Two-step cryogenic treatment process based on temperature difference effect inside structure

Abstract

The application belongs to the technical field of welding, and discloses a two-step cryogenic treatment process based on internal temperature difference effect of structure, wherein, in the first step, the base material area far from the weld of the welded joint after welding is subjected to cryogenic treatment; in the second step, the weld and the base material area near the weld are subjected to cryogenic treatment; the cryogenic treatment is performed in the following manner: after the part other than the target area is coated with heat insulation cotton or a heating pad, the target area of the welded joint is subjected to refrigerant application. The application utilizes the method of applying cryogenic treatment to the specific area of the joint in steps to generate a larger temperature difference, which can not only effectively eliminate the post-weld residual stress at the weld, but also optimize the brittle and hard structure at the weld; meanwhile, the application first provides a method of cryogenic treatment in air, which can implement cryogenic treatment on large plate materials in open places such as laboratories, workshops, outdoors and sites, and the overall operation is more convenient, which breaks through the engineering bottleneck that large plate materials are difficult to be subjected to overall post-treatment and gets rid of the size limitation of traditional cryogenic boxes.

Description

Technical Field

[0001] This invention belongs to the field of welding technology, specifically, it relates to a post-weld treatment process for welded joints. Background Technology

[0002] During the welding process, the metal in the weld area is locally and rapidly heated to a molten state. When cooling after welding, the extremely high cooling rate and large temperature gradient result in large tensile residual stress at the weld, forming a brittle and hard martensitic structure, which severely degrades the mechanical properties of the material.

[0003] The most common post-treatment method is heat treatment. Heating the material to the two-phase region or even the high-temperature single-phase region transforms the brittle and hard phase into a microstructure with better plastic deformation compatibility, thereby improving the material's mechanical properties. However, excessively high heating temperatures and long holding times can lead to softening of the base material and a decrease in strength, which in turn worsens the overall mechanical properties of the joint.

[0004] To address this, researchers have proposed using cryogenic treatment to regulate joint stress and improve joint microstructure. Typically, the entire sample is placed in a cryogenic chamber and purged with liquid nitrogen. The effect of overall material contraction upon cooling is utilized to regulate residual stress and refine the microstructure. This method, which does not sacrifice the properties of the base material, has gained popularity among researchers. However, existing research on cryogenic treatment involves overall treatment within a cryogenic chamber, requiring large-scale equipment and consuming significant energy. More importantly, this overall cryogenic treatment method keeps the entire sample at a uniform temperature, limiting weld shrinkage and resulting in less than ideal stress regulation. Summary of the Invention

[0005] This invention addresses the technical problems of microstructure coarsening and high residual tensile stress in large metal welded components during post-weld cooling due to their large cross-sectional thickness and poor heat dissipation. It provides a two-step cryogenic treatment process based on the internal temperature difference effect of the structure. By performing cryogenic treatment on specific areas of the welded joint in stages in air, a large temperature difference is generated. This not only effectively eliminates the residual stress at the weld but also optimizes the brittle and hard microstructure at the weld, breaking through the engineering bottleneck of the difficulty in post-processing large plates as a whole.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] This invention provides a two-step cryogenic treatment process based on the internal temperature difference effect of a structure, comprising the following steps:

[0008] The first step is to perform cryogenic treatment on the base material area far from the weld in the welded joint that has been welded;

[0009] The second step is to perform cryogenic treatment on the weld and the surrounding base material area.

[0010] The method of deep cryogenic treatment is as follows: after covering the area outside the target area with insulation cotton or heating pads, refrigerant is applied to the target area of ​​the weld joint.

[0011] Furthermore, the refrigerant applied is one or a combination of liquid nitrogen spraying, cryogenic nitrogen purging, attaching liquid nitrogen flow channel cold plates, and attaching flexible cold pads.

[0012] Furthermore, in the first step, the base material area away from the weld is a symmetrical strip-shaped area on both sides of the weld: in the transverse direction, it includes the area 25-250 mm away from the center line of the weld on both sides; in the longitudinal direction, it covers the entire length of the weld along the weld axis.

