# Patent

## Title
Heat treatment method for bodies that comprise a nickel based superalloy

## Abstract

In a heat treatment process for a single-crystal or directionally solidified material body comprising a nickel-based superalloy, the material body is solution-annealed and then at a first temperature γ′ particles of greater than 1 μm are precipitated in a proportion by volume with Vtot−V1 of less than 50%, where Vtot is the total amount of γ′ particles after complete heat treatment and V1 is the proportion of the γ′ particles which is greater than 1 μm, and at least at a second temperature ‘γ’ particles of less than 1 μm are precipitated. The γ′ particles are preferably precipitated in a size of 2 pin or more with a proportion by volume of 0.25<(Vtot−V1)/(100−V1)<0.55 at the first temperature. The proportion by volume Vtot of the γ′ particles will be at least 50%.

## Background
TECHNICAL FIELD
The invention relates to a heat treatment process for a single-crystal or directionally solidified material body comprising a nickel-based superalloy.
PRIOR ART
Nickel-based superalloys, as are known, for example, from U.S. Pat. No. 5,759,301, are subjected to a heat treatment using the casting process. In this case, in a first solution-annealing step the γ′ phase which has precipitated nonuniformly during the casting process is completely or partially dissolved. In a second precipitation heat treatment, this γ′ phase is precipitated again under controlled conditions. To achieve optimum mechanical properties, this precipitation heat treatment is carried out in such a manner that fine, uniformly distributed γ′ particles are precipitated.
A heat treatment of this type is known from the article Development of two Rhenium-containing superalloys for single crystal blade and directionally solidified vane applications in advanced turbine engines, Journal of Materials Engineering and Performance, 2(1993)August, No. 4, Materials Park, Ohio, US. In this case, the material CMSX-4 is subjected to a two-stage heat treatment which firstly provides for γ′ particles with a mean size of 0.45 μm to be precipitated at a temperature of 1140° C., before a second fraction of ultrafine γ′ particles with a size in the nanometer range is precipitated at a temperature of 871° C. Similar heat treatment for the precipitation of γ′ particles in nickel-based superalloys are disclosed by the documents GB-A-2 235 697, EP-A2-155 827, U.S. Pat. No. 5,100,484, U.S. Pat. No. 4,459,160 and U.S. Pat. No. 4,4643,782.
EP-A2-76 360, U.S. Pat. No. 5,154,884 and EP-A1-937 784 have likewise disclosed heat treatments of this type. These documents provide for precipitation heat treatments at various temperatures in order to precipitate γ′ particles of different sizes. By way of example, in EP-A1-76 360, a first phase of “coarse” γ′ particles is precipitated at a temperature of 1204° to 1260° C. over a period of 2 to 4 hours. Then, a second phase of “fine” γ′ particles is precipitated at a temperature of 1080° C. In a final step, a heat treatment is carried out at 649° C.
When there is a mechanical load and prolonged high-temperature load, the result is directional coarsening of the γ′ particles, a phenomenon known as rafting, and, with high γ′ contents (i.e. if the proportion by volume of γ′ is at least 50%), the microstructure is inverted, i.e. γ′ becomes the continuous phase in which the previous γ′ matrix is embedded. Since the intermetallic γ′ phase has a tendency toward environment embrittlement, under certain load conditions this leads to a huge loss of mechanical properties. Environment embrittlement occurs in particular if moisture and long holding times under a tensile load are present.
SUMMARY OF THE INVENTION
The invention is based on the object of providing a heat treatment process of the type described in the introduction which, in a simple manner, allows the described embrittlement and the associated loss of properties in nickel-based superalloys with a high γ′ content of 50% and above to be avoided.
According to the invention, in a heat treatment process, this is achieved by the fact that, at a first temperature T1, γ′ particles of larger than 1 μm are precipitated in a proportion by volume of Vtot−V1 of less than 50%, where V1 is the proportion of the γ′ particles which is larger than 1 μm, and, at least at a second temperature TA, γ′ particles of less than 1 μm are precipitated. The γ′ particles are preferably precipitated in a size of 2 μm or more with a proportion by volume of 0.25<(Vtot−V1)/(100−V1)<0.55 at the first temperature.
With a material having a composition of (% by weight) Ni—6.5% Cr—9.6% Co—0.6% Mo—6.4% W—6.5% Ta—3% Re—5.6% Al—1.0% Ti—0.2 Hf—230 ppm C—70 ppm B, the precipitation of γ′ particles of greater than 1 μm is carried out a temperature T1 of between 1180° C. and 1275° C., preferably at a temperature T1 of between 1230° C. and 1265° C., over a period of between 1 and 10 hours.
The precipitation of γ′ particles of less than 1 μm is carried out a temperature TA1 of 1050° to 1150° C. over the course of 1 to 10 hours and a temperature TA2 of 820° to 900° C. over the course of 10 to 30 hours.
Cooling from the solution-annealing temperature to the first precipitation temperature for precipitation of coarse γ′ particles with a particle size of greater than 1 μm is advantageously carried out with a cooling rate of less than 5 K/min, preferably between 2 K/min and 0.1 K/min, preferably of 0.5 K/min. Alternatively, between the solution-annealing and the precipitation of coarse γ′ particles, the material body can be cooled to room temperature and then reheated to the first temperature T1.



