In the production of
large forgings, mixed crystallization is a critical problem that affects the performance of the final product. Mixed crystallization refers to the significant inhomogeneity of grain size within the metal structure, often manifesting as differences in grain size exceeding three grades in different regions. This uneven grain structure leads to a decline in the mechanical properties of the forging, severely affecting its service life and quality. Particularly during the heat treatment process, mixed crystallization can result in unusually large grains, which in turn reduce yield strength and impact toughness, and may even complicate internal defect detection. Effectively addressing the mixed crystallization problem is therefore a key technological challenge to improve the quality and performance of large
forgings.
The occurrence of mixed crystallization is typically associated with poor control during the heating, deformation, and cooling processes of the forging. Large forgings, due to their size, present significant challenges in controlling internal temperature distribution and deformation uniformity, leading to an uneven grain structure.
Uneven Temperature Distribution: During the heating process of large forgings, temperature differences between the internal and surface areas can cause inconsistent grain growth rates, resulting in an uneven grain structure.
Inconsistent Deformation: During forging, different parts of the workpiece undergo different amounts of deformation. In thick forgings, for instance, the center often experiences less deformation compared to the outer sections, leading to inconsistent grain refinement.
Improper Heat Treatment: After initial heating and deformation, proper heat treatment is necessary. If factors such as dwell time, temperature control, or cooling rate are not properly managed, this can cause grain coarsening or uneven distribution, leading to mixed crystallization.
The effects of mixed crystallization are multifaceted. First, it directly reduces yield strength and impact toughness, making the forging more prone to fracture under high load or low temperature conditions. Second, mixed crystallization raises the brittle-ductile transition temperature, thus reducing low-temperature impact toughness and the reliability of the forging in harsh environments. Finally, mixed crystallization can also affect internal defect detection, especially with ultrasound testing, where mixed crystallization may lead to spurious or bottomless waves, compromising detection accuracy and, in severe cases, leading to rejection of the forging.
Thus, addressing the mixed crystallization issue and ensuring uniformity of grain size and stability of mechanical properties are urgent tasks in the production of large forgings.
The concept of controlled forging was introduced to tackle the mixed crystallization problem in large forgings. The goal of controlled forging is to achieve grain homogenization and refinement by carefully controlling the temperature, deformation, and cooling parameters throughout the forging process, thus preventing mixed crystallization.
The core of controlled forging technology lies in controlling thermodynamic and mechanical parameters to induce static recrystallization during the forging process, thus achieving a uniform grain structure. Unlike dynamic recrystallization, which requires significant deformation, static recrystallization occurs at a specific temperature and dwell time, where spontaneous nucleation and growth of grains refine and homogenize the grain structure.
Controlled forging technology prevents mixed crystallization through the following methods:
Temperature Control: By accurately controlling the forging temperature, it ensures that the workpiece stays within an optimal temperature range during heating and deformation, preventing excessive differences in grain size due to uneven temperatures.
Deformation Control: Properly designing the deformation amounts for each stage of the forging process ensures that all sections of the workpiece deform uniformly, avoiding areas of excessive or insufficient deformation that could cause grain inhomogeneity.
Static Recrystallization: By leveraging the principle of static recrystallization, the process involves halting or holding the workpiece at specific temperatures after deformation, allowing the grains to refine and homogenize, thus preventing grain coarsening due to improper cooling rates or excessive deformation.
In practical production, controlled forging is often achieved through several thermal processing schemes aimed at controlling mixed crystallization and achieving grain homogenization:
High-Temperature Controlled Forging (1000°C to 1100°C): In this temperature range, the workpiece undergoes static recrystallization, which allows for the formation of a relatively uniform coarse structure. Deformation is usually kept under 20%, and the subsequent treatment, such as normalizing, further refines the grain structure.
Mid-Temperature Controlled Forging (850°C to 950°C): This temperature range is suitable for steels with slower static recrystallization. By controlling the deformation amount (typically less than 10%), a fine and uniform grain distribution is achieved. The recrystallization process at this temperature is more consistent, making it ideal for high-performance forgings.
Low-Temperature Controlled Forging (800°C to 900°C): In this range, austenite undergoes rapid recrystallization. Grain refinement occurs quickly, and a fine and uniform grain structure is obtained in a short period. This process is particularly beneficial for large forgings, where minimal deformation during final shaping can still meet controlled forging goals.
Heat treatment for large forgings is a key process in controlled forging technology. First, the workpiece is heated to a uniform temperature in the furnace, then rapidly cooled to below the transformation point, followed by reheating to 900°C for uniformization. Finally, the workpiece is forged to the desired dimensions. Throughout this process, deformation is controlled to less than 10%, and static recrystallization ensures the homogenization and refinement of the grains, preventing mixed crystallization.
This approach simplifies the complex steps in traditional controlled forging methods and enables grain optimization with minimal deformation during the final finishing stage, providing great potential for large forging production.
Mixed crystallization remains a major challenge in the production of large forgings. It affects not only the mechanical properties of the forging but also internal defect detection, reducing product quality and reliability. Controlled forging technology offers an effective solution. By carefully designing the heating, deformation, and cooling processes, and controlling temperature and deformation amounts, it is possible to achieve grain homogenization and refinement, thereby preventing mixed crystallization. As the technology continues to advance, controlled forging will play an increasingly important role in large forging production, driving the forging industry towards greater efficiency and precision.
