Method for improving low temperature performance of laser welded joint of ultra-high strength stainless steel and welded joint prepared thereby

Abstract

This invention discloses a method for improving the low-temperature performance of laser-welded joints of ultra-high strength stainless steel and the welded joints prepared therefrom. First, the workpieces to be welded are laser-welded to form a joint. Then, the surface of the weld zone and the adjacent heat-affected zone undergoes a controllable strengthening and smoothing treatment, such as laser surface treatment, ultrasonic / high-speed shot peening, precision mechanical grinding and polishing, or controlled pressure sandblasting. This method effectively eliminates defects such as microcracks, spatter, and undercut on the weld surface, ensuring that the angle between the weld melting zone and the heat-affected zone does not exceed 30°, the height difference between the melting zone and the adjacent heat-affected zone does not exceed 0.3 mm, and the maximum depth of surface depression does not exceed 0.05 mm. This reduces low-temperature stress concentration, inhibits crack initiation, and improves the low-temperature mechanical properties of the welded joint.

Description

Technical Field

[0001] This invention relates to the field of metal material welding and surface modification technology, specifically to a method for improving the low-temperature performance of ultra-high strength stainless steel laser welded joints and the welded joints prepared therefrom, which is applicable to the manufacture of key load-bearing structures in aerospace, deep-sea equipment, cryogenic storage and transportation, and national defense, where there are stringent requirements for low-temperature mechanical properties. Background Technology

[0002] Ultra-high strength stainless steels (such as maraging stainless steel and strain-hardening stainless steel) have become the preferred material for critical load-bearing structures in aerospace, deep-sea equipment, cryogenic storage and transportation, and defense industries due to their excellent specific strength, good corrosion resistance, and good low-temperature performance. Laser welding technology, with its advantages of high energy density, low heat input, high speed and efficiency, and ease of automation, has shown great potential in the precision joining of these high-strength materials. However, applying laser welding to ultra-high strength stainless steel still faces a serious challenge: the low-temperature toughness of the weld joint is often significantly lower than that of the base material, making it prone to low-temperature brittle fracture. The root cause of this problem mainly comes from two aspects: internal microstructure deterioration: the rapid thermal cycle of laser welding easily forms a cast low-strength austenitic structure in the weld melting zone and heat-affected zone, significantly reducing the mechanical properties of the weld. Surface integrity destruction: more critically, the weld surface has inherent and unavoidable geometric and metallurgical defects, such as weld reinforcement, undercut, micro-splash adhesion, surface microcracks, and wrinkles formed due to rapid solidification. These defects constitute sharp stress concentration sources. At low temperatures, localized stress can easily reach the material's fracture strength, becoming the starting point for crack initiation and rapid propagation, leading to catastrophic brittle fracture. Currently, traditional methods for improving welded joint performance mainly focus on two aspects: first, optimizing welding process parameters or employing complex composite energy fields, which increases process complexity and cost; second, performing post-weld overall heat treatment (such as tempering and aging), which may damage the base material properties and is difficult to implement for large or structurally complex components. Therefore, developing a controllable, non-thermally affected post-weld surface treatment technology that matches high-quality laser welding, specifically targeting the weld area for strengthening and smoothing, and systematically eliminating defects and optimizing the state from the crack-sensitive "surface," has crucial engineering practical value and theoretical significance for overcoming the low-temperature performance bottleneck of ultra-high strength stainless steel laser-welded joints. This invention is proposed against this background. Summary of the Invention

[0003] The purpose of this invention is to overcome the problems of softening and poor low-temperature performance of cold-worked ultra-high-strength stainless steel welded joints due to numerous surface defects, which are common in existing technologies. This invention provides a method for improving the low-temperature performance of ultra-high-strength stainless steel laser-welded joints and the welded joints prepared accordingly. This method involves controlling the strengthening and smoothing treatment of the weld zone and adjacent heat-affected zone after laser welding, eliminating defects such as microcracks, spatter, and undercut on the weld surface. The angle between the weld melting zone and the heat-affected zone transition section does not exceed 30°, the height difference between the melting zone and the adjacent heat-affected zone does not exceed 0.3 mm, and the maximum surface depression depth on any side of the welded joint does not exceed 0.05 mm. This reduces low-temperature stress concentration, inhibits crack initiation, and improves the low-temperature mechanical properties of the welded joint. This invention offers flexible processing and can significantly improve the elongation and resistance to brittle fracture of the welded joint in low-temperature environments, making it suitable for manufacturing critical components with stringent low-temperature performance requirements.

