In the field of metal processing, forging is a crucial process. It shapes metal through plastic deformation under applied pressure, producing parts with specific shapes and properties. Carbon steel, as one of the widely used metal materials, receives particular attention regarding its forging process. Accurately determining the forging temperature range of carbon steel plays an indispensable role in ensuring forging quality, improving production efficiency, and optimizing product performance. This article will explore in depth the methods for determining the temperature range of carbon steel forging and the scientific principles behind it, helping readers better understand and master this key process.
The forging temperature range of carbon steel primarily includes two key parameters: the starting forging temperature and the final forging temperature. The starting forging temperature refers to the temperature of the billet when forging begins, while the final forging temperature refers to the temperature at the end of forging. Determining these two temperatures requires comprehensive consideration of multiple factors, including the chemical composition of carbon steel, its microstructure, and the requirements of the forging process.
Before delving into the entire forging temperature range, let us first focus on determining the starting forging temperature, which is a critical step in the forging process.
When determining the starting forging temperature of carbon steel, the primary principle is to ensure that the billet does not experience overburning during heating. Overburning refers to a situation in which the metal temperature becomes excessively high during heating, causing rapid grain growth or even grain boundary oxidation and melting. Once overburned, the mechanical properties of the metal will drop significantly, failing to meet usage requirements. Therefore, the starting forging temperature of carbon steel should be 150°C–250°C lower than the solidus line of the iron–carbon phase diagram. The solidus line refers to the temperature above which a liquid phase begins to appear in the metal. Maintaining a temperature below the solidus line within a certain range can effectively prevent overburning.
In addition to avoiding overburning, it is also necessary to prevent overheating. Overheating refers to a condition where the metal temperature is too high during heating, and although it does not reach the level of overburning, grain growth still occurs to a certain extent, reducing the metal's toughness and ductility. Therefore, when determining the starting forging temperature, it is necessary to comprehensively consider the chemical composition and microstructure of carbon steel, ensuring that it achieves good plasticity without overheating. Generally, the starting forging temperature of carbon steel decreases with an increase in carbon content. This is because higher carbon content lowers the melting point of the steel, increasing the risks of overburning and overheating. For alloy steels, the starting forging temperature decreases even more with increasing carbon content, as the addition of alloying elements further affects the melting point and structural stability, requiring more careful temperature control during heating.
The final forging temperature is also a critical parameter in the forging process of carbon steel, directly affecting the final performance and quality of the forged part. A reasonable final forging temperature ensures that the metal maintains good plasticity and structural performance at the end of forging, meeting subsequent processing and usage requirements.
When determining the final forging temperature, the first consideration is to ensure that the metal retains sufficient plasticity before the end of forging. If the final forging temperature is too low, the plasticity of the metal will decrease significantly, causing work hardening in the later stages of forging and potentially leading to cracks. Additionally, excessively low final forging temperatures may result in local critical deformation zones, forming coarse grains that affect the final performance of the forged part. Therefore, the final forging temperature must not be too low.
At the same time, the final forging temperature should not be too high. Excessively high temperatures can coarsen the grains of the forged part and produce abnormal structures such as Widmanstätten patterns upon cooling. These abnormal structures reduce the mechanical properties and service life of the forged component. Therefore, the final forging temperature should be controlled within a suitable range while ensuring sufficient plasticity to achieve good structural performance. Generally, the final forging temperature of steel should be slightly higher than its recrystallization temperature. Recrystallization refers to the process in which metal grains are rearranged and refined through heating after cold deformation, restoring plasticity and toughness. A final forging temperature slightly above the recrystallization temperature ensures dynamic recrystallization during forging, refining grains and achieving a uniform microstructure. According to this principle, the final forging temperature of carbon steel is approximately 25°C–75°C above the A-line of the iron–carbon phase diagram. The A-line represents the temperature at which austenite begins to form during heating. Operating above the A-line promotes the formation and stability of austenite, ensuring plasticity and uniform microstructure during forging.
After discussing the principles of determining the final forging temperature, we will further analyze the characteristics of final forging temperatures for carbon steels with different carbon contents. Carbon steels can be classified as low, medium, or high carbon steels, each having unique microstructures and performance characteristics, which result in different final forging temperature requirements during forging.
The final forging temperature of medium carbon steel lies in the austenite single-phase region. Austenite is a high-temperature phase of carbon steel with a face-centered cubic lattice, offering good plasticity and low deformation resistance. Forging in the austenite single-phase region ensures a uniform microstructure and excellent plasticity, fully meeting the requirements of final forging. At this temperature range, the steel has a single-phase structure, no phase transformation occurs during deformation, and metal flow is more uniform, resulting in high-quality forged components with good structural performance.
The final forging temperature of low carbon steel is within the austenite–ferrite dual-phase region, but since both phases have good plasticity, forging is not difficult. Ferrite, which forms at lower temperatures, has a body-centered cubic lattice, good plasticity, and higher deformation resistance. Forging in the dual-phase region allows the cooperative deformation of austenite and ferrite, ensuring sufficient plasticity. The presence of ferrite also helps refine grains to some extent, improving the strength and toughness of the forged part.
The final forging temperature of high carbon steel is within the austenite–cementite dual-phase region. Cementite is a hard carbide phase with poor plasticity. Forging in this temperature range allows plastic deformation to break cementite into fine dispersed particles, improving the metal's microstructure and properties. However, forging above the Acm line can lead to the formation of network-like cementite along grain boundaries, which reduces toughness and may cause cracking. Therefore, the final forging temperature of high carbon steel must be strictly controlled within the appropriate dual-phase range to ensure forging quality and performance.
During carbon steel forging, temperature control and lubrication are two critical aspects. Accurate control of forging temperature ensures the metal's plasticity and structural stability, while proper lubrication reduces friction between the metal and the die, improving forging efficiency and surface quality.
Hot forging: The heating temperature of carbon steel is generally 1100°C–1250°C, ensuring good plasticity and low deformation resistance for various complex forging operations. However, high temperatures may cause local overheating and overburning, so uniform heating with proper equipment, such as a soaking furnace, is necessary.
Warm forging: Heating is relatively lower, generally 800°C–950°C, reducing energy consumption. Metal flow is more stable during warm forging, achieving better dimensional accuracy and surface quality, though it requires dies with sufficient strength and wear resistance.
Cold forging: The processing temperature is generally at or slightly above room temperature. Cold forging increases strength and hardness but requires strict control of deformation to prevent defects such as cracks.
In hot forging, lubricants such as graphite are applied to dies to reduce friction between metal and die. Graphite offers excellent lubrication and high-temperature stability. In cold forging, lubricants such as mineral oils, animal oils, or vegetable oils are applied to reduce friction and wear, improving forging efficiency and surface quality.
Determining the forging temperature range of carbon steel is a complex and important process that requires comprehensive consideration of chemical composition, microstructure, and process requirements. By reasonably determining the starting and final forging temperatures, the forging process can proceed smoothly, efficiency can be improved, and the microstructure of the forged part can be optimized. Carbon steel forging has wide applications in the metal processing field and notable advantages, enabling the production of various high-quality metal parts to meet the demands of different industries.
