Heat treatment method for reducing quenching stress and improving low-temperature toughness of Cr-Ni-Mo-V steel forging

Abstract

The invention aims to provide a heat treatment method for reducing quenching stress and improving low-temperature toughness of a Cr-Ni-Mo-V steel forging, and belongs to the technical field of metal material heat treatment. The method comprises the following steps: (1) carrying out isothermal annealing heat treatment on the Cr-Ni-Mo-V steel forging; (2) carrying out normalizing heat treatment on the isothermal annealing forge piece; (3) carrying out tempering heat treatment on the normalized forge piece; (4) the tempered forge piece is subjected to step quenching treatment; and (5) carrying out rapid tempering treatment on the quenched forge piece. By means of the heat treatment method, the surface quenching residual stress of the Cr-Ni-Mo-V steel forge piece can be reduced by 30% or above, meanwhile, the strength and low-temperature impact energy of the Cr-Ni-Mo-V steel forge piece are improved by 10% or above, and the safety and reliability of the Cr-Ni-Mo-V steel forge piece are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of metal materials, and specifically relates to a heat treatment method for reducing quenching stress and improving low-temperature toughness of a Cr-Ni-Mo-V steel forging. Background Art

[0002] Large integral rotor forgings are key components of third-generation nuclear power plant equipment. The rotor blanks are heavy and large in size, and the heat treatment process in the production process is complex and has a long cycle. In order to ensure the stability and reliability of the rotor performance, higher requirements are placed on the heat treatment processes such as post-forging annealing, pretreatment and tempering. In actual production, the forging ratio of large rotor forgings is generally small, and the deformation is uneven and the heating time is long. In addition, the grain inheritance phenomenon unique to steel for large forgings causes the austenite grains of large forgings to be relatively coarse, thus affecting the low-temperature impact performance of the forgings. Moreover, during the quenching process, the inconsistent phase transformation of the inner and outer layers of large forgings and the large temperature difference induce a large gap between the organizational stress and thermal stress of the core and surface of the forgings, resulting in large residual stress on the surface of the forgings after quenching, which ultimately makes the forgings easy to crack during heat treatment and placement.

[0003] At present, the traditional heat treatment process for large Cr-Ni-Mo-V rotor steel forgings is 3-4 times of normalizing + tempering + quenching + tempering. However, as the volume and size of the rotors are getting larger and larger, the existing heat treatment process can no longer guarantee the uniformity of the grain size of the forgings, and the surface quenching stress is getting higher and higher, which seriously affects the toughness and production safety of the forgings. Summary of the invention

[0004] In view of the above problems, the object of the present invention is to provide a heat treatment method for reducing the quenching stress and improving the low-temperature toughness of Cr-Ni-Mo-V steel forgings. The present invention reduces the surface residual stress of the forgings after quenching by refining the grain size of the forgings and optimizing the quenching cooling parameters, thereby improving the strength and toughness of large forgings and inhibiting surface cracking of forgings, and ultimately improving the performance and safety of large Cr-Ni-Mo-V rotor forgings.

[0005] The present invention adopts the following technical solution: A heat treatment method for reducing quenching stress and improving low-temperature toughness of Cr-Ni-Mo-V steel forgings comprises the following steps: The first step is to perform isothermal annealing heat treatment on the Cr-Ni-Mo-V steel forging to obtain an isothermal annealed forging; The second step is to perform normalizing heat treatment on the isothermal annealed forging obtained in the first step to obtain a normalized forging; The third step is to perform a tempering heat treatment on the normalized forging obtained in the second step to obtain a tempered forging; The fourth step is to perform a graded quenching heat treatment on the tempered forging obtained in the third step to obtain a quenched forging; The fifth step is to perform rapid tempering heat treatment on the quenched forging obtained in the fourth step.

[0006] Furthermore, the Cr-Ni-Mo-V steel includes the following components in mass percentage: C≤0.35%, Si≤0.05%, Mn≤0.07%, P≤0.006%, S≤0.003%, 1.50%≤Cr≤2.00%, 3.25%≤Ni≤3.75%, 0.25%≤Mo≤0.60%, 0.07%≤V≤0.15%, Cu≤0.10%, and Al≤0.008%.

[0007] Furthermore, the diameter of the Cr-Ni-Mo-V steel forging in the first step is 1000-2000 mm.

[0008] Furthermore, the isothermal annealing heat treatment process in the first step is: heating to 650-680°C at a rate of no more than 30°C / h and keeping warm for 12-20h, continuing to heat to 830-860°C at a rate of no more than 30°C / h and keeping warm for 15-30h, then air cooling to 600-650°C and keeping warm for 120-180h, and then air cooling to below 300°C.

