Method for improving fatigue strength of welding structure

Abstract

The invention discloses a method for improving fatigue strength of a welding structure, which comprises the following steps of: aiming at a full penetration butt joint or a non-bearing angle joint, firstly, respectively establishing control optimization aiming at weld toe stress concentration caused by a welding seam geometrical shape factor, an initial fatigue crack source formed by a welding seam macroscopic or microdefect and welding residual tensile stress; on the basis of control optimization of the three, a fatigue strength mechanism represented by local microcell cyclic plastic strain accumulated damage controlled by base metal yield strength is formed, and secondly, on the basis of positive correlation between fatigue strength and static load strength of basic metal, a welding material is set as high-strength steel, so that the fatigue strength of a welding structure is improved. According to the method for improving the fatigue strength of the welding structure, the fatigue strength of the welding structure is improved on the basis of overcoming the conventional understanding prejudice of the technology in the industry through the set constraint conditions and the set treatment method.

Description

Technical Field

[0001] The present invention belongs to the technical field of welding, and particularly relates to a method for improving the fatigue strength of a welded structure. Background Art

[0002] For a complete and uniform metal material, there is a significant positive correlation between its fatigue performance under dynamic loading conditions and the yield strength of the material. Its fatigue failure mechanism is the cumulative damage behavior of local microzone cyclic plastic strain controlled by the yield strength of the base metal. However, for welded joints or structures, due to the influence of three key factors: the stress concentration at the weld toe caused by the weld geometry factor, the initial fatigue crack source formed by geometric discontinuities or welding defects in the weld toe and weld, and the superposition of welding residual tensile stress and external load, the fatigue performance will be significantly reduced compared with the base metal material. The reduction amplitude is related to the influence degree of the above three key factors. In view of this, in the early fatigue design standards for dynamic load structural components, the fatigue S-N curve classification of welded joints was carried out based on a large amount of experimental data accumulation. The fatigue strength level of welded joints or structures of steel materials is not related to the yield strength level of the material, but only related to joint form design, joint detail features, welding quality grade, and post-weld treatment status, etc. For example: in the mainstream EN 1993 standard, IIW-2259-15 standard, DNV-RP-C203 standard, etc., a set of S-N curves that can be used for fatigue design are given according to different design forms and quality grades of steel materials, and the stress range value corresponding to the cyclic number of 2*10 6 is extracted from each S-N curve and defined as the fatigue strength characteristic value, which is used as the basis for the fatigue design of dynamic load structural components. For example: in the EN 1993 standard, for a double-sided welded full-penetration retained weld joint, the S-N curve corresponds to DC90, and the fatigue strength characteristic value is 90 MPa. Other standards have almost the same definition.

[0003] The emergence of the current mainstream fatigue design standards has its specific background. At that time, there was a large gap between the metallurgical manufacturing level and the welding technology level and the current overall technology level. The superposition of factors such as purity control in the metallurgical process, inclusion segregation control, welding residual tensile stress, and welding macro and micro defects formed an initial fatigue crack source, so that welded joints or structures of different material types showed basically similar fatigue strength. However, with the continuous progress of the metallurgical manufacturing level and the welding technology level, the quality of steel materials themselves and the welding manufacturing quality are constantly improving. It can be predicted that: when the key factors that have an important impact on the fatigue strength of welded joints or structures are controlled, it becomes possible to improve the fatigue strength of joints or structures.

[0004] The invention application with the application number: CN 2005800471047 discloses "a non-detachable welded joint and method for ultrasonic impact treatment". The welded joint is made of high-strength steel or alloy with a yield strength σ greater than 500 MPa after ultrasonic impact treatment, and has a fatigue limit that is up to 30% higher than that of steel or alloy with σ less than 500 MPa. The beneficial compressive stress has a depth of 2 mm, and the magnitude at the surface is greater than 1.5 times the yield strength and fatigue limit of the untreated base material of the welded joint. The residual stress level of the welded joint is below 0.5 of the yield strength of the welded joint, the residual welding deformation is 100% or less of the predetermined dimensional tolerance of the welded joint, and / or the fatigue resistance is equal to or greater than that of the untreated base material of the welded joint. The fatigue limit of the welded joint is increased by at least 1.3 times compared to the untreated base material, and has been improved to a level equal to or greater than the fatigue resistance, yield point, ultimate strength, and impact strength of the untreated base material of the welded joint. The fatigue limit of the tack weld is at least 1.3 times that of the untreated base material of the welded joint, and the fatigue resistance, ultimate strength, and impact strength are equal to or greater than those of the untreated base material.

