ROYCEO
Metal heating and forging process introduction
1. Forging temperature range
Forging temperature range is the interval from the initial forging temperature to the final forging temperature of forgings . The initial forging temperature is the maximum allowable temperature for forging of various metal materials. Steel initial forging temperature is usually lower than the solidus 100-200 ℃. Final forging temperature is the temperature of a variety of metal materials that must stop forging. Steel final forging temperature is generally around 800 ℃. Forging temperature range of various types of steel in Table 1.
Table 1 forging temperature range of various types of steel
type of steel |
Initial temperature/℃ |
Final temperature/℃ |
Carbon structural steel |
1280 |
700 |
High-quality Carbon structural steel |
1200 |
800 |
Alloy structural steel |
1150 -1200 |
800 - 805 |
Carbon tool steel |
1100 |
770 |
Heat-resistant steel |
1100 -1150 |
850 |
Carbon steel forging, should be carried out in the single-phase austenitic region, because austenitic tissue has excellent plasticity. However, in order to expand the forging temperature range and strive for more operation time, the final forging temperature of mild steel can generally be reduced to below the GS line two-phase zone. For hypereutectoid steels, if forging is stopped above the ES line, reticular secondary cementite will appear in the forgings when cooled to room temperature, affecting the mechanical properties of the forgings. For this purpose, the forging should be stopped at 50 - 70 ° C above the PSK line to crush the reticular secondary cementite that has just precipitated.
2. Burning and decarburization
When the steel is heated, the oxidation reaction takes place between the iron element and the oxidizing medium in the furnace gas to form the scale. The oxide thickness of large forgings can up to 7-8mm. The loss due to the scale fall off in the steel heating process ,is known as burning. The amount of burning of each heating is up to 1% -3% of the quality of the steel. In addition, the high hardness of the scale, will speed up mold wear, thus affecting the quality of forgings and mold life.
In the high temperature condition of steel, not only the metal surface is strongly oxidized, but also the carbon in the surface metal is lost due to oxidation. Steel in the heating process, the surface loss due to oxidation is usually referred to as decarburization. When decarburization is serious, decarburization layer thickness is up to 1.5 -2 mm, the workpiece surface will have a shallow turtle-like cracks. The mechanical properties of decarburized layer is not good, should be cut in the cutting process.
When heating, we must control the temperature, time and furnace gas composition, to prevent serious oxidation loss and decarbonization phenomenon.
3. Overheating and over burning
Overheating means the metal is heated to too high temperature or kept for a long time at high temperature caused the phenomenon of coarse grains. The overheating billet forging performance is not good, forging mechanical properties are poor. Annealing heat treatment is often used in the production of steel to get overheating microstructure refinement.
Over burning means the heating temperature exceeds the initial forging temperature too much, appear oxidation and melting phenomenon in the grain boundaries . The plasticity of over burning metal is completely lost, will be broken when forging. Over burning is an irreparable heating defect. Production must strictly control heating temperature and holding time to prevent overheating and over burning.