Melting procedures for production of superalloys
The production of superalloys that are used in the present time is a story of the invention of advanced melting technology. Either the final product is forged or cast, the basis of superalloy’s ability is to produce the characteristics based on the suitable option and adaption of melting techniques.
Superalloy melting practices are categorized as primary (initial melt of raw material or scrap that sets the composition) or secondary (remelting of primary melt for controlled solidification structure). The melt kind or combination of chosen melt kinds depend on chemical composition of alloy, mill form and size needed,required properties and sensitivity of final part to localized inhomogeneity in alloy.
Two popular primary melt practices are argon-oxygen decarburization (AOD) treatment of electric arc processed melt arc and vacuum induction melting. The popular secondary melt procedures are vacuum arc remelting and electroslag remelting.Some alloy products are produced from cast ingots subsequent to primary melting. The industrial material size standards like AMS will describe suitable melting practices.
Argon – Oxygen Decarburizing Melting
The argon – oxygen decarburizing (AOD) treatment common among super alloys is slightly different from those methods that are utilized for stainless steels. An electric arc furnace receives initial charge and gets power from arc mounted between graphite electrode to heat up and melt the charge. Upon melting of charge, further electricity is not required, heat is supplied to the charge through supplying oxygen beneath the melt surface to react with elements like aluminum, titanium, silicon and carbon. Addition of desulfurization agent is done. The slag produced by oxidation materials and desulfurization inclusion is physically eradicated from the furnace. The deslagged charge is sent to argon oxygen decarburizing vessel that is a refractory lined steel shell with tuyeres in the base. The tuyeres are employed to send a combination of argon and oxygen in the molten bath. By controlling the argon to oxygen ratio, the specific oxidation or reduction process of elements can be done. The reduced principal element is carbon. The elements that tend to be retained in a melt portion may also be sensitive to oxidation into slag. Although inclusion of aluminum to heat will combine with slag and decrease chromium back into the molten charge. No additional and external heating source is given to argon-oxygen decarburizing tube. The molten charge is heated through inclusions of aluminum and oxidation of them. Quenching is done by inclusion of solid scrap to the bath. When the required composition and pouring temperatures are achieved, heat is extracted and sent to electrode or ingot pouring unit.Precautions are followed to decrease the reacting rate of pour stream with surrounding or natural gases.
Other common primary melt technique is VIM (Vacuum induction melting) whose furnace includes ceramic lined crucible built up around water quenching induction coils. The crucible is kept in a vacuum container that may comprise of many vacuum ports.
The charge material is sent into crucible, molds are sent to chamber, systems for discarding slag from pour stream may be provided and sample of molten metal may be produced for composition control.
The charge is classified into three parts: new, reactive and slip back. New materialis often an elemental material that may also comprise of any other material that has not been earlier vacuum melted. The reactive elements recommend the powerful oxide producing elements such as aluminum, zirconium and titanium for superalloys.VIM process includes the charging of new part of charge into crucibles. The furnace is pumped down and a calculation is performed for the increased pressure when the vacuum ports are blanked off. Power is supplied to the induction coils, magnetic field links created cause to induce current in the charge specimen,heating it. Gases are released by molten charge.
Calcium is usually added to decrease sulfur in the charge through production of CaS slag. The chemical composition of sample is evaluated. The metal is powered into the molds with the help of a system for metal transit as well as slag control and maintenance of pour rate. The cast product for straight treatment is called as ingot. Usually, VIM product is proposed for secondary melting and is called as an electrode as it is used in electrically heated form in the secondary procedures.
As compare to VIM, AOD employs more economical charge material and shows higher forging rates. Therefore arc AOD is the cheapest process available. Although it is confined to its ability to accurately monitor composition in a small rage and also creates very large oxygen content material. But choosing ESR(electroslag remelting) as a secondary melting method may recompense to some extent for increased content of oxide in arc AOD procedure.
Electroslag remelting is a commonly used secondary melt method for arc AOD electrodes. In this melting process, AC current is supplied to an electrode present in a water quenched crucible comprising of molten slag charge. The purposed circuit of current travels from electrode, molten slag, solidifying ingot, water quenched stool and symmetrically arranged by bars back to the electrode. The slag is normally a calcium fluoride based enhanced material through large inclusions of calcium oxide, magnesium oxide and alumina. The nominal inclusions of titanium oxide and zirconium oxides may be made to compensate the reduction in titanium and zirconium while melting. Electricity traveling through the slag keeps it in molten form that melts the immersed part of electrode, it is reduced to sulfur and entrained oxides are included in the slag. The completely produced drop falls through slag and is gathered in water quenched crucible. A solid film produced against the quenched crucible by slag is remelted partly through molten metal however it sustains as a film between crucible and ingot. With full use of electrode, an ingot is formed inside the crucible.
Vacuum Arc Remelting
Vacuum arc remelting is preferably followed than ESR (electroslag refining) to meet the needs of larger size ingots. This process is conducted in vacuum by supplying DC current. Electrode is melted in the water quenched crucible similar to ESR. The melting process is completed by hitting arc among electrode face and crucible stool. Electrode face is melted by arc and the molten material is collected in drops that fall into the crucible to produce an ingot. As this process doesn’t need an additional source of heat, neither the insulation slag skin on ingot, moreover heat extraction in this method is more than that of ESR therefore vacuum arc remelting is more preferred for any alloy material of any size.