Plastic-coated steel pipe welding process suitable for low-temperature working condition

Abstract

The invention discloses a plastic-coated steel pipe welding process suitable for a low-temperature working condition, belongs to the technical field of pipeline welding, is used for solving the problems of cracking of a plastic-coated layer, cold brittle fracture of a welding seam and the like in an extremely cold environment in a traditional process, and comprises an environment evaluation stage, a welding preparation stage, a metal welding stage, a welding seam detection stage and a plastic-coated repair stage. And a quality detection stage. The environment evaluation stage is used for detecting local environment data and helping to select welding parameters; in the welding preparation stage, guarantee is provided for the metal welding stage through the steps of a coating treatment technology and the like; in the metal welding stage, a bimetal composite welding technology is introduced; in the welding seam detection stage, a welding seam is protected and detected through gradient slow cooling, ultrasonic detection and other modes, and in the plastic coating repairing stage, a low-temperature-resistant epoxy resin material is used for repairing and other modes, and a low-temperature-resistant coating is formed. In the quality detection stage, a plurality of high-precision detection modes are utilized to ensure the total quality.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of plastic-coated steel pipe welding, and particularly relates to a plastic-coated steel pipe welding process suitable for low-temperature working conditions. BACKGROUND

[0002] The prior art selects matching welding materials and cleans the surface to the Sa2.5 level in the environmental assessment stage for the welding of plastic-coated steel pipes under low-temperature working conditions; sand blasting cleaning and experience preheating are adopted in the welding preparation stage without combining with low-temperature environmental dynamic regulation; double-metal welding and other methods are commonly used in the metal welding stage; conventional room temperature detection is adopted in the weld detection stage without introducing low-temperature special means such as infrared thermal imaging; and epoxy resin is used for repair in the plastic coating repair stage without optimizing the low-temperature performance. There are problems such as uneven low-temperature preheating, easy cracking of plastic coating materials, insufficient performance of welding machines, and lack of low-temperature adaptability in detection, which cannot meet the engineering requirements in high-cold regions. Existing problems: 1) No preheating or uneven local heating exists in welding under low-temperature conditions; 2) Conventional plastic coating materials crack and damage the protective layer under extreme low-temperature conditions; 3) The low-temperature performance of ordinary welding machines is insufficient and cannot meet the welding requirements in high-cold working conditions; 4) Conventional room temperature detection is usually used in the quality detection stage; In general, the existing plastic-coated steel pipe welding technology can adapt to the welding process under low-temperature special working conditions in some aspects, but still has certain problems and limitations. SUMMARY

[0003] The application aims to provide a plastic-coated steel pipe welding process suitable for low-temperature working conditions to solve the problems of uneven heating during welding, material selection not meeting low-temperature working conditions, and conventional welding equipment not meeting welding requirements.

[0004] To achieve the above-mentioned purpose, the application provides the following technical scheme: a plastic-coated steel pipe welding process suitable for low-temperature working conditions, and the specific steps are as follows: S1, environmental assessment, by collecting environmental parameters of the welding area, and then determining and matching the working conditions according to the collected parameters, the environmental parameters including temperature, humidity, and wind speed; S2, welding preparation, by coating treatment technology, intelligent low-temperature preheating regulation provides guarantee for the welding of plastic-coated steel pipes under low-temperature conditions (the coating treatment is used to ensure the cleanliness of the welding area and the accuracy of the groove, and the intelligent low-temperature preheating regulation is used to realize uniform preheating and maintain stable welding environment, thereby providing technical and environmental guarantee for the welding of plastic-coated steel pipes under low-temperature conditions.); S3, welding stage, double-metal composite welding technology is adopted to realize zero damage of the plastic coating layer and improve the cold brittleness resistance of the weld; S4, weld detection, protect and detect the formed weld by gradient slow cooling, ultrasonic detection, etc. S5, plastic coating repair, increase the bonding strength between the plastic coating layer and the base material by restoring the corrosion resistance of the welded area; S6, quality detection, ensure that the performance of the welded joint and the plastic coating layer meets the low temperature working condition requirements through new technology means, salt spray test, infrared thermal imaging detection, and grid method bonding strength verification.

