Nuclear-grade high-temperature-resistant, high-strength and high-creep-resistance iron-nickel-based alloy

Abstract

The invention discloses a nuclear-grade iron-nickel base alloy with high temperature resistance, high strength and high creep property, which comprises the following chemical components in percentage by weight: 0.085-0.10% of C, 22.0-23.5% of Cr, 0.35-0.65% of Al, 0.35-0.65% of Ti, 32.0-35.0% of Ni, less than or equal to 0.008% of N and the balance of Fe and inevitable impurities, and the contents of the components must simultaneously meet the following relational expressions: 0.02 < = B / C < = 0.06, and 0.75% < = Al + Ti < = 1.30%. The iron-nickel-based alloy has high high-temperature instantaneous performance and high-temperature creep strength, the microstructure of the iron-nickel-based alloy is an isometric crystal, the grain size is 2.0-5.0, under the high-temperature condition of 675 DEG C, the tensile strength of the iron-nickel-based alloy is larger than or equal to 420 MPa, the yield strength of the iron-nickel-based alloy is larger than or equal to 140 MPa, the ductility of the iron-nickel-based alloy is larger than or equal to 80%, and the creep strength value of 1% total strain at the temperature of 675 DEG C is larger than or equal to 69 MPa. And the method meets the use requirements in a 675 DEG C high-temperature environment, and is particularly suitable for manufacturing large forgings for main steam tube plates of fourth-generation nuclear power high-temperature gas cooled reactor steam generators.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron-nickel base alloys, and particularly relates to an iron-nickel base alloy with nuclear-grade high temperature resistance, high strength, and high creep performance. Background Art

[0002] The solution-strengthened iron-nickel base alloy represented by UNS N08810 alloy (the chemical composition of this alloy in weight percentage is: C 0.05 - 0.10%, Cr 19.0 - 23.0%, Ni 30.0 - 35.0%, Cu ≤ 0.75%, Al 0.15 - 0.60%, Ti 0.15 - 0.60%, Si ≤ 1.0%, Mn ≤ 1.5%, S ≤ 0.015%, Fe ≥ 39.5%, and the rest are inevitable impurities) has good high-temperature mechanical properties, relatively high creep rupture strength, excellent oxidation resistance, and certain corrosion resistance. When exposed to high temperature for a long time, it can maintain a stable austenite structure and has excellent creep properties. Therefore, UNS N08810 alloy is widely used in manufacturing catalytic tubes, convection tubes, quench tubes, and cracking tubes in the petrochemical industry, radiation tubes, casings, distillation kettles in metallurgical industrial furnaces, and high-temperature heat exchangers and their fittings in air-cooled nuclear reactors.

[0003] With the rapid development of nuclear power in China, the modular high-temperature gas-cooled reactor nuclear power plant has become one of the fourth-generation advanced nuclear reactor types required by the future energy market due to its high safety, simple system, high power generation efficiency, wide application, and economic competitiveness. The helium temperature at the inlet of the steam generator in the high-temperature gas-cooled reactor nuclear power plant is as high as 750 °C, which makes the working temperature of large forgings such as the main steam tube sheet of the steam generator also reach above 675 °C. Therefore, the high-temperature tensile properties required for the tube sheet at 675 °C are: tensile strength ≥ 410 MPa, yield strength ≥ 140 MPa, elongation ≥ 60%, especially the creep strength value at 1% total strain at 675 °C ≥ 67 MPa, and the grain size structure is coarser than grade 5. Ordinary austenitic stainless steel can no longer meet this usage requirement.

[0004] The traditional UNS N08810 alloy is the main candidate material for large forgings used in the steam generator of the fourth-generation nuclear power high-temperature gas-cooled reactor. Under the requirement that the grain size structure is coarser than grade 5, its room-temperature mechanical properties meet the requirements, but its high-temperature tensile strength and creep strength at 1% total strain at 675 °C cannot reach the index requirements of tensile strength ≥ 410 MPa, yield strength ≥ 140 MPa, elongation ≥ 60%, especially the creep strength value at 1% total strain at 675 °C ≥ 67 MPa.

