CN115130348B - Calculation method for maximum heating rate of 9% Cr heat-strengthening steel thick-wall pipeline after local post-welding heat treatment through medium-frequency induction heating - Google Patents
Calculation method for maximum heating rate of 9% Cr heat-strengthening steel thick-wall pipeline after local post-welding heat treatment through medium-frequency induction heating Download PDFInfo
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Abstract
The invention discloses a calculation method of a maximum heating rate of heat treatment after local welding of a 9% Cr heat-strength steel thick-wall pipeline through medium-frequency induction heating. The method comprises the following steps: firstly, establishing a transient model of a heat treatment temperature field after local welding of a 9% Cr heat-strength steel thick-wall pipeline through medium-frequency induction heating, and obtaining the maximum axial temperature gradient of different characteristic points in the whole heat treatment process; then obtaining the maximum axial temperature gradient allowed by each characteristic point of the pipeline axial through an equation of the axial temperature gradient and the induced stress; and finally, calculating the maximum heating rate of the pipelines with different wall thicknesses under the temperature gradient according to the maximum axial temperature gradient allowed by each characteristic point of the pipeline axial direction, fitting to obtain a maximum heating rate formula, and calculating to obtain the maximum heating rate according to the pipeline wall thickness. The calculation method is suitable for the medium-frequency induction heating process, calculates the maximum heating rate, reduces the heat treatment time as much as possible under the condition of ensuring safe operation, shortens the construction period and reduces the heat treatment cost.
Description
Technical Field
The invention belongs to the technical field of heat-resistant steel welding, and particularly relates to a calculation method for the maximum heating rate of a 9% Cr heat-resistant steel thick-wall pipeline when a medium-frequency induction heating mode is adopted for carrying out local postweld heat treatment, which can be suitable for welding novel 9% Cr heat-resistant steel thick-wall pipelines such as P91, P92 and G115.
Background
The novel 9% Cr hot strength steel has good high-temperature oxidation resistance, high-temperature creep resistance, high thermal conductivity and low linear expansion coefficient, and is an ideal material for manufacturing thick wall parts such as a high-parameter thermal power unit header, a main steam pipe and the like. However, the welding property of the novel 9% Cr hot-strength steel is poor, the welded joint of the novel 9% Cr hot-strength steel is subjected to phase transformation under the action of a welding thermal field, and a hard and brittle martensitic structure is formed after cooling, so that the toughness in a welded state is lower, the joint structure must be improved through post-welding heat treatment in engineering, and residual stress is eliminated. The conventional post-welding heat treatment method is flexible ceramic resistance heating during field construction, and has the advantages of small heating power, poor heating uniformity, large heating width and large thermal damage to the base metal. In addition, the method has poor heating control, so that a smaller temperature rising rate is generally selected in engineering, which increases the construction period. In view of the above disadvantages, the application of the medium frequency induction heating method in the heat treatment of novel 9% Cr hot-strength steel pipelines is focused, and the method has the advantages of good heating uniformity, high heating rate, controllable process and less damage to the parent metal.
The rate of temperature rise is an important parameter to be selected during post-weld heat treatment. The heating rate is improved, so that the heating time of the postweld heat treatment can be reduced, the construction period is shortened, the heat treatment efficiency is improved, and the cost is saved. However, the temperature rising rate is too high, so that an obvious temperature gradient can be generated in the axial direction of the pipeline, and if the axial temperature gradient is too high, new residual stress can be generated, so that the safe operation of the pipeline is affected. Therefore, the heating rate has an optimal value, namely the maximum heating rate, and the efficiency and the safe operation of the pipeline can be both achieved. At present, although some researches are carried out on an optimization method for the post-welding heat treatment heating rate of the 9% Cr heat-resistant steel pipeline, for example, patent 2019111714440 proposes an optimization calculation method for the post-welding heat treatment heating rate of the 9% Cr heat-resistant steel pipeline, but the optimization calculation method is only suitable for flexible ceramic resistance heating. For medium frequency induction heating, the heating power is high, the temperature rising process is well controlled, the axial temperature distribution of the pipeline under the same heat treatment parameters is more uniform, and the higher temperature rising rate can be allowed. The electric power industry standard DL/T819-2019 proposes a calculation formula 8000/delta (DEG C/h, delta is the wall thickness of a pipeline and mm) for selecting the temperature rising rate according to the wall thickness, but the calculation result of the formula is more conservative. In engineering, in order to shorten the construction period, it is necessary to increase the temperature rise rate as much as possible and shorten the heat treatment time. Therefore, the method for calculating the maximum heating rate of the heat treatment after the local welding of the medium-frequency induction heating of the 9% Cr heat-strength steel pipeline has obvious practical value.
