CA1074674A

CA1074674A - Multi-step heat treatment for superalloys

Info

Publication number: CA1074674A
Application number: CA259,896A
Authority: CA
Inventors: Alan D. Cetel; Robert J. Henricks; Maurice L. Gell; Hector B. Dunthorne
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1975-09-22
Filing date: 1976-08-26
Publication date: 1980-04-01
Also published as: DE2642202A1; JPS5239522A; SE7609900L; BE846329A; FR2324753A1; GB1508099A; FR2324753B3

Abstract

MULTI-STEP HEAT TREATMENT FOR SUPERALLOYS

ABSTRACT OF THE DISCLOSURE
This invention relates to a multi-step heat treatment method for increasing the elevated temperature mechanical properties of nickel base superalloys. The heat treatment method is particularly applicable to those conventional alloys which have a cast structure consisting essentially of a gamma matrix containing a gamma prime second phase precipitate and regions of low melting constituents. The effect of the process of the invention is to maximize the volume fraction of the fine gamma prime precipitate, by a solution treatment and reprecipitation, without causing significant incipient melting.

Description

~74~7~L
BACKGROUND OF THE INVENTION
Field of the Inven~ion - This inven-tion relates to the field of nickel base superalloys, which may be defined as those nickel alloys used where mechanical properties at elevated temperatures are importantO The invention is particularly applicable to nickel base alloys of the gamma/gamma prime type which also contain localized concentra-tions of low melting constituents.
Description of the Prior Art - Nickel base superalloys ... ..
are commonly used in high temperature, high stress applications such as gas turbine engines. Because such alloys have been developed for their high temperature, high strength properties it is difficult to form them into complex shapes by conventional metal working techniques. For this reason nickel base super-alloy articles of complex shapes are commonly formed by casting.
; When complex alloys such as nickel base superalloys are solidified, the solidification process occurs in a nonuniform fashion so that on a microscopic scale different portions of -~
~he casting have diferent chemical compositions. This chemical segregation generally has detrimen~al effects on the properties of the alloy and as a general rule improved proper-ties result if segregation can be minimized or eliminated.
As increasing demands have been placed on the properties and working t~mperatures of nickel base superalloys, alloys have been developed which include increasing amounts of alloying additives, particularly reactive metal additions, which have been found extremely efficient ln improving certain properties. While these more complex alloys do have generally improved properties

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they also are subject to increased segregation which can lead to problems in other property areas.
In the prior art it has been considered to be a standard technique to heat treat cast nickel base superalloy articles at elevated temperatures for periods of time so as to perrnit some degree of homogenization to occur. Homogenization is a diffusion process which is extremely temperature sensitive.
Accordingly, for success~ul homogenization to be performed in a realistic length of time the temperature employed must be as high as possible and preferably very near the melting point of the bulk composition of the alloy. A considerable amoullt of chemical segregation of low melting eutectic phases occurs in comrnon nickel base superalloys. The presence of such low melting eutectic regions poses a serious problern to the successful heat treatment of many such alloys, particularly the successful homogenization of such alloys. The melting point of some low melting areas in a nickel base superalloy may be as much as from 200 to 300F below the bulk melting point of the alloy. The melting of such segregated regions is termed incipient melting and has been found to be highly detrimental to the transverse properties of directionally solidi~ied nickel base superalloy castingsj and consequently must be avoided.
In conventionally cast superalloys, longitudinal as well as transverse properties are adversely affected.
Nickel base superalloys of a conventional nature commonly have a gamrna matrix containing a gamma prime second phase in the form of small discrete particles. This gamma prime second phase is formed by precipitation after solidification has occurred ~Q~

The size, spacing and quantity of the gamma prime second phase is largely controlled by chemistry of the alloy and the cooling rate of the alloy from the solidification temperature.
It has long been appreciated that conventional nickel base superalloys owe a good portion of their strength to the strengthening effects of the gamma prime second phase; however, methods of improving the s~renghening process by modifying the gamma prime second phase morphology, without incipient melting, have not been apparent.
It has been determined that suitable heat treatment processes can be utilized to increase the mel~ing point of the segregated regions above the solvus tempera-ture of the gamma prime phase without significantly changing the volume of fraction of the segregated regions. The present process relies on diffusion to change the composi~ion of the segregated regions sufficiently to increase their melting point rather than to completely eliminate such regions. Prior art Patents Nos.