[0005] The invention application with the application number: CN200980130008.7 discloses "an impact treatment method for improving the fatigue characteristics of a welded joint, an impact treatment device for improving its fatigue characteristics, and a welded structural member with excellent fatigue resistance characteristics". On the surface of the base metal material near the weld toe of the weld, while pressing the impact pin, it is relatively moved along the welding line direction, thereby performing peening treatment or ultrasonic impact treatment. In the impact treatment method, as the impact pin, an impact pin with a tip curvature radius of 1 / 2 or less of the thickness of the metal material and 2 - 10 mm is used; the distance from the weld toe of the weld to the center of the impact treatment position is within 2.5 times the tip curvature radius of the impact pin; and on the surface of the base metal material within the range where the impact pin does not contact the welding metal during the impact treatment, the peening or ultrasonic impact treatment is performed, thereby generating residual plastic deformation using the impact pin, such that the groove depth of the impact mark is 0.1 - 2 mm, and is below the tip curvature radius of the impact pin and 1 / 10 or less of the thickness of the metal material, and the width of the impact mark is 1.5 - 15 mm and is more than 5 times the groove depth. Summary of the Invention

[0006] The object of the present invention is to overcome the conventional technical understanding bias in the industry through set constraint conditions and a combined treatment method, in order to complete a technical solution that can establish a correlation between the fatigue strength of a welded joint or welded structure and the static load strength of the base material according to the set constraint conditions and the combined treatment method.

[0007] To achieve the above technical objectives, the present invention provides a method for improving the fatigue strength of welded structures, and the technical solution is as follows:

[0008] A method for improving the fatigue strength of welded structures,

[0009] For full penetration butt joints or non-load-bearing fillet joints, first, by establishing control optimizations for the weld toe stress concentration caused by weld geometry factors, the initial fatigue crack sources formed by macroscopic or microscopic weld defects, and the welding residual tensile stress respectively, and based on the control optimizations of the three, a fatigue strength mechanism characterized by the cumulative damage of local microzone cyclic plastic strain controlled by the base metal yield strength is formed.

[0010] Secondly, based on the positive correlation between the fatigue strength and the static load strength of the base metal, the welding material is set as high-strength steel, thereby achieving the improvement of the fatigue strength of the welded structure.

[0011] Furthermore,

[0012] The control optimization established for the weld toe stress concentration caused by weld geometry factors is specifically achieved by reducing the weld height-width ratio, increasing the weld toe transition angle, and establishing constraints on the set weighted stress concentration coefficient.

[0013] Furthermore,

[0014] For full penetration butt joints, control the ratio of the weld reinforcement height to the weld width to be less than 0.1; control the weld toe transition angle to be greater than 135°; control the weighted stress concentration coefficient to be less than 0.04;

[0015] For non-load-bearing fillet joints, control the ratio of the weld protrusion height to the weld leg width on the load-transfer side to be less than 0.15; control the weld toe transition angle on the load-transfer side to be greater than 135°; control the weighted stress concentration coefficient to be less than 0.05.

[0016] Furthermore,

[0017] The control optimization established for the initial fatigue crack sources formed by macroscopic or microscopic weld defects is specifically as follows:

[0018] By establishing the optimization of weld internal defects and the optimization of weld surface defects, a microzone cyclic plastic strain cumulative damage fatigue crack initiation stage controlled by the base metal yield strength is formed, and the direct entry of the initial fatigue crack source into the crack propagation stage is eliminated.

[0019] Furthermore,

[0020] For full penetration butt joints, the establishment of the optimization of weld internal defects is specifically as follows:

[0021] Control to prevent the occurrence of two-dimensional defects, and for three-dimensional defects, control to prevent the occurrence of aggregated three-dimensional defects. At the same time, control the maximum size of independent three-dimensional defects and the number of independent three-dimensional defects within a set spacing in the weld length direction.

[0022] Furthermore,

[0023] Specifically:

[0024] Control the maximum size of independent three-dimensional defects to be less than or equal to 2 mm;

[0025] Control the number of independent three-dimensional defects within any 300 mm range in the weld length direction to be less than or equal to 3.

[0026] Furthermore,

[0027] For full-penetration butt joints, establish optimization of weld surface defects, which is completed by controlling the depth of undercut, the width of undercut, and the number of undercuts within a set spacing in the weld length direction.

[0028] Furthermore,

[0029] Specifically:

[0030] Control the depth of undercut to be less than 0.5 mm;

[0031] Control the width of undercut to be less than 0.5 mm;

[0032] Control the number of undercuts within any 300 mm range in the weld length direction to be less than or equal to 3.

[0033] Furthermore,

[0034] For non-load-bearing fillet joints, establish optimization of weld internal defects, which is completed by establishing constraints on independent three-dimensional defects and controlling the fitting quality with the base metal floor.

[0035] Furthermore,

[0036] Establish constraints on independent three-dimensional defects, which consist of establishing a limit on the maximum size of independent three-dimensional defects, establishing a limit on the number of independent three-dimensional defects within a set spacing in the weld length direction, and establishing a limit on aggregated three-dimensional defects.

[0037] Furthermore,

[0038] Establish a limit on the maximum size of independent three-dimensional defects. Specifically: Control the maximum size of independent three-dimensional defects to be less than or equal to 3 mm;

[0039] Establish a limit on the number of independent three-dimensional defects within a set spacing in the weld length direction. Specifically:

[0040] Control the number of independent three-dimensional defects within any 300 mm range in the weld length direction so that the number of independent three-dimensional defects is less than or equal to 6;

[0041] Establish a limit for aggregated three-dimensional defects, specifically: control to prevent the occurrence of aggregated three-dimensional defects.

