A nickel-copper alloy forged bar with a diameter ≥ 350 mm and a manufacturing method thereof

Abstract

A nickel-copper alloy forged bar with a diameter ≥ 350 mm and its manufacturing method, the chemical composition of which is by weight percentage: C 0.09 - 0.12%, Cu 24.0 - 26.5%, Al 2.8 - 3.2%, Ti 0.35 - 0.60%, also containing B, and 0.2 ≤ B / C ≤ 0.6, with the rest being Ni and other inevitable impurities. Through the optimization and design of the contents of C, Cu and B / C, the present invention controls the precipitation of boron carbide in the alloy, reduces the dendrite spacing, improves the processing performance of large steel ingots with a diameter ≥ 600 mm, and solves the problem of easy cracking during forging of large steel ingots. The room temperature yield strength of the bar is ≥ 600 MPa, the elongation is ≥ 20%, the reduction of area Z is ≥ 40%, and it is tested according to the stress corrosion test of MgCl2 solution in YB / T 5632-2006. The specimen is immersed in a 42% boiling MgCl2 solution, and the time for the specimen to appear macroscopic cracks is ≥ 20 days. The bar is suitable for manufacturing large structural parts resistant to seawater environment.

Description

Technical Field

[0001] The present invention belongs to the technical field of nickel-copper alloy processing, and particularly relates to a nickel-copper alloy forged bar with a diameter ≥ 350 mm and a manufacturing method thereof. Background Art

[0002] Taking the UNS N05500 alloy (the chemical composition of this alloy in weight percentage is: C ≤ 2.5%, Cu: 27.0 - 33.0%, Ti: 0.35 - 0.85, Al: 2.3 - 3.15%, Si ≤ 0.5%, Mn ≤ 1.5%, Fe ≤ 2.0%, and the rest are Ni and inevitable impurities) as a representative age-hardening nickel-copper alloy, due to its high strength, seawater corrosion resistance and erosion resistance, and stable structure, it is widely used in fields such as oil refining, chemical industry, navigation and medical devices.

[0003] With the rapid development of China's marine equipment manufacturing industry, the demand for UNS N05500 alloy bars is increasing rapidly, especially the demand for large-sized forged bars with a diameter ≥ 350 mm is more urgent. Through the research and trial production of traditional UNS N05500 alloy materials, it is found that there are three problems in the manufacture of large-sized forged bars:

[0004] First, the manufacture of large-sized bars of UNS N05500 alloy requires large-sized ingots. However, due to the high contents of C, Al, and Ti in this alloy, large-sized ingots (diameter ≧ 500 mm) are prone to segregation of C, Al, and Ti elements during smelting and solidification, which causes a large number of brittle precipitated phases to aggregate in the core of the ingot, generating tissue stress, resulting in a large number of microcracks in the core during forging processing, leading to scrapping (as shown in the attachment Figure 1 ).

[0005] Second, the Cu content in the alloy composition is as high as 27.0 - 33.0%, which makes the dendritic crystals developed during alloy solidification and the porosity rate large. Especially in the core of large-sized ingots during solidification, the above problems are more likely to occur, which also leads to central cracking during alloy forging;

[0006] Third, the marine equipment also puts forward harsh performance requirements for the performance of UNS N05500 alloy bars, requiring the yield strength Rp0.2 ≥ 570 Mpa in the aged state of the bars used, and at the same time the reduction of area Z ≥ 40%. However, currently, bars with a diameter ≥ 350 mm cannot meet the above performance requirements simultaneously.

[0007] Chinese Patent CN107338369A discloses "a nickel-copper alloy bar and its manufacturing method". The composition of the bar is as follows by mass percentage: Cu: 30 - 32%, Si: 3.9 - 4.3%, Fe: 1.5 - 2.8%, Mn: 0.5 - 1.5%, impurity elements ≤ 0.3%, and the rest is Ni. The nickel-copper alloy bar is obtained through the preparation of master alloy ingot, casting of the bar, solution heat treatment, and aging heat treatment. The bar manufactured by this method uses precision casting, does not adopt the forging process, and has a maximum diameter of only 40 mm, and its performance cannot meet the requirements.

