Method for determining width of heating band for local heat treatment of large pressure-bearing equipment

Abstract

The invention belongs to the technical field of pressure-bearing equipment manufacturing and safe service, and particularly discloses a method for determining the width of a heating band for local heat treatment of large pressure-bearing equipment. The problems that in the prior art, the stress relieving effect is poor, and on-site implementation difficulty is large are effectively solved. Comprising the following steps: (1) determining a local heat treatment distributed deformation expression of the coupling heating band width, the heating band edge temperature and the heat treatment temperature; (2) determining axial and circumferential residual stresses of the inner surface and the center of a welding seam after local heat treatment cooling; and (3) drawing an evolution law of axial and circumferential stress of the center of the weld joint on the inner surface of the pressure-bearing equipment under different heating band widths by utilizing drawing software to obtain a general relational expression of the heating band width meeting the requirement that the residual stress elimination rate is more than 70% and a relational equation of the expected residual stress elimination rate and the heating band width. The heating band width meeting the expected stress relief rate can be determined according to the actual production requirement, and the local heat treatment cost can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of pressure-bearing equipment manufacturing and safe service, and in particular relates to a method for determining the width of a heating zone for local heat treatment of large-scale pressure-bearing equipment. Background Art

[0002] As the core infrastructure of the modern industrial system, the safety and reliability of pressure-bearing equipment are directly linked to national energy security, ecological protection, and public safety. Welding, a key technology in pressure-bearing equipment manufacturing, determines its overall performance and service life. However, the welding process is prone to generating residual stress, deformation, and microstructural changes, which can seriously affect the structural integrity and safe service life of the equipment. Post-weld heat treatment is a core technical step in eliminating welding residual stress and improving material properties. It is crucial for ensuring that pressure-bearing equipment moves from "qualified manufacturing" to "reliable service."

[0003] The development of GB / T 30583-2014, "Code for Post-weld Heat Treatment of Pressure Equipment," resolved the issue of whether China had a standard for heat treatment of pressure equipment, making a significant contribution to the pressure equipment industry. However, the 2014 edition only specified guidelines for the width of the heating zone for local heat treatment of wall thicknesses less than 50 mm, and did not specify requirements for wall thicknesses greater than 50 mm. However, the large-scale development of petrochemical plants, such as those with tens of millions of tons of oil refining capacity, millions of tons of ethylene capacity, and millions of tons of aromatics capacity, inevitably leads to larger pressure equipment, with wall thicknesses generally exceeding 50 mm. The 14 edition is unable to provide guidance for its implementation. Furthermore, as pressure equipment develops toward ultra-thick walls, ultra-large diameters, and extremely stringent operating conditions, traditional heat treatment technologies face multiple bottlenecks: Sluggish heat conduction in thick-walled vessels makes it difficult to meet temperature uniformity requirements; and the waist-constricting deformation caused by local heat treatment induces secondary stresses on the vessel's inner surface, exacerbating the risk of stress corrosion cracking. In existing technologies, the width of the heating zone is a key parameter that determines the effectiveness of local heat treatment. However, domestic and international standards and related technologies have significant limitations. For large-scale pressure-bearing equipment, local heat treatment can be difficult to perform on-site due to the heating zone being too wide. This makes it difficult to eliminate residual stress.

[0004] Therefore, a method for determining the width of the heating zone for local heat treatment of large-scale pressure-bearing equipment is urgently needed to solve key problems in the existing technology, such as poor stress relief effect and difficulty in on-site implementation. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for determining the width of the heating zone for local heat treatment of large-scale pressure-bearing equipment, which effectively solves the problems of poor stress elimination effect and difficulty in on-site implementation in the prior art.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A method for determining the width of a local heat treatment heating zone for large-scale pressure-bearing equipment comprises the following steps: S1, determining a local heat treatment distributed deformation expression for the coupled heating zone width, the heating zone edge temperature, and the heat treatment temperature:

[0008]

[0009] Where R is the radius, w is the deflection, β is the size parameter, T max is the peak temperature of heat treatment at the center of the weld, x is the distance from the weld center in the axial direction, λ is the axial temperature gradient parameter, HB is the width of the heating zone, T b is the edge temperature of the heating zone, and t is the wall thickness.

[0010] S2. Determine the axial and hoop residual stress distribution equations on the inner surface after local heat treatment cooling:

[0011]

[0012] γ=λ / β;

[0013] Where E is the elastic modulus of the material, σ x is the axial residual stress, σ θ is the hoop residual stress, and α is the thermal expansion coefficient.

