Forging process  part1

The forging temperature range refers to the interval between the forging temperature (called the initial forging temperature) and the stop forging temperature (called the final forging temperature).
Determination of initial forging temperature
Under the premise of no heating defects such as overheating and overheating, the initial forging temperature should be increased as much as possible to make the metal have good forgeability. The initial forging temperature is generally controlled at 150 to 250 ° C below the solidus line.

Determination of final forging temperature
If the final forging temperature is too high, the grain of the metal will continue to grow after the forging process stops, and the mechanical properties of the forging will also decrease. If the final forging temperature is too low, the metal recrystallization will be insufficient, the phenomenon of work hardening will be serious, and the internal stress will increase, may even cause cracks in forgings.

defects that are easily generated in metal heating
oxidation and decarbonization
After the steel is heated to a certain temperature, the iron on the surface layer and the oxidizing gas (02, C02, H20, SO2) in the furnace produce chemical reaction to form oxide scale on the surface layer of the steel (iron oxides FeO, Fe304, Fe203). This phenomenon is called oxidation. The thickness of the scale peeled off from the surface of the large forging can reach 7-8 mm, and the loss caused by the formation of scale in the heating process of the steel is called burning loss.

When the steel is heated to a high temperature, the carbon in the surface layer is oxidized by O2, C02 in the furnace or chemically reacted with hydrogen to form CO or methane and is burned off. This phenomenon in which the content of carbon in the surface layer is lowered when the steel is heated is called decarburization.

Decarburized steel softens the surface of the workpiece and reduces strength and wear resistance. The higher the mass fraction of carbon in steel, the easier it is to decarburize when heated.
Methods of reduce decarburization
apply quick heating
Shorten the heating time in the high temperature stage, and forge the heated billet as soon as possible
Apply a protective coating to the surface of the blank before heating.

overheated
Overheating refers to a phenomenon in which the metal heating temperature is too high and the heating time is too long to cause coarse grains. Overheating reduces the forgeability and mechanical properties of the billet, and it must refine the grains by annealing to eliminate the superheated structure. The billet which cannot be annealed can be repeatedly forged to improve the grain size.