Efficient Utilization of Residual Heat After Forging

What Are Forgings?

Forgings are metal components shaped through plastic deformation under applied pressure to achieve the desired shape or compression strength. This process typically uses hammers or presses. Forging builds a refined grain structure and improves the metal's physical properties. A well-designed forging can align the grain flow with the main pressure direction, resulting in components that are uniform, free of porosity, voids, inclusions, or other defects.

The Significance of Utilizing Residual Heat

The forging industry is an energy-intensive sector, with heat treatment of forgings accounting for 30%-35% of total energy consumption in forging production. In China, the energy consumption per ton of forged parts is approximately 1.0 tons of standard coal, significantly higher than industrialized nations such as Japan, where it is 0.515 tons of standard coal.

Energy consumption for forgings contributes to 8%-10% of production costs. Reducing energy usage not only lowers production costs and improves economic efficiency but also addresses critical energy sustainability issues at national and global levels. Efficient utilization of residual heat for heat treatment offers clear advantages:

• Energy savings
• Streamlined production processes
• Reduced environmental impact

Heat Treatment Using Residual Heat

Heat treatment using residual heat involves leveraging the heat retained in the forgings after shaping, eliminating the need to reheat the components before heat treatment. This approach generally falls into three categories:

1. Residual Heat Homogenization Treatment

After forging, components are directly transferred to a heat treatment furnace. Standard heat treatment procedures are followed, but the uniform temperature achieved during this process shortens the holding time. This method is particularly beneficial for complex shapes or forgings with varying cross-sections, ensuring consistent quality.

2. Direct Heat Treatment Using Residual Heat

Forged components utilize residual heat for direct heat treatment. This tight integration of forging and heat treatment eliminates the energy waste associated with reheating, significantly improving efficiency.

3. Partial Residual Heat Treatment

Forgings are cooled to 600–650°C, then reheated to the required temperature for heat treatment. This method is suitable for components requiring fine grain structures, as it saves the energy required to heat forgings from room temperature to 600–650°C.

Common Residual Heat Treatment Processes

1. Residual Heat Quenching

Residual heat quenching involves cooling the forged component (when its temperature is above the Ar3 or between Ar3 and Ar1) in a suitable quenching medium to form martensite or bainite structures.

Benefits:

• Achieves good mechanical properties after quenching and tempering.
• Saves energy by eliminating the reheating process.
• Simplifies workflows and shortens production cycles.
• Reduces equipment investment in quenching furnaces.
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Comparison with Conventional Quenching:

• Residual heat quenching combined with high-temperature tempering results in higher strength and hardness compared to conventional quenching and tempering.
• Adjusting the tempering temperature (40-80°C higher than conventional processes) can achieve comparable or superior ductility and toughness.
• Coarse grains from residual heat quenching improve machinability.

2. Residual Heat Normalizing (or Annealing)

This process involves placing forged components (when their temperature is above Ar3 for hypoeutectoid steels) into a normalizing furnace, cooling box, or annealing furnace for controlled cooling.

Characteristics:

• Due to the high forging temperatures, this method generally produces coarse grains and is used as a preparatory heat treatment.
• Generates a ferrite-pearlite structure suitable for subsequent treatments.
• Coarse grains from this process are not inherited in further treatments, enabling grain refinement later.

3. Residual Heat Isothermal Normalizing

Components are rapidly cooled after forging (when the temperature is above Ar3), then held at the isothermal temperature before being air-cooled to room temperature.

Key Parameters:

• Forging temperature: 900–1000°C
• Cooling rate: 30–42°C/min
• Isothermal temperature: 550–680°C, determined based on material properties and hardness requirements.
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Applications:
Commonly used for carburized gear steels such as SCM420H, SCM822H, SAE8620H, and 20CrMnTiH.

Control Points for Residual Heat Treatment

Residual Heat Quenching:

1. Stable and Controllable Heating System

   • Use induction heating, infrared pyrometers, and temperature sorting systems to ensure consistent heating.
   • Sort out components that do not meet temperature requirements.
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2. Appropriate Quenching Temperature

   • Determine through experiments. Control forging temperature and post-forging holding time (carbon steel: ≤60s, alloy steel: 20–60s).
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3. Efficient Quenching System

   • Choose quenching agents with slower cooling rates to prevent deformation or cracking.
   • Maintain cleanliness and performance of quenching media.
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4. Post-Quenching Tempering

   • Forgings must be tempered promptly to avoid deformation or cracking and to stabilize properties.

Residual Heat Normalizing/Annealing:

1. Pre-Furnace Temperature Control

   • Cool high-temperature components with air if necessary.
   • Ensure furnace power reserves for continuous production.
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2. Reasonable Holding Time

   • Avoid coarse grains (from excessive holding) or insufficient transformations (from short holding).

Residual Heat Isothermal Normalizing:

1. Forging Temperature Control

   • Ensure uniform temperature distribution before rapid cooling.
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2. Cooling Rate

   • Control to prevent undesirable bainite or martensite formation.
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3. Isothermal Temperature and Time

   • Select based on material properties and transformation curves.

Application Examples

Automobile Transmission Gears

Materials: 20MnCr5JV, 27MnCr5JV
Process: Partial residual heat isothermal normalizing.

• Components are reheated to 900–920°C after cooling to 550–600°C, followed by rapid cooling and isothermal holding at 580–600°C for 1 hour.
• Achieved ferrite-pearlite structures with no bainite.

Micro-Car Crankshafts

Residual Heat Quenching:

• Material: 40CrH
• Process: Quench immediately after forging using oil or PAG quenching media, followed by tempering.
• Results: Reduced energy consumption by 259kWh/t, simplified processes, and shortened production cycles.
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Residual Heat Annealing:

• Material: 40CrH
• Process: Annealing in an insulation box after forging.
• Results: Achieved ferrite-pearlite structures, improved machinability, and eliminated additional heating steps.

Conclusion

Production practices have proven that utilizing residual heat for heat treatment is viable and effective. By controlling post-forging cooling parameters, the resulting properties can meet or exceed those of conventional heat treatments.

Residual heat treatments also improve machinability, reduce energy consumption, and lower production costs, offering significant economic and environmental benefits. This process has broad application potential in modern manufacturing.