Method for judging creep fatigue damage of stainless steel

Abstract

The invention relates to the technical field of creep fatigue damage judgment, in particular to a creep fatigue damage judgment method for stainless steel, which comprises the following steps of: 1, establishing a mechanical model in finite element analysis software, and applying a design load on the mechanical model; step 2, calculating PM and PB of each part of the material in the mechanical model according to an elastic stress analysis method; if all positions in the mechanical model meet the conditions that PM is smaller than or equal to S and PM + PB is smaller than or equal to 1.5 S, the next step is carried out; otherwise, returning to the step 1; step 3, calculating values of x and y in combination with the data calculated in the step 2; if x + y < = 1, performing the next step; otherwise, returning to the step 1; 4, calculating fatigue damage DF and creep damage DC; and 5, if the total damage D does not exceed the creep-fatigue damage envelope line, judging whether the creep-fatigue damage meets conditions or not. The creep fatigue damage judgment method for the stainless steel has the advantages of being convenient to calculate, high in precision, high in applicability and the like.

Description

Technical Field

[0001] The invention relates to the technical field of creep fatigue damage determination, and in particular to a method for determining creep fatigue damage of stainless steel. Background Art

[0002] The working environment of modern high-temperature equipment is becoming increasingly complex. In addition to the constant load of steady-state operation, the key hot-end life-limiting components are usually subjected to alternating loads caused by device start-up and shutdown and temperature fluctuations. Their service process is accompanied by severe creep-fatigue load interaction. Studies have shown that the life of metal materials under creep-fatigue interaction is much lower than the material life under a single fatigue or creep mechanism, which poses new challenges to the traditional life prediction method based on a single mechanism.

[0003] The international mainstream standards have introduced a series of design specifications and assessment criteria that can be used to evaluate creep-fatigue damage. These standards are based on the stress-strain response of half-life cycles and use linear damage accumulation as a framework to extrapolate the final life.

[0004] However, these criteria have great limitations: (1) The life prediction method based on a single stress or strain parameter ignores the physical nature of the failure of metal materials during creep fatigue; (2) The life prediction of the time fraction, ductility exhaustion and its derivative models promoted internationally are highly dispersed and have poor universality for practical engineering; (3) The damage assessment and life prediction methods based on half-life cycles cannot describe the load history of high-temperature equipment during operation.

[0005] Therefore, formulating relevant standards for creep-fatigue damage assessment and life prediction methods is the key to achieving accurate life prediction of high-temperature equipment, and is of great significance to ensuring the life reliability of high-temperature equipment. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a method for determining creep fatigue damage of stainless steel in view of the above shortcomings, which has the advantages of convenient calculation, high precision and strong applicability.

[0007] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0008] A method for determining creep fatigue damage of stainless steel comprises the following steps:

[0009] Step 1: Establish a mechanical model in the finite element analysis software and apply the design load to the mechanical model.

[0010] Step 2: Calculate the P of each material in the mechanical model according to the elastic stress analysis method. M , P B ; If all positions in the mechanical model satisfy PM ≤S,P M +P B ≤1.5S, proceed to the next step; if there are local parts in the mechanical model that do not satisfy P M ≤S,P M +P B When ≤1.5S, re-optimize the design or input parameters and return to step 1;

[0011] Among them, P M is the primary film stress intensity, P B is the bending stress intensity; S is the allowable stress of the material;

[0012] Step 3: Combine the data calculated in step 2 to calculate the values ​​of x and y.

[0013]

[0014] Among them, P QM is the secondary film stress intensity, k is the section modulus of the cross section under study, S Y is the yield strength of the material in the mechanical model;

[0015] If x+y≤1 is satisfied at all positions, proceed to the next step; if x+y≤1 is not satisfied at all positions, re-optimize the design or input parameters and return to step 1;

[0016] Step 4: Calculate the fatigue damage D in the mechanical model F 、Creep damage D C ;

[0017] Step 5: According to the relationship between the total damage D and the creep-fatigue damage envelope of the material, the creep fatigue damage of the stainless steel is determined; when (D F ,D C )≤D, the result is that the creep fatigue damage of the stainless steel meets the conditions; otherwise, the result is that the creep fatigue damage of the stainless steel does not meet the conditions.

