Surface modification method for reducing intergranular segregation of laser welding seam of ferritic stainless steel based on scanning pulse laser remelting

Abstract

The invention discloses a surface modification method for reducing intergranular segregation of a laser welding seam of ferritic stainless steel based on scanning pulse laser remelting, and belongs to the technical field of laser processing. According to the method, the problems that intergranular segregation is prone to occurring in existing laser welding ferritic stainless steel welding seams, and the corrosion performance of welding joints can be affected by the intergranular segregation are solved. The method comprises the following steps: 1, preparing a ferritic stainless steel welding seam by using an optical fiber laser; secondly, a fiber laser pulse laser mode is used for being matched with a galvanometer scanning system, and single-channel linear and swinging scanning pulse laser remelting is conducted on the ferrite stainless steel weld joint. The method is used for surface modification for reducing the intergranular segregation of the laser welding seam of the ferritic stainless steel based on scanning pulse laser remelting.

Description

Technical Field

[0001] The invention belongs to the technical field of laser processing. Background Art

[0002] Ferritic stainless steel boasts advantages such as low cost, high thermal conductivity, low linear expansion coefficient, and excellent resistance to stress corrosion cracking. It is widely used in the automotive, nuclear power, and petrochemical industries. Laser welding, with its advantages of a small heat-affected zone and high welding precision, is widely used for joining stainless steel. However, laser welding of ferritic stainless steel can lead to defects such as coarse grains and intergranular segregation in welded joints. The intergranular segregation products (intergranular second phases) are primarily composed of iron-chromium compounds, resulting in chromium depletion at the grain boundaries. Furthermore, the precipitation of intergranular second phases can easily induce localized galvanic corrosion, compromising the post-weld corrosion performance of welded joints.

[0003] To address the deterioration in intergranular corrosion resistance of ferritic stainless steel after welding, current approaches focus on optimizing the composition of ferritic stainless steel. By improving smelting technology and reducing harmful carbon and nitrogen content, these efforts can mitigate post-weld intergranular segregation. Alloying elements with strong affinity for ferritic stainless steel, such as Ti and Nb, are added to ferritic stainless steel. Under certain conditions, these elements form niobium and titanium carbonitrides, effectively inhibiting the formation of chromium carbonitrides at grain boundaries and improving intergranular corrosion resistance. However, due to limitations in smelting technology and production costs, simply controlling the alloying elements of ferritic stainless steel to improve intergranular corrosion resistance is not economical. Laser remelting uses a laser beam to rapidly heat the material surface, forming a molten pool. Once the laser beam leaves the treatment area, the molten pool rapidly cools, resulting in a fine-grained surface structure and improved surface properties. Laser remelting is widely used in post-processing of additively manufactured parts, reducing surface porosity. Laser remelting can also improve the microhardness, bonding strength, and wear resistance of coatings. When laser remelting the material surface using a traditional linear laser processing path, when the laser spot is not large enough to cover the weld width, this laser processing method requires a "bow" processing path to allow the laser to act on the entire weld area. Therefore, this method requires a long processing time and is not suitable for fast-paced production. Traditional linear pulsed laser remelting processing methods have long single-point dwell times, which will lead to significant heat accumulation, and the overlapping area of ​​adjacent laser spots is prone to high temperatures, resulting in a larger heat-affected zone. Since ferritic stainless steel has a sensitization range of 400-900℃, this heat input characteristic significantly limits the process applicability of traditional linear pulsed laser remelting methods on ferritic stainless steel. Summary of the Invention

[0004] The present invention aims to solve the problem that intergranular segregation easily occurs in existing laser-welded ferritic stainless steel welds, which affects the corrosion performance of the welded joints. It further provides a surface modification method based on scanning pulse laser remelting to reduce intergranular segregation in ferritic stainless steel laser-welded welds.

