Knowledge

Section 1: Overview

 

The iron liquid filtration process refers to the procedure of removing large impurities such as inclusions and slags, as well as harmful elements and oxides from the iron liquid through special devices or materials, thereby purifying the iron liquid and enhancing the quality of castings. Its main objective is to reduce the impurity content in castings, lower the scrap rate, improve the material properties of castings, and enhance the surface quality of castings.

 

I. Filtration Principles and Methods

1. Rectification mechanism: During the casting process, a filter screen is placed. The liquid metal increases resistance when being poured, changes its flow state, and slowly flows into the mold cavity. This makes it less likely to generate vortices, ensures a stable filling process, reduces secondary oxidation and slag inclusion, and is conducive to the floating of inclusions and the slag-blocking function of the pouring system.

2. Mechanical filtration: By choosing a filter screen with an appropriate pore size, mechanical methods can be used to filter out inclusions larger than the pore size.

3. Cake filtration: During mechanical filtration, many inclusions larger than the pore size are captured on the filter screen. As the number of inclusions increases, a filter cake forms at the entrance of the filter screen. These filter cakes make the metal liquid flow thinner and have a filtering function, allowing for secondary filtration.

4. Adsorption: The filter screen has a large specific surface area. When the metal liquid flows through the filter screen, it is divided into small liquid flow units, increasing the contact area between the metal liquid and the filter medium. Harmful elements and oxides in the metal melt react chemically with the surface of the filter screen and are adsorbed on it.

 

II. Filtering Media and Equipment

1. Filtering Media: Commonly used filtering media include fiber filters (such as alkali-free cloth, high-silica glass fiber cloth, etc.) and foam ceramic filters. These media have a porous structure and can trap impurities in the molten iron.

2. Filtering Equipment: The required filtering equipment varies depending on the filtering process. For instance, for mechanical filtration and filter cake filtration, a filtering screen and corresponding support devices are usually needed; for adsorption filtration, special adsorption devices may be required.

 

Section 2: Types of Inclusions in Cast Iron Parts

 

Inclusions in cast iron parts include types such as sand particles, slag, inoculants or iron alloy particles, magnesium slag or dross, and microscopic inclusions, etc.

I. Sand Particles

Sand inclusions originate from loose sand or erosion by the metal flow. Sand particles in castings are usually caused by poor quality pattern equipment or worn sprue cups, and are also related to the interaction of sand particles and slag. Slag carried by the metal flow during pouring can also pull sand particles from the molding sand.

 

II. Slag

Slag is a glassy material formed due to oxidation, erosion of refractory materials, impurities, and the effect of molten sand. It may consist of MgO, CaO, SiO₂, Al₂O₃, MnO, FeO, MnO, eroded refractory materials, and various sulfides, which solidify into large masses and float in the metal liquid.

 

III. Inoculants or alloy particles, magnesium slag or dross

In gray cast iron and ductile iron castings, the alloy or inoculant itself is impure and contains inclusions. There is also the possibility that slag comes into contact with the inoculant and prevents its dissolution.

In ductile iron, there are two types of magnesium-based inclusions, magnesium sulfide (MgS) and magnesium oxide (MgO). These form clusters during the magnesium treatment due to the reaction between sulfur or oxygen and are very hard. Dross is a thin, sticky layer of magnesium silicate that forms in the metal liquid transportation and turbulent pouring system.

 

IV. Microscopic Inclusions

Various types of fine inclusions, such as titanium carbonitride, vanadium carbonitride, other nitrides, FeO, Al₂O₃, etc. Although the quantity of these inclusions may be large, they are usually very small and dispersed minutely. Their hardness has an impact on wear, erosion, and processing.

 

Section 3: Sources and Elimination Methods of Inclusions

 

I. Sources

1. Charge Materials: If the charge materials are severely rusted or improperly stored, the inclusion content in the molten metal may exceed the standard.

2. Melting and Solution Treatment: During processes such as inoculation treatment, spheroidization treatment, and refining treatment, if the process parameters are inappropriate or the operation is improper, slag may enter the molten metal.

3. Core and Mold Sand Strength: If the strength of the core and mold sand is insufficient to withstand the impact of high-temperature molten metal, it will cause sand flushing.

 

Secondary oxidation: Due to the structure of the casting or improper design of the gating system, the molten metal splashes violently during the pouring process, forming secondary inclusions.

 

II. Methods:

1. Use qualified and clean furnace materials and avoid open-air storage.

2. Develop a reasonable melting and intermediate treatment process and strictly follow the procedures.

3. Ensure sufficient core strength. Resin processes can be adopted, and special materials can be used in areas subject to strong thermal shock.

4. Design the gating system reasonably to prevent molten metal splashing, add slag traps, and extend the runner.

5. Use filters, etc. Among them, filters are the most effective measure to reduce and eliminate inclusions and porosity defects.

 

Vigor team have rich experience on castings. If any question or any demands we can help, please feel free to contact us at info@castings-forging.com