[0018] As an improved solution, in step 4,

[0019]

[0020] Where m is the number of loop types, f i is the number of repetitions of the action of type i cycle, F i is the number of permissible cycles of type i cycle;

[0021]

[0022] Where n is the number of loop types, t j is the creep duration at time interval j, Tj is the permissible duration at time interval j.

[0023] As an improved solution, in step 2, multiple regions should be selected in the mechanical model, and stress linearization paths should be set along the shortest direction of the wall thickness to perform mechanical analysis.

[0024] As an improved solution, when selecting multiple regions in the mechanical model in step 2, the stress linearization path should be selected with reference to the stress intensity distribution cloud map of the mechanical model.

[0025] As an improved solution, when selecting the stress linearization path, select the area with the maximum stress intensity, or the high stress intensity area, or the corresponding node position in the analysis mechanical model.

[0026] The present invention adopts the above technical solution, and has the following advantages compared with the prior art:

[0027] 1. The solution of the present invention can more accurately predict the life of materials under high temperature and stress conditions. Compared with existing methods, it can more accurately evaluate the impact of creep and fatigue interaction on material properties.

[0028] 2. The present invention designs a creep fatigue damage assessment method for stainless steel, which provides guidance for the practical use of stainless steel in the future. This method is particularly suitable for 347H stainless steel.

[0029] 3. The solution of the present invention can quickly and accurately complete creep fatigue damage assessment by programming means.

[0030] The present invention is described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart of a method for determining creep fatigue damage of stainless steel according to the present invention;

[0032] Figure 2 It is a schematic diagram of the structure of the top head opening pipe in the high temperature pressure vessel in the embodiment;

[0033] Figure 3 is a three-dimensional schematic diagram of a mechanical model in an embodiment of the present invention;

[0034] Figure 4 A schematic diagram of the mesh division of the mechanical model in an embodiment of the present invention;

[0035] Figure 5 A stress intensity distribution cloud diagram of the mechanical model in the embodiment of the present invention;

[0036] Figure 6 A stress intensity distribution cloud diagram of the mechanical model in the embodiment of the present invention;

[0037] Figure 7 It is the creep-fatigue damage envelope of stainless steel 347H. DETAILED DESCRIPTION

[0038] Example

[0039] A method for determining creep fatigue damage of stainless steel comprises the following steps:

[0040] Step 1: Establish a mechanical model in the finite element analysis software and apply the design load to the mechanical model.

[0041] like Figure 2 As shown, in this embodiment, the finite element analysis software is ansys as an example, and the mechanical model is described by taking the top head opening pipe structure in a high-temperature pressure vessel as an example, wherein the head is an elliptical head and the material is stainless steel 347H. The geometric dimensions of the top head opening pipe in the high-temperature pressure vessel are as follows: Figure 2 As shown, Figure 2 The units of the dimensions shown in are all mm. According to the symmetry of the load and structure, the mechanical model of this embodiment is selected as follows: Figure 2 A quarter model of the top head opening nozzle in a medium and high temperature pressure vessel is modeled, such as Figure 3 In the modeling process, the flange hole and insulation layer of the top pipe opening are ignored, and the connection between the head and the forged pipe is provided with local details such as chamfers.

[0042] like Figure 4 As shown, in this embodiment, preferably, when the mechanical model is meshed, four layers of units are set in the thickness direction of the shell, the total number of mesh nodes is 208051, and the total number of units is 43888.

[0043] The working condition of the mechanical model is that the temperature of all shell inner surfaces is 585℃, and the equivalent convection heat transfer coefficient of all shell outer surfaces is set to 7.85×10 -7 W / mm 2 ℃, the container only bears an internal pressure of 0.3MPa, and the corresponding equivalent axial pressure of 0.623MPa is applied on the flange end face. The material parameters of 347H are shown in Table 1.

[0044] Table 1 Material parameters of 347H

[0045]

[0046]

[0047] According to the material parameters in Table 1, the material parameters at the required temperature can be obtained by interpolation.

[0048] Adding plastic strain data of materials to a mechanical model is a common operation in the prior art. The present invention can adopt a method of adding plastic strain data of materials to a mechanical model in the prior art.

