EP0856589A1

EP0856589A1 - Age hardenable / controlled thermal expansion alloy

Info

Publication number: EP0856589A1
Application number: EP98300279A
Authority: EP
Inventors: Karl Andrew Heck; John Scott Smith; Darrell Franklin Smith, Jr.
Original assignee: Inco Alloys International Inc
Current assignee: Huntington Alloys Corp
Priority date: 1997-01-29
Filing date: 1998-01-15
Publication date: 1998-08-05
Also published as: JPH10212544A

Abstract

The alloy of the invention provides a controlled coefficient of thermal expansion alloy that age hardens. This alloy consists essentially of, by weight percent, about 32 to 55 nickel, about 6 to 16 cobalt, about 1.2 to 7 chromium, about 0.25 to 1.5 aluminum, about 0.25 to 3 titanium, about 2 to 8 total (niobium + ½ tantalum), about 0.1 to 1.2 silicon and balance iron and incidental impurities with iron being about 28 to 50.

Description

Field of the Invention

This invention relates to the field of controlled expansion superalloys. In particular, this invention relates to controlled thermal expansion nickel-iron-cobalt superalloys.

Background of the Invention

Turbine engine manufacturers rely upon controlled coefficients of thermal expansion (CTE) alloys for engine seals, rings and casings. These relatively low CTE superalloys maintain tight tolerances between turbine engine components over broad temperature ranges. This careful control of tolerances facilitates the achieving of high engine operating efficiencies. The problems associated with commercially used low CTE superalloys include: general oxidation, stress accelerated grain boundary oxidation (SAGBO) and phase stability.

Smith, Jr. et al., in U.S. Pat. No. 4,200,459 ('459), disclose controlled CTE superalloys designed for turbine engine applications. INCOLOY alloy 905, containing only 0 to 3 wt% cobalt, represents the commercial product of the '459 patent. (INCOLOY is a trademark of the Inco group of companies.) But due to its prolonged heat treating requirements, INCOLOY alloy 905 lacks cost effectiveness for turbine engine applications. For example, INCOLOY alloy 905 typically requires aging times in excess of 300 hours to ensure adequate impact strength. Because of these painfully slow aging procedures, INCOLOY alloys 907 and 909, that contain cobalt for accelerating aging kinetics, quickly replaced INCOLOY alloy 905.

Recently developed controlled thermal expansion alloys tend to contain small amounts of chromium in combination with relatively large amounts of cobalt. For example, U.S. Patent No. 5,283,032 discloses a cobalt-chromium modified low CTE alloy. A commercially available embodiment of this patent nominally contains: 29 wt% cobalt, 25 wt% nickel, 5.5 wt% chromium, 4.8 wt% niobium, 0.8 wt% titanium, 0.5 wt% aluminum, 0.3 wt% silicon and balance iron ('032 alloy).

Smith et al., in U.S. Pat. No. 5,403,547 ('547), disclose a low-CTE superalloy containing a γ matrix strengthened by the precipitation of γ' (ordered FCC) and BCC aluminide phases. The commercial alloy of the '547 patent, INCONEL® alloy 783, nominally contains: 34 wt% cobalt, 28.5 wt% nickel, 5.4 wt% aluminum, 3 wt% chromium, 3 wt% niobium, 0.1 wt% titanium and balance iron. (INCONEL is a trademark of the Inco group of companies.) Alloy 783 provides a low-CTE superalloy having good general oxidation resistance, excellent crack growth resistance, good strength and decreased density. With a nominal cobalt concentration of 34 wt% however, alloy 783 is also a relatively expensive alloy.

It is an object of this invention to provide nickel-iron-cobalt superalloys having controlled CTE, SAGBO resistance, corrosion resistance and good strength properties.

It is a further object of this invention to provide alloys with improved creep resistance that maintain phase stability and rupture ductility at elevated temperatures.

It is a further object of this invention to provide nickel-iron-cobalt-controlled-CTE superalloys readily aged with commercially practical heat treatments.

Summary of the Invention

The alloy of the invention provides a controlled-coefficient of thermal expansion alloy that age hardens. This alloy consists essentially of, by weight percent, about 32 to 55 nickel, about 6 to 16 cobalt, about 1.2 to 7 chromium, about 0.25 to 1.5 aluminum, about 0.25 to 3 titanium, about 2 to 8 total (niobium + ½ tantalum), about 0.1 to 1.2 silicon and balance iron and incidental impurities with iron being about 28 to 50.