BRIEF DESCRIPTION OF THE FIGURES
The invention is explained in more detail in the appended figures, in which:
FIG. 1 shows a material body which has been heat-treated using variant 1 in accordance with the invention, and
FIG. 2 shows a material body which has been heat treated using variant 3, which does not correspond to the invention.



Only the elements which are pertinent to the invention are shown.
WAYS OF CARRYING OUT THE INVENTION
The present invention relates to a heat treatment process for a single-crystal or directionally solidified material body which consists of a nickel-based superalloy with a volumetric γ′ content Vtot after complete heat treatment of at least 50%. A nickel-based superalloy of this type is known, for example, from U.S. Pat. No. 5,759,301. This may, for example, be a thermally loaded component, such as for example a guide vane or rotor blade of a gas turbine.
In a first step, the material body is solution-annealed at a temperature TL in order to virtually completely dissolve the γ′ particles in accordance with the prior art. This first step is used to completely or partially dissolve the γ′ phase, which has been precipitated nonuniformly during the casting process. In a further heat treatment, these γ′ particles are then precipitated again, but with a uniform distribution.
According to the invention, this γ′ phase is precipitated at various temperatures and therefore in various sizes. The mean particle diameter of a “coarse” γ′ phase which is precipitated first of all at a temperature T1 must be greater than 1 μm, preferably greater than 2 μm. One qualitative feature is the irregular morphology of these γ′ particles, which results from the fact that the particles at least partially lose their coherency with respect to the matrix. If Vtot is the total volumetric proportion of γ′ which can be precipitated, i.e. for example 70%, and V1 is the first proportion, which is to be precipitated in coarse form, of the γ′ particles, Vtot−V1 must be less than 50%. Furthermore, the following relationship should preferably be satisfied: 0.25<(Vtot−V1)/(100−V1)<0.55. These relationships should be satisfied throughout the entire volume of the material, i.e. in the dendritic and interdendritic regions. This relationship leads to a large interparticle spacing of the “coarse” γ′ phase, which increases the diffusion distance and thereby prevents rafting in this phase.
Furthermore, if the upper limit of Vtot−V1 less than 50% or (Vtot−V1)/(100−V1)<0.55) is maintained, the proportion of fine γ′ particles of less than 1 μm is reduced to such an extent that the loss of certain mechanical properties caused by environmental embrittlement described in the introduction no longer occurs, since the proportion by volume of these particles is no longer sufficient to cause matrix inversion. The preferred lower limit of 0.25<(Vtot−V1)/(100−V1) results from the fact that the strength values have to achieve a certain minimum level through the presence of a sufficient proportion by volume of fine γ′ particles.
Furthermore, in a second precipitation heat treatment at temperature TA<T1, γ′ particles of less than 1 μm are precipitated. This may also take place at two temperatures TA1 and TA2 in order to further precipitate γ′ particles in the range of a few tens of nanometers.
Furthermore, it is proposed to introduce a cooling rate v of less than 5 K/min from the solution-annealing temperature TL to the precipitation temperature T1. This promotes the desired establishment of a coarse γ′ microstructure, since slow cooling leads to a low density of γ′ nuclei. A lower limit is determined only by cost reasons based on increasing heat treatment times. The cooling rate v is preferably between 0.1 K/min and 2 K/min, with a preferred value of 0.5 K/min.
As an alternative to the abovementioned slow cooling, cooling from TL to room temperature by means of gas cooling (gas fan quenching) with subsequent reheating to T1 is also conceivable. In this case, cooling rates of at least 20 K/min are typically achieved. T1 and the heat treatment duration can once again be defined in accordance with the criteria classified above with regard to mean diameter and volumetric proportion of the coarse precipitations.
Experimental Results
The experimental results relate to the material MK4 of the following composition (% by weight) Ni—6.5% Cr—9.6% Co—0.6% Mo—6.4% W—6.5% Ta—3% Re—5.6% Al—1.0% Ti—0.2 Hf—230 ppm C—70 ppm B.
Variant 1 of the Heat Treatment According to the Invention


    
    
        Solution annealing at TL=1300° C./2.5 h and 1310° C./6 h
        Cooling at 0.5 K/min to T1=1250° C.
        Annealing at T1=1250° C./6 h
        Gas cooling to room temperature with a cooling rate of more than 20 K/min
        Precipitation heat treatment at TA1=1140° C./6 h and TA2=870° C./20 h.