[0049] Preferably, in this embodiment, when the plastic strain data of the material is added, the stress (σ) strain (ε) curve of the material is calculated by the following constitutive formula.

[0050] The total strain ε is divided into elastic strain ε e , plastic strain ε p and creep strain ε c Three parts. The elastic strain calculation formula is: ε e =σ / E. Where σ is stress and E is elastic modulus.

[0051] The plastic strain calculation formula is as follows:

[0052] ε p =γ1+γ2

[0053]

[0054] ε p ′=2.0×10 -5

[0055] m2=0.75(1-R)

[0056]

[0057] ε ys =0.002

[0058] K=1.5R 1.5 -0.5R 2.5 -R 3.5

[0059]

[0060] Where T is temperature, σ ys is the yield strength, σ uts For tensile strength.

[0061] Table 2 Values ​​of parameters A0~A4, B0~B4 in plastic strain calculation.

[0062]

[0063] The creep strain calculation formula is as follows:

[0064]

[0065]

[0066] S=log 10 [σ]

[0067] Where T is temperature and t is creep time.

[0068] Table 3 Values ​​of parameters C0~C4, D0~D4 in creep strain calculation

[0069]

[0070] Step 2: Calculate PM and PB of the material in the mechanical model according to the elastic stress analysis method; if all positions in the mechanical model satisfy PM≤S, PM+PB≤1.5S, proceed to the next step; if there are parts in the mechanical model that do not satisfy PM≤S, PM+PB≤1.5S, re-optimize the design or input parameters and return to step 1. Among them, PM is the primary film stress intensity, PB is the bending stress intensity; S is the allowable stress of the material.

[0071] The stress intensity distribution cloud diagram is obtained by calculation and analysis in the finite element analysis software. Figure 5 shown. Figure 5 The points with greater stress in the mechanical model are clearly shown.

[0072] In actual engineering calculations, the areas of maximum stress intensity, other high stress intensity areas, and corresponding nodes of the areas of concern in the mechanical model are usually analyzed, and the stress linearization path is set along the shortest direction of the wall thickness to perform mechanical analysis. Although it is impossible to analyze all parts one by one in actual applications, as many points with higher risk of creep fatigue damage as possible should be selected for analysis. Figure 6 As shown in FIG. 1 , six stress linearization paths are set in the mechanical model. Figure 5 The stress intensity distribution cloud diagram is only Figure 6 Taking path 3 passing through the maximum stress area as an example, the method for determining creep fatigue damage of stainless steel is illustrated.

[0073] By linearizing the stress on path 3, we can obtain P in path 3 M =15.429MPa, P M +P B =65.179MPa. The above calculation results can be directly exported in the stress analysis software.

[0074] Of course, the above data can also be obtained by calculation through the calculation formula. The specific calculation method first selects the point with the largest stress on path 3 for conservative processing.

[0075]

[0076] According to the above formula, we can calculate:

[0077]

[0078] According to the operating temperature, the allowable stress S of the material can be calculated by linear interpolation to be 100.92MPa. M =15.253MPa<100.92MPa,P M +P B =57.62MPa<1.5×100.92MPa=151.38MPa, which meets the conditions.

[0079] Step 3: Combine the data calculated in step 2 and calculate x and y according to the following formula.

[0080]

[0081] Among them, P QM is the secondary film stress intensity, k is the section modulus of the cross section under study, S Y is the yield strength of the material.

[0082] If x+y≤1 is satisfied, proceed to the next step; if x+y≤1 is not satisfied, re-optimize the design or input parameters and return to step one.

[0083] By linearizing the stress on the path, we can obtain P B , P QM They are 44.073MPa and 1.2386×10 -2 MPa, the calculation results can be directly exported in the software. Similarly, the point with the largest stress on the path is selected for conservative processing. The specific calculation process is as follows:

[0084]

[0085] According to the operating temperature of 585℃, the material data S can be obtained by linear interpolation. Y =114.6MPa; Section coefficient k = 1.5. Calculate the values ​​of x and y according to the following formulas.

[0086]

[0087] Therefore, x = 0.509028, y = 1.1668 × 10 -4 At this time, x+y=0.509<1, which meets the conditions and proceeds to the next step.

[0088] Step 4: Calculate the fatigue damage D in the mechanical model F 、Creep damage D C .

