Vacuum arc remelting

Vacuum arc remelting (VAR) is a secondary melting process used to produce metal ingots with increased chemical and mechanical homogeneity for demanding applications.The VAR process has revolutionized specialized traditional metallurgical technology industries and has enabled tightly controlled materials in the fields of biomedicine, aeronautics and aerospace,overviewVAR is used most frequently in high-value applications.It is an additional processing step to improve the quality of the metal.Because it is time  consuming and expensive, most commercial alloys do not use this process.Nickel, titanium and specialty steels are the materials most commonly machined using this method.Traditional titanium alloy production routes include single-pass, double-pass or even triple-pass VAR processing.Compared with traditional methods, using this technique has several advantages:

• The solidification rate of the molten material can be strictly controlled.This allows a high degree of control over the microstructure and the ability to minimize segregation.

• Gases such as nitrogen, oxygen and hydrogen that dissolve in the liquid metal when melting the metal in an open furnace are considered harmful to most steels and alloys.Under vacuum conditions,these gases escape from the liquid metal.

• Reduced concentrations of elements with high vapor pressure, such as carbon,sulfur and magnesium (often pollutants).

• Eliminates centerline porosity and segregation.

• Certain metals and alloys, such as titanium,cannot be melted in open-air furnaces.

Process description 

Alloys to be VAR are usually formed into cylinders by vacuum induction melting (VIM) or ladle refining (airmelt).This cylinder, called the electrode, is then placed in a large cylindrical closed crucible and placed in a metallurgical vacuum (0.001–0.1 mmHg or 0.1–13.3 Pa).At the bottom of the crucible is a small amount of alloy to be remelted, and the top electrode is brought close before melting begins.Several thousand amps of direct current are used to start an arc between the two pieces, creating a continuous melt.The crucible (usually made of copper) is surrounded by a water jacket to cool the melt and control the rate of solidification.To prevent arcing between the electrodes and the crucible walls, the diameter of the crucible is larger than the electrodes.Therefore, the electrode must be lowered as the melt consumes it.Controlling current, cooling water and electrode gap is critical to effectively control the process and produce defect-free material.Ideally, the melting rate remains constant throughout the process cycle, but monitoring and controlling the vacuum arc remelting process is not straightforward.This is because a complex heat transfer takes place involving conduction, radiation, convection within the liquid metal, and advection due to Lorentz forces.Ensuring consistency of the melting process in terms of melt pool geometry and melting rate is critical to ensure optimal performance of the alloy.

Electro-slag remelting

Electroslag remelting (ESR), also known as electroslag remelting, is a process for remelting and refining steel and other alloys used in mission-critical applications in aircraft, thermal and nuclear power plants, military technology and other fields.The electroslag remelting (ESR) process is used to remelt and refine steel and various superalloys to produce high-quality ingots.The process can be initiated by vacuum induction melting.The ESR process uses as-cast alloys as consumable electrodes.An electric current (usually alternating current) is passed between the electrodes and a new ingot, which is formed at the bottom of a water-cooled copper mold. The new ingot is covered with engineering slag superheated by an electric current.The electrode tip slowly melts from contact with the slag.These metal droplets travel through the slag to the bottom of the water-cooled mold, where they slowly freeze as the ingot solidifies directed upward from the bottom of the mold.The slag pool floats above the refined alloy, floating upward as the alloy solidifies.Removes impurities from the molten metal that would chemically react with the slag or otherwise float to the top of the molten pool as the droplets pass through the slag.Electroslag remelting uses highly reactive slag (calcium fluoride, lime, alumina or other oxides are often the main constituents) to reduce the amount of Type A sulfide present in biometal alloys.This is a common practice in European industry. ESR can also reduce other types of inclusions and is considered an alternative to the vacuum arc remelting (VAR) method commonly used in the US industry.An example of the use of electroslag refined (ESR) steel technology is the L30 tank gun.CrNi60WTi is a stainless steel that is best formed by electroslag remelting or vacuum arc remelting.This alloy can be used to build nuclear power plants.