Overview

In the material test certificate issued by the professional manufacturer, you will see this line:

Melt Practice: VIM + ESR

You may be confused. What is VIM and ESR? How do they relate to materials?

VIM, short for vacuum induction melting, is an advanced alloy smelting method that can better control alloying elements. The full name of ESR is electroslag remelting. It is a secondary smelting technology that can make the alloy obtain better properties.

Almost all superalloys use the VIM + ESR smelting method. In this article, we will introduce these two smelting processes in detail.

Necessary Knowledge

The Principle of Smelting

Smelting is the process of melting different metals and mixing them together. The ratio between different metals determines the different grades of alloys. When smelting, workers first place metals in a furnace in a specific order. The metal in the furnace is then melted by heating. After the metal is melted, the metal is uniformly mixed by stirring. Finally, the metal mixed solution is cooled down. After curing, the alloy is produced.

Ni

Cr

Fe

Figure 1
The Principle of Smelting

What is Slag?

During smelting, many elements that are harmful to the alloy enter the alloy. To reduce these harmful elements, some other substances are added to the alloy. These substances are called slag. Slag can absorb elements that are harmful to the alloy during smelting. At the same time, the slag is insoluble in the alloy. They float on the surface of the alloy liquid and are easily separated from the alloy.

Figure 2
The Function of Slag

What is Vacuum Induction Melting?

Induction Furnace

Induction furnaces generate heat through electromagnetic induction, which is then used to melt metals. Induction furnaces have the following features:

• 1. Induction furnaces use electromagnetic induction heating to melt metals. There is no metal contact with the heating element during the smelting process. It does not require carbon as an electrode for heating. Therefore, the induction furnace can smelt low-carbon or even carbon-free superalloys;
• 2. The induction furnace has no arc effect. The gas content of the smelted superalloy is low;
• 3. The induction furnace uses electromagnetism to stir the alloy, which is beneficial to the uniformity of alloy composition and temperature. Electromagnetic stirring can also promote the absorption of harmful elements by the slag;
• 4. When smelting in an induction furnace, the slag cannot be heated by induction. Its melting depends entirely on the heat conduction to the alloy after it is melted. Therefore, the temperature of the slag is lower, which leads to the limitation of the effect of the slag to a certain extent;
• 5. The crucible in the induction furnace has a short service life and high smelting cost;
• 6. High requirements for raw materials. The impurities in the raw materials should be as few as possible. The surface should be clean and free of rust. The size should be moderate. Should be stored in a dry place.

Vacuum Induction Melting

Vacuum induction melting (VIM) is the induction furnace melting of materials in a vacuum. Compared with ordinary induction furnace melting, vacuum induction furnace melting can further improve the quality of the alloy. It increases the hot workability and service temperature of the alloy.

Vacuum induction melting smelts superalloy ingots with accurate chemical composition and high purity through a series of physical and chemical reactions such as deoxidation, denitrification, and volatilization of impurity elements in vacuum.

Ni

Cr

Fe

Figure 3
Vacuum Induction Melting

Advantages of Vacuum Induction Furnace Melting

Precise Control of Alloy Composition

Smelting under vacuum avoids pollution caused by interaction with air, and the smelted superalloy has high purity. This makes it possible to precisely control the chemical composition of superalloys. Vacuum induction furnace smelting can strictly control the elements that are easy to react with oxygen and nitrogen (such as Al, Ti, B, Zr, Nb, rare earth elements, etc.) within a very narrow range.

For example, the content of aluminum and titanium can be controlled within the range of ±0.12% under vacuum, but can only be controlled within the range of ±0.25% at most in air. When smelting Inconel 718, the yield strength can be increased by about 10MPa for every 0.1% increase in niobium content. This precision is so strict that it can only be guaranteed by vacuum induction melting.

Volatile Harmful Elements

There are harmful impurities with low melting point in the raw materials, such as tellurium, lead, selenium, bismuth, copper, antimony, arsenic, tin, etc. These elements will vaporize after heating to a certain temperature, and part of them can be volatilized under vacuum, which will improve the purity of the material. The volatilization of impurities is related to temperature, time, surface area of alloy liquid, depth of alloy liquid and stirring.

Figure 4
Volatile Harmful Elements

Remove Gas Elements

Under vacuum, the gas elements in the alloy are more easily discharged. This further optimizes the properties of the alloy.

The Process of Vacuum Induction Melting

Charging

The raw materials are sequentially loaded into the crucible according to the melting point, density, degree of easy oxidation, volatility and quantity. Metals with a low melting point are placed on the bottom, and metals with a high melting point (tungsten, molybdenum, etc.) are placed on top. Oxidizable metals are placed in the hopper. At the same time, care should be taken to prevent chromium from contacting the crucible.

Vacuum

The smelting furnace should be sealed and the air in the furnace should be drawn out so that the furnace is in a vacuum state.

