How to Determine Overheating Status

Metallographic observation of irreversible damage such as grain boundary oxidation, voids or localised melting can be used to identify the condition of overheating. These properties are indicative of a material that has undergone permanent failure and cannot be remelted.

1. Typical Features in a Metallographic Microscope

2.2. Grain Boundary Oxidation: In the microscope , a black oxide network is shown at the grain boundaries . This shows that oxygen has penetrated along the grain borders and a severe reduction in bonding strength occurs .

Grain boundary voids: Small pores or fissures in the grain boundary area are direct evidence for enhanced atomic diffusion and weakening of grain borders at elevated temperatures.

Localised Melting (Remelted Balls) At the grain boundary junctions, rounded traces of liquid phase (remelted balls) are seen, suggesting that the temperature has exceeded the melting point of the material and micro-area melting has occurred.

Intergranular fracture: The fracture path is along the grain boundaries, the fracture surface is "stone-like fracture"-grayish white, no metallic lustre, and granular.

Judgement Criteria: GB/T 6394 is used for rating. If any of the above features are observed, it can be proven that it is overheating.

2. Macroscopic and Usage Anomaly

Surface Discolouration of Mould: The overheated part shows bluish purple, greyish white or peeling phenomena, which indicate microstructure degradation.

Cracking, repeated: Intergranular cracking in high temperature locations like hot runner nozzles and manifolds, and recurring rapidly after repair.

Sudden Performance Degradation: Hardness and impact toughness drop significantly under same working conditions, lifespan shortened by more than 50%.

Special Reminder: Once it is determined that overheating has occurred, the material is irreparable and the component must be replaced. Regular use will lead to security accidents.

3. Suggested Judgement Process

Sampling and Testing: Take samples from the suspected hot area and prepare metallographic specimen;

Corrosion observation: The grain boundary state was observed by microscope (100×–200×) after etching with 4% nitric acid alcohol;

Full verification: final decision according to heating history (temperature >1150°C or not), fracture morphology and performance test results.