[0042] Furthermore,

[0043] Control the fitting quality with the base metal bottom plate, which is achieved by controlling the size of independent slag inclusions less than 3 mm and the size of independent cavities less than 3 mm during the fitting with the base metal bottom plate.

[0044] Furthermore,

[0045] For non-load-bearing fillet joints, establish an optimization for weld surface defects, which is completed by establishing control over the undercut depth, undercut width, and the number of undercuts within a set spacing in the weld length direction.

[0046] Furthermore,

[0047] Specifically:

[0048] Control the undercut depth to be less than 0.5 mm;

[0049] Control the undercut width to be less than 0.5 mm;

[0050] Control the number of undercuts within any 300 mm range in the weld length direction to be less than or equal to 3.

[0051] Furthermore,

[0052] Establish a control optimization for welding residual tensile stress, which is achieved by establishing an optimization of the welding process that can reduce welding residual tensile stress and an optimization of the peak value of welding residual tensile stress.

[0053] Furthermore,

[0054] The optimization of the peak value of welding residual tensile stress is completed by controlling the duration of the residual stress relaxation effect generated by the superposition of welding residual tensile stress and external load.

[0055] Furthermore,

[0056] The said duration is controlled to ensure no fatigue failure occurs during 1000 - 5000 early fatigue cycles.

[0057] A method for improving the fatigue strength of welded structures realizes the improvement of the fatigue strength of welded structures on the basis of overcoming the conventional technical understanding biases in the industry through set constraints and a cooperating treatment method. The constraints refer to defining the objects to be improved as full-penetration butt joints or non-load-bearing fillet joints. The cooperating treatment method refers to aiming at controlling the establishment of the fatigue failure mechanism of welded structures by characterizing the local microzone cyclic plastic strain accumulation damage fatigue failure mechanism controlled by the yield strength of the base material. Around this purpose, control optimizations are respectively established from three aspects: the stress concentration at the weld toe caused by the weld geometry factor, the initial fatigue crack source formed by macroscopic or microscopic defects in the weld, and the welding residual tensile stress, so that the results of the comprehensive optimization jointly serve to achieve this technical purpose. Description of the Drawings

[0058] Figure 1 It is the fatigue strength curve of the full-penetration butt joint of Q355ND plain carbon steel under infinite fatigue life in the first embodiment of the present invention;

[0059] Figure 2 It is the fatigue strength curve of the full-penetration butt joint of Q500MD high-strength steel under infinite fatigue life in the first embodiment of the present invention;

[0060] Figure 3 It is the S-N curve of the finite fatigue life of the full-penetration butt joint of Q355ND plain carbon steel by the group method in the second embodiment of the present invention;

[0061] Figure 4 It is the S-N curve of the finite fatigue life of the full-penetration butt joint of Q500MD high-strength steel by the group method in the second embodiment of the present invention;

[0062] Figure 5 It is the S-N curve of the finite fatigue life of the welded structure of Q500MD high-strength steel accessories by the group method in the third embodiment of the present invention;

[0063] Figure 6 It is the S-N curve of the finite fatigue life of the welded structure of Q420MD high-strength steel accessories by the group method in the third embodiment of the present invention. Detailed Embodiments

[0064] Next, a method for improving the fatigue strength of welded structures according to the present invention will be further specifically described according to the detailed embodiments. To form a full understanding of this technical solution, it is elaborated in three steps. The first part gives an overview of the setting, implementation, and achieved effects of this technical solution. The second part specifically elaborates on the specific optimization process and the correlation of the internal mechanism. The third part is specific embodiments.

[0065] Overview of the technical solution:

[0066] A method for improving the fatigue strength of welded structures according to the present invention, aiming at full penetration butt joints and non-load-bearing fillet joints with relatively low stress concentration levels. On the basis of deeply analyzing the fatigue failure process of welded structures, through the control optimization scheme set by this technical solution, it effectively reduces or eliminates the adverse effects on the fatigue strength of welded structures caused by factors such as stress concentration caused by weld geometry factors, initial fatigue crack sources formed by macroscopic or microscopic defects in welds, and the superposition of welding residual tensile stress and external loads. Thus, the fatigue failure mechanism of high-strength steel welded joints or structures is switched to the cumulative damage behavior of local microzone cyclic plastic strain controlled by the yield strength of the base metal, giving full play to the high fatigue strength potential of high-strength steel base metal, effectively improving the fatigue strength of welded structures, and providing a theoretical basis for the popularization and application of high-strength steel and the realization of lightweight weight reduction in dynamic load fatigue design occasions in related industrial fields.

