The process is heating, and then slow cooling. The steel should be heated a little above its critical temperature; for if heated below this temperature, all strain in the steel is not released and warping takes place when the hardening heat is reached.
As is noted under "Hardening", the rapidity of cooling determines the final hardness of the steel and if the steel is cooled very slowly it will be left very soft; while if cooled rapidly, it will be left hard. This difference in the cooling time is the only difference between its hardening and its annealing. Both should be done from the same heat.
For good results in annealing high-speed steels, the following rules must be observed:
(1) Heat should be as near to 1500 degrees as possible. If the heat reached were only 1250 degrees, the steel would retain its original hardness. If heated to 1700 or 1800 degrees, the steel would be soft but would show brittleness with a coarse grain. If there were a further increase in heat, the steel would become hard and unannealed, and the fracture from this temperature would be dull and lifeless and would show a marked decarbonization.
(2) The steel should be packed in boxes or pipes, in powdered charcoal or lime, and sealed with fire clay, and in packing it should be seen that no part of the steel comes in contact with the box or pipes. If the parts come together, that does no harm.
(3) The heating should be started slowly and there should be given plenty of time for cooling. If a forge fire is used for annealing, leave the box in the fire and let it remain until cold, or if the forge fire has to be used for other purposes, the box can be put in some place out of the way and then covered with ashes, lime, or better still, charcoal. In all cases a slow cooling is necessary. If a furnace is used, heat slowly to full red or about 1500° Fahrenheit. Hold the heat long enough to be sure of penetration through the box and the steel to be annealed, and then shut off the heat and let the steel cool off with the furnace.
Another method is water annealing. If only a few pieces are to be annealed for quick use, heat the steel to between 750° and 800° Fahrenheit, plunge it into water which has been heated to about 150° or 160° Fahrenheit, and allow it to remain until it becomes the same temperature as the bath. The heating should be slow and uniform for good results. To heat rapidly will cause internal strains and is apt to increase the risk of rupture when the steel is plunged into the water.
This last method is not to be recommended as it is not effective on all high-speed steels.
The treatment of these two metals consists in heating them to a red heat and then cooling suddenly in cold water. When copper or brass is hammered to any extent, it becomes hard and springy, and, if it has to be further worked, it must be annealed or softened, otherwise it is almost sure to split.