GB2437081A

GB2437081A - Heat treatment of nickel-based superalloy components

Info

Publication number: GB2437081A
Application number: GB0607091A
Authority: GB
Inventors: Colin Neil Jones; Ian Anthony Verso; David Brown
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2006-04-08
Filing date: 2006-04-08
Publication date: 2007-10-17
Also published as: GB0607091D0

Abstract

A method of heat treating a nickel-base superalloy component, such as a gas turbine blade, by heating the component to a temperature of about 1220{C for about 30 minutes and subsequently cooling the component to below about 700 {C at a cooling rate in the range of about 110 to about 160 {C per minute. The method can be done either during component manufacture or to rejuvinate the component. Hot isolatic pressing can be performed before, after or as part of the heat treatment. The nickel-based superalloy may be IN100.

Description

HEAT TREATMENT OF NICKEL-BASE SUPERALLOY COMPONENTS

Field of the Invention

The present invention relates to heat treatment of nickel-base superalloy components such as gas turbine blades.

Background of the Invention

Heat treatment of nickel-base superalloy components is performed in order to improve the creep and fatigue (stress/strain) resistance of the components. It is typically performed at the manufacturing stage, before the component begins its service life, and as a so-called "rejuvenation" heat treatment after the component has completed part of its initial service life, so that the service life is thereby lengthened, preferably at least doubled.

"Rejuvenation procedures to recover creep properties of nickel-base superalloys by heat treatment and hot isostatic pressing techniques" J Mat Sd 26 (1991) 3409-3421, the contents of which are incorporated herein by reference, reviews known heat-treatment and hot isostatic pressing (HIP) processes to recover creep properties. A heat treatment of the superalloy IN-100 which involves heating to 1493 K(1220 C)for 3.6 ks (1 hour) followed by cooling at 0.1 K s (6 C per minute) and other steps is described in connection with Figure 9. Further details are given in Mater Sci Engng 33, (1978) 35, the contents of which are incorporated herein by reference.

The heat treatment described in the prior art is relatively complex and expensive to run, and involves a subsequent reheating for a further hour and then very rapid cooling at 4 K (240 C per minute). The published data indicate that the method was repeated several times. It is desirable to minimise the time of heating, to minimise energy consumption of the furnace. Furthermore, very rapid cooling at 240 C per minute requires consumption of relatively large volumes of argon or other inert gas, which also increases the complexity and cost of the treatment. It is therefore desirable to find a simpler and therefore cheaper heat treatment method, for example one which requires no or few repeated (periodic or cycled) heating/cooling and where the/each heating/cooling is less energy demanding.

It is therefore an object of the present invention to provide an improved or at least alternative heat treatment method for nickel-base superalloy components such as gas turbine blades.

Brief Description of the Invention

According to a first aspect of the present invention, there is provided a method of heat treating a nickel-base superalloy component which comprises heating the component to a temperature of about 1220 C for about 30 minutes and subsequently cooling the component to below about 700 C at a cooling rate in the range of about 110 to about C per minute.

According to a second aspect of the present invention, there is provided a nickel-base superalloy component which has been heat treated by the method of the first aspect of the invention.

The heat treatment according to the present invention may be performed at the manufacturing stage or as a rejuvenation treatment.

The heat treatment according to the present invention may preferably be preceded or followed by an HIP treatment, although this is not essential. An HIP treatment typically involves treatment of the component with pressurised inert gas (eg argon) at elevated pressure (eg above about 90 MPa) for a period of several hours at an elevated temperature, eg above about 1200 C, and is typically followed by further heat treatment at atmospheric pressure at a somewhat lower temperature, eg about 1100 C, and ageing for many hours (eg at least about 15 hours) at atmospheric pressure at a lower temperature still, typically below about 900 C The heat treatment may be performed at atmospheric pressure, and suitably under an inert gas such as argon. Alternatively, the heat treatment according to the present invention may be performed at an elevated pressure, for example during -and as part of -an HIP treatment in the presence of pressurised inert gas (eg argon), for example at a gas pressure above about 90 MPa.