[0013] Furthermore, in the first step, the cryogenic treatment temperature is -100 to 196°C, and the cryogenic treatment time is 1 to 60 minutes.

[0014] Furthermore, in the second step, the weld and the surrounding base material area are a strip-shaped area with the weld centerline as the axis of symmetry: the width in the transverse direction is 50-500 mm; in the longitudinal direction, it covers the entire length of the weld along the weld axis.

[0015] Furthermore, in the second step, the cryogenic treatment temperature is -100 to 196°C, and the cryogenic treatment time is 2 hours to 72 hours.

[0016] The beneficial effects of this invention are:

[0017] For the welded joints that have been welded, the present invention first performs deep cryogenic treatment on the base material region far from the weld in air to eliminate residual stress at the weld; then performs deep cryogenic treatment on the weld and the base material region nearby in air to refine the weld microstructure and promote the transformation of residual β phase to α phase.

[0018] Both cryogenic treatment steps are based on the internal temperature difference effect of the structure, manifested in the temperature difference actively created by the two cryogenic treatment steps and its effect: The first step involves cryogenic treatment of the base material area far from the weld, causing the base material to shrink upon cooling and form a significant temperature difference with the weld area. The internal stress generated by the shrinkage is transferred to the weld and transformed into compressive stress, thereby offsetting the residual welding stress. The second step involves cryogenic treatment of the weld and the nearby base material area, causing the weld area to form a temperature difference with the surrounding microstructure again. The local rapid cooling enhances the driving force of microstructure transformation, promoting the refinement of brittle and hard martensite in the weld and the transformation of residual β to α. Therefore, this invention can not only effectively eliminate post-weld residual stress at the weld, but also optimize the brittle and hard microstructure at the weld, solving the engineering bottleneck of the difficulty in post-processing large plates as a whole.

[0019] Meanwhile, this invention provides, for the first time, a method for cryogenic treatment in air, enabling cryogenic treatment of large sheet materials in open locations such as laboratories, workshops, outdoors, and on-site. The overall operation is more convenient, overcoming the size limitations of traditional cryogenic chambers and not being restricted by the size of structural components. Furthermore, compared to cryogenic treatment of entire components, it saves liquid nitrogen materials and reduces welding costs. Attached Figure Description

[0020] Figure 1 This is a flow chart of the two-step cryogenic treatment process based on the internal temperature difference effect of the structure according to the present invention;

[0021] Figure 2 This is a schematic diagram of the process parameters for the two-step cryogenic treatment based on the internal temperature difference effect of the structure according to the present invention; wherein, (a) represents the first step and (b) represents the second step;

[0022] Figure 3 This is a schematic diagram of the target area of ​​the two-step cryogenic treatment based on the internal temperature difference effect of the structure according to the present invention; wherein, (a) represents the first step and (b) represents the second step;

[0023] Figure 4 The diagram shows a comparison of residual stress between the two-step cryogenic treatment and the overall cryogenic treatment in Example 1; where (a) represents the overall cryogenic treatment and (b) represents the two-step cryogenic treatment.

[0024] Figure 5 The images show the weld microstructure before and after the two-step cryogenic treatment in Example 1; where (a) represents the weld before the two-step cryogenic treatment and (b) represents the weld after the two-step cryogenic treatment. Detailed Implementation

[0025] The present invention will be further described in detail below through specific embodiments. These embodiments will enable those skilled in the art to have a more comprehensive understanding of the present invention, but will not limit the present invention in any way.

[0026] like Figures 1-3 As shown, the present invention provides a two-step cryogenic treatment process based on the internal temperature difference effect of the structure. The workpiece to be treated is a welded joint that has been completed, and the welding method is not limited.

[0027] In this invention, the description of the welded joint is as follows: "longitudinal" refers to the direction of the base material surface parallel to the weld axis, "transverse" refers to the direction of the base material surface perpendicular to the weld axis, and "thickness direction" refers to the direction perpendicular to the base material surface.

