How long does it take for H13 material to reach stable hardness after tempering?

After the second tempering and cooling to room temperature, the H13 material achieves stable hardness. Depending on the thickness of the workpiece and the cooling technique, the full operation typically takes four to six hours.

1. Important Hardness Stabilization Time Points

Description of Stage Time Requirements

Tempering holding for at least two hours, followed by air cooling to room temperature (less than 50°C) To avoid cracking, it should be done as soon as possible after quenching (recommended ≤ 4 hours).

Cooling to room temperature after a second tempering holding period of at least two hours To guaranty full release of residual stress, this must be done after the initial tempering has fully cooled.

Time for Hardness Stabilization Following the second tempering and cooling to ambient temperature At this stage, the hardness reaches a stable condition and the microstructure modification is finished.

Real-World Example: An H13 hot runner nozzle was tempered at 580°C × 2h × 2 times, totaling about 5.5 hours (heating and chilling). The hardness only varied by 0.5 HRC after cooling, stabilizing at 49.8 HRC.

2. Elements Influencing the Speed of Hardness Stabilization

Effective Workpiece Thickness: To guaranty adequate core tempering, the holding time must be increased by 30 minutes for every 25 mm increase in thickness.

Cooling Rate: To prevent stress rebound brought on by overly quick water or air cooling, air cooling to room temperature is advised.

Furnace Temperature Uniformity: To avoid inadequate or excessive tempering in certain regions, the temperature differential within the furnace should be ≤±5°C.

Raw Material Consistency: The contents of Cr, Mo, and V must adhere to specifications (Cr: 5.0%~5.5%, Mo: 1.1%~1.75%, V: 0.8%~1.2%).

Note: To guaranty that the core performance satisfies requirements, it is advised to extend the entire cycle to 6–8 hours for thick-walled items (>75mm).

3. How Can Hardness Stabilization Be Confirmed? Multi-point hardness testing: each piece is tested at three to five points, proving an average hardness of 48–52 HRC and a range of ≤2 HRC;

Metallographic analysis: There is no untempered martensite in the microstructure, which is composed of homogenous tempered martensite and scattered carbides;

Process documentation: For traceability, cooling data and furnace temperature curves for both tempering operations are kept.

"Two tempering processes, complete cooling, and data verification" are all essential safety precepts.