3,753,790 and 3~783,032 disclose heat treatment processes which are intended to completely remove the segregated regions in nickel base superalloys. The present invention differs from these references because the process of the invention îs not intended to remove the segregated regions, and in fact does not significantly affect their amount (volume fraction), but is rather intended only to change the composition o-f the second phase regions sufficiently to raise the incipient melting point above the gamma prime solvus temperature. The present invention also contemplates further steps in the heat treatment process to solutionize the gan~a prime particles and reprecipitate them as ~07~674 a much finer particulate thereby significantly increasing the high temperature properties of the nickel base superalloys.
SUMMARY OF THE INVENTION
The present invention involves a heat treatment process useful for improving the high temperature properties of the gamma/gamma prime nickel base superalloys, especially those which contain low melting segregated areas. The heat treatment includes a first step which serves to raise the incip-ient melting temperature of the low melting regions to above the gamma prime solvus without significantly affecting their volume fraction, and a second step performed at a higher temp-exature which serves to solutionize the coarse as cast gamma prime particles. One effect of the heat treatment is to mini-mize incipient melting during the solutionizing of the gamma prime phase, thereby improving certain mechanical properties of the alloy. The gamma prime particles reprecipitate in finer form during cooling thereby improving the properties of the alloy.
More specifically, the present invention relates to a multi-step heat treatment process for gamma/gamma prime nickel base superalloys having localized regions of low melting eutec-tic material and an incipient melting temperature and a quantity of coarse gamma prime particles having a solvus temperature, including the steps of a. heating the alloy to a temperature below, but within 100F of the melting point of the eutectic for a period of time sufficient to increase the melting point of the eutectic to a temperature above the gamma prime solvus, b. heating the alloy to a temperature above the gamma prime solvus but helow the increased incipient melting temperature for a time sufficient to solutionize substan-;7~

tially all of the gamma prime material, and c. cooling the alloy at a rate which will prevent the formation of coarse gamma prime particles.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows the as cast microstructure of a typical gamma/gamma prime nickel base superalloy.
Fig. 2 shows a schematic time-temperature plot of the present invention.
Fig. 3 shows a schematic plot of an alternate embodi-ment of Fig. 2.
Fig. 4 shows a schematic plot of an alternate embodi-ment of Fig. 2.
Fig. 5 shows a schematic plot of an alternate embodi-ment of Fig. 2.

- 5a -~7~74 Fig. 6 shows the cast alloy of Fig. 1 after a heat treatment at 2200F for 4 hours.
Fig. 7 shows an electron micrograph of the g mma prime particles in Fig. 1.
Fig. 8 shows an electron micrograph of the gamma prime particles in Fig. 6.
Fig. 9 shows the alloy of Fig. 1 af~er a heat treatment at 2250F for 4 hours.
Fig. 10 shows the alloy of Fig. 1 after the application of the two step heat treatment of the present invention.
Fig. 11 shows an electron micrograph of the gamma prime particles shown in Fig. 10.
Fig. 12 shows a comparison of times to 1 percent creep a-fter various heat treatments.
Fig. 13 shows a comparison of rupture lives after various heat treatments.
Fig. 14 shows the improvements in creep rupture life which result from the application of the present invention.

DF.SCRIPTION OF THE__REFERRED EMBODIMENT
The process of the presen-t invention is suitable for application to nickel base superalloys having low melting eutectic phases. The process of the present invention is particularly suited for those alloys in which the incipient melting temperature of the low melting phases is below the solvus temperature of the gamma prime strengthening precipitate.
The process of the invention will improve both longitudinal and transverse properties in equiaxed grain cast material and will improve the transverse properties of directionally solidified , . . .

