heat treatment techniques for die casting molds guide

Heat treatment techniques are widely applied in die casting mold manufacturing; they can significantly improve the microstructure and performance of die components, extend mold life, enhance machining quality, and reduce tool wear.

Die casting molds are commonly made from alloy tool steels. Typical heat treatment sequences include spheroidizing annealing, stress relief/stabilization, quenching (or quenching and tempering) and multiple tempering operations. By selecting an appropriate combination of heat treatments, the required strength, toughness and high‑temperature performance can be achieved.

Pre‑treatment:

1. Purpose: to remove residual stresses from forging or rough machining, reduce hardness for easier cutting, and prepare a uniform structure for subsequent quenching and tempering.
2. Methods and effects: spheroidizing annealing produces a uniform microstructure with finely dispersed carbides to improve toughness. Quench-and-temper (i.e., quenching followed by tempering) often provides superior strength‑toughness balance, so for dies that require high toughness, tempering after quenching is commonly used instead of spheroidizing.
3. Recommendation: perform pre‑treatment on forgings or rough‑machined blanks and record the process batch to facilitate later quality traceability.

Stabilization treatment:

1. Purpose: to remove internal stresses generated during rough machining and to reduce the risk of distortion and cracking during quenching. This is particularly important for dies with complex shapes.
2. Typical process: heat to 650 °C to 680 °C, hold for 2 to 4 hours, then remove from the furnace and air cool. For dies with complex geometry, furnace‑cool to below 400 °C before air cooling to further reduce internal stresses.
3. Special cases: surfaces modified by EDM (electrical discharge machining) may develop a re‑cast or damaged layer and are prone to wire‑cutting cracks; a lower‑temperature stress relief anneal should be performed to remove local stresses.

Quench preheating:

1. Reason: die casting dies are often made of high‑alloy steels with poor thermal conductivity; rapid direct heating can create large temperature gradients, causing cracking or distortion. Therefore, staged preheating is necessary.
2. Preheating methods and temperatures: lower‑temperature preheating (400 °C to 650 °C) can be done in a box or air furnace. Higher‑temperature preheating is better performed in a salt bath furnace to improve heat uniformity.
3. Number of preheats and requirements: dies with low deformation control requirements may use fewer preheating stages; for strict deformation control, multiple staged preheats should be used to reduce thermal gradient stresses. A common empirical estimate for preheat time is 1 min/mm (for reference only; verify based on material and component thickness).

Quenching heating and soaking:

1. Objective: to obtain austenite of uniform composition and to dissolve carbides sufficiently, thereby improving high‑temperature strength and resistance to softening.
2. Temperature selection: quenching temperature should be chosen within the limits specified for the material. Higher quenching temperatures favor thermal stability but can promote grain growth and reduce toughness; dies requiring high toughness generally use relatively lower quenching temperatures.
3. Soaking time: to ensure homogenization, salt‑bath quenching heating typically requires a relatively long soak; a common estimate is 0.8 min/mm to 1.0 min/mm (reference value; must be validated).

Quenching cooling:

1. Choice of cooling method: dies of simple shape and with low deformation sensitivity may be oil‑quenched. Dies with complex shapes or high anti‑distortion requirements are better treated with staged (step) quenching (accelerating cooling in stages with gradual transitions).
2. Equalization before tempering: to prevent cracking and distortion, regardless of the cooling method, do not cool directly to room temperature; instead cool first to 150 °C to 180 °C for equalization. Equalization time can be estimated at 0.6 min/mm (reference), then proceed immediately to tempering.
3. Notes: control the cooling‑rate profile to avoid surface‑cool/core‑hot thermal stress concentration. When using staged or localized quenching, ensure deformation is controllable and provide corrective procedures.

Tempering:

1. Necessity: sufficient tempering removes quench residual stresses and adjusts hardness and toughness. Die casting dies generally require thorough tempering, commonly performed in three tempers.
2. The first temper temperature is usually chosen in the material’s secondary hardening range (to stabilize the structure).
3. The second temper temperature is selected to achieve the required hardness.
4. The third temper temperature is 10 °C to 20 °C lower than the second temper and is used to remove residual stresses and refine the structure.
5. Tempering cooling: after tempering, oil cooling or air cooling may be used. Single‑cycle tempering hold time should not be less than 2 hours (determine exact time based on part thickness and material requirements).