[0005] Preferably, S1, environmental assessment stage: 1) Data collection and environmental classification: Real-time collection of temperature, humidity and wind speed in the welding area through wireless weather stations, establishment of environmental parameter database, real-time uploading of data to the central controller through wireless transmission module, division of environmental grades according to "Pipeline Construction Specification in High and Cold Regions" DB54T 0181-2019, extreme cold grade (temperature ≤-20℃): match preheating temperature 150-200℃, increase welding current by 10%-15%; severe cold grade (-20℃< temperature ≤-10℃): match preheating temperature 100-150℃, increase welding current by 5%-10% 2) Preheating and welding parameter setting: dynamically match preheating temperature and welding current, dynamically adjust preheating power, central controller controls dynamic adjustment of preheating power through PID closed loop control system to ensure real-time matching of environmental parameters and welding parameters.

[0006] Preferably, S2, welding preparation stage, mainly includes: 1) Coating treatment and surface cleaning: use mechanical polishing or chemical paint remover to remove coating on the welding site, expose the metal substrate, clean the range of 10-20mm on both sides of the weld, remove oxidation scale, rust, oil stains, water marks and other contaminants; check the assembly size, groove size and quality of the positioning weld, clean the positioning weld defects. For DN greater than 500, thick plastic coated steel pipes, laser stripping technology can be used as an auxiliary means.

[0007] 2) Intelligent low temperature preheating control: based on the accurate data obtained in S1, environmental assessment stage, intelligently dynamically set the preheating temperature according to different low temperature environmental grades: when in extreme cold grade severe low temperature environment, preferentially use electric heating sheet or high efficiency flame heating equipment to preheat the welding site to an ideal interval of 150 to 200℃, while ensuring that the preheating coverage extends to 150 to 200mm on both sides of the weld, ensuring uniform heating of the entire welding area. If in severe cold grade low temperature condition, the preheating temperature is adjusted to 100 to 150℃ accordingly, and the preheating coverage is 100 to 150mm on both sides of the weld, to adapt to the requirements of different degrees of low temperature environment on welding preheating.

[0008] 3) Preheating temperature monitoring: using high-precision infrared thermometer, preheating temperature is monitored every 5 minutes, temperature fluctuation is strictly controlled within ±5℃, to ensure the stability of the preheating process. After completing the preheating operation, the welding process must be started quickly within 10 minutes to avoid the decrease of preheating effect due to heat loss.

[0009] 4) Wind and cold prevention: build a fully enclosed warm shed, the environmental temperature is maintained at ≥5℃, the shed is equipped with electric fan heater, the wind speed is ≤1m / s, to ensure the stability of the welding environment.

[0010] Preferably, S3, metal welding stage, double metal composite welding technology mainly includes the following components: 1) 20°V groove is used in the plastic coated steel pipe welding interface area, the root bevel size is 2mm, 316L stainless steel lining plate is lined in the inside of the groove, the thickness is ≥2mm, the lining plate and the inner wall of the steel pipe are metallurgically combined by argon arc welding, and E5015-G electrode is used for filling the carbon steel layer.

[0011] 2) The welding operation adopts layered welding process, and the inner layer stainless steel is welded first: argon arc welding process is used to weld the stainless steel lining plate and the inner wall of the steel pipe, the pulse argon arc welding base current is 50A, the peak current is 150A, the frequency is 2Hz, the heat input is controlled to be ≤12kJ / mm during the welding process to avoid damage to the untreated plastic coating layer area due to high temperature. The stainless steel lining plate acts as a heat buffer layer to isolate the welding heat affected zone from the external plastic coating layer, realizing zero damage to the plastic coating layer during welding.

[0012] 3) After the inner layer welding is completed, the outer layer carbon steel filling is carried out: E5015-G low hydrogen sodium type electrode is used to fill the carbon steel layer, the low temperature impact toughness of the electrode is ≥47J, the interlayer temperature is maintained at 80-120℃, the real-time monitoring is carried out by using the infrared thermometer, and the temperature is controlled by using the ceramic heating sheet, and the deviation is controlled within ±3℃ of the preheating temperature.