Description of Preferred Embodiments

The deliberate addition of aluminum, chromium, silicon, titanium and (niobium and/or tantalum) to a nickel-iron-cobalt matrix produces controlled CTE superalloys with improved creep resistance. These alloys also provide SAGBO resistance, good general corrosion resistance and good strength properties.

EXAMPLE

The ingots of Table 1 were vacuum induction melted (VIM) and cast as 4" (10.2 cm) diameter, 22 kg ingots. (This specification contains compositions expressed in weight percentage, unless specifically noted otherwise.)

A 16 hour heat treatment at 2150°F (1177°C) homogenized the ingots. After cooling the ingots to 2050°F (1121°C), hot rolling converted the ingots into 2" (5.1 cm) squares. Additional hot rolling converted these squares into 0.680" (1.7 cm) rods. These rods showed some evidence of freckling and non-uniform grain structure at the centers. (This lack of homogeneity may explain some of the variation in properties.)

The following heat treatment prepared the samples of heats 1 to 6: 1) 1825°F (996°C) for 1 hour of annealing; 2) air cooled to room temperature; 3) aged at 1375°F (746°C) for 8 hours; 4) furnace cooled to 1150°F (621°C); 5) held at 1150°F (621°C) for 8 hours of secondary aging; and 6) air cooled to room temperature. Table 2 provides inflection temperatures and coefficients of thermal expansion for the heat treated samples of heats 1 to 6 below.

All of the alloys of Table 2 had a coefficient of thermal expansion of less than 13.6 x10^-6 cm/cm/°C at 371°C. Most advantageously, the alloys have a coefficient of thermal expansion of less than 10 x10^-6 cm/cm/°C at 371°C. Heats 1 to 3 and 6 provided properties similar to '032 alloy's published specification of 580-640 °F (304-338°C) inflection temperature and a CTE of 4.25 to 4.75 x10^-6 in/in/°F at temperatures between 400 and 600°F (7.65 to 8.55 x10^-6 cm/cm°C between 204 and 316°C). These alloys however, achieve this inflection temperature range with a stable nickel-iron-cobalt matrix.

Table 3 below provides the room temperature tensile properties of alloys at room temperature given the same heat treatment as Table 2.

The room temperature properties of these alloys compare favorably to the tensile strength of the '032 alloy.

Table 4 below provides the 1250°F (677°C) properties of the alloys of Heats 1 to 6 after receiving the heat treatment of Tables 2 and 3.

Similarly, the alloys of Heats 1 to 6 provide relatively good high temperature properties with the stable nickel-iron-cobalt matrix.

Table 5 below provides the 1300°F (704°C) tensile properties of Heats 1, 2 and 4 to 6 heat treated with the heat treatment of Tables 2 to 4.

The data of Table 5 illustrate that these alloys have useful tensile properties at 1300°F (704°C).

Tables 6 and 7 below provide the elevated temperature combination smooth notch bar test data for alloys as heat treated with the heat treatment of Tables 2 to 5.

Tables 6 and 7 illustrate that the alloys of Heats 1 to 6 are notch ductile and resistant to SAGBO failure.

Tables 8 and 9 below illustrate the effect of aging treatment temperature on stress rupture properties.

The following heat treatment annealed and aged the alloys of Table 8: 1) 1825°F (996°C) for one hour of annealing; 2) air cooled to room temperature; 3) aged at 1325°F (718°C) for eight hours; 4) furnace cooled at 100°F (55.6°C) per hour to 1150°F (621°C); 5) aged at 1150°F (621°C) for eight hours; and 6) air cooled to room temperature.

The heat treatment of Table 8 annealed and aged the alloy of Table 9, except that an initial aged eight hours aging treatment at 1425°F (774°C) replaced the 1325°F (718°C) step.

Tables 8 and 9 illustrate that these alloys provide flexible annealing and aging times. Aging temperatures of 1325°F (718°C), 1375°F (746°C) or 1425°F (774°C) effectively aged the alloys.

Alloys of "about" the numerical ranges of Table 10 provide creep-resistant-controlled-CTE materials for elevated temperature applications.

Table 11 below lists multiple specific alloys that are within the scope of the alloys of the invention.

