Box Furnace for Mold Steel Quenching: Gas Annealing & Carbonitriding Tips

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Box Furnace for Mold Steel Quenching: Gas Annealing & Carbonitriding Tips

Mold steel is the core material for manufacturing stamping dies, injection molds, die-casting molds and other tools, and its hardness, wear resistance and toughness directly determine the service life and forming accuracy of molds. Box furnaces, with the advantages of precise temperature control, flexible furnace space, easy operation and suitability for batch heat treatment, have become mainstream equipment for mold steel quenching, annealing and carbonitriding. In particular, gas-fired box furnaces are widely used in small and medium-sized mold heat treatment scenarios due to the uniformity and cost advantages of gas heating. This article focuses on the practical scenario of box furnaces, detailing the key points of two core processes of mold steel gas annealing and carbonitriding, helping to improve the quality of mold steel heat treatment.

1. Core Advantages of Box Furnace for Mold Steel Heat Treatment

- Stable Temperature Control: High-precision temperature control system achieves ±1℃ temperature difference control, meeting the precise temperature requirements of mold steel heat treatment and avoiding uneven hardness caused by local overheating or underheating.

- Uniform Heating: Circulating air duct in the furnace plus direct gas heating eliminate temperature differences between mold edges and cores, ensuring consistent overall heat treatment effect.

- High Process Compatibility: Can flexibly switch multiple processes such as quenching, annealing, carbonitriding and tempering, suitable for cold work mold steel, hot work mold steel, plastic mold steel and other materials.

- Balanced Cost and Efficiency: Gas heating features low energy consumption and fast heating; the furnace can be customized in size, adapting to single-piece, small-batch and multi-batch mold heat treatment to match workshop production rhythm.

2. Key Points of Gas Annealing Process for Mold Steel in Box Furnace

Annealing is a critical pre-process for mold steel heat treatment, with the core purpose of eliminating forging internal stress, reducing steel hardness, improving machinability and refining grains, laying a foundation for subsequent quenching and carbonitriding. Gas box furnace annealing requires strict control of four core parameters: temperature, heating rate, holding time and cooling method to prevent annealing defects.

2.1 Preparatory Work

- Clean up oil stains, rust and impurities on the surface of mold steel to avoid oxide scale and decarburization layer during heating.

- Arrange workpieces reasonably with a spacing of 5-10mm to ensure air circulation in the furnace and prevent local uneven heating.

- Check the tightness of gas pipelines, accuracy of furnace thermocouples and sensitivity of temperature control system to eliminate potential safety hazards.

2.2 Core Process Parameter Control

- Heating Rate: Slow heating at low temperature section (≤300℃) with a rate of 50-80℃/h to avoid stress cracking caused by rapid heating; medium and high temperature sections can be accelerated to 100-150℃/h.

- Annealing Temperature: Customized according to mold steel materials: 860-880℃ for cold work mold steel (Cr12MoV), 840-860℃ for hot work mold steel (H13), 780-800℃ for plastic mold steel (P20).

- Holding Time: Calculated based on the effective thickness of the workpiece, 1-1.5h per 10mm thickness to ensure the core temperature of the workpiece evenly reaches the annealing temperature and fully eliminates stress.

- Cooling Method: After holding, cool slowly with the furnace to below 500℃, then take out for air cooling; rapid cooling is strictly prohibited to prevent hardness rebound and stress regeneration.

2.3 Prevention of Common Defects

High hardness after annealing: extend holding time and reduce cooling rate; surface oxidation and decarburization: pass protective gas (nitrogen) into the furnace and reduce furnace door opening times; workpiece deformation: standardize placement and strictly control heating rate.

3. Key Points of Mold Steel Carbonitriding Process in Box Furnace

Carbonitriding is the core process for surface strengthening of mold steel. By infiltrating carbon and nitrogen elements into the workpiece surface in a box furnace, a surface structure with high hardness and wear resistance is formed, while ensuring core toughness. This greatly improves the mold's wear resistance, fatigue resistance and seizure resistance, suitable for high-load and high-speed mold scenarios. Gas box furnace carbonitriding needs to control four key points: atmosphere, temperature, time and quenching connection.

3.1 Core Process Principle

Using gas as the heating source, pass carburizing medium (propane, natural gas) + nitriding medium (ammonia) into the sealed furnace to form a carbonitriding atmosphere. At 800-900℃, carbon and nitrogen atoms diffuse to the surface of mold steel, forming a 0.1-0.5mm strengthened layer. Subsequent direct quenching + low-temperature tempering realizes surface hardening.

3.2 Core Process Parameter Control

- Carburizing Temperature: Conventional control at 830-860℃; excessively high temperature easily leads to coarse grains and increased deformation, while excessively low temperature results in slow infiltration speed and uneven infiltration layer.

- Atmosphere Ratio: The ratio of ammonia to carburizing gas is controlled at 1:3-1:5 to ensure balanced carbon and nitrogen concentration, avoiding surface looseness, decarburization or brittle infiltration layer.

- Carburizing Time: Adjusted according to the required infiltration layer thickness; 2-3h holding for thin infiltration layer (0.1-0.2mm), 4-6h holding for thick infiltration layer (0.3-0.5mm).

- Quenching Connection: After carburizing and holding, take out quickly for quenching (mainly oil quenching), with quenching oil temperature controlled at 60-80℃ to avoid surface decarburization; conduct low-temperature tempering at 180-200℃ in time after quenching to eliminate quenching stress.

3.3 Key Operational Precautions

- The furnace must be kept sealed to prevent air from entering and causing oxidation and uncontrolled atmosphere combustion.

- Monitor carbon potential and nitrogen potential in real time to ensure stable atmosphere and avoid infiltration layer defects.

- Complex structure molds need reserved deformation allowance and fixed with special tooling to reduce heat treatment deformation.

4. Practical Summary of Box Furnace Mold Steel Heat Treatment

The application of box furnace in mold steel quenching, annealing and carbonitriding focuses on precise temperature control, uniform heating, strict process parameter control and standardized operation flow. Gas annealing emphasizes "slow heating, long holding, slow cooling" to eliminate stress and soften steel; carbonitriding emphasizes "balanced atmosphere, moderate temperature, rapid quenching" to strengthen surface performance. In practice, the process should be flexibly adjusted according to mold steel material, mold specification and usage scenario. With subsequent processes such as tempering and shaping, the hardness, wear resistance and service life of molds can be effectively improved, reducing production loss.

Zhengzhou Protech Technology Co.,LTD is a professional manufacturer specializing in tube furnaces, muffle furnaces, atmosphere furnaces, and vacuum furnaces. We are committed to providing targeted solutions to meet your diverse heating equipment needs.
For customized heating solutions tailored to your specific requirements, feel free to get in touch with us:
WhatsApp: +86 17719806024
Email: info@lab-furnace.com

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Box Furnace for Mold Steel Quenching: Gas Annealing & Carbonitriding Tips

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