Backing welding process curve, application and product based on narrow-gap U-groove

Abstract

This application belongs to the field of subsea pipeline welding, and specifically discloses a back bead welding process curve, application and product based on a narrow-gap U-groove. This back bead welding process curve is used to weld the narrow-gap U-groove, and on the basis of the CMT reference curve, at least one current pulse is introduced respectively before and after the short-circuit stage to increase the root bead width and avoid root depression. At the same time, the peak current and cycle time are increased to increase the welding heat input, and the peak current rising rate and peak current falling rate are reduced to extend the action time of the peak current. This application can increase the welding heat input and root bead width, achieve excellent single-sided welding with double-sided forming effect under the conditions of 0-gap butt joint and no backing, solve the problem of weld defects, and effectively improve the service performance of acidic offshore pipelines.

Description

Technical Field

[0001] This application belongs to the field of submarine pipeline welding, and more specifically, relates to a backing welding process curve, application, and product based on a narrow-gap U-groove. Background Art

[0002] In the South China Sea of our country, the geological conditions are complex and harsh, and the content of acidic media such as H 2 S / CO 2 is high. The exploitation of high-sulfur oil and gas is an important part of the development of marine oil and gas resources. Carbon steel oil and gas submarine pipelines (projects in the Middle East, Thailand, etc.) are faced with the strong coupling effects of multiple factors such as low temperature, strong sulfide corrosion, and fatigue loads. The welding technology of anti-strong acid carbon steel submarine pipelines is an important guarantee for their long-term service safety.

[0003] In the traditional submarine pipeline laying process, in order to improve the forming quality of root welding, copper backing is often used to assist in achieving high-quality root forming. However, although copper backing improves the forming effect, a large amount of copper will penetrate into the weld. As is well known, copper has better corrosion resistance than iron. The penetration of copper into the pipeline will form an electric couple, and due to its high corrosion potential, copper will accelerate the corrosion and dissolution of the pipeline (Fe), seriously deteriorating the service life of the pipeline in acidic media. Therefore, there is an urgent need to develop a backing welding process for single-sided welding with double-sided forming without copper backing and excellent root forming.

[0004] Currently, the backing welding processes for single-sided welding with double-sided forming mainly include the surface tension transfer technology (STT) and the cold metal transfer technology (CMT). When using the STT technology for pipeline backing welding, a certain gap must be maintained in the pipeline, the process is difficult to control, and the cost of STT equipment is relatively high. The CMT technology has the advantages of small heat input and stable and reliable welding process, and can achieve all-position welding of the pipeline under 0-gap conditions. However, it also faces the problem of poor misalignment adaptability caused by the narrow weld bead due to small heat input. And in the overhead welding area, due to the existence of the gravity of the molten pool metal, root depression is likely to occur. Root depression will weaken the strength of the pipeline connection, cause stress concentration, and reduce the reliability of the welded joint.

[0005] In addition, the H 2 S stress corrosion resistance of acidic marine pipelines is one of their main service indicators, and this indicator is closely related to the hardness of the joint. Usually, it is required that the maximum hardness of the joint ≤ 248 HV. Using large heat input welding is usually the most effective way to reduce the hardness of the joint. However, in the existing backing welding processes, large heat input usually corresponds to a thicker weld bead, resulting in insufficient heat transfer to the root, causing the hardness of the root weld bead to exceed the standard; if small heat input process parameters are used to reduce the weld bead thickness, it is easy to cause incomplete penetration defects, and small heat input is likely to increase the cooling rate of the molten pool, promoting the formation of martensite, causing the hardness of the root weld bead to increase instead of decrease. Summary of the Invention

[0006] In view of the deficiencies of the prior art, the present application provides a root welding process curve, application and product based on a narrow-gap U-groove, aiming to solve the problems that the heat of the existing acid ocean pipeline root welding cannot be transferred to the root, there is a concave at the root of the weld bead, and the hardness of the root weld bead exceeds the standard, resulting in unqualified SSC (stress corrosion cracking) performance.

