Heat treatment method of GH350 alloy fastener for aviation

Abstract

The invention belongs to the technical field of aviation fastener processing, and particularly relates to a heat treatment method of an aviation GH350 alloy fastener, which comprises the following steps: after upsetting and forming, sequentially carrying out two-step solution treatment and two-step aging treatment; wherein the temperature of the primary solution treatment is 1017-1023 DEG C, the heat preservation time is 115-125 minutes, the temperature of the secondary solution treatment is 945-955 DEG C, and the heat preservation time is 230-250 minutes; the temperature of the primary aging treatment is 880-890 DEG C, the heat preservation time is 50-70 min, the temperature of the secondary aging treatment is 750-770 DEG C, and the heat preservation time is 230-250 min. According to the heat treatment process, the problem that the grain size of a GH350 bolt product exceeds the standard is solved, the bolt can be controlled to meet the requirement that the grain size is grade 2-6 under the condition that other mechanical properties are met, and meanwhile the mechanical property of the bolt product is improved.

Description

Technical Field

[0001] The invention relates to a heat treatment method for a GH350 alloy fastener for aviation, and belongs to the technical field of aviation fastener processing. Background Art

[0002] GH350 alloy is a nickel-based high-temperature alloy that has high strength when used at 700°C, and has excellent corrosion resistance, good plasticity and toughness. It is suitable for manufacturing aerospace parts and military materials.

[0003] GH350 alloy is often used as turbofan disk bolts for turbofan engines in the aerospace field. The design operating temperature range of turbofan disk bolts is 560-580℃. According to the engine design requirements, the performance requirements for a certain type of turbofan disk bolts are proposed: room temperature tensile strength ≥1130Mpa, high temperature tensile strength ≥740Mpa at 750℃, stress endurance ≥50h at 412Mpa at 750℃, and grain size requirement 2-6.

[0004] GH350 bolts currently produced using conventional processing methods exhibit grade 1 grains in their metallographic microstructure, failing to meet the 2-6 grain size requirement for turbofan disk bolts. Therefore, it is necessary to optimize the heat treatment process to reduce the presence of grade 1 grains and achieve a uniform microstructure while meeting the room temperature strength and high-temperature mechanical property requirements for GH350 alloy bolts, ultimately producing bolt products that meet engine requirements. Summary of the Invention

[0005] In view of the defects of the prior art, the present invention provides a heat treatment method for GH350 alloy fasteners for aviation, which reduces the level 1 large grains in GH350 bolts and achieves a dual improvement in the room temperature strength and high temperature mechanical properties of GH350 alloy bolts.

[0006] The technical solution of the present invention to solve the above technical problems is as follows: A heat treatment method for aviation GH350 alloy fasteners comprises the following steps: after upsetting, sequentially performing a two-step solution treatment and a two-step aging treatment; wherein the temperature of the first solution treatment is 1017°C-1023°C, and the holding time is 115 min-125 min; the temperature of the second solution treatment is 945°C-955°C, and the holding time is 230 min-250 min; the temperature of the first aging treatment is 880°C-890°C, and the holding time is 50 min-70 min; the temperature of the second aging treatment is 750°C-770°C, and the holding time is 230 min-250 min.

[0007] On the basis of the above technical solution, the present invention can also make the following improvements: Furthermore, during the heating process of the first solution treatment, the temperature is maintained at 800° C. for 40 min-50 min.

[0008] Furthermore, the primary solution treatment is carried out in a vacuum furnace with a vacuum degree ranging from 0.133 Pa to 13.3 Pa. After the primary solution treatment is completed, it is first cooled in argon for a cooling time of ≥30 minutes and then air-cooled.

[0009] Furthermore, the temperature of the primary solution treatment is 1020°C.

[0010] Furthermore, the secondary solution treatment is carried out in a vacuum furnace with a vacuum degree ranging from 0.133 Pa to 13.3 Pa. After the secondary solution treatment is completed, it is first cooled in argon for a cooling time of ≥30 minutes and then air-cooled.

[0011] Furthermore, the temperature of the secondary solution treatment is 950°C.

[0012] Furthermore, the primary aging treatment is carried out in a vacuum furnace with a vacuum degree ranging from 0.133 Pa to 13.3 Pa. After the primary aging treatment is completed, the steel is first cooled in argon for a cooling time of ≥30 min, and then air-cooled.

[0013] Furthermore, the temperature of the primary aging treatment is 885°C.

[0014] Furthermore, the secondary aging treatment is carried out in a vacuum furnace with a vacuum degree ranging from 0.133 Pa to 13.3 Pa. After the secondary aging treatment is completed, it is first cooled in argon for a cooling time of ≥30 minutes, and then air-cooled.

