Melting and Primary Fabrication613

There are three major types of nickel deposits: nickel-copper-iron sulfides, nickel silicates, and nickel laterites and serpentines.5 The sulfide deposits, which are located in Canada, provide most of the Western world's supply. The nickel-copper-iron sulfide ore is crushed and ground and the iron sulfide is separated magnetically. The remaining nickel and copper ores are then separated by flotation. The nickel concentrate is roasted, smelted in a reverberatory furnace and converted to a Bessemer matte which consists mainly of nickel and copper sulfides. The copper-nickel matte is cooled under controlled conditions so that discrete crystals of nickel and copper sulfides and a nickel-copper metallic alloy are formed. After the cooled matte is crushed and ground, the metallic alloy is separated magnetically and treated. The remaining copper and nickel sulfides are separated by froth flotation. The nickel sulfide is roasted to produce various grades of nickel oxides which are then converted to pure nickel and nickel alloys.

Vacuum induction melting (VIM), vacuum arc remelting (VAR) and elec-troslag remelting (ESR) are used in the production of nickel and iron-nickel based superalloy ingots. In the VIM process, liquid metal is processed under vacuum in an induction heated crucible. VIM is used as the initial melting method to reduce interstitial gases to low levels, enable higher and more controllable levels of the reactive strengthening elements aluminum and titanium, and eliminate the slag or dross problem inherent in air melting. After initial VIM processing, the alloys are then remelted using either VAR or ESR.

Vacuum induction melting is used to produce the desired alloy composition. Feedstock for VIM includes pure elements, master alloys and recycled scrap. VIM is used to remove dissolved gases (oxygen, nitrogen and hydrogen) and other impurities before the addition of reactive alloying elements to the melt. Ceramic filters are also used to remove large oxide and nitride inclusions during the final pour. The VIM process may be the only melting process if the ingot is going to be remelted for casting; however, if the ingot is going to be hot worked, it is must be secondarily melted, because VIM ingots generally have coarse and non-uniform grain sizes, shrinkage and alloying element segregation that restricts hot workability during forging.

Vacuum arc melting and electroslag remelting are used to further refine the ingot after initial VIM processing. In the VAR process, an arc is struck between the end of the ingot electrode and the bottom of a water cooled copper crucible. The arc generates the heat to melt the end of the electrode which drips down into the crucible. In the ESR process, remelting does not occur by striking an arc; instead, the ingot is built-up in a water-cooled crucible by melting a consumable electrode that is immersed in a slag that is heated by resistance heating. The ESR process does not require a vacuum since the molten metal is protected from the atmosphere by the slag covering.

In addition to refining the composition, VAR and ESR refine the solidification structure of the resulting ingot. The ESR process is inherently capable of producing cleaner metal than VAR, while VAR is capable of producing larger ingots with fewer segregation defects. Therefore, a triple melting process (VIM-ESR-VAR) is used for producing large ingots for forging stock for gas turbine components,13 as shown in Fig. 6.8. Other triple melt processing options include double VIM followed by VAR or ESR and VIM-VAR-VAR. In some highly alloyed nickel based alloys, containing a high volume fraction of y', even VIM-VAR or VIM-ESR does not provide an ingot structure satisfactory for hot working. These alloys are normally processed by using powder metallurgy methods.

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