3Cr2Mo tool steel | P20

3. Heat Treatment

The heat treatment of 3Cr2Mo tool steel (P20) is vital for its final properties and performance in mold applications, ensuring necessary hardness, toughness, and stability.

3.1 Understanding Pre-Hardened 3Cr2Mo (P20) Steel

3Cr2Mo (P20) is often supplied pre-hardened to 28-35 HRC. This minimizes distortion after machining, as hardening and high-temperature tempering are done by the manufacturer. If reworking or starting from an annealed state, understanding the full heat treatment process is key.

3.2 Comprehensive Heat Treatment Guide for 3Cr2Mo (P20)

The following outlines typical heat treatment for 3Cr2Mo steel.

3.2.1 Preliminary Heat Treatments (If Required)

These soften the steel for machining or refine its structure.

Annealing: Targets ≤229 HBW for machinability.

• Isothermal Annealing (Option 1): Heat to 840-860°C (2-3 hrs), cool to 710-730°C (3-4 hrs), furnace cool below 500°C, then air cool.
• Isothermal Annealing (Option 2): Heat to 850°C (2-4 hrs), cool to 720°C (4-6 hrs), then furnace cool.
• High-Temperature Tempering (Softening): 720-740°C (1-2 hrs), furnace cool (≤229 HBW).
• Alternative Tempering: 650-680°C (≤225 HBW).

Normalizing: Heat to 900-920°C, cool in still air to refine grain.

3.2.2 Hardening Process for 3Cr2Mo (P20)

This transforms the steel to hard martensite.

Approximate transformation temperatures: Ac1: ~770°C; Ac3: ~825°C; Ms: ~335°C; Mf: ~180°C.

Procedure:

1. Preheating: Often recommended.
2. Austenitizing: Heat to 850-880°C (or 880-900°C). Hold for complete austenitization.
3. Quenching:

• Media: Oil is most common; water is less typical.
• Expected Hardness (Oil Quench from 850-880°C): Approx. 50-52 HRC (or ≥50 HRC).
• Isothermal Quench Option (900°C into 180-220°C medium): Approx. 47-48 HRC.

3.2.3 Tempering – Tailoring 3Cr2Mo (P20) Properties

Tempering reduces brittleness, relieves stress, and improves toughness.

• Standard Pre-Hardened (28-35 HRC): Temper at 580-640°C.
• Higher Hardness: Use lower tempering temperatures.

• Duration: Typically 1-2 hours.
• Multiple Cycles: Two or three cycles are often recommended for optimal properties.
• Cooling: Air or oil.

Effect of Tempering Temperature: Higher temperatures generally decrease hardness and strength but increase ductility and toughness.

3.3 Advanced Surface Treatments for 3Cr2Mo (P20)

For enhanced surface wear/corrosion resistance or polish:

• Methods: Carburizing, nitriding (gas/ion), carbonitriding.
• Nitriding: Can significantly increase surface hardness (e.g., 650 HV up to 950-1100 HV). Soft nitriding (570°C, 2 hrs) can yield ~700 HV surface hardness (depth ~0.1mm).
• Post-Treatment: Often followed by final polishing.

3.4 Key Considerations for 3Cr2Mo (P20) Heat Treatment

While often supplied pre-hardened, 3Cr2Mo (P20) properties are precisely adjustable through controlled austenitizing, quenching, and tempering. Surface treatments like nitriding offer further enhancements. For optimized results, consult with metallurgical experts or your steel supplier.