Process for inhibiting carbon migration of laser welding interface of nuclear power 16MnD5 and 316L stainless steel and application

Abstract

The application discloses a process for inhibiting carbon migration of a laser welding interface of nuclear power 16MnD5 and 316L stainless steel and application, and belongs to the technical field of nuclear power dissimilar metal welding.The application adopts IN690 alloy coaxial wire feeding to perform laser multi-pass tailor welding, and high-frequency pulse laser is applied at a specific included angle; meanwhile, a coaxial double-ring gas cover is adopted.The application avoids overall oxidation by reconstructing thermodynamic boundary through spatial layered airflow; and with the help of periodic energy impact and acoustic streaming effect generated by the high-frequency pulse laser in the molten pool, the molten pool can be stirred, the liquid residence time of the molten pool can be shortened, the diffusion distance and time of C elements can be inhibited, the oxygen is promoted to penetrate along the dislocation channel with the help of the high-frequency laser shock wave, titanium oxide is generated by combining with Ti as a heterogeneous nucleation core to promote the growth of NbC encapsulation to "microscopic carbon sequestration", the excess carbon is oxidized to "gas escape", the double mechanisms decompose the network carbide, purify the grain boundary, and significantly improve the high-temperature performance and crack resistance of the joint.

Description

Technical Field

[0001] This invention belongs to the field of nuclear power dissimilar metal welding technology, and particularly relates to a process for suppressing carbon migration at the laser welding interface between nuclear power 16MnD5 and 316L stainless steel. Background Technology

[0002] In the primary loop system of a pressurized water reactor nuclear power plant, main equipment such as the reactor pressure vessel and steam generator are typically manufactured using low-alloy high-strength steel 16MnD5, while the main piping and safety system piping connected to them are mostly made of austenitic stainless steel 316L. Therefore, dissimilar metal welding of 16MnD5 low-alloy steel and 316L stainless steel is an indispensable and critical manufacturing process in the construction and maintenance of nuclear power plants.

[0003] 16MnD5 steel and 316L stainless steel differ significantly in chemical composition, thermophysical properties, and crystal structure. Under the high temperatures of welding, due to the much higher carbon content and chemical activity of carbon on the 16MnD5 steel side compared to the 316L stainless steel side, carbon readily and spontaneously diffuses and migrates from the 16MnD5 base metal across the fusion line into the weld metal and the 316L side, driven by both concentration and activity gradients. This uncontrolled carbon migration leads to a large accumulation of carbon on the weld side of the fusion line, forming a high-hardness, high-brittleness "carbon-rich zone." Simultaneously, the 16MnD5 base metal near the weld develops a coarse-grained "decarburized softening layer" due to carbon loss. This drastic change in microstructure and properties significantly weakens the overall mechanical properties of the joint. Furthermore, the high hardness of the carbon-rich zone, coupled with the huge residual welding stress caused by the difference in thermal expansion coefficients of dissimilar steels, makes this area extremely prone to cold cracking. In the high-temperature and high-pressure water environment of nuclear power plants, it becomes a sensitive source of intergranular stress corrosion cracking, seriously threatening the long-term operational safety of nuclear power plants.

[0004] Currently, tungsten inert gas (TIG) welding or shielded metal arc welding (SMAW) is mainly used with nickel-based welding wires (such as Inconel 82 / 182 / 52 / 152) for transition welding. However, traditional arc welding has a large heat input and a slow cooling rate, which provides ample time for carbon diffusion, resulting in severe interfacial carbon migration. In recent years, some scholars have attempted to improve this problem by introducing laser welding with mixed shielding gases (such as Ar+CO2) or pure inert gases, but this faces irreconcilable physical contradictions. If pure argon gas is used with high-frequency pulses (such as simple plasma acoustic stirring), although the macroscopic carbon-rich region at the interface can be physically dispersed, the carbon cannot be chemically consumed, and the extremely rapid cooling rate will cause carbon to form periodic micro-segregation inside the weld. If Ar+CO2 mixed gas is directly used to cover the entire molten pool for decarburization, the 690 nickel-based welding wire will be severely oxidized during droplet transfer, and key alloying elements such as Cr will be severely burned off, sacrificing the corrosion resistance of the joint.

[0005] Therefore, there is an urgent need in this field for a new welding process that can completely eliminate the carbon-rich layer near the fusion line on the IN690 side through the synergistic effect of multiple dimensions such as thermodynamics, kinetics and metallurgical chemistry. Summary of the Invention

[0006] To address the technical problem of welded dissimilar thick plates of 16MnD5 low-alloy steel and 316L stainless steel in existing nuclear power primary loop systems, where the large carbon activity gradient leads to easy long-range carbon migration on the 16MnD5 side of the fusion line, resulting in a high-hardness carbon-rich layer, adjacent decarburization softening zone, and internal carbon segregation on the IN690 side, and the inherent contradiction between decarburization and oxidation prevention in traditional processes, which severely reduces the mechanical properties and stress corrosion resistance of the joint, this invention provides a laser-assisted filler wire welding method for 16MnD5 and 316L stainless steel based on a dual-ring stratified protective gas. This invention aims to reconstruct the thermodynamic boundary and completely eliminate the carbon-rich zone at the interface through the synergistic intervention of a multi-dimensional energy field (high-frequency laser shock) and chemical field (spatial stratified gas flow). The core mechanism lies in the following: Under the combined effect of laser shock wave and localized oxidizing atmosphere (outer ring active protective gas), the impact kinetic energy significantly promotes the microscopic diffusion of elements; oxygen atoms in the outer ring penetrate inward along the dislocation channels generated by the impact and preferentially combine with Ti elements in the system to form fine titanium oxide particles; these particles act as heterogeneous nucleation cores, promoting the subsequent growth of NbC on their surface, thereby microscopically fixing free carbon to the particle shell; simultaneously, excess free carbon atoms at the interface directly react with oxygen to generate gas that escapes from the molten pool. This invention avoids the overall oxidation risk through spatially layered airflow and relies on the dual mechanism of microscopic carbon fixation and in-situ gas escape to decompose the network carbides at the interface, effectively purifying the grain boundaries, and ultimately significantly improving the high-temperature performance and crack resistance of dissimilar steel joints.

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

[0008] A process for suppressing carbon migration at the laser welding interface between 16MnD5 and 316L stainless steel used in nuclear power plants includes the following steps:

[0009] Step 1: The nuclear power 16MnD5 steel and 316L stainless steel to be welded are beveled to ensure that the area to be welded is smooth and burr-free, remove surface impurities, and assemble the test plate.

[0010] Step 2: Using laser coaxial wire feeding welding method, adjust the laser beam and coaxial wire feeding nozzle to align with the center of the bevel, and perform multi-layer multi-pass fusion welding;

[0011] Step 3: While the main laser is working, a high-frequency pulsed laser is simultaneously applied to the side to assist in irradiating the weld pool;

[0012] The main laser power is controlled at 3000W-4000W, the wire feeding speed is 5-10 mm / s, and the scanning speed is 10~15 cm / min; a high-frequency pulsed laser is simultaneously activated for assistance, with a pulsed laser power of 500-800W, a pulse repetition frequency of 1-2MHz, a pulse width of 2-10ns, a scanning interval of 50-200μm, and a scanning width of 1-3mm.

[0013] Step 4: Throughout the laser welding process, a coaxial double-ring gas shield with a spatial composition gradient is applied to the welding area simultaneously. The double-ring gas shield includes an inner ring of pure inert gas close to the wire feed center and an outer ring of active mixed gas surrounding the inner ring.