Disclosure of Invention
The invention aims to provide a calculation method for the maximum heating rate of the heat treatment after the local welding of the medium-frequency induction heating of a 9% Cr heat-resistant steel pipeline, which is suitable for the medium-frequency induction heating process, and can obtain the maximum heating rate, reduce the heat treatment time as much as possible, shorten the construction period and reduce the heat treatment cost under the condition of ensuring safe operation.
In order to solve the technical problems, the invention adopts the following technical scheme:
the method for calculating the maximum heating rate of the heat treatment after the local welding of the medium-frequency induction heating of the 9% Cr heat-strengthening steel thick-wall pipeline comprises the following steps:
step 1: establishing a transient model of a heat treatment temperature field after local welding of the medium-frequency induction heating of the 9% Cr heat-strength steel thick-wall pipeline to obtain the maximum axial temperature gradient of different characteristic points in the whole heat treatment process;
Step 2: obtaining the maximum axial temperature gradient allowed by each characteristic point of the pipeline axial direction through an equation of the axial temperature gradient and the induced stress;
Step 3: and (3) calculating the maximum heating rate of the pipelines with different wall thicknesses under the temperature gradient according to the maximum axial temperature gradient allowed by each characteristic point of the pipeline axial direction obtained in the step (2), fitting the calculation results to obtain a maximum heating rate formula, and calculating the maximum heating rate according to the pipeline wall thickness.
According to the above scheme, in the step 1, a transient model is built by finite element software.
According to the above scheme, in the step 1, the feature points are as follows: the edge of the heating zone of the outer wall of the pipeline and the center of the welding line of the outer wall of the pipeline to the midpoint of the edge of the heating zone.
According to the above scheme, in the step 1, the proposed transient model specific method is as follows:
step 1.1, selecting a pipeline with a certain specification, selecting parameters according to a power industry standard DL/T819-2019, and modeling the specification and the parameters of the pipeline by using finite element software;
Step 1.2, coupling by utilizing an electromagnetic model and a heat transfer model, inputting transient time domain excitation, applying additional heating power excitation after a temperature control point reaches a Curie temperature, and enabling the temperature control point to reach the temperature control temperature to obtain a transient model of a heat treatment temperature field after intermediate frequency induction heating local welding;
And 1.3, selecting corresponding characteristic points of the outer wall of the pipeline by using the obtained intermediate frequency induction heating local post-welding heat treatment temperature field transient model, and determining time nodes of maximum axial temperature gradients of the characteristic points in the whole heat treatment process to obtain the maximum axial temperature gradients of different characteristic points in the whole heat treatment process.
Preferably, in the step 1.1, the specification of the pipeline is wall thickness; the parameters of the pipeline are heating width, heat preservation width, alternating current frequency, number of turns of the induction coil, turn spacing and heating rate.
Preferably, in the step 1.3, when the central point of the welding seam on the outer wall of the pipeline just reaches the steady-state temperature, the axial temperature gradient of the feature point is the largest, namely the time node of the largest axial temperature gradient of the feature point in the whole heat treatment process.
According to the above scheme, in the step 2, the specific method for obtaining the maximum axial temperature gradient allowed by each feature point in the axial direction of the pipeline comprises the following steps:
step 2.1, calculating deflection of the axisymmetric cylinder due to axial temperature gradient, and solving the deflection according to the Fourier heat transfer theorem to obtain the maximum bending stress of the axisymmetric cylinder;
step 2.2, solving according to the maximum bending stress formula obtained in step 2.1 to obtain the relation between the maximum stress and the axial temperature gradient of each characteristic point of the outer wall of the pipeline;
And 2.3, taking the value of the formula obtained in the step 2.2, and calculating to obtain the maximum axial temperature gradient allowed under the safe operation condition of each characteristic point of the outer wall of the pipeline.
Preferably, in the step 2.1, the formula of the maximum bending stress is:
Wherein E is elastic modulus, h is coefficient, R is radius of the inner wall of the pipeline, v is Poisson's ratio, and T (x) is axial temperature distribution of the pipeline.
According to the above scheme, in the step 3, the formula of the maximum heating rate is:
v=0.03081t2-6.8416t+454.73273 (12)
wherein v (DEG C/h) is the heating rate; t (mm) is the wall thickness of the pipeline.