[0028] The first step is to perform deep cryogenic treatment on the base material area far from the weld in the welded joint that has been completed. This will rapidly cool the base material area and create a temperature difference with the surrounding area. The internal stress generated by the thermal expansion and contraction effect of the temperature difference will be used to counteract the residual stress of the weld.

[0029] In the first step, the cryogenic treatment of the base material region away from the weld consists of two symmetrically arranged strip-shaped regions on both sides of the weld: in the transverse direction, it is a range 25-250 mm away from the weld centerline on both sides, based on the weld centerline. This range provides sufficient base material shrinkage to generate a compressive stress of not less than 100 MPa at the weld centerline, without directly cooling the weld or heat-affected zone; in the longitudinal direction, it covers the entire length of the weld to be treated along the weld axis; in the thickness direction, the cryogenic treatment acts directly on the surface of the base material, and the cooling effect is transmitted into the component through heat conduction along the thickness direction.

[0030] In the first step, the cryogenic treatment temperature is -100 to 196°C, and the cryogenic treatment time is 1 to 60 minutes. This invention uses finite element simulation to set the cryogenic treatment time to 1 to 60 minutes, based on the principle of achieving a compressive stress greater than 100 MPa at the weld center.

[0031] The second step involves deep cryogenic treatment of the weld and the surrounding base metal area, which rapidly cools the weld and the surrounding base metal area and creates a temperature difference with the surrounding area. The thermal expansion and contraction effect caused by the temperature difference refines the coarse and brittle martensite in the weld and the surrounding area, and the residual β phase in the middle further transforms into the α phase.

[0032] In the second step, the cryogenically treated weld and the surrounding base material area are arranged in an intermediate strip-shaped region with the weld centerline as the axis of symmetry: in the transverse direction, its transverse width is 50-500 mm, which can cover the weld and the near-weld areas on both sides, ensuring that the brittle and hard martensite is fully refined and the residual β phase is completely transformed into the α phase, and connecting with the area in the first step to avoid the occurrence of an untreated stress transition zone between the weld and the cryogenic zone of the base material; in the longitudinal direction, it covers the entire length of the weld along the weld axis; in the thickness direction, the cryogenic treatment is directly applied to the surface of the base material in the weld and near-weld areas, and the cooling effect is transmitted to the interior of the component along the thickness direction through heat conduction.

[0033] In the second step, the cryogenic treatment temperature is -100 to 196°C, and the cryogenic treatment time is 2 to 72 hours. This invention sets the cryogenic treatment time to 2-72 hours based on conventional cryogenic treatment operation times.

[0034] In this invention, the specific method for performing cryogenic treatment on the target area of ​​the welded joint is to cover the part outside the target area with insulation cotton or heating pads, and then apply refrigerant to the target area of ​​the welded joint.

[0035] Clearly, in the first step, the target area of ​​the weld joint refers to the base material area far from the weld; in the second step, the target area of ​​the weld joint refers to the weld and the base material area nearby.

[0036] Those skilled in the art can select the specific method of applying the refrigerant according to the component size, operating environment, and cooling rate requirements. This includes liquid nitrogen spraying, cryogenic nitrogen purging, attaching liquid nitrogen flow channel cold plates, attaching flexible cold pads, or a combination of the above methods. Any method that can achieve localized cryogenic treatment of the target area of ​​the welded joint in an open environment and create a temperature difference within the structure falls under the category of cryogenic treatment of the target area of ​​the welded joint according to this invention.

[0037] Example 1

[0038] This embodiment focuses on a 40 mm thick welded joint made of titanium alloy by vacuum laser butt welding, employing a two-step cryogenic treatment process.

[0039] Step 1: In the laboratory, wrap the area outside the target area of ​​the welding joint with thermal insulation cotton, and spray the target area of ​​the welding joint with liquid nitrogen at a temperature of -196℃ for 12 minutes.

[0040] The target area in step 1 is the base material area away from the weld, which is a strip-shaped area extending 250mm from both sides of the weld centerline.

[0041] Step 2: In the laboratory, wrap the area outside the target area of ​​the weld joint with thermal insulation cotton, and spray the target area of ​​the weld joint with liquid nitrogen at a temperature of -196℃ for 5 hours.

