A method of heat treating a cast nickel-iron based material

Abstract

The application belongs to the technical field of heat treatment of casting materials, and mainly relates to a heat treatment method of a nickel-iron-based material casting, strictly controls the parameter value range of key nodes of each step of heat treatment, detects the high-temperature tensile property of the casting in the temperature range of 650-700 DEG C after the completion of the heat treatment process, the tensile strength is 550-620 MPa, the yield strength is 500-550 MPa, compared with the conventional heat treatment method, the tensile strength is increased by more than 10%, the yield strength is increased by more than 25%, and the high-temperature use property and the service life of the material can be greatly ensured.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for casting materials, and mainly relates to a heat treatment method for nickel-iron-based material castings. Background Technology

[0002] Improving the steam inlet parameters of steam turbines in coal-fired power plants can significantly improve power generation efficiency, reduce greenhouse gas emissions, and create greater economic benefits. The most critical factor affecting the steam inlet parameters of a steam turbine unit is the material of its key components. Currently, both domestic and international efforts are actively developing high-temperature materials to improve the steam parameters of the units. The most advanced and mature material currently used worldwide is CB2 material for 620℃. If the main steam temperature is further increased, the requirements for materials will become even more stringent. While existing ferritic heat-resistant steel materials for 630–650℃, such as Marbn and G115, have been developed, they have not yet been used in actual project manufacturing. The reasons are that the cost is too high, the improvement in unit efficiency is relatively small, and the benefits are minimal. Furthermore, their high-temperature operational stability is poor, posing safety hazards. Countries around the world are researching nickel-based and nickel-iron-based high-temperature alloy materials for 650–700℃ in order to increase the steam temperature of the units to 650–700℃. Preliminary calculations indicate that raising the steam temperature to 700℃ will increase the unit's power supply efficiency to 50%–55%, significantly reducing carbon dioxide emissions and coal consumption, resulting in substantial economic and environmental benefits. However, current iron-nickel-based high-temperature alloy materials are still in the research and development stage, especially in the manufacturing process where key control points have not yet been established. Conventional steam turbine materials, being ferritic heat-resistant steels, achieve suitable martensitic structures through normalizing and tempering heat treatment to improve their overall mechanical properties. However, nickel-iron-based materials do not undergo phase transformation and cannot be strengthened through phase transformation to improve their overall performance. Therefore, how to improve the high-temperature performance parameters of nickel-iron-based materials through reasonable and precise heat treatment control to meet the operating conditions of 650–700℃ units is of great significance. Summary of the Invention

[0003] The purpose of this invention is to provide a heat treatment method for nickel-iron-based material castings. The chemical composition of the nickel-iron-based material castings, by weight percentage, is as follows: C≤0.08%, Si≤0.3%, P≤0.028%, S≤0.01%, Cr: 15.5~16.5%, Mo≤0.6%, Cu≤0.2%, Al: 1.3~1.5%, W≤0.6%, Ti: 1.9~2.1%, B: 0.003~0.006%, Fe: 40~45%, Ni: 40~45%, with the remainder being trace impurity elements such as Zr, Pb, Bi, Se, Sn, Ta, N, O, and H. Because this material contains a wide variety and complex range of alloying elements, especially Cr and Ni, and also some high-melting-point alloys such as B, W, Zr, and Pb. Dendrite formation is inevitable during the slow cooling process after casting. Dendrites in nickel-iron-based materials are more difficult to eliminate than in conventional materials, severely impacting their mechanical properties. Furthermore, since the matrix of nickel-iron-based materials is predominantly austenitic, prolonged high-temperature heat treatment leads to severe grain coarsening, affecting the overall mechanical properties. Therefore, precise design is required in the heat treatment process to ensure optimal overall mechanical performance. Based on these considerations, this application addresses this issue by controlling the heat treatment process and parameters at each step to obtain qualified nickel-iron-based material castings.

[0004] A heat treatment method for nickel-iron-based material castings, characterized by comprising the following steps:

[0005] S01, the casting is heated to a first solution temperature at a first set rate and held at that temperature for a first set time; the first solution temperature ranges from 880°C to 920°C.

[0006] S02, the casting is heated to a second solution temperature at a second set rate and held at that temperature for a second set time; the second solution temperature ranges from 1150°C to 1180°C.

[0007] S03, the casting is cooled to the third solution temperature at a third set rate and held at that temperature for a third set time; the third solution temperature ranges from 880°C to 920°C.

