How to Select and Design Heat Treatment Fixtures (Tooling)?

The selection and design of heat treatment fixtures is a systematic engineering task that requires comprehensive consideration of process requirements, material characteristics, production efficiency, and cost-effectiveness. Below are the key principles and steps:

1. Core Design Principles

01. High-Temperature Resistance & Thermal Fatigue Resistance

• Materials must withstand the maximum operating temperature (e.g., 1000°C for quenching, 600°C for tempering) and endure repeated heating/cooling stresses.
• Priority should be given to heat-resistant steels (e.g., Cr-Ni series: 310S/RA330 for temperatures above 1000°C; 2520-type for temperatures below 950°C).

02. Balance Between Strength and Rigidity

• Calculate workpiece weight and stacking methods to avoid deformation at high temperatures.
• Adopt truss structures or reinforcing ribs in design to reduce weight while ensuring load-bearing capacity.

03. Optimization of Heat Transfer & Atmosphere Circulation

• Avoid blocking radiation heating channels; use open structures (e.g., grids, open-area ratio ≥30%).
• Ensure uniform furnace atmosphere flow to prevent soft spots or uneven case depth on workpieces.

04. Resistance to Environmental Corrosion

• Select materials based on furnace atmosphere:
  • Carburizing/Carbonitriding: Choose high-nickel alloys (e.g., RA333) to resist carburization embrittlement.
  • Salt Bath/Vacuum Furnaces: Avoid contact between dissimilar metals to prevent low-melting eutectic reactions.
  • Oxidizing Atmospheres: Apply surface coatings (e.g., aluminosilicon diffusion coatings) for protection.

05. Workpiece Compatibility & Damage Prevention

• Minimize contact area at support points (e.g., knife-edge supports) to reduce heat transfer obstruction and sticking.
• For precision parts (e.g., gears), use contoured fixtures to prevent quenching distortion.

2. Material Selection Guide

• Cost-Efficiency Tip: Use high-performance materials only in critical high-temperature zones; combine with lower-grade materials for non-critical areas via welding.

3. Design Steps & Validation

01. Define Process Parameters

• Temperature profile, atmosphere type, loading capacity, cooling method (oil/gas quenching).

02. 3D Modeling & Simulation

• Use Thermo-Calc or ANSYS to analyze thermal stress distribution and optimize weak areas.
• Simulate furnace airflow to validate the layout of openings.

03. Key Design Details

• Weld Locations: Avoid high-stress areas; use groove welding with nickel-based electrodes (e.g., ENiCrFe-3).
• Dimensional Allowances: Account for thermal expansion coefficients (e.g., ~16×10⁻⁶/°C for 310S) with appropriate gaps.
• Lifting Structures: Add lifting lugs and reinforcing ribs for safe handling.

04. Prototype Testing

• Conduct no-load thermal cycling tests to measure deformation; trial production runs to check workpiece uniformity.

4. Common Pitfalls & Solutions

5. Full Lifecycle Management

01. Coding & Traceability System: Establish a record for each fixture, documenting material, usage cycles, and maintenance history.

02. Regular Inspection Standards:

• Mandatory correction if deformation exceeds 50% of workpiece tolerance.
• Grit blasting required if oxide scale thickness exceeds 1mm.

03. Scrap Criteria:

• Cracks appear in critical load-bearing structures.
• Weight increase >20% after multiple repairs (impacts energy efficiency).

6. Innovation Trends

• Lightweight Composite Materials: Carbon fiber-reinforced silicon carbide (C/SiC) for vacuum furnaces, reducing weight by >60%.
• 3D-Printed Conformal Cooling Channels: Designed for complex geometries to achieve uniform quenching.
• Smart Fixtures: Embedded thermocouples for real-time temperature monitoring and dynamic process adjustments.

Practical Recommendations

• “Simulate Before Manufacturing”: Conduct thermo-mechanical coupled simulations before production to avoid ~80% of early failures.
• “Zonal Design”: Use higher-grade materials or add thermal insulation in areas with steep temperature gradients (e.g., near furnace doors).
• “Maintenance as Investment”: Regular removal of carbon buildup and oxide scale can extend fixture life by over 30%.