Overheating of molten metal is a common process anomaly in casting production, and its hazards are concealed and delayed. Often, it is not exposed until subsequent processing steps, resulting in losses far greater than ordinary process fluctuations.

Overheating of the melt refers to the phenomenon where the temperature of the melt exceeds the normal melting upper limit temperature specified for this grade of alloy, and persists for a certain period of time, resulting in a deterioration of the quality and subsequent structural properties of the melt. According to the degree of overheating, it can be divided into two levels:

General overheating: When the temperature of the melt exceeds the upper limit of the process by 10-30 ℃ and lasts for less than 30 minutes, the oxidation and hydrogen absorption of the melt increase, but the deterioration of the structure is not yet severe. After treatment, it can be downgraded for use;

Severe overheating: If the melt temperature exceeds the upper limit of the process by more than 30 ℃, or if the overheating lasts for more than 1 hour, the melt will be severely oxidized and burned, the elements will evaporate severely, and even local overheating may occur. The ingot structure will irreversibly coarsen.

If the temperature exceeds the solidus temperature of the alloy, causing the melting of low melting point phases at the grain boundaries, it is called overburning, which is the most serious temperature accident, and the melt basically loses its practical value.

The harm of molten overheating is not only reflected in the molten material itself, but also transmitted to the ingot and subsequent processed products, causing quality loss throughout the entire process.

Firstly, the metallurgical defect rate of ingots has significantly increased. Overheated melt has high hydrogen content and many oxide inclusions, which can increase the defect rate of porosity, looseness, and slag inclusion in the cast ingot by 2-5 times. In severe cases, the entire furnace ingot may fail the non-destructive testing and be directly scrapped.

Secondly, the ingot has a coarse structure and decreased performance. The greater the degree of superheat, the coarser the grains after solidification, and even coarse columnar crystals may appear, resulting in a significant decrease in the plasticity and toughness of the ingot, and cracking is prone to occur during extrusion and rolling processing. For heat treated strengthened alloys, overheating can also lead to increased grain boundary segregation, resulting in grain boundary overburning after solution treatment and mechanical properties that do not meet standard requirements.

Thirdly, the quality of subsequent surface processing deteriorates. The small oxide inclusions generated by overheating will extend into strip-shaped defects after extrusion and rolling, and appear as surface stripes, pitting, and peeling after anodizing and spraying treatment, greatly reducing the surface qualification rate of the product. In addition, severely overheated melt can exacerbate lining corrosion, shorten the service life of the melting furnace, and increase equipment maintenance costs.

• Immediately cut off the heating source: turn off the burner or heating power, stop continuing to heat up, and at the same time cover the furnace tightly to reduce the entry of air and intensify oxidation;
• Rapid cooling: Add preheated clean cold furnace materials or recycled materials of the same brand into the furnace, quickly reduce the temperature of the melt by absorbing heat from the cold materials, and slowly add them during feeding to avoid splashing of aluminum liquid;
• Uniform temperature: During the cooling process, sufficient stirring is carried out to eliminate local overheating areas, making the temperature of the entire furnace melt uniform and consistent;
• Enhanced refining: After the temperature drops to the normal range, the refining time can be appropriately extended by 10% to 20%, or the amount of refining agent can be increased to enhance gas and slag removal, offsetting the inclusions and hydrogen generated by overheating;
• Quality assessment: After processing, sample and test the hydrogen content, composition, and inclusion level to evaluate the quality level. Generally, overheating can be used for ordinary products after treatment; Severely overheated melt is prohibited from being used for high-end products and should be downgraded for use or remelted; Confirmed burnt melt should be directly scrapped and returned to the furnace, and it is prohibited to flow into the next process.

• Gradual heating: If the temperature is below the lower limit of the process, gradually raise the temperature to avoid local overheating caused by instantaneous high-power heating;
• Adequate stirring: Stir regularly during the heating process to ensure uniform temperature throughout the entire furnace;
• Re evaluation: If the temperature is too low and causes local solidification of the melt, direct refining and casting are prohibited. The composition must be readjusted and refined after complete melting, and can only be used after confirmation of qualification.