[0015] Furthermore, the temperature of the secondary aging treatment is 760°C and the holding time is 240 min.

[0016] The beneficial effects of the present invention are: The present invention reduces the solution treatment temperature, controls the holding time and cooling method of the solution treatment, adds a secondary solution treatment process, and combines a two-step aging treatment to control the grain size of the GH350 alloy so that the alloy meets the requirements of grain size levels 2-6 while meeting the mechanical properties, thereby obtaining a bolt product that meets both the mechanical property and grain size requirements.

[0017] The present invention sets a heat preservation platform at 800℃, and the heat preservation platform and two solid solution treatments jointly control the grain size to make the product meet the grain size requirements. The 800℃ heat preservation platform is conducive to the overall equilibrium state of the GH350 alloy, reducing local grain coarsening. Combined with the subsequent two solid solution treatments and the use of argon cooling for ≥30min, the purpose is to quickly cool and prevent residual heat from coarsening the grains, thereby being able to control the crystal phase structure to meet the 2-6 grain size. The purpose of the two aging treatments is to precipitate a gradient strengthening phase through secondary aging. The strengthening phase hinders dislocation movement, thereby improving the room temperature strength and high-temperature durability of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a metallographic photograph of Example 1; Figure 2 This is the metallographic photograph of Example 2; Figure 3 This is the metallographic photograph of Example 3; Figure 4 This is a metallographic photograph of Comparative Example 1; Figure 5 This is the metallographic photograph of Comparative Example 2; Figure 6 This is a metallographic photograph of Comparative Example 3; Figure 7 This is a metallographic photograph of Comparative Example 4; Figure 8 This is a metallographic photograph of Comparative Example 5; Figure 9 This is a metallographic photograph of Comparative Example 6; Figure 10 This is a metallographic photograph of Comparative Example 7; Figure 11 This is a metallographic photograph of Comparative Example 8; Figure 12 This is a metallographic photograph of Comparative Example 9; Figure 13 This is a metallographic photograph of Comparative Example 10; Figure 14 This is a metallographic photograph of Comparative Example 11; Figure 15 This is the metallographic photograph of Comparative Example 12. DETAILED DESCRIPTION

[0019] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0020] The present invention aims to provide a heat treatment method for GH350 alloy fasteners for aviation applications, addressing the issue of unsatisfactory grain size in turbofan disk bolts made of GH350 alloy. By reducing the solution treatment temperature, adding a secondary solution treatment step, and combining it with a two-step aging treatment, the method achieves bolts that meet both mechanical property and grain size requirements.

[0021] The GH350 alloy of the present invention is composed of the following elements in mass fraction: Mn≤0.1%, W 1.8-2.3%, Si≤0.15%, Al 0.85-1.25%, P≤0.01%, C≤0.02%, O≤0.005%, Co 23.5-26.5%, N≤0.01%, Mo2.8-3.3%, Cr 14-17.5%, S≤0.01%, Ti 1.9-2.35%, Nb 0.9-1.3%, B≤0.03%, Cu≤0.02%, Zr≤0.1%, Ta 3.7-4.5%, Fe≤0.8%, and the balance is Ni.

[0022] Example 1 The heat treatment of the bolt after the head is hot-forging is carried out, which specifically includes the following steps: (1) Single solution treatment: The specific process is as follows: single solution treatment is carried out in a vacuum furnace with the vacuum degree set to 4.5 Pa. The bolt is heated to 1020 °C with the furnace and kept at 1020 °C for 120 min. During the heating process, a heat preservation platform is set and kept at 800 °C for 45 min. (2) After the insulation is completed, high-purity argon gas is used for cooling, and the cooling time is ≥30 minutes; (3) Secondary solution treatment: The specific process is as follows: Secondary solution treatment is carried out in a vacuum furnace with a vacuum degree set to 3.3 Pa. The bolts are heated to 950 °C with the furnace and kept warm for 240 min. (4) After the insulation is completed, high-purity argon gas is used for cooling, and the cooling time is ≥30 minutes; (5) Primary aging treatment: The specific process is as follows: the bolts after the secondary solid solution are aged in a vacuum furnace with the vacuum degree set to 0.47 Pa. The bolts are heated to 885 °C with the furnace for aging treatment, and the holding time is 60 min. (6) After the insulation is completed, high-purity argon gas is used for cooling, and the cooling time is ≥30 min. Then the bolt sample is taken out and naturally cooled to room temperature in the air; (7) Secondary aging treatment: The specific process is as follows: aging treatment is carried out in a vacuum furnace, the vacuum degree is set to 0.156 Pa, the bolts are heated to 760 °C with the furnace, and the holding time is 240 min; (8) After the insulation is completed, the sample is cooled with high-purity argon gas for ≥30 minutes. Then the sample is taken out and naturally cooled to room temperature in the air.

