A homogenization heat treatment method for iron-nickel based alloy castings

Abstract

A homogenization heat treatment method for an iron-nickel-based alloy casting belongs to the technical field of alloy materials and overcomes the defect of high-temperature homogenization process in the prior art that grains grow and cause the mechanical properties of the alloy to decline. The homogenization heat treatment method for an iron-nickel-based alloy casting of the present invention comprises the following steps: step 1, heating the iron-nickel-based alloy casting to a first step temperature and keeping it warm, wherein the first step temperature is 20 to 80°C below the Y' phase solution temperature; step 2, continuing to heat up to a second step temperature and keeping it warm, wherein the second step temperature is 200-300°C above the Y' phase solution temperature; step 3, cooling to the first step temperature and keeping it warm; step 4, steps 2 and 3 are repeated n times, wherein n is 4 to 6; step 5, cooling to room temperature. The present invention can avoid the coarse grains in the high-temperature homogenization process and thus cause the mechanical properties of the alloy to decline.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy materials, and in particular relates to a homogenization heat treatment method for an iron-nickel based alloy casting. Background Art

[0002] Improving the parameters of coal-fired power units is an important means to improve the efficiency of coal-fired power and reduce coal consumption. Increasing the steam temperature of the turbine can effectively improve the efficiency of the turbine generator set. Increasing the air inlet temperature to 700℃ can increase the efficiency of the ultra-supercritical steam turbine to more than 50%. The steam temperature of the ultra-supercritical steam turbines currently put into commercial operation in China has reached 620℃. Due to the increase in unit parameters, it is difficult to cope with the high-temperature component materials using traditional heat-resistant steel.

[0003] For 650℃ and 700℃ ultra-supercritical units, iron-nickel based high-temperature alloys are mainly used. Foreign countries have carried out relevant product trials for 650℃ and 700℃ grade steam turbine castings. For example, Europe has made progress in solid solution strengthened iron-nickel based high-temperature alloys and successfully trial-produced large steam turbine related components using In617 and In625. The United States has selected precipitation strengthened alloys Haynes282 and In740H to conduct casting process research. my country has also carried out research on 700℃ ultra-supercritical steam turbine castings.

[0004] However, for iron-nickel-based alloys, especially age-hardened alloys used at higher temperatures, the complex composition of iron-nickel-based alloys leads to significant composition segregation in their castings, which poses a risk to subsequent heat treatment processes and the final product performance. At the same time, large ingots usually need to be homogenized to increase the solid solubility of alloying elements in the matrix according to the ingot size and microstructure characteristics, which requires a complex heat treatment process. Although the existing homogenization heat treatment process uses a high-temperature homogenization process to effectively solve the above problems, it also causes significant grain growth at high temperatures, making it impossible to refine the grains through hot working like forgings, resulting in coarse grains and poor mechanical properties of the alloy. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defect in the prior art that the high-temperature homogenization process leads to coarse grains and thus a decrease in the mechanical properties of the alloy, thereby providing a homogenization heat treatment method for iron-nickel based alloy castings.

[0006] To this end, the present invention provides the following technical solutions.

[0007] A homogenization heat treatment method for an iron-nickel based alloy casting comprises the following steps:

[0008] Step 1: heating the iron-nickel based alloy casting to a first step temperature and keeping the temperature constant, wherein the first step temperature is 20 to 80° C. below the γ′ phase solution temperature;

[0009] Step 2: Continue heating to the second step temperature and keep warm. The second step temperature is 200-300°C above the γ′ phase solution temperature.

[0010] Step 3: Cool to the first step temperature and keep warm;

[0011] Step 4, steps 2 and 3 are repeated n times, where n is 4 to 6;

[0012] Step 5: Cool to room temperature.

[0013] Furthermore, the composition of the iron-nickel based alloy casting, in percentage by mass, includes Fe: 40%-48%, Cr: 14%-18%, Mo: 0.2%-1.0%, W: 0.3%-1.0%, 0.5%≤Mo+W≤1.4%, Ti: 1.0%-2.1%, Al: 1.0%-1.8%, Nb≤0.1%, Si≤0.4%, Zr≤0.03%, C: 0.03%-0.1%, B: ≤0.007%, P: ≤0.01%, and the rest is Ni.

