Method for transiting alloy elements into ferrite stainless welding seam

Abstract

The invention relates to a method for transiting alloy elements into a ferritic stainless welding seam. The method comprises the steps that a mixture composed of active welding flux and metal element powder to be transited is obtained; coating the mixture on a to-be-welded area by using an adhesive which can be completely volatilized; and welding after the adhesive is completely volatilized. According to the method, the effects of improving the structure and improving the mechanical property are achieved, the use requirement is met, and the performance of the weld joint is improved; and the corresponding active welding flux is selected according to the alloy elements needing to be transited, so that the transition efficiency of the elements is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of welding, and in particular relates to a method for transitioning alloy elements into a ferrite stainless weld. Background Art

[0002] Activating flux TIG welding (A-TIG) involves applying a thin layer of active flux (primarily oxides and fluorides) to the surface of the workpiece to be welded. This welding method not only significantly increases penetration but also improves the structure and properties of the weld. This is particularly true for ferritic stainless steels, which have poor weldability. This method not only increases the weld depth-to-width ratio and refines the grain size, but also allows for the addition of beneficial alloying elements, such as titanium, aluminum, and nickel powders, to the active flux to transition these beneficial alloys into the weld, compensating for the low alloying element content and poor weldability of ferritic stainless steel. Studies have shown that adding an appropriate amount of these elements to ferritic stainless steel welds can refine the grain size and improve the mechanical properties and corrosion resistance of the weld.

[0003] However, existing literature and technical data show that research on stainless steel active fluxes generally focuses on austenitic stainless steels, while there are few research contents and results on ferritic stainless steels. Therefore, studying the method of transitioning alloying elements to ferritic stainless steel welds is of great significance for the promotion and application of ferritic stainless steel. Summary of the Invention

[0004] In order to solve all or part of the above problems, the present invention aims to provide a method for transitioning alloy elements into ferritic stainless welds.

[0005] According to one aspect of the present invention, there is provided a method for transitioning alloying elements into a ferritic stainless weld, comprising:

[0006] Obtaining a mixture consisting of active flux and powder of the metal element to be transitioned;

[0007] Applying the mixture to the area to be welded with a completely volatile adhesive; and

[0008] Welding is performed after the adhesive is completely volatilized.

[0009] Furthermore, if the transition metal element powder is aluminum powder, the active flux comprises the following components by mass percentage: 80% to 90% of a mixture of MgO powder and TiO2 powder, and 10% to 20% of SiO2 powder.

[0010] Furthermore, the aluminum powder accounts for 25% to 50% of the total mass of the active flux.

[0011] Furthermore, if the transition metal element powder is titanium powder, the active flux comprises the following components by mass percentage: 35% to 45% of CaF2 powder, 35% to 45% of NaF powder, and 10% to 20% of SiO2 powder.

[0012] Furthermore, the titanium powder accounts for 20% to 30% of the total mass of the active flux.

[0013] Furthermore, if the transition metal element powder is nickel powder, the active flux comprises the following components by mass percentage: 20% to 25% TiO2 powder, 20% to 25% SiO2 powder, 20% to 25% CaF2 powder, and 20% to 25% NaF powder.

[0014] Furthermore, the nickel powder accounts for 20% to 40% of the total mass of the active flux.

[0015] Furthermore, the adhesive capable of completely volatilizing is industrial alcohol;

[0016] The method of applying the mixture to the area to be welded using a completely volatile adhesive is specifically: applying the mixture to the area to be welded using industrial alcohol as an adhesive, with a coating thickness of 0.1 to 0.3 mm.

[0017] Furthermore, the welding is performed after the adhesive is completely volatilized. Specifically, TIG welding is performed after the adhesive is completely volatilized.

[0018] Furthermore, after welding after the adhesive is completely volatilized, the method further includes: cleaning the welding area after welding.