[0042] The target area in step 2 is the weld and the surrounding base material area, which is a strip-shaped area with a transverse width of 400 mm covering the weld and the near weld areas on both sides.

[0043] Comparative Example 1

[0044] This comparative example demonstrates the effect of overall cryogenic treatment on stress control. The welded joint was immersed in liquid nitrogen at a temperature of -196°C for 12 minutes.

[0045] Example 2

[0046] This embodiment focuses on a 30 mm thick titanium alloy electron beam welded joint, employing a two-step cryogenic treatment process.

[0047] Step 1: In the workshop, cover the area outside the target area of ​​the welding joint with insulating cotton, and use the low temperature nitrogen generated by the vaporization of liquid nitrogen to force-purge the target area of ​​the welding joint. The temperature of the low temperature nitrogen is -160℃, and the purging time is 10 minutes.

[0048] The target area in step 1 is the base material area away from the weld, which is a strip-shaped area extending 150mm from both sides of the weld centerline.

[0049] Step 2: Cover the area outside the target area of ​​the welding joint in the workshop with insulation cotton, and use the low temperature nitrogen generated by the vaporization of liquid nitrogen to force-purge the target area of ​​the welding joint. The temperature of the low temperature nitrogen is -160℃, and the purging time is 3 hours.

[0050] The target area in step 2 is the weld and the surrounding base material area, which is a strip-shaped area with a transverse width of 200 mm covering the weld and the near weld areas on both sides.

[0051] Example 3

[0052] This embodiment focuses on a 40 mm thick welded joint made of titanium alloy TIG filler wire, employing a two-step cryogenic treatment process.

[0053] Step 1: At the construction site, cover the area outside the target area of ​​the welding joint with insulation cotton, and attach a copper / aluminum cold plate with liquid nitrogen flow channels inside to the target area of ​​the welding joint. The temperature of the copper / aluminum cold plate is -120℃ and the cooling time is 45 minutes.

[0054] The target area in step 1 is the base material area far from the weld, which is a strip-shaped area extending 230mm from both sides of the weld centerline.

[0055] Step 2: At the construction site, cover the area outside the target area of ​​the welding joint with insulation cotton, and attach a copper / aluminum cold plate with liquid nitrogen flow channels inside to the target area of ​​the welding joint. The temperature of the copper / aluminum cold plate is -120℃, and the cooling time is 24 hours.

[0056] The target area in step 2 is the weld and the surrounding base material area, which is a strip-shaped area with a transverse width of 450 mm covering the weld and the near weld areas on both sides.

[0057] Example 4

[0058] This embodiment takes a 50 mm thick titanium alloy TIG filler wire butt joint as the object and adopts a two-step cryogenic treatment process.

[0059] Step 1: On the offshore platform, cover the area outside the target area of ​​the welding joint with thermal insulation cotton and spray the target area of ​​the welding joint with liquid nitrogen. The temperature of the liquid nitrogen spray is -100℃ and the spraying time is 60 min.

[0060] The target area in step 1 is the base material area away from the weld, which is a strip-shaped area extending 250mm from both sides of the weld centerline.

[0061] Step 2: On the offshore platform, cover the area outside the target area of ​​the weld joint with insulating cotton and spray the target area of ​​the weld joint with liquid nitrogen. The temperature of the liquid nitrogen spray is -100℃ and the spraying time is 72 hours.

[0062] The target area in step 2 is the weld and the surrounding base material area, which is a strip-shaped area with a transverse width of 500 mm covering the weld and the near weld areas on both sides.

[0063] like Figure 4 The figure shows a comparison of the residual stress in Example 1 after two-step cryogenic treatment and in Comparative Example 1 after overall cryogenic treatment. From... Figure 4 As can be seen, after overall cryogenic treatment, only the weld core has a compressive stress of about 58 MPa, while the upper and lower parts are under tensile stress, with the surface tensile stress even exceeding 100 MPa. In contrast, after two-step cryogenic treatment, significant compressive stress was generated in and around the weld center, with a maximum compressive stress reaching 127 MPa, significantly reducing the residual stress in the weld center; at the same time, the width of the compressive stress area exceeding 58 MPa was 314 mm, accounting for 15.7% of the overall width.