[0008] S04, the casting is subjected to solution treatment twice in sequence according to the process of S01, S02 and S03. After completion, the casting is cooled to below 200℃.

[0009] S05, the casting is cooled to the aging temperature at a fourth set rate and held at that temperature for a fourth set time. After completion, a cooling treatment is performed to cool it to below 200°C. The aging temperature range is 700°C to 750°C.

[0010] S06, the casting is cooled to the stress relief temperature at the fifth set rate and held at the temperature for the fifth set time. After completion, a cooling treatment is performed to cool it to below 200°C. The stress relief temperature range is 350°C to 400°C.

[0011] To better implement the present invention, in S01, the first set rate is less than or equal to 60°C / h, and the first set time range is 1h to 2h.

[0012] To better implement the present invention, in step S02, the second set rate is less than or equal to 70°C / h, and the second set time range is 15h to 20h.

[0013] To better implement the present invention, in step S03, the third set rate is less than or equal to 50°C / h, and the third set time range is 2h to 5h.

[0014] To better realize the present invention, S04 further includes, after the casting undergoes two cycles of solution treatment, the temperature of the casting is raised to 1000°C to 1050°C, held for 5 hours to 10 hours, and then subjected to rapid cooling treatment.

[0015] To better implement the present invention, in step S05, the fourth set rate is less than or equal to 60°C / h, and the fourth set time range is 20h to 30h.

[0016] To better implement the present invention, in step S06, the fifth set rate is less than or equal to 50°C / h.

[0017] To better realize the present invention, in step S05, the cooling process adopts air cooling, and the air cooling rate is greater than or equal to 200℃ / h.

[0018] To better realize the present invention, in S06, the cooling rate is less than or equal to 30℃ / h.

[0019] The heat treatment method for the aforementioned nickel-iron-based castings mainly employs a stepped cyclic holding heat treatment approach. First, the casting temperature is raised to 880℃~920℃ at a rate ≤60℃ / h and held for 1h~2h. Then, the temperature is raised to 1150℃~1180℃ at a rate ≤70℃ / h and held for 15h~20h. Next, the temperature is lowered to 880℃~920℃ at a rate ≤50℃ / h and held for 2h~5h. This process is repeated three times for solution treatment to eliminate casting dendrite defects. The final temperature increase raises the casting temperature to 1000℃~1050℃ and holds for 5h~10h, followed by rapid water cooling. After high-temperature solution treatment, the casting temperature is raised to 700℃~750℃ at a rate ≤60℃ / h and held for 20h~30h for aging strengthening. Finally, strong air cooling is used to lower the temperature to room temperature, improving the high-temperature mechanical properties of the material.

[0020] This application provides a heat treatment method for nickel-iron-based material castings, particularly relating to a novel nickel-iron-based material. It offers a complete and effective heat treatment method, strictly controlling the parameter ranges of key nodes in each heat treatment step. After the heat treatment process is completed, the high-temperature tensile properties of the castings are tested within a temperature range of 650–700℃. The tensile strength is 550–620 MPa, and the yield strength is 500–550 MPa. Compared with conventional heat treatment methods, the tensile strength is increased by more than 10%, and the yield strength is increased by more than 25%, which can greatly ensure the high-temperature performance and service life of the material. Detailed Implementation

[0021] To facilitate understanding of the present invention, a more comprehensive description will be provided below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0022] This implementation takes a novel nickel-iron-based material as an example. The chemical composition (by weight percentage) of the casting material is as follows: C≤0.08%, Si≤0.3%, P≤0.028%, S≤0.01%, Cr: 15.5~16.5%, Mo≤0.6%, Cu≤0.2%, Al: 1.3~1.5%, W≤0.6%, Ti: 1.9~2.1%, B: 0.003~0.006%, Fe: 40~45%, Ni: 40~45%, with the remainder being trace impurity elements such as Zr, Pb, Bi, Se, Sn, Ta, N, O, and H. The specific heat treatment steps are as follows:

[0023] S01, the casting is heated to a first solution temperature at a first set rate and held at that temperature for a first set time; wherein the first set rate is less than or equal to 60℃ / h, the first solution temperature ranges from 880℃ to 920℃, and the first set time ranges from 1h to 2h.

[0024] S02, the casting is heated to the second solution temperature at a second set rate and held at that temperature for a second set time; wherein the second set rate is less than or equal to 70℃ / h, the second solution temperature ranges from 1150℃ to 1180℃, and the second set time ranges from 15h to 20h.