[0004] The technical solution of the present invention is as follows:

[0005] A method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints, characterized by comprising the following steps:

[0006] S1. Laser welding step: Laser welding is performed on ultra-high strength stainless steel workpieces to form a welded joint; the welded joint includes a weld melting zone, a heat-affected zone and a substrate zone;

[0007] S2. Surface morphology inspection step: Inspect the surface morphology of the welded joint to determine the transition angle between the weld melting zone and the heat-affected zone, the height difference of the weld reinforcement, and the depth of the surface depression;

[0008] S3. Strengthening and smoothing treatment step: Based on the detection results of step S2, the weld area and adjacent heat-affected zone of the welded joint are subjected to controllable strengthening and smoothing treatment to ensure that the treated welded joint meets the following surface morphology characteristics:

[0009] The angle of the transition section between the weld melting zone and the heat-affected zone shall not exceed 30°;

[0010] When the surface of the weld molten zone is lower than the surface of the adjacent substrate zone, the maximum depth of the depression on either side of the weld joint shall not exceed 0.05 mm;

[0011] When the surface of the weld molten zone is not lower than the surface of the adjacent base material zone, the maximum height difference of the weld reinforcement shall not exceed 0.3 mm;

[0012] Through the above treatment, defects such as microcracks, spatter, undercut, and wrinkles on the weld surface are eliminated or significantly reduced, low-temperature stress concentration is reduced, crack initiation is inhibited, and thus the low-temperature mechanical properties of the welded joint are improved.

[0013] Furthermore, the ultra-high strength stainless steel workpiece is a combination of equal or unequal thicknesses, and the room temperature tensile strength of any workpiece is not less than 1000 MPa.

[0014] Furthermore, visual inspection and microscopic analysis were used to determine the surface morphology of the welded joint.

[0015] Furthermore, the controllable strengthening and smoothing treatment is selected from any one or a combination of the following: laser surface treatment, ultrasonic shot peening, high-speed shot peening, precision mechanical grinding and polishing, and controllable pressure sandblasting.

[0016] Furthermore, when the treatment is laser surface treatment, a laser beam is used to scan the surface to be treated, causing the surface material to remelt, micro-melt, or undergo laser cleaning to improve the surface morphology and / or surface microstructure. Preferably, the height difference between the weld melting zone and the adjacent heat-affected zone or substrate area after laser treatment does not exceed 0.3 mm. Specifically, a combination of parameters such as laser power, scanning speed, and spot size is selected to control the laser treatment, ensuring that the surface melts without forming a laser keyhole, allowing the surface to remelt and solidify through heat conduction.

[0017] Furthermore, when the treatment is precision mechanical grinding and polishing, the maximum depth of micro-protrusions and depressions on the surface of the weld area and adjacent heat-affected zone after grinding does not exceed 0.05 mm, and the surface roughness is not higher than Ra6.4. The matching of parameters such as abrasive particle size, grinding pressure, rotation speed, and feed rate ensures low surface roughness after surface treatment and allows for efficient and economical implementation.

[0018] Furthermore, the enhanced flattening treatment covers the weld area completely and extends to the heat-affected zone at least 1 mm away from the center line of the weld on both sides, to ensure that the stress concentration sensitive area is adequately improved.

[0019] Furthermore, the strengthening and flattening treatment can be performed on only one side of the welded joint surface, or on both sides; when only one side is treated, the back side of the laser weld (i.e., the non-laser incident side) is treated first.

[0020] This invention also provides an ultra-high strength stainless steel laser-welded joint with excellent low-temperature mechanical properties, characterized in that...