[0013] 4) After welding, the stainless steel lining plate and the carbon steel base pipe are welded to form a composite layer, and the subsequent process is carried out for plastic repair, and finally the three-layer structure of carbon steel base pipe, stainless steel welding layer and plastic coating layer is restored, forming local composite reinforcement of plastic coated steel pipe in the welding process, and the low temperature toughness of stainless steel is used to compensate for the weakness of the plastic coating layer in the welding heat affected zone.

[0014] Preferably, S4, weld detection stage: 1) Gradient slow cooling process: immediately cover double-layer asbestos cloth + electric blanket after welding, set 4-hour cooling gradient: ensure that the temperature is ≥100℃ for 0-2 hours; the weld temperature is slowly cooled to room temperature for 2-4 hours, and the cooling rate is ≤5℃ / min.

[0015] 2) Phased array ultrasonic testing (PAUT): 5-10 MHz probe is used to scan the weld at a speed of ≤100 mm / s for 100% full coverage detection; the phased array image is analyzed to detect internal defects (such as cracks, incomplete fusion, porosity) in the weld; the defect evaluation is performed according to the “Nondestructive Testing of Pressure Equipment” (NB / T 47013.5-2015) Level I standard.

[0016] 3) Low-temperature impact toughness test: 3 V-notch samples are taken from every 50 welds, and impact test is performed at -20°C; the average impact absorption energy is ≥27 J, and the minimum value of a single sample is ≥20 J (GB / T 2650-2020 standard).

[0017] 4) Fluorescent penetrant testing (PT) or magnetic particle testing (MT) is used to detect surface and near-surface defects such as undercut and micro-cracks; the detection sensitivity level is not less than GB / T 5097-2005 B level requirement.

[0018] Preferably, S5, the plastic coating repair stage, the main repair methods are as follows: 1) After the plastic coating layer cools to room temperature, surface pretreatment is performed: the surface of the welding area is mechanically polished to a depth of 50-100 μm, which increases the surface roughness and improves the adhesion of the subsequent coating. Then wipe the area with acetone, and the number of wiping should not be less than 3 times, until there is no stain on the cotton ball used for wiping, so as to ensure the cleanliness of the welding area surface, free of oil stains, impurities, etc., and provide a good substrate for subsequent plastic coating work.

[0019] 2) Plastic coating operation: low-temperature resistant epoxy resin material is used for spraying. The pressure should be controlled at 0.3-0.5 MPa, which can ensure uniform spraying and effective adhesion. The single spraying thickness is 50-80 μm, and the total thickness of the coating is 150-200 μm after 2-3 layers of superposition. By layering, the problem of cracking and sagging of the coating caused by over-thick spraying can be avoided, and the quality of the coating can be ensured.

[0020] 3) Curing treatment: after spraying, forced hot air curing is required. First, maintain the temperature at 60°C for 30 minutes, then increase the temperature to 120°C and maintain for 2 hours. Through such temperature control and time setting, the low-temperature resistant epoxy resin material can be fully cured, so as to ensure that the interfacial shear strength reaches ≥250 MPa, and the bonding strength between the plastic coating layer and the base material is sufficient to meet the performance requirements of the welding process in actual application.

[0021] Preferably, S6, the quality detection stage, the specific method is as follows: 1) Salt spray resistance test: after 24 hours delay, perform "Artificial Atmosphere Corrosion Test Salt Spray Test" (GB / T10125-2021); use 5% NaCl solution, salt spray deposition 1-2 mL / 80 cm² / h, continue testing for 720 hours; judgment standard: no blistering, peeling, rusting of coating, interface thermal resistance increment ≤0.1 K・m² / W.

[0022] 2) Infrared thermal imaging detection: use an infrared thermal imager with a resolution of ≥320×240 pixels, scan the surface of the plastic coating layer, record the interface thermal resistance distribution, threshold ≤0.5 K・m² / W, and the abnormal area needs to be locally peeled off for detection of bonding strength.

[0023] 3) Bonding strength verification: use the grid method (GB / T 9286-1998), cut with a 1mm knife to form 100 2mm×2mm squares on the metal substrate; vertically tear off the 3M 610 adhesive tape, and the coating falling area ≤5% is qualified.