Variant 2 of the Heat Treatment According to the Invention

        Solution annealing at TL=1300° C./4 h
        Cooling with furnace cooling (1–3 K/min) to T1=1200° C.
        Annealing at T1=1200° C./6 h
        Gas cooling to room temperature with a cooling rate of 10 to 30 K/min
        Precipitation heat treatment at TA1=1110° C./6 h and TA2=760° C./16 h.
    
    


The present results demonstrate that the inventive purpose, that of substantially avoiding environmental embrittlement after a high-temperature mechanical load has been present, is achieved. FIG. 1 shows the microstructure after complete heat treatment. The presence of a bimodal γ′ particle distribution is clearly visible after complete heat treatment, the coarse γ′ particle fraction being characterized by a mean diameter of greater than 1 μm and an irregular morphology.
Table 1 shows characteristic values determined in a tensile test in which a heat treatment in accordance with Variants 1 and 2 were selected. For comparison purposes, the table also includes values for which the annealing at 1250° C. was omitted from the heat treatment and there was an absence of slow cooling between the solution annealing and the precipitation heat treatment (designated “conventional”). In addition, tensile tests have been carried out in which the material was exposed to previous creep preshaping at high temperatures (“degraded” material state). The creep test for the material with the heat treatment in accordance with the invention was carried out at 1050° C. and 120 MPa for 285 h. The result was a creep elongation of 3.2%.













TABLE 1











Elongation


Heat
Material
Drawing rate



at break


treatment
state
[mm/min]
T [° C.]
Rp0.2 [MPa]
Rm [MPa]
[%]





















Variant 1
Invention
1
RT
639
660
18.4


Variant 1
Invention
1
700
725
734
22.4


Variant 1
Invention,
0.0005
RT
526
>561
>21.4



“degraded”
(humid




environment)


Variant 1
Invention,
1
700
591
639
42.2



“degraded”


Variant 2
Invention
1
RT
811
917
7.6


Variant 2
Invention,
0.0005
RT
623
946
17.83



“degraded”
(humid




environment)



Conventional
1
RT
910
986



Conventional
1
700
956
1114



Conventional,
0.0005
RT
639
660
Brittle



“degraded”
(humid



fracture




environment)



Conventional,
1
700
725
734



“degraded”









It is noticeable that the tensile strength drops significantly after the heat treatment according to the invention compared to MK4 in a conventional heat treatment. After aging, as typically occurs in operation, however, the difference is significantly less, and consequently this can also be considered acceptable.
Carrying out the tensile test at a drawing rate of 0.0005 mm/min in a humid environment after the creep deformation mentioned above was interrupted at a plastic elongation of 21.4% without specimen fracture, which indicates that, unlike with the conventional heat treatment, there is no embrittlement. In the comparable specimen at room temperature for MK4 at a drawing rate of 0.0005 mm/min after prior creep deformation at 1050° C./120 MPa up to 1% plastic deformation, the specimen experiences brittle fracture without plastic deformation. A rafting structure with matrix inversion is formed.
Of course, other mechanical properties should not be reduced to an unacceptable extent by the heat treatment according to the invention.
Fatigue tests at 900° C. with a holding time of 10 min at the maximum compressive strain and Δε=0.8%; Rε=−1.
The specimen reached 1300 cycles without fracturing. For comparison purposes, the mean and minimum for MK4 with a conventional heat treatment are approx. 2500 and 1000 cycles, respectively. This demonstrates that the fatigue performance approximately corresponds to that of MK4 which has been heat treated conventionally.
Creep test at 1050° C./120 MPa up to a creep elongation of 3.2% after 285 hours. This result approximately corresponds to the minimum values for MK4 heat treated conventionally. In this case too, therefore, there is no unacceptably high loss of properties.
Variant 3 of the Heat Treatment


    
    
        Solution annealing at TL=1300° C./2.5 h and 1310° C./6 h
        Cooling at 0.5 K/min to T1=1275° C.
        Annealing at T1=1275° C./6 h
        Gas cooling to room temperature at a cooling rate of more than 20 K/min
        Precipitation heat treatment at TA1=1140° C./6 h and TA2=870° C./20 h.
    
