Hot isostatic pressing and heat treatment process for high-temperature alloys

Abstract

This invention discloses a hot isostatic pressing (HIP) and heat treatment process for high-temperature alloys. The process employs HIP followed by aging to eliminate porosity defects and improve the mechanical properties of the alloy. The treatment method consists of two steps: first, the alloy casting is subjected to HIP in a hot isostatic pressing apparatus; second, the HIP-pressed alloy component is placed in a muffle furnace under a protective atmosphere for aging treatment. This invention belongs to the field of high-temperature alloy heat treatment, specifically a HIP and heat treatment process for high-temperature alloys, aiming to eliminate internal porosity and obtain high-temperature alloys with excellent microstructure and properties.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature alloy heat treatment technology, specifically referring to a hot isostatic pressing and heat treatment process for high-temperature alloys. Background Technology

[0002] The current heat treatment regime for the alloy is solution treatment followed by aging. Solution treatment involves holding at 1160℃ for 2 hours and then furnace cooling to 1080℃ followed by air cooling. Aging treatment involves holding at 760℃ for 16 hours followed by air cooling. Currently, most alloy components undergo standard heat treatment before being put into use. However, due to the tendency for shrinkage cavities and porous structures to form inside the castings during the casting process, these defects can easily develop into crack initiations during component service, leading to component failure. Hot isostatic pressing (HIP) is a common method for removing porosity defects in high-temperature alloys and improving the mechanical properties and reliability of components. However, there are currently no corresponding standards for HIP and subsequent treatments of alloys. Summary of the Invention

[0003] In view of the above situation and to overcome the defects of the prior art, the present invention provides a hot isostatic pressing and heat treatment process for high-temperature alloys, which aims to eliminate the loose structure inside the alloy and obtain a high-temperature alloy with excellent structure and properties.

[0004] The technical solution adopted in this invention is as follows: This solution uses hot isostatic pressing to replace the solution process in the standard heat treatment system, and uses hot isostatic pressing and subsequent aging heat treatment system to eliminate the porosity defects of the alloy while improving the mechanical properties of the alloy.

[0005] The processing method consists of two steps: the first step is to perform hot isostatic pressing on the alloy casting in a hot isostatic pressing equipment, and the second step is to place the hot isostatically pressed alloy parts in an atmosphere-protected muffle furnace for aging treatment.

[0006] The hot isostatic pressing process includes the following steps:

[0007] Step 1: Place the cast alloy parts into the working chamber of the hot isostatic pressing (HIP) equipment. After evacuating the HIP furnace, introduce protective gas and simultaneously raise the temperature. Maintain the temperature and pressure at a constant level. Control the heating rate at 5 K / min and raise the temperature to 1180℃. This temperature is slightly higher than the standard solution treatment temperature of the γ' strengthening phase of the alloy. At this temperature, the pore closure effect of the alloy is the best, and since this temperature does not reach the initial melting temperature of the alloy, no harmful initial melting structure will appear in the finished alloy product.

[0008] While heating up, argon gas is evenly introduced into the cavity, so that the pressure inside the cavity rises evenly to 140 MPa.

[0009] Step 2: After the temperature and pressure inside the working chamber of the isostatic pressing equipment stabilize, the heat preservation and pressure holding stage is entered. The temperature and pressure of heat preservation and pressure holding are 1180℃ and 140MPa, respectively. After heat preservation and pressure holding for 2~4 hours, the furnace is cooled, and the cooling rate is about 5K / min.

[0010] Under high temperature and high pressure for 2-4 hours, defects such as porosity and pores inside the alloy are healed by creep. The healing effect of the porosity is related to the holding time. The longer the holding time, the better the pore healing effect and the lower the micro porosity number.

[0011] In addition, hot isostatic pressing improves the segregation of elements such as Ti and Mo between dendrites in the alloy. The γ' strengthening phase is refined after hot isostatic pressing, and the proportion of MC carbides is reduced. Under the effect of precipitation strengthening and solid solution strengthening, the mechanical properties of the alloy are improved.

[0012] The timeliness processing includes the following steps:

[0013] The hot isostatically pressed alloy sample was placed in an atmosphere muffle furnace, and argon gas was introduced for atmosphere protection while the temperature was raised at a rate of 8 K / min to 760 °C. The sample was then held at this constant temperature for 16 h. After the holding period, the sample was rapidly cooled at a rate of 60 K / min to a final cooling temperature of 80–120 °C.