A specific Ni-Fe-Co alloy for replacing the '032 alloy consists essentially of about 39.2 to 40 nickel, about 10 to 11.7 cobalt, about 4.8 to 5.2 niobium, about 3.3 to 3.7 chromium, about 1.2 to 1.6 titanium, about 0.4 to 0.65 aluminum, about 0.35 to 0.45 silicon, about 0 to 0.3 copper, about 0 to 0.2 molybdenum, about 0 to 0.02 carbon, about 0 to 0.015 phosphorus, about 0 to 0.002 sulfur, about 0.0001 to 0.0025 boron and balance iron and incidental impurities.

These age hardenable Ni-Fe-Co alloys provide superalloys having controlled CTE, SAGBO resistance, corrosion resistance and good strength properties. In addition, these Ni-Fe-Co alloys provide improved creep resistance while maintaining phase stability and rupture ductility at elevated temperatures. Finally, treating these Ni-Fe-Co alloys with relatively quick heat treatments produces material suitable for demanding turbine engine applications.

While in accordance with the provisions of the statute, this specification illustrates and describes specific embodiments of the invention. Those skilled in the art will understand that the claims cover changes in the form of the invention and that certain features of the invention provide advantages without the use of other features.

Claims

A controlled thermal expansion superalloy consisting essentially of, by weight percent, about 32 to 55 nickel, about 6 to 14.5 cobalt, about 1.2 to 4.2 chromium, about 0.25 to 1.5 aluminum, about 0.25 to 3 titanium, about 2 to 8 total (niobium + ½ tantalum), about 0.2 to 1.2 silicon, about 0 to 2 manganese, about 0 to 0.03 boron, about 0 to 0.2 carbon, 0 to 0.5 magnesium, about 0 to 0.5 cerium, about 0 to 0.5 calcium, about 0 to 2 tungsten, about 0 to 2 molybdenum, about 0 to 2 vanadium, about 0 to 1 copper, about 0 to 1 total yttrium and rare earths, less than about 0.01 phosphorus less than about 0.01 sulfur and balance iron and incidental impurities with iron being about 28 to 50.
The alloy of claim 1 wherein said alloy contains about 34 to 52 nickel, about 7 to 14.5 cobalt, about 2 to 4.2 chromium, about 0.25 to 1 aluminum, about 0.25 to 2 titanium, about 2.5 to 6 total (niobium + ½ tantalum), about 0.2 to 1 silicon and balance iron and incidental impurities with iron being about 30 to 48.
The alloy of claim 2 wherein said alloy contains about 0 to 1.5 manganese, about 0 to 0.02 boron and about 0 to 0.15 carbon.
The alloy of claim 2 wherein said alloy contains about 0 to 0.2 magnesium, about 0 to 0.2 cerium and about 0 to 0.2 calcium.
The alloy of claim 1 wherein said alloy contains about 36 to 48 nickel, about 8 to 14.5 cobalt, about 2.8 to 4.2 chromium, about 0.3 to 0.8 aluminum, about 0.5 to 1.6 titanium, about 3 to 5.3 niobium, about 0.2 to 0.6 silicon, about 0 to 1 manganese, about 0 to 0.01 boron, about 0 to 0.1 carbon, and balance iron and incidental impurities with iron being about 34 to 44.
The alloy claim 5 wherein said alloy contains about 0 to 0.1 magnesium, about 0 to 0.1 cerium, about 0 to 0.1 calcium, about 0 to 1 tungsten, about 0 to 1 molybdenum, about 0 to 1 vanadium, about 0 to 0.5 copper, about 0 to 0.5 total (yttrium and rare earths), less than about 0.01 phosphorus and less than about 0.01 sulfur.
The alloy of claim 1 wherein said alloy contains about 39.2 to 40 nickel, about 10 to 11.7 cobalt, about 4.8 to 5.2 niobium, about 3.3 to 3.7 chromium, about 1.2 to 1.6 titanium, about 0.4 to 0.65 aluminum, about 0.35 to 0.45 silicon, about 0 to 0.3 copper, about 0 to 0.2 molybdenum, about 0 to 0.02 carbon, about 0 to 0.015 phosphorus, about 0 to 0.002 sulfur, about 0.0001 to 0.0025 boron and balance iron and incidental impurities.
Use of the alloy of claims 1 to 7 to form parts of turbine engines.