Method for grain refinement and homogenization of 9-12%Cr rotor steel

Abstract

The present invention relates to a method for grain refinement and homogenization of 9-12% Cr rotor steel, and belongs to the technical field of heat-resistant steel materials. The method for grain refinement and homogenization of 9-12% Cr rotor steel comprises: after forging of 9-12% Cr rotor steel, a forging is obtained, the diameter of the forging is more than 700 mm, the forging is subjected to material waiting at 600-750°C for more than 100 mm / 1h, after the material waiting is completed, the temperature is reduced to 160-350°C, the temperature is kept for more than 100 mm / 2h, the temperature is then increased to 1000-1110°C, the temperature is kept for more than 100 mm / 1.5h, the temperature is then reduced to 680-750°C, the temperature is kept to make the forging undergo a balanced phase change, and then the temperature is furnace cooled to below 250°C on the surface of the forging. The present invention can achieve the refinement and homogenization of the grain size of 9-12% Cr rotor steel with large diameter.

Description

Technical Field

[0001] The invention relates to a method for refining and homogenizing crystal grains of 9-12% Cr rotor steel, belonging to the technical field of heat-resistant steel materials. Background Art

[0002] With the introduction of concepts such as "carbon peak" and "carbon neutrality", ultra-supercritical coal-fired technology has become the mainstream of the development of the coal-fired power industry due to its high power generation efficiency. The rotor of the ultra-supercritical unit is a key component of the ultra-supercritical unit. Therefore, the research and development and manufacturing of 9-12% Cr rotor steel materials have become particularly important.

[0003] 9-12% Cr material is martensitic heat-resistant steel, which has strong organizational inheritance, and the size of the ultra-supercritical unit rotor is large. During the forging process, it is easy to cause coarse grains and mixed crystals inside the workpiece, which will greatly affect the performance of the workpiece, resulting in unqualified mechanical properties. During the flaw detection, coarse grains or mixed crystals will also cause noise in the flaw detection results, resulting in unqualified flaw detection. All of these will cause great difficulties in the production and manufacturing of ultra-supercritical unit rotors, so it is very important to achieve the refinement and homogenization of the material grain size.

[0004] The commonly used methods for grain refinement of large castings and forgings are multiple normalizing, critical zone rapid heating, and isothermal transformation of supercooled austenite. The method of rapid heating in the critical zone to refine grains requires that during heat treatment, the temperature is raised to the austenitizing temperature at a heating rate of not less than 100°C / min, and the residence time at the lower austenitizing temperature is reduced to obtain spherical austenite, thereby refining the grains. However, it is extremely difficult to obtain such a fast heating rate during the heat treatment of large castings and forgings; the method of isothermal transformation of supercooled austenite to refine grains requires that the workpiece undergo a balanced decomposition process after forging, such as obtaining pearlite structure. When heated again, only spherical austenite is formed in the pearlite, and the original orientation is not retained, thereby obtaining refined grains. However, the 9-12% Cr material is a martensitic heat-resistant steel with a high alloy content. After the pearlite transformation is completed, a large amount of carbides are precipitated and unevenly distributed. When heated again, the grains grow abnormally and are unevenly distributed, resulting in mixed crystal problems. The method of multiple normalizing grain refinement mainly utilizes spontaneous recrystallization of austenite. However, since the heating temperature of spontaneous recrystallization of austenite in martensitic steel is high and for martensitic steel, it is difficult to achieve sufficient homogenization due to the strong inheritance of the structure, the method of multiple normalizing grain refinement also has defects.

[0005] Therefore, this patent solves the problems of mixed crystals, coarse grains and tissue inheritance that occur in the production and manufacturing process of ultra-supercritical unit rotors by means of fine-tuning post-forging heat treatment and changing performance heat treatment, while refining the grains.

[0006] Song Chuanbao, Jin Jiayu, Liu Zhiying, et al. Study on heat treatment process for grain refinement and homogenization of 300MW steam turbine low pressure rotor [J]. Heavy Castings and Forgings, 1998, (4): 34-37. The grain inheritance phenomenon of 30Cr2NI4MoV steel was discussed, and the heat treatment methods that may eliminate or reduce grain inheritance such as multiple normalizing, pretreatment and preliminary treatment in the critical zone were simulated and analyzed. According to the simulation test results, the production test of grain refinement heat treatment was carried out on the 300MW steam turbine low pressure rotor, and a reasonable and effective heat treatment process was determined. However, the process showed coarse grain and mixed grain phenomenon.