[0005] How to redesign the alloy composition, regulate the alloy microstructure and properties, and further improve the high-temperature instantaneous properties and creep strength of the alloy material at 675°C while maintaining the original cost of the alloy has become the development direction for realizing the performance potential of this type of iron-nickel-based alloy. Summary of the Invention

[0006] The object of the present invention is to provide an iron-nickel-based alloy with nuclear-grade high temperature resistance, high strength, and high creep performance. This alloy has high high-temperature instantaneous properties and high-temperature creep strength. The grain size of the microstructure of this iron-nickel-based alloy is coarser than grade 5. Under the high-temperature condition of 675°C, its tensile strength ≥ 420 MPa, yield strength ≥ 140 MPa, elongation ≥ 80%, and the creep strength value at 1% total strain at 675°C ≥ 69 MPa. It meets the usage requirements of the 675°C high-temperature environment and is especially suitable for the manufacture of large forgings for the main steam tube sheet of the steam generator of the fourth-generation nuclear power high-temperature gas-cooled reactor.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] An iron-nickel-based alloy with nuclear-grade high temperature resistance, high strength, and high creep performance, the weight percentage of its chemical composition is: C: 0.085 - 0.10%, Cr: 22.0 - 23.5%, Al: 0.35 - 0.65%, Ti: 0.35 - 0.65%, Ni: 32.0 - 35.0%, N ≤ 0.008%, and the rest includes Fe and inevitable impurities. And the above component contents must simultaneously satisfy the following relational expressions: 0.02 ≤ B / C ≤ 0.06, 0.75% ≤ Al + Ti ≤ 1.30%.

[0009] Preferably, 0.85% ≤ Al + Ti ≤ 1.20%.

[0010] Furthermore, the rest is Fe and inevitable impurities.

[0011] The microstructure of the iron-nickel-based alloy of the present invention is equiaxed grains, and the grain size is 2.0 - 5.0.

[0012] In the iron-nickel-based alloy of the present invention, boron carbide secondary phase and γ' strengthening phase are distributed in the matrix under the high-temperature condition of 675°C. The volume fraction of the boron carbide secondary phase is 0.05 - 0.10%, and the volume fraction of the γ' strengthening phase is 1.5 - 3.0%.

[0013] The iron-nickel-based alloy of the present invention under the high-temperature condition of 675°C: tensile strength ≥ 420 MPa, yield strength ≥ 140 MPa, elongation ≥ 80%, and the creep strength value at 1% total strain at 675°C ≥ 69 MPa.

[0014] In the composition design of the iron-nickel-based alloy of the present invention:

[0015] The design concept of the present invention is to optimize the composition system of the traditional UNS N08810 alloy through composition design, adjust the proportion of the second phase, the ratio of solid solution elements and the microstructure, improve the instantaneous properties and creep strength of the alloy at 675°C, and expand the service range of the alloy in high-temperature long-term environments.

[0016] The traditional UNS N08810 alloy is not suitable for manufacturing large forgings for the main steam tube sheets of the steam generators of the fourth-generation nuclear power high-temperature gas-cooled reactors, which require high strength and high creep performance at high temperatures. The main reasons are:

[0017] The strengthening mechanism of the traditional UNS N08810 alloy is mainly solid solution strengthening, carbide dispersion strengthening and γˊ strengthening phase (Ni 3 Al and Ni 3 Ti) strengthening. Through solution heat treatment, the strengthening element Cr is fully dissolved into the matrix, which causes lattice distortion and improves the strength of the alloy. At the same time, the precipitated M 6 C, M 23 C 6 and γ′ phase also play a strengthening role; however, the solid solution strengthening elements of the traditional UNS N08810 alloy are only Cr, the carbides are mainly Cr carbides and a small amount of titanium-containing carbides, and a small amount of γˊ strengthening phase is precipitated. Therefore, the strengthening effect is poor, especially the strength of the alloy will decrease significantly in high-temperature environments.

