Knowledge

Porosity control in casting process: During the casting process, the casting temperature and casting speed should be strictly controlled to avoid porosity caused by overheating or undercooling of the molten metal or by too fast or too slow casting speed.

 

To reduce the hydrogen content in molten iron and minimize porosity in ductile iron, we can take multiple approaches. Here are some effective measures:

 

1. Control of raw materials and smelting process

Selection of low-hydrogen raw materials: Ensure that the raw materials used (such as scrap steel, pig iron, etc.) have a low hydrogen content to reduce the introduction of hydrogen during the smelting process.

Strict control of smelting process: Optimize smelting process parameters, such as smelting temperature and smelting time, to reduce the dissolution and diffusion of hydrogen in the molten iron.

 

2. Vacuum degassing treatment

Vacuum degassing technology: Use vacuum degassing equipment to degas the molten iron. By reducing the gas partial pressure above the molten iron, hydrogen dissolved in the molten iron can escape, thereby reducing the hydrogen content in the molten iron. This method is highly effective but requires corresponding equipment investment and operational skills.

 

3. Addition of dehydrogenation agents

Addition of rare earth elements: Rare earth elements such as cerium (Ce) and lanthanum (La) have deoxidizing, desulfurizing, and dehydrogenating effects, which can reduce the hydrogen content in the molten iron. Adding an appropriate amount of rare earth elements to the molten iron before casting can effectively prevent the formation of sub-surface gas holes.

Use of other dehydrogenation agents: Such as tellurium (Te), etc., can also be used as dehydrogenation agents. Through chemical reactions, they can reduce the hydrogen content in the molten iron.

 

4. Control of casting process

Selection of appropriate casting temperature: Avoid falling into the dangerous casting temperature range to prevent the molten iron from absorbing more hydrogen due to temperature fluctuations during casting.

Accelerate casting speed: Under the premise of ensuring casting quality, try to shorten the casting time to reduce the contact time between the molten iron and air, thereby reducing the hydrogen content in the molten iron.

 

5. Control of mold factors

Management of wet molding sand: Control the moisture content of wet molding sand within an appropriate range (generally less than 5%) to avoid excessive moisture causing the molten iron to absorb hydrogen during casting.

Addition of additives: Adding additives such as coal powder, hematite powder, and ammonium difluoride to the sand mold can help prevent the formation of sub-surface gas holes.

 

6. Other measures

Strict control of the original hydrogen content in the molten iron: Practice has shown that when the hydrogen content in the molten iron reaches 4-5 ppm, sub-surface gas holes are prone to occur. Therefore, the original hydrogen content in the molten iron should be strictly controlled below 2-2.5 ppm.

Management of hydrogen content after furnace opening: Within 1.5 to 3.5 hours after furnace opening, the hydrogen content in the molten iron is relatively high and is not suitable for casting. Production plans should be reasonably arranged to avoid casting operations during this period.

 

To reduce the porosity in ductile iron through the design of sand molds, the following aspects can be considered:

 

I. Sand Mold Quality Control

 

Mold sand moisture control: The moisture content of wet mold sand should be strictly controlled within a certain range, typically recommended to be between 4.5% and 5.0%, to reduce the gas evolution of the mold sand. When the moisture content is too high, such as exceeding 5%, it will significantly increase the probability of subcutaneous gas holes in ductile iron.

 

Additives addition: Adding appropriate amounts of coal powder (4% to 6%), hematite powder (2%), and ammonium difluoride (2% to 2.5%) and other additives to the sand mold can help prevent the formation of subcutaneous gas holes. During casting, the coal powder can form a reducing gas film at the metal mold interface, inhibiting the reaction of water vapor at the interface and thereby reducing subcutaneous gas holes.

 

New sand addition and old sand reuse: Regularly add new sand, reduce the clay content of old sand, and install sand cooling devices and sufficient sand storage bins in the sand treatment system to ensure that there are no problems with hot molding sand during production.

 

The compactness and permeability of molding sand: Under the premise of ensuring the effective bentonite content of the molding sand, appropriately reducing the compactness of the molding sand can improve its permeability, which is conducive to the discharge of gases in the casting.

 

II. Sand Mold Structure Design

 

Exhaust system design: In the sand mold, add an exhaust system, such as exhaust grooves and exhaust pins, to double the exhaust area compared to the inner gate area, which is conducive to the discharge of gases in the mold cavity. Especially in the design of the pattern and template, more exhaust grooves and exhaust pins should be opened and set to improve the exhaust efficiency.

 

Sand mold hardness control: Under the premise of ensuring the hardness of the sand mold's horizontal and vertical surfaces, appropriately reduce the compaction specific pressure of the upper box to enhance the exhaust efficiency. At the same time, control the Baumé degree of the coating within an appropriate range and standardize the spraying operation method to prevent coating accumulation from affecting exhaust.

 

III. Other Relevant Measures

 

Melting and pouring control: Raising the tapping temperature and pouring temperature is conducive to the escape of gases and the removal of inclusions in the molten iron. At the same time, strictly control the chemical composition of the molten iron, especially the content of sulfur, magnesium, aluminum, titanium and other elements, to reduce the formation of subcutaneous pores.

 

The use of spheroidizing agent and inoculant: Low rare earth spheroidizing agent is adopted and its addition amount is strictly controlled to reduce the residual magnesium and rare earth content's influence on subcutaneous porosity. Meanwhile, appropriate inoculant is used to improve the microstructure and properties of the castings.

 

Improving the purity of molten iron: By implementing measures such as desulfurization outside the furnace and slag collection, the purity of molten iron is enhanced, reducing the content of slag and oxide inclusions, and thereby minimizing the occurrence of subcutaneous porosity.

 

Vigor has more than 18 years of experience in castings, we know how to avoid defects when production. If you have any questions or demand for product development or ito mprove your supply chain, please feel free to contact us at info@castings-forging.com