~7~6~4 material.
A typical as cast microstructure of a nickel base superalloy is shown in Figo 1. The nominal alloy composition is 9% Cr, 10%
Co, 2% Ti, 5% Al, .11% C, 12.5V/o W, 1.0% Cb, .015% B, 2.0% Hf, balance nickel. The regions denoted A consist of a low melting `
phase, a eutectic between the gamma and gamma prime phases. The regions denoted as B consist oE coarse gamma prime particles in a gamma matrix.
This invention will be explained with reference to Fig. 2 which shows the process of the invention in schematic form.
The as cast starting material has an incipient melting tempera-ture, denoted Tl, and a gamma prime solvus temperature denoted T2. Although these temperatures are referred to as single values, both the incipient melting temperature and the gamma prime solvus temperature are composition sensitive. Thus di~ferent areas in a casting may have differen~ incipient melting temperatures and different gamma prime solvus tempera-tures, however the spread of these temperatures is usually not great. Above Tl, localized areas of the as cast material comprising a low melting eutectic composition will melt;
however, in order to dissolve the gamma prime material it is necessary to exceed T2. In the as cast material it is not possible to exceed T2 without exceeding Tl. Incipient melting is detrimental to certain mechanical properties, especially transverse properties in directionally-solidified alloys, hence it has heretofore not been practical to fully solutionize the gamma prime precipitates in many nickel base superalloys.

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The first step of the invention consists of heating the alloy to a temperature below but preferably within 50F of the incipient melting temperature Tl; this temperature is denoted T3, at this temperature diffusion will occur relatively -rapidly and the incipient melting temperature Tl, will gradually increase as the composition of the low melting regions changes.
Step I is continued at T3 until the incipient melting temperature increases to a temperature T4 above ~he gamma prime solvus temperature T2. T4 preferably exceeds T2 by a~ least 20F. The material is then cooled from T3. Step II is then performed.
Step II consists of heating the article to a temperature T5 which lies above the gamma prime solvus T2 but below the modified incipient melting temperature T4. At this temperature, T5, the gamma prime particles will dissolve into the matrix.
When dissolution is substantially complete~ the material is cooled to room temperature at a rate which will minimize the formation of coarse gamma prime particles. During this cooling step and/or during subsequent high temperature exposures, the dissolved gamma prime material will reprecipitate in the form of fine particles.
Other variations are possible, for example, during step I
the temperature T3 may be increased gradually~ or stepwise, so long as T3 is always less than the incipient melting temperature.
These variations are shown in Fig. 3 and Fig. 4. Fig. 3 depicts the situation where a gradual temperature -increase is used and Fig. 4 shows the case of a stepwise temperature increase. Since diffusion rates are very temperature sensitive, the embodiments shown in Fig. 3 and Fig. 4 may permit the incipient melting ~ (b7467~
temperature to be increased at a more rapid rate than the embodiment shown in Fig. 1. This potential improvement must be balanced against the increased probability of inadvertently exceeding the incipient melting ~emperature.
Fig. 5 shows another embodiment in which the cooling portion of step I and the heating portion of step I are combined into step III, by proceeding directly from step I to step II. Of course this embodimen~ may be combined with the embodiments shown in Figs. 3 and 4.
Having now generally described the invention, certain critical parameters will now be discussed and specified in greater detail, with reference to the general embodiment schematically described by Fig. 2. With regard to step I, the heating and cooling rates are not particularly critical, but the time and temperature are important. The temperature must be determined with respect to each alloy but must always be less than the incipient melting temperature. The temperature at which step I is performed should be as close to the incipient melting temperature, preferably within 50F and never more than 100F away from the incipient melting tempera~ure. The time at temperature for step I must be determined experimentally since it depends on the size and composition of the low meltlng eutectic phase. Generally the time for step I will lie between 2 and 20 hours.
The heating rate for step II is not especially critical, but the time and temperature are critical and the cooling rate is also important. The temperature must be selected so that it exceeds the gamrna prime solvus but does not exceed the incipient melting temperature. The gamma prime solvus temperature _g_ ~74~7~