[0014] The inner ring pure inert protective gas is argon with a purity of not less than 99.99%, and the gas flow rate is controlled at 8~12 L / min;

[0015] The outer ring active mixed protective gas is a mixture of argon and carbon dioxide, with carbon dioxide accounting for 15% to 30% by volume. The outer ring gas flow rate is controlled at 15 to 20 L / min, and the outflow velocity of the outer ring gas is greater than that of the inner ring gas.

[0016] Step 5: In the multi-layer, multi-pass welding process, after each weld is completed, the surface of the current weld bead is cleaned. Repeat the above steps until all welds are completed.

[0017] Furthermore, in step one, the thickness of both the nuclear power 16MnD5 steel and 316L stainless steel is 15mm.

[0018] Furthermore, in step one, the beveling process is as follows: the beveling angle on one side is 20°, the total beveling angle on both sides after the butt joint assembly is 40°, and the assembly gap between the nuclear power 16MnD5 steel and the 316L stainless steel test plate is 1.2mm.

[0019] Furthermore, the surface roughness Ra of the bevel is ≤5μm.

[0020] Furthermore, in step three, the spatial angle between the high-frequency pulsed laser and the main laser is between 10-20°.

[0021] Furthermore, in step three, the distance between the high-frequency pulsed laser beam and the main laser beam that feeds the coaxial wire is 5-8 mm.

[0022] According to the aforementioned process, the laser welding interface between 16MnD5 and 316L stainless steel in nuclear power plants has no brittle carbon-rich region.

[0023] Furthermore, the life assessment method for the laser-welded interface between 16MnD5 and 316L stainless steel in nuclear power plants includes the following steps:

[0024] At the 16MnD5 / IN690 interface, the activity coefficient γ of carbon in the multi-component alloy c satisfy:

[0025] ;

[0026] Where, γ c The activity coefficient representing carbon; X represents the activity coefficient of carbon in an infinitely diluted state in the matrix solvent; c X represents the mole fraction of carbon atoms. Cr X represents the mole fraction of chromium atoms. Ni The mole fraction representing nickel atoms. This represents the carbon-to-carbon self-interaction coefficient, with a value of +8.0. The value represents the activity interaction coefficient of chromium with carbon, and is -12. The value represents the activity interaction coefficient between nickel and carbon, and is +4.8.

[0027] Heterogeneous driving force amplification factor The difference between the natural logarithms of the activity coefficients on both sides of 16MnD5 / IN690: , where γ1 represents the activity coefficient of carbon atoms on the 16MnD5 base metal side, and γ2 represents the activity coefficient of carbon atoms on the IN690 weld side.

[0028] ;

[0029] In the formula, η is the heat absorption rate, P is the power, and v is the energy. scan For the scanning speed, λ eff T is the equivalent thermal conductivity of the interface. c T0 is the critical activation temperature, and T0 is the ambient temperature.

[0030] The classical solubility product equation for NbC in austenite / liquid phase is:

[0031] ;

[0032] Following the modified Darken equation:

[0033] ;

[0034] In the formula, D eff R is the effective diffusion coefficient of carbon in nickel-based solid solutions. c To achieve an effective residual free carbon ratio, W eff The thickness of the brittle carbon-rich layer;

[0035] ;

[0036] ;

[0037] In the formula, C(W) eff C represents the interface dynamic fatigue crack propagation constant. base The parameter represents the fracture mechanics parameter, λ represents the embrittlement sensitivity coefficient, and a0 represents the initial crack size, which is taken as 0.5 mm. c Represents the critical instability fracture size, a c Take 20mm, m represents the fatigue crack propagation index, m is taken as 3.0, da represents the integral over a, a represents the crack length variable, Y represents the geometry factor, Y=1.0 or 1.12, Δσ represents the specific stress, Δσ is taken as 200MPa, N f This represents the remaining number of iterations.

[0038] Furthermore, the aforementioned process is applied to dissimilar metal welding in nuclear power plants.

[0039] Furthermore, the application of laser-welded interfaces between 16MnD5 and 316L stainless steel in the primary loop system of pressurized water reactor nuclear power plants.

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

[0041] This invention relates to the field of dissimilar metal welding technology for nuclear power plants, specifically a laser filler wire welding method for 16MnD5 steel and 316L stainless steel used in nuclear power plants. The invention discloses a laser welding method for 16MnD5 and 316L dissimilar steels based on a double-ring layered shielding gas, relating to the field of special welding for nuclear power equipment. Addressing the technical problem of easily forming a carbon-rich zone at the interface and internal carbon segregation during the welding of dissimilar thick plates in nuclear power plants, and the incompatibility between decarburization and oxidation prevention, the technical solution adopted in this invention is as follows: using IN690 alloy coaxial wire feeding for multi-pass laser welding, and applying high-frequency pulsed laser at a specific angle; the inventive point lies in the use of a coaxial double-ring gas shield, with the inner ring of pure argon gas protecting the wire feeding center, and the outer ring of active mixed gas covering the edge of the fusion line. This invention avoids overall oxidation by reconstructing the thermodynamic boundary through spatial layered airflow; and by utilizing the periodic energy impact and acoustic flow effect generated by high-frequency pulsed laser in the molten pool, it can not only stir the molten pool, but also shorten the liquid residence time of the molten pool, suppress the diffusion distance and time of C element, and promote oxygen penetration along dislocation channels by high-frequency laser shock wave, combining with Ti to generate titanium oxide as a heterogeneous nucleation core to promote NbC encapsulation growth for "microscopic carbon fixation" and "gas escape" of excess carbon oxidation. This dual mechanism decomposes network carbides, purifies grain boundaries, significantly improves the high-temperature performance and crack resistance of the joint, and increases the service life of the joint. Attached Figure Description

[0042] Figure 1 Schematic diagram of a double-ring layered protective gas for coaxial laser wire feeding; Figure 2(a) is a tissue morphology diagram of the joint in Example 1; Figure 2(b) is an elemental distribution diagram of element C in Example 1; Figure 2(c) shows the line scan results at the connector in Example 1; Figure 3(a) shows the tissue morphology at the joint in Comparative Example 1; Figure 3(b) is the element distribution diagram of element C in Comparative Example 1; Figure 3(c) shows the line scan results at the joint in Comparative Example 1; Figure 4(a) shows the tissue morphology at the joint in Comparative Example 2; Figure 4(b) is the element distribution diagram of element C in Comparative Example 2; Figure 4(c) shows the line scan results at the joint in Comparative Example 2; Figure 5(a) shows the tissue morphology at the joint in Comparative Example 3; Figure 5(b) is the elemental distribution diagram of element C in Comparative Example 3; Figure 5(c) shows the line scan results at the joint in Comparative Example 3; Figure 6 This diagram illustrates the molten pool fluctuations and the transition between the welding wire and the molten pool under the action of a high-frequency pulsed laser. Figure 7 A cross-sectional view of the double-ring layered protective gas for coaxial laser wire feeding. Detailed Implementation

[0043] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0044] This invention discloses a process for suppressing carbon migration at the laser welding interface between 16MnD5 and 316L stainless steel used in nuclear power plants, comprising the following steps:

[0045] Step 1: The nuclear power 16MnD5 steel and 316L stainless steel to be welded are beveled to ensure that the area to be welded is smooth and burr-free, remove surface impurities, and assemble the test plate.