[0021] The welded joint is made of ultra-high strength stainless steel by laser welding; the welded joint includes a weld melting zone, a heat-affected zone and a base material zone;

[0022] The transition angle between the weld melting zone and the heat-affected zone shall not exceed 30°, the height difference between the weld melting zone and the adjacent heat-affected zone or substrate area shall not exceed 0.3 mm, and the maximum depth of the depression on any side of the weld joint shall not exceed 0.05 mm.

[0023] Preferably, the angle between the transition zone of the weld fusion zone and the heat-affected zone does not exceed 20°, and the height difference does not exceed 0.2mm.

[0024] Preferably, the maximum width of the heat-affected zone of the welded joint does not exceed 0.8 mm.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] The method of this invention significantly improves the mechanical properties of the joint through post-weld surface treatment; and the use of methods such as mechanical grinding and laser remelting makes it easy to implement and more economical, with high production efficiency. The selected methods can be integrated into the welding system to form a complete system equipment. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 The figure shown is a cross-sectional schematic diagram of the welded joint involved in this invention.

[0029] Figure 2 The diagram shown is a schematic representation of the welding process involved in this invention.

[0030] Figure 3 The diagram shown illustrates the cross-sectional changes at the weld joint during the welding process involved in this invention.

[0031] Figure 4 The diagram shown is a schematic representation of the welding process involved in this invention.

[0032] Figure 5 The diagram shown is another schematic representation of the cross-sectional changes at the weld during the welding process involved in this invention.

[0033] Figure 6 The diagram shown is a schematic representation of the final cross-section of the welded joint involved in this invention.

[0034] Figure 7 The diagram shown is another schematic representation of the final cross-section of the welded joint involved in this invention.

[0035] Figure 8 The image shows the metallographic profile of the welded joint section obtained in Comparative Example 1.

[0036] Figure 9 The image shows the metallographic profile of the welded joint obtained in Example 1.

[0037] Figure 10 The metallographic structure of the welded joint section obtained in Comparative Example 2 is shown.

[0038] Figure 11 The image shows the metallographic profile of the welded joint obtained in Example 2.

[0039] Figure 12 The metallographic structure of the welded joint section obtained in Comparative Example 3 is shown.

[0040] Figure 13 The metallographic structure of the welded joint section obtained in Example 3 is shown.

[0041] Figure 14 The image shows the metallographic profile of the welded joint obtained in Example 4.

[0042] Reference numerals: 1-Final welded joint; 10-Final weld melting and solidification zone; 2-Final weld heat-affected zone; 11-Final weld front reinforcement; 12-Weld back reinforcement; 110-Initial weld surface; 1001-Initial melting zone; 101-Remelting zone; 120-Initial weld back surface; 21-Heat-affected zone on one side of the initial weld; 22-Heat-affected zone on the other side of the initial weld; B2-Width of the initial weld heat-affected zone; 31-First high-strength stainless steel... Steel workpiece, 32 - second high-strength stainless steel workpiece, h10 - final weld reinforcement on one side, h11 - final weld reinforcement on the other side, h - final weld joint surface depression depth, h21 - remelting depth, B10 - initial weld front width, B20 - initial weld back width, 4 - laser beam, B11 - final weld front width, B21 - final weld back width, α - transition angle between the final weld joint surface reinforcement and the adjacent heat-affected zone, b - assembly gap. Detailed Implementation

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the present invention is further described below in conjunction with specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, the accompanying drawings are schematic diagrams; therefore, the apparatus and device of the present invention are not limited by the size or scale of the schematic diagrams.

[0044] It should be noted that in the claims and specification of this patent, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0045] This invention provides a method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints. After the welded joint is formed by laser welding, the surface of the weld zone and the adjacent heat-affected zone is subjected to controllable strengthening and smoothing treatment, so that the treated welded joint meets specific surface morphology parameters, thereby significantly reducing low-temperature stress concentration, inhibiting crack initiation, and improving low-temperature mechanical properties.