[0023] Example 2 The difference from Example 1 is that the primary solution temperature of step (1) of this embodiment is 1023°C, the holding time is 115 min, and the vacuum degree of the vacuum furnace is set to 4.1 Pa. The secondary solution temperature of step (3) is 945°C, the holding time is 230 min, and the vacuum degree of the vacuum furnace is set to 3 Pa. The primary aging temperature of step (5) is 880°C, the holding time is 50 min, and the vacuum degree of the vacuum furnace is set to 0.16 Pa. The secondary aging temperature of step (7) is 750°C, the holding time is 230 min, and the vacuum degree of the vacuum furnace is set to 0.33 Pa. Other conditions are the same as those in Example 1.

[0024] Example 3 The difference from Example 1 is that the primary solution temperature of step (1) of this embodiment is 1017°C, the holding time is 125 min, and the vacuum degree of the vacuum furnace is set to 3.9 Pa. The secondary solution temperature of step (3) is 955°C, the holding time is 250 min, and the vacuum degree of the vacuum furnace is set to 2.8 Pa. The primary aging temperature of step (5) is 890°C, the holding time is 70 min, and the vacuum degree of the vacuum furnace is set to 0.155 Pa. The secondary aging temperature of step (7) is 770°C, the holding time is 250 min, and the vacuum degree of the vacuum furnace is set to 0.88 Pa. Other conditions are the same as those in Example 1.

[0025] Comparative Example 1 After the head is hot-forged, the bolt undergoes conventional heat treatment, which specifically includes the following steps: (1) The solution treatment process is carried out in an air furnace, with the temperature rising to 1030°C and holding for 120 minutes. After the sample is held in the air, it is air-cooled. In the heat treatment of this comparative example, the bolts are only subjected to one solution treatment. (2) The primary aging treatment process is as follows: the sample is placed in an air furnace, heated to room temperature, and aged in a furnace at 885°C for 60 minutes. The sample is then taken out and naturally cooled to room temperature in the air.

[0026] (3) The secondary aging treatment process is as follows: the sample is placed in an air furnace, put into the furnace when it reaches the temperature, and aged in the furnace at 760°C for 240 minutes. After that, the sample is taken out and naturally cooled to room temperature in the air.

[0027] Comparative Example 2 The bolts after the head is hot-forged are heat-treated. The difference from Example 1 is that only the first solid solution treatment is performed in this comparative example, with a solid solution temperature of 1020°C and a holding time of 120 minutes. No secondary solid solution treatment is performed. Other conditions are the same as Example 1.

[0028] Comparative Example 3 The bolts after the head is hot-forging and formed are heat-treated. The difference from Example 1 is that the primary solution treatment temperature of this comparative example is 1030° C. and the holding time is 120 min. Other conditions are the same as Example 1.

[0029] Comparative Example 4 The bolts after the head is hot-forged are heat-treated. The difference from Example 1 is that the primary solution treatment temperature of this comparative example is 1010° C. and the holding time is 120 min. Other conditions are the same as Example 1.

[0030] Comparative Example 5 The bolts after the head is hot-forged are heat-treated. The difference from Example 1 is that the holding time of the first solution treatment in this comparative example is 60 minutes, and the holding time of the second solution treatment is 120 minutes. The rest is the same as Example 1.

[0031] Comparative Example 6 The bolts after the head is hot-forging are subjected to heat treatment. The difference from Example 1 is that the holding time of the secondary solution treatment in this comparative example is 120 minutes, and the rest is the same as Example 1.

[0032] Comparative Example 7 The bolts after the head is hot-forged are heat-treated. The difference from Example 1 is that the holding time of the first solution treatment in this comparative example is 180 minutes, and the holding time of the second solution treatment is 120 minutes. The rest is the same as Example 1.

[0033] Comparative Example 8 The bolts after the head is hot-forged are heat-treated. The difference from Example 1 is that the holding time of the first solution treatment in this comparative example is 240 minutes, and the holding time of the second solution treatment is 120 minutes. The rest is the same as Example 1.

[0034] Comparative Example 9 The bolts after the head is hot-forged are heat-treated. The difference from Example 1 is that the holding time of the first solid solution treatment in this comparative example is 60 minutes, and the rest is the same as Example 1.