[0046] Step 2: Using laser coaxial wire feeding welding method, adjust the laser beam and coaxial wire feeding nozzle to align with the center of the bevel, and perform multi-layer multi-pass fusion welding;

[0047] The welding wire fed coaxially is 690 nickel-based welding wire;

[0048] Step 3: While the main laser is working, a high-frequency pulsed laser is simultaneously applied to the side to assist in irradiating the weld pool;

[0049] The main laser power is controlled at 3000W-4000W, the wire feeding speed is 5-10 mm / s, and the scanning speed is 10~15 cm / min; a high-frequency pulsed laser is simultaneously activated for assistance, with a pulsed laser power of 500-800W, a pulse repetition frequency of 1-2MHz, a pulse width of 2-10ns, a scanning interval of 50-200μm, and a scanning width of 1-3mm.

[0050] Step 4: Throughout the laser welding process, a coaxial double-ring gas shield with a spatial composition gradient is applied to the welding area simultaneously. The double-ring gas shield includes an inner ring of pure inert gas close to the wire feed center and an outer ring of active mixed gas surrounding the inner ring.

[0051] The inner ring pure inert protective gas is argon with a purity of not less than 99.99%, and the gas flow rate is controlled at 8~12 L / min;

[0052] The outer ring active mixed protective gas is a mixture of argon and carbon dioxide, with carbon dioxide accounting for 15% to 30% by volume. The outer ring gas flow rate is controlled at 15 to 20 L / min, and the outflow velocity of the outer ring gas is greater than that of the inner ring gas.

[0053] Step 5: In the multi-layer, multi-pass welding process, after each weld is completed, the surface of the current weld bead is cleaned. Repeat the above steps until all welds are completed.

[0054] This invention also discloses a method for assessing the lifespan of the laser-welded interface between 16MnD5 and 316L stainless steel in nuclear power plants, comprising the following steps:

[0055] At the 16MnD5 / IN690 interface, the activity coefficient γ of carbon in the multi-component alloy c satisfy:

[0056] ;

[0057] Where, γ c The activity coefficient representing carbon; X represents the activity coefficient of carbon in an infinitely diluted state in the matrix solvent; c X represents the mole fraction of carbon atoms. Cr X represents the mole fraction of chromium atoms. Ni The mole fraction representing nickel atoms. This represents the carbon-to-carbon self-interaction coefficient, with a value of +8.0. The value represents the activity interaction coefficient of chromium with carbon, and is -12. The value represents the activity interaction coefficient between nickel and carbon, and is +4.8.

[0058] Heterogeneous driving force amplification factor The difference between the natural logarithms of the activity coefficients on both sides of 16MnD5 / IN690: , where γ1 represents the activity coefficient of carbon atoms on the 16MnD5 base metal side, and γ2 represents the activity coefficient of carbon atoms on the IN690 weld side.

[0059] ;

[0060] In the formula, η is the heat absorption rate, P is the power, and v is the energy. scan For the scanning speed, λ eff T is the equivalent thermal conductivity of the interface. c T0 is the critical activation temperature, and T0 is the ambient temperature.

[0061] The classical solubility product equation for NbC in austenite / liquid phase is:

[0062] ;

[0063] Following the modified Darken equation:

[0064] ;

[0065] In the formula, D eff R is the effective diffusion coefficient of carbon in nickel-based solid solutions. c To achieve an effective residual free carbon ratio, W eff The thickness of the brittle carbon-rich layer;

[0066] ;

[0067] ;

[0068] In the formula, C(W) eff C represents the interface dynamic fatigue crack propagation constant. base The parameter represents the fracture mechanics parameter, λ represents the embrittlement sensitivity coefficient, and a0 represents the initial crack size, which is taken as 0.5 mm. c Represents the critical instability fracture size, a c Take 20mm, m represents the fatigue crack propagation index, m is taken as 3.0, da represents the integral over a, a represents the crack length variable, Y represents the geometry factor, Y=1.0 or 1.12, Δσ represents the specific stress, Δσ is taken as 200MPa, N f This represents the remaining number of iterations.

[0069] This invention overcomes the limitations of homogenization in traditional mixed protective gases by forming a "corrugated inner and active outer" physical spatial isolation through a coaxial double-ring gas shield. The resulting technical effects are as follows:

[0070] 1. Reducing carbon concentration at the interface. This invention precisely purges an outer ring of active protective gas containing CO2 to the outer fusion line region. At the edge of the high-temperature plasma, CO2 micro-dissociates into active oxygen, which undergoes an in-situ micro-decarburization reaction with transboundary carbon atoms (generating CO that escapes). This significantly reduces the local carbon concentration and molar fraction at the interface, fundamentally flattening the chemical potential gradient at the interface, making the thermodynamic driving force for carbon diffusion into the depth of the weld approach zero.

[0071] 2. Kinetic freezing (cut-off diffusion time) of inner ring inert protection and intercalation transition:

[0072] An inner ring of pure argon gas creates an oxygen-free isolation chamber at the wire feeding center, ensuring that the Cr element in the IN690 welding wire is not burned off. Simultaneously, optimized wire feeding and scanning speeds maintain the welding wire in an intercalation-pool transition mode. This mode not only eliminates spatter caused by droplet transition, but the continuously inserted welding wire also acts as an in-situ cold source, significantly reducing the molten pool temperature gradient and providing a surge of external nucleation particles, effectively interrupting the continuous growth of coarse columnar crystals. According to the diffusion kinetics model, the effective diffusion distance is:

[0073] ;

[0074] Where L represents the effective diffusion distance, which is the ideal distance that a carbon atom can travel due to its own thermal motion at a given temperature and time; L is an ideal value; the thickness W of the subsequent brittle carbon-rich layer... eff It is the distance that can be moved under actual conditions. It depends not only on temperature and time, but also on chemical potential and the number of carbon atoms that actually participate in diffusion. t represents the high-temperature residence time, D represents the effective diffusion coefficient, D0 represents the diffusion constant (obtained from the Handbook of Physical Chemistry of Metallurgy), Q represents the diffusion activation energy (an intrinsic constant of the material), R represents the ideal gas constant, and T represents the thermodynamic temperature of the system.

[0075] The combined effect of the cold source in the molten pool and the megahertz acoustic current oscillation of high-frequency pulses (1-2MHz) cause the cooling rate of the central molten pool to surge to 10. 3 -10 5 K / s. The high-temperature residence time t is compressed to near 0, and the residual carbon atoms attempting to cross the interface are instantly frozen in situ by the solid-liquid phase transition, effectively limiting the macroscopic enrichment to a safe submicron scale.

[0076] 3. Space aerodynamics to eliminate internal segregation:

[0077] The outflow velocity of the outer ring gas is greater than that of the inner ring, forming an outward hydrodynamic curtain at the boundary between the two gas layers, effectively blocking the entrainment of active oxygen towards the center of the molten pool. The Marangoni convection induced by the high-frequency pulse, combined with this gas flow topology, compensates for the internal solute segregation defects caused by the dispersion without consumption of pure argon gas, thus achieving deep homogenization of carbon elements.

[0078] In order to conduct a quality quantification evaluation and service life prediction of the double-ring layered shielding gas laser welding process described in this invention, and to demonstrate the outstanding technical advantages of the "double-ring layered shielding gas high-frequency pulsed laser welding process" described in this invention in solving the problem of carbon "uphill diffusion" at the extreme heterogeneous interface of 16MnD5 / IN690, eliminating the brittle carbon-rich layer, and improving service life, this invention constructs a set of theoretical deduction and evaluation models based on classical physicochemical laws.

