Energy-saving welding method

Abstract

The invention discloses an energy-saving welding method applied to pressure pipelines and pressure containers in the petrochemical industry, and belongs to the technical field of low alloy steel welding. According to the method, a tempering weld bead technology is used for replacing a traditional postweld heat treatment process, and the method comprises three stages of preparation before welding, welding process control and postweld quality detection. Before welding, the preheating temperature is determined through the base metal carbon equivalent, and a base metal-working condition-welding material matching system is established; spiral welding bead arrangement is adopted in the welding process, the heat input quantity (the precision is + / -0.5 kJ / cm) and the interlayer temperature (150-220 DEG C) are accurately controlled, a tempering welding bead is additionally welded and strengthened after capping, and a 1 / 2 tempering layer is reserved; and performing multi-dimensional detection after welding. According to the method, the energy consumption per unit length is reduced by 81.1%, the construction efficiency is improved by more than 50%, the qualification rate of a welded joint reaches 99.2%, the problems of poor welding material matching, inaccurate parameter control and the like in the prior art are solved, and the method is suitable for welding of low alloy steel such as 15CrMo and P91 under high-temperature, high-pressure and corrosion working conditions.

Description

Technical Field

[0001] This invention relates to the field of petrochemical equipment manufacturing and maintenance technology, and in particular to an energy-saving welding method. Specifically, it relates to an energy-saving welding method for low-alloy steel welds in pressure pipelines and pressure vessels, which is particularly suitable for weld construction scenarios where heat treatment is required after welding. It can replace the traditional post-weld heat treatment process with tempering weld technology, thereby achieving energy saving, high efficiency and high quality in the welding process.

[0002] In the petrochemical industry, pressure pipelines and pressure vessels are core equipment for transporting and storing flammable, explosive, high-temperature, and high-pressure media. The quality of their welds directly affects the safe and stable operation of the entire production system. Low-alloy steel, due to its excellent mechanical properties and corrosion resistance, is widely used in the manufacturing and maintenance of such equipment. However, low-alloy steel welds are prone to defects such as coarse-grained structures and residual stress during welding. Traditional processes require post-weld heat treatment (PWHT) to eliminate these defects, a process that is not only energy-intensive and time-consuming but may also adversely affect the properties of the base material. This invention addresses this industry pain point by developing an energy-saving welding method that effectively solves the above problems. It is applicable to low-alloy steel weld construction in multiple fields such as petrochemicals, nuclear power, and electric power, and has broad application prospects. Background Technology

[0003] After long-term service under harsh conditions such as high temperature, high pressure, and corrosion, low-alloy steel is prone to cracking, wear, and performance degradation in its weld area, requiring welding repair to restore the equipment's performance. During the welding repair process, the weld and heat-affected zone (HAZ) undergo complex thermal cycling, leading to uneven microstructure, the formation of coarse-grained regions and hardened structures, and the generation of high residual stress. These defects significantly reduce the mechanical properties of the welded joint, such as impact toughness and fatigue resistance. If not effectively treated, they may cause weld failure, resulting in serious safety accidents and economic losses.

[0004] The traditional solution is post-weld heat treatment, which involves heating, holding, and cooling to refine the weld microstructure and reduce residual stress. Common post-weld heat treatment methods include overall high-temperature tempering and localized heating and tempering. Overall high-temperature tempering is the most effective at relieving stress, but for large pressure pipelines and pressure vessels, overall heating consumes a lot of energy, and the heating equipment is bulky and has a long construction period. While localized heating can reduce some energy consumption, temperature control is difficult, and uneven heat treatment is prone to occur. At the same time, it still requires a significant investment of manpower and equipment costs.

[0005] Tempering welding technology, as a novel energy-saving welding technique, utilizes the welding heat cycle of subsequent weld passes to temper the previous weld and its heat-affected zone. By precisely controlling the heat input during the welding process, the coarse-grained zone of the previous weld is refined, and residual stress is released, thus replacing traditional post-weld heat treatment processes. This technology requires no additional heating equipment and can simultaneously optimize the microstructure and relieve stress during the welding process, offering significant energy-saving and high-efficiency advantages.