[0035] Comparative Example 10 The bolts after the head is hot-forging are heat-treated. The difference from Example 1 is that the holding time of the first solid solution treatment in this comparative example is 180 minutes, and the rest is the same as Example 1.

[0036] Comparative Example 11 The bolts after the head is hot-forging and forming are subjected to heat treatment. The difference from Example 1 is that the first solution holding time of this comparative example is 240 minutes, and the rest is the same as Example 1.

[0037] Comparative Example 12 The bolts after the head is hot-forging are heat-treated. The difference from Example 1 is that no 800° C. insulation platform is provided during the first solution treatment in this comparative example. Other aspects are the same as Example 1.

[0038] The optical metallographic observation and mechanical property measurement of the products after treatment of the embodiment and the comparative example are shown in Table 1.

[0039] Table 1 Product performance data

[0040] The data in the table above show that the room temperature tensile strength of the bolt products of Examples 1-3 of the present invention ranges from 1271 MPa to 1299 MPa, exceeding that of Comparative Examples 1-12. The bolt products of Examples 1-3 can achieve a durability of 71 hours to 72 hours, significantly superior to Comparative Examples 1-12. This indicates that the heat treatment of the bolts according to the present invention significantly impacts product performance. Fine grains of grades 7 and 8 were observed in Comparative Examples 1-4, failing to meet product requirements. Large grains of grades 1 and 0 were observed in Comparative Examples 5-12 (the presence of grade 1 grains is indicated in parentheses following the grain size in Table 1), failing to meet product requirements. After heat treatment using the method of the present invention, the bolts of Examples 1-3 had a grain size of grades 2-6, with no grade 1 grains present. These bolts meet both mechanical performance and grain size requirements, meeting product requirements.

[0041] According to the Hall-Petch formula: s =σ o + Kd -1 / 2 Where: σ s represents the yield strength of the material, σ o represents the lattice friction force required for the generation of a single dislocation, K is a constant, and d represents the average grain size.

[0042] The Hall-Petch equation shows that the yield strength of a material at room temperature is inversely proportional to grain size: the finer the grains, the higher the material's strength. However, at high temperatures, grain boundary defects reduce alloy strength. Coarse grains at high temperatures have higher creep limit and rupture strength. The operating temperature of GH350 turbofan disk bolts is 560°C-580°C, a temperature range where grain boundary strengthening dominates. Coarse, level 1 grains lead to low grain boundary density, weakening the effect of hindering dislocation motion, resulting in reduced alloy strength. Furthermore, numerous low-angle grain boundaries (LAGBs) appear at the grain boundaries of large level 1 grains. These LAGBs accumulate at these boundaries. As the bolt deforms, continued deformation creates stress concentrations at the intersections of the large grains and surrounding smaller grains, leading to crack initiation and ultimately part failure. The present process, by controlling the holding temperature, holding time, and cooling method, and adding a secondary solution step, can control the grain size of the GH350 alloy, ensuring that it meets the requirements for grain sizes 2-6 while maintaining other mechanical properties.

[0043] Figure 1-Figure 3 It shows that the bolt grain size of Examples 1-3 of the present invention is 2-6 and meets the product requirements. Figure 4-Figure 15 Metallographic photographs of Comparative Examples 1-12 are presented. Figure 4 Comparative Example 1 is a conventional process, that is, during the heating process, no insulation platform is set at 800℃, and no secondary solution treatment is performed. Only one 1030℃ / 120min solution treatment is performed. The product grain size is level 7 or finer, indicating that the energy provided by the heat treatment of Comparative Example 1 is not enough to make the grains grow to level 2-6. Figure 5 Comparative Example 2 was subjected to a 1020℃ / 2h solution treatment, and the product grain size was 7 or finer. This shows that only a single solution heat treatment at 1020℃ does not provide enough energy to grow the grains to 2-6. Figure 4 Compared with the comparative example 1 heat treated at 1030℃, Figure 5 The grains of Comparative Example 2 are finer and denser, indicating that when only one heat treatment is performed, the higher the temperature, the more conducive it is to atomic diffusion and grain boundary movement, and the more conducive it is to further grain growth. Figure 6 Comparative Example 3 is a metallographic structure that has undergone two solution treatments at 1030℃ / 120min+950℃ / 240min, with a grain size of 1-8. This shows that after two solution treatments, the grain size has further grown, but the grain structure is uneven, and there are abnormally grown level 1 grains. Figure 7 Comparative Example 4 is a metallographic image obtained after two solid solution treatments at 1010℃ / 120min+950℃ / 240min. The grain size is 2-8, and there are small grains of grade 8, indicating that it is necessary to increase the temperature or extend the holding time to provide energy to promote the growth of small grains of grade 8. Figure 6It was found that when the first step heat treatment temperature was increased to 1030°C, abnormally large grains would appear. Therefore, controlling the first step heat treatment temperature between 1010°C and 1030°C is beneficial to controlling the uniformity of grain size.

