RU2803779C1 - Cast corrosion-resistant nickel-based polycrystalline superalloy - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to casting corrosion-resistant nickel-based polycrystalline heat-resistant alloys containing cobalt and chromium and can be used for the manufacture of parts with a polycrystalline structure by casting, for example, turbine blades operating at a temperature of 1000°C. Cast corrosion-resistant polycrystalline heat-resistant nickel-based alloy comprises, wt.%: chromium 14.5-19.5, tungsten 5.2-7.2, aluminium 2.5-3.5, titanium 4.0-5.0, cobalt 11.0-14.0, boron 0, 01-0.05, carbon 0.05-0.13, molybdenum 0.6-1.6, hafnium 0.1-0.3, cerium 0.01-0.05, yttrium 0.01-0.05 , lanthanum 0.01-0.05, rhenium 0.5-1.5, tantalum 0.7-1.7, manganese 0.05-0.3, silicon 0.05-0.3 and the rest is nickel, with the following ratios: 1≤ Al/(Ti+Ta)≤ 1.5, where Al, Ti and Ta is the content of these components, at.%, 1≤ Ti/Al≤ 1.5, where Al, Ti is the content of these components, wt.%.

EFFECT: alloy has a high resistance to sulphide corrosion combined with high structural and phase stability.

1 cl, 2 tbl

Description

The invention relates to metallurgy, in particular, to cast corrosion-resistant polycrystalline heat-resistant nickel-based alloys containing cobalt and chromium, and can be used for the manufacture of casting parts with a polycrystalline structure, for example, turbine blades operating at a temperature of 1000°C.

Increasing the efficiency of promising power gas turbine plants and marine gas turbine engines leads to an increase in operating temperatures, which in turn requires the use of corrosion-resistant structural materials with higher operating temperatures.

Currently, work on the creation of new corrosion-resistant heat-resistant alloys is being carried out in two directions: the creation of alloys with a single-crystalline and polycrystalline structure.

The monocrystalline structure has a number of disadvantages. First of all, the technology for producing single-crystal casting is more complex, expensive and has significant technological limitations associated with the dimensions of the castings and their geometry. The monocrystalline structure is ensured by directional crystallization of the casting, while it is necessary to ensure high temperature gradients at the crystallization front. As the dimensions of the casting increase, the temperature gradient decreases and the crystallization zone expands. Also, an increase in the mass of the casting requires an increase in the thickness of the casting molds to meet the strength requirements and prevent the destruction of mold blocks as a result of hydrostatic pressure of the melt in the mold or water hammer during the process of filling the molds with liquid metal. During crystallization, the mold acts as a heat insulator and further reduces the temperature gradient at the crystallization front. It should also be noted that sharp geometric transitions, for example, shroud flanges of blades or casting of blade sectors, lead to the formation of parasitic grains in the transition zone due to restrictions on the growth rate of the lateral branches of the dendrites of the crystal. Also, single-crystal castings have extremely low repairability due to the impossibility of obtaining a single-crystal structure by surfacing the area being restored, as well as the sensitivity of single-crystal castings to work hardening during machining and impacts, which can lead to recrystallization of the part at operating temperatures.

For this reason, single-crystal alloys are used to a limited extent in marine and power gas turbine engines and installations. This is especially true for turbine nozzle blades, for which strength indicators are not the main ones; operating temperatures, corrosion resistance and manufacturability are more critical. The need to create polycrystalline corrosion-resistant alloys operating at elevated temperatures is obvious.

A nickel heat-resistant alloy for single-crystal casting is known (RF patent for invention RU2700442 dated 06/04/2019, published on 09/17/2019 bulletin No. 26 IPC C22C19/05), containing carbon, chromium, cobalt, tungsten, molybdenum, aluminum, tantalum, hafnium, yttrium, lanthanum, cerium, silicon, manganese and boron, rhenium, with the following ratio of components, wt.%:

WITH 0.002-0.1 Cr 4.0-8.0 Co 6.0-12.0 W 3.0-8.0 Mo 4.0-8.0 Al 4.6-6.6 Ta 6.5-11.0 Hf 0.1-1.0 Re 1.0-3.0 Y 0.001-0.1 La 0.001-0.1 Xie 0.001-0.1 Si 0.01-0.2 Mn 0.01-0.2 IN 0.005-0.03 Ni rest

and subject to the following conditions: 15.4W - 0.9WTa + 28.8Re - 1.7TaRe ≥ (1.0W2 + 3.1ReC + 2.1Re2) ≥ 16.1W - 1.2WTa +17.5Re - 1.3TaRe .

