DESCRIPTION

The present invention relates to a method for nickel-based alloy manufacturing with post heat-treatment to increase the mean grain size within the alloy of fine-grained structure comprising second-phase particles. The present invention re- lates further to a component manufactured using the method, particularly a component within a turbine.

Nickel-based alloys, so-called superalloys are extensively used for example for high-duty components. These components are comprised inter alia by combustion engines and gas turbines. The so-called superalloys have superior mechanical properties and a high corrosion/oxidation resistance at elevated temperatures. An example for a superalloy is the Alloy 718, which is one of the most widely used nickel-based superalloys. The mechanical properties of Ni-based superalloys are very much depending on grain size of the fee gamma (γ) phase matrix. Life time and creep resistance increase with grain size. On the other hand, excessively large grains lower the tensile strength. In superalloys an equilibrium balance should be kept in order to achieve superior mechanical performance at elevated temperatures . The effect of grain size on time to rupture for a homogeneous microstructure is for example known from B. Pieraggi, J.F. Uginet, "Fatigue and creep properties in relation with alloy 718 microstructure in: Superalloys 718, 625, 706 and various derivatives", Ed. E.A. Loria, The Minerals, Metals & Materi- als Society, 1994. Recently, a variety of additive-layer manufacturing technologies employing electron and laser beam melting of metal powder precursors have been described. They have demonstrated novel prospects for developing complex and multifunctional components with applications in biomedical, aerospace, and automotive areas. More homogeneous microstructures suitable for high-temperature applications have been obtained by powder metallurgy, as for example described in D. Furrer and H. Fecht, „Ni-based superalloys for turbine disks", JOM 51

(1999), 14-17.

However, compared to conventionally produced Ni-base superalloys for example in cast and wrought form, additive layer manufacturing can result in more fine-grained microstructures which are less advantageous in terms of fatigue and creep- rupture properties.

Typically, components produced from powder metal gamma-prime (γ' ) precipitation-strengthened nickel-base superalloys are consolidated, such as by hot isostatic pressing (HIP) and/or extrusion consolidation. Then, the resulting billet is iso- thermally forged at temperatures slightly below the γ' solvus temperature of the alloy, to approach superplastic forming conditions. That allows the filling of the die cavity without the accumulation of significant metallurgical strains. In order to improve fatigue crack growth resistance and mechanical properties at elevated temperatures, these alloys are then heat treated above their γ' solvus temperatures. This is generally referred to as a super-solvus heat treatment, to cause uniform coarsening of the grains.

In case of gamma double prime (γ' ' ) precipitation-hardened nickel-base superalloys, for example Alloy 718, heat treatment is similar in concept to that of γ' -hardened superalloys. In Alloy 718, both γ' ' and delta (δ) phases are present in the microstructure . The first one is the principal

strengthening phase, while the δ phase has an important role in controlling the grain size in the metal matrix. Alloy 718 can be worked and heat treated above the δ phase solvus temperature, which is at 1010°C, or at a temperature between the δ phase solvus and the γ' ' phase solvus temperature, which is at 910°C, for grain-size control. The grain-size control is an important aspect of current high-strength superalloy production .

The conventional heat treatment method for Alloy 718 compris- es for example the following steps:

- solution annealing at 927 to 1010°C for 1 to 2 hours, followed by rapid cooling, and

- aging at 719°C for 8 hours, followed by furnace cooling to 621°C, and

- aging at 621°C for a total aging time of 18 hours, followed by air cooling.

A method of increasing intergranular stress corrosion cracking of Alloy 718 in water reactor environments is described in US 5,244,515. The alloy is heat treated at a high solution annealing temperature, substantially at 1093°C, to dissolve grain boundary precipitates formed during thermomechanical processing. This step is followed by subsequent aging at two separate temperatures. However, it is not possible to dis- solve all the grain-boundary precipitates in nickel-base sup- eralloys containing carbon and/or boron during solution annealing, since they contain a certain amount of metal carbides and borides which remain stable at supersolvus temperatures. These inert precipitates can considerably suppress grain growth, thus limiting uniform coarsening of the matrix grains .

The object of the present invention is to present a method for nickel-based alloy manufacturing to increase the mean grain size within the alloy of fine-grained structure which comprises pinning second-phase particles. Particularly an object of the present invention is to dissolve the grain- boundary precipitates in nickel-base superalloys, containing for example carbon and/or boron and to increase the size of the grains above a value of limiting grain size D _L existing without using the method according to the present invention. A further object of the present invention is to present a component of an arrangement manufactured using the method according to the present invention with desired properties, particularly within heavy-duty use.

