Manufacturers must know the characteristics of their different materials, including stainless steel. It’s crucial to understand how metals will react to the various circumstances that will be present in their operating environments. The performance of a particular grade in terms of corrosion resistance, grain structure, mechanical properties, and general cleanliness can be enhanced during the manufacturing process.
How certain grades are produced, and the mechanical and physical properties of stainless steel they must meet are governed by specifications, such as those established by the American Meteorological Society (AMS), the American Society for Testing and Materials (ASTM), and individual businesses like General Electric, and even entire industries. Melting is one of the manufacturing procedures essential to the characteristics of stainless steel. In today’s post, we’ll examine several techniques for melting stainless steel.
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Melting in a Vacuum Induction
A method that uses electric currents to melt the metal in a vacuum is vacuum induction melting (VIM). Dr Wilhelm Rohn and Heraeus Vacuumschmelze created this technique for the first time in 1920. The method was designed to refine speciality metals like cobalt and nickel. Stainless steel has been added to its list of applications, and VIM is primarily used in nuclear and aerospace applications.
A primary melting procedure called the VIM process uses an electromagnetic induction furnace inside a vacuum-sealed chamber. According to the induction melting principle, a primary electromagnetic coil’s high voltage electrical source causes a low voltage, a high current condition in the metal or secondary coil. Simply put, induction heating is a technique for transferring heat energy. Melting and casting are done at low pressures to regulate the entire alloy chemistry process.
Utilizing electrodes made in VIM furnaces, the VAR process is used in some crucial industrial applications, such as producing stainless steel suitable for medical implants. This VIM/VAR combined practice yields some of the most homogenous and defect-free stainless steel available.
Another choice for remelting and refining steel is electro-slag remelting (ESR). Typically, it will be applied to alloys made for military, aerospace, power generation applications and nuclear power plants.
The procedure is used for remelting and refining stainless steel to produce ingots of superior quality. An electrode made of the consumable subject material receives an electric current. The electrode tip melts into a pool of molten engineering slag at the base of a copper baseplate. Metal droplets travel through the slag to the bottom of the mould and cool as the electrode tip slowly melts. The water-cooled copper sleeve moves vertically as the electrode melts from the bottom up.
As the alloy solidifies, the slag rises, purifying the metal of impurities. Typically, highly reactive slags like calcium fluoride, lime, alumina, or other oxides are used in this process. As a result, the amount of sulfide in the alloy is reduced, the grain structure is greatly improved, and the amount of alloy segregation is eliminated.
Arc Vacuum Remelting
Vacuum arc remelting is another method of melting that is frequently used with stainless steel (VAR). This method of secondary melting yields metal ingots with a high degree of chemical and mechanical homogeneity. It is frequently found in sectors like healthcare and aerospace.
This procedure is an extra step that can be taken to improve the alloy’s quality; it is typically only used with high-quality stainless steel, titanium, and nickel alloys. It has many benefits. The melted metal’s solidification rate can be precisely controlled, and the vacuum cleans the metal of dangerous gases. Very clean material is produced by eliminating issues like inclusions, alloy segregation, and centerline porosity.
The VAR procedure also utilizes a cast or forged consumable electrode. After that, a vacuum is introduced into a sealed chamber with the metal inside. An engineered slag layer is placed on the copper baseplate, and an electric current is introduced to begin a continuous melt through it. Like the ESR unit, the water-cooled sleeve moves upward as the electrode is consumed. The standard EAF/AOD method, ESR remelted electrodes, or the VIM process—which we’ll look at next—can all produce electrodes for use in VAR.
Electric Arc Furnace
The electric arc furnace (EAF), the most common primary melting technique currently in use, uses a hearth, shell, and roof lined with refractory to contain the liquid metal. The furnace is first loaded with scrap metal, and then long, cylindrical graphite electrodes are lowered through the roof into the scrap charge. The electrodes deliver a strong electrical current into the charge, melting the scrap. On-grade chemistry can be achieved by adding and combining elements.
The EAF process has clear advantages over traditional techniques like the blast furnace. Unlike the blast furnace, which must run continuously, electric arc furnaces can easily be stopped from producing. Additionally, 100% of scrap input can be processed by EAF units, enhancing sustainability and bringing down production costs. A staggering 240 tonnes of material can be processed by the largest EAF unit in the world, demonstrating the technology’s ability to produce melts on a large scale.