How to determine the optimal heat treatment process for mold materials?

The determination of the optimal heat treatment process for mold materials requires focusing on three core objectives: high hardness and wear resistance, high temperature strength and fatigue resistance, and processability and corrosion resistance.

Cold work mold steel: with high hardness and high wear resistance as the core

Suitable for metal forming at room temperature, such as punching, cold heading, cold extrusion, etc., requiring materials to have extremely high hardness (58-64 HRC) and anti chipping ability.

Heat treatment process path

Preparation heat treatment:
Adopting isothermal spheroidization annealing (heating at 840-870 ℃, isothermal at 700-760 ℃ for 4-6 hours) to evenly distribute carbides, reduce hardness to ≤ 220 HBS, and improve cutting performance.
Final heat treatment:
Quenching: Heat to 980-1040 ℃, hold for heat, then oil cool or gas quench to obtain martensitic structure.
Low temperature tempering: Temper 1-2 times at 150-250 ℃ to eliminate stress and maintain high hardness, avoiding hardness decrease caused by medium high temperature tempering.

Key points of process optimization

For high carbon and high chromium steel (such as Cr12 type), the "low quenching and low recovery" process (quenching 1050-1080 ℃, tempering 180-220 ℃) can be used to improve toughness and prevent brittle cracking.
Vacuum heat treatment is recommended for complex molds to reduce oxidation and decarburization, and the deformation can be reduced by 30% -40%.

Hot work mold steel: focus on high temperature strength and thermal fatigue resistance

Used for high-temperature forming processes such as hot forging and die casting, the surface temperature of the mold can reach over 500 ℃, and it needs to have good red hardness, thermal conductivity, and resistance to thermal cracking.

Heat treatment process path

Preparation heat treatment:
After forging, slow cooling and annealing treatment (such as furnace cooling after 870 ℃ insulation) are required to eliminate forging stress and improve tissue uniformity.
Final heat treatment:
Quenching: Heat to 1020-1050 ℃, oil cooled or gas quenched to ensure complete hardening of the core.
High temperature tempering: 2-3 tempering cycles at 500-620 ℃, utilizing the secondary hardening effect to increase hardness to 48-52 HRC and avoid tempering brittle zones.

Key points of process optimization

The number of tempering shall not be less than two times to prevent size changes caused by residual austenite transformation.
Large molds can be quenched in stages (first air cooled to 740-760 ℃ and then oil cooled) to reduce thermal stress and deformation.

Plastic mold steel: emphasizing processability, polishability, and corrosion resistance

The working temperature is usually below 200 ℃, and the main failure modes are wear, corrosion, and surface scratches. The material is required to be easy to process, easy to polish, and corrosion-resistant.

Heat treatment process path

Pre hardened steel:
The factory has undergone quenching and tempering treatment (quenching+high-temperature tempering), with a hardness of 32-38 HRC, and can be directly processed to avoid deformation during subsequent heat treatment.
Carbonized steel:
The carburizing temperature is 900-920 ℃, the depth of the carburizing layer is 0.8-1.5mm, followed by quenching and low-temperature tempering, and the surface hardness reaches 58-64 HRC.
Corrosion resistant steel:
By using solid solution and aging treatment (solid solution at 1020-1080 ℃, aging at 420-480 ℃), a hardness of 40-44 HRC is obtained, which combines strength and corrosion resistance.

Key points of process optimization

Precision molds are recommended to undergo vacuum heat treatment, resulting in a smooth and oxidation free surface that is conducive to mirror polishing.
Steel grades containing easily cutting elements such as sulfur and calcium can achieve excellent cutting performance in the annealed state and are suitable for high surface quality molds.

General Process Control Principles

Regardless of the type of mold steel, the following key points must be strictly controlled:

Heating uniformity: Large modules should be preheated to 600-650 ℃ before reaching the austenitizing temperature to prevent cracking.
Cooling method selection:
Gas cooling is suitable for deformation sensitive parts (such as 0.6 MPa high-pressure gas quenching);
Oil quenching is suitable for large section workpieces, but the cooling rate needs to be controlled to prevent deformation.
Tempering adequacy: The tempering temperature should be higher than the actual working temperature of the mold, and the time should not be less than 60 minutes to prevent performance degradation during service.
Deformation control: For complex structures, isothermal quenching or graded quenching is preferred, and the deformation can be controlled within 0.05 mm.