[0019] It can be seen from the above technical solution that the method for transitioning alloying elements into ferritic stainless welds provided by the present invention has the following beneficial effects:

[0020] The method of the present invention for transitioning alloying elements into ferritic stainless welds improves the structure and mechanical properties, meets the use requirements, and improves the performance of the welds. The embodiment of the present invention selects the corresponding active flux according to the alloying elements to be transitioned, thereby improving the transition efficiency of the elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the mechanism of increasing the penetration depth of the active agent;

[0022] Figure 2 The present invention is a flowchart of a method for transitioning alloying elements into a ferritic stainless weld according to an embodiment of the present invention. DETAILED DESCRIPTION

[0023] In order to better understand the purpose, structure and function of the present invention, the following is a further detailed description of a method for transitioning alloy elements into a ferritic stainless weld of the present invention in conjunction with the accompanying drawings.

[0024] An embodiment of the present invention relates to a method for transitioning alloying elements into a ferritic stainless steel weld, including determining the proportions of active fluxes corresponding to the addition of aluminum, titanium, and nickel elements to the ferritic stainless steel weld, that is, preparing different active fluxes for different metal elements to be transitioned. The embodiment of the present invention is proposed based on research and summary of the transition characteristics of different active elements to different alloying elements, while taking into account feasibility and economy, and illustrating the effectiveness of different active fluxes in transitioning the above-mentioned metal elements.

[0025] In ferritic stainless steel welds, some of these metal elements, such as aluminum, titanium, and nickel, form interstitial phases such as oxides or carbonitrides with interstitial elements such as carbon, nitrogen, and oxygen. These second-phase particles are dispersed throughout the ferrite matrix. These metal elements not only strengthen the base metal, but their atomic presence also enhances the material's corrosion resistance. More importantly, these second-phase particles, which are stable even at high temperatures, prevent excessive grain growth during the weld thermal cycle, thereby minimizing the tendency for weld performance to deteriorate.

[0026] However, with the exception of nickel, all of these elements are susceptible to oxidation and burning at high welding temperatures. Therefore, the content of microalloying elements in the weld must be lower than that in the parent material, inevitably leading to grain coarsening and performance degradation in the weld. However, adding excessive trace elements to the ferritic stainless steel parent material is neither economical nor necessary. Local alloying of the weld metal is the ideal solution.

[0027] The main function of active flux in the welding process is to increase the penetration depth (depth-to-width ratio) of the weld under the same current conditions. There are mainly several theories about its mechanism. It is generally believed that the increase in penetration depth is the result of the combined action of the following factors.

[0028] 1. Anode spot shrinkage theory:

[0029] After adding sulfides, chlorides, or oxides to the molten pool, the anode spot on the molten pool is significantly reduced, while a greater penetration depth is achieved. The theory is that after adding a high-melting-point activator, the amount of metal vapor generated on the molten pool surface is suppressed. When there is less metal vapor, only a small area of metal near the anode spot is vaporized and ionized, which tightens the arc conductive path, increases the arc pressure, and accelerates, increases, and deepens the flow of metal liquid within the molten pool, thereby increasing the weld penetration depth.

[0030] The active flux widens the current lines of the liquid metal in the molten pool, forming a Lorenz force. Due to the change in the cross section of the conductor, an axial force pointing to the larger cross section is generated, and its size can be determined by formula (1):

[0031]

[0032] Where p is the Lorenz force, in N; I is the welding current, in A; R1 and R2 are the radii of the current-carrying cross section, in mm; and μ is the magnetic permeability of a given volume. Equation (1) shows that the magnitude of the electromagnetic force depends on the welding current and the degree of current line widening. In active flux welding, the radius of the polar spot is much smaller than in conventional welding, and the axial electromagnetic force P increases. It is this force that causes the liquid metal in the molten pool to flow in a directional manner from the surface to the bottom of the pool. Under the action of this high-temperature liquid metal flow, the weld penetration increases.

