The mechanical properties of stainless steel forgings after residual heat quenching are influenced by multiple factors, including heating temperature, forging deformation temperature, deformation amount, deformation speed, holding time, quenching medium, and tempering temperature. By rationally controlling and optimizing these factors, the mechanical properties of stainless steel forgings can be significantly improved, thereby meeting the requirements of various industrial applications. This article will explore these influencing factors in detail and their practical applications in production, aiming to provide systematic and comprehensive theoretical support and technical guidance for the residual heat quenching process of stainless steel forgings.
The Influence of Heating Temperature on Forging Performance
Heating temperature is one of the key factors affecting the mechanical properties of stainless steel forgings after residual heat quenching. During the heating process, the forging undergoes austenitization. As the heating temperature increases, the austenitization process gradually progresses. For hypoeutectoid steel, pearlite first transforms into austenite, followed by the gradual dissolution of ferrite into austenite, ultimately forming a uniform austenitic structure. When the temperature further increases, austenite grains grow. Although coarse austenite grains can be refined through deformation or recrystallization, their grain size is generally larger than that achieved through conventional quenching and tempering.
1. The Effect of Austenitization Temperature
As the austenitization temperature changes, the strength and hardness of the forging remain relatively stable, but plasticity and toughness significantly improve. For example, when the austenitization temperature of 40Cr steel decreases from 1280°C to 1070°C, its impact toughness increases by 40% to 100%, elongation increases by 31% to 77%, and reduction of area increases by 26% to 121%. This improvement is primarily due to the fact that lower austenitization temperatures refine the carbides precipitated during tempering, thereby enhancing the forging's plasticity and toughness. For general structural steels, the austenitization temperature is typically set around 1200°C, which does not significantly affect the strength after residual heat quenching but helps improve plasticity and toughness.
2. Optimal Heating Temperature
In summary, the optimal heating temperature during forging should be maintained around 1200°C. This temperature range helps optimize the forging's plasticity and toughness without negatively impacting its strength and hardness. Excessive heating temperatures may lead to overly coarse grains, reducing the mechanical properties of the forging, especially in applications requiring high strength and toughness. Therefore, controlling the heating temperature is particularly important.
The Relationship Between Forging Deformation Temperature and Plasticity/Toughness
The forging deformation temperature directly affects the microstructure and mechanical properties of the forging. By lowering the forging deformation temperature (e.g., initial and final forging temperatures), the strengthening effect of the forging can be preserved, thereby enhancing its strength. However, excessively low deformation temperatures may reduce the material's plasticity and toughness, making it crucial to control the deformation temperature appropriately.
1. Advantages of Low-Temperature Deformation
Low-temperature deformation helps refine the carbides precipitated during tempering, significantly improving the forging's strength and toughness. The combination of low-temperature austenitization and low-temperature deformation is an effective way to enhance the forging's plasticity and toughness. In practical production, low-temperature austenitization and deformation are recommended for forgings requiring improved plasticity and toughness.
2. Recrystallization After Deformation
After deformation, the forging undergoes recrystallization during tempering at an appropriate temperature, which helps improve its microstructure. For alloy steel forgings with low deformation temperatures and small deformation amounts, recrystallization is less likely to occur. Therefore, controlling the holding time is necessary to form a polygonal grain structure that enhances strength.
The Influence of Post-Forging Holding Time on Mechanical Properties
After forging, the forging typically undergoes trimming and correction, during which there is a certain holding time. The length of this holding time directly affects the forging's mechanical properties, particularly its strength, plasticity, and toughness. Generally, shorter holding times make it easier to retain the strengthening effect, but the optimal holding time varies depending on the material and forging process.
1. Relationship Between Holding Time and Material Type
For carbon steel, excessive post-forging holding time can lead to dynamic recrystallization and grain coarsening, so quenching should be performed as soon as possible. For alloy steels, especially those subjected to low-temperature deformation and small deformation amounts, an appropriate holding time helps form a polygonal grain structure, thereby improving the forging's strength and toughness. However, excessively long holding times can reduce strength, so the holding time between forging and quenching should be controlled based on specific performance requirements.
2. Optimization of Post-Forging Holding Time
In general, the post-forging holding time should not be too long. Experiments can determine the optimal holding time for the forging's performance requirements. Optimizing this process is crucial for ensuring consistent and stable forging performance.
The Advantages of Residual Heat Quenching
Residual heat quenching is a process in which the forging is quenched in an appropriate medium at a temperature above a certain critical point to obtain a martensitic or bainitic structure. Compared to conventional quenching and tempering, residual heat quenching offers the following advantages:
Energy Savings: Residual heat quenching saves energy and reduces production costs.
Increased Strength and Hardness: Forgings subjected to residual heat quenching typically exhibit higher strength and hardness than those treated with conventional quenching and tempering.
Simplified Process: Residual heat quenching simplifies the production process, shortens the production cycle, and reduces equipment investment and labor costs.
After residual heat quenching, the forging's strength and hardness are significantly improved while maintaining good plasticity and toughness. An appropriate tempering temperature (usually 40°C to 80°C higher than conventional tempering) helps improve plasticity and toughness, making it comparable to or slightly better than conventional quenching and tempering.
Applications of Different Residual Heat Treatment Methods
Residual heat treatment involves heating the forging and controlling the temperature to achieve specific microstructures and mechanical properties. Common residual heat treatment methods include residual heat normalizing and residual heat isothermal normalizing.
1. Residual Heat Normalizing
Residual heat normalizing involves cooling the forging in a normalizing furnace, cooling chamber, or annealing furnace after heating to a certain temperature, resulting in a pearlitic and ferritic equilibrium structure. Due to the high forging heating temperature, the resulting grains are coarse, making this method suitable for preliminary heat treatment but not for forgings requiring strict grain size control.
2. Residual Heat Isothermal Normalizing
This process involves rapidly cooling the forging to an isothermal temperature after heating, holding it for a period, and then air-cooling it to room temperature. Controlling the cooling rate is key to this process, and adjusting the cooling conditions ensures uniform cooling of the forging. The isothermal temperature is selected based on the material type and desired hardness, typically at the inflection point of the pearlite transformation curve to shorten the holding time.
Conclusion
The residual heat quenching process for stainless steel forgings offers significant advantages in improving mechanical properties, saving energy, and shortening production cycles. By rationally controlling key parameters such as heating temperature, deformation temperature, holding time, and tempering temperature, the strength, plasticity, toughness, and hardness of forgings can be effectively enhanced to meet the needs of various applications. Additionally, different residual heat treatment methods provide multiple options for optimizing production processes and achieving the best material performance.