    


As can be seen from FIG. 2, the precipitation heat treatment resulted in the coarse γ′ particles not being established in the dendrite cores at T1=1275° C. for the material MK4 with Vtot of approx. 70%. This is less favorable, since as a result not all regions of the material are effectively protected from the embrittlement phenomenon described above. Consequently, this temperature was no longer taken into account for the further experimental data. In concrete terms, what this means is that T1=1275° C. is already too high, since no coarse γ′ precipitations have formed in the dendrite cores.
Variant 4 of the Heat Treatment


    
    
        Solution annealing at TL=1300° C./2.5 h and 1310° C./6 h
        Cooling at 0.5 K/min to T1=1180° C.
        Annealing at T1=1180° C./6 h
        Gas cooling to room temperature at a cooling rate of more than 20 K/min
        Precipitation heat treatment at TA1=1140° C./6 h and TA2=870° C./20 h.
    
    


With a precipitation heat treatment at T1=1180° C., coarse γ′ particles were formed throughout. However, the subsequent heat treatment at TA only leads to a certain amount of precipitation of fine γ′ particles. There is therefore only a certain extent of a bimodal microstructure. Therefore T1=1180° C. is too low, since practically the entire precipitation volume is in the form of a coarse phase, and the finer particle fraction is required in order to ensure that sufficient strength is achieved.
Therefore, for the material MK4 used, there is a temperature window between 1180° C. and 1275° C., with a preferred range from 1230° C. to 1265° C.
LIST OF REFERENCE SYMBOLS
TL Solution-annealing temperature
T1 Precipitation temperature of the “coarse” γ′ phase
TA Precipitation temperature of a “fine” γ′ phase
TA1 Precipitation temperature of a first “fine” γ′ phase
TA2 Precipitation temperature of a second “fine” γ′ phase
v Cooling rate
Vtot Volumetric content of the total γ′ phase
V1 Volumetric content of the “coarse” γ′ phase

## Claims

1. A heat treatment process for a single-crystal or directionally solidified material body comprising a nickel-based superalloy with a volumetric γ′ content Vtot after complete heat treatment of at least 50%, the material body being solution-annealed and then γ′ particles being precipitated in various sizes at different temperatures wherein, at a first temperature, γ′ particles of larger than 1 μm are precipitated in a proportion by volume of Vtot−V1 of less than 50%, where V1 is the proportion of the γ′ particles which is larger than 1 μm, and, at least at a second temperature, γ′ particles of less than 1 μm are precipitated, wherein the precipitation of γ′ particles of greater than 1 μm in a material with a composition of (% by weight) Ni—6.5% Cr—9.6% Co—0.6% Mo—6.4% W—6.5% Ta—3% Re—5.6% Al—1.0% Ti—0.2 Hf—230 ppm C—70 ppm B is carried out a temperature of between 1180° C. and 1275° C.

2. The heat treatment process as claimed in claim 1, wherein first of all γ′ particles of greater than 2 μm are precipitated in a proportion by volume of 0.25<(Vtot−V1)/(100−V1)<0.55.

3. The heat treatment process as claimed in claim 1, wherein the precipitation of γ′ particles of less than 1 μm is carried out at a temperature (TA1) of 1050° to 1150° C. over the course of 1–10 hours and a temperature (TA2) of 820° to 900° C. over the course of 10 to 30 hours.

4. The heat treatment process as claimed in claim 1, wherein a cooling rate of less than 5 K/min is selected between the solution annealing and the precipitation of coarse γ′ particles at the first temperature.

5. The heat treatment process as claimed in claim 4, wherein a cooling rate of between 2 K/min and 0.1 K/min is selected between the solution annealing and the precipitation of coarse γ′ particles.

6. The heat treatment process as claimed in claim 5, wherein a cooling rate of 0.5 K/min is selected between the solution annealing and the precipitation of coarse γ′ particles.

7. The heat treatment process as claimed in claim 1, wherein between the solution annealing and the precipitation of coarse γ′ particles the material body is cooled to room temperature and is then reheated to the first temperature.

8. A heat treatment process for a single-crystal or directionally solidified material body comprising a nickel-based superalloy with a volumetric γ′ content Vtot after complete heat treatment of at least 50%, the material body being solution-annealed and then γ′ particles being precipitated in various sizes at different temperatures wherein, at a first temperature, γ′ particles of larger than 1 μm are precipitated in a proportion by volume of Vtot−V1 of less than 50%, where V1 is the proportion of the γ′ particles which is larger than 1 μm, and, at least at a second temperature, γ′ particles of less than 1 μm are precipitated, wherein the precipitation of γ′ particles of greater than 1 μm in a material with a composition of (% by weight) Ni—6.5% Cr—9.6% Co—0.6% Mo—6.4% W—6.5% Ta—3% Re—5.6% Al—1.0% Ti—0.2. Hf—230 ppm C—70 ppm B is carried out a temperature of between 1230° C. and 1265° C. over a period of between 1 and 10 hours.