[0014] Furthermore, in step 1, the temperature is raised from room temperature to the first step temperature at a heating rate of less than 100°C / h.

[0015] Furthermore, in the process of heating to the first step temperature in step 1, the temperature is first raised to 550-650°C, kept warm, and then raised to the first step temperature; the heating rate is less than 100°C / h.

[0016] Furthermore, after heating to 550-650°C, the holding time is ≤5h.

[0017] Furthermore, in step 1, the holding time at the first step temperature is 0.6*d min, where d is the thickness of the iron-nickel based alloy casting in mm.

[0018] Furthermore, in step 2, the temperature is raised from the first step temperature to the second step temperature, and the heating time is controlled to be ≥4h.

[0019] Furthermore, in step 2, the holding time at the second step temperature is 10 to 15 hours.

[0020] Furthermore, in step 3, the insulation time is 2 to 5 hours.

[0021] Furthermore, the iron-nickel based alloy casting is produced by EAF+LF+VOD smelting.

[0022] The technical solution of the present invention has the following advantages:

[0023] 1. The homogenization heat treatment method for an iron-nickel-based alloy casting provided by the present invention comprises the following steps: step 1, heating the iron-nickel-based alloy casting to a first step temperature and keeping it warm, wherein the first step temperature is 20 to 80°C below the γ′ phase solution temperature; step 2, continuing to heat the casting to a second step temperature and keeping it warm, wherein the second step temperature is 200 to 300°C above the γ′ phase solution temperature; step 3, cooling the casting to the first step temperature and keeping it warm; step 4, repeating steps 2 and 3 n times, where n is 4 to 6; and step 5, cooling the casting to room temperature.

[0024] The present invention controls the homogenization heat treatment temperature and adopts multi-stage homogenization treatment, so that large-sized ingots can be effectively dissolved back during the homogenization process, avoiding the high-temperature homogenization process that causes grain coarsening and thus a decrease in the mechanical properties of the alloy; at the same time, it ensures that the alloy is not overburned, effectively preventing a sharp decrease in the plasticity of the ingot and the formation of intergranular cracks in the ingot during the embryo opening process.

[0025] 2. In the homogenization heat treatment method of the iron-nickel based alloy casting provided by the present invention, the composition of the iron-nickel based alloy casting, in mass percentage, includes Fe: 40%-48%, Cr: 14%-18%, Mo: 0.2%-1.0%, W: 0.3%-1.0%, 0.5%≤Mo+W≤1.4%, Ti: 1.0%-2.1%, Al: 1.0%-1.8%, Nb≤0.1%, Si≤0.4%, Zr≤0.03%, C: 0.03%-0.1%, B: ≤0.007%, P: ≤0.01%, and the rest is Ni.

[0026] The iron-nickel-based alloy ingot has the following characteristics: (1) It does not contain low-melting-point phases such as the Laves phase; (2) It contains Ti, which easily combines with C to form larger carbides. The present invention can effectively eliminate element segregation in the ingot through appropriate heating temperature and holding time. Specifically, the present invention rationally regulates the distribution and content of carbides through a homogenization heat treatment process, improves the hot forming performance and plasticity of the alloy, and facilitates subsequent heat treatment. (3) The alloy of the present invention adopts a cyclic process, using low-temperature precipitation phases to pin the grain boundaries, followed by high-temperature homogenization, and achieves grain refinement through cyclic heat treatment to improve overall performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1It is the microstructure morphology of the iron-nickel based alloy casting before homogenization heat treatment;

[0029] Figure 2 This is the microstructure morphology of the alloy after homogenization heat treatment in Example 1;

[0030] Figure 3 This is the microstructure of the alloy after homogenization heat treatment in Example 1;

[0031] Figure 4 This is the microstructure morphology of the alloy after homogenization heat treatment in Example 2. DETAILED DESCRIPTION

[0032] The following examples are provided for a better understanding of the present invention and are not intended to limit the best mode of implementation. They do not limit the content and scope of protection of the present invention. Any product identical or similar to the present invention obtained by anyone under the guidance of the present invention or by combining the features of the present invention with other prior arts shall fall within the scope of protection of the present invention.