varies with alloy composition and the incipient melting tempera-ture varies as a function of alloy composition, processing history, and the parameters chosen for step I of the process.
Thus the temperature at which step II will be performed must be experimentally determined for each alloy composition. The time which the material must be held at temperature varies as a function of the morphology of the coarse gamma prime material.
The larger the particle size~ the longer the time required, but typically the time will range from 1 to 10 hours. The material must be cooled from the temperature at which step II is performed at a rate which will allow ~he precipitation of fine gamma prime particles withou~ the formation of coarse gamma prime particles, typically this rate will be on the order of 250F per hour.
The particle size of the gamma prime particles in the as cast material is typically on the order of about 2 to 5 microns, while after the heat treatment of the present invention, the particle size of the gamma prime phase will be on the order of less than about 1 micron.
The present invention will be clarified through reference to the following illustrative example. An alloy designated as PWA 1422 was utilized having a nominal chemical composition of 9% chromil~m, 10% cobalt, 2% titanium, 5% aluminum, 12.5%
tungsten, .15% carbon, 1% columbium, .05% zirconium, .015%
boron, 1.5% hafnium, balance essentially nickel. The alloy was directionally solidified in shell molds to form articles in the shape of gas turbine blades. A metallographic examination ~07467~

was conducted on a series of cast parts which had been heat treated at dif~erent temperatures and it was determined that the incipient melting temperature of the alloy was about 2210F
and that the gamma prime solvus temperature was about 2243F.
Through experimentation it was determined that a 10 hour heat treatment at 2200F would raise the incipient melting tempera-ture of the alloy to about 2260F During this heat treatment there was no sign of significant incipient melting. The following heat treatment was then applied to the cast parts, a heat treatment at 2200F for 10 hours, cool to room tempera-ture, followed by a heat treabment at 2250F for 4 hours, cool to room temperature. The effect of this two-step hea~
treatment was to raise the incipient melting temperature o the alloy to above the gamma prime solvus temperature during the first step thereby permitting complete solutionizing of the gamma prime second phase precipitate during the second step with-out significant incipient melting. The effect of several heat treatments are shown in the figures. Fig. 1 shows a photomicro-graph of the as cast microstructure; tke regions denoted by the letter A are the gamma/gamma prime eutectic particles which formed immediately during the solidification process and the regions denoted by the letter B are the coarse gamma prime precipitates. Fig. 6 shows a photomicrograph of the same alloy after exposure to a temperature of 2200F ~ r 4 hours.
Again, the regions labeled A are the low melting eutectic r~gions of ~he alloy and the regions labeled B are the coarse gamma prime particles formed durlng the solidification process.
Comparison of Fig~ 1 and Fig. 6 shows that a heat treatment for ~Q7~67~

hours at 2200F has no observable effect on the segregated regions, but does solutionize about half of the coarse gamma prime precipitate in the matrix. The size of the coarse gamma prime particles in both Fig. 1 and Fig. 6 is on the order of 3 to 5 microns. Fig. 7 shows a replica elec~ron micrograph of the gamma prime particles in the as cast material and Fig. 8 shows the same type of photograph of the material after a heat treatment o~ 2200F for ~ hours. Fig. 9 shows a photomicrograph of the same alloy after a heat treatment at temperature oE
2250F ~or ~ hours. It is noteworthy that after this heat treatment the gamma prime particles are not visible at optical magnifications and it is ~urther observed that incipient melting D has occurred in the low melting eutectic regions labeled A.
Fig. 10 shows a photomicrograph of the same alloy after the two-step heat treatment of the present invention has been performed. The alloy was heat treated at 2200F for 10 hours followed by heat treatment at 2250F for a temperature of ~
hours. The effect o~ the ~irst part o~ this heat treatment is to increase the incipient melting temperature of the low melting regions without significantly affecting their visible appearance.
The second step of the heat treatment process is performed at a temperature slightly above the gamma prime solvus so as to solutionize the gamma prime particles. During subsequent cooling these particles reprecipitate in a much finer form, so fine that they are not readily visible to the naked eye under the magni~ication conditions of this photomicrograph. In Fig. 10, the regions labeled A are the low melting eutectic phase and the regions labeled C are areas of solutionized gamma prime.