[0006] From a technical perspective, the essence of tempering weld bead technology lies in utilizing the secondary thermal cycle effect during the welding process. When welding subsequent weld beads, the high temperature generated by the electric arc raises the temperature of the previous weld and the heat-affected zone to a certain range (usually the tempering temperature range below Ac1). At this temperature, the coarse-grained structure in the weld undergoes recrystallization, refining into uniform grains, while the hardened structure transforms into more ductile structures such as sorbite and troostite. Simultaneously, residual stress is released through thermoplastic deformation. Compared to traditional post-weld heat treatment, tempering weld bead technology transforms "post-weld treatment" into "in-weld treatment," achieving process integration and efficiency.

[0007] Despite the significant advantages of tempering welding technology, existing technologies both domestically and internationally still have many problems, making it difficult to meet the high-quality and high-stability requirements of the petrochemical industry for low-alloy steel welds. These problems are specifically reflected in the following aspects: The selection of welding consumables lacks a systematic approach: Current technologies rely heavily on experience-based judgment, failing to establish a matching system based on base metal composition and operating conditions (such as temperature, pressure, and media corrosivity). For example, some companies still use general-purpose welding consumables for 15CrMo steel welds under high-temperature and high-pressure conditions, resulting in low mechanical property matching between the weld and the base metal, and performance degradation during long-term service. Furthermore, there is a lack of clear regulations regarding the pretreatment process of welding consumables (such as drying temperature and holding time), allowing impurities like moisture and oil in the consumables to easily lead to defects such as porosity and slag inclusions in the weld.

[0008] Low precision in heat input control: Heat input is a core control parameter in tempering weld technology, directly determining the tempering effect. In existing technologies, heat input calculations often rely on empirical formulas, failing to consider the coupled effects of multiple factors such as welding material specifications, welding speed, and arc voltage. This leads to significant deviations between actual and theoretical heat input values. When heat input is insufficient, the coarse-grained zone of the previous weld cannot be sufficiently refined, resulting in incomplete elimination of residual stress. When heat input is too high, the weld temperature exceeds Ac1, easily leading to coarse grains and reducing the strength and hardness of the welded joint.

[0009] Inappropriate weld bead layout and lap parameters: A reasonable weld bead layout and lap width are crucial for ensuring uniform tempering of the weld. In existing technologies, weld bead layouts often employ simple parallel arrangements without optimization based on weld thickness and bevel type, leading to insufficient tempering at weld edges. Regarding lap width control, there is a lack of clear quantitative standards; insufficient lap can result in incomplete fusion defects, while excessive lap can cause weld beads on the weld surface, affecting subsequent weld bead layout and weld formation quality.

[0010] Inadequate interpass processing: Interpass processing includes two key steps: interpass temperature control and weld bead grinding. In existing technologies, interpass temperature is largely determined by manual experience, without the use of precise temperature measurement equipment for real-time monitoring. When the interpass temperature is too low, cold cracking of the weld is likely to occur; when the interpass temperature is too high, grain growth will be accelerated. Regarding weld bead grinding, there is a lack of standardized grinding thickness and range. Insufficient grinding will result in residual defects on the weld surface, while excessive grinding will damage the tempered layer of the weld, affecting overall performance.

[0011] The quality inspection and evaluation system is inadequate: Existing technologies for quality inspection of welded joints are mostly limited to visual inspection and routine mechanical property tests (such as tensile strength and yield strength tests), lacking systematic testing of key indicators such as weld microstructure, microhardness distribution, and residual stress. Furthermore, the lack of quality evaluation standards for tempered welds makes it impossible to comprehensively and accurately assess the performance of welded joints, increasing the risks in engineering applications. Summary of the Invention

[0012] In view of the shortcomings of the existing technology, the purpose of this invention is to provide an energy-saving welding method to overcome the problems of poor welding material matching, low heat input control accuracy, unreasonable weld parameters, imperfect interlayer treatment and unsound quality evaluation system in the existing tempering welding technology.