[0005] A surface modification method for reducing intergranular segregation in laser welded ferritic stainless steel based on scanning pulse laser remelting is carried out in the following steps:

[0006] 1. Preparation of ferritic stainless steel welds using fiber lasers;

[0007] 2. Using the fiber laser pulse laser mode with a galvanometer scanning system, under the conditions of laser power of 300W~700W, pulse frequency of 500Hz~10000Hz, swing amplitude of 0.8mm~2mm, duty cycle of 50%~80% and swing frequency of 150Hz~500Hz, the ferritic stainless steel weld is subjected to single-pass straight and swinging scanning pulse laser remelting, thus completing the surface modification method of reducing intergranular segregation of ferritic stainless steel laser welds based on scanning pulse laser remelting.

[0008] The beneficial effects of the present invention are:

[0009] Since ferritic stainless steel has a sensitization range of 400℃~900℃, the formation of chromium carbides is promoted in this temperature range, and these carbides tend to segregate between grains, which can easily lead to chromium depletion. The present invention uses a fiber laser pulsed laser mode combined with a galvanometer scanning system to modify the surface of ferritic stainless steel welds. This method is different from the stirred molten pool in laser welding and laser cladding:

[0010] 1. The present invention adopts a pulsed laser with low heat input. The pulsed laser beam irradiates the surface of the material, causing it to melt instantly to form a small molten pool. The heat is more easily dissipated through the base material by rapid heat transfer. After the laser beam stops acting on the processing area, the molten pool rapidly cools and solidifies, with a faster cooling rate, thereby refining the alloy structure, reducing intergranular segregation, improving the quality of the oxide film on the weld surface, and enhancing the surface corrosion resistance of the metal material or workpiece.

[0011] 2. The pulse laser of the present invention is combined with the high-speed swing of the galvanometer. Because of its fast scanning speed and short single-point action time, the laser energy is dispersed by the swing of the galvanometer, thereby reducing heat accumulation. It can reduce the residence time of the remelted metal on the weld surface in the sensitization range of 400℃~900℃, further avoid local overheating, thereby improving the intergranular segregation phenomenon and avoiding the risk of intergranular corrosion of low-cost ferritic stainless steel under conditions such as salt spray.

[0012] 3. The present invention effectively increases the effective area of ​​the laser beam by controlling the swing amplitude, so that a single-channel linear pulse laser surface treatment can act on the entire weld surface area, thereby improving processing efficiency and being suitable for fast-paced production. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a surface forming diagram of a 430 ferritic stainless steel welding specimen prepared as a comparative example;

[0014] Figure 2 Surface forming diagram of the 430 ferritic stainless steel sample after laser surface treatment prepared in Implementation 1;

[0015] Figure 3 The salt spray corrosion results of the 430 ferritic stainless steel samples prepared in 1 to 2 after laser surface treatment and the 430 ferritic stainless steel welding samples prepared in the comparative example at different time intervals were statistically analyzed;

[0016] Figure 4 The cross-sectional structure of the weld of the 430 ferritic stainless steel welding sample prepared for the comparative example;

[0017] Figure 5 Cross-sectional structure of the weld of the 430 ferritic stainless steel sample prepared in Implementation 1 after laser surface treatment. DETAILED DESCRIPTION

[0018] Specific embodiment 1: This embodiment is a surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding welds based on scanning pulse laser remelting, which is carried out in the following steps:

[0019] 1. Preparation of ferritic stainless steel welds using fiber lasers;

[0020] 2. Using the fiber laser pulse laser mode with a galvanometer scanning system, under the conditions of laser power of 300W~700W, pulse frequency of 500Hz~10000Hz, swing amplitude of 0.8mm~2mm, duty cycle of 50%~80% and swing frequency of 200Hz~500Hz, the ferritic stainless steel weld is subjected to single-pass straight and swinging scanning pulse laser remelting, thus completing the surface modification method of reducing intergranular segregation of ferritic stainless steel laser welds based on scanning pulse laser remelting.

[0021] This implementation method innovatively integrates welding and post-processing processes into the same workstation, effectively solving the efficiency loss problem caused by traditional segmented operations.

