Melting is one of the casting processes. The process in which metal materials and other auxiliary materials are put into the heating furnace for melting and tempering, and certain physical and chemical changes occur to the materials in the high-temperature furnace to produce coarse metals or metal concentrates and slag, that is usually molten metal in casting. What is the melting process in casting and why is it important? In this article, we’ll go through the basic requirements of molten iron and common steps of the melting process.

The Importance of Melting in Casting

Melting molten iron is an important phase in the production of iron castings. Casting quality includes internal quality, appearance quality and whether defects are formed, which are directly related to molten iron. For example, the fluidity of molten iron, the formability of thin-walled and structurally complex castings, and cold shut defects are affected by the temperature of molten iron. Whether the chemical composition of molten iron meets the requirements has a direct impact on the mechanical properties of castings. The content of gas and non-metallic inclusions in molten iron not only affects the strength of cast iron and the density of castings but also is related to the formation of defects such as pores and cracks in castings. 

 

Cast iron is required to be thin-walled and high-strength. The minimum wall thickness of castings is reduced from 4~6mm to 2~3mm, which requires that the pouring temperature of molten iron be increased accordingly. The temperature of molten iron also has an important influence on the internal quality of iron castings. For example, the quality index of gray iron castings has a clear relationship with the temperature of molten iron. In the production of nodular cast iron, the temperature of molten iron and the original sulfur content become the prerequisites for the success of spheroidization.

Basic Requirements for Molten Iron

1. Outlet temperature

The pouring temperature range of different grades of gray iron castings is approximately 1330-14100 degrees. In general, the tapping temperature of molten iron shall be at least 500 degrees higher than the pouring temperature, so the tapping temperature of molten iron shall not be lower than 1380-14600 degrees according to the specific conditions of cast iron brand and casting structure conditions. When extra thin (2-4mm) castings need to be poured, the tapping temperature should also be increased by 20-300 ℃. In order to meet the needs of casting, the tapping temperature of malleable cast iron of different grades shall not be lower than 1460-14800 ℃. For nodular cast iron and other modified cast iron, the temperature of molten iron will decrease significantly in the process of nodularization inoculation. In order to compensate for the temperature loss of molten iron, it is necessary to increase the tapping temperature of molten iron accordingly.


2. Chemical composition

The chemical composition of molten iron obtained by melting shall meet the specification requirements of castings. When smelting with cupola, burden calculation is the primary link to ensure that the chemical composition of molten iron meets the requirements. That is, according to the requirements of molten iron chemical composition, considering the changes of elements in the cupola smelting process and the actual situation of the charge, the mix proportion of various metal charges is calculated.


3. Harmful ingredients

During the melting process of cast iron, harmful elements (phosphorus, sulfur, and other trace elements that interfere with the normal crystallization and microstructure control of cast iron) must be controlled below the limit.

– Desulfurization. The sources of sulfur in molten iron in cupola smelting are the inherent sulfur in the charge and the sulfur absorbed from coke. Acid cupola has no desulfurization capacity, while alkaline cupola can play the role of desulfurization to a certain extent. When the basicity of slag is increased within a certain range, it is beneficial to reduce the sulfur content of molten iron; When the temperature increases, the amount of sulfur added in molten iron decreases in the smelting process; When the oxidizability of the furnace gas is strong, the FeO content in the slag increases, which is not conducive to the desulfurization reaction. Properly increasing the coke iron ratio and reducing air supply intensity is beneficial to desulfurization. However, when producing nodular iron castings, in addition to using a hot blast cupola for desulfurization inside the furnace, the measures of desulfurization outside the furnace are often used. The basic point of desulfurization outside the furnace is to maximize the contact area between desulfurized and molten iron to enhance the desulfurization effect. Common methods include shaking ladle desulfurization, jet desulfurization, mechanical desulfurization, mechanical stirring desulfurization, and multi-empty plug desulfurization.

– Dephosphorization. Phosphorus is harmful to the mechanical properties of cast iron, especially the toughness of nodular cast iron and malleable cast iron. Therefore, the phosphorus content of cast iron should be strictly controlled. The dephosphorization capacity of cupola smelting is very weak. Therefore, the phosphorus content of molten iron can only be controlled by batching. A certain proportion of low phosphorus pig iron and scrap steel shall be used for batching.


4. The molten iron is pure and contains less slag, gas and inclusions.

In order to remove the inclusions formed in the cupola smelting from the molten iron, a certain amount of limestone CaCO3 is often added as the solvent according to the burdened weight during the smelting process. Limestone decomposes at high temperature and combines with sediment and ash to form a complex compound with a low melting point – slag. The molten slag is easy to separate from the molten iron for easy removal. When the viscosity of slag is high, some fluorite (CaF2) can be added to reduce the melting point of slag.

Steps of Melting Process in Casting

1) According to the alloy brand specified in the technical requirements of castings, the chemical composition range of the alloy can be found and the chemical composition can be selected.

2) According to the burning rate of elements and composition requirements, the burden calculation is carried out to obtain the addition amount of various furnace charges, and the furnace charge is selected. If the furnace charge is polluted, it needs to be treated to ensure that all furnace charges are clean and free of rust, and preheated before feeding.

3) Check and prepare chemical appliances, paint and preheat to prevent pollution by gas, inclusions, and harmful elements.

4) Feeding. The general charging sequence is as follows: return charge, master alloy, and metal charge; metal charge with low melting point and easy to oxidize, such as magnesium, is added after the furnace charge is melted.

5) In order to reduce the gettering and oxidation pollution of the alloy liquid, it should be melted as soon as possible to prevent overheating. According to the needs, some alloy liquid must be protected with a covering agent.

6) After the furnace charge is melted, it is refined to purify the alloy liquid, and the refining effect is tested.

7) Modify and subdivide the structure as required to improve the performance, and inspect the treatment effect.

8) Adjust the temperature and pour. Some alloys should be stirred before pouring to prevent specific gravity segregation.