[0067] The method for improving the fatigue strength of welded structures described in this technical solution is applicable to full penetration butt joints and non-load-bearing fillet joints that inherently have relatively low stress concentration levels, and is not applicable to some welded joints or structural forms that inherently belong to high stress concentration levels, such as: positive load-bearing fillet joints and lap joints. The reason is that the inherently high stress concentration attribute determines the fatigue failure mechanism dominated by high stress concentration, and it is impossible to switch to the cumulative damage fatigue failure behavior of local microzone cyclic plastic strain controlled by the yield strength of the base metal through the optimization measures established in this technical solution, thus unable to give full play to the high fatigue strength potential of high-strength steel base metal and achieve the improvement of the fatigue strength of welded structures.

[0068] And this technical solution realizes the conditional improvement of the fatigue strength of high-strength steel welded structures by optimizing the key factors that have an important impact on the fatigue strength of welded structures, breaking through the technical limitations and obstacles of existing fatigue design standards that do not consider the material strength level and only consider the details of welded structures, and providing a theoretical basis and technical support for the application of high-strength steel in dynamic load service scenarios and the high-strength lightweight weight reduction of structures. By replacing plain carbon steel with high-strength steel, it promotes the material upgrade and replacement in related fields and reduces the material cost, having important economic value and social value.

[0069] The specific optimization process and principle of this technical solution:

[0070] For full penetration butt joints and non-load-bearing fillet joints commonly used in dynamic load industrial structures with relatively low stress concentration levels, considering the adverse effects of three key factors on the fatigue strength of welded joints or structures, namely, stress concentration caused by weld geometry factors, initial fatigue crack sources formed by macroscopic or microscopic defects in the weld, and the superposition overload of welding residual tensile stress and external loads, based on a large number of experimental studies, data accumulation and integration, a method is revealed to control and reduce or eliminate the influence of these three key factors on the fatigue strength of welded joints or structures through welding process technology, enabling the fatigue failure mechanism of welded joints or structures to switch to the local microzone cyclic plastic strain cumulative damage behavior controlled by the yield strength of the base metal, thereby exerting the high fatigue strength characteristics of high-strength steel base metal and effectively improving the fatigue strength of the overall welded structure.

[0071] For low-strength plain carbon steel, even if the above three key factors affecting the fatigue strength of joints or structures are controlled and ensured, and the fatigue failure mechanism of welded joints or structures switches to the local microzone cyclic plastic strain cumulative damage behavior controlled by the material yield strength, due to the relatively low yield strength of low-strength plain carbon steel itself, the fatigue strength of welded joints or structures cannot be effectively improved.

[0072] Specifically as follows:

[0073] (1) Control of stress concentration caused by weld geometry factors

[0074] During the loading process of welded structural components under dynamic loads, the stress transmission line will change direction and concentrate at the sudden change of geometry. This stress concentration effect causes the actual stress borne by this part to be much greater than the nominal stress value, and even exceeds the yield strength of the base metal at some moments, which is very likely to cause early low-life fatigue failure. If the sudden change of weld geometry in welded joints or structures is reduced through the control and optimization of the welding process, the stress concentration effect will be changed to a great extent, thereby improving the anti-fatigue failure ability of welded joints or structures. The control and optimization of the welding process include two paths:

[0075] First, reduce the weld height-width ratio (h / b). For full penetration butt joints, h represents the weld reinforcement height, b represents the weld width, and control h / b < 0.1; for non-load-bearing fillet joints, h represents the convex height of the fillet weld, b represents the weld leg width on the side of the transmitted load, and control h / b < 0.15;

[0076] Second, increase the weld toe transition angle θ. For full penetration butt joints, θ represents the weld toe transition angle, and ensure θ > 0.75π; for non-load-bearing fillet joints, θ represents the weld toe transition angle on the side of the transmitted load, and ensure θ > 0.75π.

[0077] To control the stress concentration effect of welded joints or structures as a whole, the concept of weighted stress concentration coefficient F is proposedsc ,here,

[0078] For full penetration butt joints, F sc <0.04; for non-load bearing corner joints, F sc <0.05.

[0079] (2) Initial fatigue crack source formed by macro or micro defects in welds

[0080] Welding is a metallurgical process of continuous cooling, solidification and structural transformation in a non-equilibrium state, which inevitably forms macroscopic or microscopic discontinuities, generally referred to as welding defects. Welding defects that exceed the acceptance standards of engineering projects are called defects. Macroscopic or microscopic welding defects preferentially become the initial fatigue crack source during dynamic load fatigue service, so there is no fatigue crack initiation stage dominated by micro-area cyclic plastic strain, and it directly enters the fatigue crack propagation stage, thereby reducing the fatigue strength and fatigue life of the welded joint or structure. If the welding process is controlled and optimized to minimize or even partially eliminate macroscopic and microscopic welding defects, the fatigue failure mode in the welded joint or structure can be changed, and the micro-area cyclic plastic strain fatigue crack initiation stage controlled by the yield strength of the parent material can be increased, which helps to extend the fatigue life of the joint or structure and improve its fatigue strength. The control of macroscopic or microscopic welding defects mainly includes the following two aspects:

