A cutting method to prevent cracking of nickel-iron-based castings

Abstract

This invention belongs to the field of casting cutting technology, and mainly relates to a cutting method for preventing cracking of nickel-based superalloy castings. It changes the cutting process and process control parameters in existing technologies, providing a complete and effective cutting method. It strictly controls the parameter values ​​of key nodes in each cutting step, avoiding the casting from passing through the brittle phase precipitation temperature range, improving the casting's plasticity and toughness, reducing stress, and preventing crack formation. The casting undergoes annealing after molding to eliminate casting stress, providing a better cutting condition for subsequent riser cutting. The cutting method provided by this application effectively prevents casting cracking during cutting, making casting manufacturing quality controllable and reducing rework costs. After annealing, the elongation is significantly increased by about 500%, and the plasticity is significantly improved, effectively reducing the risk of cracking.

Description

Technical Field

[0001] This invention belongs to the field of casting cutting technology, and mainly relates to a cutting method for preventing cracking of nickel-based high-temperature alloy castings. Background Technology

[0002] Currently, both domestic and international efforts are actively developing high-temperature materials to improve the steam parameters of power units. Increasing the intake 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 intake parameters of steam turbine units is the material of the key components themselves. Currently, the most advanced and mature material 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 projects for 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. At present, various countries are researching nickel-based / nickel-iron-based high-temperature alloy materials for 650–700℃ in order to increase the steam temperature of power 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, and key control points, especially in the manufacturing process, have not yet been established. The production cost of these materials is approximately 100,000–150,000 RMB per ton, and serious quality problems will lead to significant losses and quality risks. In particular, preventing cracking during the cutting process of nickel-iron-based material castings urgently needs to be addressed. Summary of the Invention

[0003] The purpose of this invention is to provide a cutting method to prevent cracking of nickel-iron-based material castings. The chemical composition (weight percentage) of the nickel-iron-based material casting 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. Due to the large and complex variety of alloying elements contained in this material, especially Cr and Ni, a large number of harmful elements will precipitate during the slow cooling process after casting, mainly in the form of Sigma phase and M. 23The main phases are C6, η, and γ', with Sigma phase accounting for the largest proportion. According to theoretical calculations using relevant simulation software, the maximum precipitation of Sigma phase reaches 30%. In actual production, due to the inconsistent wall thickness of different parts of the casting, the solidification and cooling process is slower in thicker parts, resulting in more Sigma phase precipitation. Sigma phase precipitation severely reduces the plasticity and toughness of the material, and the material has a larger volume shrinkage rate than other materials. This results in significant casting stress during solidification shrinkage, making it highly susceptible to cracking under thermal stress during cutting, affecting the quality of the casting and increasing rework costs. Based on the above, this application addresses the problem of cutting cracking in existing technologies by controlling the cutting process and process parameters at each step.

[0004] A cutting method for preventing cracking of nickel-iron-based material castings includes the following steps:

[0005] Boxing: After the casting is poured, the temperature is measured to be within the range of 900℃ to 1100℃ and then boxed. This temperature range is higher than the precipitation temperature range of brittle phases in the casting, and a certain safety margin is left. If the temperature drops too much during the process from boxing to entering the furnace, more brittle phases will precipitate.

[0006] Annealing: The castings after being packed are placed in a heat treatment furnace for annealing to improve the plasticity and toughness of the casting material, while eliminating casting stress and providing a better cutting condition for riser cutting.

[0007] Cutting: The riser is cut when the casting temperature drops below 400℃. During the cutting process, the casting temperature is less than 400℃ to prevent the cutting area from staying in the brittle temperature range for too long, which would cause the casting to become brittle and crack.

[0008] To better realize the present invention, the heat treatment furnace is first preheated to 700°C to 750°C during the annealing process. Then the casting is placed in the heat treatment furnace for annealing treatment. This ensures that the casting enters the furnace at high temperature and the temperature rises rapidly. It prevents the casting temperature from dropping due to the low furnace temperature, which would lead to the precipitation of brittle phases. At the same time, it can also reduce the thermal stress during the heating process and reduce the risk of casting cracking.

[0009] To better realize the present invention, the temperature of the casting when it is loaded into the heat treatment furnace during the annealing step is greater than 700°C, so as to meet the temperature range above which the brittle phase of the casting precipitates, and to prevent the precipitation of the brittle phase.

[0010] To better realize the present invention, after the recast part is placed in the heat treatment furnace during the annealing step, it is first held at a temperature range of 700°C to 750°C for 3 to 5 hours to make the overall temperature of the casting uniform and reduce the thermal stress caused by temperature difference; then a slow heating control method is adopted, with the heating rate of the heat treatment furnace less than or equal to 35°C / h; after the casting is heated to between 1100°C and 1200°C, it is held for 10 to 20 hours to greatly improve the plasticity and toughness of the casting and improve the optimal cutting state of the casting.

