The most reliable method to determine if hot runner grain size is too large is through metallographic examination combined with grain size rating standards, such as the national standard GB/T 6394 or ASTM E112. The result is determined by comparing the standard charts under a 100x microscope.
1. Metallographic Microscopy Method (Core Method)
Operation Procedure:
Cut a sample from the most severely heated area of the hot runner mold;
Prepare a metallographic sample after grinding, polishing, and etching;
Observe the grain morphology under a 100x optical microscope.
Judgment Criteria:
If the grains are significantly coarse and unevenly distributed, exhibiting a "large engulfing small" phenomenon;
Refer to the GB/T 6394-2017 standard rating chart. If the grain size grade is ≤4, it is considered coarse grain and deemed too large.
Reliability: This method is the industry gold standard, and the results are intuitive and reliable.
Note: The grain size of mold steel under normal service conditions should be within the range of grade 5-8 (fine grain). Exceeding this range warrants caution.
2. Auxiliary Judgment Methods
|
Method |
Judgment Logic |
Limitations |
|
Mechanical Property Testing |
Excessively large grains lead to decreased strength and plasticity. Tensile tests can be used to check if the elongation is significantly reduced. |
Cannot pinpoint the specific location; only for indirect reference. |
|
Heating History Retrospective |
Long-term operation at excessively high temperatures (e.g., H13 steel > 600°C) easily leads to grain coarsening. |
Only a risk warning, not direct evidence. |
|
Fracture Analysis |
The appearance of a "stone-like fracture" or intergranular fracture characteristics indicates grain boundary weakening. |
Requires metallographic confirmation. |
Note: Hardness testing and eddy current testing cannot effectively reflect grain size and are prone to misjudgment due to interference from various factors.
3. Practical Application Recommendations
Regular Sampling Inspection: Conduct metallographic inspections of key components every 6–12 months;
Establish Benchmark Files: Perform grain size testing before putting a new mold into use as a comparison benchmark;
Integrate with Process Control: If grain growth is observed, immediately check whether the heating temperature setting and holding time are compliant.
Coarser grains at high temperatures make the material more brittle. For hot runner systems, a grain size ≤4 is considered too large and requires repair or replacement.