[0014] Furthermore, it is difficult to control the strain rate during compression in hot isostatic pressing (HIP), so the pressure must be greater than the maximum flow stress at the maximum strain rate. Simultaneously, the temperature should be maintained within the plastic deformation range of the material to facilitate the diffusion of alloying elements, and the temperature should not be too high to prevent initial melting of the casting.

[0015] In this invention, a hot isostatic pressing (HIP) process at 1180℃ is used to replace the solution treatment step in standard heat treatment. The high temperature and high pressure environment of HIP facilitates the diffusion of alloying elements and promotes microstructure homogenization, resulting in an increase in the precipitation of small-sized γ' strengthening phases during subsequent aging. With the reduction of the microporous structure, the alloy components become denser, improving their mechanical properties and increasing their reliability.

[0016] This invention eliminates porosity and gas defects inside high-temperature alloy castings by hot isostatic pressing and subsequent heat treatment, thereby improving the plasticity and strength of the alloy.

[0017] The beneficial effects of this invention using the above structure are as follows: This solution provides a hot isostatic pressing and heat treatment process for high-temperature alloys. Cast nickel-based high-temperature alloys, due to their wide solidification range, exhibit elemental segregation and microporous defects in their internal structure. These pores, not connected to the external environment, are prone to becoming crack initiation points under high temperature and high load, leading to component failure. The effectiveness of hot isostatic pressing is related to temperature, pressure, and time parameters. Higher temperatures result in better plastic deformation capacity of the alloy at high temperatures; longer holding times lead to better pore healing. Excessively high temperatures can cause overheating, and excessively long holding times can lead to grain growth, resulting in a decrease in the alloy's mechanical properties.

[0018] The alloy exhibits good plastic deformation capacity at 1180℃, and holding at 140MPa for 2-4 hours effectively closes pores and reduces loose microstructure. DSC differential thermal analysis shows the alloy's solidus temperature to be approximately 1223.0℃. Hot isostatic pressing (HIP) at 1180℃ avoids the formation of initial melting structures and promotes homogenization of alloy element diffusion. 1180℃ is slightly higher than the alloy's solution treatment temperature (1160℃). At this temperature, grain growth is minimal, and HIP promotes the dissolution of the γ' phase, leading to the precipitation of more small-sized γ' phases during subsequent aging, thus enhancing the second-phase precipitation strengthening effect. In summary, HIP at 1180℃ can eliminate porosity and looseness in castings while achieving the effects of a solution treatment process, effectively replacing the solution treatment step in standard heat treatment. Attached Figure Description

[0019] Figure 1 These are metallographic images of high-temperature alloys after hot isostatic pressing and aging treatment in the embodiments and comparative examples of this invention;

[0020] Figure 2 These are SEM images of high-temperature alloys after hot isostatic pressing and aging treatment in the embodiments and comparative examples of this invention.

[0021] Figure 3 These are closed-loop SEM images of high-temperature alloy defects at different hot isostatic pressing temperatures compared to the embodiments and comparative examples of this invention.

[0022] Figure 4 These are room temperature tensile properties of alloys after different heat treatments in the embodiments and comparative examples of the present invention.

[0023] in, Figure 1 Metallographic images showing alloys in different heat treatment states after hot isostatic pressing at different temperatures and subsequent aging treatments. Figure 1 (a) is the original as-cast alloy; Figure 1 (b) is an alloy subjected to hot isostatic pressing at 1140℃ / 140MPa / 2h and aging treatment; Figure 1(c) is an alloy subjected to hot isostatic pressing at 1160℃ / 140MPa / 2h and aging treatment; Figure 1 (d) is an alloy subjected to hot isostatic pressing at 1180℃ / 140MPa / 2h followed by aging. The black raised precipitates in the figure are MC carbides, which typically precipitate between dendrites in the alloy.

[0024] Figure 2 The SEM images show the strengthening phase γ' of alloys under different heat treatment states after hot isostatic pressing at different temperatures followed by aging treatment. After hot isostatic pressing, the alloying elements of the alloy are homogenized and the dendrites are basically eliminated. Figure 2 (a) is the original as-cast alloy; Figure 2 (b) Alloy subjected to hot isostatic pressing at 1140℃ / 140MPa / 2h followed by aging treatment; Figure 2 (c) Alloy subjected to hot isostatic pressing at 1160℃ / 140MPa / 2h followed by aging treatment; Figure 2 (d) is an alloy subjected to hot isostatic pressing at 1180℃ / 140MPa / 2h followed by aging treatment.

