Will the hardness of H13 material rebound after tempering?

After tempering, the hardness of H13 material often does not recover. However, residual stress may not be completely released if the process is done incorrectly (e.g., with only one tempering or inadequate cooling), which could result in an unstable microstructure during service and performance degradation that shows up as cracking or early failure rather than a hardness rebound.

1. Why is "hardness rebound" misinterpreted?

Tempering is an irreversible procedure that releases internal stress by converting quenched martensite into more stable tempered martensite.

Hardness change trend: The hardness only falls, not rises, as the tempering temperature rises. The hardness stabilizes at 48–52 HRC following conventional tempering (580℃×2h×2 times).

The so-called "rebound" is essentially an anomaly caused by inadequate tempering (low temperature, short duration, few tempering cycles), which leaves behind an unstable residual microstructure that changes with further processing or heating and eventually fails.

Correct comprehension: "There is no rebound, only stability."

 

2. The True Reasons for "False Hardness Rebound"

Reasons

Mechanisms

Consequences

Just One Tempering

metastable microstructure and partial release of residual stress.

Microcrack propagation is triggered by stress release during service.

Prior to cooling to room temperature, a second tempering

unequal core transformation and superposition of thermal stress.

localized brittleness and irregular microstructure.

Low Temperature (less than 550°C)

High-carbon martensite remains after inadequate tempering.

ongoing change when heated again, causing cracking.

Uneven or Overly Quick Cooling

creates additional stress, upsetting the homogeneity of the microstructure.

During machining, wire cutting deforms or splits.

Case Study: Within a week of operation, three brittle fractures of H13 mold cores occurred because a plant failed to complete the second tempering phase. This was mistakenly identified as "hardness rebound," but it was actually the result of inadequate tempering.

 

3. How Can Abnormalities Be Prevented and Stable Hardness Ensured? Conduct two tempering procedures: Prior to the second tempering, the furnace needs to be air-cooled to room temperature (less than 50°C).

Accurate temperature control: Use a PID temperature-controlled furnace, set the temperature to 580℃±5℃, and make sure the temperature difference is ≤±5℃ by taking several temperature readings.

Adequate holding time: Each tempering operation should last at least two hours; for thicker sections, add thirty minutes for every twenty-five millimeters of thickness.

Process traceability and documentation: Keep track of operator data, hardness records, and furnace temperature curves.

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