[0008] Chinese Patent CN10564827A discloses "a nickel-copper alloy with excellent low-temperature impact performance", and also discloses its production process. The composition of the nickel-copper alloy by weight percentage is: Cu: 28.5 - 32.5%, Al: 2.74 - 3.2%, Ti: 0.47 - 0.6%, Mn: 1.18 - 1.28%, Si: 0.33 - 0.38%, C: 0.057 - 0.085%, S ≤ 0.002%, and the balance is Ni. The alloy invented in this patent is mainly applied to the low-temperature shaft of LNG ships, aiming to improve the low-temperature impact performance of the nickel-copper alloy. Therefore, the alloy controls a relatively low C content. It is difficult to manufacture high-strength bars using this method.

[0009] Chinese Patent CN102925753A discloses "a nickel-copper alloy with high strength and high corrosion resistance and its manufacturing method". The composition of the nickel-copper alloy by weight percentage is: Cu: 36 - 44%, Fe: 0.2 - 1.0%, Mn: 1.25 - 2.25%, Si: 0.15 - 0.1%, Mg: 0.05 - 0.1%, Ti: 0.1 - 0.15%, and the balance is Ni and inevitable impurities, with inevitable impurities ≤ 0.5%. Its tensile strength Rm is 670 - 720 MPa, and the elongation is 6 - 7.5%. When using this method to manufacture the bar with a diameter ≥ 350 mm of the present invention, its elongation cannot meet the requirements.

[0010] Therefore, it is necessary to redesign the composition of UNS N05500 alloy and provide a full-process manufacturing method for large-sized forged bars, overcoming the existing technical difficulties to manufacture nickel-copper alloy forged bars with a diameter ≥ 350 mm that meet the requirements of high comprehensive performance, so as to meet the rapid development of the marine equipment manufacturing industry. Summary of the Invention

[0011] The object of the present invention is to provide a nickel-copper alloy forged bar with a diameter ≥ 350 mm and its manufacturing method, which improves the processing performance of the alloy, solves the problem that large steel ingots made of the alloy are prone to cracking during forging, and at the same time enables the obtained bar to have high mechanical properties and corrosion resistance; the room-temperature yield strength of the bar is ≥ 600 MPa, the elongation is ≥ 20%, the reduction of area Z is ≥ 40%, and it is tested according to the stress corrosion test of MgCl2 solution in YB / T 5632-2006. The specimen is immersed in a 42% boiling MgCl2 solution, and the time for the specimen to show macroscopic cracks is ≥ 20 days, which is suitable for manufacturing large structural parts for seawater environments.

[0012] To achieve the above object, the technical solution of the present invention is as follows:

[0013] A nickel-copper alloy forged bar with a diameter ≥ 350 mm, the chemical composition of which by weight percentage is: C: 0.09 - 0.12%, Cu: 24.0 - 26.5%, Al: 2.8 - 3.2%, Ti: 0.35 - 0.60%, and it also contains B, and 0.2 ≤ B / C ≤ 0.6, and the rest are Ni and other inevitable impurities.

[0014] Preferably, in the chemical composition of the bar, 0.3 ≤ B / C ≤ 0.4.

[0015] The microstructure of the bar of the present invention is equiaxed crystals, the grain size is 6.0 - 8.0 grades, and boron carbide precipitation phases and strengthening phase γ' are dispersedly distributed in the equiaxed crystal matrix.

[0016] The room-temperature yield strength of the bar of the present invention is ≥ 600 MPa, the elongation is ≥ 20%, the reduction of area Z is ≥ 40%, and it is tested according to the stress corrosion test of MgCl2 solution in YB / T 5632-2006. The specimen is immersed in a 42% boiling MgCl2 solution, and the time for the specimen to show macroscopic cracks is ≥ 20 days.

[0017] In the composition design of the nickel-copper alloy bar of the present invention:

[0018] The main design idea of the present invention is to optimize the traditional UNS N05500 alloy. Through the optimization and design of key components C and B, the precipitation of boron carbide in the alloy is regulated, the dendrite spacing during alloy solidification is controlled to be ≤ 85 μm, the processing performance of large steel ingots with a diameter ≥ 600 mm is improved, and the problem of easy cracking of large steel ingots during forging is solved; at the same time, by controlling the whole-process processing technology, large-sized bars with a diameter ≥ 350 mm are manufactured, and their mechanical properties such as yield strength, elongation, and reduction of area, as well as corrosion resistance, can also meet the requirements of marine equipment.

[0019] The traditional UNS N05500 alloy is not suitable for manufacturing large-sized forged bars with a diameter ≥ 350 mm, which is mainly determined by the following aspects:

[0020] 1. In the traditional UNS N05500 alloy, the content of Cu element is high. When the alloy solidifies, the dendritic crystals are extremely developed, and the dendrite spacing reaches 168.03 μm, resulting in a large solidification porosity rate, which exacerbates the segregation of the alloy, especially the segregation in the core of the large ingot, reducing the grain boundary bonding strength and thus the processing performance.

