VI.
HEATING FOR FORGING; FOR HARDENING;
FOR WELDING.
BURNING, OVERHEATING, RESTORING.
From what has been said already about the effects of heat it follows without further argument that heating is one of the most important, or perhaps more properly the most important of all, of the operations to which steel has to be subjected.
The first and vital thing to be borne in mind is that all heating should be uniform throughout the mass. It has been shown that heat affects the grain, the structure, as surely as it moves the mercury-column, and such being the case it is plain that as perfect uniformity as it is possible to attain is the first essential for all heating, no matter what the ultimate object may be.
In heating for forging the limit lies between the point of recalescence, the beginning of true plasticity, and the granular condition, the end of plasticity; these temperatures lie between dark or medium orange for all steels and medium or light lemon on the upper limit, depending on the carbon content, or lower if it be an alloy steel.
If there is much work to be done upon a piece of steel, it is well to heat at first to as high a temperature as is safe, and then to forge or work heavily at the higher heat, reducing the blows or passes as the piece is reduced and the temperature falls. Although this high heating will raise the grain of the steel, the heavy working will bring it back to a fine, compact structure.
If little work is to be done, then it is better to heat as low as may be safe, and allow the work to be done without letting the heat down below orange red, so that the steel may not be crushed in the grain.
Below orange red, the so called “dark cherry,” steel should not be forged, except that in forging for fine tools it is well to give many light and rapid blows until black begins to show in order to hammer-refine it; this must be done with extreme care so as not to crush the steel and cause cracking in the subsequent hardening, or crumbling in the hardened tool.
HEATING FOR HARDENING.
When a piece of steel is to be hardened by quenching in water or any quick-cooling medium, it should be heated with great care to the exact temperature to produce the required hardness.