[0014] S3. According to the axial and circumferential residual stress distribution equations on the inner surface, the axial residual stress and circumferential residual stress generated at the center of the weld are solved as follows:

[0015]

[0016] S4. Based on the calculation equations of the axial residual stress and hoop residual stress generated in the weld center, use the drawing software to draw the evolution law of the axial and hoop stress of the inner surface weld center of the pressure-bearing equipment with different heating zone widths. The horizontal axis of the curve is Normalization processing.

[0017] S5. According to the regular curve fitted in step S4, a general relationship between the width of the heating zone and the edge temperature of the heating zone for achieving the overall heat treatment stress relief effect, as well as a relationship equation between the expected residual stress relief rate and the width of the heating zone are obtained.

[0018] The general relationship between the heating zone width and the heating zone edge temperature is:

[0019]

[0020] Where R represents the container radius and t represents the wall thickness.

[0021] The relationship equation between the expected residual stress relief rate and the heating zone width is:

[0022]

[0023] Where η is the stress relief rate, A1, A2, κ, and τ are parameters related to the heating method. When the local heat treatment of the ceramic sheet and the local heat treatment of the cassette furnace are used, A1, A2, κ, and τ are -0.357, 1.00, 1.445, and 1.95, respectively; when the induction heating method is used, A1, A2, κ, and τ are -0.434, 1.00, 0.935, and 1.915, respectively.

[0024] Furthermore, in step S1, the axial temperature distribution equation T(x) of the local heat treatment insulation stage is established according to the maximum heat treatment temperature and the edge temperature of the heating zone:

[0025] T(x)=T max e -λx (cosλx+sinλx).

[0026] Furthermore, the deflection differential equation of the local heat treatment cooling stage is established:

[0027]

[0028] Where w is the deflection and R is the radius.

[0029] Furthermore, in step S1, according to the boundary conditions of zero slope and zero shear force at the center of the weld, that is, the position x=0, the first-order derivative and the third-order derivative of w are 0, thereby obtaining the local heat treatment distributed deformation expression.

[0030] Furthermore, in step S2, the axial and circumferential residual stresses on the inner surface after local heat treatment and cooling are solved by the deflection differential equation based on the geometric relationship between strain and displacement and the physical relationship between stress and strain.

[0031] Furthermore, according to actual production, the edge temperature of the heating zone of large-scale pressure-bearing equipment is at least greater than 60% of the heat treatment peak temperature. If the local heat treatment heater is considered to be an ideal heat source, that is, the edge temperature of the heating zone is equal to the center temperature of the heating zone, under the above different boundary conditions, the heating zone width solved according to the general relationship between the heating zone width and the heating zone edge temperature needs to meet the criteria:

[0032]

[0033] Furthermore, if the production requirement does not require a stress relief rate of 70% or requires a higher stress relief rate, the heating zone width that meets the expected stress relief rate can be solved using the relationship equation between the expected stress relief rate and the heating zone width:

[0034]

[0035] Furthermore, according to the criteria that the width of the heating zone needs to meet solved by the general relationship, when performing local heat treatment on a certain pressure-bearing equipment, the heat treatment heating and cooling rate, holding temperature and holding time are first determined based on the heat treatment object, combined with the heat treatment technical documents and standard specifications.

[0036] Combined with the general relationship between the heating zone width and the heating zone edge temperature, and based on the characteristics of the local heat treatment heater, the local heat treatment heating zone width that meets the stress relief effect required for production is selected.

[0037] According to the width of the local heat treatment heating zone, when the edge temperature of the heating zone is lower than the calculated temperature value, the heating rate should be reduced or the thickness or length of the insulation cotton should be increased to ensure that the edge temperature of the heating zone during the heat treatment insulation stage is greater than or equal to the value required by the general relationship between the width of the heating zone and the edge temperature of the heating zone.

[0038] Furthermore, when using ceramic sheets or cassette furnaces for heating, the ratio of the heating zone edge temperature to the heating zone center temperature is determined to be 0.7; when using induction heating, the ratio of the heating zone edge temperature to the heating zone center temperature is selected to be 0.8; or the ratio of the heating zone edge temperature to the heating zone center temperature of different heaters is determined through preliminary experiments.

