Zhao Cui Lu Xiaofeng (School of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing 210009, China)
Abstract: Using the finite element software ABAQUS, a sequential coupled thermal stress calculation program for fillet welds was developed. The distribution of stress field of plate welded joints in hydrogenation high pressure heat exchangers was studied. Based on the numerical simulation results, combined The orthogonal test design method was used to analyze the influence of welding process current, arc voltage, welding speed and preheating temperature on the welding residual stress field, and find a set of optimal welding process parameters to minimize the welding residual stress. The results show that The welding speed has the greatest influence on the welding residual stress. The highest value of the residual stress of the fillet weld is concentrated near the weld and the heat affected zone. The research result is to optimize the welding process of the plate welded seal of the hydrogenation and high pressure heat exchanger, and prevent the welded seal. The cracking failure of the joint provides a theoretical basis.
Key words: superhydrogenation heat exchanger; sealing joint; welding process parameters; orthogonal optimization test; residual stress
CLC number: TG115.28 Document code: A Article ID: 0253-360X(2009)07-0101-04
0 Preface
With the development of petroleum and chemical equipment towards large-scale and high-parameters, heat exchangers are indispensable equipment for energy balance and energy recovery of plant systems. Their long-term safe operation has been increasingly valued by equipment management engineers. The high-pressure heat exchanger in the hydrogenation unit is the main equipment of the unit, and is responsible for the heat exchange between the feedstock oil and the reaction-generated oil. The pressure and temperature parameters of the hydrogenation heat exchanger are high, and the medium is flammable and explosive. Hydrogen, once the heat exchanger leaks, the consequences will be very serious. Therefore, the sealing structure of the hydrogenation high-pressure heat exchanger uses a fillet weld welded joint [1-3], which is a zero-leakage seal. The seal is reliable and has a long service life. However, in the long-term high temperature, high pressure and hydrogen environment, the weld structure is prone to cracks and even cracks and cause leakage [4]. Therefore, the residual stress at the welded joint is studied [5] and its influence. Factors, and optimization of welding process parameters, have important engineering practical significance for extending the life of the sealing structure and ensuring long-term safe operation of the hydrogenation unit.
Four welding process parameters of welding current, arc voltage, welding speed and welding preheating temperature were selected as test objects. A set of orthogonal test schemes of L25(56) was designed and used by large nonlinear finite element software ABAQUS[6]. The welding residual stress of the welded joint of hydrogen high pressure heat exchanger was numerically simulated. The orthogonal analysis of the test results obtained the optimum process parameters of a set of fillet weld welded joint electrode arc welding, in order to further study the welded joint The cracking provides a reference.
1 Calculation model establishment
1.1 Geometrical model of fillet weld welded joint
The research object is the flat-plate welded sealing structure of the hydrogenation high pressure heat exchanger tube box flange. The material of the tube box flange and cover plate is 16Mo5. The solid round plate size is 1 212 mm×10 mm, and the material is 0Cr18Ni9.
The surface of the pipe box flange has a 5 mm thick 0Cr18Ni9 surfacing layer. The calculation model is shown in Figure 1. It is assumed that the material is a linearly strengthened elastoplastic model [7-9]. Figure 2 is a detailed view of the fillet weld of the welded sealing structure. The welding is completed by three weldings, and the welding process parameters are shown in Table 1 [10].
The first welding is tungsten gas shielded welding, and the process parameters are all determined. Therefore, the orthogonal test design is only for the second and third electrode arc welding.
1.2 Finite Element Model of Plate Welding Seal Structure
Using the finite element software ABAQUS 6.5, a sequential coupled thermal stress calculation program was developed to simulate the welding residual stress of the plate welded joint structure. Firstly, thermal analysis was performed, and the calculation results of the temperature fields of each node were output to the result file as the force analysis. Pre-defined field, in the force analysis process, read the temperature of each node from this predefined field, and perform interpolation calculation. Simulate multi-pass welding by applying the model change remove or add by applying the endogenous heat to simulate the heating of the arc. The situation. Thermal analysis and force analysis use the same elements and nodes.
Taking into account the symmetry of the model, take 1/2 of the model for analysis. The model is built with the central plane as the symmetry plane, and the structural grid is used to generate 2 966 nodes and 2 815 units. The thermal simulation of ABAQUS uses four-node symmetry. Unit DCAX4, the residual stress simulation uses CAX4 unit, the meshing is shown in Figure 3. The temperature near the weld changes greatly, the mesh is dense; the temperature change is small away from the weld, and the mesh is sparse.
2 Orthogonal test design and results analysis
2.1 Orthogonal test design
In order to reduce the number of simulations as much as possible, the effects of welding current, arc voltage, welding speed and welding preheating temperature on welding residual stress are investigated. Orthogonal test method should be used [11]. Take the above four welding process parameters as factors, each The factor takes 5 values ​​as the level, and a total of 25 sets of tests are performed. The L25(56) orthogonal table designed in this way is shown in Table 2.
