Metal treatment – Stock – Nickel base
Reexamination Certificate
1999-01-28
2001-05-15
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Nickel base
C148S675000, C148S555000, C420S446000, C420S449000, C420S450000
Reexamination Certificate
active
06231692
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to nickel base superalloys and castings made therefrom and, more particularly, to a nickel base superalloy and casting having improved machinability while retaining beneficial alloy mechanical properties.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 3,615,376 describes a nickel base superalloy having a composition consisting essentially of, in weight %, 0.1 to 0.3% C, greater than 13% to less than 15.6% Cr, greater than 5% to less than 15% Co, 2.5% to 5% Mo, 3% to 6% W, 2% to 4% Al, 4% to 6% Ti, 0.005% to 0.02% B, up to 0.1% Zr, and balance essentially nickel with the ratio of Ti to Al being greater than 1 but less than 3; the sum of Ti and Al being 7.5%-9 weight %; and the sum of Mo and half of the W being 5 to 7 weight %. Carbon concentrations of 0.08 weight % and below are said to be insufficient to achieve high temperature alloy strength properties.
This nickel base superalloy exhibits improved high temperature stability, strength, and corrosion resistance. However, large gas turbine engine blades and vanes of industrial gas turbine (IGT) engines conventionally cast (e.g. equiaxed casting microstructure) from this superalloy exhibit inadequate machinability as a result of the cast microstructure containing large equiaxed grains, chemical segregation in thicker sections of the IGT castings, and undesirable carbide formation at the grain boundaries that embrittles the grain boundaries and can result in cracking or carbide/grain pull out during subsequent machining of the casting by such machining processes as grinding.
As a result, current machining practice for such IGT castings involves greatly increasing machining times by reducing machining feed rates to reduce cracking and carbide/grain pullout and produce a satisfactory machined surface finish. For example, the machining time of a large IGT equiaxed cast gas turbine engine blade cast from the above superalloy typically consumes 270 minutes.
An object of the present invention is to modify the above nickel base superalloy to unexpectedly and substantially improve its machinability, especially machinability of large equiaxed IGT castings produced from the modified superalloy, without adversely affecting the desirable alloy high temperature mechanical properties.
SUMMARY OF THE INVENTION
The present invention involves modifying the carbon content of the nickel base superalloy described hereabove in a manner discovered to unexpectedly and significantly improve its machinability, especially when conventionally cast and heat treated to produce large cross-section, equiaxed grain castings, such as IGT blades and vanes. In accordance with the present invention, the carbon content of the aforementioned superalloy composition is reduced to an amount effective to substantially improve machinability without adversely affecting the desirable alloy high temperature mechanical properties. The carbon concentration is controlled below about 0.08 weight %, preferably from about 0.055% to about 0.075% by weight of the superalloy composition to this end.
A preferred nickel base superalloy in accordance with an embodiment of the present invention consists essentially of, in weight %, of about 12.5% to 15% Cr, about 9.00% to 10.00% Co, about 3.70% to 4.30% Mo, about 3.70% to 4.30% W, about 2.80% to 3.20% Al, about 4.80% to 5.20% Ti, about 0.005% to 0.02% B, up to about 0.10% Zr, about 0.055% to 0.075% C and balance essentially Ni. The modified nickel base superalloy can be cast as equiaxed grain castings pursuant to conventional casting techniques to produce large castings, such as IGT blades and vanes, that exhibit a surprising and significant improvement in machinability (e.g. 33% reduction in machining time) after appropriate heat treatment as compared to the same superalloy casting similarly heat treated with higher carbon content.
The above objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.
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Corrigan John
Esser Winfried
Mihalisin John R.
Pickert Ursula
Vogt Russell G.
Howmet Research Corporation
Yee Deborah
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