[0064] The weld microstructure after two-step cryogenic treatment in Example 1 is as follows: Figure 5 As shown. From Figure 5 As can be seen, the grains of the weld structure have undergone a significant refinement process. According to the thermodynamic stability of the β phase, as the temperature decreases, more of the residual β phase will further transform into the α phase, so the residual β phase in the original acicular martensite is significantly reduced.

[0065] Furthermore, the results of Examples 2-4 all showed that a compressive stress greater than 100 MPa was generated at the weld center, along with needle-like α-phase refinement and residual β-phase transformation to α-phase.

[0066] The process verified in this embodiment demonstrates that this two-step cryogenic treatment, compared to traditional integral cryogenic treatment, overcomes the size limitations of traditional cryogenic chambers. It allows for cryogenic treatment of large plates in open environments such as laboratories, workshops, outdoor locations, and engineering sites, significantly improving operational convenience and scenario adaptability. Simultaneously, it leverages the internal temperature difference effect of the component to achieve a synergistic improvement in stress control and microstructure optimization. First, cryogenic treatment is applied to the base material far from the weld, utilizing the temperature gradient formed by the base material's shrinkage to transform internal stress into weld compressive stress, effectively offsetting residual welding stress. Then, cryogenic treatment is applied to the weld and the near-weld zone, strengthening the phase transformation driving force through the secondary local temperature difference, promoting weld martensite refinement and the transformation of residual β phase to α phase.

[0067] In addition, the process of this application can be simplified to a single-step mode as needed. If stress relief is emphasized, the first step is performed separately to perform deep cryogenic treatment on the base material area far away from the weld. If microstructure is emphasized, the second step is performed separately to perform deep cryogenic treatment on the weld and the base material area nearby, which can flexibly adapt to different working conditions.

[0068] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many specific modifications under the guidance of the present invention without departing from the spirit and scope of the claims, and these modifications all fall within the scope of protection of the present invention.

Claims

1. A two-step cryogenic treatment process based on the internal temperature difference effect of a structure, characterized in that, Includes the following process: The first step is to perform cryogenic treatment on the base material area far from the weld in the welded joint that has been welded; The second step is to perform cryogenic treatment on the weld and the surrounding base material area. The method of deep cryogenic treatment is as follows: after covering the area outside the target area with insulation cotton or heating pads, refrigerant is applied to the target area of ​​the weld joint.

2. The two-step cryogenic treatment process based on the internal temperature difference effect of a structure according to claim 1, characterized in that, The refrigerant applied is one or a combination of liquid nitrogen spraying, cryogenic nitrogen purging, attaching liquid nitrogen flow channel cold plates, and attaching flexible cold pads.

3. The two-step cryogenic treatment process based on the internal temperature difference effect of a structure according to claim 1, characterized in that, In the first step, the base material area away from the weld is a symmetrical strip-shaped area on both sides of the weld: in the transverse direction, it includes the area 25-250 mm away from the center line of the weld on both sides; in the longitudinal direction, it covers the entire length of the weld along the weld axis.

4. The two-step cryogenic treatment process based on the internal temperature difference effect of a structure according to claim 1, characterized in that, In the first step, the cryogenic treatment temperature is -100 to 196℃, and the cryogenic treatment time is 1 to 60 minutes.

5. The two-step cryogenic treatment process based on the internal temperature difference effect of a structure according to claim 1, characterized in that, In the second step, the weld and the surrounding base material area are a strip-shaped area with the weld centerline as the axis of symmetry: the width in the transverse direction is 50-500 mm; in the longitudinal direction, it covers the entire length of the weld along the weld axis.

6. The two-step cryogenic treatment process based on the internal temperature difference effect of a structure according to claim 1, characterized in that, In the second step, the cryogenic treatment temperature is -100 to 196℃, and the cryogenic treatment time is 2 h to 72 h.

Images