[0033] 2. Surface tension gradient change theory:

[0034] There are three forces inside the liquid molten pool: electromagnetic force, buoyancy and surface tension. The flow field generated by the electromagnetic force conducts heat from the center of the arc to the bottom of the molten pool to form a deep molten pool; while the buoyancy and negative surface tension gradient cause the molten pool to flow upward to form a shallow and wide weld, and the effect of buoyancy is minimal. The simultaneous action of these three forces results in two circulation modes inside the molten pool: one on the surface of the molten pool, determined by surface tension; the other inside the molten pool, determined by electromagnetic force. The two cycles have different rates, and the second cycle is much smaller than the first cycle. Therefore, the heat conduction from the heat source to the root of the molten pool is interrupted by the first cycle, forming a shallow and wide weld. However, when there are surfactant elements on the surface of the molten pool, the surface tension gradient is made positive, which changes the direction of the first cycle. The flow of the molten pool increases heat conduction, forming a deep and narrow weld, such as Figure 1 shown.

[0035] 3. Arc contraction theory:

[0036] The components of the active flux are all polyatomic molecules, so thermal dissociation occurs in the arc atmosphere. Since thermal dissociation is an endothermic reaction, the arc shrinks according to the minimum voltage principle.

[0037] Based on the above principles and the influence of active flux on the arc and molten pool, adding microalloying elements to the active flux to achieve alloying of the weld metal is a feasible and relatively ideal alloy transition method. However, the transition effect of different active fluxes or active flux combinations on different alloying elements needs further research, so that different proportions of active fluxes can be used according to the alloying elements required for transition.

[0038] It needs to be further clarified that although active fluxes such as TiO2 and Al2O3 contain titanium and aluminum elements, these oxides have very high melting points (1840°C and 2054°C respectively) and high-temperature stability. Although tests show that the titanium and aluminum elements in the weld metal obtained by using fluxes containing the above substances are increased, most of them exist in the form of oxides rather than carbonitrides or atomic forms that can prevent grain coarsening. Therefore, the effect of pure active flux on refining weld performance and improving performance is limited. Transitioning alloy elements into the weld through active flux is an ideal solution.

[0039] Although activated fluxes without alloying powders can somewhat refine the metal grains in stainless steel welds, the primary purpose of developing activated fluxes was to improve the depth-to-width ratio of austenitic stainless steel welds. For ferritic stainless steels, incorporating appropriate amounts of microalloying elements into the weld, based on specific requirements, can further improve microstructure and performance.

[0040] The present invention is based on research on the effects of microalloying elements in ferritic stainless steel on the microstructure and properties of welded joints and on the application of active flux in stainless steel welding. The invention aims to obtain an effective and economical method for transitioning alloying elements into ferritic stainless steel welds, and summarizes the corresponding relationships between the transition capabilities of different active fluxes for various alloying elements to achieve the effectiveness and economy of the active flux. The corresponding active flux formula is selected according to the alloying elements to be transitioned, thereby improving the transition efficiency of the metal elements.

[0041] Specifically, such as Figure 2 As shown, a method for transitioning alloy elements into a ferritic stainless weld according to an embodiment of the present invention includes the following steps:

[0042] Step S001: obtaining a mixture of active flux and metal element powder to be transitioned;

[0043] Step S002: applying the mixture to the area to be welded using a completely volatilizable adhesive; and

[0044] Step S003: welding is performed after the adhesive is completely volatilized.

[0045] Ferritic stainless steel is generally applied in the form of thin plates (thickness ≤ 3mm), and TIG welding is the most commonly used method. Therefore, it is highly scientific and feasible to achieve alloying of weld metal through the A-TIG welding process. Beneficial alloying elements of ferritic stainless steel include aluminum, titanium and nickel. The content of these elements in ferritic stainless steel is generally less than 0.3%, but they play an important role in improving the mechanical properties, corrosion resistance and welding performance of stainless steel. These trace elements can exist in stainless steel in the form of one or a combination of several elements, and play a role in improving the structure and enhancing the mechanical properties in different applications. Therefore, in some specific applications, technicians hope to transition specific alloying elements to the weld metal during the welding process to meet the use requirements and further improve the performance of the weld. In order to obtain the transition efficiency of different substances to each element, the following experiment was designed to summarize the relationship between active substances and transition elements.