[0046] like Figure 1 As shown, the final welded joint 1 is obtained by laser welding a first high-strength stainless steel workpiece 31 (with a thickness of t1) and a second high-strength stainless steel workpiece 32 (with a thickness of t2), followed by controlled strengthening and flattening treatment of the welded joint after welding. The final welded joint 1 includes a final weld melting and solidification zone 10, a final weld heat-affected zone 2, and a base material zone. The formed welded joint has a relative final weld front side reinforcement height 11 and a weld back side reinforcement height 12. The maximum angle α between the final weld melting and solidification zone 10 and the final weld heat-affected zone 2 does not exceed 30°, and the maximum values ​​of the reinforcement heights h10 and h11 on one side of the formed final weld do not exceed 0.3 mm. In a preferred embodiment, the maximum values ​​of h10 and h11 do not exceed 0.25 mm. In a more extreme case, both h10 and h11 do not exceed 0.15 mm. At the same time, the maximum depth of the surface depression on either side of the weld does not exceed 0.05 mm.

[0047] The first and second stainless steel billets are cold-rolled austenitic stainless steels that have undergone plastic deformation. These materials, after solution annealing, are strengthened by cold rolling deformation ranging from 10% to 90%. Their microstructure is typical deformed austenite, containing high-density dislocation entanglements, deformation twins, or deformed martensite. Through this work hardening mechanism, their room temperature tensile strength can be significantly increased to over 800 MPa, and even to over 1200 MPa, thus meeting the high strength requirements of structures such as rocket propellant tanks. Commonly used grades include austenitic stainless steels such as 301, 304, 304L, 308, 308L, 316, 316L, 321, and 347. The main elements in the billet matrix are present in the following mass percentages: 0.05%≤C≤0.3%, 0.5%≤Mn≤3.0%, 0.1%≤Si≤2.0%, 12%≤Cr≤25%, 3%≤Ni≤12%, N≤0.6%; the balance is iron and impurities generated during manufacturing. Preferably, 0.05%≤C≤0.2%. These grades of stainless steel can achieve the aforementioned high strength levels in the cold-rolled state, and their inherent austenitic structure ensures excellent toughness even at ultra-low temperatures. A typical base material for steel plates is 301, whose elemental composition by mass is mainly as follows: 0.1%≤C≤0.2%; 1%≤Mn≤2.5%; 0.5%≤Si≤1.5%; 16%≤Cr≤18%; 6%≤Ni≤8%; N≤0.15%; the remainder being Fe and unavoidable impurities. Its thickness is generally between 0.5 and 5.0 mm.

[0048] During welding, the thickness and composition of the two stainless steel billets can be the same or different. The assembly can be formed by butt joint or lap joint, and the resulting weld joint can be a lap fillet weld or a butt weld with unequal thicknesses. An assembly gap b can be present during welding, typically 0 to 0.5 mm. Laser welding can employ either solderless laser self-fusion welding or laser filler welding with simultaneous solder addition. When using laser filler wire welding, the wire composition is generally Cr ≤ 25% and Ni ≤ 15% by mass, ensuring that the austenite content in the final weld is not less than 5%, preferably not less than 10%.

[0049] Generally, the laser beam used in laser welding has a wavelength of 0.3-10 μm, especially 0.5-3 μm infrared lasers, and more commonly 900-1100 nm wavelengths, which are common in laser processing. The laser beam is generally emitted by a laser, which can be of various types, such as solid-state lasers or gas lasers. Specifically, it can include fiber lasers, disk lasers, semiconductor diode lasers, and Nd:YAG type solid-state lasers, or CO2 gas lasers. Other types are also possible, as long as they can generate a laser beam and melt the solder and workpiece to form a weld pool. During the welding process, the laser beam can travel along the welding direction in a fixed or synchronous high-speed motion, such as oscillation. The oscillation shape can include circular, zigzag, figure-eight, and infinity shapes, with an oscillation frequency generally between 50-500 Hz and an oscillation amplitude between 0.1-1.5 mm. During the welding process, various single or mixed shielding gases can be added, such as Ar, N2, He, etc., or the welding can be performed without a shielding gas. The power of the laser beam is generally 500W-10KW, preferably 1000W-6KW; and the travel speed of the laser beam relative to the workpiece assembly is generally 1-10m / min, preferably 1.5-8m / min.

