Chain sintering furnace annealing softens steel, while quenching hardens it. Compared to annealing, normalizing produces a lamellar pearlitic microstructure, improving machinability and wear resistance, while minimizing cracking, deformation, and easing operation. However, normalizing is relatively challenging because it relies on air cooling, which can vary due to seasonal changes, workpiece size, or airflow. To maintain uniform cooling, measures such as shading, enclosing with curtains, pits, or fans can be used.
Normalizing vs Annealing
Normalizing:
- Heat steel to 40–60°C above the A3 or Acm point.
- Hold to allow the microstructure to become uniform (mostly ferrite-pearlite).
- Air cool to room temperature.
- Subcritical steel: Refines grain size, improving strength and toughness.
- Hyper-eutectoid steel: Prevents networked cementite at ferrite grain boundaries, preserving toughness.
Full Annealing:
- Heat to 20–30°C above A3 for hypo-eutectoid steel or 30–50°C above A1.
- Hold until a full ferrite-pearlite structure forms.
- Cool slowly below A1 point (~50°C) to allow lamellar pearlite transformation.
- Goal: soften steel, improve machinability.
Stress-Relief Annealing:
- Heat below transformation temperature (450–650°C) and cool slowly to room temperature.
- Removes residual stress from cutting, stamping, casting, or welding.
- Reference heating temperatures:
- Carbon steel: 625±25°C
- Alloy steel: 700±25°C
- Holding time:
- Carbon steel: 1 hour per 25 mm thickness
- Alloy steel: 2 hours per 25 mm thickness
- Cooling rate: ≤ 275°C/hour per 25 mm thickness
Preventing Heating Deformation
- Use slow heating and preheating steps.
- Preheating guidelines:
- Below transformation temperature (e.g., 650–700°C for ordinary steel, 800–850°C for high-speed steel).
- Around 100°C for low-intensity preheat.
- Two-step preheat: e.g., 500°C for first stage, then raise below A1.
- Three-step preheat for large, high-alloy steel (like high-speed steel): third stage 1000–1050°C.
This approach ensures uniform properties, minimizes distortion, and improves steel machinability.