[0013] The above-mentioned objective of this invention is achieved through the following technical solutions: This invention provides an energy-saving welding method for the construction of low-alloy steel welds in pressure pipelines and pressure vessels in the petrochemical industry. This method replaces the traditional post-weld heat treatment process with tempering welding technology, and includes a pre-weld preparation stage, a welding process control stage, and a post-weld quality inspection and evaluation stage. The specific steps are as follows: Step 1, Pre-welding preparation stage: This includes base metal pretreatment, welding material selection and pretreatment, and welding equipment debugging. The base metal pretreatment temperature is determined based on the carbon equivalent of the base metal: 100℃±20℃ when carbon equivalent Ceq≤0.45%, 150℃±20℃ when 0.45%<Ceq≤0.6%, and 200℃±20℃ when Ceq>0.6%. Welding material selection is based on a matching system between the base metal composition and operating conditions, determining the welding material type according to the low-alloy steel type, service temperature, pressure, and media corrosivity. Step 2, Welding Process Control Stage: Perform root pass welding, fill pass welding, and cover pass welding in sequence. The fill pass welding adopts a spiral weld bead arrangement, and the overlap width of the weld bead is 1 / 3 ± 0.5 mm. The interpass temperature is monitored and controlled in real time at 150-220℃ using an infrared thermometer. After the cover pass welding is completed, an additional layer of strengthening tempering weld bead is welded, with a heat input of 70%-80% of that of the cover pass weld. After the additional weld, the tempering layer with a thickness of 1 / 2 is ground off. Step 3, Post-weld quality inspection and evaluation stage: A multi-dimensional inspection system is adopted, including visual inspection, non-destructive testing, microstructure inspection, mechanical property testing, residual stress testing, and corrosion resistance testing, to ensure that the quality of the welded joint meets the standards.

[0014] According to one embodiment of the present invention, the pretreatment of the base material further includes surface cleaning and beveling design: the surface roughness of the welding area needs to reach Ra12.5μm or less, and ultrasonic testing is used to confirm the removal of defects; when the base material thickness is ≤10mm, a V-shaped beveling is used; when the thickness is 10mm < ≤20mm, an X-shaped beveling is used; when the thickness is >20mm, a U-shaped beveling is used; after beveling, magnetic particle testing is used to confirm that there are no crack defects.

[0015] According to one embodiment of the present invention, the specific principles for selecting welding materials are as follows: for 15CrMo and 12Cr1MoV pearlitic low-alloy steels under operating conditions of 300℃-500℃ and 10MPa-20MPa, ER5515-1CM or ER55-B2 welding wire is selected; for P91 and P92 martensitic heat-resistant steels under operating conditions of 500℃-650℃ and 20MPa-35MPa, E9015-B91 or ER90S-B9 welding materials is selected; for Q345R low-alloy high-strength steels under operating conditions of room temperature and 1MPa-10MPa, ER50-6 welding wire is selected; for low-alloy steel welds in corrosive media service environments, ER55-Ni3 welding materials containing Ni and Cu elements are selected.

[0016] According to one embodiment of the present invention, the welding material pretreatment step is as follows: the welding wire is wiped with alcohol to remove oil and then dried. For welding materials of type ER5515-1CM and E9015-B91, the drying temperature is 350℃±20℃ and the holding temperature is 2h. For welding materials of type ER50-6, the drying temperature is 150℃±20℃ and the holding temperature is 1h. After drying, the welding material is exposed to air for no more than 2h and is re-dried ≤3 times.

[0017] According to one embodiment of the present invention, the heat input during the welding process is monitored in real time by a computer with a control accuracy of ±0.5kJ / cm; the heat input for filler welding is determined according to the type of base material, which is 12-18kJ / cm for 15CrMo steel, 10-15kJ / cm for P91 steel, and 15-20kJ / cm for Q345R steel.

[0018] According to one embodiment of the present invention, the root pass welding adopts gas metal arc welding with a welding wire diameter of 2.5 mm, a welding current of 100-130 A, a voltage of 20-23 V, and a heat input of 12-17 kJ / cm; the fill pass welding adopts a segmented back-welding method, dividing the weld into several sections of 400 mm / segment, and welding from both ends to the middle.

[0019] According to one embodiment of the present invention, the non-destructive testing includes penetrant testing, ultrasonic testing, and radiographic testing. The penetrant testing and ultrasonic testing have a sensitivity of Level II, and the radiographic testing evaluation level of important welds is ≥ Level II. In the mechanical property test, the tensile strength of the welded joint is not less than 90% of the base material, the yield strength is not less than 85% of the base material, and the impact absorption energy at -20℃ is ≥47J.

[0020] According to one embodiment of the present invention, the residual stress detection is performed using an X-ray stress meter, and the residual stress at the weld center is ≤30% of the yield strength of the base material, ≤25% in the transition zone, and ≤20% in the heat-affected zone; under corrosive conditions, the weld needs to pass salt spray test and electrochemical corrosion test, with a 72h salt spray corrosion rate ≤0.01mm / year and a corrosion potential ≥-0.6V in 3.5% NaCl solution.

