RU2361011C1 - Processing method of casts made of heat-resistant nickel alloys for single-crystal foundaring - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to alloys metallurgy, particularly to manufacturing of alloys on the basis of nickel, used for parts with single-crystal structure, for instance, turbine blades, operating at high temperatures. For increasing of endurance strength and strength properties of products there are treated casts from alloy, containing nickel, chrome, cobalt, tungsten, aluminium, tantalum, rhenium, ittrium, lanthanum and cerium. Quantitative content of alloy components, wt %, is chosen from following conditions: W > Ta >Re and 15.1 ≤ (W+Ta+Re) ≤ 31.0. Cast is subject to homogenising by means of stepped heating up to temperature (Tts.-15°C) ≤ Thom. ≤ (Tts.+10°C), where Tts. - temperature of total solution γ'- phase in alloy, isolation at particular temperature and cooling at a rate 50-100 grad./min. Then it is implemented aging at two stages. Before thermal treatment casts can be subject to high-temperature gas-lubricated pressing for removing of foundry microporosity.

EFFECT: increasing of endurance strength and strength properties of products.

5 cl, 2 tbl, 2 dwg, 2 ex

Description

The invention relates to the metallurgy of alloys, in particular to the production of nickel-based alloys used for the manufacture of parts with a single crystal structure, for example, turbine blades, and installations operating at high temperatures.

Known heat-resistant nickel alloy for single crystal casting containing chromium, cobalt, aluminum, tungsten, niobium, molybdenum, tantalum, cerium, yttrium, lanthanum and nickel subject to the condition 10.5≈ (1 / 2W + 1 / 2Ta + Nb + Mo) ≈11.5 (RF patent No. 1776076, MKI C22C 19/05, publ. 1990), - analogue.

Known heat-resistant nickel alloy for single crystal casting containing cobalt, chromium, molybdenum, tungsten, aluminum, titanium, hafnium, rhenium, provided (rhenium + chromium) not less than 4.0 wt.%, (Rhenium + molybdenum + tungsten + chromium) not less than 18 wt.%, and the method of its manufacture (application US 2002/0062886, publ. 30.05.2002) - analogue.

Known alloys have insufficient heat resistance, as they are not optimally balanced in the quantitative composition of the components.

Known heat-resistant alloy CMSX-4, which is also used as a material for single-crystal vanes and is a carbon-free single-crystal rhenium-containing alloy (US patent No. 4643782, IPC С22С 19/05, 1987.02.17) - analogue.

The known alloy has the following chemical composition (wt.%): Cobalt - 9.3-10.0, chromium - 6.4-6.8, molybdenum - 0.5-0.7, tungsten - 6.2-6, 6, tantalum - 6.3-6.7, aluminum - 5.45-5.75, titanium - 0.8-1.2, hafnium - 0.02-0.12, rhenium - 2.8-3, 2, nickel - the rest is up to 100%.

The known alloy adopted as a prototype also has insufficient heat resistance (the hourly strength limit at a temperature of 1000 ° C is 26 kgf / mm ² ) and, in addition, it exhibits phase instability associated with the release of topologically close packed (TPU) phases. Products obtained from the CMSX-4 alloy have an insufficient level of heat resistance during long-term operation in the temperature range of 900-1100 ° С.

There is a known method of heat treatment used for single-crystal castings of heat-resistant nickel alloys, which includes three stages: at the first stage, the parts are heated to the temperature of complete dissolution of the γ'-phase (solvus temperature for the γ'-phase in this alloy), can withstand from several minutes to several hours and cooled at a rate of more than 100 ° C / min; at the second stage, the part is heated to a temperature close to operating temperatures of 1000-1050 ° C, maintained and cooled at a speed of more than 100 ° C / min; at the third stage, the parts are heated to a temperature of 870-900 ° C, maintained and cooled (Kablov E.N. Cast blades of gas turbine engines (alloys, technologies, coatings). - M .: MISIS, 2001, p.110-115) - analogue.

The disadvantage of this method is the increase in the size of casting micropores and the total number of micropores as a result of heat treatment, which leads to a decrease in strength and fatigue characteristics of the alloy, for example, such as long-term strength.

The technical result to which the claimed invention is directed is to increase the endurance limit and strength characteristics of products made of nickel alloys by the claimed method, for example, long-term strength σ ₁₀₀ ^{1000 of} at least 300 MPa.