According to the above scheme, in the step 3, the specific method is as follows:
step 3.1, calculating the maximum heating rate of pipelines with different specifications (wall thickness) according to the temperature field transient model obtained in the step 1 and the maximum axial temperature gradient allowed by each characteristic point obtained in the step 2;
And 3.2, fitting the data obtained in the step 3.1 to obtain a selection curve and a calculation formula of the maximum heating rate.
The beneficial effects of the invention are as follows:
The invention provides a calculation method of the maximum heating rate of the heat treatment after the local welding of the medium-frequency induction heating of the 9% Cr heat-strength steel thick-wall pipeline, which is suitable for the medium-frequency induction heating process, can obtain the maximum heating rate of pipelines with different wall thicknesses on the premise of ensuring the axial temperature gradient, can reduce the heat treatment time as much as possible on the basis of ensuring the safe operation, shortens the construction period, improves the heat treatment efficiency, reduces the heat treatment cost, has important industrial application value, and can be suitable for welding novel 9% Cr heat-strength steel thick-wall pipelines such as P91, P92, G115 and the like.
Drawings
FIG. 1 is a graph fitted with maximum heating rates for pipes of different wall thicknesses obtained in the example of the present invention.
FIG. 2 is a simulated flow chart of a post induction heating heat treatment heating phase in an embodiment of the invention.
Detailed Description
The technical scheme of the invention is further specifically described below by way of examples and with reference to the accompanying drawings.
The embodiment of the invention provides a calculation method of the maximum heating rate of the medium-frequency induction heating local postweld heat treatment of a 9% Cr hot-strength steel pipeline, which comprises the steps of firstly establishing a medium-frequency induction heating local postweld heat treatment temperature field transient model of the 9% Cr hot-strength steel pipeline, and finding out the node of the maximum axial temperature gradient in the whole heat treatment process; and then determining the maximum axial temperature gradient allowed under the safe operation condition through an equation of the axial temperature gradient and the induced stress. Based on the established transient model, calculating the maximum heating rate of pipelines with different specifications under the allowable maximum axial temperature gradient, fitting the calculation result, and finally obtaining a calculation formula of the maximum heating rate of the heat treatment after the medium-frequency induction heating local welding; the method specifically comprises the following steps:
In the step 1, a transient model of a heat treatment temperature field after local welding of medium-frequency induction heating is established, the edge of a heating zone of the outer wall of a pipeline and the midpoint between the center of a welding seam of the outer wall of the pipeline and the edge of the heating zone are taken as characteristic points, and the maximum axial temperature gradient time node of the characteristic points in the whole heat treatment process is calculated, wherein the specific method comprises the following steps:
Step 1.1, modeling pipeline specifications and parameters in finite element software when the P91 pipeline OD 540X 85mm with the specification is 800mm, the heat preservation width is 1000mm, the frequency of the input alternating current is 1500Hz, the number of turns and the inter-turn distance of an induction coil are 17 and 30mm, and the heating rate is controlled to be 100 ℃/h;
Step 1.2, coupling by utilizing an electromagnetic model and a heat transfer model, inputting transient time domain excitation to obtain a local post-welding heat treatment transient model of intermediate frequency induction heating, and applying heating power which is adjusted in real time according to the temperature of a temperature control point to a heating zone after the temperature of the temperature control point reaches 750 ℃ of a Curie point so as to enable the temperature of the temperature control point to reach 760 ℃;
And 1.3, utilizing the obtained intermediate frequency induction heating local post-welding heat treatment temperature field transient model, taking the edge of the heating zone of the outer wall of the pipeline and the midpoint between the center of the welding seam of the outer wall of the pipeline and the edge of the heating zone as characteristic points, and determining the maximum axial temperature gradient occurrence time node of each characteristic point in the whole heat treatment process, particularly when the center point of the welding seam of the outer wall of the pipeline just reaches the steady-state temperature, wherein the axial temperature gradient of the characteristic points is the maximum.
In the step 2, the maximum axial temperature gradient allowed under the safe operation condition is determined by an equation of the axial temperature gradient and the induced stress, and the specific method is as follows:
Step 2.1, calculating the deflection ω of the axisymmetric cylinder due to the axial temperature gradient can be expressed as:
ω″′(x)+4β4ω(x)=-4β4αR[T(x)-T] (1)
Wherein alpha is the heat transfer coefficient of the material, R is the radius of the inner wall of the pipeline, T is the ambient temperature, and T (x) is the temperature distribution along the axial direction of the pipeline.