[0025] S03, the casting is cooled to the third solution temperature at a third set rate and held at that temperature for a third set time; wherein the third set rate is less than or equal to 50℃ / h, the third solution temperature ranges from 880℃ to 920℃, and the third set time ranges from 2h to 5h.

[0026] S04, the casting is subjected to solution treatment twice in sequence according to the process of S01, S02 and S03; then the temperature of the casting is raised to 1000℃ to 1050℃, held for 5h to 10h, and then rapidly cooled down to below 200℃ by water cooling.

[0027] S05, the casting is cooled to the aging temperature at a fourth set rate and held at that temperature for a fourth set time. After completion, it is cooled by air cooling to below 200℃. The fourth set rate is less than or equal to 60℃ / h, the aging temperature range is 700℃ to 750℃, and the fourth set time range is 20h to 30h. The air cooling rate is greater than or equal to 200℃ / h.

[0028] S06. The casting is cooled to the stress-relieving temperature at the fifth set rate and held at that temperature for the fifth set time. After completion, it is cooled by furnace cooling to below 200℃. The fifth set rate is less than or equal to 50℃ / h, the stress-relieving temperature range is 350℃ to 400℃, and the fifth set time is calculated using the formula: maximum wall thickness of the casting * 25 min / mm. The furnace cooling rate is less than or equal to 30℃ / h.

[0029] It should be noted that S01 to S04 involve solution treatment. The process parameters were designed as follows: due to the high content of alloying elements in this material, severe segregation occurs during solidification and cooling after casting, resulting in significant as-cast dendrites, which reduces the material's strength and high-temperature performance. If segregation and dendrite effects are not eliminated, the material will fracture at the dendrite sites during long-term high-temperature use, affecting its service life and equipment safety. Furthermore, compositional segregation leads to uneven distribution of γ' atoms in the microstructure, resulting in significant differences in high-temperature performance across different parts, also impacting the overall service life of the equipment. Therefore, the primary goal of heat treatment is to eliminate as-cast dendrites, achieving compositional and microstructure homogenization and improving high-temperature performance. Considering the material's excellent high-temperature resistance and high content of high-melting-point metals such as W, B, Ti, Mo, and Al, eliminating as-cast dendrites and achieving homogenization is very challenging. Research into the characteristics of this type of material suggests that achieving these goals requires higher holding temperatures, while the holding time needs to be selected based on the temperature. Generally, higher temperatures require shorter holding times to achieve homogenization. Another issue is that existing heat treatment furnaces generally do not operate at temperatures exceeding 1200℃, and their operating time is also limited. Otherwise, the steel structure of the furnace can easily soften, affecting the equipment's lifespan and safety. To eliminate cast dendrites and achieve compositional homogenization, while also ensuring equipment safety, a method is adopted where the temperature is first increased to 880℃~920℃ at a rate of ≤60℃ / h and held for 1h~2h. Within this temperature range, homogenization ensures that all precipitated γ' phases dissolve and re-transform into austenite, thereby reducing structural stress caused by differences in microstructure and preventing cracking. Then, the temperature is increased to 1150℃~1180℃ at a rate of ≤70℃ / h and held for 15h~20h, followed by a decrease to 880℃~920℃ at a rate of ≤50℃ / h and held for 2h. Repeat the above cycle three times, ensuring a total holding time of 50-60 hours between 1150℃ and 1180℃ to eliminate casting dendrite defects and achieve compositional homogenization. The holding time should not be too long, otherwise the grains will become coarser, reducing the material's ductility and toughness. The final heating to 1000℃-1050℃ and holding for 5-10 hours will prevent excessive coarsening of the γ phase, achieving appropriate grain refinement. Afterward, rapid water cooling ensures the temperature remains above the brittle phase precipitation temperature while minimizing quenching thermal stress and cracking, significantly improving the material's strength and ductility.