[0046] Typical active materials selected for the experiment include oxides SiO2, TiO2, and MgO, and fluorides CaF2 and NaF. The stainless steel used was medium-chromium ultrapure ferritic stainless steel SUS441 (022Cr18NbTi), a typical ferritic stainless steel with a dual-stable niobium-titanium structure. The stainless steel plate used in the experiment was 3 mm thick. Bead-on-plate welding was performed using a tungsten inert gas arc welding method with direct current (DC) connection. The welding current was 120-150 A, the welding speed was 300 mm / min, and the heat input range was 0.25-0.3 kJ / mm. The activator + metal powder combinations listed in Table 1 were tested, with the metal powder mass representing 10-30% of the corresponding activator. After completion, samples were taken and optical microscope specimens were prepared. The metallographic structure was observed to qualitatively determine the weld grain size. The results were compared with samples without flux. If the grain size changed significantly, it was considered that the element had effectively transitioned into the weld metal.

[0047] Table 1: Different active agent + metal powder combinations and corresponding results

[0048]

[0049] The results in Table 1 show that different active materials exhibit significant differences in their transition properties for each alloying element. It can be concluded that oxides have a better transition effect on aluminum, fluorides have a better transition effect on titanium, and both oxides and fluorides are suitable for transitioning nickel. To achieve effective transition of the required elements, active materials with good transition properties should be selected, rather than those with poor transition properties. While SiO2 is the active material most effective in increasing weld penetration, excessive amounts can increase the Si content in the weld, leading to poor performance. Therefore, its content should be kept to a minimum (≤25%).

[0050] In summary, for S001, a mixture of active flux and transition metal element powder is obtained: depending on the different transition metal elements, the active flux selected is also different. The following discussion takes the transition metal element powders as aluminum powder, titanium powder and nickel powder respectively:

[0051] If the transition metal element powder is aluminum powder, the active flux comprises the following components by mass percentage: 80% to 90% of a mixture of MgO powder and TiO2 powder, and 10% to 20% of SiO2 powder, wherein the aluminum powder accounts for 25% to 50% of the total mass of the active flux.

[0052] If the transition metal powder is titanium powder, the active flux comprises the following components by mass: 35% to 45% CaF2 powder, 35% to 45% NaF powder, and 10% to 20% SiO2 powder, wherein the titanium powder accounts for 20% to 30% of the total mass of the active flux.

[0053] If the transition metal element powder is nickel powder, the active flux comprises the following components by mass: 20% to 25% TiO2 powder, 20% to 25% SiO2 powder, 20% to 25% CaF2 powder, and 20% to 25% NaF powder. The nickel powder accounts for 20% to 40% of the total mass of the active flux.

[0054] For S002, the mixture is applied to the area to be welded with a completely volatile adhesive: the completely volatile adhesive is industrial alcohol; applying the mixture to the area to be welded with a completely volatile adhesive specifically includes: applying the mixture to the area to be welded with industrial alcohol as the adhesive, with a coating thickness of 0.1 to 0.3 mm.

[0055] S003: welding after the adhesive is completely volatilized. Specifically, TIG welding is performed after the adhesive is completely volatilized.

[0056] After welding is performed after the adhesive is completely volatilized in S003, the method of the embodiment of the present invention further includes: cleaning the welding area after welding.

[0057] The method of the embodiment of the present invention is verified below with reference to examples to confirm the effectiveness of the active flux formulation design in the embodiment of the present invention.

[0058] The experimental stainless steel used was 3mm thick SUS441 (022Cr18NbTi) ferritic stainless steel. The purpose of the experiment was to introduce an appropriate amount of aluminum into the weld to refine the grain size and improve the mechanical properties. Relevant literature indicates that an aluminum content of around 0.3% in stainless steel is most effective in improving the microstructure and performance of the weld. To enhance the contrast, the aluminum content was significantly increased in this experiment. To further verify the aforementioned conclusions regarding the effect of aluminum transition on different active materials, the following experiment was designed: Seven oxides and fluorides were selected, mixed with aluminum powder in a 1:1 ratio, and then evenly mixed with alcohol before being applied to the welded area of the stainless steel plate. Welding was performed after the alcohol had completely evaporated. The polarity was DC positive, the welding current was 120-150A, the welding speed was 300mm / min, and the heat input range was 0.25-0.3kJ / mm. The seven oxides and fluorides are SiO2, TiO2, Cr2O3, MgO, NaF, CaF2, and ZnF. In addition, Al2O3 without aluminum powder was selected. Experiments were conducted with the above eight substances individually to investigate whether they could play an alloying role.