Beta 4 can be expressed as:
Wherein, v is poisson ratio, t is pipeline wall thickness;
the maximum bending stress for an axisymmetric cylinder can be expressed as:
σxmax=6Mx/t (3)
Wherein:
Mx=-Dω″(x) (4)
Wherein:
wherein E is the elastic modulus;
From the fourier heat transfer law, equation 2 can be solved:
ω(x)=-αR[T(x)-T] (6)
taking the second derivative of equation 6 and substituting it into equation 3, the maximum bending stress can be expressed as:
And 2.2, calculating the relation between the bending stress and the axial temperature gradient of each characteristic point of the outer wall of the pipeline, wherein the temperature of the central point of the welding seam of the outer wall of the pipeline can be recorded as T 0, the temperatures of the characteristic points are T 1 and T 2, T 1 refers to the temperature of the edge of the heating area of the outer wall, and T 2 refers to the temperature from the center of the welding seam of the outer wall to the midpoint of the edge of the heating area. The distance from the center point of the outer wall weld of the pipeline to the edge of the heating zone is denoted as s, and the distance from the outer wall surface to the center point of the outer wall weld is denoted as x, then the temperature distribution in the axial direction of the pipeline can be expressed as:
substituting the second derivative of equation 8 into equation (7) yields:
Also because the pipe is cylindrical, there are:
Step 2.3, the formula obtained in step 2.2 is valued, and the reciprocal of the axial temperature gradient can be expressed as:
The maximum bending stress is twice the yield strength at constant temperature (namely temperature control temperature), and other material attribute values are substituted, so that the maximum allowable axial temperature gradient at the edge of the heating area of the pipeline can be obtained to be about 2.1; and replacing T 1 in the formula with T 2 to obtain the maximum allowable axial temperature gradient from the center of the welding line of the outer wall of the pipeline to the midpoint of the edge of the heating zone of the outer wall of the pipeline, wherein the maximum allowable axial temperature gradient is about 1.26.
In step 3, calculating the maximum heating rate of the pipelines with different specifications (wall thicknesses) under the condition according to the maximum axial temperature gradient allowed by each characteristic point of the pipeline axial direction obtained in step 2, and fitting the calculation result, wherein the specific method comprises the following steps:
Step 3.1, calculating the maximum heating rate of the pipelines with different wall thicknesses (20 mm-120 mm) according to the transient model obtained in the step 1 and the maximum allowable axial temperature gradient at the edge of the pipeline heating zone obtained in the step 2, wherein the maximum allowable axial temperature gradient is about 2.1 from the center of a welding seam of the outer wall of the pipeline to the midpoint of the edge of the pipeline heating zone;
Step 3.2, fitting the data obtained in the step 3.1 to obtain a selection curve with the maximum heating rate, wherein the obtained fitting formula is as follows:
v=0.03081t2-6.8416t+454.73273 (12)
wherein v (DEG C/h) is the heating rate; t (mm) is the wall thickness of the pipeline. And calculating the maximum heating rate under different wall thicknesses through a fitting formula.
Examples
For P91 pipelines with two specifications of OD 540X 35mm and OD 540X 55mm, the maximum allowable axial temperature gradient and the maximum heating rate during the local postweld heat treatment of the medium-frequency induction heating are calculated according to the method of the invention. And carrying out a post-welding heat treatment test of the two specifications of pipelines at the calculated maximum heating rate, actually measuring the axial temperature gradient, and comparing the calculated result of the allowable maximum value of the axial temperature gradient with the actually measured value to verify the accuracy of the invention. The comparison result is shown in Table 1, and it can be seen that under the maximum heating rate given by the method, the deviation between the calculated value and the measured value of the axial temperature gradient of the P91 pipeline with two specifications is very small, thus indicating the accuracy of the calculated result of the method.
TABLE 1 verification of the accuracy of the calculation results of the maximum heating Rate of the invention
The maximum heating rates of the P91 pipelines with the two specifications obtained by the invention are compared with the maximum heating rates obtained according to the power industry standard DL/T819-2019, and the maximum heating rates are shown in Table 2.
TABLE 2 comparison of the calculation results of the maximum heating rate of the invention with the existing standard
Pipeline specification/mm | OD540×35mm | OD540×55mm |
The maximum temperature rising rate/. Degree.C/h obtained by the invention | 253.0 | 171.7 |
DL/T819-2019 recommended rate of rise/DEG C/h | 228.6 | 145.5 |
The results show that the maximum heating rate obtained by using the invention is greater than the power industry standard, and the improvement amplitude is very obvious for pipelines with two specifications. Therefore, the invention can save heat treatment time, shorten construction period and reduce cost.