[0025] Figure 3 SEM images of the pore structure of the alloy after treatment at different hot isostatic pressing temperatures are shown. Figure 3 (a) is the original as-cast alloy; Figure 3 (b) is an alloy subjected to hot isostatic pressing at 1140℃ / 140MPa / 2h; Figure 3 (c) Alloys subjected to hot isostatic pressing at 1160℃ / 140MPa / 2h; Figure 3 (d) is an alloy subjected to hot isostatic pressing at 1180℃ / 140MPa / 2h.

[0026] Figure 4 The effects of different hot isostatic pressing (HIP) temperatures plus aging treatments on the microstructure and properties of the alloy are shown. 1140 HIP + aging is the sample treated in Comparative Example 1, 1160 HIP + aging is the sample treated in Comparative Example 2, and 1200 HIP + aging is the sample treated in Example 1. Detailed Implementation

[0027] This invention discloses a hot isostatic pressing and heat treatment process for a high-temperature alloy. The as-cast high-temperature alloy is prepared using a vacuum induction melting furnace. The main chemical composition of the high-temperature alloy used is (by mass percentage): C: 0.05-0.09%, Cr: 14.0-15.25%, Co: 14.00-16.00%, Al: 4.00-4.60%, Ti: 3.00-3.70%, Mo: 3.90-4.50%, Fe≤0.50%, Si≤0.20%, with the balance being Ni.

[0028] The cast alloy parts are placed in a hot isostatic pressing (HIP) apparatus for hot isostatic pressing treatment.

[0029] The specific hot isostatic pressing process is as follows:

[0030] The cast alloy component was placed in the working chamber of a hot isostatic pressing (HIP) furnace. After evacuating the furnace, a protective gas was introduced, and the temperature was simultaneously raised and maintained at a constant temperature and pressure. The heating rate was controlled at 5 K / min, reaching 1180℃. This temperature exceeds the γ' strengthening phase solution temperature (1160℃) of the alloy's standard heat treatment, where the pore closure effect is optimal. Subsequent observation of the alloy microstructure showed no initial melting structure in the HIP sample at this temperature, indicating that the alloy product prepared using this process will not exhibit harmful low-melting-point structures.

[0031] While heating up, argon gas is evenly introduced into the cavity, so that the pressure inside the cavity rises evenly to 140 MPa.

[0032] After the temperature and pressure inside the working chamber of the hot isostatic pressing equipment stabilize, it enters the heat preservation and pressure preservation solution treatment stage. The heat preservation and pressure preservation temperature and pressure are 1180℃ and 140MPa, respectively. After heat preservation and pressure preservation for 2~4 hours, it is cooled with the furnace at a rate of about 5K / min.

[0033] Under high temperature and high pressure for 2-4 hours, defects such as porosity and pores inside the alloy are healed by creep. The healing effect of the porosity is related to the holding time. The longer the holding time, the better the pore healing effect and the lower the micro porosity number.

[0034] High-temperature alloy parts that have undergone hot isostatic pressing are then subjected to subsequent aging treatment.

[0035] The specific time-sensitive processing steps are as follows:

[0036] The hot isostatically pressed alloy sample was placed in a muffle furnace, and argon gas was introduced for atmosphere protection while the temperature was raised at a rate of 8 K / min to 760 °C. The sample was then held at this constant temperature for 16 h. After the holding period, argon gas was introduced for rapid cooling at a rate of 60 K / min, with the final cooling temperature being 80–120 °C.

[0037] This invention eliminates porosity and air holes inside the cast alloy by hot isostatic pressing and heat treatment, homogenizes the internal structure of the alloy, and improves the plasticity of the alloy.

[0038] The combination of hot isostatic pressing and subsequent aging treatment promoted the precipitation of the γ' strengthening phase and refined the γ' phase, significantly improving the comprehensive mechanical properties of the high-temperature alloy.

[0039] Example 1

[0040] This embodiment provides a high-temperature alloy heat treatment method based on hot isostatic pressing, including:

[0041] Step 1) Hot Isostatic Pressing: Place the cast sample into the working chamber of the hot isostatic pressing (HIP) equipment. After evacuating the HIP furnace, introduce protective gas and simultaneously raise the temperature. Maintain the temperature and pressure at a constant level. The heating rate is controlled at 5 K / min, raising the temperature to 1180℃. While heating, uniformly fill the chamber with argon gas to uniformly raise the pressure inside the chamber to 140 MPa.

[0042] After the temperature and pressure inside the working chamber of the hot isostatic pressing equipment stabilize, it enters the heat preservation and pressure preservation solution treatment stage. The heat preservation and pressure preservation temperature and pressure are 1180℃ and 140MPa, respectively. After heat preservation and pressure preservation for 2 hours, it is cooled with the furnace at a rate of about 60K / min, and the final temperature of cooling is 80~120℃.

