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Vacuum Induction Melt Electroslag Refine Vacuum Arc Remelt

Vacuum Induction Melt Electroslag Refine Vacuum Arc Remelt

Cobalt based alloys generally do not require vacuum melting and are melted in air induction melting furnaces; however, these alloys are quite frequently remelted using either VAR or ESR. Cobalt based alloys containing aluminum, titanium, zirconium and tantalum must be vacuum melted using VIM. Vacuum melting of the other cobalt based alloys can be used to enhance strength and ductility as a result of improved cleanliness and compositional control.

Initial ingot breakdown is conducted well above the y' solvus to allow chemical homogenization and microstructural refinement; however, care must be taken to make sure that the temperature does not exceed the incipient melting temperature. Cast ingots are initially refined by cogging (Fig. 6.9), a forging operation that uses open dies and hydraulic presses. New cogging equipment and tighter, more reproducible controls are being used to produce more uniform billets. Automated equipment is used to manipulate the billet and carry out the deformation producing uniform deformation along the length of the billet. Continued deformation is conducted at successively lower temperatures to give greater microstructural refinement. Producing a fine, uniform grained billet is important for at least three reasons: (1) fine grained billets lead to fine grained forgings which improves fatigue life; (2) fine grained billets yield lower flow stresses and uniform reductions during forging; and (3) fine grained billets are easier to ultrasonically inspect assuring increased levels of cleanliness.14

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