The specified technical result is achieved by the fact that in the method of processing castings of heat-resistant nickel alloy for single crystal casting containing nickel, chromium, cobalt, tungsten, aluminum, tantalum, rhenium, yttrium, lanthanum and cerium under the conditions of 15.1≤ (W + Ta + Re) ≤31.0 wt.%, The casting is subjected to heat treatment, homogenization and aging are used as the heat treatment, moreover, homogenization is carried out by stepwise heating to a temperature of (Tp. -15 ° C) ≤ Tgom. ≤ (Tp. + 10 ° C), where Tpr. - the temperature of complete dissolution of the γ'-phase in the alloy, exposure at a given temperature and cooling at a speed of 50-100 deg./min

Before heat treatment, high-temperature gas-static pressing of single-crystal castings can be performed to remove casting microporosity.

The aging operation is carried out in at least two stages, characterized by heating temperatures T ₁ > T ₂ . At the first stage of aging, the casting is heated to a higher temperature T ₁ in the two-phase γ-γ 'region, exposure at which ensures the formation of the main amount of the strengthening γ'-phase and the formation of a regular γ / γ' structure.

The second stage of aging is carried out at a lower temperature T ₂ <T ₁ in the two-phase γ-γ 'region, which ensures the release from the solid solution of an additional amount of the strengthening γ'-phase in the alloy.

A casting is processed for which the condition (Cr + Co + W + Al + Ta) ≤37.8 wt.% Is fulfilled.

The inventive method is based on the following premises.

The successes in the development of high-temperature nickel alloys (ZhS) of recent generations are largely associated with alloying alloys with a large amount of rhenium (for example, 9.3 wt.% Re in ZhS 47 alloy) and an element of the platinum group of ruthenium (for example, 6 wt.% Ru in TMS alloy -162) (E.N. Kablov, N.V. Petrushin, I.L. Svetlov. Modern cast nickel heat-resistant alloys. In collection of proceedings of the International scientific and technical conference. M: VIAM, 2006, p. 43). Since Re, and especially Ru, are very expensive and scarce metals, questions arise as to whether the indicated direction of doping is optimal and whether the possibilities for improving the liquid content by doping with traditional alloying elements are completely exhausted.

The authors conducted theoretical and experimental studies on the possibility of optimizing the traditional alloying system for heat-resistant nickel alloys intended for single-crystal casting, for example, gas turbine engine blades, by analyzing the effect of alloying elements on the cohesive strength of nickel alloys.

The cohesion energy of alloys was chosen as a fundamental parameter determining the characteristics of the heat resistance of structural materials:

where E _atom is the energy of a free atom

E _cryst - energy of a substance in a crystalline state.

The cohesion energy so defined is the work needed to destroy a solid to an atomic state. The energy E of the ground state of a crystalline substance (alloy, metal) was used as _Eryst . To find this energy, the exact muffin-ting orbitals (TMTO) method was used. This method is mainly intended for the calculation of alloys, so the energy of free atoms for pure metals was calculated as:

where E _{coh was} taken from experimental data (Kittel C. Introduction to Solid State Physics, 7th ed. -Wiley, New York, 1996), and E _{cryst was} calculated using the TMTO method.

The following alloying elements were included in the generalized alloying system for nickel ZhS:

Co, Cr, V, Ti, Al, Ru, Mo, Nb, Zr, Hf, Ta, W, Re, Os, Ir.

Figure 1 shows the dependence of the calculated values of the cohesion energy of nickel alloys on the content of alloying elements - Al, Co, Cr, Hf, Ir, Mo, Nb, Os, Pt, Re, Ru, Sc, Ta, Ti, V, W, Zr . From _figure 1 it follows that for most elements the dependence of E _coh on concentration are linear:

It is natural to use χ = ∂ E _coh / ∂c as a parameter characterizing the effectiveness of the influence of alloying elements on the cohesion energy. Positive values of the χ parameter indicate an increase in the cohesion energy upon doping with this element, negative ones indicate the opposite effect. The larger the positive value of the χ parameter, the stronger this element increases the cohesive strength of nickel alloys.

The distribution of alloying elements according to the parameter χ in nickel alloys is shown in Fig.2.

The data shown in figures 1, 2, allow you to select the basic group of elements, which should be used primarily for alloying the Ni-Al system. These include alloying elements, which correspond to the largest positive values of the parameter χ. The first five basic elements include W, Ta and Re.