[0021] 2. To improve the strength of the alloy, high contents of C, Al, and Ti elements are added to the composition system. However, C, Al, and Ti elements are also prone to segregation elements. Therefore, these elements are likely to aggregate in the developed dendritic crystals and solidification porosity during the solidification of the steel ingot, thus causing a large number of brittle precipitation phases and tissue stresses, reducing the processing performance of the alloy.

[0022] The design characteristics of the nickel-copper alloy of the present invention are as follows: By adjusting the ratio of alloying elements, reducing the segregation of the alloy and the aggregation and precipitation of brittle phases, improving the processing performance of the alloy, solving the problem of easy cracking during the forging of large steel ingots, and at the same time enabling the obtained bars to have high comprehensive performance.

[0023] The present invention mainly considers the selection of elements, the control of element contents, and the coordinated matching adjustment among elements to improve the processing performance of the alloy:

[0024] 1. Selection of elements: The B element can form boron carbide particles in the nickel-copper alloy, thereby refining the as-cast structure and improving the plasticity of the alloy.

[0025] 2. Adjust the content of the matrix element Cu, improving the solidification quality of the alloy while also taking into account the corrosion resistance.

[0026] 3. Reasonable matching and control of the contents of C element and B element, regulating the dendritic crystal size during the solidification of the alloy, controlling the dendrite spacing ≤ 85 μm, reducing dendrite porosity and segregation, and improving the grain boundary bonding force.

[0027] Specifically as follows:

[0028] Carbon (C): The C content lower than that of UNS N05500 alloy is adopted. Element C is an important alloying element that strongly forms, stabilizes, and expands austenite, and is also an important strengthening element. It is found that in large ingots of nickel-copper alloy, on the one hand, too high C content will form a large amount of primary carbide TiC with Ti, and the precipitation of excessive brittle carbide TiC is one of the main reasons for the cracking of the ingot; on the other hand, the solubility of C in nickel-copper alloy is very small. When the content exceeds 0.15%, C will appear in the form of graphite strips along the grain boundaries or precipitate in clusters within the grains, resulting in cold brittleness of the alloy and reducing its corrosion resistance. Therefore, the C content in the alloy should not be too high. Reducing the C content is a main way to reduce primary carbides, but too low C content will significantly reduce the strength, and the C content cannot be too low. Therefore, in the present invention, the C content is controlled within 0.09 - 0.12%.

[0029] Copper (Cu): On the one hand, with a relatively high copper content, copper-rich dendrites precipitate first from the melt during solidification, and the last part to solidify is relatively copper-rich. Its typical structure is a single-phase solid solution, forming dendrites and segregation in the as-cast state. On the other hand, element Cu is a key element for seawater corrosion resistance. Therefore, in the present invention, the content of element Cu is controlled within 24.0 - 26.5%.

[0030] Boron (B): In the present invention, element B is additionally added. The addition of an appropriate amount of B element can form boron carbide particles with carbon in the alloy. The boron carbide particles can refine the as-cast structure of the nickel-copper alloy, disperse in the molten steel, and form crystallization nucleation points within the grains, thereby refining the size of the as-cast dendrites and improving the plasticity of the alloy. However, the B element should not be excessive. Excessive borides aggregate 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.

[0031] For the cooperation between B and C elements, see Figure 1 , the influence of B / C content on the plasticity of the alloy. It can be seen from the curve in the figure that as the B / C value increases, the cross-sectional shrinkage rate of the alloy first increases, then reaches the peak and starts to decrease, indicating that the plasticity of the material first increases and then decreases. The main reason is that as the B content increases, at the beginning, B forms nucleation points with carbon and distributes in the molten steel, achieving the purpose of refining the size of the as-cast dendrites and improving the plasticity of the alloy. However, as the B content further increases, the boron carbide in the molten steel reaches saturation and will aggregate at the grain boundaries, reducing the grain boundary bonding force and thus reducing the plasticity of the alloy. Therefore, based on a large amount of experimental data, in the present invention, B / C is controlled within 0.2 - 0.6, and the preferred range of B / C is 0.3 - 0.4.

