I. Heat Treatment Process Flow
Annealing Process
Isothermal Annealing: Heat to 880℃, soak, then cool to 750℃ for isothermal treatment. Finally, furnace-cool to ≤550℃ before air cooling. Post-annealing hardness ≤230 HBW.
Soft Annealing: Maintain 750–800℃, followed by slow furnace cooling, suitable for reducing hardness to facilitate machining.
Quenching Process
Preheating: Two-stage preheating (600–650℃ + 830–850℃), each held for 2 hours to minimize thermal stress.
Heating & Cooling:
Quenching temperature: 1000–1050℃ (typically 1020–1030℃);
Cooling method: Oil cooling or interrupted quenching at 500–550℃. During oil cooling, remove parts when smoke appears but before ignition;
Post-quench hardness: 54–57 HRC.
Tempering Process
Temperature Range: 530–650℃ (conventionally 530–600℃), with ≥2 tempering cycles, each soaking ≥2 hours;
Performance Adjustment:
Tempering at 530–560℃: Hardness 50–54 HRC;
Tempering at 600℃: Significantly enhances toughness (impact toughness +31.8%) and plasticity (reduction of area +177.2%);
Secondary Hardening Peak: Achieves maximum hardness at 500–550℃.
II. Critical Control Points
Cooling Rate: Oil cooling requires precise timing to prevent cracking or uneven hardness; air cooling suits large molds to minimize deformation.Tempering Cycles: Mandatory ≥2 tempering cycles to fully eliminate retained austenite and quenching stress.
Temperature Uniformity: Large molds require extended soaking times to ensure complete core transformation.
III. Performance Enhancement Techniques
Nitriding: Gas nitriding at 525℃ increases surface hardness to 1250 HV, improving wear resistance.Toughness-Optimized Process: Double tempering at 600℃ (soaking 2.5 hours) enhances both strength (tensile strength +14.1%) and toughness.
IV. Key Precautions
Avoid Grain Coarsening: Quenching temperature must not exceed 1050℃ to prevent reduced toughness from grain growth.Stress Relief: After machining, hold at 650℃ for 1 hour followed by slow cooling to reduce deformation risks.
Microstructure Uniformity: Forged materials require specialized processes (e.g., patented methods) to optimize carbide distribution.
Quick-Reference Parameter Table
Stage Temp. Range Cooling Method Target Hardness/PerformanceAnnealing 880℃→750℃ Furnace cool ≤550℃ ≤230 HBW
Quenching 1020–1030℃ Oil/Interrupted 54–57 HRC
1st Temp. 540–600℃ Air cool 50–54 HRC
2nd Temp. 540–600℃ Air cool Residual stress elimination
Note: For aerospace components (e.g., landing gear) requiring high impact toughness, double tempering at 600℃ is recommended.