[0022] The beneficial effects of this embodiment are:

[0023] Since ferritic stainless steel has a sensitization range of 400℃~900℃, the formation of chromium carbides is promoted in this temperature range. These carbides tend to segregate between grains, which can easily lead to chromium depletion. This embodiment uses a fiber laser pulsed laser mode combined with a galvanometer scanning system to modify the surface of ferritic stainless steel welds. This method is different from the stirred molten pool in laser welding and laser cladding:

[0024] 1. This embodiment uses a pulsed laser with low heat input. The pulsed laser beam irradiates the surface of the material, causing it to melt instantly to form a small molten pool. Heat is more easily dissipated through the base material through rapid heat transfer. After the laser beam stops acting on the processing area, the molten pool rapidly cools and solidifies, with a faster cooling rate, thereby refining the alloy structure, reducing intergranular segregation, improving the quality of the oxide film on the weld surface, and enhancing the surface corrosion resistance of the metal material or workpiece.

[0025] 2. In this embodiment, the pulsed laser is combined with the high-speed swing of the galvanometer. Because of its fast scanning speed and short single-point action time, the laser energy is dispersed by the swing of the galvanometer, thereby reducing heat accumulation. It can reduce the residence time of the remelted metal on the weld surface in the sensitization range of 400℃~900℃, further avoid local overheating, thereby improving the intergranular segregation phenomenon and avoiding the risk of intergranular corrosion of low-cost ferritic stainless steel under conditions such as salt spray.

[0026] 3. This embodiment effectively increases the effective area of ​​the laser beam by controlling the swing amplitude, so that a single linear pulse laser surface treatment can act on the entire weld surface area, thereby improving processing efficiency and being suitable for fast-paced production.

[0027] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that the ferritic stainless steel in step 1 is 430 stainless steel. Other aspects are the same as specific embodiment 1.

[0028] Specific embodiment 3: This embodiment differs from either specific embodiment 1 or 2 in that, in step 1, a fiber laser is used to prepare the ferritic stainless steel weld under conditions where the side-blowing shielding gas flow rate is 10 L / min to 30 L / min and the back-blowing shielding gas flow rate is 5 L / min to 15 L / min. Other aspects are the same as specific embodiments 1 or 2.

[0029] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that the side-blowing shielding gas and the back-blowing shielding gas are both argon with a purity of 99.9%. Other aspects are the same as specific embodiment 3.

[0030] Specific embodiment 5: This embodiment differs from Specific embodiments 1 to 4 in that, in step 1, a fiber laser is used to prepare the ferritic stainless steel weld under the conditions of a welding power of 450W to 1000W, a defocus of 0mm to 6mm, and a welding speed of 0.1m / min to 3m / min. Other aspects are the same as Specific embodiments 1 to 4.

[0031] Specific embodiment 6: This embodiment differs from specific embodiments 1 to 5 in that the laser processing speed in step 2 is 0.5 m / min to 3 m / min. Other aspects are the same as specific embodiments 1 to 5.

[0032] Specific embodiment 7: This embodiment differs from specific embodiments 1 to 6 in that the laser processing defocusing amount in step 2 is 2 mm to 6 mm. Other aspects are the same as specific embodiments 1 to 6.

[0033] Specific Embodiment 8: This embodiment differs from Specific Embodiments 1 to 7 in that, in step 2, a single-pass linear and oscillating scanning pulse laser remelting is performed on the ferritic stainless steel weld under the condition of a shielding gas flow rate of 10 L / min to 30 L / min. Otherwise, the process is the same as Specific Embodiments 1 to 7.

[0034] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that the protective gas is argon with a purity of 99.9%. Other aspects are the same as specific embodiments 1 to 8.

[0035] Specific embodiment 10: This embodiment differs from any one of specific embodiments 1 to 9 in that the oscillation pattern in the single-track linear and oscillating scanning pulse laser remelting described in step 2 is circular, square, figure-8, or infinity. Other aspects are the same as specific embodiments 1 to 9.

[0036] The following examples are used to verify the beneficial effects of the present invention:

[0037] Implementation 1:

[0038] A surface modification method for reducing intergranular segregation in laser welded ferritic stainless steel based on scanning pulse laser remelting is carried out in the following steps:

[0039] 1. Ferritic stainless steel welds were prepared using a fiber laser under the conditions of a side shielding gas flow rate of 20 L / min, a back shielding gas flow rate of 5 L / min, a laser wavelength of 1064 nm, a welding power of 450 W, a defocus of +2 mm, and a welding speed of 3 m / min.