[0007] Wang Xiaofang. Research on grain refinement heat treatment process of steel for 620℃ turbine rotor forgings[J]. Heavy Castings and Forgings, 2016(2):4. Combined with the material characteristics of 9%-10%Cr ferritic steel, the grain refinement heat treatment process of COST-FB2 steel for 620℃ turbine rotor forgings was experimentally studied and analyzed. The research results show that the COST-FB2 steel sample with the original structure of martensite and grain size greater than 00 can obtain ferrite plus carbide diffusion transformation structure after long-term isothermal transformation treatment at 660-760℃. The diffusion transformation structure sample can obtain 6-1 grain size after austenitization at 1080-1100℃ and quenching, and the coarse original structure inheritance is cut off. This can provide a basis for the formulation of heat treatment process for rotor forging products. The results in this document were obtained using laboratory-grade specimens. The initial microstructure of the specimens was quite different from that of actual large forgings. The austenitizing holding time was 12 hours, which was shorter than that of actual products. In addition, the grain size was only level 1 when the austenitizing temperature was 1100°C in the document.

[0008] Xiang Chong, Liang Gang, Wang Kun, et al. Research on heat treatment process of 12% Cr material for welding rotor [J]. Oriental Steam Turbine, 2022 (000-002). The article takes 12% Cr material for welding rotor as the test object. By comparing different secondary tempering temperatures and different quenching temperatures, the change law of the mechanical properties of the material under different heat treatment systems is obtained, which provides data support for the post-weld heat treatment system of welding rotor. The structure after secondary tempering is tempered bainite structure. With the increase of secondary tempering (580-700℃), the grain size does not change significantly, all of which are 4-6 levels. The process parameters of quenching in the process described in the literature are used for experiments on small samples, and the process parameters are not suitable for the production and manufacturing of large-size workpieces. Summary of the invention

[0009] The purpose of the present invention is to provide a new method for refining and homogenizing the grains of 9-12% Cr rotor steel.

[0010] To achieve the purpose of the present invention, the method for grain refinement and homogenization of 9-12% Cr rotor steel comprises:

[0011] After forging of the 9-12% Cr rotor steel, a forging is obtained. The diameter of the 9-12% Cr rotor steel is greater than 700 mm. The forging is subjected to material waiting at 600-750° C. for more than 100 mm / 1 hour. After the material waiting is completed, the temperature is lowered to 160-350° C., and the temperature is kept for more than 100 mm / 2 hours. The temperature is then raised to 1000-1110° C., and the temperature is kept for more than 100 mm / 1.5 hours. The temperature is then lowered to 680-750° C., and the temperature is kept to make the forging undergo a balanced pearlite phase transformation. The furnace is then Cool until the surface of the forging is below 250℃, then heat it to 480-600℃ and keep it warm for more than 100mm / 0.2h, then heat it to 760-810℃ and keep it warm for more than 100mm / 0.5h, then heat it to 800-840℃ and keep it warm for more than 100mm / 0.5h, then heat it to 950-1060℃ and keep it warm for more than 100mm / 0.5h, then heat it to 1040-1110℃ and keep it warm for more than 100mm / 1h, and then cool it with quenching medium.

[0012] The material waiting time of 100mm / 1h is calculated based on the maximum diameter size, and the same applies to the following.

[0013] 160~350℃, keep warm for more than 100mm / 2h, and ensure that the core temperature drops to the temperature range of 160~350℃.

[0014] 1000~1110℃, keep warm for more than 100mm / 1.5h, to ensure the complete austenitization transformation of the rotor. After the quenching medium is cooled, tempering can be carried out. Tempering is an existing conventional tempering process. The tempering temperature can be appropriately adjusted according to the strength requirements of the forgings.

[0015] In a specific embodiment, the cooling rate to 160-350° C. is below 30° C. / h.

[0016] In a specific embodiment, the heating rate of heating to 1000-1110° C. is less than 60° C. / h.

[0017] In a specific embodiment, the cooling rate to 680-750° C. is below 30° C. / h.