The disadvantages of the presented alloy are that the alloy has an insufficient level of resistance to sulfide corrosion. This is due to the chromium concentration, which does not exceed 8% by weight, and the absence of titanium in the alloy. Satisfactory resistance of nickel alloys to sulfide corrosion begins to appear when they contain at least 12 wt.% chromium. Titanium partially replaces aluminum in the strengthening γ'-phase based on Ni ₃ Al, significantly slows down the rate of coagulation of particles of the γ'-phase and, as a result, has a more active effect on its strengthening. Titanium is the only element contained in the γ'-phase in significant quantities and increases resistance to sulfide corrosion.

An alloy with high oxidation resistance is also known (Patent US 7169241 dated March 19, 2004, IPC C22C19/05, published January 30, 2007), which contains carbon, boron, hafnium, cobalt, tantalum, chromium, molybdenum, tungsten, titanium , aluminum, niobium, vanadium, zirconium, rhenium, platinum group elements, and rare earth materials in the following ratio of components, % by weight:

Chromium 1.5-16 Tungsten 5-14 Aluminum 2.5-7 Titanium 0.1-4.75 Cobalt 0.8-15 Bor 0.01-0.04 Carbon 0.01-0.5 Molybdenum <1.0 Hafnium 0.1-2.5 Cerium 0-0.1 Yttrium 0-0.1 Lanthanum 0-0.1 Rhenium 0-9 Tantalum <8.5 Nickel basic

The disadvantage of this alloy is the unbalanced alloying complex, which does not provide sufficient corrosion resistance when operating in marine gas turbine engines. In particular, the minimum chromium concentration in this alloy is 1.5%, which is not enough to ensure not only resistance to sulfide corrosion, but also heat resistance of the alloy. The alloying criterion of the above-described alloy, which affects the corrosion resistance, ranges from 0.37 to 20.4. To ensure corrosion resistance, this value should not exceed 0.2 (Getsov L.B. Materials and strength of gas turbine parts, book 1, Rybinsk - 2010).

The closest is a heat-resistant nickel alloy for casting parts (RF invention patent RU2520934 dated March 15, 2013, published June 27, 2014 Bulletin No. 18, IPC C22C19/05), containing chromium, cobalt, tungsten, molybdenum, aluminum, titanium, tantalum , boron, yttrium, rhenium, niobium, lanthanum, cerium, hafnium, manganese, silicon and magnesium in the following ratio of components, wt.%:

chromium 9-16 cobalt 10-16 tungsten 4-9 molybdenum 0.2-3.0 aluminum 1.8-4.5 titanium 2.0-4.5 tantalum 2.5-7.0 niobium 0.01-1.5 boron 0.01-0.5 lanthanum 0.01-0.5 yttrium 0.01-0.2 cerium 0.01-0.2 rhenium 0.5-5.0 hafnium 0.1-1.0 manganese 0.05-1.0 silicon 0.05-1.0 magnesium 0.01-0.2 nickel rest

The disadvantages of this alloy are the lack of carbon, which provides strengthening of grain boundaries. Thus, the feasibility of producing parts with a polycrystalline structure from this alloy is completely excluded, since this will lead not only to a sharp decrease in the performance characteristics of the parts, but also to defective workpieces during manufacturing due to the formation of cracks along the grain boundaries as a result of thermal stresses during the crystallization process.

The technical result to be achieved by the invention is the development of a cast corrosion-resistant polycrystalline heat-resistant nickel-based alloy with high resistance to sulfide corrosion in combination with high structural-phase stability and manufacturability, for the use of this alloy under conditions of exposure to a sea salt environment.

The technical result is achieved by the fact that a cast corrosion-resistant polycrystalline heat-resistant nickel-based alloy containing chromium, tungsten, aluminum, titanium, cobalt, boron, manganese, molybdenum, hafnium, cerium, yttrium, lanthanum, rhenium, tantalum, silicon and nickel, unlike the known alloy, it additionally contains carbon in the following ratio of components, wt.%

Chromium 14.5-19.5 Tungsten 5.2-7.2 Aluminum 2.5-3.5 Titanium 4.0-5.0 Cobalt 11.0-14.0 Bor 0.01-0.05 Carbon 0.05-0.13 Molybdenum 0.6-1.6 Hafnium 0.1-0.3 Cerium 0.01-0.05 Yttrium 0.01-0.05 Lanthanum 0.01-0.05 Rhenium 0.5-1.5 Tantalum 0.7-1.7 Manganese 0.05-0.3 Silicon 0.05-0.3 Nickel rest

when the relations are fulfilled:

,

Where , , - content of the specified components in at. %,

,

Where , - content of these components in wt.%.

The chemical composition of the proposed alloy differs from the prototype in the presence of carbon, as well as the concentration of chromium, titanium, and tantalum.

