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

Abstract

A gamma prime precipitation-strengthened nickel-base superalloy and method of forging an article from the superalloy to promote a low cycle fatigue resistance and high temperature dwell behavior of the article. The superalloy has a composition of, by weight, 16.0-22.4% cobalt, 6.6-14.3% chromium, 2.6-4.8% aluminum, 2.4-4.6% titanium, 1.4-3.5% tantalum, 0.9-3.0% niobium, 1.9-4.0% tungsten, 1.9-3.9% molybdenum, 0.0-2.5% rhenium, greater than 0.05% carbon, at least 0.1% hafnium, 0.02-0.10% boron, 0.03-0.10% zirconium, the balance nickel and incidental impurities. A billet is formed of the superalloy and worked at a temperature below the gamma prime solvus temperature of the superalloy so as to form a worked article, which is then heat treated above the gamma prime solvus temperature of the superalloy to uniformly coarsen the grains of the article, after which the article is cooled to reprecipitate gamma prime. The article has an average grain size of not coarser than ASTM 7 and is substantially free of critical grain growth.

Description

BACKGROUND OF THE INVENTION[0001]The present invention generally relates to nickel-base superalloys and methods for processing such superalloys. More particularly, this invention relates to a nickel-base superalloy and a method of forging an article from the nickel-base superalloy to 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 and exhibits improved low cycle fatigue behavior.[0002]The turbine section of a gas turbine engine is located downstream of a combustor section and contains a rotor shaft and one or more turbine stages, each having a turbine disk (rotor) mounted or otherwise carried by the shaft and turbine blades mounted to and radially extending from the periphery of the disk. Components within the combustor and turbine sections are often formed of superalloy materials in order to achieve acceptable mechanical properties while at elevated temperatures resu...

AI Technical Summary

Benefits of technology

[0010]The present invention provides a gamma prime precipitation-strengthened nickel-base superalloy and a method of forging an article from the superalloy to 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 and exhibits improved low cycle fatigue behavior.

[0012]In view of the above, the superalloy has a sufficiently high carbon content and is forged at sufficiently high local strain rates so that, following a supersolvus heat treatment, the resulting forged component is characterized by a fine and substantially uniform grain size distribution. Also preferably avoided is 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. As a result, the forged component is capable of exhibiting improved mechanical properties, particularly low cycle fatigue behavior. Though not wishing to be held to any particular theory, it is believed that formulating a superalloy to have a chemistry similar to R104 but formulated to contain relatively high carbon levels, especially carbon levels above the upper limit of R104 (0.10 weight percent), allows the use of high strain rates, resulting in a forged component 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 can be particularly improved within a temperature range of about 400° F. to about 800° F. (about 200° C. to about 425° C.) relative to R104 with a conventional carbon content of up to 0.10 weight percent. 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.

Images