[0050] The equivalent circular diameter of the area irradiated by the laser beam on the surface of the assembly is generally 0.2-1.2 mm, preferably 0.3-0.8 mm. To achieve efficient melting, the laser heat source has a high power density, which can be achieved using a single-mode laser with a good beam quality factor or a high-power multimode laser. The energy distribution of the laser beam can be uniform, Gaussian, or other configurations. The shape of the irradiated area on the surface of the assembly by each laser beam can be circular, rectangular, annular, or a combination of straight lines and curves.

[0051] After laser welding is completed, the weld area undergoes surface hardening and smoothing treatment on at least one side, preferably both sides. When only one side is treated, the back side (the side opposite the laser incident light) is treated first. The hardening and smoothing treatment is any one or a combination of laser surface treatment, ultrasonic / high-speed shot peening, precision mechanical grinding and polishing, or controlled pressure sandblasting. When the treatment is laser surface treatment, refer to... Figure 2An auxiliary laser beam 4 is used to scan the weld area to improve the initial weld surface 110 and / or surface microstructure, making the surface smooth. The irradiation area of ​​the laser beam 4 at the initial weld surface at least covers a portion of the original weld initial melting zone 1001, forming 101. The laser acts on the initial melting zone 1001, causing surface remelting, shot peening, cladding, or cleaning effects. The laser surface treatment can use pulsed laser or continuous laser, and by controlling parameters such as laser power and scanning speed, remelting, micro-melting, or laser cleaning removal of the surface material of the weld area can be achieved. The laser beam 4 and the main welding laser beam can be the same laser beam acting twice, or two separate independent laser beams. The light source characteristics such as beam quality and spot size of the two laser beams can be the same or different. The two laser beams can be integrated on one welding device or two independently controlled devices.

[0052] When it is surface remelting, the shape of the surface reinforcement area on at least one side of the weld region changes. After remelting and solidification, the maximum angle α between the final weld molten zone and the adjacent heat-affected zone does not exceed 30°, and the height of the molten zone is not lower than that of the heat-affected zone, and h10 does not exceed 0.3 mm. More preferably, after remelting and solidification, the angle between the final weld and the adjacent heat-affected zone does not exceed 20°, and the height of the molten zone is not lower than that of the heat-affected zone, and h10 does not exceed 0.1 mm. Figure 3 As shown, the remelting depth h21 of the final weld generally does not exceed the melting depth of the initial weld joint, while the weld width B11 on the face of the final weld is generally greater than the weld width B10 on the face of the initial weld, such as... Figure 4 As shown, the molten zone covers at least a portion of the heat-affected zones 21 and 22 on one side of the initial weld, and after remelting, forms the final weld heat-affected zone 2. Generally, at least one side of the initial weld is remelted; the back surface 120 of the initial weld may or may not be remelted. The maximum transition angle between the surface reinforcement of the final weld and the heat-affected zone should not exceed 30°, and h10 should not exceed 0.3 mm. It is worth noting that laser surface treatment can be performed once or multiple times, meaning that the final weld width B11 on the front side after final remelting can be obtained by multiple melting processes. Figure 3 The diagram shows that remelting is performed only on the front side, so the final weld width B21 on the back side is the same as the initial weld width B20 on the back side and remains unchanged.