[0021] According to one embodiment of the present invention, the debugging of the welding equipment also includes precise control of the protective gas ratio. Under normal operating conditions, the protective gas is a mixture of Ar and CO2, and under high temperature and high pressure conditions, the protective gas is a mixture of Ar and CO2, and the purity of the gas is not less than 99.99%.

[0022] According to one embodiment of the present invention, the weld bead grinding in the interlayer treatment is carried out using an angle grinder with a grit size of 80-120 mesh. After grinding, the surface roughness of the weld bead is controlled to be below Ra6.3μm, and the tempering layer of the adjacent completed weld bead shall not be damaged.

[0023] According to one embodiment of the present invention, the heat input control of the enhanced tempering weld bead adopts a real-time feedback adjustment mechanism, which ensures that the heat input fluctuation does not exceed ±0.3kJ / cm through the linkage adjustment of welding current and speed.

[0024] According to one embodiment of the present invention, the flow rate of the protective gas is dynamically adjusted according to the diameter of the welding wire. The flow rate is 15-20 L / min when the diameter of the welding wire is 2.5 mm, and the flow rate is 20-25 L / min when the diameter of the welding wire is 3.2 mm, with a flow control accuracy of ±1 L / min.

[0025] In summary, compared with the prior art, the present invention has at least one of the following beneficial technical effects: Compared with existing technologies and traditional welding processes, this invention achieves energy saving, precision, and high quality in low-alloy steel weld seams through systematic technological innovation and process optimization. Its core beneficial effects are reflected in the following six aspects: This invention, through the precise application of tempering welding technology, completely replaces the energy-intensive post-weld heat treatment step in traditional welding processes. Traditional high-temperature tempering processes require approximately 800-1200 kWh of electricity to process a single 10m long, 20mm thick low-alloy steel weld. However, this invention, through the self-tempering effect during welding, eliminates the need for additional heating equipment. The total energy consumption for welding a single weld of the same specification can be reduced to 60%-70% of that of traditional processes, resulting in a direct energy cost reduction of over 30%. Simultaneously, it eliminates the costs of equipment rental, site occupancy, and labor for post-weld heat treatment, reducing overall construction costs by 25%-40%. This invention is particularly suitable for batch welding of large-scale petrochemical equipment, significantly reducing energy consumption and construction costs.

[0026] To address the problems of poor welding consumable matching and low heat input control precision in existing technologies, this invention establishes a multi-dimensional welding consumable selection system and a precise heat input control method, significantly improving the mechanical properties of welded joints. Experimental verification shows that 15CrMo steel joints welded using this method exhibit stable tensile strength of 460-490 MPa and impact absorption energy ≥55 J, both exceeding the basic requirements of 90% and 47 J for the base metal. The hardened microstructure content of P91 steel joints is controlled below 3%, avoiding performance fluctuations caused by uneven heat treatment in traditional processes. Furthermore, by strengthening the combination of tempering weld beads and a multi-dimensional detection system, the residual stress elimination rate of the weld reaches 60%-75%, 20%-30% higher than traditional local heat treatment, effectively reducing the risk of cracking during weld service and improving the quality stability and reliability of welded joints.

[0027] In traditional processes, the heat treatment holding time after welding is typically 2-4 hours per 10mm thickness. For a 40mm thick low-alloy steel weld, a single heat treatment process requires 8-16 hours. This invention integrates "post-weld treatment" into "in-weld simultaneous treatment," eliminating the separate heat treatment step and shortening the construction cycle of a single weld by 40%-60%. Taking the circumferential weld of a 1m diameter, 20mm thick Q345R pressure vessel as an example, the traditional process requires 72 hours to complete welding and heat treatment, while this invention only requires 28-36 hours to complete all processes. Furthermore, it eliminates the need to wait for the weld to cool to room temperature before treatment, enabling continuous construction. This is particularly suitable for emergency situations such as repairs in petrochemical plants, significantly improving construction efficiency and project deadline assurance.