Melt

The raw material charged into the crucible is melted. Oxygen, nitrogen and hydrogen are continuously removed from the superalloy liquid during the melting process. At the same time harmful impurities are removed. In the initial stage of melting, the metal material will melt layer by layer. This layer-by-layer melting is very beneficial for removing air and impurities. Therefore, a high degree of vacuum and a slow melting speed should be maintained during melting.

Refining

The main task of the refining period is to make the superalloy melt continue to remove gas elements and remove harmful impurity elements through vacuum volatilization. At the same time, the content of alloying elements is adjusted to meet the specified range of the standard and achieve the optimal content. Refining can also ensure uniform alloy composition and reduce composition segregation.

During refining, an appropriate amount of carbon needs to be added to the alloy for deoxidation. Under vacuum conditions, carbon reacts with oxygen, and a large amount of CO gas is precipitated from the alloy liquid. At the same time, harmful impurities are volatilized. During refining, the refining temperature, time and vacuum degree should be reasonably controlled according to the specific requirements of the alloy.

Pouring

When the superalloy composition meets the requirements and the temperature is suitable, the alloy liquid can be poured into the mold to form an ingot. Due to the better fluidity of the superalloy liquid under vacuum, the pouring temperature can be appropriately reduced. Pouring speed depends on grade and shape of ingot. In the early stage, pouring should be done at a slower rate. When the alloy in the mold reaches a certain amount, the pouring speed can be accelerated. The use of ceramic filters for filtration of superalloys during casting can further reduce impurity content and oxygen and nitrogen content and improve mechanical properties.

What is Electroslag Remelting

Electroslag Remelting (ESR) is a secondary melting process. After vacuum induction melting (VIM), electroslag remelting of the alloy is generally performed in order to further improve the purity of the superalloy.

Electroslag remelting is a smelting method that uses the resistance heat generated by the electric current through the molten slag to melt the alloy. The heated and melted alloy passes through the slag in the form of droplets and then falls into the water cooling device to solidify to form an ingot.

In vacuum induction melting, slag is mixed in the alloy liquid to absorb harmful impurities in the alloy. However, in this case, it is difficult to ensure that the slag and the alloy liquid are in full contact.

In electroslag remelting, the alloy liquid passes through the molten slag one by one in the form of droplets. This fully guarantees that every part of the alloy can be fully contacted with the slag. Thereby removing impurities to the greatest extent possible.

Figure 5
Electroslag Remelting

Selection of Slag

In the electroslag remelting process, slag is mainly used to remove harmful impurities. Slag needs to meet the following requirements:

• 1. Have good liquidity;
• 2. The boiling point should be higher than the temperature in the furnace, and the melting point should be lower than the melting point of the metal;
• 3. Appropriate electrical conductivity and high resistance (to ensure the stability of the electroslag remelting process and increase the heat required for remelting);
• 4. If there is a need to remove sulfur, a highly alkaline slag needs to be used.

Commonly used slag materials for electroslag remelting of superalloys are CaF2, Al2O3, MgO, CaO, TiO2, etc. The following table lists common slag combinations:

The following are the roles of various substances in the slag:

• CaF2: Lowers the melting point of slag, improves conductivity and fluidity. It is the base ingredient of many slags.
• Al2O3: Increases electrical resistance, thereby increasing furnace temperature and melting speed.
• CaO: Reduces the conductivity of CaF2, improves alkalinity and fluidity, and has a desulfurization effect.
• MgO: Lowers the melting point of the CaF2-Al2O3-TiO2 slag and lowers the slag surface temperature. Prevent Al, Ti, Mg and other elements in the alloy from being removed. It has antioxidant properties.
• TiO2: Adding a small amount of TiO2 to the slag can prevent the titanium in the alloy from being eliminated by Al2O3. At the same time, TiO2 can increase the fluidity of slag.

The Process of Electroslag Remelting

• 1. Ensure that the surface of the material is smooth, rust-free and uniform in composition.
• 2. Select the material and quantity of slag.
• 3. Melt the slag. The purpose of this step is to maintain a high temperature before melting the alloy.
• 4. Pour the molten liquid slag into the furnace for remelting.
• 5. Determine the melting current and voltage. Then electroslag remelting is performed.

Conclusion

Superalloys are complex in composition and have strict requirements on the composition range. In order to meet this requirement, almost all superalloys use the smelting method of VIM + ESR.

VIM is an acronym for Vacuum Induction Melting. This smelting method can achieve non-contact heating and stirring, and can precisely control the composition range of the alloy and harmful elements.

ESR is an acronym for Electroslag Remelting. This smelting method can fully react the alloy with the slag to remove impurity elements. After vacuum induction melting, the properties of superalloys can be further optimized by electroslag remelting.

We have high quality vacuum induction furnaces and electroslag furnaces. The composition of the alloy can be customized for you through the smelting method of VIM + ESR. Please contact us if you have any enquiries for superalloys.