[0011] To better realize the present invention, after holding the casting at a temperature for 10 to 20 hours during the annealing process, rapid cooling measures are taken to quickly cool the casting temperature to below 400°C before proceeding with the cutting process. This allows the casting to quickly pass through the brittle zone after annealing, ensuring that the casting temperature is below 400°C before the riser is cut.

[0012] To better realize the present invention, after the casting is boxed, a sand removal process is performed, and the sand removal time is less than 1 hour to avoid the casting temperature decaying too much and entering the brittle range.

[0013] To better realize the present invention, the casting temperature is kept below 400°C during the cutting process, and the cutting allowance is 30mm to 50mm. The excess allowance is subsequently removed by using a carbon rod with low heat input and grinding.

[0014] This application provides a cutting method to prevent cracking in nickel-iron-based material castings, specifically relating to a novel nickel-iron-based material. It modifies the cutting process and control parameters in existing technologies, providing a complete and effective cutting method. This method strictly controls the parameter ranges at key nodes in each cutting step, avoiding the casting from passing through the brittle phase precipitation temperature range, improving the casting's ductility and toughness, reducing stress, and preventing crack formation. After the casting is packaged, it undergoes annealing to eliminate casting stress, providing a better cutting condition for subsequent riser cutting. The cutting method provided in this application effectively prevents casting cracking during cutting, ensuring controllable casting manufacturing quality and reducing rework costs. After annealing, the elongation is significantly increased by approximately 500%, and the plasticity is significantly improved, effectively reducing the risk of cracking. Detailed Implementation

[0015] 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.

[0016] 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 cutting steps are as follows:

[0017] High-temperature annealing: The casting is pretreated by high-temperature annealing and stress relief in the furnace. After the casting is poured, the temperature is measured in the range of 900 to 1100℃ and the casting is annealed. In order to avoid the casting temperature from falling too much and entering the brittle range, coarse sand removal is required within 1 hour to clean up the thick sand blocks. After that, the casting is loaded into the furnace for annealing.

[0018] It should be noted that when designing the mold-making process parameters, the solidification and cooling curve of the material was simulated, and combined with actual production tests. Slow cooling of the material in the 400–700℃ range resulted in a sharp drop in plasticity. Therefore, based on the experimental and simulation results, the material became brittle in this range, mainly due to the precipitation of a large amount of Sigma phase. Simultaneously, due to the material's larger volume shrinkage rate than conventional materials during solidification and cooling, significant casting stress was generated. Therefore, the precipitated brittle phases combined with the high casting stress greatly increased the risk of cracking after mold-making. To avoid this risk, rapid cooling through this range is necessary to reduce the content of brittle phase precipitation. However, rapid cooling is difficult to implement. Firstly, the casting is difficult to cool rapidly in the sand mold; secondly, if the casting is removed from the sand mold at high temperature, a large amount of sand remains on the surface, making high-temperature cleaning difficult. This makes it difficult to ensure rapid and uniform cooling of all parts, and instead adds thermal stress, further increasing the risk. Therefore, a high-temperature mold-making and furnace annealing stress-relief method is used to pre-treat the casting. This means the casting needs to be annealed in the furnace at a temperature above 700℃ to relieve stress. Specifically, Magma simulation software is used to simulate the solidification and cooling rate of the casting. A high-temperature annealing process is performed within the temperature range of 900-1100℃, leaving a certain safety margin above the precipitation temperature range of brittle phases to prevent excessive temperature drop during the annealing and furnace loading process, which would result in the precipitation of too many brittle phases. After annealing, the surface of the casting will have some sand adhering to it. Based on actual high-temperature annealing experience, to avoid excessive temperature decay and entry into the brittle zone, coarse sand removal is required within 1 hour to remove thick and large sand blocks. Then, the casting is loaded into the furnace for annealing. Even if the temperature is slightly lower during this process, the short residence time in the brittle phase zone prevents the precipitation of a large amount of brittle phases, making the casting relatively safe and the risk controllable.

[0019] Annealing to relieve stress: First, preheat the heat treatment furnace to 700-750℃, then place the casting in the heat treatment furnace and hold it at 700-750℃ for 3-5 hours, using slow temperature control, with the heating rate of the heat treatment furnace less than or equal to 35℃ / h; after the casting is heated to between 1100-1200℃, hold it for 10-20 hours.

[0020] Specifically, after the casting is kept at a constant temperature for 10 to 20 hours, rapid cooling measures are taken to quickly cool the casting temperature to below 400°C before proceeding with the cutting process. This allows the casting to quickly pass through the brittle zone after annealing, ensuring that the casting temperature is below 400°C before the riser is cut.

[0021] It should be noted that this type of nickel-iron-based material contains a large and complex variety of alloying elements, resulting in severe compositional segregation and significant dendrite growth during solidification and cooling. This significantly reduces the material's plasticity and toughness, leading to brittleness and hardness, which is detrimental to cutting. Furthermore, this material has a relatively large volumetric shrinkage rate compared to conventional materials, exhibiting a strong shrinkage tendency. The casting stress generated during solidification and cooling is much greater than that of conventional castings. If cut directly without annealing, the as-cast material will have an uneven composition, high carbide content, and will be brittle and hard, with an elongation of only about 8%. Therefore, annealing is necessary to improve the material's plasticity and toughness while eliminating casting stress, providing a better cutting condition for riser cutting.