[0007] According to one aspect of the present application, there is provided a root welding process curve based on a narrow-gap U-groove, which is used to weld the narrow-gap U-groove, and on the basis of the CMT reference curve, two current pulses are respectively introduced before and after the short-circuit stage to increase the root fusion width and avoid root depression, and at the same time increase the peak current and cycle time to increase the welding heat input, and reduce the peak current rising rate and peak current falling rate to extend the peak current action time, wherein:

[0008] The root face thickness b of the narrow-gap U-groove is 1.45±0.1 mm, the single-side angle α of the groove is 3±0.5°, and the distance a between the top of the transition arc and the root face is 2.95±0.1 mm;

[0009] In the CMT reference curve, the curve peak current is 270 A, the period is 5.5 ms, the current rising rate is 220 A / ms, and the current falling rate is 130 A / ms. Compared with the CMT reference curve, the root welding process curve increases the peak current by 30% - 50%, increases the cycle time by 300% - 500%, reduces the peak current rising rate by 30% - 50%, reduces the peak current falling rate by 20% - 40%, the duration of a single introduced current pulse is 5.5 ms - 7.5 ms, and the pulse peak current is 220 A - 300 A.

[0010] Through the above technical solution conceived by the present application, compared with the prior art, the present application can increase the welding heat input and root fusion width by introducing current pulses and optimizing multiple parameters, and can achieve excellent single-sided welding with double-sided forming effect under the condition of 0-gap butt joint, thereby improving the SSC performance of acid ocean pipelines.

[0011] According to another aspect of the present application, there is provided an application of the root welding process curve based on a narrow-gap U-groove. First, the groove of the pipeline to be welded is designed as a narrow-gap U-groove, and then the above root welding process curve is used for welding.

[0012] According to still another aspect of the present application, there is provided an acid ocean pipeline prepared by applying the root welding process curve based on a narrow-gap U-groove.

[0013] Generally speaking, compared with the prior art through the above technical solution conceived by the present application, the following technical advantages are mainly possessed:

[0014] 1. The present application optimizes the back cover welding process curve, which can increase the welding heat input and the root fusion width. Under the conditions of 0-gap butt joint and without backing, excellent single-sided welding with double-sided forming effect can be achieved. Even in the overhead welding area, a root pass with a certain reinforcement height can be obtained, solving the problem of weld defects, greatly reducing the butt joint difficulty, improving the misalignment adaptability of back cover welding, and enabling the energy to reach the root more easily, ensuring that the molten pool cools and solidifies sufficiently, reducing the hardness of the back cover weld, and improving the service performance of acidic offshore pipelines;

[0015] 2. In particular, the present application optimizes the size of the U-groove. Compared with the existing groove, the bottom is wider and the upper part is narrower, thereby reducing the thickness of the back cover weld pass, making the arc energy reach the root more easily, and reducing the cooling rate of the molten pool metal. While avoiding the defect of incomplete penetration, the hardness of the back cover weld pass is reduced, and the defect of lack of fusion on the side wall caused by insufficient fluidity of the molten pool metal during welding can also be avoided;

[0016] 3. In addition, the present application optimizes the specific parameters of the back cover welding process curve, which can further improve the service performance of acidic offshore pipelines. Description of the Drawings

[0017] Figure 1 is a schematic diagram of the narrow-gap U-groove in the back cover welding process curve provided by the present invention;

[0018] Figure 2 is the current waveform of the CMT reference curve used in Comparative Example 1 of the present invention;

[0019] Figure 3 is the root pass forming diagram of the acidic offshore pipeline obtained in Comparative Example 1 of the present invention;

[0020] Figure 4 is the weld microstructure diagram of the acidic offshore pipeline obtained in Comparative Example 1 of the present invention;

[0021] Figure 5 is the weld hardness test result of the acidic offshore pipeline obtained in Comparative Example 1 of the present invention;

[0022] Figure 6 is the weld SSC test result of the acidic offshore pipeline obtained in Comparative Example 1 of the present invention;

[0023] Figure 7 is the current waveform of the back cover welding process curve used in Example 1 of the present invention;

[0024] Figure 8 is the root pass forming diagram of the acidic offshore pipeline obtained in Example 1 of the present invention;

[0025] Figure 9It is the weld formation diagram of the acidic offshore pipeline obtained in Embodiment 1 of the present invention;

[0026] Figure 10 It is the weld hardness test result of the acidic offshore pipeline obtained in Embodiment 1 of the present invention;

[0027] Figure 11 It is the weld SSC test result of the acidic offshore pipeline obtained in Embodiment 1 of the present invention. Specific embodiments