[0039] Furthermore, during local heat treatment, the heating belts are arranged symmetrically with the weld as the center, and the thermocouples are welded, and the heating plates and insulation cotton are installed. Note that insulation cotton should be arranged on both the inner and outer walls. In addition to meeting the standard requirements, temperature measuring thermocouples should be arranged at the edge of the heating belt to monitor the temperature changes at the edge of the heating belt.

[0040] Compared with the prior art, the beneficial technical effects of the present invention are:

[0041] The present invention takes into account the impact of shrinkage deformation that will occur during the cooling stage of local heat treatment, and proposes a distributed deformation calculation formula and a distributed stress calculation formula for local heat treatment that couples the width of the heating zone-the edge temperature of the heating zone-the heat treatment temperature. On the basis of the distributed theory, the local heat treatment process is optimized, and a local heat treatment process criterion that considers the residual stress elimination effect is proposed, which effectively solves the problems of poor stress elimination effect and difficulty in on-site implementation in the existing technology. On the one hand, it is conducive to achieving the residual stress elimination effect of the overall heat treatment, that is, the residual stress elimination rate is more than 70%. At the same time, the width of the heating zone that meets the expected residual stress elimination rate can be determined according to actual production needs. On the other hand, it is conducive to significantly reducing the cost of local heat treatment, shortening the manufacturing cycle, and improving the safety and reliability of pressure-bearing equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1This is the evolution law of the axial residual stress in the center of the weld in Example 1 with the width of the heating zone.

[0043] Figure 2 This is the evolution law of the circumferential residual stress in the center of the weld in Example 1 with the width of the heating zone.

[0044] Figure 3 It is the axial residual stress test value after local heat treatment using the heating zone width determination method of the present invention in Example 1.

[0045] Figure 4 It is the circumferential residual stress test value after local heat treatment using the heating belt width determination method of the present invention in Example 1.

[0046] Figure 5 These are the test values ​​of axial and circumferential residual stresses after local heat treatment using the method for determining the width of the heating belt of the present invention in Example 2. DETAILED DESCRIPTION

[0047] This invention takes into account the effects of shrinkage deformation during the cooling phase of local heat treatment, which will generate new secondary axial and hoop stresses on the inner surface of the container. It proposes a distributed deformation calculation formula for local heat treatment, coupling the heating zone width, heating zone edge temperature, and heat treatment temperature. Based on distributed theory, the local heat treatment process is optimized, and a local heat treatment process criterion that considers the effect of residual stress elimination is proposed. This can achieve the same residual stress elimination effect as overall heat treatment, meaning a residual stress elimination rate of over 70%. At the same time, the heating zone width that meets the expected residual stress elimination rate can be determined based on actual production needs. This can significantly reduce local heat treatment costs, shorten manufacturing cycles, and improve the safety and reliability of pressure-bearing equipment.

[0048] The present invention provides a method for determining the width of a heating zone for local heat treatment of large-scale pressure-bearing equipment. The method comprises the following steps: S1. Determining a local heat treatment distributed deformation expression for the coupled heating zone width, the heating zone edge temperature, and the heat treatment temperature.

[0049] The expansion deformation generated during the local heat treatment and insulation stage will increase the radius of the container, but the expansion deformation generated by large pressure-bearing equipment is much smaller than the radius of the container, so the change in radius can be ignored when solving the stress. The differential equation of the moment theory for the axisymmetric problem of cylindrical shells is:

[0050]

[0051] Where R is the radius, t is the wall thickness, w is the deflection, E is the elastic modulus of the material, x is the distance from the center of the weld in the axial direction, D, p z denote the bending stiffness and temperature load respectively.

[0052]

[0053] Where α is the thermal expansion coefficient; μ is the Poisson's ratio, which is 0.3; T x is the temperature of each axial point during the insulation stage; T i It is room temperature.

[0054] Due to T i Much smaller than T x , therefore, (T i -T x ) can be used to calculate the axial temperature distribution equation -T x Directly expressed, then:

[0055]

[0056] In summary, the deflection differential equation of the local heat treatment cooling stage is established:

[0057]

[0058] Furthermore, the axial temperature distribution equation T(x) of the local heat treatment insulation stage is established according to the maximum heat treatment temperature and the edge temperature of the heating zone:

[0059] T(x)=T max e -λx (cosλx+sinλx).