[0030] S05 is an aging treatment. The process parameters are designed as follows: Since the matrix structure of this material is austenitic, it is a precipitation-strengthened material. During high-temperature long-term heat treatment, the grains will inevitably coarsen to some extent, affecting the mechanical properties of the material. Therefore, aging treatment is required to precipitate smaller, uniformly distributed γ' phases, thereby improving the high-temperature strength of the material. Nickel-iron based materials precipitate a large amount of Sigma phase in the 400℃~700℃ range, leading to brittleness. Since this material is used in an environment of 650℃~700℃, the aging strengthening temperature range of 700℃~750℃ is selected to stabilize the size and quantity of the γ' phase, ensuring its relatively stable existence below 700℃. The temperature is first increased to 700℃~750℃ at a rate of ≤60℃ / h. Too short an aging holding time will affect the amount of precipitated phase, while too long an aging time will cause the precipitated phase to grow excessively. The holding time is selected as 20h~30h. Cooling is achieved through strong airflow, ensuring a cooling rate of ≥200℃ / h, and quickly passing through the danger zone.

[0031] After aging as described above, air cooling will inevitably be accompanied by some thermal stress. Therefore, low-temperature stress relief should be carried out after aging is completed.

[0032] S06 is a low-temperature stress-relieving treatment. The process parameters are designed such that the stress-relieving temperature range is 350℃~400℃, avoiding the brittle range. First, the temperature is increased to 350℃~400℃ at a rate of ≤40℃ / h, and the holding time is calculated according to the formula of maximum wall thickness * 25min / mm. Then, the temperature is reduced at a rate of ≤30℃ / h to reduce thermal stress.

[0033] The above-mentioned heat treatment method effectively improves the high-temperature performance of this novel nickel-iron-based material, providing a material for turbine units operating at temperatures between 650℃ and 700℃, meeting usage requirements and improving power generation efficiency.

[0034] In the description of this invention, it should be understood that the terms "first", "second", "third", "fourth" and "fifth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number or order of the indicated technical features.

[0035] The embodiments described above merely illustrate specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method of heat treatment of a cast of a nickel-iron based material, characterized in that, The method comprises the following steps: S01, the casting is heated to a first solution temperature at a first setting rate of less than or equal to 60 ℃ / h, and is kept for a first setting time of 1 h to 2 h; the first solution temperature ranges from 880 ℃ to 920 ℃; S02, the casting is heated to a second solution temperature at a second setting rate of less than or equal to 70 ℃ / h, and is kept for a second setting time of 15 h to 20 h; the second solution temperature ranges from 1150 ℃ to 1180 ℃; S03, the casting is cooled to a third solution temperature at a third setting rate of less than or equal to 50 ℃ / h, and is kept for a third setting time of 2 h to 5 h; the third solution temperature ranges from 880 ℃ to 920 ℃; S04, the casting is subjected to solution treatment by recycling twice in sequence according to the processes of S01, S02 and S03, and is cooled after completion; S05, the casting is cooled to an aging temperature at a fourth setting rate, and is kept for a fourth setting time, and is cooled after completion; the aging temperature ranges from 700 ℃ to 750 ℃; S06, the casting is cooled to a stress relief temperature at a fifth setting rate, and is kept for a fifth setting time, and is cooled after completion; the stress relief temperature ranges from 350 ℃ to 400 ℃.

2. The heat treatment method of a ferronickel base material casting according to claim 1, characterized by, In the S04, after the casting is subjected to two-cycle solution treatment, the temperature of the casting is raised to 1000 ℃ to 1050 ℃, and is kept for 5 h to 10 h, and then rapid cooling treatment is performed.

3. The heat treatment method of the ferronickel base material casting according to claim 1, characterized by, In the S05, the fourth setting rate is less than or equal to 60 ℃ / h, and the fourth setting time ranges from 20 h to 30 h.

4. The heat treatment method of a ferronickel base material casting according to claim 1, characterized by, In the S06, the fifth setting rate is less than or equal to 50 ℃ / h.

5. The heat treatment method of a ferronickel base material casting according to claim 1, characterized by, In the S05, the cooling treatment adopts air cooling, and the air cooling rate is greater than or equal to 200 ℃ / h.

6. The heat treatment method of a ferronickel base material casting according to claim 1, characterized by, In the S06, the cooling rate is less than or equal to 30 ℃ / h.

7. The heat treatment method of a cast piece of a nickel-iron-based material according to any one of claims 1 to 6, characterized in that, The chemical composition of the casting contains, by weight, C≤0.08%, Si≤0.3%, P≤0.028%, S≤0.01%, Cr: 15.5-16.5%, Mo≤0.6%, Cu≤0.2%, Al: 1.3-1.5%, W≤0.6%, Ti: 1.9-2.1%, B: 0.003-0.006%, Fe: 40-45%, Ni: 40-45%, and the rest is trace impurity elements.