[0022] After annealing and loading the castings into the furnace, they are held at 700–750℃ for 3–5 hours to ensure uniform temperature throughout the casting and reduce thermal stress caused by temperature differences. A slow, controlled heating method is used, with the heating rate ≤35℃ / h. Due to the variety of alloying elements in the material and the high melting points of the resulting compounds, coupled with significant dendrite growth in the as-cast state, a lower annealing temperature is detrimental to the homogenization of composition and microstructure. Therefore, a holding temperature of 1050–1200℃ is chosen to reduce the adverse effects of dendrites and better achieve compositional homogenization. Nickel-iron based materials are primarily strengthened by the γ' phase precipitated during high-temperature heat treatment. However, with increasing high-temperature treatment time, the γ' phase continues to grow, along with other precipitated phases such as M23C6 and G, which can affect the material's ductility and toughness, leading to brittleness. Therefore, the holding time is determined based on actual production test results, and is set at 10–20 hours.

[0023] Riser cutting: Since the casting material becomes brittle in the range of 400-700℃ and precipitates a large amount of Sigma phase, after the casting is annealed and taken out of the furnace, rapid cooling measures are taken to quickly cool the casting temperature to below 400℃ before riser cutting. In order to reduce the residence time in the brittle range, the cutting is carried out at room temperature for short time and is controlled by water mist cooling. The temperature of the casting is kept below 400℃ throughout the cutting process. The cutting allowance is 30-50mm. The excess allowance is subsequently removed by using a carbon rod with low heat input and grinding.

[0024] It should be noted that iron-nickel-based materials become brittle in the 400–700℃ temperature range, mainly due to the large precipitation of the Sigma phase. Cutting temperature can be controlled either above 700℃ or below 400℃. Cutting above 700℃ is difficult in practice and the process is hard to control. Therefore, the second temperature control method, cutting below 400℃, is the best choice. However, the casting temperature will inevitably pass through the 400–700℃ brittle range during cutting. Therefore, it is necessary to control the material temperature during cutting and reduce the residence time in the brittle range. Cutting is carried out at room temperature for short periods, supplemented by water mist cooling. That is, after cutting the riser area at room temperature for 3–5 minutes, a water gun is used for rapid cooling to prevent the cut area from remaining in the 400–700℃ brittle range for too long, causing brittleness and cracking, which would affect the casting quality. To further prevent cutting cracks from initiating on the casting body, the cutting allowance is controlled at 30-50mm. Any excess allowance is subsequently removed using a carbon rod with low heat input and grinding.

[0025] The above control method effectively prevents castings from cracking during cutting, ensuring controllable casting manufacturing quality and reducing rework costs. Appendix Table 1 compares the data between the annealed and as-cast states. After annealing, the elongation rate increases significantly by about 500%, and the plasticity is significantly improved, effectively reducing the risk of cracking. However, because the material is held at excessively high temperatures, grain growth is more pronounced, which has a certain impact on strength, but does not reduce the material's ductility and toughness.

[0026] Table 1 Comparison of performance data under different heat treatment conditions

[0027]

[0028] The cutting method using the above-mentioned novel nickel-iron-based material can effectively solve the problem of casting cracking during cutting, avoid major quality problems, improve casting quality, and reduce production costs.

[0029] 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 cutting method for preventing cracking of nickel-iron-based material castings, characterized in that, The chemical composition of the casting, 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; the cutting method of the casting includes the following steps: Boxing: After the casting is poured, the box is opened when the temperature is measured to be within the range of 900℃ to 1100℃; Annealing: Preheat the heat treatment furnace to 700℃ to 750℃, then place the casting in the heat treatment furnace and hold it at 700℃ to 750℃ for 3 to 5 hours. Using a slow temperature control method, raise the casting to between 1100℃ and 1200℃ and hold it for 10 to 20 hours; then perform the annealing treatment. Cutting: The riser is cut when the casting temperature drops below 400℃. During the cutting process, the casting temperature is less than 400℃.

2. The cutting method for preventing cracking of nickel-iron-based material castings according to claim 1, characterized in that, During the annealing step, the heating rate of the heat treatment furnace is less than or equal to 35°C / h.

3. The cutting method for preventing cracking of nickel-iron-based material castings according to claim 1, characterized in that, During the annealing process, after holding the casting at a temperature for 10 to 20 hours, rapid cooling measures are taken to quickly cool the casting temperature to below 400°C before proceeding with the cutting process.

4. The cutting method for preventing cracking of nickel-iron-based material castings according to claim 1, characterized in that, After the castings are packed into boxes, they undergo a sand removal process, which takes less than 1 hour.

5. The cutting method for preventing cracking of nickel-iron-based material castings according to claim 1, characterized in that, The cutting step includes leaving a cutting allowance of 30mm to 50mm.

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