[0024] Technical effects and advantages of the present application: through dynamic matching of environmental assessment parameters, intelligent preheating and wind prevention in the welding preparation stage, and double-metal composite technology in the metal welding stage, the coating plastic layer is zero-damaged, the welds are cold brittle, the low-temperature resistant material and gradient curing are used to improve the bonding strength in the coating plastic repair stage, and the multi-stage high-precision detection ensures the quality, solving the problems of coating plastic layer cracking and weld cold brittle fracture in traditional process in extremely cold environment, suitable for water, oil and gas transportation and chemical pipeline engineering in high-cold regions, with low-temperature adaptability, corrosion resistance reliability and construction efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 It is a coating plastic steel pipe welding process flowchart suitable for low-temperature and high-cold regions; Figure 2 It is a process flowchart of environmental assessment stage; Figure 3 It is a process flowchart of welding preparation stage; Figure 4 It is a process flowchart of weld detection stage; Figure 5 It is a process flowchart of coating plastic repair stage; Figure 6 It is a process flowchart of quality detection stage. DETAILED DESCRIPTION

[0026] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0027] The present application provides a plastic coated steel pipe welding process 1 suitable for low temperature working conditions as shown in the figure, and the specific steps are as follows: S1, environmental assessment, by collecting the environmental parameters of the welding area, and then according to the collected parameters, the working condition grade is determined and matched, and the environmental parameters include temperature, humidity, and wind speed; S2, welding preparation, through coating treatment technology, intelligent low temperature preheating control provides guarantee for low temperature condition of plastic coated steel pipe metal welding (through coating treatment to ensure the cleanliness of the welding area and the groove accuracy, intelligent low temperature preheating control to realize uniform preheating and maintain stable welding environment, to provide technical and environmental guarantee for low temperature condition of plastic coated steel pipe metal welding.) ; S3, welding stage, using double metal composite welding technology to realize zero damage of plastic coating layer and improve the cold brittleness resistance of weld; S4, weld detection, through gradient slow cooling, ultrasonic detection and other ways to protect and detect the formed weld; S5, plastic repair, by restoring the corrosion resistance of the welding area, the interface bonding strength between the plastic coating layer and the base material is increased; S6, quality detection, through new technology means, salt spray test, infrared thermal imaging detection, and grid method combined strength verification, to ensure that the performance of the welded joint and the plastic coating layer meets the requirements of low temperature working condition. Example 1: 1. Environmental assessment stage (S1): Parameter collection: the wireless weather station monitors the temperature of the welding area in real time-30℃, the wind speed is 3m / s, and the relative humidity is 60%, which is determined as extremely cold working condition.

[0028] Parameter matching: the central controller sets the preheating temperature to 180℃ according to the standard of DB54T0181-2019, increases the welding current by 12%, and adjusts the current of 4.0mm welding rod from 160A to 180A. The preheating power is automatically increased by 30% through the PID closed loop system.

[0029] 2. Welding preparation stage (S2): Coating treatment: laser stripping technology is used to remove the weld and 25mm coating on both sides, mechanical polishing to Sa2.5 level, acetone wiping 3 times to no stain on cotton ball; 20°V type groove is processed, blunt edge is 2mm, assembly error is 1.2mm, ≤10% wall thickness.

[0030] Intelligent preheating: Cover the area of 200mm on both sides of the weld with electric heating film, and raise the temperature to 180℃ within 2 hours. Monitor every 5 minutes with an infrared thermometer, and the temperature difference should be ±3℃. Build a fully enclosed warm shed, and maintain the temperature at 8℃ inside with an electric heater. The wind speed should be less than or equal to 1m / s.

[0031] 3. Metal welding stage (S3): Double-metal composite welding: Insert a 3mm-thick 316L stainless steel liner on the inside of the groove, and weld it with argon arc welding (base current 50A, peak current 150A, frequency 2Hz). The heat input should be controlled at 10kJ / mm to achieve metallurgical bonding between the stainless steel liner and the carbon steel base pipe. Use E5015-G welding rod to fill the outer layer, maintain the interlayer temperature at 100℃ (deviation ±3℃), and weld in 3 layers with each layer being 3mm thick. Plastic coating protection: The stainless steel liner acts as a thermal buffer layer, and the plastic coating is not damaged by high temperature during welding (surface temperature ≤60℃).