[0060] Where, T max is the peak temperature of heat treatment at the center of the weld, x is the distance from the weld center in the axial direction, and λ is the axial temperature gradient parameter. The value of λ is related to the width of the heating zone HB, the peak temperature of heat treatment at the center of the weld T max and the edge temperature of the heating zone T b The expressions are as follows:

[0061]

[0062] According to the boundary conditions of zero slope and zero shear force at the center of the weld, that is, the position x = 0, that is, the first-order derivative and the third-order derivative of w are 0, the local heat treatment distributed deformation expression can be obtained:

[0063]

[0064] Where β is the size parameter.

[0065] S2. Based on the geometric relationship between strain and displacement and the physical relationship between stress and strain, the deflection differential equation is used to solve the axial and circumferential residual stress distribution equations on the inner surface after local heat treatment and cooling:

[0066]

[0067] Where E is the elastic modulus of the material, σ x is the axial residual stress, σ θ is the hoop residual stress; γ=λ / β, which can be expressed as γ and and the width of the heating zone, i.e.

[0068] S3. According to the axial and circumferential residual stress distribution equations on the inner surface, the axial residual stress and circumferential residual stress generated at the center of the weld are solved as follows:

[0069]

[0070] S4. Based on the calculation equations of the axial residual stress and hoop residual stress generated in the weld center, use Originpro or other drawing software to draw the evolution law of the axial and hoop stress of the inner surface weld center of the pressure-bearing equipment with different heating zone widths. The horizontal axis of the curve is Normalization processing, that is, the specific value of the horizontal axis represents the width and size parameters of the heating zone The vertical axis is the multiple relationship of residual stress.

[0071] Each edge temperature can be plotted against the heat treatment peak temperature (i.e., the center temperature of the heating zone) to form a separate curve. Furthermore, based on the overall heat treatment residual stress relief effect, i.e., a residual stress relief rate of 70%, the heating zone width values ​​that meet the above requirements at different heating zone edge temperatures can be obtained.

[0072] S5. According to the regular curve fitted in step S4, the general relationship between the width of the heating zone and the edge temperature of the heating zone to achieve the overall heat treatment stress relief effect (residual stress relief rate of more than 70%), as well as the relationship equation between the expected residual stress relief rate and the width of the heating zone are obtained.

[0073] (1) The general relationship between the width of the heating zone and the edge temperature of the heating zone is:

[0074]

[0075] According to actual production, the edge temperature of the heating zone of large pressure-bearing equipment is greater than 60% of the heat treatment peak temperature. If the local heat treatment heater is considered to be an ideal heat source, that is, the edge temperature of the heating zone is equal to the center temperature of the heating zone, then the width of the heating zone solved according to the general relationship between the width of the heating zone and the edge temperature of the heating zone that meets the residual stress elimination rate of more than 70% needs to meet the following criteria:

[0076]

[0077] When the edge temperature is 60%, take the larger value on the right side of the inequality; when it is an ideal heat source, the edge temperature is equal to the center temperature, take the smaller value on the left side of the inequality.

[0078] According to the above criteria, when performing local heat treatment on a certain pressure-bearing equipment, first determine the heat treatment heating and cooling rate, holding temperature and holding time based on the heat treatment object, combined with the heat treatment technical documents and standard specifications;

[0079] Based on the general relationship between heating zone width and heating zone edge temperature, and taking into account the characteristics of the local heat treatment heater, select a local heat treatment heating zone width that meets the stress relief effect required for production. The specific selection method is as follows: When using ceramic plates or a cassette furnace for heating, the ratio of the heating zone edge temperature to the heating zone center temperature can be determined as 0.7; when using induction heating, the ratio of the heating zone edge temperature to the heating zone center temperature can be selected as 0.8. Alternatively, preliminary experiments can be conducted to determine the heating zone edge temperature to heating zone center temperature ratio for different heaters.

[0080] Furthermore, according to the heating band width selected by the general relationship between the heating band width and the heating band edge temperature, when the heating band edge temperature is lower than the calculated temperature value, the heating rate should be reduced or the thickness or length of the insulation cotton should be increased to ensure that the heating band edge temperature during the heat treatment insulation stage is greater than or equal to the value required by the general relationship between the heating band width and the heating band edge temperature.

[0081] (2) If the production requirement does not require a stress relief rate of 70% or requires a higher stress relief rate, the heating zone width value that meets the expected stress relief rate is solved through the relationship equation between the expected stress relief rate and the heating zone width.