[0032] The key control of the composition design of the present invention is the contents of C, Cu and B / C. Through the associated matching design of the C element, Cu element and B element in the composition, the solidification segregation tendency of large ingots is controlled, the central porosity is improved, and at the same time, the size of the solidification dendritic structure is refined, so as to improve the solidification structure quality of large steel ingots and improve the forging performance of large steel ingots. The above three aspects influence each other and are indispensable. If any one of the designs is unreasonable, it is impossible to manufacture large-sized bars with a diameter ≥ 350 mm using the alloy described in the present invention, and make its mechanical properties such as yield strength, elongation and reduction of area and corrosion resistance also meet the requirements of marine equipment.

[0033] The manufacturing method of the forged bar of nickel-copper alloy with a diameter ≥ 350 mm described in the present invention includes the following steps:

[0034] 1) Smelting:

[0035] Smelt into an ingot by the double process of vacuum induction + electroslag remelting according to the above composition, and the diameter of the ingot ≥ 600 mm;

[0036] 2) Homogenization annealing

[0037] Carry out high-temperature homogenization annealing on the ingot, the homogenization annealing temperature: 1150 - 1200 °C, the homogenization annealing time = D × 0.02 h, D is the diameter of the ingot, and the unit is mm;

[0038] 3) Forging

[0039] Forging the homogenized ingot, controlling the reduction per pass: 30 - 40%, the forging temperature: 1080 - 1120 °C, forging to the finished bar, the final forging temperature ≥ 900 °C, and then water-cooling to room temperature;

[0040] 4) Heat treatment

[0041] The obtained bar is subjected to segmented heat treatment. In the first stage, the heat treatment temperature is 1010 - 1050 °C, and the holding time is 30 - 60 min; in the second stage, the heat treatment temperature is 580 - 650 °C, and the holding time is 12 - 16 h, and then it is cooled with the furnace at a speed of 12 - 16 °C / h to below 480 °C, and then taken out of the furnace and air-cooled to room temperature.

[0042] Preferably, in step 4), after the heat treatment in the first stage, the bar is cooled with the furnace to 580 - 650 °C

[0043] In the manufacturing method of the forged bar of nickel-copper alloy with a diameter ≥ 350 mm described in the present invention:

[0044] In step 1) of the present invention, smelting is carried out by the double process of vacuum induction + electroslag remelting to obtain an ingot with a diameter ≥ 600 mm, and the obtained large steel ingot does not have problems such as solidification segregation, central porosity and coarse solidification dendritic size.

[0045] In step 2), homogenization annealing is carried out on the obtained large ingot. The homogenization annealing temperature is 1150 - 1200 °C, and the homogenization annealing time = D × 0.02 h, where D is the diameter of the ingot in mm. Controlling the homogenization temperature and time mainly aims to solve the segregation of carbon elements in the core of the large steel ingot, making the composition more evenly distributed in the steel ingot, effectively preventing cracking during the forging process of the large steel ingot, and improving the comprehensive properties.

[0046] In step 3), the reduction per pass during forging is controlled at 30 - 40%: the forging temperature is 1080 - 1120 °C, and the finished product bar is gradually forged. The final forging temperature is ≥900 °C, and then it is rapidly water-cooled. Combining with the controlled composition of the alloy of the present invention, a large number of thermal simulation tests show that the alloy has the best plasticity during hot working at 1080 - 1120 °C and is particularly suitable for hot working deformation. Controlling the reduction per pass of the deformation pass at 30 - 40%, on the one hand, too large a reduction will cause excessive deformation and cracking in the core of the steel ingot, and on the other hand, too small a reduction will result in the problem that the core of the large steel ingot cannot be deformed, thus affecting the microstructure and properties of the alloy.

[0047] In step 4), the bar after forging is subjected to two-stage superimposed heat treatment, mainly aiming to improve the comprehensive properties of the alloy. The first-stage heat treatment is solution heat treatment, and the temperature is 1010 - 1050 °C. At this temperature, carbides can be fully dissolved without affecting the growth of grains. Solution heat treatment mainly has two purposes: one is to fully dissolve the carbides precipitated during the forging process to improve the corrosion resistance; the other is to control the grain size of the bar at 6.0 - 8.0 grades to improve the strength of the alloy. The second-stage heat treatment is aging treatment, so that the alloy is dispersed with strengthening phase γ', further improving the strength of the alloy.