[0081] First, weld internal defect control

[0082] For full penetration butt joints, no two-dimensional defects of any size are allowed, such as lack of fusion at the edge or between layers, lack of penetration, various types of welding cracks, etc. The maximum size of independent three-dimensional defects shall not exceed 2mm, including welding pores, slag inclusions, etc. There shall be no more than 3 independent three-dimensional defects within any 300mm range along the length of the weld, and aggregated three-dimensional defects are not allowed. Aggregated three-dimensional defects are defined as two or more independent defects with a spacing less than their own maximum size;

[0083] For non-load-bearing fillet joints, which are generally non-penetration type, the maximum size of independent three-dimensional defects does not exceed 3mm, including welding pores, slag inclusions, etc., and within any 300mm range along the length of the weld, there are no more than 6 independent three-dimensional defects, and aggregated three-dimensional defects are not allowed. At the same time, for the non-load-bearing fillet joints, the incomplete weld position needs to be closely fitted with the base plate of the parent material, and large slag inclusions or voids are not allowed. This is achieved by controlling the size of independent slag inclusions less than 3mm and the size of independent voids less than 3mm between the fit with the base plate of the parent material.

[0084] Second, weld surface defect control

[0085] After the macroscopic defects such as porosity on the weld surface and unfilled grooves are controlled, the main surface defect of the weld that has an important impact on the fatigue strength of the welded joint or structure is the undercut at the weld toe. Whether it is the weld toe of a full-penetration butt weld or the weld toe on the load-transfer side of a non-load-bearing fillet weld, the undercut depth d u < 0.5 mm, and the undercut width d w < 0.5 mm, and within any 300 mm range along the weld length, the number of undercuts does not exceed 3.

[0086] (3) Control of the superposition of welding residual tensile stress and external load

[0087] Due to the inherent non-equilibrium solidification and solid-state phase transformation characteristics of the welding process, relatively large residual tensile stresses will be generated at the joint, which cannot be released through deformation in large structural components and remain in the joint or structure after welding. The superposition of welding residual tensile stress and external load during dynamic load service increases the actual load borne by the joint or structure, and even exceeds the yield strength of the base metal at some moments, easily triggering early low-cycle fatigue failure, which is similar to the effect of stress concentration. Although there are currently mature technologies for eliminating welding residual stress after welding, such as overall stress relief heat treatment of structural components after welding, high-frequency mechanical peening of weld toes, etc., these post-weld treatment methods all increase the manufacturing processes and manufacturing costs of structural components, and are not operable in some structural components. The mechanism for enhancing the fatigue strength of the high-strength steel welded joint or structure described in the present invention mainly realizes the control of the superposition of welding residual stress and external load through the following two paths:

[0088] First, optimize and control the welding process to reduce welding residual tensile stress

[0089] On the premise of ensuring the design and manufacturing requirements of structural components, through the optimization and control of the welding process, the welding residual tensile stress can be greatly reduced, including but not limited to process methods such as multi-layer and multi-pass welding with low heat input, double-sided symmetric welding, and symmetric welding of complex structural components in space position. For some structural components or key parts with high requirements, welding materials with high alloy and low phase transformation points can be used to change the residual tensile stress at the welded joint to residual compressive stress.

[0090] Second, eliminate the peak value of welding residual tensile stress during the early fatigue cycle of the structural component during service

[0091] For fully penetrated butt joints and non-load-bearing fillet joints with complete structures, after the peak value of welding residual tensile stress is superimposed with the external load, the actual load borne by the joint or structure increases. Even under the fatigue failure mechanism of cumulative damage of cyclic plastic strain in the local micro-region controlled by the yield strength of the base metal, it is easy to cause fatigue crack initiation and exacerbate the propagation process, reducing the fatigue life and fatigue strength. However, during the early fatigue cyclic loading in the service process of structural components, there is a residual stress relaxation effect, which can partially eliminate the peak value of welding residual tensile stress, maintain the welding residual tensile stress at a relatively low level, and make the internal stress of the structure tend to be balanced. As a result, the actual load borne by the joint or structural component is reduced. Under the fatigue failure mechanism of cumulative damage of cyclic plastic strain in the local micro-region controlled by the yield strength of the base metal, with the increase of the yield strength of the base metal, the fatigue life and fatigue strength of the joint or structure increase accordingly. According to the different types and loading characteristics of structural components, the number of fatigue cycles required to achieve welding residual stress relaxation ranges from 1000 to 5000 times. In other words, when improving the fatigue performance of joints or structures based on the mechanism of eliminating the peak value of welding residual tensile stress through early fatigue cycles during the service process, it is necessary to ensure that no fatigue failure occurs during 1000 - 5000 early fatigue cycles in order to exert the subsequent effect that the fatigue strength of the joint or structure increases with the increase of the yield strength of the base metal after welding residual stress relaxation. And finally achieving the goal that the fatigue strength of the structure increases with the increase of the yield strength of the base metal is based on the combined effect of the above three factors.