[0079] 4. A thermodynamic driving model for carbon diffusion at heterogeneous interfaces (based on the Wagner activity interaction model)

[0080] At the 16MnD5 / IN690 interface, the fundamental driving force for carbon atom diffusion is the chemical potential gradient. According to the Wagner activity interaction model, the activity coefficient γ of carbon in the multi-component alloy... c satisfy:

[0081] ;

[0082] Where, γ c The activity coefficient representing carbon; The activity coefficient representing carbon in an infinitely diluted state in the matrix solvent is used to evaluate the "carbon uphill diffusion" at the 16MnD5 / IN690 interface. This is because the study focuses on the "difference" (driving force) of the chemical potentials on both sides of the interface, rather than the absolute chemical potential value on one side. When establishing the thermodynamic model, a common reference scale must be selected for both sides of the interface (low-alloy steel side and nickel-based alloy side) for consistent comparison (usually defined as the austenitic reference state of a face-centered cubic (FCC) structure). This means that regardless of whether the carbon atom is on the 16MnD5 side or the IN690 side, the activity coefficient used in the formula is the same. They are numerically identical constants; X i The mole fraction representing each alloying element is determined based on the actual chemical composition of the base material and the welding wire; that is, X c X represents the mole fraction of carbon atoms. Cr X represents the mole fraction of chromium atoms. Ni The mole fraction representing nickel atoms. This represents the carbon-to-carbon self-interaction coefficient. This represents the activity interaction coefficient between chromium and carbon. This represents the activity interaction coefficient between nickel and carbon.

[0083] The parameter is the intrinsic thermodynamic constant of the material, which can be obtained by consulting classical metallurgical thermodynamics handbooks or using the CALPHAD (phase diagram calculation) standard database. In a face-centered cubic (FCC) matrix at 1000℃ (1273K), this constant is typically taken as +8.0; and The intrinsic thermodynamic constants of the material can be found by consulting the Metallurgical Thermodynamics Standard Handbook or the CALPHAD Modern Phase Diagram Database, and can be taken as -12 and +4.8 respectively.

[0084] and These are the activity interaction coefficients of chromium and nickel to carbon, respectively. Because the IN690 side is rich in the strong carbide-forming element Cr (… (<0 and extremely large absolute value), resulting in a strong carbon chemical potential well at the interface. This invention defines a heterogeneous driving force amplification factor. The difference between the natural logarithms of the activity coefficients on both sides: γ1 represents the activity coefficient of carbon atoms on the 16MnD5 base metal side, and γ2 represents the activity coefficient of carbon atoms on the IN690 weld side.

[0085] It is used to correct the thickness W of brittle carbon-rich layers. eff Correction parameters;

[0086] 5. Heat transfer model of high-temperature heat cycle in welding (based on Rosenthal's analytical solution)

[0087] The long-range diffusion depth of carbon is limited by the residence time t at the interface in the high-temperature region (typically above 1000°C). high Based on the Rosenthal moving line heat source model, for a specific heat input, the effective residence time approximately satisfies:

[0088] ;

[0089] In the formula, η is the heat absorption rate, P is the power, and v is the energy. scan For the scanning speed, λ eff T is the equivalent thermal conductivity of the interface. c T0 is the critical activation temperature, and T0 is the ambient temperature.

[0090] 6. The present invention provides a carbon source dissipation kinetic model at the ultra-high temperature liquid / solidification front (based on reaction kinetics and the solubility product law).

[0091] Unlike traditional solid-state diffusion models, the process of this invention forcibly strips the carbon source at the molten pool stage, involving both a gas-phase chemical dissipation mechanism and a solid-phase microscopic pinning mechanism. The gas-phase chemical dissipation mechanism involves the dissociation of outer-ring CO2 into highly reactive oxygen species under the action of laser plasma, resulting in a dominant microscopic decarburization reaction at the molten pool boundary. The solid-phase micro-pinning mechanism targets Nb and Ti elements in the system. The acoustic cavitation effect induced by high-frequency pulsed laser completely breaks down the heterogeneous nucleation barrier of high-melting-point carbides, forcing them to reach thermodynamic solubility product equilibrium instantaneously at the solidification front. The classical solubility product equation for NbC in austenite / liquid phase is:

[0092] ;

[0093] 7. Modified effective carbon-rich layer and fatigue life prediction model (based on Darken equation and Paris law)

[0094] After forced dissipation in the liquid phase, the "effective residual free carbon ratio" that allows long-range diffusion at the interface upon entering the solid phase is defined as R. c The final brittle carbon-rich layer thickness W eff Following the modified Darken equation:

[0095] ;

[0096] In the formula, D eff Let W be the effective diffusion coefficient of carbon in a nickel-based solid solution. The thickness W of this brittle, carbon-rich layer is... eff This directly determines the degree of grain boundary embrittlement. Substituting this into the Paris fatigue crack propagation life integral model:

[0097] ;

[0098] ;

[0099] In the formula, C(W) eff C represents the interface dynamic fatigue crack propagation constant. base Represents fracture mechanics parameters, λ represents the embrittlement sensitivity coefficient, and N represents the fracture mechanics parameter. f This represents the remaining number of welding cycles for the weld joint, and a0 represents the initial crack size, obtained according to the ASME standard for nuclear power or the ultrasonic testing limits. a0 is taken as 0.5 mm. c Represents the critical instability fracture size; determined by the material's fracture toughness K. IC and maximum working stress σ max Calculation yields ( ), a cThe value is approximately 20 mm. m represents the fatigue crack propagation index, which is set to 3.0. da represents the integral over a, where a represents the crack length variable. Y represents the geometry factor, obtained from a fracture mechanics stress intensity factor handbook; in engineering estimations, it is usually simplified to Y = 1.0 or 1.12. Δσ represents a specific stress, representing the alternating stress amplitude / stress range; determined based on actual service conditions. Typically 100-300 MPa; in this invention, Δσ is set to 200 MPa. N f This represents the remaining number of iterations.

[0100] Traditional welding suffers from the presence of a carbon-rich layer (i.e., a brittle carbon-rich layer with a thickness W). eff The material constant C value at its interface (i.e., C(W)) eff Extremely high (highly prone to cracking). This invention purifies grain boundaries through a dual-ring layered protective gas and microscopic carbon fixation mechanism. Experimental calibration shows a significantly reduced C value and optimized m value under this process. Substituting these parameters into the life integral equation, the remaining cycle number N of the joint under a specific stress Δσ can be predicted. f This predictive model provides crucial data support for the safe operation and periodic maintenance of nuclear power equipment.

[0101] 8. Service life and safety margin demonstration based on nuclear power design basis transients:

[0102] In the design specifications for the safety end of the primary loop main pipeline in third-generation nuclear power plants, the macroscopic service life of the joint must strictly meet the Miner linear fatigue cumulative damage criterion of the ASME or RCC-M specifications (i.e., For 16MnD5 / IN690 dissimilar metal welded joints, one of the most severe challenges they face during their 60-year design life comes from the combined effect of high-frequency thermal fluctuations of the main coolant and fluid-induced vibration (FIV).

[0103] According to the typical third-generation nuclear power plant design transient specifications, under a specific reference alternating stress frequency band (e.g., Δσ≈200MPa), the number of statutory baseline cycles (n) for this operating condition during a 60-year service life is approximately 2.0 × 10⁻⁶. 6 Second-rate.

