Selection of Melting procedures for Super Alloys
Super alloys are not only made in standard mill forms like bar, sheet, plate, tape and ribbon, but these may be formed as particular module shapes by forging or casting. It is crucially important that the feature needs of a module be completely recognized such as if a module is chosen to prevent corrosion, it may not require to be melted in a process that provides fatigue resistance.
The major crucial attribute of melt practice selection is the module size that needs to be made from an ingot. The bigger modules need bigger ingot sizes. The alloys containing higher compositions are tougher to produce in larger ingot sizes. The forged modules concern should not be made only of the final module size but also consider the strength of the ingot to be adequately deformed to produce the characteristics of a forged shape. Inner spotlessness is also a factor to consider in choosing the melting process.
Normally, for fatigue prone modules, a melt process is chosen that will assure the prevention of freckles from the module structure. For bigger size parts VAR is preferred than ESR. Although the prevention of freckles may be assured, none of VAR processes can assure the prevention of dirty spots in the resulted material. Therefore any material designed with VAR, a particular fault size is available in the new module.
For melting of alloys like Waspaloy that may be formed in significant no freckle ingot size the practice VIM (vacuum induction melting) and ESR may be chosen for producing mill products which do not need ingots in bigger sizes.
For sheet and plates, the guaranteed prevention of freckles is not crucial, however feasible pinhole corrosion at freckles should be taken into account. The alloys as sheets and plates are widely produced by following melt VIM and ESR or AOD and ESR methods.
Vacuum Arc remelting (VAR)
In vacuum arc remelting or VAR the boundary among ingot and crucible does not possess slag skin as observed in electroslag refining (ESR). Rather, the surface of ingot, an initial material for solidification has less solute elements. Crucially, the oxides or nitrides from electrode are moved into this surface layer upon melting the electrode and on the surface of the molten material. This layer is known as shelf. The non uniform areas are light and are called as dirty white spots. However strong VAR process factors and high quality electrode can decrease the frequency of production of dirty white sport but their production cannot be prevented completely. Therefore the probable availability of dirty regions in a material should be taken into account while choosing a material process for a superalloy.