[0092] A method for improving the fatigue strength of welded structures according to the present invention has the beneficial effects formed by the above comprehensive optimization settings as follows:

[0093] (1) It subverts the understanding of the fatigue performance of welded joints in the industry for many years, breaks through the fatigue strength design mode in existing mainstream fatigue design standards that only considers the joint structure form and details without considering the yield strength of the base metal, and creatively proposes a new concept that the fatigue strength of welded joints or structures can be conditionally improved with the increase of the yield strength of the base metal, leading the development and progress in this field.

[0094] (2) The fatigue strength improvement mechanism of welded joints or structures described in the present invention provides a theoretical basis for the popularization and application of high-strength steel in dynamic load industrial structural components to achieve lightweight weight reduction, and helps the material upgrading and high-quality development of related industries.

[0095] (3) The present invention promotes the popularization and application of high-strength steel in the industrial field of dynamic load service. Using high-strength steel to replace plain carbon steel, in the long run, both the manufacturing process and the service process have the effect of being green and low-carbon, and it will make a lasting contribution to energy conservation and carbon reduction in the industrial field.

[0096] Embodiment

[0097] The following three embodiments are listed and supported by experimental methods and the presentation of experimental result comparisons. Through the optimization control established by this technical solution, it can be achieved that the fatigue strength of the welded structure increases with the increase of the yield strength of the base material.

[0098] Embodiment 1

[0099] When the three key influencing factors are controlled and improved, the fatigue strength of the full-penetration butt joint of high-strength steel under infinite life conditions is significantly improved compared with that of low-strength plain carbon steel. Specifically as follows:

[0100] Q500MD high-strength steel and Q355ND plain carbon steel with a wall thickness of 20 mm were used, and butt joints were prepared by double-sided submerged arc automatic welding. Considering the stress concentration factor caused by the weld geometry factor, the weld height-width ratio h / b < 0.1, and the weld toe transition angle θ > 0.75π. At the same time, ensure the weighted stress concentration factor F sc < 0.04; Considering the initial fatigue crack source factor formed by macroscopic or microscopic welding defects, ensure that there are no two-dimensional defects, three-dimensional defects in the weld, and the surface weld toe undercut meets the limiting conditions of the present invention; Considering the superposition factor of welding residual tensile stress and external load, Q500MD high-strength steel and Q355ND plain carbon steel adopt exactly the same welding process, which basically offsets the influence of welding residual tensile stress. At the same time, under the low-stress and high-fatigue life experimental conditions, no early fatigue failure occurred. After the peak relaxation of the welding residual tensile stress, the adverse effect of the residual stress on the fatigue strength was reduced. When the above conditions are ensured, according to the requirements of GB / T 3075 standard, the fatigue strength test of the butt joints of two different strength levels based on infinite fatigue life was carried out by the staircase method, and the set number of cycles was 1*10 7 , Figure 1 and Figure 2 The fatigue strength curves of the Q355ND plain carbon steel and Q500MD high-strength steel butt joints under the staircase method of infinite fatigue life are shown respectively. According to GB / T 24176 for data processing and fatigue strength calculation, under the conditions of 95% survival rate and 95% confidence level, the conditional fatigue strength of the Q355ND plain carbon steel butt joint Δσ(95%, 95%) = 133.2 MPa, and the conditional fatigue strength of the Q500MD high-strength steel butt joint Δσ(95%, 95%) = 164.7 MPa, with a promotion ratio of 23.65%.

[0101] This embodiment fully proves that: when the three influencing factors of the fatigue strength of the welded joint or structure are controlled and ensured, its fatigue failure mechanism switches to the local microzone cyclic plastic strain cumulative damage behavior controlled by the yield strength of the base material, and the fatigue strength of the welded joint or structure increases significantly with the increase of the yield strength of the base material.

[0102] Embodiment 2

[0103] When the three key influencing factors are controlled and improved, the fatigue strength characteristic values of the full penetration butt joints of high-strength steel based on the S-N curve are significantly improved compared with those of low-strength plain carbon steel and the extraction values of the standard fatigue design curve, while the fatigue strength characteristic values of the plain carbon steel joints are basically equivalent to the extraction values of the standard design curve. Specifically as follows:

[0104] Q500MD high-strength steel and Q355ND plain carbon steel with a wall thickness of 20 mm were used, and butt joints were prepared by automatic submerged arc welding on both sides. Considering the stress concentration factor caused by the weld geometry factor, the weld height-width ratio h / b < 0.1, and the weld toe transition angle θ > 0.75π, while ensuring the weighted stress concentration factor F sc < 0.04; Considering the initial fatigue crack source factor formed by macroscopic or microscopic welding defects, ensure that the weld does not have two-dimensional defects, three-dimensional defects, and the surface weld toe undercut meets the limiting conditions of the present invention; Considering the superposition factor of welding residual tensile stress and external load, Q500MD high-strength steel and Q355ND plain carbon steel adopt exactly the same welding process, basically offsetting the influence of welding residual tensile stress. At the same time, under the experimental conditions of low stress and high fatigue life, no premature fatigue failure occurred. After the peak relaxation of the welding residual tensile stress, the adverse effect of the residual stress on the fatigue strength was reduced. When the above conditions are ensured, in accordance with the requirements of GB / T 3075 standard, the series stress level finite fatigue life tests of two different strength grade welded joints were carried out by the group method, and the set number of cycles was 1*10 7 , Figure 3 and Figure 4 The group method finite fatigue life S-N curves of Q355ND plain carbon steel and Q500MD high-strength steel welded joints are shown respectively (in the figure, the abscissa represents the number of loading cycles, and the ordinate represents the stress amplitude). The fatigue strength characteristic values corresponding to a survival rate of 97.7% and a fatigue cycle number of 2 million times were extracted from the S-N curve. For Q355ND plain carbon steel, FAT(97.7%) = 92.63 MPa. For Q500MD high-strength steel, FAT(97.7%) = 115 MPa. It can be seen that with the increase of the yield strength of the base metal, the fatigue strength characteristic value of the full penetration butt joint of Q500MD high-strength steel is increased by 24.1% compared with that of Q355ND plain carbon steel, while the fatigue strength characteristic value of the Q355ND plain carbon steel welded joint is basically equivalent to the fatigue strength characteristic value of 90 MPa of the same structural form extracted from the standard design curve. This also shows that: even if the three key influencing factors are controlled and ensured, the fatigue strength of the full penetration butt joint of plain carbon steel will not be significantly improved, which is mainly related to the relatively low yield strength of plain carbon steel itself.

[0105] Example Three

[0106] When the three key influencing factors are controlled and improved, the fatigue strength characteristic values of the welded structure of high-strength steel non-load-bearing accessories based on the S-N curve increase significantly with the increase of the yield strength of the base metal, and are significantly higher than the characteristic fatigue strength values of the standard fatigue design curve, as follows:

[0107] Q500MD high-strength steel and Q420MD high-strength steel with a wall thickness of 20 mm are used, and rectangular plate-shaped accessories with a wall thickness of 16 mm and a length of 50 mm are matched. Fillet welds are completed by semi-automatic gas shielded welding with flux-cored wire. Considering the stress concentration factor caused by the weld geometry factor, the weld height-width ratio h / b < 0.15, and the weld toe transition angle θ > 0.75π. At the same time, the weighted stress concentration factor F sc < 0.05; Considering the initial fatigue crack source factor formed by macroscopic or microscopic welding defects, ensure that the three-dimensional defects and surface weld toe undercut meet the limiting conditions of the present invention. At the same time, ensure that the non-load-bearing accessory is closely attached to the base metal bottom plate without large-scale slag inclusions or voids; Considering the superposition factor of welding residual tensile stress and external load, Q500MD high-strength steel and Q420MD high-strength steel adopt exactly the same welding process, which basically offsets the influence of welding residual tensile stress. At the same time, under the experimental conditions of low stress and high fatigue life, no premature fatigue failure occurred. After the peak relaxation of the welding residual tensile stress, the adverse effect of the residual stress on the fatigue strength was reduced. When the above conditions are ensured, according to the requirements of GB / T 3075 standard, the finite fatigue life tests of the series stress levels of the welded structures of the two different strength levels of accessories are carried out by the group method, and the set number of cycles is 1*10 7 , Figure 5 and Figure 6 The finite fatigue life S-N curves of the welded structures of Q500MD high-strength steel and Q420MD high-strength steel accessories by the group method are shown respectively (in the figure, the abscissa represents the number of loading cycles, and the ordinate represents the stress amplitude). The fatigue strength characteristic values corresponding to a survival rate of 97.7% and a fatigue cycle number of 2 million times are extracted from the S-N curve. For Q500MD high-strength steel, FAT(97.7%) = 110 MPa. For Q420MD high-strength steel, FAT(97.7%) = 91 MPa. It can be seen that with the increase of the yield strength of the base metal, the fatigue strength characteristic value of the welded structure of Q500MD high-strength steel accessories is 20.9% higher than that of Q420MD, and both are significantly higher than the fatigue strength characteristic value of 80 MPa of the welded structure of accessories extracted from the standard design curve.

Claims

1. A method for enhancing the fatigue strength of a welded structure, characterized in that: For a full penetration butt joint or a non-load-bearing fillet joint, first, by respectively establishing control optimizations for the weld toe stress concentration caused by weld geometry factors, the initial fatigue crack sources formed by macroscopic or microscopic defects in the weld, and the welding residual tensile stress, a fatigue strength mechanism characterized by the cumulative damage of local microzone cyclic plastic strain controlled by the yield strength of the base metal is formed based on the control optimizations of the three. Secondly, based on the positive correlation between the fatigue strength and the static load strength of the base metal, the welding material is set as high-strength steel, thereby realizing the enhancement of the fatigue strength of the welded structure.

2. The method for enhancing the fatigue strength of a welded structure according to claim 1, characterized in that: The control optimization established for the weld toe stress concentration caused by weld geometry factors is specifically achieved by reducing the weld height-width ratio, increasing the weld toe transition angle, and establishing constraints on the set weighted stress concentration coefficient.