[0033] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0034] The iron-nickel based alloy castings used in the embodiments and comparative examples have the following mass percentages: Fe: 45%, Cr: 16%, Mo: 0.5%, W: 0.5%, Mo+W: 1%, Ti: 1.9%, Al: 1.4%, Nb: 0.1%, Si: 0.3%, Zr: 0.03%, C: 0.05%, and the rest is Ni.

[0035] The raw materials were subjected to EAF+VOD+LF and cast to obtain an iron-nickel based alloy casting. The solid solution temperature of the γ′ phase was 923°C and the thickness of the iron-nickel based alloy casting was 300 mm. The microstructure of the iron-nickel based alloy casting is shown in FIG. Figure 1 As shown in the figure, there are obvious dendrites in the casting structure and obvious carbides between the dendrites.

[0036] Example 1

[0037] This embodiment provides a homogenization heat treatment method for an iron-nickel based alloy casting, comprising the following steps:

[0038] Step 1: The temperature of the iron-nickel based alloy casting is raised from room temperature to the first step temperature of 860° C. for 9 hours, and the temperature is kept at 860° C. for 3 hours.

[0039] Step 2: Continue heating from the first step for 4 hours to the second step temperature of 1150°C, and keep warm for 15 hours;

[0040] Step 3: Cool to the first step temperature of 860°C and keep warm for 3 hours;

[0041] Step 4, step 2, 3 are repeated 4 times;

[0042] Step 5: Air cool to room temperature.

[0043] The alloy structure morphology after homogenization heat treatment in Example 1 is as follows Figure 2 As shown, the dendrites completely disappeared.

[0044] Example 2

[0045] This embodiment is basically the same as embodiment 1, except that steps 2 and 3 of this embodiment are repeated 5 times.

[0046] Example 3

[0047] This embodiment is basically the same as embodiment 1, except that steps 2 and 3 of this embodiment are repeated 6 times.

[0048] Example 4

[0049] This embodiment provides a homogenization heat treatment method for an iron-nickel based alloy casting, comprising the following steps:

[0050] Step 1: The iron-nickel based alloy casting is heated from room temperature to the first step temperature of 850° C. for 9 hours, and kept at 850° C. for 3 hours.

[0051] Step 2: Continue heating from the first step for 6 hours to the second step temperature of 1200°C, and keep warm for 10 hours;

[0052] Step 3: Cool to the first step temperature of 850°C and keep warm for 3 hours;

[0053] Step 4, step 2, 3 are repeated 4 times;

[0054] Step 5: Air cool to room temperature.

[0055] Example 5

[0056] This embodiment provides a homogenization heat treatment method for an iron-nickel based alloy casting, comprising the following steps:

[0057] Step 1: The temperature of the iron-nickel based alloy casting is raised from room temperature to the first step temperature of 890° C. for 20 h, and then kept at 890° C. for 3 h.

[0058] Step 2: Continue heating from the first step for 5 hours to the second step temperature of 1180°C, and keep warm for 15 hours;

[0059] Step 3: Cool to the first step temperature of 890°C and keep warm for 3 hours;

[0060] Step 4, step 2, 3 are repeated 4 times;

[0061] Step 5: Air cool to room temperature.

[0062] Example 6

[0063] This embodiment provides a homogenization heat treatment method for an iron-nickel based alloy casting, comprising the following steps:

[0064] Step 1: Raise the temperature of the iron-nickel alloy casting from room temperature to 600°C, hold for 3 hours, then continue raising the temperature to the first step temperature of 860°C, and hold at 860°C for 3 hours. In step 1, the time for heating to 600°C, holding, and heating to 860°C is 16 hours.

