Method of controlling and refining final grain size in supersolvus heat treated nickel-base superalloys

Abstract

A method of forming a component from a gamma prime precipitation-strengthened nickel-base superalloy. The method entails formulating the superalloy to have a sufficiently high carbon content and forging the superalloy at sufficiently high local strain rates so that, following a supersolvus heat treatment, the component is characterized by a fine and substantially uniform grain size distribution, preferably finer than ASTM 7 and more preferably in a range of about ASTM 8 to 10.

Description

BACKGROUND OF THE INVENTION[0001]The present invention generally relates to methods for processing nickel-base superalloys. More particularly, this invention relates to a method of forging an article from a nickel-base superalloy, in which increased local strain rates in combination with increased carbon content promote a more controlled grain growth during supersolvus heat treatment, such that the article is characterized by a microstructure with a finer uniform grain size.[0002]Gamma prime (γ′) precipitation-strengthened nickel-base superalloys contain chromium, tungsten, molybdenum, rhenium and / or cobalt as principal elements that combine with nickel to form the gamma (γ) matrix, and contain aluminum, titanium, tantalum, niobium, and / or vanadium as principal elements that combine with nickel to form the desirable gamma prime precipitate strengthening phase, principally Ni3(Al, Ti). Gamma prime precipitation-strengthened nickel-base superalloys (hereinafter, gamma prime nickel-bas...

AI Technical Summary

Benefits of technology

[0010]The present invention provides a method of forming components from gamma prime nickel-base superalloys. The method entails formulating such a superalloy to have a sufficiently high carbon content and forging the superalloy at sufficiently high local strain rates so that, following a supersolvus heat treatment, the component is characterized by a fine and substantially uniform grain size distribution, preferably an average grain size finer than ASTM 7 and more preferably in an average range of about ASTM 8 to 10. The present invention is further capable of avoiding critical grain growth that would produce individual grains or small regions of grains having grain sizes of more than five and preferably three ASTM units coarser than the average grain size in the component, or large regions that are uniform in grain size but with a grain size coarser than a desired grain size range of about two ASTM units.

[0012]A significant advantage of this invention is that, in addition to avoiding critical grain growth and avoiding the necessity to forge superplastically, the higher strain rate limit of the process window for working the billet has been shown to achieve significant control of the average grain size in the component and achieve a uniform grain size distribution within a desired narrower range that is significantly finer than previously possible. In this manner, mechanical properties of the component, including low cycle fatigue and tensile strength, can be improved. Though not wishing to be held to any particular theory, it is believed that formulating a superalloy such as R88DT to contain a carbon level above its conventional upper limit (0.060 weight percent) allows the use of strain rates beyond the upper strain rate limit of 0.010 per second (s−1) typically associated with R88DT, and even above the upper strain rate limit of 0.032 s−1 previously permitted by Yoon et al. and Raymond et al. for R88DT, resulting in components capable of exhibiting a more refined average grain size and substantially free of critical grain growth, which together improve the low cycle fatigue life of the component. Low cycle fatigue life is particularly improved within a temperature range of about 400° F. to about 750° F. (about 200° C. to about 400° C.) relative to R88DT with a conventional carbon content of up to 0.060 weight percent.

[0013]Improvements in low cycle fatigue life are believed possible, with the further benefit of higher temperature properties achieved with powder metallurgy alloys such as R88DT. Other benefits of the finer average grain size achieved with this invention include improved sonic inspection capability due to lower sonic noise, and improved yield behavior in service due to improved yield strength with finer grain size.

Problems solved by technology

Critical grain growth can also have a negative impact on other mechanical properties, such as tensile strength.

Though not wishing to be held to any particular theory, critical grain growth is believed to be driven by excessive stored energy within the worked article, and may involve individual grains, multiple individual grains within a small region, or large areas of adjacent grains.

Disks and other critical gas turbine engine components forged from billets produced by powder metallurgy and extrusion consolidation have appeared to exhibit a lesser propensity for critical grain growth than if forged from billets produced by conventional cast and wrought processing or spraycast forming techniques, but in any event are susceptible to critical grain growth during supersolvus heat treatment.

Though prior forging practices of the type described above have achieved grain sizes in a range of ASTM 5 to 8, less than optimal mechanical properties can still result.

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