[0028] In order to make the objectives, technical solutions and advantages of the present application clearer, the following further describes the present application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0029] According to one aspect of the present application, a backing welding process curve based on a narrow-gap U-groove is provided, which is used to weld the narrow-gap U-groove. On the basis of the CMT reference curve, at least one current pulse is introduced before and after the short-circuit stage. The pulsed current introduced before the short-circuit stage is used to increase the welding heat input and the heat coverage range, promote the increase of the root penetration width, and improve the misalignment adaptability of the backing welding; the pulsed current introduced after the short-circuit stage is used to extend the action time of the arc pressure, promote the molten pool metal to flow towards the root, and solve the problem of root depression caused by the gravity drop of the molten pool metal, especially in the overhead welding state, and a root bead with a certain reinforcement can be obtained; at the same time, the peak current and the cycle time need to be increased to increase the welding heat input, and the peak current rising rate and the peak current falling rate are reduced to extend the action time of the peak current, so that the increased heat input can more easily reach the root, while increasing the root penetration width and reinforcement, reducing the cooling rate of the molten pool metal, thereby achieving the purpose of reducing the hardness of the backing bead, so as to improve the anti-H 2 S stress corrosion of the acidic offshore pipeline obtained by welding.

[0030] The present application optimizes the backing welding process curve, can increase the welding heat input and the root penetration width, and can achieve excellent single-sided welding with double-sided forming effect under the conditions of 0-gap butt joint and no precipitation, and then obtain an acidic offshore pipeline with high anti-H 2 S stress corrosion.

[0031] Further, as Figure 1As shown, the root face thickness b of the narrow-gap U-groove is 1.45 ± 0.1 mm, the single-side angle α of the groove is 3 ± 0.5°, and the distance a between the top of the transition arc and the root face is 2.95 ± 0.1 mm. In a preferred embodiment of the present application, the radius R of the transition arc is 3.2 mm, and a suitable radius of the transition arc can be selected in actual applications. By optimizing the parameters of the narrow-gap U-groove, the lower part of the narrow-gap U-groove in the present application is wider and the upper part is narrower. The wide bottom space design can reduce the thickness of the back seal bead, make the arc energy easier to reach the root, and reduce the cooling rate of the molten pool metal. While avoiding the lack of penetration defect, the hardness of the back seal bead is reduced; the narrow upper groove design can avoid the sidewall lack of fusion defect caused by insufficient fluidity of the molten pool metal during the welding process.

[0032] Furthermore, compared with the CMT reference curve, the peak current (I b ) of the back seal welding process curve is increased by 30% - 50%, the cycle time (T) is increased by 300% - 500%, the rising rate of the peak current (d_boostup) is reduced by 30% - 50%, and the falling rate of the peak current (d_boostup) is reduced by 20% - 40%. When the peak current and the cycle are too large, the heat input is too large, which is likely to cause leakage in the back seal welding; when the peak current and the cycle are too small, it is likely to cause lack of penetration in the back seal welding, and the heat input is small, which is likely to cause high hardness; when the rising rate of the peak current is too large, the welding current reaches the peak value in a very short time, which may lead to unstable welding arc and poor droplet transfer. When it is too small, the time for the welding current to reach the peak value is long, and insufficient energy is provided to penetrate the workpiece, affecting the quality of the weld seam; when the falling rate of the peak current is too large, the weld seam cools rapidly, resulting in high hardness of the weld seam and affecting the service performance. When the falling rate of the peak current is too small, the weld seam is overheated, resulting in coarse grains and affecting the weld seam performance.

[0033] Furthermore, the number of current pulses introduced before and after the short-circuit stage does not need to be the same, and a suitable number of current pulses can be introduced according to the actual situation. Preferably, two current pulses are introduced before and after the short-circuit stage respectively. The duration of a single introduced current pulse is 5.5 ms - 7.5 ms, and the peak pulse current is 220 A - 300 A. When the pulse current and the duration are too large, the heat input increases, which is likely to cause problems such as leakage and penetration of the weld, and coarse grains, affecting the weld formation and performance; when the pulse current and the duration are too small, the heat input is insufficient, and the heat is difficult to reach the root, resulting in high hardness at the root and affecting the service performance of the acid-resistant submarine pipeline.

[0034] According to another aspect of the present application, there is provided an application of a backing welding process curve based on a narrow-gap U-groove, specifically: designing the groove of the pipeline to be welded as a narrow-gap U-groove, and then using the above-mentioned backing welding process curve to weld the narrow-gap U-groove to obtain an acidic offshore pipeline.

[0035] In the present application, the layer thickness of the backing weld bead is reduced by the wide bottom space design of the narrow-gap U-groove. On this basis, by means of increasing the peak current and period, reducing the peak current rising rate and current falling rate, and introducing two current pulses before and after the short-circuit stage respectively through the backing welding process curve, it can be promoted that the increased heat input is more likely to reach the root. While increasing the root fusion width and reinforcement, the cooling rate of the molten pool metal is reduced, so as to achieve the purpose of reducing the hardness of the backing weld bead.