[0009] Furthermore, the normalizing heat treatment process in the second step is: heating to 650-680°C at a rate of no more than 30°C / h and keeping warm for 12-20h, continuing to heat to 850-900°C at a rate of no more than 30°C / h and keeping warm for 15-30h, and then air cooling to below 300°C.

[0010] Furthermore, the tempering heat treatment process in the third step is: heating to 650-680°C at a rate of no more than 30°C / h, keeping the temperature for 20-35h, and then air cooling to below 300°C.

[0011] Furthermore, the graded quenching heat treatment process in the fourth step is: heating to 650-680°C at a rate of no more than 30°C / h and keeping warm for 12-20h, continuing to heat to 840-880°C and keeping warm for 15-30h, and then oil cooling for 30-60min+air cooling for 60-90min+oil cooling for 30-60min to room temperature.

[0012] Furthermore, the rapid tempering heat treatment process in the fifth step is: heating to 550-600°C at a rate of not less than 120°C / h, keeping the temperature for 20-35h, and then water cooling to room temperature.

[0013] The beneficial effects of the present invention are as follows: The above heat treatment process can reduce the surface quenching residual stress of Cr-Ni-Mo-V steel forgings by more than 30%, and improve its low-temperature toughness by more than 20%.

[0014] The present invention provides a heat treatment method for reducing quenching stress and improving low-temperature toughness of Cr-Ni-Mo-V steel forgings. The new heat treatment method includes post-forging pretreatment (isothermal annealing + normalizing + tempering) and quenching and tempering treatment (graded quenching + rapid tempering). In the present invention, the post-forging pretreatment refines the grains by isothermal annealing + normalizing + tempering instead of 3-4 normalizing + tempering. For large-volume and large-size forgings, the isothermal annealing pearlite transformation is more effective than multiple normalizing bainite transformation recrystallization and grain refinement. Moreover, the pearlite transformation causes the original austenite phase to re-beat irregularly, hindering the formation of tissue inheritance, which is beneficial to the subsequent normalizing phase transformation recrystallization and refinement. The purpose of tempering heat treatment after normalizing is to precipitate carbides and hinder the coarsening of grains when Cr-Ni-Mo-V steel is quenched and austenitized. Therefore, the purpose of the pretreatment process of isothermal annealing + normalizing + tempering is to first refine the grains and hinder the inheritance of the organization through isothermal annealing, then refine the grains again through the normalizing bainite phase transformation, and finally temper to precipitate carbides to inhibit the coarsening of the quenched high-temperature austenite grains, and finally achieve the effect of grain refinement. Compared with ordinary quenching, the graded quenching treatment in the present invention reduces the difference in the transformation rate and temperature of bainite and martensite in the core and surface of the forging by changing the cooling medium, so that the difference in internal and external organizational stress and thermal stress of the forging is reduced, thereby reducing the quenching residual stress on the surface of the forging. Compared with ordinary tempering treatment, the rapid tempering in the present invention shortens the tempering time by increasing the heating rate, and the less dislocation annihilation leads to more carbide precipitation nucleation sites, so that the carbides are finely dispersed during the tempering process, which is ultimately beneficial to the improvement of impact toughness. In summary, the present invention promotes the homogenization of internal and external structures through process control, controls the phase transformation and thermal stress of the internal and external structures to reduce the residual stress on the quenched surface of the forging, improves the low-temperature toughness of the forging by refining the grains and finely dispersing carbides, and ultimately improves the production quality of large forgings and reduces costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a schematic diagram of the first step of isothermal annealing process of the present invention; Figure 2 It is a schematic diagram of the normalizing heat treatment in the second step and the tempering heat treatment in the third step of the present invention; Figure 3 It is a schematic diagram of the fourth step of graded quenching heat treatment and the fifth step of rapid tempering heat treatment process of the present invention. DETAILED DESCRIPTION

[0016] The method of the present invention will be described in further detail below in conjunction with specific examples. It should be understood that the following examples are only exemplary descriptions and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are included in the scope that the present invention is intended to protect.

[0017] The experimental methods used in the following examples are conventional methods unless otherwise specified; the materials used in the following examples are commercially available unless otherwise specified.

[0018] Example 1 The diameter of Cr-Ni-Mo-V steel forging is 1500mm, and its heat treatment process is as follows: (1) Annealing after forging The temperature was raised to 650°C at a rate of 30°C / h and kept at this temperature for 20h. The temperature was further raised to 830°C at a rate of 30°C / h and kept at this temperature for 30h. The mixture was then air-cooled to 600°C and kept at this temperature for 180h. The mixture was then air-cooled to room temperature.

[0019] (2) Normalizing The temperature was raised to 650°C at a rate of 30°C / h and kept at that temperature for 20h. The temperature was further raised to 850°C at a rate of 30°C / h and kept at that temperature for 30h. The temperature was then air-cooled to below 300°C.