[0089] The fatigue damage D is calculated according to the following formula: F and creep damage D C Perform calculations.

[0090]

[0091] Where m is the number of loop types, f i is the number of repetitions of the action of type i cycle, F i is the number of permissible cycles for type i cycles.

[0092]

[0093] Where n is the number of loop types, t j is the creep duration at time interval j, T j is the permissible duration for time interval j.

[0094] In this embodiment, m=1, n=1, and the number of cycles f i =4000, the number of cycles allowed is F i =1000000, creep duration t j =100000 hours, allowable duration T j = 300,000 hours. Calculated: D F =4000 / 1000000=0.004,D C =100000 / 300000=0.333.

[0095] Step 5: According to the relationship between the total damage D and the creep-fatigue damage envelope of the material, the creep fatigue damage of the stainless steel is determined; when (D F ,D C )≤D, the result is that the creep fatigue damage of the stainless steel meets the conditions; otherwise, the result is that the creep fatigue damage of the stainless steel does not meet the conditions.

[0096] The value of D can be obtained by Figure 7 The creep-fatigue damage envelope of 347H is determined. F , D C ) corresponds to the point (0.004, 0.333) located below the curve, so (D F ,D C )<D, and the result is that the creep fatigue damage of stainless steel meets the conditions.

[0097] Otherwise, the result is that the creep fatigue damage of stainless steel does not meet the conditions.

[0098] In summary, the creep fatigue damage determination method of stainless steel of the present invention firstly performs stress limit assessment and strain limit assessment on the structure, and after the assessment is passed, creep damage and fatigue damage are calculated respectively, and then creep-fatigue damage is assessed according to the creep-fatigue damage envelope; the method has the advantages of convenient calculation, high precision, strong applicability, etc.

[0099] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for determining creep fatigue damage of stainless steel, characterized in that: The following steps are involved: Step 1: Establish a mechanical model in the finite element analysis software and apply the design load to the mechanical model. Step 2: Calculate the P of each material in the mechanical model according to the elastic stress analysis method. M , P B ; If all positions in the mechanical model satisfy P M ≤S,P M +P B ≤1.5S, proceed to the next step; if there are local parts in the mechanical model that do not satisfy P M ≤S,P M +P B When ≤1.5S, re-optimize the design or input parameters and return to step 1; Among them, P M is the primary film stress intensity, P B is the bending stress intensity; S is the allowable stress of the material; Step 3: Combine the data calculated in step 2 to calculate the values ​​of x and y. Among them, P QM is the secondary film stress intensity, k is the section modulus of the cross section under study, S Y is the yield strength of the material in the mechanical model; If x+y≤1 is satisfied at all positions, proceed to the next step; if x+y≤1 is not satisfied at all positions, re-optimize the design or input parameters and return to step 1; Step 4: Calculate the fatigue damage D in the mechanical model F 、Creep damage D C ; Step 5: According to the relationship between the total damage D and the creep-fatigue damage envelope of the material, the creep fatigue damage of the stainless steel is determined; when (D F ,D C )≤D, the result is that the creep fatigue damage of the stainless steel meets the conditions; otherwise, the result is that the creep fatigue damage of the stainless steel does not meet the conditions.

2. The method for determining creep fatigue damage of stainless steel according to claim 1, characterized in that: In the step 4, Where m is the number of cycle types, fi is the number of action repetitions of type i cycle, and Fi is the number of permissible cycles of type i cycle; Where n is the number of cycle types, tj is the creep duration at time interval j, and Tj is the permissible duration at time interval j.

3. The method for determining creep fatigue damage of stainless steel according to claim 1, characterized in that: In the step 2, multiple regions should be selected in the mechanical model, and stress linearization paths should be set along the shortest direction of the wall thickness to perform mechanical analysis.

4. The method for determining creep fatigue damage of stainless steel according to claim 3, characterized in that: When multiple regions are selected in the mechanical model in step 2, the stress linearization path should be selected with reference to the stress intensity distribution cloud diagram of the mechanical model.

5. The method for determining creep fatigue damage of stainless steel according to claim 4, characterized in that: When selecting the stress linearization path, select the area with the maximum stress intensity, or the high stress intensity area, or the corresponding node position in the analysis mechanical model.

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