In modern manufacturing, low-carbon forgings are widely used in various types of machinery equipment due to their excellent mechanical and machining properties. From automotive components to key parts in the aerospace field, low-carbon forgings play an indispensable role. As a crucial process to enhance the performance of low-carbon forgings, the importance of heat treatment is self-evident. This article will delve into the commonly used heat treatment processes for low-carbon forgings, including recrystallization annealing, normalizing, and spheroidizing annealing, and analyze their applications in actual production.
Low-carbon forgings such as 20 forgings and A105 are typically supplied to the market in the form of hot-rolled or cold-rolled profiles. Hot-rolled profiles are rolled at high temperatures, with the final rolling temperature above the material's recrystallization temperature. After final rolling, the material is slowly cooled through the recrystallization zone, a process that is essentially equivalent to recrystallization annealing. The grain size and shape of the hot-rolled profiles have been restored to their original state, resulting in good uniformity and stability.
Cold-rolled profiles, on the other hand, are produced by pressure working at room temperature. During cold rolling, dislocation movement occurs within the material's microstructure, leading to work hardening. If there is significant residual internal stress within the material, recrystallization annealing is usually required to restore the elongated or deformed grain size. Recrystallization annealing can not only eliminate work hardening but also effectively remove internal stress, bringing the material back to a more desirable state.
In the matrix microstructure of low-carbon forging profiles, the volume fraction of ferrite is usually above 70%. Ferrite, a body-centered cubic phase, has good ductility and toughness. However, the relatively large interlamellar spacing of ferrite and cementite in the small amount of pearlite results in a relatively low hardness of the material. During machining, chips tend to adhere to the cutting tool, affecting machining efficiency and surface quality. Appropriate heat treatment processes, such as normalizing, can effectively increase the material's hardness, improve machinability, and eliminate abnormal microstructures in the original material.
Heat treatment processes are crucial for improving the performance of low-carbon forgings, and recrystallization annealing, as one of the important processes, is worth detailed study.
Principle and Function: Recrystallization annealing is an important heat treatment process, the core of which lies in the recrystallization and growth of grains in the material through heating. During cold rolling or cold drawing, the grains within the material are subjected to severe deformation, leading to lattice distortion and the accumulation of internal stress. Recrystallization annealing involves heating the material to a certain temperature to allow the grains to reorient and form fine equiaxed grains. This process not only eliminates work hardening but also effectively removes internal stress, restoring the material's ductility and toughness.
Influencing Factors: The recrystallization annealing temperature is influenced by various factors, including the material's chemical composition, the degree of cold deformation, and the annealing holding time. For low-carbon forgings, the critical degree of deformation is generally between 6% and 10%. The greater the degree of deformation, the lower the recrystallization annealing temperature. When recrystallization annealing is carried out at the critical degree of deformation, abnormal grain growth may occur, which requires special attention to control. For pure metals, the recrystallization annealing temperature can be estimated using the formula T=0.47T(K). As the carbon content increases and the number of alloying elements rises, the recrystallization temperature of low-carbon forgings gradually increases.
Process Parameters: For cold-rolled profiles and cold-drawn rod materials of low-carbon forgings, after cold stamping processing, a recrystallization annealing temperature of 650℃-700℃ is generally appropriate. The holding time depends on the loading method, the amount of material loaded, and the type of furnace. For example, the holding time in a salt bath furnace is relatively short, in an air furnace it is shorter than in a salt bath furnace, and in a vacuum furnace it is shorter than in an air furnace. When performing recrystallization annealing in an air furnace, the holding time should be more than 2 hours. For large workpieces or when the amount of material loaded is high, the holding time should be extended appropriately. The specific holding time can be determined through process trials. After high-temperature holding, the material can be directly removed from the furnace and air-cooled.
In addition to recrystallization annealing, normalizing is also an essential process for enhancing the performance of low-carbon forgings. Normalizing can further optimize the material's hardness and machinability, eliminate abnormal microstructures in the original material, and provide a better basis for subsequent processing and use.
Principle and Function: Normalizing is a commonly used heat treatment process aimed at improving the microstructure and properties of the material through heating and cooling. It can increase the material's hardness, improve machinability, and eliminate abnormal microstructures in the original material. For low-carbon forging profiles, normalizing can effectively increase their hardness, making them less likely to stick to the cutting tool during machining, thereby improving machining efficiency and surface quality.
Process Parameters: The commonly used normalizing temperature is between 900℃ and 960℃. After holding, the material should be air-cooled. The holding time should be more than 1 hour. The hardness after normalizing is generally around 143HBW. Through normalizing treatment, the microstructure of low-carbon forgings is optimized, and their hardness and machinability are significantly improved.
Spheroidizing annealing can cause the carbides in the material to be distributed in a spherical shape, significantly reducing hardness while improving toughness and machinability, preparing for subsequent processes.
Principle and Function: Spheroidizing annealing is a heat treatment process specifically used to improve the microstructure of low-carbon forgings. Its purpose is to distribute the carbides in the material in a spherical shape through heating and cooling processes, thereby reducing the material's hardness and improving machinability. The material after spheroidizing annealing has good machinability and mechanical properties, making it suitable for subsequent cold working and heat treatment processes.
Process Parameters: For 10, 15, and 20 forgings, isothermal spheroidizing annealing treatment should be carried out at 20-30℃ above or below the Ac1 line. Through this treatment, a spheroidal structure can be obtained, with the hardness controlled around 130HBW. For larger rivets, the hardness is usually controlled between 110-120HBW. The spheroidizing annealing process generally involves heating at a rate of 100-150℃/h, holding for a period in the isothermal zone, and then slowly cooling to room temperature. The microstructure after spheroidizing annealing should meet the specific requirements of JB/T 5074-2007 "Rating of Spheroidal Structures in Low and Medium Carbon Forgings", and it is generally advisable to control it at levels 4-6.
Carburizing can significantly increase the surface hardness and wear resistance of low-carbon forgings, enabling them to perform well under high-stress and wear conditions. Therefore, understanding the application of low-carbon forgings in carburizing processes is crucial for fully leveraging their performance advantages.
Process Requirements: Low-carbon forgings such as 15, 20, and 10 forgings are often used as parts for carburizing or carbonitriding. These parts are generally forged into shape. After forging, a normalizing treatment at a temperature higher than the carburizing temperature is required. The purpose of normalizing is to eliminate defects produced during the forging process, increase the material's hardness for easier subsequent machining, and verify whether the grains will grow after normalizing to ensure that the material's microstructure and properties meet the requirements.
Practical Application: In actual production, after normalizing treatment, the hardness and machinability of low-carbon forgings are significantly improved. This makes subsequent machining smoother and ensures machining efficiency and surface quality. At the same time, by controlling the normalizing temperature and holding time, abnormal grain growth can be effectively avoided, ensuring the uniformity and stability of the material's microstructure. This is of great significance for improving the service life and reliability of the parts.
Low-carbon forgings, with their excellent properties, have found widespread application in modern manufacturing. Through rational heat treatment processes such as recrystallization annealing, normalizing, and spheroidizing annealing, the microstructure and properties of low-carbon forgings can be effectively improved to meet the requirements of different application scenarios. In actual production, appropriate heat treatment process parameters should be selected based on the specific material composition, processing technology, and usage requirements to ensure the quality and performance of low-carbon forgings. With continuous technological advancements, the heat treatment processes of low-carbon forgings will become more refined and efficient, providing stronger support for the development of the manufacturing industry.