The heat treatment according to the present invention includes cooling beyond about 700 C, most preferably to substantially below about 700 C, for example to about room temperature, at a cooling rate up to about 160 C per minute. It is most preferred that a lower cooling rate than 110 C per minute, for example in the range about 5 to about C per minute, is used below about 700 C.

The heat treatment according to the present invention may, if desired, be repeated. If it is to be repeated, it is preferred that full cooling to about room temperature is not performed between each heating/cooling cycle. Conventional other treatments and procedures may, if desired, be performed on the component between repetitions of the heat treatment according to the present invention. It is most preferred, however, that the heat treatment according to the present invention is not repeated, but is applied only once.

The heat treatment according to the present invention is suitably performed in a standard heat treatment furnace under an atmosphere of argon or other suitable inert gas. The apparatus and operating conditions will be well understood by those skilled in the art, or apparent from the review article mentioned above and the literature referred to in it, and further details are not required here.

The nickel-base superalloy component may, for example, contain greater than about 40 weight% nickel, for example greater than 50%, for example greater than 60%. The alloy may contain other metallic and non-metallic elements. For example, the alloy may contain one or more other materials selected from chromium, cobalt, molybdenum, titanium, carbon, boron aluminium, vandium and zirconium. One superalloy which may particularly be mentioned is IN 100. IN 100 is nickel-based with, in weight%, nominally 15% Cr, 10% Co, 5.5% Al, 3.6% Ti, 3% Mo, 1% V, 0.15% C, 0.05% Zr and 0.006% B, the remainder being nickel or incidental impurities.

The heat treatment according to the present invention may be preceded or followed by any other appropriate treatment procedure for the component, as will be well known to those skilled in the art. For example, the heat treatment may be followed by provision of an appropriate diffusion coating (eg an aluminium-rich surface layer of the superalloy), ageing of the component, machining of the component, or any combination thereof. The apparatus and operating conditions for such other treatment(s), if present, will be well understood by those skilled in the art, or apparent from the review article mentioned above and the literature referred to in it, and further details are not required here.

It is found that the components treated according to the present invention have substantially greater resistance to stress rupture (measured as Life to Failure on a stress rupture test at 950 C and 230 MPa stress) than comparable untreated components. For example, the life to rupture may be at least doubled using only a single application of the heat treatment according to the present invention.

Without wishing to be bound by theory, it is believed that during cooling from 1220 C the gamma prime phase that is partially dissolved at 1220 C precipitates from the gamma phase solution. The gamma prime particles also grow after precipitation when the parts are at relatively high temperatures in the cooling range (above about 1050 C).

The cooling rate will therefore change the size and morphology of the gamma prime particles. Optimising the gamma prime particles through control of the cooling rate optimises the creep rupture properties at any condition.

Brief Description of the Figure

For further understanding of the present invention, and to show how the same may be put into effect, embodiments will now be described, purely by way of illustration and without limitation, with reference to the accompanying Figure, which shows for a number of different samples of gas turbine blades constructed from the same nickel-base superalloy the Life to Failure (in hours) on the stress rupture test described below, plotted against the cooling rate applied to cool the component from 1220 C to below 700 C (cooling rate in C per minute).

Tests Relating to the Invention In the tests performed on the present invention, unused HlPed 1st stage high-pressure (HP1)turbine blades, formed of cast lN-100 and which had not been provided with a diffusion coating, were subjected in a conventional argon furnace to a range of heat treatments involving heating to a temperature of 1220 C for 30 minutes and subsequently cooling to below room temperature at a cooling rate which was varied for different tests between below about 5 C per minute to about 215 C per minute.

The blades were then subjected to a heat treatment at 1100 C for one hour to simulate the effect of a thermal cycle on the micro structure and properties of the blade material commensurate with that which the blade would have experienced had it been provided with an aluminide diffusion coating. Such a coating is provided on production or "in service" components to protect against oxidation and corrosion. In practice the diffusion coating is applied to the components by deposition of aluminium by a vapour deposition technique onto the surface of the component, followed by heating to about 1100 C to form an aluminium-rich surface region of the superalloy in conventional manner.

The coating does not have a significant beneficial effect during the relatively short life of the test compared to in service life, where the tests typically last about 110 hours and service lives are typically over 2,000 hours. However the application of the coating causes the component to undergo a heat cycle (i.e. heated to 1100 C) which changes its metallurgical structure, and hence the reason for simulating the application of the coating.