[0044] See also Figures 8-14 , the temperature of the first solution treatment is fixed at 1020℃, the temperature of the second solution treatment is fixed at 950℃, and the holding time of the two-step heat treatment is explored. Figure 8 and Figure 12 It was found that the holding time of the first solution treatment in Comparative Examples 5 and 9 was 60 minutes, and there were still 8-level small grains. Therefore, the holding time of the first solution treatment should be no less than 60 minutes. Figures 8-11 As the holding time of the single solution treatment of comparative examples 5, 6, 7, and 8 was gradually extended from 60 min, 120 min, 180 min, and 240 min, the grain size of the products was 3-8 (1), 5-6 (1), 2-7, and 2-6 (1), and the grains grew gradually. Figure 13 and Figure 14 The holding time of the secondary solution treatment of comparative example 10-11 is fixed at 240 min, and the holding time of the primary solution treatment is 180 min and 240 min respectively. The grain size of the product is 4-7 (1) and 1-7.5, with abnormal grade 1 grains. Therefore, the primary solution time should not exceed 180 min. Figure 15 In the comparative example 12, no heating platform was set at 800°C during the heating process of the solid solution treatment. Other aspects were the same as those of the example 1. Figure 15 Table 1 shows that the mechanical properties of the sample are inferior to those of the example due to the presence of large Class 0 grains. This is because the sample lacks a heating platform at 800°C, leading to rapid growth of local grains during the heating process. Dislocations accumulate at the junction of large Class 0 grains and small grains, easily forming cracks. Ultimately, crack propagation leads to fracture, resulting in poor mechanical properties.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A heat treatment method for GH350 alloy fasteners for aviation, characterized in that: The following steps are involved: After upsetting, two-step solution treatment and two-step aging treatment are carried out in sequence; the temperature of the first solution treatment is 1017℃-1023℃, the holding time is 115min-125min, the temperature of the second solution treatment is 945℃-955℃, and the holding time is 230min-250min; the temperature of the first aging treatment is 880℃-890℃, the holding time is 50min-70min, and the temperature of the second aging treatment is 750℃-770℃, and the holding time is 230min-250min.

2. The heat treatment method of the aviation GH350 alloy fastener according to claim 1, characterized in that: During the heating process of the first solution treatment, the temperature is kept at 800°C for 40-50 minutes.

3. The heat treatment method of the aviation GH350 alloy fastener according to claim 1 or 2, characterized in that: The primary solution treatment is carried out in a vacuum furnace with a vacuum degree range of 0.133Pa-13.3Pa. After the primary solution treatment is completed, it is first cooled in argon for a cooling time of ≥30min and then air-cooled.

4. The heat treatment method of the aviation GH350 alloy fastener according to claim 3, characterized in that: The temperature of the first solution treatment is 1020°C.

5. The heat treatment method of the aviation GH350 alloy fastener according to claim 1 or 2, characterized in that: The secondary solution treatment is carried out in a vacuum furnace with a vacuum range of 0.133Pa-13.3Pa. After the secondary solution treatment is completed, it is first cooled in argon for a cooling time of ≥30min and then air-cooled.

6. The heat treatment method of the aviation GH350 alloy fastener according to claim 5, characterized in that: The temperature of the secondary solution treatment is 950°C.

7. The heat treatment method of the aviation GH350 alloy fastener according to claim 1 or 2, characterized in that: The primary aging treatment is carried out in a vacuum furnace with a vacuum degree range of 0.133Pa-13.3Pa. After the primary aging treatment is completed, it is first cooled in argon for a cooling time of ≥30min and then air-cooled.

8. The heat treatment method of the aviation GH350 alloy fastener according to claim 7, characterized in that: The temperature of the primary aging treatment is 885°C.

9. The heat treatment method of the aviation GH350 alloy fastener according to claim 1 or 2, characterized in that: The secondary aging treatment is carried out in a vacuum furnace with a vacuum range of 0.133Pa-13.3Pa. After the secondary aging treatment is completed, it is first cooled in argon for a cooling time of ≥30min and then air-cooled.

10. The heat treatment method of the aviation GH350 alloy fastener according to claim 9, characterized in that: The temperature of the secondary aging treatment is 760℃ and the holding time is 240min.

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