. After cooling from forging reheat to about 1,450°F. and cool slowly so as to remove forging strains.
Chrome-nickel Steel.—Forging heat of chrome-nickel steel depends very largely on the percentage of each element contained in the steel. Steel containing from 1/2 to 1 per cent chromium and from 1½ to 3½ per cent nickel, with a carbon content equal to the chromium, should be heated very slowly and uniformly to approximately 1,600° F., or salmon color. After forging, reheat the steel to about 1,450° and cool slowly so as to remove forging strains. Do not attempt to harden the steel before such annealing.
A great deal of steel is constantly being spoiled by carelessness in the forging operation. The billets may be perfectly sound, but even if the steel is heated to a good forging heat, and is hammered too lightly, a poor forging results. A proper blow will cause the edges and ends to bulge slightly outwards—the inner-most parts of the steel seem to flow faster than the surface. Light blows will work the surface out faster; the edges and ends will curve inwards. This condition in extreme cases leaves a seam in the axis of the forging.
Steel which is heated quickly and forging begun before uniform heat has penetrated to its center will open up seams because the cooler central portion is not able to flow with the hot metal surrounding it. Uniform heating is absolutely necessary for the best results.