3. The method for enhancing the fatigue strength of a welded structure according to claim 2, characterized in that: For a full penetration butt joint, control the ratio of the weld reinforcement height to the weld width to be less than 0.1; control the weld toe transition angle to be greater than 135°; control the weighted stress concentration coefficient to be less than 0.04; For a non-load-bearing fillet joint, control the ratio of the weld protrusion height to the weld leg width on the load-transfer side to be less than 0.15; control the weld toe transition angle on the load-transfer side to be greater than 135°; control the weighted stress concentration coefficient to be less than 0.

05.

4. The method for enhancing the fatigue strength of a welded structure according to claim 1, characterized in that: The control optimization established for the initial fatigue crack sources formed by macroscopic or microscopic defects in the weld is specifically as follows: By establishing the optimization of internal weld defects and the optimization of surface weld defects, a microzone cyclic plastic strain cumulative damage fatigue crack initiation stage controlled by the yield strength of the base metal is formed, and the direct entry of the initial fatigue crack source into the crack propagation stage is prevented.

5. The method for enhancing the fatigue strength of a welded structure according to claim 4, characterized in that: For a full penetration butt joint, the establishment of the optimization of internal weld defects is specifically as follows: Control to prevent the occurrence of two-dimensional defects, and for three-dimensional defects, control to prevent the occurrence of aggregated three-dimensional defects, and at the same time control the maximum size of independent three-dimensional defects and the number of independent three-dimensional defects within a set spacing in the weld length direction.

6. The method for enhancing the fatigue strength of a welded structure according to claim 5, characterized in that: Specifically: Control the maximum size of independent three-dimensional defects to be less than or equal to 2 mm; Control the number of independent three-dimensional defects within any 300 mm range in the weld length direction of independent three-dimensional defects to be less than or equal to 3.

7. The method for enhancing the fatigue strength of a welded structure according to claim 4, characterized in that: For a full penetration butt joint, the establishment of the optimization of surface weld defects is completed by establishing control over the undercut depth, undercut width, and the number of undercuts within a set spacing in the weld length direction.

8. The method for enhancing the fatigue strength of a welded structure according to claim 7, It is characterized in that: Specifically: Control the undercut depth to be less than 0.5 mm; Control the undercut width to be less than 0.5 mm; Control the number of undercuts within any 300 mm range in the weld length direction to be less than or equal to 3.

9. A method for enhancing the fatigue strength of a welded structure according to claim 4, It is characterized in that: For non-load-bearing fillet joints, optimize the internal defects of the weld by establishing constraints on independent three-dimensional defects and combining with controlling the fitting quality with the base metal bottom plate.

10. A method for enhancing the fatigue strength of a welded structure according to claim 9, It is characterized in that: Establish constraints on independent three-dimensional defects, which consist of establishing a limit on the maximum size of independent three-dimensional defects, establishing a limit on the number of independent three-dimensional defects within a set spacing in the weld length direction, and establishing a limit on aggregated three-dimensional defects.

11. A method for enhancing the fatigue strength of a welded structure according to claim 10, It is characterized in that: Establish a limit on the maximum size of independent three-dimensional defects. Specifically: control the maximum size of independent three-dimensional defects to be less than or equal to 3 mm; Establish a limit on the number of independent three-dimensional defects within a set spacing in the weld length direction. Specifically: Control the number of independent three-dimensional defects within any 300 mm range in the weld length direction to be less than or equal to 6; Establish a limit on aggregated three-dimensional defects. Specifically: control to prevent the occurrence of aggregated three-dimensional defects.

12. A method for enhancing the fatigue strength of a welded structure according to claim 9, It is characterized in that: Control the fitting quality with the base metal bottom plate by controlling the size of independent slag inclusions less than 3 mm and the size of independent cavities less than 3 mm between the fitting with the base metal bottom plate.

13. A method for enhancing the fatigue strength of a welded structure according to claim 4, It is characterized in that: For non-load-bearing fillet joints, optimize the surface defects of the weld by establishing control over the undercut depth, undercut width, and the number of undercuts within a set spacing in the weld length direction.

14. A method for enhancing the fatigue strength of a welded structure according to claim 13, It is characterized in that: Specifically: Control the undercut depth to be less than 0.5 mm; Control the undercut width to be less than 0.5 mm; Control the number of undercuts within any 300 mm range in the weld length direction to be less than or equal to 3.

15. A method for enhancing the fatigue strength of a welded structure according to claim 1, It is characterized in that: Establish control optimization for welding residual tensile stress by establishing optimization of the welding process that can reduce welding residual tensile stress and combining with establishing optimization of the peak value of welding residual tensile stress.

16. A method for enhancing the fatigue strength of a welded structure according to claim 15, It is characterized in that: Optimize the peak value of welding residual tensile stress by controlling the duration of the residual stress relaxation effect generated by the superposition of welding residual tensile stress and external load.

17. A method for enhancing the fatigue strength of a welded structure according to claim 16, It is characterized in that: The duration is completed without fatigue failure when controlling the early fatigue cycles from 1000 to 5000 times.

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