[0028] This invention develops customized process solutions for commonly used low-alloy steels in China, such as 15CrMo, P91, and Q345R, solving the problems of foreign technologies being incompatible with local conditions and the need for domestic technology to be simply copied. For example, for the welds of 12Cr1MoV steel in high-sulfur crude oil pipelines, a composite process using ER5515-1CM welding material and a Ni-containing corrosion-resistant layer reduces the corrosion rate of the weld in a 5% NaCl salt spray environment to below 0.008 mm / year, meeting the requirements for use in acidic media conditions. Furthermore, the process parameters of this invention can be flexibly adjusted according to the base metal thickness, bevel type, and service conditions, making it applicable to welding and repair projects in multiple fields such as petrochemicals, nuclear power, and electric power, enhancing the technology's adaptability and the scope of its engineering applications.

[0029] This invention transforms traditionally experience-based operations into standardized processes through clearly defined quantitative standards (such as weld overlap width being 1 / 3 of the previous pass width and interpass temperature control accuracy of ±5℃) and supporting equipment monitoring schemes. During welding, the coordinated control of the heat input calculator and weld seam tracker keeps parameter deviation within 5%, reducing error risk by 15%-20% compared to manual operation. Simultaneously, the multi-dimensional quality inspection system covers the entire chain of indicators from surface defects to internal structure, avoiding the problem of "emphasizing appearance while neglecting internal structure" in traditional inspections, providing systematic assurance for welding quality, and reducing operational difficulty and quality control risks.

[0030] Against the backdrop of national "dual carbon" targets and tightening environmental policies, the energy-saving characteristics of this invention have significant environmental benefits. A single 10m long weld can reduce CO2 emissions by approximately 300-500 kg (calculated based on the carbon emission coefficient of thermal power plants). If fully implemented in the annual welding projects of companies such as Sinopec and PetroChina, the annual emission reduction could reach tens of thousands of tons. Simultaneously, this invention reduces noise pollution and exhaust emissions from heating equipment during post-weld heat treatment, meeting the construction requirements for green factories in the petrochemical industry. It possesses significant social benefits and industrial promotion value, aligning with green environmental protection and industrial policy guidelines. Attached Figure Description

[0031] Figure 1 This is a flowchart of the present invention.

[0032] Figure 2 This is a welding schematic diagram of Embodiment 1 of the present invention. Detailed Implementation

[0033] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0034] In the description of this application, it should be noted that the terms "upper," "lower," "inner," "outer," "top / bottom," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0035] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installed," "equipped with," "sleeved / connected," "connected," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0036] This invention discloses an energy-saving welding method, specifically addressing the following technical problems: Establish a welding material selection system based on the composition of the base material and the working conditions to achieve precise matching between welding materials and base materials, thereby improving the mechanical properties and corrosion resistance of welded joints; Develop a precise method for controlling heat input, and combine it with multi-factor coupling analysis to determine the optimal range of heat input, so as to ensure that the coarse grain zone of the previous weld is fully refined and tempered. Optimize the weld bead layout and overlap width parameters, and design a personalized weld bead layout scheme based on the weld thickness and groove form to avoid defects such as incomplete fusion and weld beads; Improve the interpass processing technology, realize real-time monitoring and precise control of interpass temperature, clarify the thickness and range standards of weld grinding, and improve the weld formation quality. Establish a comprehensive quality inspection and evaluation system, covering multiple dimensions such as weld microstructure, mechanical properties, and residual stress, to ensure that welded joints meet the requirements of engineering applications; To improve the adaptability of the technology, we will develop customized welding process solutions for commonly used low-alloy steels and complex working conditions in China, so as to achieve a precise match between the technology and the actual application scenarios.

[0037] By solving the above-mentioned technical problems, this invention aims to completely replace the traditional post-weld heat treatment process with tempering welding technology, thereby reducing energy consumption and cost in the welding process and improving construction efficiency while ensuring the quality of the welded joint, and providing technical support for the welding and repair of low alloy steel welds in my country's petrochemical industry.

[0038] To achieve the above objectives, this invention provides an energy-saving welding method for low-alloy steel welds in pressure pipelines and pressure vessels in the petrochemical industry. Through the coordinated design of precise matching of welding materials, precise control of heat input, optimization of weld parameters, interpass strengthening treatment, and comprehensive quality inspection, it achieves a dual improvement in weld quality and welding efficiency. (Refer to...) Figure 1 The method specifically includes the following steps: Step 1: Preparation stage before welding This stage forms the foundation of the welding process and directly affects the subsequent welding quality. It includes three key steps: base material pretreatment, welding material selection and pretreatment, and welding equipment debugging.

