The hardness fluctuation standards for H13 material after tempering are as follows: The hardness range of a single workpiece within the same batch should be ≤2 HRC, and the average hardness of the entire batch should be stable within the range of 48~52 HRC. Exceeding this range is considered a process out of control and poses a quality hazard.
1. Hardness Fluctuation Acceptance Criteria
|
Indicator |
Acceptance Criteria |
Explanation |
|
Single Workpiece Hardness Range |
≤2 HRC |
The maximum hardness difference between different measuring points on the same workpiece, reflecting uneven cooling or inconsistent microstructure. |
|
Entire Batch Hardness Range |
48~52 HRC |
All workpiece measurements should be concentrated within this range. A deviation of >1 HRC from the average value requires a warning. |
|
Batch Consistency |
Range ≤1.5 HRC |
When multiple batches are produced continuously, the average hardness fluctuation should be smaller, reflecting process stability. |
Practical Application: A company in Wuhan requires that the hardness range of 5 measuring points per piece be ≤1.8 HRC after tempering of H13 hot runner nozzles. Non-conforming products are immediately isolated for re-inspection.
2. Risks and Consequences of Excessive Fluctuations
>2 HRC: Indicates the presence of localized soft spots or insufficiently tempered areas, leading to stress concentration during service and potentially causing early cracking or deformation;
<48 HRC: Indicates over-tempering or incomplete quenching, resulting in insufficient strength and susceptibility to wear and collapse;
>52 HRC: Indicates insufficient tempering, high residual stress, high brittleness, and a significantly increased risk of sudden fracture;
Large batch dispersion: Indicates unstable process control, potentially affecting the consistency of mold life.
Case Study: Due to abnormal temperature control in the tempering furnace, a batch of H13 manifolds exhibited hardness dispersion ranging from 46 to 53 HRC. Two pieces cracked within three days of operation.
3. Key Measures to Ensure Consistent Hardness
Perform two tempering cycles: After the first tempering, cool to room temperature before performing the second to prevent stress rebound;
Standardize furnace loading: Maintain a distance of ≥5cm between workpieces, avoid stacking, and ensure uniform hot air circulation;
Use a PID temperature-controlled furnace: Control the temperature difference within the furnace within ±5℃, and maintain a complete furnace temperature curve for each furnace;
Pre-inspect raw materials: Confirm that the Cr, Mo, and V compositions meet standards using XRF to avoid affecting the tempering response due to material fluctuations.