[0104] 9. Calculation of the decay of thermodynamic effective pinning rate

[0105] Under pure argon gas protection, the interference of a 1MHz high-frequency pulsed laser on carbon atoms degenerates from an ideal state of purely thermodynamic absolute pinning to a composite process of thermodynamic partial pinning and kinetic mechanical dispersion. This embodiment, based on multiphysics coupling theory, studies the comprehensive effective rejection rate R under pure argon gas conditions. total Corrections have been made:

[0106] The overall retention rate is determined by the thermodynamic effective pinning rate η. thermoWith mechanical dispersion retention rate η mech The superposition structure has the following probabilistic coupling equation:

[0107] ;

[0108] In a pure argon atmosphere, photo-induced plasma shielding is readily generated. According to the inverse bremsstrahlung theory, the transmittance of the pulsed laser reaching the molten pool surface after penetrating the plasma cloud drops sharply to about 35%. The pulse energy attenuation leads to a significant reduction in the initial ultrasonic pressure excited within the liquid metal, enabling it to exceed the liquid phase cavitation threshold (P > P). threshold The effective volume of the molten pool shrinks drastically. The actual volume fraction of the molten pool undergoing strong acoustic cavitation is calculated by integrating the acoustic field energy attenuation model. It accounts for only about 32% of the total molten pool.

[0109] Therefore, the ideal maximum phase change pinning rate of 94% is significantly reduced, and its actual thermodynamic effective pinning rate is corrected to:

[0110] ;

[0111] That is, only about 30% of the free carbon is bound in nano-NbC. The remaining nearly 70% of the free carbon is freed from thermodynamic binding.

[0112] For the 70% of free carbon that has escaped thermodynamic constraints, although the energy of the 1MHz high-frequency acoustic current is weakened, the macroscopic mechanical vibrations it generates still induce a significant grain boundary effect in the solidification region. According to ultrasonic solidification kinetics, the shear force of the high-frequency acoustic current forcibly breaks up the leading edge of the growing columnar crystals, promoting significant grain refinement. Under conventional pure laser conditions, the average grain size at the weld is d0 (approximately 126 μm), while under the action of pulsed acoustic current, the average grain size is refined to d0. u (Approximately 92 μm).

[0113] Grain boundary specific surface area S v The grain boundary multiplication factor β is defined as being inversely proportional to the grain size.

[0114] ;

[0115] Because the total area of ​​the grain boundaries increased by nearly 1.37 times, the concentration of free carbon segregated at the grain boundaries was greatly diluted. The effective carbon concentration capable of forming a continuous network of brittle phases was dispersed. The proportion of harmless free carbon dispersed to the newly added grain boundaries and unable to form a network is the mechanical dispersion rejection rate η. mech :

[0116] ;

[0117] That is, of the free carbon that escaped thermodynamic pinning, about 27% was mechanically shattered and diluted in a large number of grain boundaries, losing its ability to form a macroscopic brittle carbon-rich layer.

[0118] Substituting the corrected results of the two independent physical effects into the probabilistic coupling equation:

[0119] R total =30.08%+(1-30.08%)×27%≈49% The laser-welded interface between nuclear power 16MnD5 and 316L stainless steel obtained by the process of the present invention has no brittle carbon-rich region.

[0120] The application of the process of this invention in dissimilar metal welding in nuclear power plants.

[0121] The application of the laser-welded interface between 16MnD5 and 316L stainless steel obtained by this invention in the primary loop system of a pressurized water reactor nuclear power plant.

[0122] Example 1

[0123] like Figure 1 , Figure 6 and Figure 7As shown, 16MnD5 low-alloy steel plates and 316L austenitic stainless steel plates, both 15mm thick, for nuclear power applications, were selected. The weldable ends of the two base materials were machined, with a single-sided bevel angle of 20°. After grinding and cleaning the bevel and its surrounding areas with anhydrous ethanol, the two base materials were butt-jointed flush, forming a V-shaped bevel with a total angle of 40° on both sides. Inconel 690 alloy was selected as the filler wire. The laser welding head was adjusted so that the main laser beam and the coaxial wire feed nozzle were aligned with the center of the bevel; then, the position and orientation of the high-frequency pulse laser head were adjusted so that the high-frequency pulse laser beam and the main laser beam formed a 10° angle in space, ensuring that the pulse energy was injected obliquely into the molten pool and that the two laser heads did not physically interfere. The distance between the two laser beams was 5mm. The protective gas valve was opened in advance, and 99.99% pure argon gas was introduced into the inner ring at a flow rate of 10 L / min; a mixture of 80% Ar and 20% CO2 (by volume) was introduced into the outer ring at a flow rate of 18 L / min. The coaxial wire-feeding main laser was activated, with a power of 3000W, a wire feed speed of 5 mm / s for the Inconel 690 alloy welding wire, and a scanning speed of 15 cm / min along the weld direction. While the main laser melted the welding wire and the micro-melted base material, a high-frequency pulsed laser was simultaneously activated to assist in irradiating and stirring the molten pool. The pulsed laser parameters were set as follows: power 500W, pulse repetition frequency 1MHz, pulse width 3ns, scanning interval 100μm, and scanning width 2mm. Under these coordinated parameters, the root pass weld was completed. Due to the 15mm thickness of the base material, a multi-layer, multi-pass welding process was required. After each weld pass is completed, pause the laser beam and wire feed, and use a brush to thoroughly clean the cooled weld surface, removing any trace oxides and spatter residue. After cleaning, repeat the co-welding process in step three, stacking layers for filler welding. After each weld pass, strictly adhere to the brush cleaning procedure until the Inconel 690 alloy deposited metal completely fills the 40° bevel, completing the overall welding of 16MnD5 and 316L.

[0124] In order to conduct a quality quantification evaluation and service life prediction of the double-ring layered protective gas laser welding process described in this invention, we input experimental parameters into the calculation model.

[0125] First, some fundamental constants and experimental parameters are determined. Wagner interaction coefficient (1000℃): =-12.0, =+4.8. IN690 side mole fraction X Cr =0.30, X Ni =0.60; 16MnD5 side X Cr =0, X Ni =0.007. The calculated heterogeneous driving force factor is... =0.753. Heat transfer constant (i.e., equivalent thermal conductivity at the interface): λ eff =25 W / (m·K), T c - T0=980℃. Diffusion coefficient: Effective diffusion coefficient of carbon at IN690 grain boundaries at 1000℃: D eff =2.0×10 -11 m 2 / s. Fracture mechanical parameters: C base =1.0×10 -12 The embrittlement sensitivity coefficient λ = 1.5 × 10 5 m -1 m=3.0. The sum of the constant terms in the mechanical integral is approximately 9.57×10. -6 The sum of the constant terms in the mechanical integral is... The main laser power is 3000W, the scanning speed is 15 cm / min (2.50 mm / s), and the absorptivity η=0.6. The outer annular mixed gas flow rate is 18 L / min, with a CO2 volume fraction of 20%. The high-frequency pulsed laser power is 500W, and the frequency is 1MHz.

[0126] The decarbonization behavior of this invention occurs in the liquid phase, and a dual mechanism is deduced:

[0127] Mechanism 1: Absolute stoichiometry surplus of peripheral reactive gas

[0128] A mixed gas containing 20% ​​CO2 is introduced into the outer ring at a rate of 18 L / min, providing a molar supply rate of 2.67 × 10⁻⁶ CO2 reactive oxygen species. -3 mol / s. And based on the volume of a conventional laser melt pool (approximately 10 mm²),... 3 Based on the carbon content (0.20%) of 16MnD5, the total molar amount of carbon in the molten pool is only about 1.16 × 10⁻⁶. -5 The oxygen supply exceeded the total carbon content by 230 times, and under the highly activated state of plasma, thermodynamically forced removal of free carbon at the interface was achieved.