[0048] The room temperature yield strength of the bar obtained by the present invention is ≥600 MPa, the elongation is ≥20%, the reduction of area Z is ≥40%, the bar diameter is ≥350 mm, the bar microstructure is equiaxed crystal, and the grain size is 6.0 - 8.0 grades; for the corrosion resistance experiment, the stress corrosion test of MgCl2 solution in the standard YB / T5632 - 2006 is adopted. The bar specimen is immersed in 42% boiling MgCl2 solution, and the time for the bar specimen to appear macroscopic cracks is ≥20 days, proving that the bar obtained by the present invention has good seawater corrosion resistance and is suitable for manufacturing large structural parts in seawater environment.

[0049] The beneficial effects of the present invention:

[0050] 1. Compared with the traditional UNS N05500 nickel-copper alloy, the alloy of the present invention reduces the segregation of the alloy, controls the aggregation and precipitation of boron carbide, regulates the dendrite size during solidification of the alloy, and controls the dendrite spacing ≤ 85 μm by adjusting the correlation matching of elements C, Cu, and B in the composition, effectively controlling the problems of solidification segregation and central porosity of large ingots with a diameter ≥ 600 mm, thereby improving the solidification quality of large ingots with a diameter ≥ 600 mm, greatly improving the hot working performance of large ingots, and solving the problem of easy cracking in the center during the forging process of large steel ingots.

[0051] 2. Based on the composition design, the present invention can melt and obtain large ingots with a diameter ≥ 600 mm, and then obtain large-sized bars with a diameter ≥ 350 mm and no cracks in the center by controlling the forging temperature and reduction ratio. Subsequently, through two-stage superimposed heat treatment and adopting a triple composite strengthening mode, namely solution strengthening + fine grain strengthening + aging strengthening, the corrosion resistance and mechanical properties of the bars are improved, not only manufacturing large-sized bars with a diameter ≥ 350 mm, but also having good mechanical properties and corrosion resistance.

[0052] 3. Compared with the traditional UNS N05500 nickel-copper alloy, the mechanical properties such as yield strength, elongation, and reduction of area of the large-sized bars with a diameter ≥ 350 mm obtained by forging the alloy of the present invention can also meet the requirements of marine equipment, with a room temperature yield strength ≥ 600 MPa, an elongation ≥ 20%, and a reduction of area Z ≥ 40%. Moreover, for the corrosion resistance experiment, using the MgCl2 solution stress corrosion test in standard YB / T5632-2006, soaking the bar specimens in 42% boiling MgCl2 solution, the time for the bar specimens to show macroscopic cracks ≥ 20 days, proving that the bars obtained by the present invention have good seawater corrosion resistance, and the bars are suitable for manufacturing large structural parts in a seawater environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a schematic diagram of the influence of B / C content on the plasticity of the alloy obtained by the present invention.

[0054] Figure 2 It is a photo of a large number of microcracks generated in the center of a bar with a diameter of 200 mm made of traditional UNS N05500 alloy.

[0055] Figure 3 It is a photo of the bar prepared in Example 5 of the present invention.

[0056] Figure 4 It is an optical photo of the specimen in Example 5 of the present invention.

[0057] Figure 5 It is a scanning electron microscope photo of the specimen in Example 5 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0058] The technical solution of the present invention will be further described in detail below in conjunction with specific embodiments.

[0059] The process flow of the embodiment of the present invention is: smelting → homogenization annealing → forging → heat treatment.

[0060] Table 1 shows the composition of the nickel-copper alloy in the embodiment of the present invention, and Table 2 shows the process parameters of the embodiment of the present invention. For the comparison of the specifications and properties of the nickel-copper alloy bars prepared in Examples 1-6 of the present invention and the UNS N05500 alloy bars, see Table 3.

[0061] Figure 2 is a photo of a bar with a diameter of 200 mm made of the traditional UNS N05500 alloy. From Figure 2 it can be seen that a large number of microcracks are generated in the core of the bar.

[0062] Figure 3 is a photo of the bar prepared in Example 5 of the present invention. From the photo, it can be seen that the bar obtained by the present invention has good quality and no cracking, indicating that the bar described in the present invention can manufacture large-sized bars and ensure that there is no cracking in the core.

[0063] Figure 4 is an optical photo of the sample in Example 5 of the present invention. From the photo, it can be seen that the microstructure is equiaxed grains and the grain size is 6.5 grades.

[0064] Figure 5 is a scanning electron microscope photo of the sample in Example 5 of the present invention. From this photo, it can be seen that strengthening phase γ' is dispersedly distributed in the matrix.