[0018] The design features of the present invention are: through the adjustment of the ratio of alloying elements, on the basis of solid solution strengthening and carbide dispersion strengthening of the alloy, second-phase strengthening is introduced, and the precipitation of more γ′ strengthening phases is promoted, so that the three strengthening mechanisms of the alloy act together at high temperatures, thereby improving the high-temperature resistance of the alloy.

[0019] The present invention mainly considers the selection of elements, the control of element content and the coordinated matching adjustment between elements, so as to improve the high-temperature performance of the alloy:

[0020] 1. Under high-temperature conditions, the B element is prone to segregate and precipitate at the grain boundaries of the material to form B compounds, which play a pinning role at the grain boundaries of the alloy, thereby effectively inhibiting the intergranular fracture of the alloy and improving the high-temperature creep strength of the alloy.

[0021] 2. Under high-temperature conditions, the segregation of the B element at the grain boundaries is also beneficial to the precipitation of eutectic γˊ strengthening phase at the grain boundaries. In addition, the B element is an interstitial solid solution element in the crystal lattice. The introduction of the B element leads to lattice distortion, and the lattice distortion can induce the precipitation of eutectic γ′ strengthening phase in the grains. The volume fraction of the γˊ strengthening phase is increased from the original 0.04 - 1.0% to 1.5 - 3.0%, thereby improving the strength and creep strength of the alloy under high-temperature conditions.

[0022] 3. Strengthen the control of elements, adjust the content of Al+Ti elements, so that more Ni is formed when the alloy is used at high temperature for a long time. 3 Al and Ni 3 Ti second phase, so as to improve the high-temperature strength and creep strength of the alloy while avoiding adverse effects on plasticity.

[0023] Carbon (C): The C element is an important alloying element that strongly forms, stabilizes and expands austenite, and is also an important strengthening element. It is found that increasing the C content in the alloy can significantly increase the number of carbides, thereby improving the strength of the alloy. Therefore, in the present invention, the C content is controlled at 0.085-0.10%.

[0024] Chromium (Cr): Cr is an indispensable alloying element in superalloys, and almost all superalloys contain Cr. In solid-solution strengthened nickel-based and iron-based alloys, the Cr element dissolves into the matrix, causing lattice distortion and generating an elastic stress field for strengthening, playing a role in solid-solution strengthening; Cr can also reduce the stacking fault energy of the solid solution, significantly improving the high-temperature creep strength; Cr can also form a series of carbides with C, playing a role in precipitation strengthening. Therefore, in the present invention, the content of the Cr element is controlled at 22.0-23.5%.

[0025] Aluminum and titanium (Al, Ti and Al+Ti): In superalloys, titanium and aluminum elements are the main elements for forming the γ'-strengthening phase (Ni 3 Al and Ni 3 Ti). From the perspective of the microstructure, Ti mainly forms Ni with a hexagonal close-packed structure 3 Ti, while aluminum mainly forms the Ni phase with a face-centered cubic structure 3 Al. In practice, the larger the number of γ'-strengthening phases, the greater the strengthening effect. As the total amount of Al+Ti in the alloy increases, the number of γ'-strengthening phases increases, and the hot strength performance improves. However, excessive increase in the total amount of Al+Ti in the alloy makes the alloy difficult to process and prone to cracking problems. Through a large number of experimental studies, in the present invention, the element contents of Al, Ti and Al+Ti are respectively controlled at Al: 0.35-0.65%, Ti: 0.35-0.65%, and 0.75% ≤ Al+Ti ≤ 1.30%, preferably 0.85% ≤ Al+Ti ≤ 1.20%.