The introduction of carbon into the alloy will ensure the formation of carbides in the alloy necessary for strengthening the grains, which will make it possible to obtain a casting with a polycrystalline structure. In addition, carbides are present in the grains themselves, increasing creep resistance, inhibiting diffusion processes and the movement of dislocations. Carbon also has a positive effect on the fluidity of the melt and improves manufacturability when producing castings due to better filling of molds.

Increased resistance to sulfide corrosion is achieved by increasing the chromium content and the optimal ratio of the main elements that affect corrosion resistance.

Condition fulfilled ,% wt. provides an optimal balance of structural-phase stability and high resistance to sulfide corrosion.

At the alloy is unsuitable for use in marine gas turbine engines and power gas turbine units using diesel fuel and natural gas contaminated with sulfur as fuel, since an excess of aluminum relative to titanium will increase heat resistance, but will negatively affect resistance to sulfide corrosion.

At the alloy becomes prone to the formation of brittle η-phases (Ni ₃ Ti), which weaken the alloy, structural-phase stability, and consequently, as a result, the performance of parts made from this alloy decreases.

Condition fulfilled , % atomic percent, allows us to ensure structural-phase stability, which means the absence in the alloy of negative phases that negatively affect the characteristics of the alloy, as well as the likelihood of their formation during operation.

If this ratio is , then the structural-phase stability of the alloy is not achieved, as a result the alloy becomes prone to the formation of brittle η-phases (Ni ₃ Ti) and (Ni ₃ Ta), which negatively affect the ductility and strength characteristics of the alloy,

If this ratio is , then it is possible to increase the rate of coagulation of the strengthening γ'-phase, which will accelerate the formation of the raft structure and reduce the service life of the alloy. This also leads to an overall increase in aluminum in the alloy, which negatively affects the alloy’s resistance to sulfide corrosion.

To confirm the effectiveness of the proposed cast corrosion-resistant heat-resistant alloy, experimental studies were carried out on resistance to sulfide corrosion with different concentrations of carbon in the alloy. For this purpose, two experimental alloy compositions were smelted, the content of components in which is given in Table 1.

For each of the presented alloy compositions, the main characteristics were determined (Table 2).

The main characteristics of cast heat-resistant nickel alloys resistant to sulfide and high-temperature salt corrosion are:

1. Minimum concentration of chromium - the alloying element that most effectively provides corrosion resistance ;

2. Vγ', % - volume fraction of the strengthening γ'-phase based on Ni ₃ Al. Al in this phase can be replaced by Ti, Ta. Exceeding the total concentration of these elements above the limit value leads to the formation of a eutectic γ'-phase, the strengthening effect of which is much lower. In addition, aluminum, unlike titanium, has a negative effect on resistance to sulfide corrosion. At the same time, the efficiency of titanium in this indicator is not comparable with chromium, therefore the resistance to sulfide corrosion of the strengthening γ'-phase is lower than that of the alloy matrix. Tantalum and titanium significantly slow down the rate of coagulation of γ'-phase particles and, as a result, have a more active effect on the strengthening of the γ'-phase.

3. - temperature of complete dissolution of the strengthening γ'-phase. The temperature that must be reached for complete dissolution of the strengthening γ'-phase. This indicator should be higher than operating temperatures, but its excessive increase narrows the technological window of heat treatment - the temperature interval between the complete dissolution of the strengthening phase and the beginning of melting of the alloy, which negatively affects the manufacturability of the alloy.

4. Ratio shows resistance to sulfide corrosion of the alloy, which is achieved by increasing the chromium content and the optimal ratio of the main elements that affect corrosion resistance. Ratio should not exceed 0.2 (Getsov L.B. Materials and strength of gas turbine parts, book 1, Rybinsk - 2010).