[0036] According to yet another aspect of the present application, there is provided an acidic offshore pipeline prepared by using the application of the above-mentioned backing welding process curve based on a narrow-gap U-groove. Using the narrow-gap U-groove and the backing welding process curve can achieve zero-defect welding of acidic offshore pipelines, and the maximum hardness of the joint is less than 248 HV10, and the stress corrosion cracking (SSC) test is qualified, meeting the service requirements of acidic offshore pipelines.

[0037] The following further illustrates the technical solutions provided by the present application according to specific embodiments.

[0038] Example 1

[0039] Design the groove of the pipeline to be welded as a narrow-gap U-groove, and then use the backing welding process curve to weld the narrow-gap U-groove to obtain an acidic offshore pipeline, where:

[0040] The root face thickness b of the narrow-gap U-groove is 1.45 mm, the single-side groove angle α is 3°, the transition arc radius R is 3.2 mm, and the distance a between the top of the transition arc and the root face is 2.95 mm.

[0041] In the CMT reference curve, the curve peak current is 270 A, the period is 5.5 ms, the current rising rate is 220 A / ms, and the current falling rate is 130 A / ms. On the basis of this CMT reference curve, the backing welding process curve increases the peak current (I b ) by 33.3%, increases the period time (T) by 370%, reduces the peak current rising rate (d_boostup) by 40%, reduces the peak current falling rate (d_boostup) by 30%. In addition, two current pulses with a duration of 7 ms are introduced before and after the short-circuit stage respectively, and the pulse peak current is 270 A.

[0042] Example 2

[0043] Design the groove of the pipeline to be welded into a narrow-gap U-groove, and then use the back-bead welding process curve to weld the narrow-gap U-groove to obtain an acidic offshore pipeline, where:

[0044] The root face thickness b of the narrow-gap U-groove is 1.35 mm, the single-side angle α of the groove is 2.5°, the radius of the transition arc R is 3.2 mm, and the distance a from the top of the transition arc to the root face is 2.85 mm.

[0045] In the CMT reference curve, the peak current of the curve is 270 A, the period is 5.5 ms, the current rising rate is 220 A / ms, and the current falling rate is 130 A / ms. On the basis of this CMT reference curve, the back-bead welding process curve increases the peak current (I b ) by 30%, increases the period time (T) by 300%, reduces the peak current rising rate (d_boostup) by 30%, and reduces the peak current falling rate (d_boostup) by 20%. In addition, two current pulses with a duration of 5.5 ms are introduced before and after the short-circuit stage respectively, and the peak current of the pulse is 220 A.

[0046] Example 3

[0047] Design the groove of the pipeline to be welded into a narrow-gap U-groove, and then use the back-bead welding process curve to weld the narrow-gap U-groove to obtain an acidic offshore pipeline, where:

[0048] The root face thickness b of the narrow-gap U-groove is 1.55 mm, the single-side angle α of the groove is 3.5°, the radius of the transition arc R is 3.2 mm, and the distance a from the top of the transition arc to the root face is 3.05 mm.

[0049] In the CMT reference curve, the peak current of the curve is 270 A, the period is 5.5 ms, the current rising rate is 220 A / ms, and the current falling rate is 130 A / ms. On the basis of this CMT reference curve, the back-bead welding process curve increases the peak current (I b ) by 50%, increases the period time (T) by 500%, reduces the peak current rising rate (d_boostup) by 50%, and reduces the peak current falling rate (d_boostup) by 40%. In addition, two current pulses with a duration of 7.5 ms are introduced before and after the short-circuit stage respectively, and the peak current of the pulse is 300 A.

[0050] Example 4

[0051] Design the groove of the pipeline to be welded into a narrow-gap U-groove, and then use the back-bead welding process curve to weld the narrow-gap U-groove to obtain an acidic offshore pipeline, where:

[0052] The root face thickness b of the narrow-gap U-groove is 1.45 mm, the single-side groove angle α is 3°, the radius R of the transition arc is 3.2 mm, and the distance a between the top of the transition arc and the root face is 2.95 mm.

[0053] In the CMT reference curve, the peak current of the curve is 270 A, the period is 5.5 ms, the current rising rate is 220 A / ms, and the current falling rate is 130 A / ms. Based on this CMT reference curve, for the back-bead welding process curve, the peak current (I b ) is increased by 40%, the period time (T) is increased by 400%, the rising rate of the peak current (d_boostup) is reduced by 40%, and the falling rate of the peak current (d_boostdown) is reduced by 30%. In addition, two current pulses with a duration of 6.5 ms are introduced before and after the short-circuit stage respectively, and the peak current of the pulse is 260 A.