[0026] Boron (B): In the present invention, B element is additionally added. With the addition of an appropriate amount of B element, under high-temperature conditions, on the one hand, the B element segregates at the grain boundaries, forming a second phase of B compound (boron carbide particles), which also facilitates the precipitation of the eutectic γˊ strengthening phase at the grain boundaries, effectively suppressing the intergranular fracture of the alloy and improving the strength and high-temperature creep strength of the alloy; on the other hand, the B element has an interstitial solid solution element in the lattice. The introduction of the B element leads to lattice distortion, and the lattice distortion can induce the precipitation of the eutectic phase γ′ strengthening phase within the grains, thereby increasing the strength of the alloy. However, the amount of B element should not be excessive. On the one hand, excessive boride aggregates at the grain boundaries in front of the liquid-solid interface, reducing the grain boundary bonding force and thus reducing the plasticity of the alloy; on the other hand, as the content of the B element further increases, in addition to the precipitation of spherical or island-shaped Ni 3 Al eutectic structure, it may also precipitate eutectic phase boride in the Ni 3 Al matrix, thus having an adverse effect on the performance of the alloy. Therefore, in the present invention, B / C is controlled within 0.02 - 0.06, enabling the precipitation of the second phase of boron carbide in the alloy, and the volume fraction of the second phase of boron carbide is 0.05 - 0.10%.

[0027] Nitrogen (N): In the present invention, the N element is strictly controlled. Excessive N element will form TiN particles with Ti. On the one hand, the TiN particles have high strength and poor plasticity, easily forming crack sources, thereby reducing the high-temperature creep strength; on the other hand, excessive N element will consume a large amount of Ti element, thus reducing the precipitation of the Ni 3 Ti second phase and reducing the high-temperature strength. Therefore, in the present invention, the N content is controlled ≤0.008%.

[0028] The key to the composition design of the present invention is to control the contents of C, Cr, Al, Ti, Al+Ti, B / C, and N. Through the associated matching design of Al+Ti and B / C in the composition, on the basis of solid solution strengthening and carbide dispersion strengthening of the alloy, second-phase strengthening is introduced, and the precipitation of the eutectic γ′ strengthening phase within the grains is induced, enabling the three strengthening mechanisms to act together at high temperatures, thereby enhancing the high-temperature resistance of the alloy. The above aspects influence each other and are indispensable. If any one design is unreasonable, it is impossible to achieve high strength and high creep strength of the alloy under high-temperature conditions.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] 1. Compared with the traditional UNS N08810 alloy, through the adjustment of the proportion of alloying elements, on the basis of solid solution strengthening and carbide dispersion strengthening of the alloy, by adding element B, the second-phase strengthening of boron carbide is introduced, and the volume fraction of boron carbide is 0.05 - 0.10%. At the same time, the contents of Al, Ti and Al + Ti are controlled, and the precipitation of eutectic γ′ strengthening phase at grain boundaries and within grains is induced by B. The volume fraction of γˊ strengthening phase is increased from the original 0.04 - 1.0% to 1.5 - 3.0%, enabling the three strengthening mechanisms to act together at high temperatures, thereby improving the high-temperature instantaneous properties and creep strength of the alloy. The tensile strength of this nickel-based alloy at 675°C is ≥420 MPa, the yield strength is ≥140 MPa, the elongation is ≥80%, and the creep strength value at 1% total strain at 675°C is ≥69 MPa. The use range of the alloy in high-temperature long-term environments is extended to meet the usage requirements of high-temperature environments at 675°C.

[0031] 2. Compared with the traditional UNS N08810 alloy, the alloy of the present invention does not require the addition of expensive metal elements. On the basis of improving the alloy properties, the manufacturing cost of the product does not increase, which is conducive to mass production and industrial promotion, and is particularly suitable for manufacturing large forgings for the main steam tube sheets of the steam generators of the fourth-generation nuclear power high-temperature gas-cooled reactors. Specific Embodiments

[0032] The present invention will be further described below in conjunction with the embodiments.

[0033] The composition of the Fe-Ni-based alloy in the embodiment of the present invention is shown in Table 1, and the balance of the composition is Fe; the corresponding nickel-based alloy is prepared by a vacuum induction + electroslag remelting process.

[0034] The nickel-based alloy BG8810 prepared in Embodiments 1 - 8 of the present invention and the traditional UNS N08810 alloy are subjected to the same solution heat treatment, and then their high-temperature mechanical properties are tested. The specific performance parameters are shown in Table 2.

[0035] It can be seen from the data in Table 2 that the yield strength and tensile strength of the Fe-Ni-based alloy of the present invention at 675°C are significantly higher than those of the traditional UNS N08810 alloy, and it is more suitable for manufacturing large forgings for the main steam tube sheets of the steam generators of the fourth-generation nuclear power high-temperature gas-cooled reactors with high-temperature creep performance and high-temperature performance requirements.