[0129] Mechanism 2: High-frequency pulse forced solubility product equilibrium

[0130] Under the cavitation effect induced by a 1MHz high-frequency pulse, the system is forced to reach NbC solubility product equilibrium at the solidification front (1600K, i.e., 1327℃): log[Nb][C] = 2.96 - 7520 / 1600 = -1.74. The equilibrium solubility product of the system is 10. -1.74 =0.018. Given that [Nb] ≥ 1.5 in the welding wire, what is the maximum allowable free carbon concentration [C] in the liquid phase? free=0.018 / 1.5=0.012%. Compared to the initial carbon concentration of 0.20% in 16MnD5, the pulse consolidation mechanism locks approximately 94% of the carbon in the ultra-high melting point nanoscale NbC during solidification.

[0131] After forced dissipation through the above two mechanisms, upon entering the solid-state diffusion temperature range (1000℃), the "effective residual free carbon ratio" R at the interface... c →0. Therefore, although the Wagner model indicates a large chemical potential difference, due to the almost absence of free carbon in this embodiment, the final macroscopically observed effective carbon-rich layer thickness, i.e., the brittle carbon-rich layer thickness W, is... eff =0µm.

[0132] The interface is restored to the ideal matrix purity, and the fracture constant (i.e., the dynamic fatigue crack propagation constant C(W)) is restored. eff =1.0 × 10 -12 The predicted fatigue life jumps to N f =9.57×10 6 Secondly, under the same working conditions, its single transient fatigue damage level drops sharply to U. 0.21. Meets the Miner linear fatigue cumulative damage criterion of ASME or RCC-M standards (i.e. ).

[0133] As shown in Figures 2(a)-2(c), the combination of coaxial laser wire feeding and high-frequency pulsed laser, along with the use of a double-ring layered protective gas, resulted in a joint with uniform carbon distribution at the interface and diffuse niobium carbide distribution between dendrites, without significant enrichment, thus completely suppressing the formation of carbon-rich regions. The experimental results are consistent with the predicted formula results.

[0134] The laser-welded interface between nuclear power 16MnD5 and 316L stainless steel obtained by the process described in this embodiment has no brittle carbon-rich region.

[0135] The process described in this embodiment is applied to dissimilar metal welding in nuclear power plants.

[0136] The application of the laser-welded interface between 16MnD5 and 316L stainless steel obtained in this embodiment in the primary loop system of a pressurized water reactor nuclear power plant.

[0137] Comparative Example 1

[0138] The difference between this comparative example and Example 1 is that the laser coaxial wire feeding + high-frequency pulsed laser was replaced with pure TIG welding. Ar shielding gas was continuously introduced into the welding area at a flow rate controlled at 10 L / min. The specific TIG welding process parameters are as follows: arc starting current 190A, base current 200A, peak current 200A, scanning speed 20cm / min, and wire feed speed 8mm / s.

[0139] That is, the difference between this comparative example and Example 1 is that the laser coaxial wire feeding + high-frequency pulsed laser is replaced with TIG, and the protective gas is replaced with Ar.

[0140] The experimental parameters from this comparative example were substituted into the prediction calculation model. The experimental parameters and constants are as follows: equivalent power 3000W (200A×15V), scan speed 20 cm / min, absorption rate η=0.7. No shielding gas chemical consumption, no pulse. Substituting into the Rosenthal equation, the equivalent high-temperature dwell time t for TIG welding... high =4.10s. Due to the lack of a carbon dissipation mechanism in the ultra-high temperature liquid phase stage in traditional processes, the proportion of residual free carbon R c =100%=1.0. Substituting into the modified Darken equation, the theoretical carbon-rich layer thickness (i.e., the brittle carbon-rich layer thickness W) is... eff ):

[0141] ;

[0142] Theoretical proof shows that under a huge chemical potential difference ( Driven by a coefficient of 0.753, traditional TIG welding inevitably produces a micron-sized, heavily carbon-rich layer.

[0143] Life prediction: Embrittlement constant, i.e. fracture constant (i.e., interfacial dynamic fatigue crack propagation constant C(W)). eff = 2.78 × 10 -12 Predicting fatigue life N f（TIG） =3.44×10 6 This single operating condition alone has resulted in localized fatigue damage as high as U. 0.58. This result means that the joint has consumed nearly 60% of its fatigue life. After being subjected to extremely low-cycle high-stress transients such as reactor start-up and shutdown, it is highly likely to cause the total damage U>1.0, making it impossible to guarantee 60 years of safe operation. As shown in Figures 3(a)-3(c), the carbon element distribution at the interface of the joint welded by TIG is extremely uneven, with a significant carbon-rich zone at the interface. This is almost consistent with the predicted results.

[0144] Comparative Example 2

[0145] The difference between this comparative example and Example 1 is that the laser coaxial wire feeding + high-frequency pulsed laser was replaced with pure laser coaxial wire feeding welding. Ar gas was continuously introduced into the welding area at a flow rate controlled at 10 L / min as a protective gas.

[0146] That is, the only difference between this comparative example and Example 1 is that the laser coaxial wire feeding + high-frequency pulsed laser is replaced with pure laser coaxial wire feeding, and the protective gas is replaced with Ar.

[0147] The main laser power was 3000W, and the scanning speed was 15 cm / min (2.50 mm / s). 100% pure argon gas was used (decarburization dissipation was reduced to zero), and the high-frequency pulse was turned off (solid-phase pinning was reduced to zero). Heterogeneous chemical potential driving force factor: =0.753; Effective diffusion coefficient: D eff =2.0×10 -11 m 2 / s; Three-dimensional chilling equivalent high-temperature residence time: t Laser =0.80s; Effective residual free carbon ratio: R c =1.0 (i.e., 100% residue); Reference constant (i.e., fracture mechanics parameter): C base =1.0×10 -12 The embrittlement sensitivity coefficient λ = 1.5 × 10 5 m -1 The sum of the life integral constant term, i.e., the mechanical integral constant term, is approximately 9.57 × 10⁻⁶. -6 .

[0148] In this process, free carbon is suppressed entirely by physical quenching. Substituting into the modified Darken diffusion equation:

[0149] ;

[0150] Since this comparative example has a carbon-rich region of 3.01 μm, it is substituted into the Paris dynamic fatigue damage model:

[0151] Predicting fatigue life:

[0152] Next, the local fatigue damage U caused by this single working condition 0.33.

[0153] As shown in Figures 4(a)-4(c), the joints welded using the pure coaxial wire feeding method have a more uniform C element distribution compared to Comparative Example 1, but there are still obvious carbon-rich areas. This is almost consistent with the predicted results.

[0154] Comparative Example 3

[0155] The difference between this comparative example and Example 1 is that Ar is continuously introduced into the welding area at a flow rate controlled at 10 L / min as a protective gas.

[0156] That is, the only difference between this comparative example and Example 1 is that the double-ring layered protective gas is replaced with Ar gas.

[0157] The experimental parameters of this comparative example were substituted into the prediction calculation model. The experimental parameters and constants are as follows: the main laser power and scanning speed are the same as in Example 1. The high-frequency pulse parameters are also the same as in Example 1. The protective gas parameter is pure argon. Therefore, the CO2 volume fraction... Substituting into the Rosenthal equation, the equivalent high-temperature residence time t high =4.68s. Heterogeneous driving force factor. =0.753, effective diffusion coefficient D eff =2.0×10 -11 m 2 / s.

