Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-08-27
2001-09-04
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S671000, C148S421000
Reexamination Certificate
active
06284070
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the heat treatment of titanium alloys, and, more particularly, to the heat treatment of alpha-beta titanium-base alloys to improve their dwell fatigue performance.
BACKGROUND OF THE INVENTION
An alpha-beta titanium-base alloy exhibits an alpha-plus-beta phase field in its temperature-composition equilibrium phase diagram. These alpha-beta titanium-base alloys may be heat treated for improved performance. Alpha-beta titanium-base alloys are used in applications requiring good mechanical performance at intermediate temperatures, coupled with their relatively low density. For example, such alpha-beta titanium-base alloys are used in compressor blades, disks, and structures of aircraft engines, where the article is expected to perform at temperatures of up to about 1100° F.
Alpha-beta titanium-base alloys are potentially susceptibility to dwell fatigue damage. In dwell fatigue, the material is loaded and held with the load applied for a period of time, and then unloaded. The loading and unloading cycle is repeated numerous times. Such loading conditions are experienced in typical situations of use of the alpha-beta titanium-base alloys. Under these conditions, the alpha-beta titanium-base alloy may crack and fail prematurely.
There is a need for an approach that reduces the incidence of dwell fatigue in alpha-beta titanium-base alloys, while retaining the other beneficial properties of the material. The present invention fulfills this need, and further provides related advantages.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method for heat treating an alpha-beta titanium-base alloy to reduce its susceptibility to dwell fatigue damage. Other beneficial properties of the alpha-beta titanium-base alloy are retained, such as good strength, ductility, fracture toughness, crack growth resistance, and machinability. The heat treatment is accomplished with conventional equipment.
A heat treatment is provided for an alpha-beta titanium-base alloy capable of forming mixtures of alpha and beta phases and having a beta transus between an alphaplus-beta phase field and a beta phase field of a temperature-composition equilibrium phase diagram of the alpha-beta titanium-base alloy. The method for heat treating the alpha-beta titanium-base alloy comprises the steps of first heating the alpha-beta titanium-base alloy to a first heat-treatment temperature within the alpha-plus-beta phase field and which produces a volume fraction of primary alpha phase of less than about 30 percent within a beta phase matrix, and thereafter quenching the alpha-beta titanium-base alloy at a rate sufficient to suppress the epitaxial regrowth of the primary alpha phase during cooling and to produce a transformed beta morphology in the beta phase. The alpha-beta titanium-base alloy is thereafter second heated to a second heat-treatment temperature less than a growth temperature at which a primary alpha phase level is substantially affected by epitaxial growth and greater than an ordering temperature at which an ordering reaction occurs, and thereafter cooled at a rate sufficient to avoid ordering reactions in the alpha-beta titanium-base alloy.
The first heating produces a microstructure having a low volume fraction of primary alpha phase, and the quenching suppresses the growth of the alpha phase. The result is a microstructure having a relatively small amount of primary alpha phase and a Widmanstatten or martensitic transformed beta morphology. The second heating is conducted at a temperature whereat the alpha phase does not significantly coarsen, and the transformed beta phase coarsens. The result is an improved balance in mechanical properties with an accompanying microstructure having low susceptibility to dwell fatigue. The alpha-beta titanium-base alloy is thereafter cooled at a slow or intermediate rate sufficient to avoid ordering reactions in the alpha-beta titanium-base alloy.
The heat treatment may be utilized with a wide variety of alpha-beta titanium-base alloys, with examples being Ti-6242 alloy and Alloy 834. In practice, the first heat-treatment temperature is preferably in a first range of from about 70° F. below a beta transus temperature of the alpha-beta titanium-base alloy to the beta transus temperature of the alpha-beta titanium-base alloy, more preferably from about 70° F. below the beta transus temperature of the alpha-beta titanium-base alloy to about 10° F. below the beta transus temperature of the alpha-beta titanium-base alloy. The quenching is typically at a rate of greater than 200° F. per minute to a temperature of less than an aging temperature for the alloy, which is about 1100° F. for Ti-6242 alloy and about 1300° F. for Alloy 834. The step of second heating is preferably accomplished by heating the alpha-beta titanium-base alloy to a second heat-treatment temperature in a second range of from about 100° F. to about 400° F. below the beta transus temperature of the alpha-beta titanium-base alloy. The step of cooling is preferably accomplished by cooling the alpha-beta titanium-base alloy to ambient temperature at a rate of from about 10° F. per minute to about 200° F. per minute.
After the heat treatment described above, the alpha-beta titanium-base alloy may be further heat treated by aging the alpha-beta titanium-base alloy, typically at a temperature of from about 950° F. to about 1350° F., depending upon the alloy and properties desired.
The result of this heat treatment is a desirable balance of properties including good strength, ductility, fracture toughness, crack growth resistance, and machinability, accompanied by good resistance to dwell fatigue. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
REFERENCES:
patent: 3833363 (1974-09-01), Bomberger, Jr. et al.
patent: 3901743 (1975-08-01), Sprague et al.
Gorman Mark D.
Link Barbara A.
Woodfield Andrew P.
General Electric Company
Hess Andrew C.
Narciso David L.
Oltmans Andrew L.
Sheehan John
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