[0082] The relationship equation between the expected residual stress relief rate and the heating zone width is:

[0083]

[0084] Where η is the stress relief rate, A1, A2, κ, and τ are parameters related to the heating method. When the local heat treatment of the ceramic sheet and the local heat treatment of the cassette furnace are used, A1, A2, κ, and τ are -0.357, 1.00, 1.445, and 1.95, respectively; when the induction heating method is used, A1, A2, κ, and τ are -0.434, 1.00, 0.935, and 1.915, respectively.

[0085] During local heat treatment, the heating zones are arranged symmetrically around the weld seam. Thermocouples are then welded, and the heating plates and insulation are installed. Note that insulation should be installed on both the inner and outer walls. In addition to meeting standard requirements, thermocouples should be placed at the edges of the heating zones to monitor temperature changes at the edges.

[0086] Example 1: The local heat treatment of a tower is studied. The material is Q345R and the geometric dimensions are Φ4400mm×66mm×56910mm. The local heat treatment heating method used is ceramic resistor heating. First, Figure 1 and Figure 2 The figures respectively represent the calculation equations for the axial residual stress and the circumferential residual stress generated at the weld center by using the present invention, and the evolution law of the axial residual stress and the circumferential residual stress at the weld center with the width of the heating zone of the equipment at different heating zone widths is plotted.

[0087] After testing, the edge temperature of the ceramic heating plate used on site is 76.0% of the peak temperature of the heat treatment. According to the formula provided by the present invention: The calculated heating zone width HB that satisfies the stress relief effect is Specifically, it is 1040mm, the insulation width is 2HB, and the maximum insulation temperature is 600℃. After residual stress test and local heat treatment, Figure 3 As shown in Figure 2, the axial residual stress of the inner wall decreases by 71%. Figure 4 As shown in the figure, the hoop residual stress is reduced by 81%, achieving the overall heat treatment effect.

[0088] Example 2: The closed annular seam of a container was used as the verification object, with a diameter of 3048mm, a wall thickness of 16mm, and a total length of 142100mm. The residual stress before and after heat treatment was tested using the indentation energy method. After testing, the ceramic heating used on site had an edge temperature of the heating zone of 70% of the peak temperature of the heat treatment. According to the method for determining the width of the heating zone provided by the present invention, the width of the heating zone HB was calculated to be Specifically, the inner and outer walls are insulated with a thickness of 487.2 mm, the width of the insulation belt is 2HB, and the insulation temperature is 600°C. Figure 5 The results of axial and hoop residual stress tests on the inner surface before and after heat treatment are presented. The maximum axial as-welded stress was 200.3 MPa. After heat treatment, the axial residual stress was significantly reduced and evenly distributed, with the maximum stress dropping to 47.1 MPa, a decrease of 73.5%. The maximum hoop residual stress was 229.3 MPa, but after heat treatment, it dropped to 49.1 MPa, a decrease of 78.4%. This confirms the rationality of the heating zone width criteria proposed in this invention.

[0089] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the scope of protection of the present invention.

Claims

1. A method for determining the width of a heating zone for local heat treatment of large pressure-bearing equipment, characterized in that: The following steps are involved: S1. Determine the local heat treatment distributed deformation expression of the coupled heating zone width, heating zone edge temperature and heat treatment temperature: Where R is the radius, w is the deflection, β is the size parameter, T max is the peak temperature of heat treatment at the center of the weld, x is the distance from the weld center in the axial direction, λ is the axial temperature gradient parameter, HB is the width of the heating zone, T b is the edge temperature of the heating zone, t represents the wall thickness; S2. Determine the axial and hoop residual stress distribution equations on the inner surface after local heat treatment cooling: γ=λ / β; Where E is the elastic modulus of the material, σ x is the axial residual stress, σ θ is the hoop residual stress, α is the thermal expansion coefficient; S3. According to the axial and circumferential residual stress distribution equations on the inner surface, the axial residual stress and circumferential residual stress generated at the center of the weld are solved as follows: S4. Based on the calculation equations of the axial residual stress and hoop residual stress generated in the weld center, use the drawing software to draw the evolution law of the axial and hoop stress of the inner surface weld center of the pressure-bearing equipment with different heating zone widths. The horizontal axis of the curve is Normalization processing; S5. Based on the regular curve fitted in step S4, a general relationship between the width of the heating zone required to achieve the overall heat treatment stress relief effect and a relationship equation between the expected residual stress relief rate and the width of the heating zone are obtained; The general relationship between the width of the heating zone is: Where R represents the container radius and t represents the wall thickness; The relationship equation between the expected residual stress relief rate and the heating zone width is: Where η is the stress relief rate, A1, A2, κ, and τ are parameters related to the heating method. When the local heat treatment of the ceramic sheet and the local heat treatment of the cassette furnace are used, A1, A2, κ, and τ are -0.357, 1.00, 1.445, and 1.95, respectively; when the induction heating method is used, A1, A2, κ, and τ are -0.434, 1.00, 0.935, and 1.915, respectively.