[0158] Under uninterrupted conditions, the theoretical thickness of the solid-state diffusion layer caused by a 3000W laser thermal input is:

[0159] ;

[0160] Because the protective gas is pure argon, the system lacks an active oxygen source. The gas-phase decarburization reaction at the molten pool boundary terminates. Chemical dissipation accounts for 0%. Regarding microscopic pinning, due to the strong photo-induced plasma shielding effect easily generated in the pure argon environment, the dense plasma cloud above the molten pool absorbs a large amount of energy from the 1MHz pulsed laser. The pulse energy cannot be effectively coupled to the bottom of the molten pool, resulting in a significant attenuation of high-frequency acoustic cavitation in the liquid phase. Therefore, the system cannot be forced to reach perfect NbC theoretical solubility product equilibrium at the solidification front. However, the 1MHz high-frequency ultrasound still excites a significant "acoustic flow fragmentation and grain boundary multiplication effect" in the lower part of the molten pool. The intense high-frequency vibration physically tears apart the continuous carbide network nucleating at the grain boundaries, dispersing it; at the same time, it significantly refines the solidification structure, causing the total grain boundary area to increase geometrically, greatly diluting the carbon-rich concentration per unit area.

[0161] After comprehensive correction using acoustic field physics, the "effective pinning and dispersion rejection rate" under a pure argon pulse is approximately 49%. The effective residual free carbon ratio R... c =100%-0%-49%=51%.

[0162] Although the pulsed laser significantly weakened the carbon source, due to the lack of reactive gas, 51% of the free carbon still diffused into the IN690 side in a solid state driven by the huge chemical potential difference.

[0163] Substituting into the modified Darken equation:

[0164] ;

[0165] The submicron-scale residual carbon-rich layer with a thickness of 3.71 µm in this comparative example, although insufficient to constitute a severe continuous network brittle phase, still introduces slight stress concentration sources at the micrograin boundaries.

[0166] ;

[0167] C in the above formula inv_Ar That is, C(W) eff ).

[0168] Predicting fatigue life: The local fatigue damage U caused by this single working condition 0.36.

[0169] As shown in Figures 5(a)-5(c), the weld joints using a combination of coaxial laser wire feeding and high-frequency pulsed laser, with Ar gas as the shielding gas, exhibit uneven carbon distribution despite the absence of a clearly defined carbon-rich region. This is almost consistent with the predicted results.

[0170] Example 2

[0171] The only difference between this embodiment and Embodiment 1 is that:

[0172] A process for suppressing carbon migration at the laser welding interface between 16MnD5 and 316L stainless steel used in nuclear power plants, characterized by comprising the following steps:

[0173] Step 1: The nuclear power 16MnD5 steel and 316L stainless steel to be welded are beveled to ensure that the area to be welded is smooth and burr-free, remove surface impurities, and assemble the test plate.

[0174] Step 2: Using laser coaxial wire feeding welding method, adjust the laser beam and coaxial wire feeding nozzle to align with the center of the bevel, and perform multi-layer multi-pass fusion welding.

[0175] The welding wire fed coaxially is 690 nickel-based welding wire.

[0176] Step 3: While the main laser is working, a high-frequency pulsed laser is simultaneously applied to the side to assist in irradiating the weld pool;

[0177] The main laser power is controlled at 4000W, the wire feeding speed is 10 mm / s, and the scanning speed is 10 cm / min; a high-frequency pulsed laser is simultaneously activated for assistance, with a pulsed laser power of 800W, a pulse repetition frequency of 2MHz, a pulse width of 10ns, a scanning interval of 200μm, and a scanning width of 3mm.

[0178] The spatial angle between the high-frequency pulsed laser and the main laser is 20°; the distance between the beam of the high-frequency pulsed laser and the beam of the coaxial wire-feeding main laser is 5mm.

[0179] Step 4: Throughout the laser welding process, a coaxial double-ring gas shield with a spatial composition gradient is applied to the welding area simultaneously. The double-ring gas shield includes an inner ring of pure inert gas close to the wire feed center and an outer ring of active mixed gas surrounding the inner ring.

[0180] The inner ring pure inert protective gas is argon with a purity of not less than 99.99%, and the gas flow rate is controlled at 8 L / min.

[0181] The outer ring active protective gas is a mixture of argon and carbon dioxide, with carbon dioxide accounting for 15% by volume, and the outer ring gas flow rate is controlled at 15 L / min.

[0182] Step 5: In the process of multi-layer and multi-pass welding, after each weld is completed, the surface of the current weld bead is cleaned. Repeat the above steps until all welding is completed.

[0183] Example 3

[0184] The only difference between this embodiment and Embodiment 1 is that:

[0185] A process for suppressing carbon migration at the laser welding interface between 16MnD5 and 316L stainless steel used in nuclear power plants, characterized by comprising the following steps:

[0186] Step 1: The nuclear power 16MnD5 steel and 316L stainless steel to be welded are beveled to ensure that the area to be welded is smooth and burr-free, remove surface impurities, and assemble the test plate.

[0187] Step 2: Using laser coaxial wire feeding welding method, adjust the laser beam and coaxial wire feeding nozzle to align with the center of the bevel, and perform multi-layer multi-pass fusion welding.

[0188] The welding wire fed coaxially is 690 nickel-based welding wire.

[0189] Step 3: While the main laser is working, a high-frequency pulsed laser is simultaneously applied to the side to assist in irradiating the weld pool;

[0190] The main laser power is controlled at 3500W, the wire feed speed is 8 mm / s, and the scanning speed is 12cm / min; a high-frequency pulsed laser is simultaneously activated for assistance, with a pulsed laser power of 600W, a pulse repetition frequency of 1.5MHz, a pulse width of 2ns, a scanning interval of 50μm, and a scanning width of 1mm.

[0191] The spatial angle between the high-frequency pulsed laser and the main laser is 15°; the distance between the beam of the high-frequency pulsed laser and the beam of the coaxial wire-feeding main laser is 8mm.

[0192] Step 4: Throughout the laser welding process, a coaxial double-ring gas shield with a spatial composition gradient is applied to the welding area simultaneously. The double-ring gas shield includes an inner ring of pure inert gas close to the wire feed center and an outer ring of active mixed gas surrounding the inner ring.

[0193] The inner ring pure inert protective gas is argon with a purity of not less than 99.99%, and the gas flow rate is controlled at 12 L / min.

[0194] The outer ring active protective gas is a mixture of argon and carbon dioxide, with carbon dioxide accounting for 30% by volume, and the outer ring gas flow rate is controlled at 20 L / min.

[0195] Step 5: In the multi-layer, multi-pass welding process, after each weld is completed, the surface of the current weld bead is cleaned. Repeat the above steps until all welds are completed.

[0196] Example 4

[0197] The only difference between this embodiment and Embodiment 1 is that:

[0198] A process for suppressing carbon migration at the laser welding interface between 16MnD5 and 316L stainless steel used in nuclear power plants, characterized by comprising the following steps:

[0199] Step 1: The nuclear power 16MnD5 steel and 316L stainless steel to be welded are beveled to ensure that the area to be welded is smooth and burr-free, remove surface impurities, and assemble the test plate.

[0200] Step 2: Using laser coaxial wire feeding welding method, adjust the laser beam and coaxial wire feeding nozzle to align with the center of the bevel, and perform multi-layer multi-pass fusion welding.

[0201] The welding wire fed coaxially is 690 nickel-based welding wire.