[0065] From the data in Table 3, it can be seen that the yield strength of the nickel-copper alloy bar of the present invention is equivalent to that of the traditional UNS N05500 alloy, but the elongation and reduction of area are significantly higher than those of the traditional one. Moreover, no cracking occurs in the large-sized bars with a diameter ≥ 350 mm prepared by the present invention, while cracks will appear in the bars with a diameter ≥ 200 mm made of the traditional UNS N05500 alloy, indicating that the nickel-copper alloy bar prepared by the present invention has excellent performance and is more suitable for manufacturing large marine equipment.

[0066] The corrosion resistance experiments of the bars made of the traditional UNS N05500 alloy and the alloy described in the present invention were tested according to the MgCl2 solution stress corrosion test in the standard YB / T 5632-2006. The bar samples were respectively immersed in 42% boiling MgCl2 solution, and the time when cracks appeared in the samples was observed.

[0067] The time when cracks appear in the bars obtained by the present invention is ≥ 20 days, while the bars prepared from the traditional UNS N05500 alloy showed cracks in 15 days, indicating that the bars alloy obtained by the present invention has more excellent corrosion resistance. Moreover, the bars of the present invention have a larger specification, and the bars of the present invention are suitable for manufacturing large structural parts for seawater environments.

[0068] 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.

[0069]

[0070]

[0071]

Claims

1. A nickel-copper alloy forged bar with a diameter ≥ 350 mm, the chemical composition of which is by weight percentage: C: 0.09 - 0.12%, Cu: 24.0 - 26.5%, Al: 2.8 - 3.2%, Ti: 0.35 - 0.60%, and also contains B, and 0.2 ≤ B / C ≤ 0.6, with the rest being Ni and other inevitable impurities.

2. The nickel-copper alloy forged bar with a diameter ≥ 350 mm according to claim 1, characterized in that In the chemical composition of the bar, 0.3 ≤ B / C ≤ 0.

4.

3. The nickel-copper alloy forged bar with a diameter ≥ 350 mm according to claim 1 or 2, characterized in that The microstructure of the bar is equiaxed crystal, the grain size is 6.0 - 8.0 grades, and boron carbide precipitation phase and strengthening phase γ' are dispersed in the equiaxed crystal matrix.

4. The nickel-copper alloy forged bar with a diameter ≥ 350 mm according to claim 1 or 2, characterized in that The room temperature yield strength of the bar is ≥ 600 MPa, the elongation is ≥ 20%, the reduction of area Z is ≥ 40%. It is tested according to the stress corrosion test of MgCl2 solution in YB / T 5632 - 2006. The specimen is immersed in 42% boiling MgCl2 solution, and the time for the specimen to show macroscopic cracks is ≥ 20 days.

5. The nickel-copper alloy forged bar with a diameter ≥ 350 mm according to claim 3, characterized in that The room temperature yield strength of the bar is ≥ 600 MPa, the elongation is ≥ 20%, the reduction of area Z is ≥ 40%. It is tested according to the stress corrosion test of MgCl2 solution in YB / T 5632 - 2006. The specimen is immersed in 42% boiling MgCl2 solution, and the time for the specimen to show macroscopic cracks is ≥ 20 days.

6. A manufacturing method of the nickel-copper alloy forged bar with a diameter ≥ 350 mm according to any one of claims 1 - 5, which comprises the following steps: 1) Smelting: Smelt into an ingot by using a vacuum induction + electroslag remelting double melting process according to the composition described in claim 1 or 2, and the diameter of the ingot ≥ 600 mm; 2) Homogenization annealing Carry out high-temperature homogenization annealing on the ingot, the homogenization annealing temperature: 1150 - 1200 °C, the homogenization annealing time = D × 0.02 h, where D is the diameter of the ingot, in mm; 3) Forging Forge the homogenized ingot, control the pass reduction: 30 - 40%, the forging temperature: 1080 - 1120 °C, forge to the finished bar, and the finish forging temperature ≥ 900 °C, then water-cool to room temperature; 4) Heat treatment Carry out stacking heat treatment on the obtained bar. In the first stage, the heat treatment temperature is 1010 - 1050 °C, and the holding time is 30 - 60 min; in the second stage, the heat treatment temperature is 580 - 650 °C, and the holding time is 12 - 16 h, then cool with the furnace at a speed of 12 - 16 °C / h to below 480 °C, and then take out of the furnace and air-cool to room temperature.

7. The manufacturing method of the nickel-copper alloy forged bar with a diameter ≥ 350 mm according to claim 6, characterized in that In step 4), after the first-stage heat treatment is completed, the bar is furnace-cooled to 580 - 650 °C.

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