2. The method for determining the width of the heating zone for local heat treatment of large-scale pressure-bearing equipment according to claim 1 is characterized in that: In step S1, the axial temperature distribution equation T(x) of the local heat treatment insulation stage is established according to the maximum heat treatment temperature and the edge temperature of the heating zone: T(x)=T max e -λx (cosλx+sinλx)。 3. The method for determining the width of the heating zone for local heat treatment of large-scale pressure-bearing equipment according to claim 2, characterized in that: Establish the deflection differential equation during the local heat treatment cooling stage: Where w is the deflection and R is the radius.

4. A method for determining the width of a heating zone for local heat treatment of large-scale pressure-bearing equipment according to claim 3, characterized in that: In step S1, according to the boundary conditions of zero slope and zero shear at the weld center, that is, x=0, the first-order derivative and the third-order derivative of w are 0, and the local heat treatment distributed deformation expression is obtained.

5. The method for determining the width of the heating zone for local heat treatment of large-scale pressure-bearing equipment according to claim 4 is characterized in that: In step S2, the axial and circumferential residual stresses on the inner surface after local heat treatment and cooling are solved by the deflection differential equation based on the geometric relationship between strain and displacement and the physical relationship between stress and strain.

6. A method for determining the width of a heating zone for local heat treatment of large-scale pressure-bearing equipment according to claim 5, characterized in that: According to actual production, the edge temperature of the heating zone of large pressure equipment is at least greater than 60% of the heat treatment peak temperature. If the local heat treatment heater is considered as an ideal heat source, that is, the edge temperature of the heating zone is equal to the center temperature of the heating zone, under the above different boundary conditions, the heating zone width solved according to the general relationship of the heating zone width needs to meet the criteria:

7. A method for determining the width of a heating zone for local heat treatment of large-scale pressure-bearing equipment according to claim 6, characterized in that: If the production requirements do not require a stress relief rate of 70% or require a higher stress relief rate, the relationship equation between the expected stress relief rate and the heating zone width can be used to solve the heating zone width value that meets the expected stress relief rate:

8. A method for determining the width of a heating zone for local heat treatment of large-scale pressure-bearing equipment according to any one of claims 6 or 7, characterized in that: According to the criteria that the heating zone width needs to meet, when performing local heat treatment on pressure-bearing equipment, first determine the heat treatment heating and cooling rate, holding temperature and holding time based on the heat treatment object, combined with the heat treatment technical documents and standard specifications; Combined with the general relationship of the heating zone width and according to the characteristics of the local heat treatment heater, the local heat treatment heating zone width that meets the stress relief effect required by production is selected; According to the width of the local heat treatment heating zone, when the edge temperature of the heating zone is lower than the calculated temperature value, the heating rate should be reduced or the thickness or length of the insulation cotton should be increased to ensure that the edge temperature of the heating zone during the heat treatment insulation stage is greater than or equal to the value required by the general relationship of the heating zone width.

9. A method for determining the width of a heating zone for local heat treatment of large-scale pressure-bearing equipment according to claim 8, characterized in that: When using ceramic sheets or cassette furnaces for heating, the ratio of the heating zone edge temperature to the heating zone center temperature is determined to be 0.7; when using induction heating, the ratio of the heating zone edge temperature to the heating zone center temperature is selected to be 0.8; or the ratio of the heating zone edge temperature to the heating zone center temperature of different heaters can be determined through preliminary experiments.

10. A method for determining the width of a heating zone for local heat treatment of large-scale pressure-bearing equipment according to claim 9, characterized in that: During local heat treatment, the heating belts should be arranged symmetrically with the weld as the center, and the thermocouples should be welded, and the heating plates and insulation cotton should be installed. Note that insulation cotton should be arranged on both the inner and outer walls. In addition to meeting the standard requirements, temperature measuring thermocouples should be arranged at the edge of the heating belt to monitor the temperature changes at the edge of the heating belt.

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