[0053] In the laser processing, pulsed or continuous lasers can be used. The laser beam irradiating the surface can form one or more spots, as long as the final molten area covers the initial molten width. Alternatively, it can be obtained by processing different locations once or multiple times. Similarly, the wavelength of the surface treatment laser beam is 0.3-10 μm, particularly emitted by an infrared laser with a wavelength of 0.5-3 μm, and more commonly, a wavelength of 900-1100 nm. The laser beam is generally emitted by a laser, which can include various types, such as solid-state laser beams or gas laser beams. Specifically, it can include fiber lasers, disk lasers, semiconductor diode lasers, and Nd:YAG type solid-state lasers, or CO2 gas lasers. Other types can also be included, as long as they can generate a laser beam and melt the solder and workpiece to form a weld pool. During the welding process, the laser beam can be fixed or simultaneously oscillate at high speed along the welding direction. The oscillation shape can include various forms such as circles, broken lines, figure-eights, and infinity symbols. The oscillation frequency is generally 50-500 Hz, and the oscillation amplitude is between 0.1-1.5 mm. During the remelting process, various single or mixed shielding gases can be added, such as Ar, N2, and He, or the process can be carried out without a shielding gas. The power of the laser beam is generally 300W-5KW, preferably 500W-4KW; and the travel speed of the laser beam relative to the workpiece assembly is generally 1-10 m / min, preferably 1.5-5 m / min. By controlling parameters such as laser power and scanning speed, the surface material of the weld zone can be remelted and modified. During the laser remelting modification process, solder can also be added simultaneously to change the morphology of the surface molten layer. Similarly, when laser filler wire remelting is used, the composition of the added welding material generally meets the following requirements: Cr≤25%, Ni≤15%, so that the average content of austenite in the final weld zone is not less than 5%, preferably not less than 10%.

[0054] Alternatively, after laser welding to obtain the initial weld seam, its surface can be treated by mechanical grinding. Mechanical grinding alters the surface reinforcement shape on at least one side of the initial weld seam to meet certain requirements. Figure 1 The required morphology is shown. More preferably, mechanical grinding is used to reduce the surface elevation to approximately flush with the adjacent heat-affected zone, and the maximum depth h of the surface depression does not exceed 0.05 mm. Figure 5 As shown. At this point, after grinding the initial heat-affected zone and the melting zone, the final weld heat-affected zone 2 and the final weld melting and solidification zone 10 are formed. The final weld width B11≤B10, B21≤B20. Generally, the change in melting width during grinding is relatively small. In practice, at least one side of the initial weld can be mechanically ground, preferably the back side after the initial laser welding (relative to the laser incident direction), such as... Figure 6 The diagram shows the final weld cross-section obtained when only the back side of the weld is mechanically ground.

[0055] It is worth noting that when performing mechanical grinding, a staged grinding process or a single-stage method can be adopted. This involves first using coarse abrasive grains (such as alumina grinding discs) to quickly remove burrs or large amounts of material, and then switching to fine abrasive grains (such as silicon carbide grinding discs or scouring pads) for surface finishing, thus balancing efficiency and surface quality. The grinding process can be automated mechanical grinding or manual grinding. Coolant can be added during the grinding process to prevent deformation or reduce dust. The grinding process can be completed using flexible or rigid devices with pressure or displacement sensors. Generally, after grinding, the maximum height difference h between the molten zone and the adjacent heat-affected zone does not exceed 0.2 mm, preferably not more than 0.1 mm, and more preferably not more than 0.05 mm. The molten zone and the adjacent heat-affected zone have a smooth transition, and the surface roughness does not exceed Ra6.4. Figure 7 As shown.

[0056] It is also worth noting that after the welded joint is formed by laser welding, the surface can be smoothed by a combination of methods, such as thermal / mechanical methods, including rolling, ultrasonic shot peening, or sandblasting. Ultimately, this alters the cross-sectional morphology of the welded joint, provided that its surface morphology characteristics meet the scope described in this invention.

[0057] Example 1

[0058] This embodiment compares the low-temperature mechanical properties of the joints after grinding different surfaces of the weld to be basically flush with the substrate surface, based on Comparative Example 1. Table 1 shows the laser welding process parameters used in different embodiments. The substrate used was 1.5mm thick 3 / 4H temper ultra-high strength 301 stainless steel for butt welding. The comparative example was direct laser autofusion welding, with a laser focused spot size of 0.5mm, a laser power of 2000W, and a welding speed of 3.0m / min. The resulting welded joint cross-section is shown below. Figure 8 As shown, there are surface depressions on both sides of the weld that are lower than the base material, with a maximum value h of approximately 0.1 mm. The low-temperature (-196℃) mechanical properties of the welded joint are shown in Table 1, with an average tensile strength of only about 1299 MPa. In Example 1, based on the comparative example, both surfaces of the weld area were mechanically ground after welding, resulting in a final joint with a maximum surface depression of only about 0.04 mm at the weld. Figure 9 The cross-sectional view of the weld zone is shown in Table 2. The final low-temperature mechanical tensile strength of the welded joint reached approximately 2207 MPa. The results indicate that mechanical grinding of the welded joint after laser autofusion welding until the depression is reduced can significantly improve the low-temperature tensile strength of the joint.