[0065] Step 2: Continue heating from the first step for 4 hours to the second step temperature of 1180°C, and keep warm for 15 hours;

[0066] Step 3: Cool to the first step temperature of 860°C and keep warm for 3 hours;

[0067] Step 4, step 2, 3 are repeated 4 times;

[0068] Step 5: Air cool to room temperature.

[0069] Comparative Example 1

[0070] This comparative example provides a homogenization heat treatment method for an iron-nickel based alloy casting, comprising the following steps:

[0071] Step 1: The temperature of the iron-nickel based alloy casting is raised from room temperature to the first step temperature of 860° C. for 9 hours, and the temperature is kept at 860° C. for 3 hours.

[0072] Step 2: Continue heating from the first step for 4 hours to the second step temperature of 1150°C, and keep warm for 60 hours;

[0073] Step 3: Air cool to room temperature.

[0074] Comparative Example 1 The alloy structure morphology after homogenization heat treatment is as follows Figure 3 As shown in Figure 2, although the dendrites have completely disappeared, Figure 2 、 Figure 3 As can be seen from the comparison, Example 1 has more grain boundaries and a significantly smaller grain size than Comparative Example 1. Table 1 also shows that reducing the grain size can further improve strength and plasticity.

[0075] Table 1 is the mechanical properties table

[0076]

[0077] Comparative Example 2

[0078] This comparative example is basically the same as Example 1, except that in this comparative example, steps 2 and 3 are repeated 3 times. The microstructure of the alloy after homogenization heat treatment in this comparative example is as follows: Figure 4 As shown, according to Figure 1 and Figure 4 It can be clearly seen from the structures before and after homogenization that the dendrites in Comparative Example 2 have not completely disappeared.

[0079] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A homogenization heat treatment method for an iron-nickel based alloy casting, characterized in that: The following steps are involved: Step 1: heating the iron-nickel alloy casting to a first step temperature and keeping it warm, wherein the first step temperature is 20 to 80° C. below the Y′ phase solution temperature; Step 2: Continue heating to the second step temperature and keep warm. The second step temperature is 200-300°C above the Y′ phase solution temperature; Step 3: Cool to the first step temperature and keep warm; Step 4, steps 2 and 3 are repeated n times, where n is 4 to 6; Step 5: Cool to room temperature; In step 1, the holding time at the first step temperature is 0.6*d min, where d is the thickness of the iron-nickel based alloy casting in mm; In step 2, the holding time at the second step temperature is 10 to 15 hours; In step 3, the heat preservation time is 2 to 5 hours.

2. The homogenization heat treatment method for an iron-nickel based alloy casting according to claim 1, characterized in that: The composition of the iron-nickel based alloy casting, in percentage by mass, includes Fe: 40%-48%, Cr: 14%-18%, Mo: 0.2%-1.0%, W: 0.3%-1.0%, 0.5%≤Mo+W≤1.4%, Ti: 1.0%-2.1%, Al: 1.0%-1.8%, Nb≤0.1%, Si≤0.4%, Zr≤0.03%, C: 0.03%-0.1%, B: ≤0.007%, P: ≤0.01%, and the rest is Ni.

3. The homogenization heat treatment method for an iron-nickel based alloy casting according to claim 1, characterized in that: In the step 1, the temperature is raised from room temperature to the first step temperature at a heating rate of less than 100° C. / h.

4. The homogenization heat treatment method for an iron-nickel based alloy casting according to claim 1, characterized in that: In the process of heating to the first step temperature in step 1, the temperature is first raised to 550-650° C., kept warm, and then raised to the first step temperature; the heating rate is less than 100° C. / h.

5. The homogenization heat treatment method for an iron-nickel based alloy casting according to claim 4, characterized in that: After heating to 550-650℃, the holding time should be ≤5h.

6. The homogenization heat treatment method for an iron-nickel based alloy casting according to any one of claims 1 to 5, characterized in that: In the step 2, the temperature is raised from the first step temperature to the second step temperature, and the heating time is controlled to be ≥4h.

7. The homogenization heat treatment method for an iron-nickel based alloy casting according to any one of claims 1 to 5, characterized in that: The iron-nickel based alloy casting is made by EAF+LF+VOD smelting.

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