Larger, more aggressive jet engine and industrial gas turbine designs have increased the demand for large diameter premium superalloy billets for rotating component applications. Forging suppliers are requesting larger diameter billet with structures and properties equivalent to smaller diameter billet. This requires larger diameter ingots since grain size in forging billet is strongly dependent on the amount of work imposed to the starting ingot structure. However, ingot diameter has been limited by segregation tendencies. This paper summarizes the development of larger diameter superalloy ingots for these applications. Much of the work focuses on alloy 718, but advances gained in this alloy system have been applied to alloys 706, 720, and Waspaloy. Extensive process development has been necessary to establish robust practices with defined process windows for each alloy. All steps of the total melt process must be evaluated but special emphasis has been placed on the final melt process where several melt parameters need to be evaluated. Sometimes thermal treatments of electrodes are necessary to prevent melt rate cycles during remelting. Development programs now underway are also presented.

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The largest 718 ingots ever produced have been forged into 2000 mm (79 inch) diameter disks for land based turbines. The 686 mm (27 inch) diameter ingots are made by a triple melt process and weigh as much as 9000 kg (20,000 pounds). VAR melting parameters were varied in a development program to establish the processing window. Freedom from harmful positive (freckle) segregation has been verified by destructive evaluation of several full size ingots. Fine grain sizes of ASTM 9 to 11 have been achieved in the forgings through a multi-step billet and closed die forging practice. This fine grain size permits detection of small indications via ultrasonic testing, in all areas of each forging. Testing of material obtained through destructive evaluation of full size disks has shown mechanical properties to be within the same range as is found in smaller aircraft engine disks.

Triple melting (TM) by VIM+ESR+VAR has been highly successful for Alloy 718, large diameter ingots of Alloy 706, and high hardener alloys such as Alloy 720. Frequencies of ultrasonic indications for TM 718 are significantly lower than for VIM+VAR melt 718 because fewer clusters of inclusions are present, The cleanliness associated with TM 718 was verified by microscopy and by electron beam melting of buttons after VIM, VIM+ESR, and TM. Furthermore, white spot frequencies were found to be reduced for TM 718. Ultrasonic defect frequencies of Alloy 706 and Alloy 720 are also low. A major advantage of triple melting Alloy 706 is that ingots up to 36 inches (914mm) in diameter can be melted without freckles or positive segregation problems.