[0059] Example 2

[0060] This embodiment compares the low-temperature mechanical properties of the joints after grinding different surfaces of the weld seam to be flush with the substrate surface, based on Comparative Example 2. Table 1 shows the laser welding process parameters used in different embodiments. The substrate used was 1.5mm thick 3 / 4H temper ultra-high strength 301 stainless steel for butt welding.

[0061] Table 1 Process parameters for different embodiments

[0062]

[0063] Comparative Example 2 is a direct laser-guided wire-filler welding method. The laser focused spot size is 0.5 mm, the laser power is 2400 W, the welding speed is 3.0 m / min, the welding material is 1.0 mm 308L welding wire, and the wire feed speed is 2.0 m / min. The resulting welded joint cross-section is shown below. Figure 10 As shown, both sides have excess height after welding, with the maximum excess height located on the front side, h10 being 0.32 mm, and the maximum surface forming angle α being approximately 40°. The low-temperature (-196℃) mechanical properties of the welded joint are shown in Table 2, with an average tensile strength of only about 1193 MPa. Example 3, based on Comparative Example 2, involves mechanically grinding both surfaces of the weld area after welding, resulting in a final joint h10 reduced to approximately 0.05 mm and a surface forming angle α reduced to approximately 18°. Figure 11 The cross-sectional view of its weld zone is shown. The final low-temperature mechanical tensile strength of the welded joint can reach approximately 2047 MPa, as shown in Table 2.

[0064] Table 2. Connector performance of different embodiments

[0065]

[0066] The results show that mechanical grinding of the welded joint after laser wire filler welding to a predetermined height can significantly improve the low-temperature tensile strength of the joint.

[0067] Examples 3 and 4

[0068] Example 3, based on Comparative Example 3, involves grinding different surfaces of the weld seam to be flush with the substrate surface and comparing the low-temperature mechanical properties of the joints. Table 1 shows the laser welding process parameters used in different examples. The substrate used was 1.5mm thick 3 / 4H temper ultra-high strength 301 stainless steel for butt welding. Comparative Example 3 used direct laser filler wire welding, with a laser focusing spot size of 0.6mm, a laser power of 2600w, a welding speed of 3.0m / min, and a 1.0mm diameter high-strength welding wire (Cr < 15%) with a wire feed speed of 1.5m / min. The resulting welded joint cross-section is shown below. Figure 12 As shown, the maximum surface depression at the weld is approximately 0.06 mm on the back side, and h10 is approximately 0.23 mm on the front side. The surface forming angle α is approximately 28°, and the austenite content in the joint after welding is greater than 5%. The low-temperature (-196℃) mechanical properties of the welded joint are shown in Table 2, with an average tensile strength of only about 1160 MPa. Example 3 involves mechanically grinding the back side of the joint in the weld area after welding, based on Comparative Example 3. The maximum surface depression at the front side of the weld in the final joint is approximately 0.01 mm, h10 is approximately 0.22 mm on the front side, and the surface forming angle is approximately 28°. The front side of the weld is higher than the substrate, with no obvious depression. Figure 13 The cross-sectional view of its weld zone is shown. The final low-temperature mechanical tensile strength of the welded joint can be increased to approximately 1921 MPa, as shown in Table 2. Example 4, based on Comparative Example 3, involves mechanically grinding both sides of the weld zone joint after welding, resulting in a maximum indentation value h at the weld seam of the final joint of approximately 0.02 mm; as shown in Table 2. Figure 14 The cross-sectional view of the weld zone is shown. The final low-temperature mechanical tensile strength of the welded joint can be increased to approximately 1943 MPa, as shown in Table 2. The results show that mechanical grinding of the welded joint to a predetermined height after laser wire filler welding can significantly improve the low-temperature tensile strength of the joint.

