Manufacturers need to be well versed in the properties of the various materials they work with, including stainless steel. It’s important to know how metals will respond to the different conditions they will be exposed to in their operating environments. Manufacturing processes can improve a specific grade’s performance with regards to corrosion resistance, grain structure, mechanical properties and overall cleanliness.
Specifications like those created by the American Meteorological Society (AMS), the American Society for Testing and Materials (ASTM), individual companies such as General Electric and even entire industries dictate how certain grades are produced and the mechanical and physical properties they must meet. One of the manufacturing processes integral to the properties of stainless steel is melting. There are several methods for melting stainless steel, which we’ll be looking at in today’s post.
Electric Arc Furnace
The most popular primary melting method in use today, the electric arc furnace (EAF) involves a refractory-lined hearth, shell and roof to contain the molten metal. Scrap metal is first loaded into the furnace; long, cylindrical graphite electrodes are then lowered into the charge of scrap through the roof. A powerful electrical current is delivered into the charge through the electrodes, melting the scrap. Alloying elements can be added to achieve on-grade chemistry.
There are distinct advantages to the EAF process over older methods, such as the blast furnace. Electric arc furnaces can be taken out of production with ease, unlike the blast furnace, which must remain in continuous operation. EAF units also allow for the processing of 100% scrap input, which increases sustainability and lowers the cost of production. This technology allows for melt production on a massive scale; the largest EAF unit in the world can process a staggering 240 tons of material per melt.
Argon Oxygen Decarburization
Argon Oxygen Decarburization, generally abbreviated as AOD, is a process well-suited to the production of stainless steel alloys. It was invented by the Union Carbide Corporation in 1954 and is now used in approximately 75% of all stainless steel production.
Through the decarburization process the amount of carbon in the melted metal is refined. Oxygen and inert gas (such as nitrogen or argon) are blown into the stainless steel in precise ratios. This propagates the release of carbon monoxide from the metal, and the process will be repeated until the desired carbon level is reached.
The introduction of inert gases into the melt prevents the unwanted oxidation of necessary elements such as chromium, an important component in the chemistry of stainless steel. Reactive elements like aluminum and silicon can also be added to pull oxides from those same critical alloys.
Desulfurization is another aspect of the AOD process. A high lime concentration, combined with low oxygen activity in the metal bath, helps to dilute the sulfur. If necessary, more aluminum or silicon can be added to further release required metals from their oxides. After the desired sulfur levels have been reached the slag is then removed; the metal bath is able to be tapped and is now ready for additional processing.
Electroslag Remelting (ESR) is another option for remelting and refining steel. It will typically be used for alloys destined for aerospace, power generation, nuclear power plants and military applications.
The process is used to remelt and refine stainless steel into high-quality ingots. An electric current is passed through a consumable electrode of the subject material. The electrode tip melts into a pool of molten engineering slag at the bottom of a copper baseplate. As the electrode tip is slowly melted, metal droplets pass through the slag to the bottom of the mold and cool. A water-cooled copper sleeve traverses vertically as the electrode melts from the bottom up.
The slag floats upwards as the alloy solidifies, which allows the metal to be cleaned of impurities. This process generally relies on highly reactive slags such as calcium fluoride, lime, alumina, or other oxides. This leads to a reduction in the amount of sulfide in the alloy, produces a much better grain structure and eliminates the amount of alloy segregation present.
Vacuum Arc Remelting
Another melting process often used with stainless steel is vacuum arc remelting (VAR). This is a secondary melting process that produces metal ingots that have an elevated chemical and mechanical homogeneity. It is commonly found in industries such as medical and aerospace.
This process is an additional step that can be done to increase the quality of the alloy, and is typically reserved for premium stainless steel, titanium and nickel alloys. It offers several advantages. The solidification rate of the melted metal can be precisely controlled, and the vacuum process removes harmful gases from the metal. Problems such as centerline porosity, alloy segregation and inclusions are eliminated; the end result is material with a very high degree of cleanliness.
Much like ESR, the VAR process uses a cast or forged consumable electrode. The metal is then placed in a sealed chamber and a vacuum is introduced. An electric current is introduced to start a continuous melt through an engineered slag layer atop the copper baseplate. The water-cooled sleeve then travels upward as the electrode is consumed, same as in the ESR unit. VAR can use electrodes produced by the standard EAF/AOD method, ESR remelted, or the VIM process, which we will examine next.
Vacuum Induction Melting
Vacuum induction melting (VIM) is another process that relies on electric currents to melt the metal within a vacuum. This method was first developed in 1920 by Heraeus Vacuumschmelze and Dr. Wilhelm Rohn. The process was originally intended for refining specialty metals such as cobalt and nickel. Its use has expanded to include stainless steel and VIM is primarily used for aerospace and nuclear applications.
The VIM process is a primary melting procedure comprised of an electromagnetic induction furnace in a sealed vacuum chamber. The principle of induction melting is that a high voltage electrical source from a primary electromagnetic coil induces a low voltage, high current in the metal, or secondary coil. Induction heating is simply a method of transferring heat energy. To control the entirety of alloy chemistry process, the melting and casting are conducted at low pressures.
Certain critical industrial applications, such as medical implant quality stainless steels, involve the VAR process using electrodes produced in VIM furnaces. The result from this VIM/VAR combined practice is some of the most defect free and homogenous stainless steels available.
Your Technical Services Professional
The melting and remelting of stainless steel alloys will occur well before the manufacturer ever touches a piece of metal. It’s important to understand the entire alloying process and to only work with suppliers that you trust so you can rest assured that the finished metal you are purchasing is of the highest quality. At Clinton Aluminum, we are dedicated to providing premium stainless steel plate, sheet, bar and more for our customers.
Our knowledgeable and friendly sales team is standing by to discuss your stainless steel needs and answer any questions that you might have. Contact us today to learn more.