[0043] Step 2) Aging treatment: The hot isostatically pressed alloy sample is placed in a muffle furnace, and argon gas is introduced for atmosphere protection while the temperature is raised at a rate of 8K / min to 760℃. The sample is then held at this constant temperature for 16 hours. After the holding period, argon gas is introduced for rapid cooling at a rate of 60K / min to a final cooling temperature of 80-120℃.

[0044] Comparative Example 1

[0045] K424 as-cast alloy with the same composition and casting process as in Example 1 was used for hot isostatic pressing and aging heat treatment. The difference was that the holding temperature of the hot isostatic pressing process was set to 1140℃.

[0046] Step 1) Hot Isostatic Pressing: Place the cast sample into the working chamber of the hot isostatic pressing (HIP) equipment. After evacuating the HIP furnace, introduce protective gas and simultaneously raise the temperature. Maintain the temperature and pressure at a constant level. The heating rate is controlled at 5 K / min, raising the temperature to 1140℃. While heating, uniformly fill the chamber with argon gas to uniformly raise the pressure inside the chamber to 140 MPa.

[0047] After the temperature and pressure inside the working chamber of the hot isostatic pressing equipment stabilize, it enters the heat preservation and pressure preservation solution treatment stage. The heat preservation and pressure preservation temperature and pressure are 1140℃ and 140MPa, respectively. After heat preservation and pressure preservation for 2 hours, it is cooled with the furnace at a rate of about 60K / min, and the final cooling temperature is 80~120℃.

[0048] Step 2) Aging treatment: The hot isostatically pressed alloy sample is placed in a muffle furnace, and argon gas is introduced for atmosphere protection while the temperature is raised at a rate of 8K / min to 760℃. The sample is then held at this constant temperature for 16 hours. After the holding period, argon gas is introduced for rapid cooling at a rate of 60K / min to a final cooling temperature of 80-120℃.

[0049] Comparative Example 2

[0050] K424 as-cast alloy with the same composition and casting process as in Example 1 was used for hot isostatic pressing and aging heat treatment. The difference was that the holding temperature of the hot isostatic pressing process was set to 1160℃.

[0051] Step 1) Hot Isostatic Pressing: Place the cast sample into the working chamber of the hot isostatic pressing (HIP) equipment. After evacuating the HIP furnace, introduce protective gas and simultaneously raise the temperature, maintaining it at a constant temperature and pressure. The heating rate is controlled at 5 K / min, raising the temperature to 1160℃. Simultaneously, argon gas is uniformly introduced into the chamber to uniformly raise the pressure to 140 MPa.

[0052] After the temperature and pressure inside the working chamber of the hot isostatic pressing equipment stabilize, it enters the heat preservation and pressure preservation solution treatment stage. The heat preservation and pressure are 1160℃ and 140MPa, respectively. After heat preservation and pressure preservation for 2 hours, it is cooled with the furnace at a rate of about 60K / min, and the final cooling temperature is 80~120℃.

[0053] Step 2) Aging treatment: The hot isostatically pressed alloy sample is placed in a muffle furnace, and argon gas is introduced for atmosphere protection while the temperature is raised at a rate of 8K / min to 760℃. The sample is then held at this constant temperature for 16 hours. After the holding period, argon gas is introduced for rapid cooling at a rate of 60K / min to a final cooling temperature of 80-120℃.

[0054] Effect verification:

[0055] Sampling was performed on different parts of the alloy castings prepared in the examples and comparative examples, and metallographic characterization and room temperature tensile mechanical property testing were conducted.

[0056] Figure 1 Metallographic images (100×) of alloys under different heat treatment conditions without corrosion. Figure 1 (a) shows the original cast alloy microstructure, where MC carbides precipitate along dendrites and exhibit a clear strip-like arrangement. Figure 1 (b) The tissue sample of Comparative Example 1, after being subjected to hot isostatic pressing at 1140℃ / 140MPa for 2 hours and then aged, showed a reduction in the precipitation of MC carbides. Figure 1 (c) and Figure 1 (d) are metallographic images of the samples from Comparative Example 2 and Example 1, respectively. As the temperature of hot isostatic pressing increases, the distribution of the precipitated carbides becomes more uniform.

[0057] Figure 2 The effect of hot isostatic pressing followed by aging at different temperatures on the γ' phase of the alloy's strengthening microstructure is shown. Figure 2(a) shows that the γ' strengthening phase inside the initial cast alloy is an irregular butterfly shape. Under hot isostatic pressing at 1140℃ and 1160℃, the shape of the γ' phase does not change significantly. Under hot isostatic pressing at 1180℃ and aging treatment, the γ' phase is significantly refined into uniform spherical particles with a particle size of 0.144 μm due to dissolution and re-precipitation. In the original dendritic intergranular region around the carbide, there is a small amount of larger γ' phase.