[0069] The method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints provided by this invention is flexible, cost-controllable, and highly effective, and can be directly integrated into existing laser welding production lines. It is particularly suitable for manufacturing key components such as aerospace tanks, deep-sea pressure hulls, cryogenic storage and transportation containers, and defense equipment, which have stringent low-temperature performance requirements, and has broad industrial application prospects.

[0070] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various improvements and modifications without departing from the spirit and principles 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 method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints, characterized in that, Includes the following steps: S1. Laser welding step: Laser welding is performed on ultra-high strength stainless steel workpieces to form a welded joint, wherein the welded joint includes a weld melting zone, a heat-affected zone and a substrate zone; S2. Surface morphology inspection step: Inspect the surface morphology of the welded joint to determine the transition angle between the weld melting zone and the heat-affected zone, the height difference of the weld reinforcement, and the depth of the surface depression. S3. Strengthening and smoothing treatment step: Based on the detection results of step S2, the surface of the weld melting zone and adjacent heat-affected zone of the weld joint is subjected to controllable strengthening and smoothing treatment to ensure that the treated weld joint meets the following surface morphology characteristics: The controllable strengthening and leveling treatment is any one or a combination of laser surface treatment, ultrasonic shot peening, high-speed shot peening, precision mechanical grinding and polishing, or controllable pressure sandblasting. The angle between the transition zone of the weld molten zone and the heat-affected zone shall not exceed 30°; When the surface of the weld molten zone is lower than the surface of the adjacent substrate area, the maximum depth of the depression on either side of the weld joint shall not exceed 0.05 mm; When the surface of the weld molten zone is not lower than the surface of the adjacent substrate zone, the maximum height difference of the weld reinforcement does not exceed 0.3 mm.

2. The method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints according to claim 1, characterized in that, The ultra-high strength stainless steel workpieces are of equal or unequal thickness, and the room temperature tensile strength of any workpiece is not less than 1000 MPa.

3. The method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints according to claim 1, characterized in that, When the treatment is laser surface treatment, a laser beam is used to scan the surface to be treated, causing the surface material to remelt or micromelt, so as to improve the surface morphology and surface microstructure at the same time.

4. The method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints according to claim 3, characterized in that, After laser surface treatment, the height difference between the weld melting zone and the adjacent heat-affected zone or substrate area does not exceed 0.3 mm, and the remelted melting zone covers at least a portion of the original heat-affected zone.

5. The method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints according to claim 1, characterized in that, When the controllable strengthening and leveling treatment is precision mechanical grinding and polishing, the maximum depth of micro-protrusions and depressions on the surface of the weld area and adjacent heat-affected zone after grinding does not exceed 0.05mm.

6. The method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints according to claim 1, characterized in that, The enhanced smoothing treatment covers the weld area completely and extends to the heat-affected zone at least 1 mm away from the center line of the weld on both sides.

7. The method for improving the low-temperature performance of ultra-high strength stainless steel laser-welded joints according to claim 1, characterized in that, The enhanced flattening treatment is performed on one side of the welded joint surface or on both sides; when only one side surface is treated, the back side of the laser weld is treated first.

8. An ultra-high strength stainless steel laser-welded joint prepared by the method according to any one of claims 1 to 7, characterized in that: The welded joint includes a weld melting zone, a heat-affected zone, and a substrate zone; the transition angle between the weld melting zone and the heat-affected zone does not exceed 30°, and the height difference between the weld melting zone and the adjacent heat-affected zone or substrate zone does not exceed 0.3 mm; the maximum depth of the depression or excess height on any side surface of the welded joint does not exceed 0.05 mm.

9. The ultra-high strength stainless steel laser-welded joint according to claim 8, characterized in that: The angle between the transition zone of the weld fusion zone and the heat-affected zone shall not exceed 20°, and the height difference shall not exceed 0.2 mm.

10. The ultra-high strength stainless steel laser-welded joint according to claim 8, characterized in that: The maximum width of the heat-affected zone of the welded joint shall not exceed 0.8 mm.

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