[0032] 4. Weld quality detection stage (S4) Gradient slow cooling: Immediately after welding, cover with double-layer asbestos cloth + electric blanket, maintain at 120℃ for 0-2 hours, and cool to room temperature for 2-4 hours (cooling rate 4℃ / min).

[0033] Non-destructive testing: 100% detection of no cracks and incomplete fusion by phased array ultrasonic; -30℃ low temperature impact test results are 32J, 30J, 28J (average value 30J ≥27J); no undercut and micro-cracks on the surface by fluorescent penetrant testing.

[0034] 5. Plastic coating repair stage (S5) Surface treatment: After cooling to room temperature, mechanically polish to a depth of 80μm, and wipe with acetone 3 times.

[0035] Coating repair: Spray low-temperature resistant epoxy resin in 2 layers (1st layer 80μm, 2nd layer 120μm, total thickness 200μm), and the spraying pressure is 0.4MPa; forced hot air curing: 60℃×30min→120℃×2h, and the actual interface shear strength is 265MPa ≥250MPa.

[0036] 6. Quality detection stage (S6) Salt spray resistance test: Test for 720 hours after a 24-hour delay, and the coating has no blistering or rusting, and the interface thermal resistance value is 0.08K・m² / W ≤0.1K・m² / W.

[0037] Bonding strength verification: The grid method test shows that the coating shedding area is 3% ≤5%, which meets the GB / T9286 standard.

[0038] Implementation effect: 98% of the first pass rate of the weld, the corrosion resistance of the repaired plastic coating meets the standard, the construction period is shortened by 20% compared with the traditional process, and it is suitable for oil and gas pipeline engineering in extremely cold areas below-30℃; Example two: engineering background: ambient temperature-40℃, DN800x10mm L360N plastic coated steel pipe (epoxy coating 300μm).

[0039] 1. Environmental assessment stage (S1): Through the real-time collection of the welding area temperature-32℃, wind speed 4.2m / s, humidity 58% by wireless weather station, it is determined as extremely cold working condition according to DB54T0181-2019 standard. The central controller dynamically matches the preheating temperature 190±5℃, the welding current is increased from the reference value 180A to 203A by 13%, and the preheating power is automatically adjusted by PID closed loop system to ensure real-time optimization of parameters.

[0040] 2. Welding preparation stage (S2): Laser stripping technology is used to accurately remove the coating within 25mm range on both sides of the weld, and mechanical polishing is used to reach Sa2.5 level surface treatment standard (GB / T 13288.1), and acetone is used to wipe three times to achieve residual oil ≤5mg / m² (ISO 8502-3). The central controller dynamically matches the preheating temperature 190±5℃, the welding current is increased from the reference value 180A to 203A by 13%, and the preheating power is automatically adjusted by PID closed loop system to ensure real-time optimization of parameters.

[0041] 3. Metal welding stage (S3): 20°V groove structure (ISO 9692-1) is used, and 3mm thick 316L stainless steel lining plate (UNSS31603) is set at the root. Pulse argon arc welding process is used for inner layer welding, with base current 52A, peak current 155A, frequency 2.1Hz, and heat input strictly controlled at 9.8kJ / mm. E5015-G electrode (-50℃ impact energy ≥54J) is used for three-layer filling, and ceramic heating sheet PID control is used for interlayer temperature 105±2℃. The surface temperature of the plastic coating during the whole process is ≤65℃, realizing zero damage.

[0042] 4. Weld detection stage (S4): Immediately after welding, double-layer asbestos cloth and electric blanket are used for gradient slow cooling: 0-2 hours maintain 115±3℃, 2-4 hours slow cooling to room temperature at the rate of 4℃ / min. 7.5MHz phased array probe is used for 100% scanning, and the detection sensitivity reaches Φ0.8mm equivalent. The results of-40℃ low temperature impact test are 36J / 34J / 38J, and the average is much higher than the standard requirement of 27J.

[0043] 5. Plastic coating repair stage (S5): The surface was mechanically polished to an anchor pattern depth of 85±5μm, and then deeply cleaned three times with acetone. Bisphenol A type epoxy resin with a Tg of 45℃ was applied in three layers (75±5μm / layer) under a pressure of 0.48MPa, achieving a total thickness of 225μm. The curing process involved holding at 60℃ for 30 minutes, then raising the temperature to 120℃ and maintaining it for 120 minutes. The measured interfacial shear strength was 272MPa.

