Info

Scrap/Revert

Scrap/Revert

Casings Blades Vanes Integral Wheel

Blades

Disks

Rolled Rings

Casings Blades Vanes Integral Wheel

Blades

Disks

Rolled Rings

Fig. 6.4. Process Flow for Superalloy Components8

but better creep and stress rupture properties. Accordingly, wrought alloys are used where tensile strength and fatigue resistance are important, such as disks, and cast alloys are used where creep and stress rupture are important, such as turbine blades. It should also be noted that the amount of alloying is so high in some superalloys that they cannot be produced as wrought products (which are produced from large diameter ingots); they must be produced as either castings or by powder metallurgy methods. In general, the more heat resistant the alloy, the more likely it is to be prone to segregation and brittleness, and therefore formable only by casting or by using powder metallurgy techniques.

Two processes are used for the production of forged superalloys, ingot metallurgy and powder metallurgy. Ingot metallurgy9 often involves triple melt technology which includes three melting steps followed by homogenizing and hot working to achieve the desired compositional control and grain size. The three melting steps include (1) VIM to prepare the desired alloy composition, (2) ESR to remove oxygen containing inclusions and (3) VAR to reduce compositional segregation that occurs during ESR solidification. Melting is followed by homogenizing and hot working to achieve the desired ingot homogeneity and grain size.

Powder metallurgy (PM)9 is often required for high volume fraction y' strengthened alloys, such as René 95 and Inconel 100, which cannot be made by conventional ingot metallurgy and forging without cracking. The PM process (Fig. 6.5) includes (1) VIM to prepare the desired alloy composition, (2) remelt-ing and atomizing to produce powder, (3) sieving to remove large particles and inclusions (>50-100 ^m in diameter), (4) canning to place the powder in a container suitable for consolidation, (5) vacuum degassing and sealing to remove the atmosphere and (6) hot isostatic pressing (HIP) or extrusion to consolidate the alloy to a billet. During HIP, small (<20 ^m) pores containing argon can be collapsed and tend to stay closed if subsequent processing temperatures are not significantly above the original HIP temperature. Billets are then subsequently forged to final part shape.

Creep failures are known to initiate at transverse grain boundaries, and it is possible to eliminate them in cast turbine blades to obtain further improvements in creep and stress rupture resistance. This can be achieved by directional solidified (DS) castings with columnar grains aligned along the growth direction, with no grain boundaries normal to the high stress direction. Further, by incorporating a geometric constriction in the mold or by the use of a seed crystal, it is possible to eliminate grain boundaries entirely and grow the blade as one single crystal (SX). The elimination of grain boundaries also removes the necessity for adding grain boundary strengthening elements, such as carbon, boron, zirconium and hafnium.10 The removal of these elements raises the melting point and allows higher solution heat treatment temperatures with improvements in chemical homogeneity and a more uniform distribution of y' precipitates.7

Vacuum Induction Melt

Remelt and Atomize

Vacuum Induction Melt

Remelt and Atomize

Vacuum

Screen

Screen

Vacuum

Sieve

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