[0054] Comparative Example 1

[0055] The groove of the pipeline to be welded is designed as a narrow-gap U-groove, and then the narrow-gap U-groove is welded using the CMT reference curve in Example 1 to obtain an acidic offshore pipeline, where the narrow-gap U-groove is the same as that in Example 1.

[0056] The hardness of the welds obtained in Example 1 and Comparative Example 1 is tested. The test positions are 2 mm below the top base metal, the middle of the weld, and 2 mm above the bottom base metal. The test force is 10 kN, the holding time is 15 s, the test spacing in the heat-affected zone is 1 mm, and the test spacing at the center of the base metal and the weld is 2 mm. The stress corrosion cracking (SSC) test is carried out on the welds, the test duration is 30 days, the H 2 S concentration is greater than 2300 mg / L, the test stress is not lower than 80% of the actual yield strength (AYS), and the test temperature is 24 ± 3°C.

[0057] As Figure 3 shown, the back-bead welding of Comparative Example 1 shows obvious concavity in the overhead welding area; as Figure 4 shown, due to the too fast cooling rate, the weld microstructure is coarse martensite, resulting in the hardness of the back-bead welding significantly exceeding 248 HV10, as Figure 5 shown. Moreover, the weld cracks after 30 days of SSC test, as Figure 6 shown.

[0058] The back bead welding process curve of Example 1 increases the peak current and cycle, increases the heat input and energy of the back bead welding, realizes uniform penetration of the back bead welding, and at the same time increases the arc thrust, which can promote the droplet transfer; by reducing the current rising rate in the acceleration stage and the current falling rate after the acceleration stage, the time for the arc to continuously impact the molten pool during the current rising and falling stages is extended. Therefore, more molten pool metal is extruded to the root of the weld bead and spreads out under the continuous action. The formation effect of the root weld bead is as Figure 8 shown: The molten pool metal is extruded to reach the root of the weld bead to form a reinforcement, and the transition with the base metal is uniform and smooth. As Figure 9 shown, the weld formation is good, the layer thickness is uniform, and there are no defects such as lack of fusion, bottom depression, pores and cracks.

[0059] The wide bottom space design of the narrow gap U-groove can reduce the layer thickness of the back bead weld. On this basis, by means of increasing the peak current and cycle, reducing the peak current rising rate and the current falling rate, and introducing two current pulses respectively before and after the short-circuit stage in the back bead welding process curve, it can be promoted that the increased heat input is more easily reached at the root. While increasing the root fusion width and reinforcement, the cooling rate of the molten pool metal is reduced, and the weld is fully melted, cooled and solidified, so as to achieve the purpose of reducing the hardness of the back bead weld and improving the service performance of the sour submarine pipeline. As Figure 10 and Figure 11 shown, the weld hardness is less than 248HV10, and there is no cracking after 30 days of SSC test.

[0060] It is easy for those skilled in the art to understand that the above are only the preferred embodiments of the present application, and are not used to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. The bottom welding process curve based on narrow gap U-shaped groove is characterized by: The bottom welding process curve is used for welding narrow gap U-shaped grooves. On the basis of the CMT reference curve, two current pulses are introduced before and after the short circuit stage to increase the root molten width and avoid root depression. At the same time, the peak current and cycle time are increased to increase the welding heat input, and the peak current rise rate and peak current drop rate are reduced to extend the peak current action time. The thickness b of the blunt edge of the narrow gap U-shaped groove is 1.45±0.1mm, the single-side angle α of the groove is 3±0.5°, and the distance a between the top of the transition arc and the blunt edge is 2.95±0.1mm; The peak current of the CMT benchmark curve is 270A, the cycle is 5.5ms, the current rise rate is 220A / ms, and the current drop rate is 130A / ms. Compared with the CMT benchmark curve, the bottom welding process curve increases the peak current by 30% to 50%, increases the cycle time by 300% to 500%, reduces the peak current rise rate by 30% to 50%, reduces the peak current drop rate by 20% to 40%, and the duration of the introduced single current pulse is 5.5ms to 7.5ms, and the pulse peak current is 220A to 300A.

2. The application of the bottom welding process curve based on the narrow gap U-shaped groove is characterized by: First, the groove of the pipe to be welded is designed as a narrow gap U-shaped groove, and then the bottom sealing welding process curve as claimed in claim 1 is used for welding.

3. A sour marine pipeline manufactured by applying the bottom welding process curve based on narrow gap U-shaped groove as claimed in claim 2.

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