Vacuum Induction Melting Furnace Melting Process

The vacuum induction melting process is primarily divided into several stages: loading, melting, refining, and casting.

3.1 Filling Materials

3.1.1 Material Requirements

(1) Precisely control the chemical composition of various metallurgical raw materials; no confusion allowed.

(2) The raw material contains low levels of S and P, and the low melting point non-ferrous metal impurities such as Pb, Bi, and Sn should be reduced; (3) The gas content in the raw material should be minimized:

(4) Raw materials must be particularly clean, free of rust and oil stains:

(5) Raw materials are stored in a dry environment to prevent the introduction of gases during the melting process that could cause spatter.

(6) Control raw material dimensions based on furnace capacity and power frequency.

(7) Materials can include recycled materials, high-grade steel, pure metals, and intermediate alloys.

3.1.2 Material Requirements

Vacuum induction melting takes place within a vacuum chamber, where the charging process differs from conventional induction melting. All raw materials are divided into two parts; one is directly loaded into the crucible, and the other is placed into an alloy material box for later addition to the melting pot during the melting process.

(1) Basic materials such as Fe, NiW, MoCo, V, and C can be directly loaded into crucibles.

(2) The temperature inside the crucible is uneven, with a higher temperature at the bottom and a lower temperature at the top. In principle, refractory metals and elements in larger quantities should be placed in the high-temperature zone.

(3) Place small pieces at the bottom of the crucible to quickly form a melt pool, and place larger pieces in the upper part of the crucible for easier preheating and collapse; (4) Ensure that the material is loosely packed on top and tightly packed at the bottom to prevent the upper crucible material from being stuck or welded, causing a "bridging" during the melting process; (5) Tilt the crucible slightly forward while loading, avoiding flat placement of the upper material and concentrating it towards the side for steel extraction.

Under hot-bath conditions, rapid loading is required to prevent condensation on the water-cooled surface of the inductor, which increases crucible aspiration, extends the vacuum pump time, lowers the temperature, and affects the crucible's lifespan.

Materials primarily included in the alloy material boxes are:

(1) Active and oxidizable elements, as well as trace elements (such as Al, Ti, Ce, Zr, and B), should be added under conditions of good deoxidation in the metal melt. (2) When adding elements with high vapor pressure and volatility (such as Mn), the melting chamber should first be filled with inert gas Ar to control the internal pressure of the furnace.

3.2 Melting

The primary tasks during the melting phase include melting the charge, degassing, removing low-ashpoint harmful impurities and non-metallic inclusions, and ensuring the metal liquid reaches an appropriate temperature. The vacuum level in the refining pond must meet the required standards to facilitate the refining process. The melting phase is time-consuming throughout the entire smelting process. Key technologies during the melting phase are as follows:

Vacuum Degree

After furnace integration, the high straightness degree of s10Pa is used for power heating, maintaining a high straightness degree before the charge is melted. During the melting period, the pool depth gradually increases, with the new metal surface continuously exposing, and the high straightness degree is beneficial for gas management.

(2) Power Supply System

During the initial melting phase, due to the skin effect of the induced current, the charge material melts layer by layer. Layer-by-layer melting of the charge material is beneficial for degassing and removing non-metallic inclusions, thus, maintaining a high vacuum and a slow melting rate during the melting period is essential. A rational power supply system is required; at the start of melting, input power should be gradually increased according to the different characteristics of the metal charge material, allowing the charge material to melt at an appropriate rate. If melting is too rapid, gases may precipitate abruptly from the metal melt, causing

Spraying during Melting Period

Rapid melting speed, large chunks of cold material falling into the melt pool, too rapid addition of material or excessive gas content, and excessively high metal liquid temperature can all cause massive splashing of the metal liquid during the melting period. Most of the splashed metal liquid adheres to the upper part of the crucible wall, forming a ring-shaped shell. This not only results in a significant loss of metal material but can also, in severe cases, prevent the smelting process from continuing. In the event of splashing, measures can be taken to reduce the melting speed (decrease input power) or slightly increase the pressure in the melting chamber (close the vacuum valve).

3.3 Refinement

The primary tasks during the refining period include deoxygenation, degassing, removal of volatile impurities, temperature adjustment, and composition adjustment. Throughout the refining process, careful control of refining temperature, vacuum level, vacuum duration, and alloying techniques is essential.

Refined Temperature

Highly refined temperatures are beneficial for the carbon-oxygen reaction and the decomposition and volatilization of inclusions, but excessively high temperatures can exacerbate the reaction between the crucible and the metal, increasing the volatilization loss of alloy elements. Therefore, the refining temperature of alloy steel is usually controlled at 100°C above the melting point of the metal being refined.

(2) Direct空虚度

Improved vacuum promotes carbon-oxygen reaction, as CO bubbles rise and are expelled, it facilitates the precipitation of [H] and [N], the upward movement of non-metallic inclusions, the decomposition of nitrides, and the volatilization of trace harmful elements. However, excessively high vacuum intensifies the reaction between the crucible and the metal, and increases the volatilization loss of alloy elements. Therefore, for large vacuum induction furnaces, the vacuum during the refining period is typically controlled between 15~150Pa, while for small furnaces, it is kept below 10Pa.

(3) Vacuum Time

The oxygen content in the molten metal first decreases and then increases, so the time it takes to reach the oxygen content is the refining time, which for a 500kg vacuum induction melting furnace is between 50 to 70 minutes. After the furnace charge is baked clean, it is immediately mixed with an appropriate amount of lumps of stone or other high-carbon materials to initiate a carbon-oxygen reaction.

Alloying

After the refining process, after thorough deoxygenation, degassing, and volatilization of impurities, active metals and trace additives are added. The composition is adjusted, and the alloying process is conducted. The general sequence of addition is typically Al, T, Zr, B, Re, Mg, and Ca.

Add uniformly and slowly to prevent splashing. Mix with high power for 1-2 minutes after addition to accelerate the alloy's refining and even distribution. Due to the high volatility of Mn, it is generally added 3-5 minutes before the steel is tapped.

3.4 Casting

The molten steel's pouring temperature and casting temperature are crucial for the metallurgical quality of the finished product. Insulation caps or thermal insulation boards can be used during casting.

(1) Casting Temperature

During the casting and metal solidification process, the overflow of gases in the molten steel, the floating of inclusions, and the shrinkage compensation during solidification require the molten steel to have a certain superheat to maintain good fluidity. However, if the temperature is too high, the columnar grain region...

Slump has decreased, the area of the coarse product zone has expanded, inclusions have increased, and porosity is severe, leading to short-length waste products. Therefore, the pouring temperature is generally controlled above the liquidus point by 50~80℃.

(2) Live Casting

The purpose of electrically charged casting: Firstly, it pushes the slag towards the back of the crucible during casting to prevent it from flowing into the steel ingot and electrode; secondly, it maintains a uniform temperature of the molten steel, reducing temperature differences.

Post-casting, maintain vacuum

Do not apply vacuum immediately after casting; generally, vacuum should be applied 5 to 15 minutes after the steel is poured to prevent oxidation of the red-hot casting and the crucible wall from adhering metal, lighten the burden of smelting in the next furnace, ensure alloy quality, and can avoid

The oxidation and mold casting of red-hot metals, and the normal crystallization process. For complex high-temperature alloys, cooling can be conducted under vacuum after casting.