[0036] The measured data of the creep strength values at 1% total strain at 675°C of the nickel-based alloy BG8810 prepared in Embodiments 1 - 8 of the present invention and the traditional UNS N08810 alloy are shown in Table 3.

[0037] As can be seen from Table 3, compared with the traditional UNS N08810 alloy, the creep strength value of the nickel-based alloy of the present invention at 1% total strain at 675°C is significantly higher than that of the traditional UNS N08810 alloy, indicating that the alloy of the present invention is more suitable for manufacturing large forgings for the main steam tube sheet of the fourth-generation nuclear power high-temperature gas-cooled reactor steam generator with high-temperature creep performance and high-temperature performance requirements.

[0038] Those of ordinary skill in the art should recognize that the above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. As long as it is within the scope of the spirit of the present invention, changes and modifications to the above embodiments will fall within the scope of the claims of the present invention.

[0039] Table 1 Unit: weight percentage

[0040] C Cr Ni Ti Al Al + Ti B / C N Example 1 0.085 22.0 32.0 0.35 0.65 1.00 0.02 0.008 Example 2 0.100 23.5 35.0 0.62 0.37 0.99 0.06 0.001 Example 3 0.090 23.0 33.0 0.42 0.35 0.77 0.03 0.003 Example 4 0.095 22.5 32.5 0.65 0.62 1.27 0.04 0.005 Example 5 0.088 23.2 34.0 0.55 0.55 1.10 0.022 0.007 Example 6 0.092 22.2 34.5 0.60 0.60 1.20 0.025 0.006 Example 7 0.098 23.4 32.1 0.50 0.45 0.95 0.035 0.002 Example 8 0.086 22.8 33.5 0.40 0.40 0.80 0.05 0.004

[0041] Table 2

[0042]

[0043] Table 3

[0044]

Claims

1. A nickel-iron-based alloy with nuclear-grade high temperature resistance, high strength, and high creep resistance, the chemical composition of which is in weight percentage: C: 0.085 - 0.10%, Cr: 22.0 - 23.5%, Al: 0.35 - 0.65%, Ti: 0.35 - 0.65%, Ni: 32.0 - 35.0%, N ≤ 0.008%, the rest includes Fe and inevitable impurities, and the above component contents must simultaneously satisfy the following relational expressions: 0.02 ≤ B / C ≤ 0.06, 0.75% ≤ Al + Ti ≤ 1.30%.

2. The nickel-iron-based alloy with nuclear-grade high temperature resistance, high strength, and high creep resistance according to claim 1, characterized in that, 0.85% ≤ Al + Ti ≤ 1.20%.

3. The nickel-iron-based alloy with nuclear-grade high temperature resistance, high strength, and high creep resistance according to claim 1 or 2, characterized in that, the rest is Fe and inevitable impurities.

4. The nickel-iron-based alloy with nuclear-grade high temperature resistance, high strength, and high creep resistance according to claim 1 or 2 or 3, characterized in that, in the nickel-iron-based alloy, there are boron carbide second phases and γ'-strengthening phases distributed in the matrix under the high temperature condition of 675°C, the volume fraction of the boron carbide second phases is 0.05 - 0.10%, and the volume fraction of the γ'-strengthening phases is 1.5 - 3.0%.

5. The nickel-iron-based alloy with nuclear-grade high temperature resistance, high strength, and high creep resistance according to claim 1 or 2 or 3 or 4, characterized in that, the microstructure of the nickel-iron-based alloy is equiaxed crystal, and the grain size is 2.0 - 5.

0.

6. The nickel-iron-based alloy with nuclear-grade high temperature resistance, high strength, and high creep resistance according to claim 1 or 2 or 3 or 4 or 5, characterized in that, under the high temperature condition of 675°C for the nickel-iron-based alloy: the tensile strength ≥ 420 MPa, the yield strength ≥ 140 MPa, the elongation ≥ 80%, and the creep strength value at 1% total strain at 675°C ≥ 69 MPa.

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