Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-03-17
2001-02-27
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S676000
Reexamination Certificate
active
06193823
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the processing of nickel-iron-base alloys, and, more particularly, to the refinement of ingot grains through the precipitation of delta-phase precipitates.
A semifinished wrought nickel-base alloy article is conventionally made by first melting blends of the constituent elements of the alloy. The final cast ingot is produced using one or more cast and remelt steps. Primary thermomechanical working of the ingot (termed “ingot conversion”) converts the structure of the cast ingot from a coarse as-cast dendritic grain structure to a fine equiaxed billet grain structure (termed a “cast-wrought” structure). The primary conversion step in highly alloyed metals such as superalloys also serves to break up interdendritic regions, thereby reducing the segregation that occurs during solidification of the ingot. The semifinished billet is then further processed into its final geometry by forging, heat treating, and machining. An example of a nickel-iron-base alloy manufactured into articles by this approach is alloy 718.
Using conventional casting methods the average dendrite grain size in as-cast ingots is typically coarser than ASTM 1 and often coarser than ASTM 00 grain size. If such a material were processed directly into the shape of the final product, an insufficiently high level of strain would be introduced to allow uniform recrystallization of the ingot structure. The result would be non-recrystallized regions that retain the large as-cast grain structure of the ingot. Because grain size is one of the primary parameters controlling properties in these materials, the non-recrystallized regions will typically either fail the microstructural requirements for the final product or, if not detected, will lead to accelerated mechanical failure of the product. The thermomechanical ingot conversion of the ingot into billet prevents the presence of as-cast grains in the billet and also mitigates associated chemical segregation in the billet, ensuring a fine-grain structure in the billet.
Conventional ingot conversion processing is an expensive operation. The function of the primary thermomechanical working is to recrystallize the material using the strain added during each working operation to nucleate new grains at existing grain boundaries. Commercial ingots of nickel-iron-base superalloys often weigh thousands of pounds. Because of the large size of these ingots, it is difficult to achieve a uniformly high level of strain for recrystallization in a single thermomechanical operation. The large-grain areas in the ingot, having a lower grain boundary density, require more cumulative strain to achieve a uniform final grain size. The ingot conversion therefore typically requires multiple upset and draw operations. In theory, the coarser the starting grain size the more thermomechanical operations will be required to refine the grain size.
The equipment to accomplish the multistep thermomechanical conversion in commercial practice is large in size and expensive. The rough thermomechanical working also requires skilled operators, and careful control over the processing practices. This equipment requirement and extensive processing cannot be avoided or significantly reduced in scale under conventional practice because of the non-uniformity in as-cast grain structure due to differences in solidification rates across the large cross-section of the large ingots.
There is a need for an improved approach to the mechanical processing of ingots into final articles, which reduces the required processing and still results in an article of acceptable, or even improved, final quality. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a method of fabricating an article of a nickel-iron-base superalloy. The method of the invention eliminates the need for the multiple primary thermomechanical ingot conversion operations of conventional practices, replacing them with a single, but fully effective, heat treatment of the ingot. The present approach may be used with undeformed ingot material, or with ingot material that has been deformed by a thermomechanical treatment. In either case, the resulting ingot is suitable for final mechanical working into the shape of the semifinished article, and thereafter processing into the final finished article. The present approach may be used with nickel-iron-base alloys that precipitate the non-coherent orthorhombic delta phase Ni
3
Nb, such as the widely used alloy 718.
In accordance with the invention, a method of fabricating an article comprises the steps of providing an homogenized ingot of a nickel-iron-base alloy having a composition comprising at least about 10 weight percent iron, from about 4.5 weight percent niobium-plus-tantalum to about 5.5 weight percent niobium-plus-tantalum, more nickel by weight than any other element, and capable of forming delta-phase precipitates. Alloy 718 is such a material, having a nominal composition of about 20 weight percent iron, 5 weight percent niobium and a low tantalum content, and about 52.5 weight percent nickel. The ingot also has fewer than about 1 grain per square inch at 100× magnification. In many locations, the ingot typically has fewer than about 0.06 grains per square inch at 100× magnification. The ingot is preferably in the as-cast, homogenized, undeformed state, but it may be deformed to any total strain but not recrystallized prior to precipitation. An array of predominantly intragranular, coarse, noncoherent delta-phase precipitates is precipitated within the ingot, preferably by heating the ingot to a temperature of from about 1600° F. to about 1675° F. The heat treatment temperature should be approached from a lower temperature by heating rather than from a higher temperature by cooling, to avoid the formation of a high volume fraction of intergranular delta phase precipitates that form at higher temperatures. Desirably, the delta-phase precipitates are present in a volume fraction of at least about 20 volume percent. The ingot having the array of intragranular delta-phase precipitates is then deformed at a temperature near to but below the delta-phase solvus temperature of the nickel-iron-base alloy, typically from about 1800° F. to about 1850° F. The deformation may be accomplished by any conventional approach, such as, for example, forging, rolling, extrusion, upsetting, or cogging. After the deformation to produce the semifinished article is complete, final processing such as heat treating and machining may be performed.
In this processing, the array of intragranular, coarse delta-phase precipitates produced by the precipitation heat treatment provides nucleation sites for new, fine grains during the deformation step. Because the heat treatment produces a uniform distribution of delta phase in both fine and coarse grain regions of the ingot, the density of grain nucleation sites results in a uniformly recrystallized grain structure that is substantially independent of the starting grain size. These new fine grains produced during deformation are randomly oriented. The result is that more uniform plastic flow occurs during the subsequent deformation.
The present approach provides an important advance in the fabrication technology of the operable nickel-iron-base alloys. The conventional primary ingot conversion processing that requires multiple thermomechanical steps to produce a fine grain structure is eliminated, and replaced by a thermal treatment of the ingot to precipitate an array of delta phase precipitates that serve as the nucleation sites for fine grains during subsequent deformation. The present approach reduces the number and complexity of processing steps, thereby reducing processing costs.
In the present approach, the delta phase precipitates allow grain nucleation at non-coherent phase boundaries, with the result that the grain nucleation on the delta phase occurs at lower strains than does conventional grain boundary nucleation and recryst
Sheehan John
Wyman Gordon Company
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