The above objects are achieved by a method for nickel-based alloy manufacturing according to claim 1 and a component of an arrangement according to claim 10.

Advantageous embodiments of the present invention are given in dependent claims. Features of the main claims can be combined with each other and with features of dependent claims, and features of dependent claims can be combined together.

A method for nickel-based alloy manufacturing according to the present invention comprises a post heat-treatment to increase the mean grain size D within the alloy of fine-grained structure with second-phase particles. The second-phase particles can be for example carbon and/or boron containing compounds accumulated at the grain boundaries. An annealing step is comprised for a defined period of time t _an neai in hours respectively h, at a temperature T in the range of more than 1000°C.

During the post-heat treatment in form of annealing the second-phase particles are dissolved, particularly totally in the material matrix. Metal carbides and/or borides, particularly accumulated at the grain boundaries are dissolved at temperatures higher than 1000°C. The reduction and/or total solving of precipitates from the grain boundaries enables a grain size growth above a limiting grain size value D _L , which exists in the presence of pinning second-phase particles. The defined period of time t _an neai can be substantially in the range of 3 hours and more, particularly 3 hours. With a post heat-treatment with annealing times t _an neai longer or in the range of 3 hours, the grain size D grows above the before ex- isting limiting grain size value D _L . A longer period of time tanneai is correlated with more energy used. For a cost and energy effective method a post heat-treatment time t _an neai of 3 hours can be advantageous. The temperature T during the annealing step can be in the range of 1140°C to 1150°C. With a temperature T below 1000°C metal carbides and/or borides are not dissolved. Very high temperatures T, for example above 1200°C lead to an increase of grain size to values, where the material shows a strongly reduced tensile strength.

The nickel-based alloy can be a gamma double prime (γ' ' ) precipitation-hardened nickel-base superalloy, particularly Alloy 718. This kind of alloy is widely used in components re- quiring high mechanical strength at high temperatures.

A heat treatment, particularly before the post heat-treatment can be comprised with the following steps:

- solution annealing at substantially 927°C to 1010°C for 1 to 2 hours, followed by rapid cooling, and/or

- aging at 719°C for 8 hours, followed by furnace cooling to 621°C, and/or

- aging at 621°C for a total aging time of 18 hours, followed by air cooling. This heat treatment partly solves precipi- tates in the metal matrix and leads to nickel-based alloys with a limiting grain size D _L . The post heat-treatment according to the present invention particularly follows to the heat treatment in time. A coarsening of particles, particularly of second-phase metal carbide particles can be caused by the post heat-treatment. The mean grain size D within the alloy can be increased by the post heat-treatment to a predefined value, particularly greater than the value of the limiting grain size D _L without post heat-treatment. The predefined value D can be in the range of 20 to 45 pm. A material with this value can show high tensile strength and a high life time as well as creep resistance. The value of grain size D can be a good compromise between values with very high tensile strength and values with very high life time as well as creep resistance.

A component of an arrangement according to the present inven tion is manufactured with the method described before. The component can be comprised by a turbine, particularly by a gas turbine. It can be for example a rotor shaft, a turbine blade or an other component.

The advantages in connection with the described component of an arrangement according to the present invention are simila to the previously, in connection with the method for nickel- based alloy manufacturing described advantages and vice versa .

The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG 1 illustrates the effect of grain size D in m on time to rupture t _R in hours for a homogeneous mi- crostructure known from the state of the art, see B. Pieraggi et al., and

FIG 2 illustrates the calculated correlation of the lim- iting grain size D _L in μπι on second-phase particle radius r _P in m for a random distribution of sec- ond-phase particles within the metal matrix of Al- loy 718, and FIG 3 illustrates the correlation between temperature T in °C and the mean second-phase particle radius r _M in μπι for annealing time t _an neai 1 hour and for 3 hours calculated for Alloy 718.

The mechanical properties of Ni-based superalloys, particularly the widely used Alloy 718, depend much on the grain size of the fee gamma (γ) phase matrix. As mentioned before, the life and creep resistance increase with the grain size D. In FIG 1 the correlation between the time to rupture t _R in hours and the grain size D for a homogeneous microstructure is shown, at a temperature T of 650°C and a pressure of 730 MPa. This correlation is known from the state of the art, see B. Pieraggi et al.

As shown in FIG 1, the time to rupture t _R decreases as the grain size D decreases, or the AST grain size number D _ASTM increases. The crack growth rate is inversely proportional to the grain size D. Excessively large grains can lower the ten- sile strength. So, for high-duty components for example in combustion engines or gas turbines, where materials are required with long life time, high creep resistance and high tensile strength, a balanced material is used. The grain size is large enough to give a long life time and high creep re- sistance, but low enough to give a high tensile strength.