2.2 Calculation and analysis of orthogonal test results
Taking the extreme value of stress in each set of simulation results as the target, the influence of welding current, arc voltage, welding speed and welding preheating temperature on welding residual stress can be analyzed according to the range analysis method. The results of orthogonal analysis are shown in Table 3. It is shown that the unit of σmises stress is MPa. Ki (i = 1, 2, 3, 4, 5) is the sum of the σmises stress extremes of each column factor at each level, the unit is MPa.ki (i= 1 2,3,4,5) is the average value of Ki, the unit is MPa; Δi is the extreme difference Δi=maxki-minki, the unit is MPa.
Take the first column factor in Table 3 as an example.
K1I= K1+K2+K3+K4+K5
K2I= K6+K7+K8+K9+K10
K3I= K11+K12+K13+K14+K15
K4I= K16+K17+K18+K19+K20
K5I= K21+K22+K23+K24+K25
Table 3 lists the calculation of the stress extreme value and Δi in each set of simulation results. From Table 3, △III>△IV>△I>△II can be obtained. This shows that among the four factors, the welding speed versus the welding residue The influence of stress is the largest, the influence of preheating temperature is second, the influence of welding current is again, and the influence of arc voltage is the smallest. Therefore, the optimal process parameter with the minimum residual stress is the combination of III1IV5I4II4, ie welding speed 21 cm/min, preheating The temperature is 240 ° C, the welding current is 114 A, and the arc voltage is 29 V.
3 Numerical simulation results analysis
3.1 Numerical simulation results of optimal welding process parameters
According to the arrangement of the orthogonal test table, the optimal welding process parameters of a theoretical minimum welding residual stress are obtained, namely welding speed 21 cm/min, preheating temperature 240 °C, welding current 114 A, arc voltage 29 V. Since this set of parameters did not appear in the test, in order to verify the correctness of the orthogonal test, the optimal welding process parameter combination was numerically simulated. Through the calculation of the finite element software ABAQUS, the welding residual stress under the optimal welding process parameters was obtained. The simulation results are shown in Fig. 4. The definition σs11 is the radial residual stress of the model, σs22 is the axial welding residual stress, and σs33 is the circumferential welding residual stress.
From the results of the welding residual stress of the optimal process parameters in Fig. 4, the optimal welding process parameters obtained by the orthogonal test did obtain the minimum welding residual stress of 233.9 MPa. The maximum σ welding residual stress can be seen from Fig. 4. And the maximum hoop tensile stress is distributed in the weld and heat affected zone, and the maximum radial compressive stress appears in the vicinity of the heat affected zone. This part is the most complicated and the force is extremely uneven, which is prone to crack, which leads to The leak has failed.
3.2 Analysis of welding residual stress in different paths
In order to visually and clearly express the welding residual stress near the weld under the optimal welding process parameters, the path 1, path 2 and path 3 shown in Fig. 2 are respectively compared, and the welding residual stress distribution is shown in Fig. 5.
It can be seen from Fig. 5 that the maximum stress is concentrated in the weld and the heat affected zone. As the distance from the center of the weld increases, the residual stress of the weld becomes smaller. Path 2 is the contact point between the weld and the gasket. At the center of the weld at 0·004 m, the radial stress and hoop stress change from tensile stress to compressive stress. Path 3 is at 0·02 m from the weld, and the maximum compressive stress is 168·0 MPa. The maximum circumferential residual stress near the weld 0·01 m is 308·9 MPa. The path 1 is the part where the weld is in contact with the surfacing layer, and the maximum radial direction appears at 0·02 m from the center of the weld. The tensile stress is 228·6 MPa. The residual stress, circumferential residual stress, axial residual stress and radial residual stress at these locations are complicated and the stress values ​​are relatively large, which is prone to cracks, resulting in leakage failure. Should pay attention to the project.
4 Conclusion
(1) Using the finite element software ABAQUS, the welding thermal stress calculation program of sequential coupling was programmed, and the welding process and cooling process of the welded joints of the fillet welds of the hydrogenation high pressure heat exchanger under different welding conditions were obtained. Welding residual stress.
(2) By orthogonal test analysis and numerical simulation method, a set of L25 (56) orthogonal test was designed, and the extreme value of the residual stress of the welded joint of the fillet weld of the hydrogenation heat exchanger was obtained by the range analysis method. Excellent process parameters. The welding speed has the greatest influence on the welding residual stress, the influence of the preheating temperature is second, the influence of the welding current is again, and the influence of the arc voltage is the smallest.
(3) Numerical simulation of the welding residual force under the optimal welding process parameters, the maximum residual stress of the fillet welded joint is mainly concentrated in the weld and heat affected zone, and the force of these parts is complicated and the force is uneven. It is prone to cracks. The necessary post-weld heat treatment should be taken to reduce the welding residual stress and prevent leakage failure.
(4) The research results can provide reference for the actual welding process parameters selection of the welded joints of the fillet welds of hydrogenation high pressure heat exchangers, and provide a theoretical basis for further research and analysis of the cracking failure of welded joints of such heat exchangers.
References: slightly
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