Hot forging refers to the forging process performed above the metal's recrystallization temperature. It is also known as hot die forging. In this process, the metal undergoes significant deformation and flow. The contact time between the workpiece and the die is relatively long. Therefore, the die material must have high thermal stability, high-temperature strength and hardness, impact toughness, thermal fatigue resistance, and wear resistance. It should also be easy to machine. For hot forging dies with lighter workloads, low-alloy steel is often sufficient.
The core of the hot forging process is to heat the metal billet. Heating increases the metal's plasticity and reduces its deformation resistance. This allows the metal to form smoothly in the die. By controlling factors such as forging temperature, die design, and pressure, high-quality structural properties can be achieved. Hot forging is widely used in industries such as aerospace, automotive, and machinery manufacturing. It plays an irreplaceable role in producing high-strength and high-precision products.
Importance of Pre-Forging Heating
The heating process of metal billets before forging is essential. It improves metal plasticity, reduces deformation resistance, enhances flowability, and ensures the final forged microstructure. Heating not only makes the metal more plastic but also reduces friction and wear during forging. This improves the quality and production efficiency of the forged parts. A good heating process can also save energy and reduce production costs. Therefore, pre-forging heating directly affects forging productivity, quality control, and energy conservation.
The main goal of pre-forging heating is to ensure that the metal billet can form smoothly at an appropriate temperature. This avoids poor metal properties or excessive oxidation caused by temperatures that are too low or too high. The choice of heating method depends on the material, size, and forging requirements of the billet.
Flame Heating
Flame heating uses fuel (such as coal, coke, heavy oil, diesel, or gas) in a furnace to generate high-temperature gases. These gases transfer heat to the metal billet through convection and radiation. Flame heating is one of the most common heating methods in forging. It has advantages such as a wide range of fuel sources, simple equipment, and low cost.
1. Convection and Radiation Heat Transfer
When the temperature is below 600–700°C, heating is mainly achieved through convection. In convection, the flame flows around the billet, transferring heat to the metal surface through thermal exchange between the hot gases and the metal. When the temperature exceeds 700–800°C, radiation heat transfer becomes the main method. In radiation, heat is exchanged between the high-temperature gases and the furnace wall, converting into radiative energy, which is then absorbed by the billet.
2. Advantages and Disadvantages of Flame Heating
Flame heating has several advantages. These include convenient fuel sources, simple furnace structure, and wide applicability, all at a relatively low cost. However, it also has drawbacks. These include poor working conditions, slow heating speed, low thermal efficiency, and difficulty in precise temperature control. Despite these disadvantages, flame heating is still widely used for heating various billets, especially during rough processing and preheating stages of forging.
Electric Heating
Electric heating uses electrical energy to generate heat for heating metal billets. Common methods of electric heating include induction heating, contact heating, resistance furnace heating, and salt bath furnace heating. Electric heating is known for its fast heating speed and high efficiency, although it requires higher equipment investment and precise operational control.
1. Induction Heating
Induction heating involves passing alternating current through an induction coil. This creates an alternating magnetic field that induces eddy currents in the metal billet, heating it. Since the electric current is denser near the metal surface, the surface layer of the billet heats first. The inner layers receive heat through conduction. To increase heating speed, lower current frequencies are used for larger billets, while higher frequencies are used for smaller billets.
2. Contact Heating
Contact heating involves passing low-voltage, high-current directly through the metal billet. Metal resistance generates heat as the current passes through. This heating method is suitable for small, uniformly shaped billets. It offers fast heating, high energy efficiency, and low cost. Contact heating is commonly used for heating long billets either entirely or in specific areas. However, it requires a smooth surface finish and precise shape, and it is difficult to control temperature accurately.
Forging Temperature Range
The forging temperature range refers to the interval between the starting forging temperature (initial forging temperature) and the ending forging temperature (final forging temperature). A reasonable forging temperature range ensures that the metal has high plasticity and low deformation resistance during forging, resulting in high-quality forged products. In general, a wider forging temperature range is beneficial. It improves production efficiency and reduces the number of heating cycles.
The forging temperature range for steel is generally determined according to its phase diagram. For carbon steel, the iron-carbon phase diagram can be directly referenced. For alloy steels, the forging temperature can often be estimated by referring to carbon steels with similar carbon content. However, for high-alloy steels with poor plasticity or steels that do not undergo phase transformation (such as austenitic steels or pure iron steels), the appropriate forging temperature range must be determined experimentally.
Conclusion
Hot forging processes and heating methods play a critical role in metal forging. The proper selection of heating methods improves forging efficiency and enhances the quality and performance of forged parts. Flame heating and electric heating each have their advantages and disadvantages. Selecting the most suitable method can optimize the forging process to meet different production needs. Scientifically setting the forging temperature range improves metal plasticity, reduces deformation resistance, and produces high-quality forged parts, thus enhancing production efficiency. In hot forging, optimizing heating methods and temperature ranges is key to improving overall forging quality. In modern industrial production, effectively utilizing energy, improving equipment efficiency, and precisely controlling production processes are crucial factors in increasing productivity and product quality.