[0058] The proportions and grain sizes of the second phases, such as carbides and γ' phases, in the alloys after different heat treatments were statistically analyzed. The results are shown in Table 1. After hot isostatic pressing followed by aging, the contents of both carbides and γ' phases in the alloy decreased. After heat treatment at 1180℃, the grain size of the γ' phase decreased significantly, from 0.627 μm in the as-cast state to 0.144 μm, which is only one-quarter of the original size. The refined γ' phase has a significant effect on improving the tensile properties of the alloy.

[0059] Figure 3 SEM images of the microstructure of porous components after hot isostatic pressing at 140 MPa for 2 hours at different temperatures (1140℃, 1160℃, 1180℃). Figure 3 (a) It can be seen that the block sample used contains some pores, with a pore ratio of 1.5% and a pore size of 15~50μm. Under hot isostatic pressing at 1140℃ and 1160℃, a large number of pores are eliminated, but a small number of unclosed pores still exist. No pores were found in the hot isostatic pressing sample at 1180℃, indicating that the pore defects in the alloy are closed under hot isostatic pressing at 1180℃. Hot isostatic pressing at 1180℃ can effectively eliminate the pore defects inside the alloy.

[0060] from Figure 4 It can be seen that the yield strength and tensile strength of the sample treated with hot isostatic pressing at 1180℃ / 140MPa for 2 hours followed by aging are significantly better than those of the original cast alloy. The yield strength and tensile strength of the sample treated with hot isostatic pressing at 1180℃ followed by aging are similar to those of the sample treated with hot isostatic pressing at 1160℃ followed by aging. However, hot isostatic pressing at 1180℃ is more conducive to eliminating porosity defects in the casting. In summary, the method proposed in this invention, which uses hot isostatic pressing at 1180℃ / 140MPa for 2 hours to replace the solution treatment in the standard heat treatment followed by aging, can effectively eliminate porosity in the alloy. 1180℃ is slightly higher than the solution treatment temperature in the standard heat treatment (1160℃), achieving the same effect as solution treatment. Furthermore, combined with the subsequent aging treatment, a finer γ' strengthening phase is obtained, improving the yield strength and tensile strength of the alloy.

[0061] Table 1. Statistics on the percentage and particle size of the second phase precipitated in the alloys after different heat treatments in the examples.

[0062]

[0063] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0064] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

[0065] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention; the actual structure is not limited thereto. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the invention, such designs should fall within the protection scope of the present invention.

Claims

1. A hot isostatic pressing and heat treatment process for a high-temperature alloy, characterized in that: The main chemical composition of the high-temperature alloy (by mass percentage) is as follows: C: 0.05-0.09%, Cr: 14.0-15.25%, Co: 14.00-16.00%, Al: 4.00-4.60%, Ti: 3.00-3.70%, Mo: 3.90-4.50%, Fe≤0.50%, Si≤0.20%, with the balance being Ni. The alloy comprises the following steps: Step 1: Place the cast alloy component into the working chamber of the hot isostatic pressing (HIP) equipment. After evacuating the HIP furnace, introduce protective gas and simultaneously raise the temperature. Maintain the temperature and pressure at a constant level. The heating rate is controlled at 5 K / min, and the temperature is raised to 1180℃. This temperature is higher than the solid solution temperature of the γ' strengthening phase of the alloy. While raising the temperature, argon gas is uniformly introduced into the chamber to uniformly raise the pressure in the chamber to 140 MPa. Step 2: After the temperature and pressure inside the working chamber of the hot isostatic pressing equipment stabilize, the heat preservation and pressure holding stage is entered. The temperature and pressure of heat preservation and pressure holding are 1180℃ and 140MPa, respectively. After heat preservation and pressure holding for 2 to 4 hours, the furnace is cooled, and the cooling rate is about 5K / min. Step 3: Place the hot isostatically pressed alloy sample into an atmosphere muffle furnace, introduce argon gas for atmosphere protection, and simultaneously raise the temperature at a rate of 8 K / min until it reaches 760℃. Hold the sample at this constant temperature for 16 hours. After holding, perform rapid cooling at a rate of 60 K / min until the final cooling temperature is 80–120℃. During hot isostatic pressing, the pressure must be greater than the maximum flow stress at the maximum strain rate. At the same time, the temperature must be kept within the plastic deformation range of the material. The temperature should not be too high to prevent initial melting of the casting.

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