[0020] (3) Tempering The temperature was raised to 650°C at a rate of 30°C / h and kept at this temperature for 35h, then air-cooled to below 300°C.

[0021] (4) Graded quenching Raise the temperature to 650℃ at a rate of 30℃ / h and keep it for 20h. Continue to raise the temperature to 840℃ and keep it for 30h. Then, cool it with oil for 30min + air for 60min + oil for 30min to room temperature.

[0022] (5) Rapid tempering The temperature was raised to 550°C at a rate of 120°C / h and kept at that temperature for 35h, then water-cooled to room temperature.

[0023] Example 2 The diameter of Cr-Ni-Mo-V steel forging is 1500mm, and its heat treatment process is as follows: (1) Annealing after forging The temperature was raised to 665°C at a rate of 30°C / h and kept at this temperature for 160h. The temperature was further raised to 8450°C at a rate of 30°C / h and kept at this temperature for 22.5h. The temperature was then air-cooled to 6250°C and kept at this temperature for 150h. The temperature was then air-cooled to room temperature.

[0024] (2) Normalizing The temperature was raised to 665°C at a rate of 30°C / h and kept at this temperature for 16 hours. The temperature was further raised to 875°C at a rate of 30°C / h and kept at this temperature for 17.5 hours. The temperature was then air-cooled to below 300°C.

[0025] (3) Tempering The temperature was raised to 665°C at a rate of 30°C / h and kept at this temperature for 27.5h, and then air-cooled to below 300°C.

[0026] (4) Graded quenching Raise the temperature to 665℃ at a rate of 30℃ / h and keep it for 16h. Continue to raise the temperature to 860℃ and keep it for 22.50h. Then, cool it with oil for 45min + air for 75min + oil for 45min to room temperature.

[0027] (5) Rapid tempering The temperature was raised to 575°C at a rate of 120°C / h and kept at that temperature for 27.5h, then water-cooled to room temperature.

[0028] Example 3 The diameter of Cr-Ni-Mo-V steel forging is 1500mm, and its heat treatment process is as follows: (1) Annealing after forging The temperature was raised to 680°C at a rate of 30°C / h and kept at this temperature for 12h. The temperature was further raised to 860°C at a rate of 30°C / h and kept at this temperature for 15h. The mixture was then air-cooled to 650°C and kept at this temperature for 120h. The mixture was then air-cooled to room temperature.

[0029] (2) Normalizing The temperature was raised to 680°C at a rate of 30°C / h and kept at this temperature for 12h. The temperature was further raised to 900°C at a rate of 30°C / h and kept at this temperature for 15h. The temperature was then air-cooled to below 300°C.

[0030] (3) Tempering The temperature was raised to 680°C at a rate of 30°C / h and kept at this temperature for 20h, and then air-cooled to below 300°C.

[0031] (4) Graded quenching Raise the temperature to 680℃ at a rate of 30℃ / h and keep it for 12h, then continue to raise the temperature to 880℃ and keep it for 15h, then oil cool for 30min + air cool for 60min + oil cool for 30min to room temperature.

[0032] (5) Rapid tempering The temperature was raised to 550°C at a rate of 120°C / h and kept at that temperature for 20h, then cooled to room temperature with water.

[0033] Comparative Example 1 The diameter of Cr-Ni-Mo-V steel forging is 1500mm, and its heat treatment process is as follows: (1) First normalizing The temperature was raised to 680°C at a rate of 30°C / h and kept at this temperature for 12h. The temperature was further raised to 920°C at a rate of 30°C / h and kept at this temperature for 15h. The mixture was then air-cooled to 650°C and kept at this temperature for 120h. The mixture was then air-cooled to room temperature.

[0034] (2) Second normalizing The temperature was raised to 680°C at a rate of 30°C / h and kept at this temperature for 12h. The temperature was further raised to 900°C at a rate of 30°C / h and kept at this temperature for 15h. The temperature was then air-cooled to below 300°C.

[0035] (3) Third normalizing The temperature was raised to 680°C at a rate of 30°C / h and kept at this temperature for 12h. The temperature was further raised to 890°C at a rate of 30°C / h and kept at this temperature for 15h. The temperature was then air-cooled to below 300°C.

[0036] (4) Tempering The temperature was raised to 680°C at a rate of 30°C / h and kept at this temperature for 20h, and then air-cooled to below 300°C.

[0037] (5) Quenching Raise the temperature to 680℃ at a rate of 30℃ / h and keep it for 12h, then continue to raise the temperature to 880℃ and keep it for 15h, then oil cool for 30min + air cool for 60min + oil cool for 30min to room temperature.

[0038] (6) Tempering The temperature was raised to 550°C at a rate of 120°C / h and kept at that temperature for 20h, then cooled to room temperature with water.