[0018] In a specific embodiment, the insulation time at 680-750° C. is more than 240 hours.

[0019] In a specific embodiment, the furnace is cooled to 250° C. at a cooling rate of less than 20° C. / h.

[0020] In a specific embodiment, the heating rate of heating to 480-600° C. is less than 30° C. / h.

[0021] In a specific embodiment, the heating rate of heating to 760-810° C. is less than 40° C. / h.

[0022] In a specific embodiment, the heating rate of heating to 800°C to 840°C is 50°C / h or less.

[0023] In a specific embodiment, the heating rate of heating to 950-1060° C. is less than 60° C. / h.

[0024] In a specific embodiment, the heating rate of heating to 1040-1110° C. is less than 60° C. / h.

[0025] In a specific implementation, the diameter of the 9-12% Cr rotor steel is 700 mm to 1300 mm.

[0026] Beneficial effects:

[0027] 9-12% Cr material is martensitic heat-resistant steel with high alloy content, large amount of carbides precipitated and unevenly distributed, resulting in uneven grain growth and mixed crystal problems. The process of the present invention can cut off the inheritance of organization, and during the austenitization process, the nucleation rate is high, the alloy elements diffuse evenly, the austenite nucleation and growth are synchronized, the mixed crystal is reduced, and the grains are refined.

[0028] The method of the present invention is applicable to large-sized workpieces with 9-12% Cr, particularly to large-sized workpieces with a diameter of more than 700 mm, and especially to large-sized workpieces with a diameter of 700 mm-1300 mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a heat treatment process for existing workpieces.

[0030] Figure 2 This is a heat treatment process of the present invention.

[0031] Figure 3 This is a metallographic photograph of Example 1 of the present invention.

[0032] Figure 4 This is a metallographic photograph of Example 2 of the present invention.

[0033] Figure 5 This is a metallographic photograph of Example 3 of the present invention.

[0034] Figure 6 This is the metallographic photograph of Comparative Example 1. DETAILED DESCRIPTION

[0035] To achieve the purpose of the present invention, the method for grain refinement and homogenization of 9-12% Cr rotor steel comprises:

[0036] After forging of the 9-12% Cr rotor steel, a forging is obtained. The diameter of the 9-12% Cr rotor steel is greater than 700 mm. The forging is subjected to material waiting at 600-750° C. for more than 100 mm / 1 hour. After the material waiting is completed, the temperature is lowered to 160-350° C., and the temperature is kept for more than 100 mm / 2 hours. The temperature is then raised to 1000-1110° C., and the temperature is kept for more than 100 mm / 1.5 hours. The temperature is then lowered to 680-750° C., and the temperature is kept to make the forging undergo a balanced pearlite phase transformation. The furnace is then Cool until the surface of the forging is below 250℃, then heat it to 480-600℃ and keep it warm for more than 100mm / 0.2h, then heat it to 760-810℃ and keep it warm for more than 100mm / 0.5h, then heat it to 800-840℃ and keep it warm for more than 100mm / 0.5h, then heat it to 950-1060℃ and keep it warm for more than 100mm / 0.5h, then heat it to 1040-1110℃ and keep it warm for more than 100mm / 1h, and then cool it with quenching medium.

[0037] The material waiting time of 100mm / 1h is calculated based on the maximum diameter size, and the same applies to the following.

[0038] 160~350℃, keep warm for more than 100mm / 2h, and ensure that the core temperature drops to the temperature range of 160~350℃.

[0039] 1000~1110℃, keep warm for more than 100mm / 1.5h, to ensure the complete austenitization transformation of the rotor. After the quenching medium is cooled, tempering can be carried out. Tempering is an existing conventional tempering process. The tempering temperature can be appropriately adjusted according to the strength requirements of the forgings.

[0040] In a specific embodiment, the cooling rate to 160-350° C. is below 30° C. / h.

[0041] In a specific embodiment, the heating rate of heating to 1000-1110° C. is less than 60° C. / h.

[0042] In a specific embodiment, the cooling rate to 680-750° C. is below 30° C. / h.

[0043] In a specific embodiment, the insulation time at 680-750° C. is more than 240 hours.

[0044] In a specific embodiment, the furnace is cooled to 250° C. at a cooling rate of less than 20° C. / h.