[0044] 6. Quality Inspection Stage (S6): A 720-hour salt spray test (5% NaCl, 35℃) showed that the coating met ISO 4628-1 standard level 0 corrosion. Infrared thermal imager (FLIR T860) scanning showed a maximum thermal resistance difference of 0.4 K·m² / W, and the coating peeling rate was only 2.8% using the cross-cut test. Comparative Example 1: Welding operations were performed under the same conditions as in Example 1, using traditional processes in the same engineering environment: fixed preheating at 120℃ (without dynamic control), with local temperature differences reaching ±28℃. During welding with a single layer of E4315 welding rods, the peak temperature in the heat-affected zone reached 380℃, leading to complete carbonization of the plastic coating. After natural cooling, the weld exhibited a martensitic hard and brittle phase (content 23%), with an impact energy of only 19J at -30℃. Salt spray testing showed 42% corrosion after 240 hours, and after two rework cycles, the construction cycle reached 35 hours per weld pass. Failure analysis indicated that uneven preheating and uncontrolled cooling rate (18℃ / min) were the main causes of plastic coating damage and weld brittleness. Comparative Example 2: Welding operations were performed under the same conditions as in Example 2, using the latest industry solution: induction preheating to 150°C and simultaneous welding with dual welding torches. Although the preheating uniformity was improved to ±12°C, the lack of a heat buffer layer resulted in 18 microcracks / cm in the coating layer due to heat conduction during welding. Rapid cooling at 18°C / min resulted in an impact energy of 24J, which did not meet the 27J standard. The interface strength of the epoxy resin repaired at room temperature was 118MPa, and localized blistering appeared during salt spray testing. The construction efficiency of 28 hours / crossing was still lower than that of this patent. The core performance comparison of the four sets of examples is as follows:

[0045] Through the test of examples and comparative examples, the coating plastic steel pipe welding process suitable for low temperature working condition can be found, the innovative coating plastic layer protection mechanism, through the heat insulation effect of 316L stainless steel lining plate (≥3mm), the temperature of the welding heat affected zone is reduced from 380℃ of the traditional process to below 65℃, which fundamentally avoids the carbonization or cracking of the coating plastic layer, and solves the long-existing problem of thermal damage in the industry. The low temperature toughness of the weld is improved, the gradient slow cooling process (≤5℃ / min) promotes the formation of ≥95% acicular ferrite in the weld structure, so that the impact energy at-40℃ reaches 36J, which is increased by 89% compared with the traditional process (19J), which is significantly higher than the industry safety threshold of 27J. The reliability of the corrosion protection system is improved, the 120℃ gradient curing process makes the interfacial strength of the low temperature resistant epoxy resin reach 272MPa, and the salt spray protection life is significantly improved compared with the industry scheme. In summary, the present application dynamically matches parameters through environmental assessment, intelligent preheating and wind prevention in the welding preparation stage, and realizes zero damage of the coating plastic layer and cold brittle resistance of the weld through the bimetallic composite technology in the metal welding stage, and the combination of low temperature resistant material and gradient curing to improve the bonding strength in the coating plastic repair stage, and multi-stage high precision detection to ensure quality, solves the problems of coating plastic layer cracking and weld cold brittle fracture in the traditional process in extremely cold environment, and is suitable for water transportation, oil and gas transportation and chemical pipeline engineering in high cold region, and has low temperature adaptability, corrosion resistance reliability and construction efficiency.

[0046] Finally, it should be noted that: the above only for the preferred embodiments of the present application, and not for limiting the present application, although the foregoing detailed description of the present application is made with reference to the foregoing embodiments, for those skilled in the art, it still can modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part of the technical features, any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application, should be included in the protection scope of the present application.