[0059] The elemental analysis of the weld metal after cleaning the surface slag using an electrospark spectrometer showed that MgO had the most significant transfer effect on aluminum, with the aluminum content in the weld reaching a maximum of 1.5%. TiO2 also had a strong transfer effect on aluminum, with the aluminum content ranging from 0.75% to 0.85%. The aluminum content of the sample using Cr2O3 was 0.7%. The transfer effect of fluoride on aluminum was relatively low, with the aluminum content in the weld ranging from 0.14% to 0.4%. The aluminum content of the single-component Al2O3 sample was 0.015%, which was basically the same as that in the base metal, that is, no aluminum transition to the weld occurred. In addition, while aluminum was transferred to the weld through SiO2 and TiO2, the Si and Ti elements in the weld also increased (Si increased from 0.35 to 1.05; Ti increased from 0.13 to 0.61). Excessive Si in ferritic stainless steel will increase brittleness, so the SiO2 content in the active flux of ferritic stainless steel should be controlled.

[0060] It should be noted that, unless otherwise specified, the technical or scientific terms used in this application should have the common meanings understood by those skilled in the art to which the present invention belongs.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and description of the present invention. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. A method for transitioning alloying elements into ferritic stainless welds, characterized in that: include: Obtaining a mixture consisting of active flux and powder of the metal element to be transitioned; Applying the mixture to the area to be welded with a completely volatilizable adhesive; as well as Welding is performed after the adhesive is completely volatilized.

2. The method for transitioning alloying elements into ferritic stainless welds according to claim 1, characterized in that: The mixture of the active flux and the transition metal element powder is specifically: If the transition metal element powder is aluminum powder, the active flux comprises the following components by mass percentage: 80% to 90% of a mixture of MgO powder and TiO2 powder, and 10% to 20% of SiO2 powder.

3. The method for transitioning alloying elements into ferritic stainless welds according to claim 2, characterized in that: The aluminum powder accounts for 25% to 50% of the total mass of the active flux.

4. The method for transitioning alloying elements into ferritic stainless welds according to claim 1, characterized in that: The mixture of the active flux and the transition metal element powder is specifically: If the transition metal element powder is titanium powder, the active flux comprises the following components by mass percentage: 35% to 45% of CaF2 powder, 35% to 45% of NaF powder, and 10% to 20% of SiO2 powder.

5. The method for transitioning alloying elements into ferritic stainless welds according to claim 4, characterized in that: The titanium powder accounts for 20% to 30% of the total mass of the active flux.

6. The method for transitioning alloying elements into ferritic stainless welds according to claim 1, characterized in that: The mixture of the active flux and the transition metal element powder is specifically: If the transition metal element powder is nickel powder, the active flux comprises the following components by mass percentage: 20% to 25% of TiO2 powder, 20% to 25% of SiO2 powder, 20% to 25% of CaF2 powder, and 20% to 25% of NaF powder.

7. The method for transitioning alloying elements into ferritic stainless welds according to claim 6, characterized in that: The nickel powder accounts for 20% to 40% of the total mass of the active flux.

8. The method for transitioning alloying elements into ferritic stainless welds according to claim 1, characterized in that: The adhesive capable of completely volatilizing is industrial alcohol; The method of applying the mixture to the area to be welded using a completely volatile adhesive is specifically: applying the mixture to the area to be welded using industrial alcohol as an adhesive, with a coating thickness of 0.1 to 0.3 mm.

9. The method for transitioning alloying elements into ferritic stainless welds according to claim 1, characterized in that: The welding is performed after the adhesive is completely volatilized. Specifically, TIG welding is performed after the adhesive is completely volatilized.

10. The method for transitioning alloying elements into ferritic stainless welds according to claim 1, characterized in that: After welding after the adhesive is completely volatilized, the method further includes: cleaning the welding area after welding.

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