How to Determine if Grain Growth Has Occurred in a Material

The primary approach for assessing if grain development has happened in a material is metallographic microscopy with assessment according to the GB/T 6394 standard. This needs complete identification by changes in microstructure, grain size rating and performance anomalies.

 

1. Direct Observation by Metallographic Microscope (Most Reliable Method)

Sample Preparation Requirements: Samples should be collected from the critical areas of the hot runner (e.g. nozzles or manifolds) and ground, polished and etched (usually with 4% nitric acid alcohol).

Observational Properties:

Grains are significantly coarse: Larger than the normal size within the normal range at 100× or 200× magnification, with the structure of "large engulfing small".

Sparse and clear grain boundaries, reduced number of grain boundaries, diminished binding force, easy to produce stress concentration spots.

Abnormal structure: Widmanstätten, secondary recrystallisation or mixed crystal phenomena are found.

Judgement basis: The grain size assessment is carried out pursuant to GB/T 6394-2017. If the existing grain size is more than 2 levels lower than the initial benchmark (e.g. from level 8 to level 5), it is considered that grain growth has happened.

 

2. Comparison with original benchmark data (Critical step)

Original File Establishment: The first metallographic inspection should be done before the new mould is put into use, and the original grain size should be recorded (level 5-8 is better);

Regular Sampling and Comparison Inspection every 6-12 months, this period is shortened to 3-6 months under high temperature and high load situations;

Trend Judgement: If grain size continues to coarsen or localised abnormal coarsening occurs, it suggests material degradation.

Practical Suggestion: Use image analysis software (e.g. Image-Pro Plus) to achieve automatic rating and improve inspection uniformity and efficiency.

 

3. Performance Testing and Verification of the Auxiliary

Test question

Performance Abnormalities

Correlation

Hardness Testing

Large variation or general reduction in the hardness value

Grain coarsening causes decreased strength

Impact Toughness

Brittle properties of fracture surface are significantly lowered

Coarse granules reduce material toughness

Tensile Test

Decreased elongation following fracture, increased intergranular fracture.

Grain boundary weakening is a common sign

Special Reminder: If the fracture surface is a "stone-like fracture" (greyish white, no metallic lustre), it means it may have developed overheated, and the material is irreparable.

 

4. Proposed Actual Judgement Process

Sampling and Testing: Sampling of suspicious locations, offline or online;

Standard rating: GB/T 6394 Determination of grain size;

Historical Comparison: Compare with original benchmark data to see if coarsening has happened;

Final Assessment: Carry out final assessment using temperature records, service time and performance test findings.