`4Ei~4 In comparison with Fig. 9, it can be seen that the amount of incipient melting is greatly reduced. Fig. 11 is a replica electron micrograph showing the gamma prime particles in the sample which was shown in Fig. 10. The reprecipitated particles which are visible in the electron microgxaph have a size on the order of .3 to .5 microns.
Mechanical properties of a directionally solidified nickel base superalloy were evaluated after several different heat treatments. The alloy had a nominal composition described earlier with reference to Fig. 1.
The standard heat treatment, shown at 2200F was used as a basis for comparison. One series of test samples was heat treated for 4 hours at different temperatures from 2200F to 2300F. Another series of test samples was given a two step heat treatment of the type described in this disclosure. These samples were tested at 1800F under a load of 32 ksi. Figure 12 shows a plot of percent change in time to 1 percent creep as a function of temperature (the abscissa shows the percent change using the value obtained after a heat treatment at Z200F as the standard~. The curve 1 shows how the solution temperature, in a single step process, affects the time to reach 1 percent creep. A maximum improvement of about 60 percent is reached at about 2250F, followed by a decrease at higher temperatures.
The curve 2 shows how the temperature of the second heat treatment step, in a two step process, affects the time to reach 1 percent creep. A maximum improvement is rea~hed at about 2260F, and it is noteworthy that this maximum is about 150%, a~

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incremental increase of about 90% over the behavior of the single step heat treatment material.
The same tests were continued and the rupture life was measured. These resul~s are plotted in Figo 13, which is drawn in much the same fashion as Fig. 12, except that the abscissa shows the percent change in rupture life as compared with material treated at 2200F. With a single step treatment a maximum increase of about 50% is obtained at 2250F, wh;le a two step heat treatment shows a maximum increase of about 100% at 2260F. Thus an incremental improvement of about 50%
in rupture life can be obtained by using a two step heat treatment process.
The effect of the heat treatment of the present invention on the creep-rupture strength of alloy PWA 1422 is shown in Fig.
14 alongwith comparative data for a conventional heat treatment process. Rupture life in hours is shown for four different conditions of stress and temperature. It can be seen that the process of the invention provides a significant improvement in life under all conditions tested.
Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed and defined as follows:

1. A multi-step heat treatment process for gamma/gamma prime nickel base superalloys having localized regions of low melting eutectic material and an incipient melting temperature and a quantity of coarse gamma prime particles having a solvus temperature, including the steps of a. heating the alloy to a temperature below, but within 100°F of the melting point of the eutectic for a period of time sufficient to increase the melting point of the eutectic to a temperature above the gamma prime solvus, b. heating the alloy to a temperature above the gamma prime solvus but below the increased incipient melting temperature for a time sufficient to solutionize substantially all of the gamma prime material, and c. cooling the alloy at a rate which will prevent the formation of coarse gamma prime particles.

2. A process as in claim 1 wherein the alloy is heated to within 50°F of the incipient melting temperature in step a.

3. A process as in claim 1 wherein the alloy is held at temperature for a period of from about 2 to about 20 hours in step a.

4. A process as in claim 1 wherein the incipient melting temperature is increased to at least about 20 F
above the gamma prime solvus temperature.

5. A process as in claim 1 wherein the alloy is held at temperature for a period of from about 1 to about 10 hours in step b.

6. A heat treatment process for refining the morphology of the gamma prime phase in nickel base super-alloys, which have low melting eutectic regions which melt below the gamma prime solvus temperature, from an as cast size on the order of from about 2 to about 5 microns to a final size of less than about 1 micron, including the steps of a. heating the alloy to a temperature below, but within 100°F of the melting point of the eutectic for a period of time suffi-cient to increase the melting point of the eutectic to a temperature above the gamma prime solvus, b. heating the alloy to a temperature above the gamma prime solvus but below the increased incipient melting temperature for a time sufficient to solutionize substantially all of the gamma prime material, and c. cooling the alloy at a rate which will prevent the formation of coarse gamma prime particles, whereby the elevated temperature mechanical properties of the superalloy are significantly increased.