[0040] The ferritic stainless steel is 430 stainless steel, which does not contain Nb or Ti elements. The welding method is to butt-join two 100mm×100mm×0.8mm stainless steel plates to obtain a workpiece with a thickness of 0.8mm. The side-blowing shielding gas and the back-blowing shielding gas are both argon with a purity of 99.9%.

[0041] Second, using a fiber laser pulse laser mode with a galvanometer scanning system, under the conditions of a shielding gas flow rate of 20 L / min, a laser wavelength of 1064 nm, a laser power of 450 W, a laser processing speed of 3 m / min, a laser processing defocus of +4 mm, a pulse frequency of 500 Hz, an oscillation amplitude of 1 mm, a duty cycle of 50%, and an oscillation frequency of 250 Hz, a single-pass straight and oscillating scanning pulse laser remelting of the ferritic stainless steel weld was performed to obtain a 430 ferritic stainless steel post-weld laser surface treated sample;

[0042] The protective gas is argon with a purity of 99.9%;

[0043] The oscillation pattern in the single-track linear and oscillating scanning pulse laser remelting is circular.

[0044] Example 2: This example differs from Example 1 in that, in step 2, a single-pass linear and oscillating scanning pulse laser remelting of a ferritic stainless steel weld was performed under the following conditions: a shielding gas flow rate of 20 L / min, a laser wavelength of 1064 nm, a laser power of 500 W, a laser processing speed of 3 m / min, a laser processing defocus of +4 mm, a pulse frequency of 700 Hz, an oscillation amplitude of 1 mm, a duty cycle of 50%, and an oscillation frequency of 196 Hz. Other procedures were the same as in Example 1.

[0045] Comparative Example: This comparative example differs from Example 1 in that step 2 is omitted and a 430 ferritic stainless steel welding specimen is obtained in step 1. Other steps are the same as Example 1.

[0046] Figure 1 This is a surface profile of a 430 ferritic stainless steel weld specimen prepared in the comparative example. As can be seen from the figure, the weld surface of the laser-welded 430 stainless steel in the comparative example is uniform and exhibits a "fish-scale" morphology.

[0047] Figure 2 This figure shows the surface of a 430 ferritic stainless steel sample after laser surface treatment, prepared in Example 1. The figure shows that the surface modification of the ferritic stainless steel weld using a fiber laser in pulsed mode coupled with a galvanometer scanning system results in a "pebble-like" morphology. This is due to the laser remelting technique, which rapidly heats the material surface with a laser beam, forming a small molten pool. When the laser beam leaves the treated area, the molten pool cools rapidly, forming a discontinuous molten pool.

[0048] 20 samples of 430 ferritic stainless steel post-weld laser surface treatment samples prepared in Implementation 1 to 2 and 20 samples of 430 ferritic stainless steel welding samples prepared in the comparative example were respectively intercepted by wire cutting. The sample size was 10 mm × 10 mm × 0.8 mm, and the weld was located in the center of the sample. A neutral salt spray corrosion test was performed on them. The solution used was a 5% sodium chloride aqueous solution. During the test, the test chamber temperature and the brine barrel temperature were maintained within the range of 35±1°C, and the pressure barrel temperature was within the range of 47±1°C. The salt spray test lasted for 96 hours. The corrosion conditions were observed every 24 hours, and the corrosion conditions of the samples were statistically analyzed. Figure 3 The following table summarizes the salt spray corrosion results of 430 ferritic stainless steel samples prepared in Implementations 1 and 2 and the comparative example for post-weld laser surface treatment at different time intervals. As shown in the figure, after 96 hours of salt spray testing, nine laser-welded samples in the comparative example exhibited corrosion, two samples in Implementation 1 exhibited corrosion, and one sample in Implementation 2 exhibited corrosion. Under the specific post-weld laser surface treatment process, the corrosion probability of the laser-treated samples was reduced by 89% compared to the laser-welded samples. Corrosion onset for samples in Implementations 1 and 2 and the comparative example was mostly between 0 and 24 hours. Compared to laser-welded samples subjected to the 96-hour salt spray test, the laser-treated samples exhibited superior corrosion resistance in the salt spray environment.