The specific embodiments described herein are offered by way of example only to illustrate the spirit of the invention. Those skilled in the art may make various modifications or additions to the described embodiments or substitutions thereof without departing from the spirit of the invention or exceeding the scope of the invention as defined in the accompanying claims.
Claims (7)
1. The calculation method of the maximum heating rate of the heat treatment after the local welding of the medium-frequency induction heating of the 9% Cr heat-strengthening steel thick-wall pipeline is characterized by comprising the following steps of:
Step 1: establishing a transient model of a heat treatment temperature field after local welding of the medium-frequency induction heating of the 9% Cr heat-strength steel thick-wall pipeline to obtain the maximum axial temperature gradient of different characteristic points in the whole heat treatment process;
Step 2: obtaining the maximum axial temperature gradient allowed by each characteristic point of the pipeline axial direction through an equation of the axial temperature gradient and the induced stress; the specific method comprises the following steps:
step 2.1, calculating deflection of the axisymmetric cylinder due to axial temperature gradient, and solving the deflection according to the Fourier heat transfer theorem to obtain the maximum bending stress of the axisymmetric cylinder;
step 2.2, solving according to the maximum bending stress formula obtained in step 2.1 to obtain the relation between the maximum stress and the axial temperature gradient of each characteristic point of the outer wall of the pipeline;
Step 2.3, the formula obtained in the step 2.2 is valued, and the maximum axial temperature gradient allowed under the safe operation condition of each characteristic point of the outer wall of the pipeline is obtained through calculation;
step 3: calculating the maximum heating rate of the pipelines with different wall thicknesses under the temperature gradient according to the maximum axial temperature gradient allowed by each axial characteristic point of the pipeline obtained in the step 2, fitting the calculation results to obtain a maximum heating rate formula, and calculating the maximum heating rate according to the pipeline wall thickness; wherein:
the formula of the maximum heating rate is:
In the method, in the process of the invention, The heating temperature rise rate is the heating temperature rise rate; t is the wall thickness of the pipeline.
2. The method according to claim 1, wherein in step 1, the transient model is built by finite element software.
3. The method according to claim 1, wherein in the step 1, the feature points are: the edge of the heating zone of the outer wall of the pipeline and the center of the welding line of the outer wall of the pipeline to the midpoint of the edge of the heating zone.
4. The method according to claim 1, wherein in the step 1, the proposed transient model concrete method is:
step 1.1, selecting a pipeline with a certain specification, selecting parameters according to a power industry standard DL/T819-2019, and modeling the specification and the parameters of the pipeline by using finite element software;
Step 1.2, coupling by utilizing an electromagnetic model and a heat transfer model, inputting transient time domain excitation, applying additional heating power excitation after a temperature control point reaches a Curie temperature, and enabling the temperature control point to reach the temperature control temperature to obtain a transient model of a heat treatment temperature field after intermediate frequency induction heating local welding;
And 1.3, selecting corresponding characteristic points of the outer wall of the pipeline by using the obtained intermediate frequency induction heating local post-welding heat treatment temperature field transient model, and determining time nodes of maximum axial temperature gradients of the characteristic points in the whole heat treatment process to obtain the maximum axial temperature gradients of different characteristic points in the whole heat treatment process.
5. The method according to claim 4, wherein in the step 1.1, the pipeline specification is wall thickness; the parameters of the pipeline are heating width, heat preservation width, alternating current frequency, number of turns of the induction coil, turn spacing and heating rate.
6. The method according to claim 4, wherein in the step 1.3, when the central point of the welding seam of the outer wall of the pipeline just reaches the steady-state temperature, the axial temperature gradient of the characteristic point is the maximum, namely the time node of the maximum axial temperature gradient of the characteristic point in the whole heat treatment process.
7. The method according to claim 1, wherein in the step 3, the specific method is as follows:
step 3.1, calculating the maximum heating rate of pipelines with different wall thicknesses according to the temperature field transient model obtained in the step 1 and the maximum axial temperature gradient allowed by each characteristic point obtained in the step 2;
And 3.2, fitting the data obtained in the step 3.1 to obtain a selection curve and a calculation formula of the maximum heating rate.
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CN105844027A (en) * | 2016-03-28 | 2016-08-10 | 武汉工程大学 | Method for rising temperature of large-diameter thick flange joint at high temperature |
CN108866314A (en) * | 2018-07-04 | 2018-11-23 | 苏州热工研究院有限公司 | A method of guaranteeing that major diameter thick-walled pipe weld seam is heat-treated uniformity |
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