[0045] In a specific embodiment, the heating rate of heating to 480-600° C. is less than 30° C. / h.

[0046] In a specific embodiment, the heating rate of heating to 760-810° C. is less than 40° C. / h.

[0047] In a specific embodiment, the heating rate of heating to 800°C to 840°C is 50°C / h or less.

[0048] In a specific embodiment, the heating rate of heating to 950-1060° C. is less than 60° C. / h.

[0049] In a specific embodiment, the heating rate of heating to 1040-1110° C. is less than 60° C. / h.

[0050] In a specific implementation, the diameter of the 9-12% Cr rotor steel is 700 mm to 1300 mm.

[0051] The specific implementation modes of the present invention are further described below in conjunction with embodiments, but the present invention is not limited to the scope of the embodiments.

[0052] Example 1

[0053] Use Figure 2 The process shown in the figure has a rotor size of φ1155*6510mm. After the forging of the 9% Cr rotor steel is completed, the forging is kept at 730±20℃ for 12h. After the waiting is completed, the forging is cooled to 240±20℃ at a rate of no more than 15℃ / h and kept warm for 24h. Then the temperature is raised to 1100±10℃ at a rate of 40-50℃ / h and kept warm for 19h to undergo complete austenitization transformation. Then the temperature is cooled to 730±10℃ at a rate of no more than 30℃ / h and kept warm for 280h after the workpiece undergoes complete pearlite transformation. The rotor is heated to 500±20℃ at 20~30℃ / h and kept at this temperature for 5h. Then, it is heated to 800±10℃ at 30~40℃ / h and kept at this temperature for 16h. Then, it is heated to 820±10℃ at 30~40℃ / h and kept at this temperature for 12h. Then, it is heated to 1050±10℃ at 40~50℃ / h and kept at this temperature for 10h. Then, it is heated to 1100±10℃ at 40~50℃ / h and kept at this temperature for 19h. Then, it is cooled by quenching medium.

[0054] The metallographic photographs of Example 1 are shown in Figure 3 , the grain size rating was 3.5, and the flaw detection results were also improved.

[0055] Example 2

[0056] Compared with Example 1, Example 2 is similar to Example 1 and both use Figure 2 The process shown in the figure has a rotor size of φ1085*8360mm. After the forging of the 9% Cr rotor steel is completed, the forging is kept at 730±20℃ for 11h. After the waiting is completed, the forging is cooled to 240±20℃ at a rate of no more than 15℃ / h and kept warm for 24h. Then the temperature is raised to 1100±10℃ at a rate of 40-50℃ / h and kept warm for 17h to undergo complete austenitization transformation. Then the temperature is cooled to 730±10℃ at a rate of no more than 30℃ / h. After the temperature is evenly balanced, the temperature is kept warm for 260h to make the workpiece undergo complete pearlite transformation. Then the temperature is raised to 1100±10℃ at a rate of no more than 40℃ / h and kept warm for 17h to undergo complete austenitization transformation. Cool to 250℃ at 20℃ / h, then take out of the furnace. After completing the pearlite phase transformation, perform performance heat treatment. Heat the rotor to 500±20℃ at 20-30℃ / h, keep warm for 5h, then heat to 800±10℃ at 30-40℃ / h, keep warm for 14h, then heat to 820±10℃ at 30-40℃ / h, keep warm for 10h, then heat to 1050±10℃ at 40-50℃ / h, keep warm for 9h, then heat to 1100±10℃ at 40-50℃ / h, keep warm for 16h, then cool with quenching medium.

[0057] The metallographic photographs of Example 2 are shown in Figure 4 , the grain size rating was 3.0, and the flaw detection results were also improved.