Stress rupture test pieces were then machined from the blades and aged by relatively low temperature heating at about 850 C for a period of about 16 hours, again in a conventional manner. The ageing has a moderate positive effect on the Life to Failure, but the method of the invention is applicable whether or not an ageing treatment is performed on the piece.

The data for the treated test pieces is shown in the accompanying figure. The Life to Failure of all the comparison blades was in the range of 25 to 45 hours, whereas the Life to Failure of the treated blades according to the present invention (i.e. where the rate of cooling from 1220 C to below about 700 C was in the range of 110 to 160 C per minute) was consistently above 50 hours, as shown in the figure.

The HIPing of all the blades was performed before the batch was split into test pieces and comparison pieces, and consisted of a conventional HIP treatment process at a temperature of 1220 C for 4 hours at an argon pressure of 1O3MPa, followed by cooling at a rate of 3 to 8 C min1. The blades were then heat-treated at 1100 C for I hour and aged for 16 hours at 850 C. Since the HIPing was essentially identical for all the specimens, whether test pieces or comparison pieces, its effects are cancelled in the tests. Generally speaking, HIPed nickel-base superalloy components have improved fatigue resistance compared with non-H IPed components, so the method of the present invention will typically and preferably be used on components that have been HiPed, and/or in association with an HIP procedure, whether at the manufacturing stage or in a rejuvenation treatment.

Stress Rupture Test The stress rupture test was performed using standard laboratory equipment for stress rupture and creep strain testing. A test piece was machined from the blade with threaded grips at the ends. The test piece was screwed into a loading arrangement and placed in a furnace. A uni axial stress load of 230 Mpa was then applied to the test piece at a temperature of 950 C. The test piece was maintained under this stress and temperature until the test piece fractured. The life to failure was recorded.

The foregoing broadly describes the present invention without limitation. Variations and modifications as will be readily apparent to those skilled in this art are intended to be included within the scope of this application and subsequent patent(s).

Claims

CLAIMS

1 A method of heat treating a nickel-base superalloy component which comprises heating the component to a temperature of about 1220 C for about 30 minutes and subsequently cooling the component to below about 700 C at a cooling rate in the range of about 110 to about 160 C per minute.

2 A method according to claim 1, wherein the method is performed at the manufacturing stage of the component.

3 A method according to claim 1, wherein the method is a rejuvenation treatment of the component.

4 A method according to any one of claims I to 3, wherein the method is preceded by a hot isostatic pressure (HIP) treatment of the component.

A method according to any one of claims I to 4, wherein the method is followed by an HIP treatment of the component.

6 A method according to any one of claims 1 to 4, wherein the method is performed as part of an HIP treatment of the component.

7 A method according to any one of the preceding claims, wherein the method includes cooling beyond about 700 C.

8 A method according to claim 7, wherein the cooling beyond about 700 C is to about room temperature.

9 A method according to claim 7 or claim 8, wherein the cooling beyond about 700 c is performed at a cooling rate below 110 C per minute, for example in the range about 5 to about 100 C per minute.

A method according to any one of the preceding claims, when performed only once on the component.

11 A method according to any one of the preceding claims, when performed repeatedly on the component.

12 A method according to any one of the preceding claims, wherein the nickel-base superalloy contains greater 60% nickel.

13 A method according to any one of the preceding claims, wherein the nickel-base superalloy contains contains other metallic and/or non-metallic materials selected from chromium, cobalt, molybdenum, titanium, carbon and boron.

14 A method according to any one of the preceding claims, wherein the nickel-base superalloy comprises IN 100.

A method according to any one of the preceding claims, wherein the heat treatment is preceded and/or followed by one or more other treatment procedure for the component, selected from provision of an appropriate diffusion coating, ageing of the component, machining of the component, and any combination thereof.

16 A method according to any one of the preceding claims, wherein the component is a gas turbine blade. -10-

17 A nickel-base superalloy component which has been prepared by a method according to any one of the preceding claims.

18 A nickel-base superalloy component according to claim 17, which is a gas turbine blade.