The results presented in the diagram show that tungsten should be considered the first element contributing to the greatest increase in nickel cohesion energy. Therefore, the basic alloying system for nickel alloys must first contain tungsten, and the amount of W should be kept at a high level, limited from above only by the solubility limit of tungsten in the nickel alloy.

The next candidate for doping nickel alloys is tantalum, which is expediently introduced into the alloys against the background of a high tungsten content, controlling the possibility of precipitation of phases based on (W, Ta) in the nickel alloy.

Following tantalum is rhenium, the cohesion energy of which is also almost two times greater than that for pure nickel.

Thus, according to the effect on cohesion energy, it is advisable to observe the following quantitative hierarchy of basic alloying elements: wt.% W> wt.% Ta> wt.% Re.

We will determine the content of the base alloying elements W, Ta, and Re in the alloy.

We will start estimating the total number of basic elements in the ZhS of the indicated type with the claimed processing with W. We will proceed from the fact that the maximum amount of W introduced into the known ZhS with a multicomponent alloying system (without isolation from the solid solution as an independent phase with a bcc lattice based on W ), is 16-20 wt.% [Kishkin S.T., Stroganov GB, Logunov A.V. Nickel-based heat-resistant foundry alloys. M., Engineering, 1987. - 116 s], the minimum content of W in the alloy is set at W≥ (10-12) wt.%, Since below this value is not provided the claimed increase in heat resistance.

The tantalum content can vary widely, given that with an increase in tantalum, the cohesive strength of the alloy increases and, therefore, heat resistance.

As for the rhenium content, on the one hand, its cohesion energy is quite high, and on the other hand it is an expensive and scarce metal.

The authors found that in order to achieve the claimed technical result with the claimed method for processing monocrystal castings from nickel heat-resistant alloy, the total amount of tungsten, rhenium and tantalum is determined separately for each case, however, as a rule, their minimum value should be no less than 15.0 wt. %, since otherwise the planned increase in heat resistance will not be provided. The maximum value of the total amount of tungsten, rhenium and tantalum for nickel ZhS processed by the claimed method should not exceed 31.0 wt.% And is limited from above by the solubility limit of the total amount of tungsten, tantalum and rhenium in nickel ZhS.

The optimum content is such a rhenium content, which in combination with the selected amount of tantalum and tungsten, when the claimed conditions for the total content of these elements are met, will provide long-term strength at a temperature of 1000 ° C for 100 hours at least 300 MPa.

To achieve the optimal parameters of the FS, i.e. for the formation of optimal parameters of the complex heterophase structure of the alloy, it should be possible to introduce other elements specified in the formula into the alloy, for example, molybdenum, niobium, titanium, etc.

The total content of the three main components of the Nickel ZhS is essential, but not the only condition necessary to achieve the claimed technical result. Another condition is the fulfillment of the claimed conditions for processing alloys, for example, single-crystal castings from nickel alloys, since the choice of heat treatment conditions is essential to increase the heat resistance of the alloy.

Charge billets of a heat-resistant nickel alloy (for example, KS-2 or KS-3) containing nickel, chromium, cobalt, tungsten, aluminum, tantalum, rhenium, yttrium, lanthanum and cerium under the conditions of 15.0≤ (W + Ta + Re ) ≤31.0 wt.%, Cast using the vacuum induction method; single-crystal castings were then obtained from charge billets by directional crystallization. To form an optimal γ-γ 'microstructure, single-crystal castings of experimental alloys were subjected to heat treatment, which consisted of homogenization and two-stage aging. The homogenization temperature was chosen from the condition (Tp. -15 ° C) ≤Tgom. ≤ (Tpr. + 10 ° C), where Tpr. is the temperature of complete dissolution of the γ'-phase in the alloy. If the homogenization temperature is lower (Tp. -15 ° C), then under conditions of underheating, a strong coarsening of the undissolved γ'-phase occurs. The effect of coarsening is not eliminated during subsequent aging and leads to a decrease in the level of heat resistance. If the homogenization temperature is higher (Tp. + 10 ° С), then the likelihood of uncontrolled processes of local fusion increases, which negatively affect the heat resistance.

The exposure time and aging parameters of the casting depend on its size, the nature of the microstructure and are selected on the basis of a metallographic study of the microstructure of the control samples.