[0058] Example 3

[0059] Compared with Example 1, Example 3 uses 12% Cr rotor steel. After forging, the forging with a rotor size of φ1150*6120mm is kept at 700±20℃ for 12h. After the waiting, it is furnace cooled to 240±20℃ at a rate of no more than 15℃ / h, kept warm for 24h, and then heated to 1060±10℃ at 40-50℃ / h, kept warm for 18h (, after complete austenitization transformation, it is furnace cooled to 700±10℃ at a rate of no more than 30℃ / h, kept warm for 280h after equalization, so that the workpiece undergoes pearlite transformation, and then heated to no more than 15℃ / h at a rate of no more than 15℃ / h. After cooling to 250℃ at 20℃ / h, the furnace is taken out of the furnace. After the pearlite phase transformation is completed, the performance heat treatment is carried out. The rotor is heated to 500±20℃ at 20~30℃ / h, and the heat preservation time is 5h. After heating to 780±10℃ at 30~40℃ / h and keeping it for 16h, it is heated to 820±10℃ at 40~50℃ / h and keeping it for 12h. After heating to 1000±10℃ at 50~60℃ / h and keeping it for 10h, it is heated to 1070±10℃ at 50~60℃ / h and keeping it for 16h, and then cooled by quenching medium. The metallographic photos of Example 3 are shown in detail. Figure 5 , the grain size rating was 3.0, and the flaw detection results were also improved.

[0060] Comparative Example 1

[0061] Use Figure 1According to the process shown, after forging of 9% Cr rotor steel, the rotor with a size of 1055*6960mm is kept at 650±10℃ for 12h. After waiting, the temperature is raised to 1100±10℃ at 40-50℃ / h and kept at this temperature for 16h to undergo complete austenitization transformation. The workpiece is then furnace cooled to 730±10℃ at a rate of no more than 30℃ / h. After being evenly heated, the workpiece is kept at this temperature for 260h to cause pearlite transformation. The workpiece is then furnace cooled to 250℃ at a rate of no more than 20℃ / h and removed from the furnace. After the pearlite transformation is completed, a performance heat treatment is performed. The rotor is heated to 500±20℃ at 20-30℃ / h and kept at this temperature for 5h. The rotor is then heated to 1100±10℃ at 40-50℃ / h and kept at this temperature for 16h before being cooled by a quenching medium. The metallographic photographs taken under the original heat treatment process mode of comparative example 1 are as follows. Figure 6 As shown, the grain size ratings are 00 accounting for 40%, 1.5 accounting for 20%, and 5.5 accounting for 40%.

Claims

A method for grain refinement and homogenization of 1.9-12% Cr rotor steel, characterized in that: The method comprises: After forging of the 9-12% Cr rotor steel, a forging is obtained. The diameter of the 9-12% Cr rotor steel is greater than 700 mm. The forging is subjected to material waiting at 600-750° C. for more than 100 mm / 1 hour. After the material waiting is completed, the temperature is lowered to 160-350° C., and the temperature is kept for more than 100 mm / 2 hours. The temperature is then raised to 1000-1110° C., and the temperature is kept for more than 100 mm / 1.5 hours. The temperature is then lowered to 680-750° C., and the temperature is kept for a balanced phase change of the forging. The furnace is then cooled to The surface of the forging is lower than 250℃, then it is heated to 480-600℃ and kept warm for more than 100mm / 0.2h, then it is heated to 760-810℃ and kept warm for more than 100mm / 0.5h, then it is heated to 800-840℃ and kept warm for more than 100mm / 0.5h, then it is heated to 950-1060℃ and kept warm for more than 100mm / 0.5h, then it is heated to 1040-1110℃ and kept warm for more than 100mm / 1h, and then it is cooled with quenching medium.

2. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1, characterized in that: The cooling rate to 160-350° C. is below 30° C. / h.

3. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1 or 2, characterized in that: The heating rate of heating to 1000-1110° C. is less than 60° C. / h.

4. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1 or 2, characterized in that: The cooling rate to 680-750° C. is less than 30° C. / h; the insulation time at 680-750° C. is preferably more than 240 hours.

5. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1 or 2, characterized in that: The furnace is cooled to below 250° C. at a temperature reduction rate of below 20° C. / h.

6. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1 or 2, characterized in that: The heating rate to 480-600° C. is below 30° C. / h.

7. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1 or 2, characterized in that: The heating rate to 760-810° C. is below 40° C. / h.

8. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1 or 2, characterized in that: The heating rate to 800°C to 840°C is below 50°C / h.

9. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1 or 2, characterized in that: The heating rate to 950-1060°C is 60°C / h or less; the heating rate to 1040-1110°C is preferably 60°C / h or less.

10. The method for grain refinement and homogenization of 9-12% Cr rotor steel according to claim 1 or 2, characterized in that: The diameter of the 9-12% Cr rotor steel is 700 mm to 1300 mm.

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