Introduction to Forging and Residual Heat
Forgings are metal components shaped under pressure through plastic deformation to achieve desired forms or appropriate compressive strengths. This force is typically applied using hammers or presses. The forging process refines the grain structure of metals and enhances their physical properties. In practical applications, a well-designed forging ensures that the grain flow aligns with the primary stress direction. Forgings are expected to be uniform, without any porosity, voids, inclusions, or defects.
Importance of Utilizing Residual Heat
The forging industry is a major consumer of energy, with heat treatment alone accounting for around 30%–35% of the total energy consumed in forging production. In China, the energy consumption per ton of die forgings is approximately 1.0 tons of standard coal, which is significantly higher than that of industrially advanced countries like Japan (0.515 tons per ton).
Energy accounts for about 8%–10% of forging production costs. Therefore, reducing energy consumption can lower production costs and improve enterprise profitability. More importantly, energy sustainability is a key issue tied to national development and global human survival. Utilizing residual forging heat for subsequent heat treatment presents clear advantages: it saves energy, shortens processing cycles, improves efficiency, and protects the environment.
Residual Heat Treatment After Hot Die Forging
Residual heat treatment involves using the heat retained in the forged part immediately after forging, bypassing the reheating step usually required before heat treatment. The following are three main types of residual heat treatments:
Equalizing Residual Heat Treatment
In this method, forged parts are immediately placed into a heat treatment furnace while still hot. They undergo standard heat treatment procedures. After temperature equalization, uniformity is achieved throughout the component, reducing soaking time. This method is especially suitable for complex-shaped forgings with large cross-sectional differences to ensure consistent quality.
Direct Residual Heat Treatment
Here, the forging and heat treatment processes are integrated. Forged parts are treated while still hot, avoiding the energy loss from reheating and significantly improving energy efficiency.
Partial Residual Heat Treatment
This method involves cooling forged parts to 600–650°C, then reheating them to the desired temperature for heat treatment. It refines the grain structure and saves the energy otherwise required to raise parts from room temperature to 600–650°C. It is ideal for forgings with high grain size requirements.
Common Residual Heat Treatment Processes
Residual Heat Quenching
This involves quenching forged parts when their temperature is above the Ar3 point (or between Ar3 and Ar1), resulting in martensitic or bainitic structures. Forgings treated with residual heat quenching and subsequent tempering exhibit enhanced mechanical properties, simplified processes, and shorter production cycles.
Compared to conventional treatments, residual heat quenching followed by high-temperature tempering yields higher strength and hardness, with slightly lower ductility and toughness. The coarser grain structure also improves machinability.
Residual Heat Normalizing or Annealing
If the forging temperature is above Ar3 (for hypoeutectoid steels), parts are placed into a normalizing or annealing furnace to form a pearlite-ferrite equilibrium structure. While the grain is coarser, the resulting structure is ideal for pre-treatment as it avoids structural inheritance, allowing grains to be refined in later processes.
Isothermal Normalizing with Residual Heat
This method rapidly cools forged parts (at 30–42°C/min) from 900–1000°C to an isothermal temperature (typically 550–680°C) and holds them before air cooling. The isothermal temperature is chosen based on the pearlite transformation curve to reduce holding time.
Isothermal normalizing is commonly used in carburized gear steels like SCM420H, SCM822H, SAE8620H, and 20CrMnTiH.
Key Control Points in Residual Heat Treatment
Residual Heat Quenching
- Stable Heating System: Use induction heating and infrared thermometers with sorting to control billet temperature.
- Quenching Temperature Control: Defined through trials. Carbon steel: ≤60s delay; Alloy steel: 20–60s.
- Effective Quenching System: Use slower quenching media (like oil or PAG) to prevent deformation or cracking.
- Post-Quenching Tempering: Timely tempering prevents deformation or cracking due to residual stresses. Tempering is done collectively in the heat treatment workshop.
Residual Heat Normalizing/Annealing
- Controlled Entry Temperature: Use air cooling before furnace entry if parts are too hot.
- Reasonable Holding Time: Avoid grain coarsening (if too long) or incomplete transformation (if too short).
Isothermal Normalizing
- Temperature Management: Ensure temperature remains above Ar3 post-forging. Use equalization if there’s fluctuation.
- Controlled Cooling Rate: Maintain uniform cooling (30–42°C/min) to avoid Widmanstätten structures.
- Post-Cooling Temperature: Must stay above Bs to avoid bainite formation. Recommended: Bs + 80–100°C.
- Isothermal Temperature Selection: Affects final hardness. Typically, Bs + 50–80°C.
- Sufficient Holding Time: Ensure pearlite transformation is complete; otherwise, martensite or bainite may form.
Application Cases
Isothermal Normalizing for Passenger Car Gear Forgings
Forgings made of 20MnCr5JV and 27MnCr5JV required a fine-grained pearlite-ferrite microstructure. The optimized process: After forging, parts are cooled to 550–600°C, reheated to 900–920°C, rapidly cooled, then isothermally held at 580–600°C for 1 hour. Result: Proper hardness, no bainite, excellent machinability, and energy savings of ~150 kWh/t.
Residual Heat Quenching for Micro Car Crankshafts
Forgings of 40CrH required tempered martensite (241–285 HBW). Traditional treatment included reheating to 850°C. The new residual heat quenching directly quenches forged parts, followed by tempering. The process met all customer specs and saved ~259 kWh/t in heating energy.
Residual Heat Annealing for Micro Car Crankshafts
Forgings of 40CrH were previously normalized at 860°C. With residual heat annealing, crankshafts were placed in insulated boxes post-forging. The structure remained pearlite + ferrite, with no abnormal phases. Grain coarseness improved machinability without affecting mechanical properties. It also saved energy and reduced furnace operating time.
Conclusion
Practical experience shows that residual heat treatment after forging is feasible and effective. With proper control of post-forging cooling parameters, forgings can achieve or surpass the performance levels of traditional treatments. Coarser grains improve machinability, and eliminating reheating reduces energy use and production costs, delivering strong economic benefits and wide industrial applications.