5. Ensuring structural and phase stability , % atm.

6. Ensuring resistance to sulfide corrosion ,% wt.

Table 1 Alloying elements Content of alloying elements, wt.% RU2700442 US7169241 Prototype RU2520934 Smelting the alloy according to the invention No. 1 No. 2 Chromium Cr 4.0-8.0 1.5-16 9-16 14.5 19.5 Tungsten W 3.0-8.0 5-14 4-9 5.2 7.2 Aluminum Al 4.6-6.6 2.5-7 1.8-4.5 2.5 3.5 Titanium Ti - 0.1-4.75 2.0-4.5 4.0 5.00 Cobalt Co 6.0-12.0 0.8-15 10-16 11.0 14.0 Bor B 0.005-0.03 0.01-0.04 0.01-0.5 0.01 0.05 Carbon C 0.002-0.1 0.01-0.5 - 0.05 0.13 Molybdenum Mo 4.0-8.0 <1.0 0.2-3.0 0.6 1.56 Hafnium Hf 0.1-1.0 0.1-2.5 0.1-1.0 0.1 0.3 Cerium Ce 0.001-0.1 0-0.1 0.01-0.2 0.01 005 Yttrium Y 0.001-0.1 0-0.1 0.01-0.2 0.01 0.05 Lanthanum La 0.001-0.1 0-0.1 0.01-0.5 0.01 0.05 Rhenium Re 1.0-3.0 0-9 0.5-5.0 0.5 1.5 Tantalum Ta 6.5-11.0 < 8.5 2.5-7.0 0.7 1.7 Manganese Mn 0.01-0.2 No 0.05-1.0 0.05 0.3 Silicon Si 0.01-0.2 No 0.05-1.0 0.05 0.3 Nickel Ni ost. ost. ost. ost. ost. Zirconium Zr - 0-0.1 - - - Niobium Nb - <4 0.01-1.5 - - Vanadium V - 0-1.0 - - - Platinum group - 0-0.5 - - - Magnesium Mg - - 0.01-1.0 - -

table 2 Alloy characteristics Known alloys Smelting according to the invention RU
2700442 US
7169241 Prototype RU2520934 No. 1 No. 2 _{Cr min} 4.0 1.5 9.0 14.5 19.5 Vγ', % 63-65 38-70 60 51 49 , °С 1314-1323 1259-1244 1220 1230 1158 Ti missing 0.37-20.41 0.22 0.17 0.12 , % atm. 4.02-4.74 1.77-44.35 1.22 1.06 1.14 ,% wt. 0 0.04-0.68 1.03 1.60 1.43

The samples were tested under conditions of a salt solution Na ₂ SO ₄ 75% + NaCl 25% at a temperature of 1000°C.

Analysis of the results obtained allowed us to establish that the proposed alloy has much higher corrosion resistance, structural-phase stability and manufacturability. The test results of samples of the proposed alloy showed that its corrosion rate under salt solution conditions does not exceed 0.0421 mg/cm ² .

According to the data in Table 2, it can be seen that smelting according to the proposed composition of alloy No. 1 and No. 2 have an increased chromium content compared to analogues and the prototype, thereby increasing resistance to sulfide corrosion while maintaining structural and phase stability, as evidenced by the indicators , .

The proposed alloy is superior to analogue alloys and the prototype alloy in terms of resistance to sulfide corrosion. The alloy described in the prototype has an alloying criterion that affects the corrosion resistance of 0.22. And the proposed alloy has the most effective indicator not exceeding 0.2.

During the research, the volume fraction of the strengthening γ'-phase Vγ' was determined. With an increase in the volume fraction of the strengthening phase, the strength and heat resistance of the alloy increases, but the ductility of the alloy decreases. And also some technological properties deteriorate, such as fluidity, tendency to crack formation during casting and welding/surfacing. Based on this, taking into account the purpose of the alloy, the amount of the strengthening γ'-phase in the developed alloy was limited to 50 at.%. Thus, the proposed alloy has an optimal indicator of the strengthening γ'-phase, according to Table. 2.

Thanks to carbon, the proposed alloy has no technological limitations in terms of dimensions, geometry and maintainability of parts and castings, which the prototype with a monocrystalline structure has.

The proposed content of chromium, titanium, tantalum in the alloy, as well as the addition of carbon to the alloy, made it possible to obtain an alloy with high resistance to sulfide corrosion. The alloy is highly stable and has operating temperatures of up to 1000°C under conditions of exposure to the sea salt environment and combustion products of low-quality diesel fuel, as shown by studies (melts No. 1, No. 2).

Claims (7)

Cast corrosion-resistant polycrystalline heat-resistant nickel-based alloy containing chromium, tungsten, aluminum, titanium, cobalt, boron, manganese, molybdenum, hafnium, cerium, yttrium, lanthanum, rhenium, tantalum, silicon and nickel, characterized in that it additionally contains carbon in the following ratio of components, wt. %: Chromium 14.5-19.5 Tungsten 5.2-7.2 Aluminum 2.5-3.5 Titanium 4.0-5.0 Cobalt 11.0-14.0 Bor 0.01-0.05 Carbon 0.05-0.13 Molybdenum 0.6-1.6 Hafnium 0.1-0.3 Cerium 0.01-0.05 Yttrium 0.01-0.05 Lanthanum 0.01-0.05 Rhenium 0.5-1.5 Tantalum 0.7-1.7 Manganese 0.05-0.3 Silicon 0.05-0.3 Nickel rest when the relations are fulfilled: 1 ≤ Al/(Ti+Ta) ≤ 1.5, where Al, Ti and Ta are the content of these components, at. %, 1 ≤ Ti/Al ≤ 1.5, where Al, Ti - content of these components, wt. %.