[0049] Metallographic preparation of the weld cross section of the 430 ferritic stainless steel welded specimen prepared in the comparative example and the 430 ferritic stainless steel post-weld laser surface treatment specimen prepared in Example 1: A specimen with a size of 20 mm × 20 mm × 0.8 mm was taken by wire cutting with the weld as the center. The weld cross section was ground to 3000# with SiC sandpaper and mechanically polished. Finally, the specimen was electrolytically etched using a 10 wt.% oxalic acid solution at a constant voltage of 5 V for 15 s. The metallographic structure after electrolytic etching was observed. Figure 4 The cross-sectional structure of the weld of the 430 ferritic stainless steel welding specimen prepared for comparative example. Figure 5 Figure 1 shows the microstructure of the weld seam of a 430 ferritic stainless steel sample prepared in Example 1 after laser surface treatment. The cross-section of the weld seam of the comparative laser-welded sample exhibits a large number of second-phase precipitates at the grain boundaries and heat-affected zone; however, no significant second phases are observed at the grain boundaries or in the original heat-affected zone of the sample after laser surface treatment, effectively eliminating intergranular segregation. Electrolytic corrosion of the cross-section of the laser-welded sample reveals clear intergranular corrosion in the weld seam, while no significant intergranular corrosion occurs in the surface area of ​​the sample after laser surface treatment. This demonstrates that the corrosion resistance of the ferritic stainless steel weld has been significantly improved by the laser surface treatment in Example 1.

Claims

1. A surface modification method for reducing intergranular segregation in laser welded ferritic stainless steel based on scanning pulse laser remelting, characterized in that It is carried out in the following steps:

1. Preparation of ferritic stainless steel welds using fiber lasers; 2. Using the fiber laser pulse laser mode with a galvanometer scanning system, under the conditions of laser power of 300W~700W, pulse frequency of 500Hz~10000Hz, swing amplitude of 0.8mm~2mm, duty cycle of 50%~80% and swing frequency of 150Hz~500Hz, the ferritic stainless steel weld is subjected to single-pass straight and swinging scanning pulse laser remelting, thus completing the surface modification method of reducing intergranular segregation of ferritic stainless steel laser welds based on scanning pulse laser remelting.

2. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 1 is characterized in that The ferritic stainless steel described in step 1 is 430 stainless steel.

3. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 1 is characterized in that In step 1, a fiber laser is used to prepare the ferritic stainless steel weld under the conditions of a side-blowing shielding gas flow rate of 10 L / min to 30 L / min and a back-blowing shielding gas flow rate of 5 L / min to 15 L / min.

4. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 3 is characterized in that The side-blowing shielding gas and the back-blowing shielding gas are both argon with a purity of 99.9%.

5. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 1 is characterized in that In step 1, a fiber laser is used to prepare the ferritic stainless steel weld under the conditions of a welding power of 450W to 1000W, a defocus of 0mm to 6mm, and a welding speed of 0.1m / min to 3m / min.

6. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 1 is characterized in that The laser processing speed in step 2 is 0.5m / min~3m / min.

7. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 1, characterized in that The laser processing defocus amount in step 2 is 2mm~6mm.

8. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 1 is characterized in that In step 2, under the condition of a shielding gas flow rate of 10 L / min to 30 L / min, the ferritic stainless steel weld is subjected to single-pass linear and oscillating scanning pulse laser remelting.

9. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 8, characterized in that The protective gas is argon with a purity of 99.9%.

10. The surface modification method for reducing intergranular segregation in ferritic stainless steel laser welding seams based on scanning pulse laser remelting according to claim 1, characterized in that The oscillation pattern in the single-track linear and oscillating scanning pulse laser remelting described in step 2 is circular, square, figure 8 or ∞.

Images