Claims

1. A process for welding a plastic coated steel pipe suitable for low temperature service, characterized in that: The specific steps are as follows: S1, environmental assessment, by collecting the welding area environmental parameters, and then according to the collected parameters to determine the working condition level and matching, environmental parameters include temperature, humidity, wind speed; S2, welding preparation, through coating treatment technology, intelligent low temperature preheating control and other steps to provide protection for the metal welding of plastic coated steel pipe under low temperature conditions; S3, welding stage, using double metal composite welding technology to realize zero damage of plastic coating layer and improve the cold brittleness resistance of weld; S4, weld detection, through gradient slow cooling, ultrasonic detection and other ways to protect and detect the formed weld; S5, plastic repair, by restoring the corrosion resistance of the welding area, realizing the increase of the interfacial bonding strength of the plastic coating layer and the base material; S6, quality detection, through salt spray test, infrared thermal imaging detection, and grid method combined strength verification, to ensure that the performance of the welded joint and the plastic coating layer meets the requirements of low temperature working condition.

2. The process as claimed in claim 1, wherein the process is applicable for coating of steel pipes for low temperature service. S1 is: through the real-time monitoring of the welding area temperature by the wireless weather station, -30℃, wind speed 3m / s, relative humidity 60%, it is determined that it is an extremely cold working condition; Then match the parameters: according to the central controller, according to the standard of DB54T0181-2019, dynamically set the preheating temperature to 180℃, increase the welding current by 12%, the current of 4.0mm welding rod is adjusted from 160A to 180A, and the preheating power is automatically increased by 30% through PID closed loop system.

3. The process as claimed in claim 1, wherein the process is applicable for painting of steel pipes for low temperature service. S2 is: laser stripping technology is used to remove the weld and 25mm coating on both sides, mechanical polishing to Sa2.5 level, acetone wiping 3 times to cotton ball without stain; processing 20°V type groove, blunt edge 2mm, assembling error 1.2mm, ≤10% wall thickness; Then intelligent preheating, using electric heating sheet to cover the welding seam on both sides of 200mm area, within 2 hours to 180℃, infrared thermometer every 5 minutes monitoring, temperature difference ±3℃; build a fully enclosed warm shed, built-in electric heater to maintain the temperature in the shed 8℃, wind speed ≤1m / s.

4. The process as claimed in claim 1, wherein the process is applicable for painting of steel pipes for low temperature service. The double metal composite welding technology in S3 is: 3mm thick 316L stainless steel liner is embedded in the inside of the groove, argon arc welding, heat input control 10kJ / mm, stainless steel liner and carbon steel base pipe realize metallurgical bonding; the outer layer uses E5015-G welding rod to fill, the interlayer temperature is maintained at 100℃, and the welding is carried out in 3 layers, each layer thickness is 3mm; plastic coating layer protection: stainless steel liner as heat buffer layer, plastic coating layer is not damaged by high temperature in the welding process, the high temperature here is ≤60℃.

5. The process as claimed in claim 1, wherein the process is applicable for painting of steel pipes for low temperature service. The gradient slow cooling in S4 is: immediately cover double layer asbestos cloth + electric blanket after welding, maintain 120℃ for 0-2 hours, and reduce to room temperature for 2-4 hours; Then nondestructive testing: 100% detection of no cracks and incomplete fusion by phased array ultrasonic; the result of low temperature impact test at-30℃ is 32J, 30J and 28J; the surface of fluorescent penetrant testing has no undercut and micro crack.

6. The process as claimed in claim 1, wherein the process is applicable for painting of steel pipes for low temperature service. The plastic repair in S5 specifically includes: surface treatment: after cooling to room temperature, mechanical polishing to anchor mark depth 80μm, acetone wiping 3 times; Coating repair: 2 layers of low-temperature resistant epoxy resin are sprayed, specifically 80 μm for the first layer and 120 μm for the second layer, with a total thickness of 200 μm, and the spraying pressure is 0.4 MPa; forced hot air curing: 60°C for 30 min and then 120°C for 2 h, and the interface shear strength is actually measured to be 265 MPa ≥ 250 MPa.

7. The process as claimed in claim 1, wherein the process is applicable for painting of steel pipes for low temperature service. The quality detection in S6 specifically includes salt spray resistance test: 720 h test is performed after a delay of 24 h, the coating has no blistering and rusting, and the interface thermal resistance increment is 0.08 K·m² / W ≤ 0.1 K·m² / W; Bond strength verification: grid method test shows that the coating shedding area is 3% ≤ 5%, which meets the GB / T9286 standard.

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