[0202] Step 3: While the main laser is working, a high-frequency pulsed laser is simultaneously applied to the side to assist in irradiating the weld pool;

[0203] The main laser power is controlled at 3500W, the wire feed speed is 6 mm / s, and the scanning speed is 13cm / min; a high-frequency pulsed laser is simultaneously activated for assistance, with a pulsed laser power of 700W, a pulse repetition frequency of 1MHz, a pulse width of 6ns, a scanning interval of 150μm, and a scanning width of 2mm.

[0204] The spatial angle between the high-frequency pulsed laser and the main laser is 15°; the distance between the beam of the high-frequency pulsed laser and the beam of the coaxial wire-feeding main laser is 6mm.

[0205] Step 4: Throughout the laser welding process, a coaxial double-ring gas shield with a spatial composition gradient is applied to the welding area simultaneously. The double-ring gas shield includes an inner ring of pure inert gas close to the wire feed center and an outer ring of active mixed gas surrounding the inner ring.

[0206] The inner ring pure inert protective gas is argon with a purity of not less than 99.99%, and the gas flow rate is controlled at 10 L / min.

[0207] The outer ring active protective gas is a mixture of argon and carbon dioxide, with carbon dioxide accounting for 25% by volume, and the outer ring gas flow rate is controlled at 18 L / min.

[0208] Step 5: In the multi-layer, multi-pass welding process, after each weld is completed, the surface of the current weld bead is cleaned. Repeat the above steps until all welds are completed.

[0209] In the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof. However, this disclosure should not be construed as reflecting an intention that the claimed invention requires more features than expressly recited in each claim. Rather, as reflected in the claims, the inventive aspect lies in fewer than all features of the foregoingly disclosed embodiments. Therefore, the claims, following the detailed description, are hereby expressly incorporated into that detailed description, wherein each claim itself is a separate embodiment of the invention.

[0210] Although the invention has been described with reference to a limited number of embodiments, those skilled in the art will understand from the foregoing description that other embodiments are conceivable within the scope of the invention described herein. Furthermore, it should be noted that the language used in this specification has been chosen primarily for readability and instructional purposes, and not for the purpose of interpreting or limiting the subject matter of the invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the invention is illustrative and not restrictive, and the scope of the invention is defined by the appended claims.

[0211] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A process for suppressing carbon migration at the laser welding interface between 16MnD5 and 316L stainless steel used in nuclear power plants, characterized in that, Includes the following steps: Step 1: The nuclear power 16MnD5 steel and 316L stainless steel to be welded are beveled to ensure that the area to be welded is smooth and burr-free, remove surface impurities, and assemble the test plate. Step 2: Using laser coaxial wire feeding welding method, adjust the laser beam and coaxial wire feeding nozzle to align with the center of the bevel, and perform multi-layer multi-pass fusion welding; Step 3: While the main laser is working, a high-frequency pulsed laser is simultaneously applied to the side to assist in irradiating the weld pool; The main laser power is controlled at 3000W-4000W, the wire feeding speed is 5-10 mm / s, and the scanning speed is 10~15 cm / min; a high-frequency pulsed laser is simultaneously activated for assistance, with a pulsed laser power of 500-800W, a pulse repetition frequency of 1-2MHz, a pulse width of 2-10ns, a scanning interval of 50-200μm, and a scanning width of 1-3mm. Step 4: Throughout the laser welding process, a coaxial double-ring gas shield with a spatial composition gradient is applied to the welding area simultaneously. The double-ring gas shield includes an inner ring of pure inert gas close to the wire feed center and an outer ring of active mixed gas surrounding the inner ring. The inner ring pure inert protective gas is argon with a purity of not less than 99.99%, and the gas flow rate is controlled at 8~12 L / min; The outer ring active mixed protective gas is a mixture of argon and carbon dioxide, with carbon dioxide accounting for 15% to 30% by volume. The outer ring gas flow rate is controlled at 15 to 20 L / min, and the outflow velocity of the outer ring gas is greater than that of the inner ring gas. Step 5: In the multi-layer, multi-pass welding process, after each weld is completed, the surface of the current weld bead is cleaned. Repeat the above steps until all welds are completed.

2. The process according to claim 1, characterized in that, In step one, the thickness of both the nuclear power 16MnD5 steel and 316L stainless steel is 15mm.

3. The process according to claim 1, characterized in that, In step one, the beveling process is as follows: the beveling angle on one side is 20°, and the total beveling angle formed after the butt joint assembly is 40°. The assembly gap between the nuclear power 16MnD5 steel and the 316L stainless steel test plate is 1.2mm.

4. The process according to claim 3, characterized in that, The surface roughness of the bevel Ra ≤ 5 μm.

5. The process according to claim 1, characterized in that, In step three, the spatial angle between the high-frequency pulsed laser and the main laser is between 10-20°.

6. The process according to claim 1, characterized in that, In step three, the distance between the high-frequency pulsed laser beam and the main laser beam that feeds the coaxial wire is 5-8 mm.

7. The laser-welded interface between nuclear power plant 16MnD5 and 316L stainless steel obtained by the process according to any one of claims 1-6, characterized in that, Non-brittle carbon-rich region.

8. The life assessment method for the laser-welded interface of 16MnD5 and 316L stainless steel in nuclear power plants according to claim 7, characterized in that, Includes the following steps: At the 16MnD5 / IN690 interface, the activity coefficient γ of carbon in the multi-component alloy c satisfy: ； Where, γ c The activity coefficient representing carbon; X represents the activity coefficient of carbon in an infinitely diluted state in the matrix solvent; c X represents the mole fraction of carbon atoms. Cr X represents the mole fraction of chromium atoms. Ni The mole fraction representing nickel atoms. This represents the carbon-to-carbon self-interaction coefficient, with a value of +8.

0. The value represents the activity interaction coefficient of chromium with carbon, and is -12. The value represents the activity interaction coefficient between nickel and carbon, and is +4.

8. Heterogeneous driving force amplification factor The difference between the natural logarithms of the activity coefficients on both sides of 16MnD5 / IN690: , where γ1 represents the activity coefficient of carbon atoms on the 16MnD5 base metal side, and γ2 represents the activity coefficient of carbon atoms on the IN690 weld side. ； In the formula, η is the heat absorption rate, P is the power, and v is the energy. scan For the scanning speed, λ eff T is the equivalent thermal conductivity of the interface. c T0 is the critical activation temperature, and T0 is the ambient temperature. The classical solubility product equation for NbC in austenite / liquid phase is: ; Following the modified Darken equation: ; In the formula, D eff R is the effective diffusion coefficient of carbon in nickel-based solid solutions. c To achieve an effective residual free carbon ratio, W eff The thickness of the brittle carbon-rich layer; ; ; In the formula, C(W) eff C represents the interface dynamic fatigue crack propagation constant. base The parameter represents the fracture mechanics parameter, λ represents the embrittlement sensitivity coefficient, and a0 represents the initial crack size, which is taken as 0.5 mm. c Represents the critical instability fracture size, a c Take 20mm, m represents the fatigue crack propagation index, m is taken as 3.0, da represents the integral over a, a represents the crack length variable, Y represents the geometry factor, Y=1.0 or 1.12, Δσ represents the specific stress, Δσ is taken as 200MPa, N f This represents the remaining number of iterations.

9. The application of the process according to any one of claims 1-6 in the welding of dissimilar metals in nuclear power plants.

10. The application of the laser-welded interface between 16MnD5 and 316L stainless steel as described in claim 7 in the primary loop system of a pressurized water reactor nuclear power plant.

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