Cooling at a speed of 50 to 100 deg / min is due to the fact that with this regulation of the cooling rate, on the one hand, it is possible to fix the microstructure of the alloy formed at the heat treatment at room temperature (this requires rapid cooling - quenching), and on the other hand - minimize the negative effects of rapid cooling (for example, the formation of microcracks).

To remove the casting porosity of single-crystal castings, they can be subjected to high-temperature gas-static treatment (HIP) according to specially developed modes that provide compaction of the casting material without local recrystallization in the volume of samples (A.V. Logunov and others. High-temperature gas-static compaction of single crystals of heat-resistant nickel alloys. Light alloy technology. , No. 1-4, 2005, p. 71-77), followed by heat treatment under the stated conditions.

Examples of specific performance.

Example 1

Single-crystal cast (growth axis [100]) billets made of KS-2 alloy (12W, 8Ta, 2Re, 5.6Al, 2.5Cr, 2Co, remaining Ni (wt.%) (Total content W + Ta + Re = 22 wt.%) to remove casting porosity, they were subjected to high-temperature gas-static pressing according to the regime: stepwise heating to a temperature of 1250 ° С, pressure Р = 150 MPa, holding for 2.5 hours, cooling in a gas bath.

After that, the workpiece was subjected to heat treatment according to the regime:

- homogenization, which was carried out by stepwise heating to a temperature of 1365 ° C, holding for 3 hours, followed by cooling at a speed of 60 ° C / min;

- aging at a temperature of 1055 ° C for 5.5 hours, cooling in air;

- aging at a temperature of 875 ° C for 9 hours, cooling in air.

Table 1 shows the key parameters for the KS-2 alloy, whose chemical composition is close to the basic alloying system (Ni-Al) -W, Ta, Re.

From table 1 it follows that the proposed alloy KS-2 has the best design characteristics of heat resistance compared to the alloy-analogue. A small negative value of the parameter δ in the KS-2 alloy can be changed to a similar positive value by alloying the KS-2 alloy with a small amount of additional elements in accordance with the formula of the present invention.

Example 2

Single-crystal cast (growth axis [100]) billets made of KS-3 alloy, the nominal chemical composition of which is 10W, 8Ta, 6Re, 5.5Al, 1.5Cr, 2Co, ost. Ni (wt.%), The total amount (W + Ta + Re) = 24 wt.%, Is close to the optimal base alloying system (Ni-Al) -W, Ta, Re.

The blanks were subjected to heat treatment according to the regime:

- homogenization, which was carried out by stepwise heating to a temperature of 1360 ° C, holding for 3 hours, followed by cooling at a speed of 70 ° C / min;

- aging at a temperature of 1045 ° C for 6 hours, cooling in air;

- aging at a temperature of 865 ° C for 11 hours, cooling in air.

Table 2 shows the values of the key parameters for the KS-3 alloy, including the experimental values of T _γ , _solvus and heat resistance σ ₁₀₀ ¹⁰⁰⁰ .

From table 2 it follows that the proposed alloy KS-3 has the best characteristics of heat resistance in comparison with the alloy-analogue. A small negative value of the parameter δ in the KS-3 alloy can be changed to a similar positive value by alloying the KS-3 alloy with a small amount of additional elements in accordance with the formula of the present invention.

Claims (5)

1. The method of processing castings of heat-resistant nickel alloy for single crystal casting containing nickel, chromium, cobalt, tungsten, aluminum, tantalum, rhenium, yttrium, lanthanum and cerium under the conditions, wt.% W> wt.% Ta> wt.% Re and 15.1≤ (W + Ta + Re) ≤31.0 wt.%, Consisting in the fact that the casting is subjected to heat treatment, homogenization and aging are used as the heat treatment, and homogenization is carried out by heating to a temperature (T 15 ° С) ≤Тгом≤ (Тпр + 10 ° С), where Тпр is the temperature of complete dissolution of the γ'-phase in the alloy, holding at constant temperature and cooling at a speed of 50-100 ° C / min. 2. The method according to claim 1, characterized in that before the heat treatment gas-static pressing of the casting is carried out. 3. The method according to claim 1, characterized in that the heating is carried out stepwise. 4. The method according to claim 1, characterized in that the aging is carried out stepwise, at least in two stages. 5. The method according to claim 1, characterized in that the casting is processed for which the condition (Cr + Co + W + Al + Ta) ≤37.8 wt.% Is fulfilled.

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