With the method according to the present invention a nickel- based alloy with balanced properties can be obtained. For example for Alloy 718 a grain size in the range of 20 to 45 μπι is suitable for components used particularly in gas turbine engines .

In metallic alloys, dispersions of second-phase particles like metal carbides (MC) can significantly inhibit grain growth, by exerting a pinning force on migrating grain boundaries during thermal treatment. This effect is widely known as Zener pinning. As a result, the final grain size is re- stricted to an upper limit, the so-called limiting grain size D _L , beyond which normal grain growth ceases. In general, the relationship between the limiting grain size and the dispersed second phase can be expressed as

D _L = k-r _P / f ⁿ where r _P and f are the size and volume fraction of the second-phase particles, and k and n are constants.

FIG 2 shows the effect of the second-phase particle radius on the limiting grain size for the case of random particle- boundary intersection. The lower the size of second-phase precipitates is, to the greater extent the matrix grain growth is limited. In addition, the limiting grain size decreases with increasing volume fraction of the second-phase.

A post-heat treatment according to the present invention can ensure sufficient coarsening of for example metal carbide particles, allowing to adjust the matrix grain sizes of finegrained materials to a desired value. The value can be selected according to ranges derived from FIG 1. For gas turbine components the desired grain size range is often limited to values between 20 to 45 ym, which corresponds to ASTM val- ues of 6 to 8. According to FIG 2, to achieve the appropriate grain growth in the presence of second-phase particles, the mean particle size should exceed 0.2 ym.

Figure 3 illustrates the dependence of the calculated mean MC particle size, i.e. mean particle radius r _M in pm, as a function of temperature T in °C for two different annealing times. The curve marked with squares corresponds to an annealing time tgnneai 1 hour and the curve marked with circles corresponds to an annealing time t _an n _ea i of 3 hours. Both curves were calculated for Alloy 718. Based on these results of precipitation kinetic simulations, the appropriate coarsening of metal carbide particles for materials with desired properties according to FIG 2 can be achieved by annealing in the temperature range 1140°C to 1150°C for 3 hours or more. Much longer annealing times increase costs and consume energy, so an annealing time t _ann eai of 3 hours can be optimal to achieve the desired grain size for example for gas turbine components .

The post-heat treatment according to the present invention can further facilitate a rapid relief of internal stresses, induced during additive manufacturing.

Nickel-base superalloys can contain 15 to 20% Fe, 17 to 20% Cr, 4.75 to 5.5% Nb, 2.8 to 3.3% Mo, 0.65 to 1.15% Ti, up to 0.6% Al, up to 0.05% C, up to 0.006% B, up to 0.015% P, the balance being nickel. Other compositions and components are possible too. The nickel-base superalloys can be produced for example by additive manufacturing techniques, e.g. selective laser melting or electron beam melting.

The nickel-base superalloys are used i.a. as material to produce components of arrangements like turbines, such as gas turbines, other engines and parts of heavy duty equipment. The process for improving the mechanical properties, such as fatigue and creep-rupture resistance, includes heat treatment of as-fabricated components in the temperature range 1140°C to 1150°C for 3 hours or more, so as to enable grain growth in the presence of inert second-phase particles up to a desired size range. The component can be then subjected to a two-step aging at temperatures of 719°C and 621°C, respectively, and finally air cooled to room temperature.

The above described features of embodiments according to the present invention can be combined with each other and/or can be combined with embodiments known from the state of the art. For example, the method can be used for other nickel-based alloys like Inconel 939 and so on. Components of an arrangement according to the present invention can be turbine discs of air craft turbines or parts of combustion engines. Much longer annealing times t _an neai than 3 hours can be used depending on the material composition and desired grain size. For different applications of components, the balance between properties like long life time, high creep resistance on one side and high tensile strength on the other can be chosen, and desired grain size values can accordingly be achieved.

A main advantage of the method of the present invention is that according to desired properties of the nickel-based alloy of fine-grained structure comprising pinning particles, a grain size can be adjusted with a post heat-treatment even to values of the mean grain size D above a limiting grain size D _L , which exists without the post heat-treatment.

LIST OF REFERENCE CHARACTERS

DASTM ASTM grain size

rupture time in hours respectively h DL limiting grain size in μπ\

rp particle radius in μιη

r _M mean particle radius in pm

T temperature in °C

tanneal annealing time in hours h