Utilization of Forging Residual Heat

A plethora of practical applications demonstrate that by reasonably controlling the cooling parameters after forging, the microstructure and properties of forgings can achieve or even surpass the levels attained through conventional heat treatment, exhibiting excellent process stability and reproducibility. Employing residual heat for heat treatment obviates the need for reheating during the treatment process, leading to significant energy savings and reduced investments and maintenance costs for heat treatment equipment.

Significance of Residual Heat Utilization

The forging industry is a major consumer of energy, with heat treatment of forgings constituting a substantial portion of the total energy consumption, accounting for approximately 30% to 35% of the entire forging production energy expenditure. In China, the energy consumption per ton of die forging is about 1.0 tons of standard coal, which lags significantly behind industrialized nations like Japan, where it's around 0.515 tons of standard coal per ton of die forging. Forging energy consumption typically represents 8% to 10% of the forging cost. Reducing energy consumption not only lowers forging production costs, enhancing enterprise economic benefits, but also addresses energy sustainability, a crucial factor for a country's development and even global survival. Thus, fully leveraging residual heat in forging for heat treatment offers evident advantages in energy conservation, efficiency enhancement, and environmental protection, concurrently saving energy, shortening process flows, and preserving the environment.

Residual Heat Heat Treatment Techniques

1. Forging Residual Heat Quenching: This method involves quenching the forged component in a suitable quenching medium when its temperature is above Ar3 or within the range of Ar3 to Ar1, to obtain martensitic or bainitic structures. Following forging residual heat quenching and tempering, not only can superior comprehensive mechanical properties be achieved, but energy savings, process simplification, shortened production cycles, reduced labor, and saved investment in quenching furnaces can also be realized.

2. Forging Residual Heat Normalizing (Annealing): After forging, when the temperature exceeds Ar3 (for hypo-eutectoid steels), the component is subjected to normalizing or controlled cooling in a normalizing furnace, cooling chamber, or annealing furnace to obtain a normalizing structure. As the forging heating temperature is high, using this method results in coarse grains in the forgings and is generally employed for pre-heat treatment, unsuitable for forgings with high grain size requirements. Moreover, the obtained structure is a pearlite + ferrite equilibrium structure, with no residual structure inherited from the coarse grains, allowing for grain refinement in subsequent heat treatments.

3. Forging Residual Heat Isothermal Normalizing (Annealing): After forging, when the temperature is above Ar3 (for hypo-eutectoid steels), the component is rapidly cooled and then maintained at an isothermal temperature for a certain period before air cooling to room temperature. The temperature after forging generally ranges from 900 to 1000°C, and the rapid cooling rate is typically controlled between 30 to 42°C/min. Rapid cooling is a critical step in this process, ensuring uniform cooling of the forgings by adjusting cooling airflow, speed, temperature, and direction. The isothermal temperature is determined based on material type and desired hardness requirements, usually selected at the nose of the pearlite transformation curve to shorten the isothermal soaking time. Forging residual heat isothermal normalization is commonly used for carburized gear steels, such as SCM420H, SCM822H, SAE8620H, and 20CrMnTiH.

Key Points in Controlling Residual Heat Treatment Processes

1. Residual Heat Quenching:

   • Ensure a stable and controllable heating system, employing medium-frequency induction heating, infrared thermometers, and a three-channel temperature sorting system for billet heating. This allows for easy control of heating and sorting out billets with inadequate heating temperatures.
   • Determine suitable quenching temperatures and effectively control them. The appropriate forging residual heat quenching temperature needs to be determined through experiments. In practice, it can be achieved by controlling the forging heating temperature and the post-forging dwell time. It's recommended that the post-forging dwell time for carbon steel should not exceed 60 seconds, and for alloy steel, it should range between 20 to 60 seconds.
   • Configure infrared thermometers and temperature sorting systems to identify billets below the quenching temperature and remove them. When both the forging heating temperature and the forging process are stable, a process time measurement and alarm system can be configured to control the quenching temperature by controlling the process time.

2. Post-Quench Tempering and Positioning of Tempering Furnaces:

   • After quenching, internal stresses in the forgings may lead to significant deformation or even cracking during placement. To prevent this, the forgings should be promptly tempered after quenching. The dwell time for the forgings after quenching depends on the material, shape, and ambient temperature and needs to be determined through experiments. For energy conservation and increased utilization of tempering furnaces, as well as reduced insulation energy consumption, forgings treated with residual heat quenching are typically tempered collectively in the heat treatment workshop.

3. Residual Heat Normalizing (Annealing):

   • Properly control the preheating temperature of the forgings before entering the furnace. When the forging temperature is high, it needs to be cooled by blowing air to reduce the temperature to the required annealing temperature. Additionally, the furnace power should have a certain surplus, and preheating should be conducted before production starts or when a small number of forgings have low temperatures.
   • Determine the reasonable soaking time. Excessive soaking time will result in coarse grains, while insufficient soaking time will lead to incomplete structural transformation. The soaking time can be determined through experiments based on the material, shape, and size of the forgings.

4. Residual Heat Isothermal Normalizing (Annealing):

   • Control the temperature of the forgings after forging. The temperature of the forgings after forming must be above Ar3 (for hypo-eutectoid steels). Rapid cooling can be used when the forging temperature is stable after forging. However, if the temperature fluctuates significantly or the forging has a large cross-section variation, an equalizing process must be added to ensure uniform temperature before rapid cooling. Otherwise, significant temperature differences between different sections or after rapid cooling may lead to abnormal structures (bainite or martensite).
   • Control the cooling rate during rapid cooling. Rapid cooling requires quick cooling of the forgings, while ensuring uniform or similar temperatures for the same or batch of forgings after cooling. The cooling rate should generally be controlled between 30 to 42°C/min to prevent the formation of bainitic structures in the forgings.
   • Control the temperature after rapid cooling. After rapid cooling, it's essential to ensure that the forging temperature is within the pearlite transformation range and not lower than the start temperature of bainite transformation (Bs). Otherwise, bainite (or granular bainite) may form in the structure. The temperature after rapid cooling is generally controlled above the material's Bs temperature, around 80 to 100°C higher.
   • Selection of Isothermal Temperature: The choice of isothermal temperature directly impacts the hardness of forgings after isothermal annealing. A higher isothermal temperature results in lower hardness, while a lower isothermal temperature leads to higher hardness. The isothermal temperature typically ranges from 50 to 80°C above the Bs temperature of the forging material, with the specific temperature determined through experimentation based on the material and shape of the forgings.
   • ​Determination of Isothermal Soaking Time: The transformation to pearlite occurs during the isothermal process, necessitating sufficient soaking time. If the soaking time is too short, incomplete transformation of retained austenite into pearlite may occur, leading to the subsequent formation of bainite or martensite during cooling, resulting in unsatisfactory structure and high hardness after isothermal treatment. The isothermal soaking time can be initially determined based on the material's isothermal transformation curve and adjusted according to experimental results.

Production practices demonstrate the feasibility of utilizing residual heat for heat treatment. By effectively controlling the cooling parameters after forging, the microstructure and properties of forgings can meet or exceed those achieved through conventional heat treatment methods. Additionally, leveraging the characteristic of coarse grains during residual heat treatment improves the machinability of forgings. Utilizing residual heat for heat treatment not only saves a considerable amount of energy consumed during the heating process but also reduces production costs, leading to significant economic benefits and promising applications in various industries.