[0039] Examples 1-3 and Comparative Example 1 are specific heat treatment processes for Cr-Ni-Mo-V steel forgings with a diameter of 1500 mm. Examples 1, 2, 3 and Comparative Example 1 are comparisons between the heat treatment process of the present invention and the traditional heat treatment process. The comparisons of room temperature tensile properties, low temperature longitudinal impact energy and surface quenching residual stress of Examples 1-3 and Comparative Example 1 are shown in Tables 1-3, respectively.

[0040] It can be seen from Table 1 that, relative to Ratio 1, the tensile strength and yield strength of Examples 1-3 are both increased by more than 10%; it can be seen from Table 2 that, relative to Ratio 1, the -20°C and -40°C impact energies of Examples 1-3 are both increased by more than 10%; it can be seen from Table 3 that, relative to Ratio 1, the surface quenching residual stresses of Examples 1-3 are all reduced by more than 30%.

[0041] Table 1 Room temperature tensile properties of Examples 1-3 and Comparative Example 1 Table 2 Low temperature longitudinal impact energy of Examples 1-3 and Comparative Example 1 Table 3 Surface quenching residual stress of Examples 1-3 and Comparative Example 1 Only the preferred embodiments of the present invention are described in detail above, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention, and various changes should be included in the protection scope of the present invention.

Claims

1. A heat treatment method for reducing quenching stress and improving low-temperature toughness of Cr-Ni-Mo-V steel forgings, characterized in that: The steps include: The first step is to perform isothermal annealing heat treatment on the Cr-Ni-Mo-V steel forging to obtain an isothermal annealed forging; The second step is to perform normalizing heat treatment on the isothermal annealed forging obtained in the first step to obtain a normalized forging; The third step is to perform a tempering heat treatment on the normalized forging obtained in the second step to obtain a tempered forging; The fourth step is to perform a graded quenching heat treatment on the tempered forging obtained in the third step to obtain a quenched forging; The fifth step is to perform rapid tempering heat treatment on the quenched forging obtained in the fourth step.

2. A heat treatment method for reducing quenching stress and improving low temperature toughness of Cr-Ni-Mo-V steel forgings according to claim 1, characterized in that: The Cr-Ni-Mo-V steel includes the following components in mass percentage: C≤0.35%, Si≤0.05%, Mn≤0.07%, P≤0.006%, S≤0.003%, 1.50%≤Cr≤2.00%, 3.25%≤Ni≤3.75%, 0.25%≤Mo≤0.60%, 0.07%≤V≤0.15%, Cu≤0.10%, and Al≤0.008%.

3. A heat treatment method for reducing quenching stress and improving low temperature toughness of Cr-Ni-Mo-V steel forgings according to claim 1, characterized in that: The diameter of the Cr-Ni-Mo-V steel forging in the first step is 1000-2000 mm.

4. A heat treatment method for reducing quenching stress and improving low temperature toughness of Cr-Ni-Mo-V steel forgings according to claim 1, characterized in that: The isothermal annealing heat treatment process in the first step is: heating to 650-680°C at a rate of no more than 30°C / h and keeping warm for 12-20h, continuing to heat to 830-860°C at a rate of no more than 30°C / h and keeping warm for 15-30h, then air cooling to 600-650°C and keeping warm for 120-180h, and then air cooling to below 300°C.

5. The heat treatment method for reducing quenching stress and improving low temperature toughness of Cr-Ni-Mo-V steel forgings according to claim 1, characterized in that: The normalizing heat treatment process in the second step is: heating to 650-680°C at a rate of no more than 30°C / h and keeping warm for 12-20h, continuing to heat to 850-900°C at a rate of no more than 30°C / h and keeping warm for 15-30h, and then air cooling to below 300°C.

6. A heat treatment method for reducing quenching stress and improving low temperature toughness of Cr-Ni-Mo-V steel forgings according to claim 1, characterized in that: The tempering heat treatment process in the third step is: heating to 650-680°C at a rate of no more than 30°C / h, keeping the temperature for 20-35h, and then air cooling to below 300°C.

7. A heat treatment method for reducing quenching stress and improving low temperature toughness of Cr-Ni-Mo-V steel forgings according to claim 1, characterized in that: The graded quenching heat treatment process in the fourth step is: heating to 650-680°C at a rate of no more than 30°C / h and keeping warm for 12-20h, continuing to heat to 840-880°C and keeping warm for 15-30h, then oil cooling for 30-60min+air cooling for 60-90min+oil cooling for 30-60min to room temperature.

8. The heat treatment method for reducing quenching stress and improving low-temperature toughness of Cr-Ni-Mo-V steel forgings according to claim 1, characterized in that: The rapid tempering heat treatment process in the fifth step is: heating to 550-600°C at a rate of not less than 120°C / h, keeping the temperature for 20-35h, and then cooling to room temperature with water.

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