[0039] Step 1.1, Pretreatment of base material: First, the welding area of ​​the low-alloy steel base material is cleaned using an angle grinder to remove rust, oil, scale, and other impurities from the welding area (including the bevel and a 50mm radius on both sides), ensuring a surface roughness of Ra12.5μm or less. For base materials with defects such as cracks or dents, the defects must be repaired first, and ultrasonic testing (UT) is used to confirm that the defects have been completely removed.

[0040] Secondly, the bevel type is designed according to the base material thickness and welding requirements. When the base material thickness is ≤10mm, a V-shaped bevel is used, with a bevel angle of 60°±5°, a blunt edge thickness of 2mm±0.5mm, and a gap of 2mm±0.5mm. When the base material thickness is >10mm and ≤20mm, an X-shaped bevel is used, with a bevel angle of 60°±5°, a blunt edge thickness of 2mm±0.5mm, and a gap of 2mm±0.5mm. When the base material thickness is >20mm, a U-shaped bevel is used, with a bevel bottom radius of 8mm±2mm, a bevel angle of 50°±5°, a blunt edge thickness of 3mm±0.5mm, and a gap of 3mm±0.5mm. After the bevel processing is completed, magnetic particle inspection (MT) is used to confirm that there are no cracks or other defects on the bevel surface.

[0041] Finally, the base material is preheated. The preheating temperature is determined based on the carbon equivalent of the base material: when the carbon equivalent Ceq ≤ 0.45%, the preheating temperature is 100℃ ± 20℃; when 0.45% < Ceq ≤ 0.6%, the preheating temperature is 150℃ ± 20℃; when Ceq > 0.6%, the preheating temperature is 200℃ ± 20℃. Preheating is performed using electric heating elements for localized heating, with a heating range of 100mm on each side of the bevel. The heating rate is controlled within 50℃ / h, and the holding time is 30min to ensure uniform preheating.

[0042] Step 1.2, Welding Material Selection and Pretreatment: A welding material selection system based on base metal composition and operating conditions is established, with the following specific selection principles: For pearlitic low-alloy steels such as 15CrMo and 12Cr1MoV, and when the service conditions are high temperature (300℃-500℃) and high pressure (10MPa-20MPa), select ER5515-1CM or ER55-B2 low-alloy steel welding wire. These welding materials contain alloying elements such as Cr and Mo, which can form a good metallurgical bond with the base metal, ensuring the high temperature strength and creep resistance of the welded joint. For martensitic heat-resistant steels such as P91 and P92, and when the service conditions are ultra-high temperature (500℃-650℃) and ultra-high pressure (20MPa-35MPa), E9015-B91 or ER90S-B9 welding materials should be selected. These welding materials contain higher levels of alloying elements such as Cr, Mo, and V, which can refine the weld structure and improve the high-temperature hardness and corrosion resistance of the welded joint. For low-alloy high-strength steels such as Q345R, and when the service conditions are normal temperature and medium pressure (1MPa-10MPa), ER50-6 welding wire is selected. This type of welding material has a low cost and good mechanical properties matching with the base material, which can meet the requirements of conventional working conditions. For low-alloy steel welds operating in corrosive media (such as sulfur-containing crude oil and sour natural gas), select welding materials containing corrosion-resistant elements such as Ni and Cu, such as ER55-Ni3, to ensure the corrosion resistance of the welded joint.

[0043] After selecting the welding materials, strict pretreatment is required: ① Wipe the welding wire with alcohol to remove surface oil stains, and then put it into a drying oven for drying. The drying temperature is determined according to the type of welding material. For low alloy steel welding wires such as ER5515-1CM and E9015-B91, the drying temperature is 350℃±20℃ and the holding time is 2h; for ordinary welding wires such as ER50-6, the drying temperature is 150℃±20℃ and the holding time is 1h; ② After drying, the welding materials should be placed in an insulated container and used as needed. The exposure time in the air should not exceed 2h. If it exceeds this time, it needs to be dried again, and the number of drying times should not exceed 3.

[0044] Step 1.3, Welding equipment debugging: Select either gas metal arc welding (MIG / MAG) equipment or manual arc welding equipment, and adjust the parameters according to the welding process requirements. For MIG / MAG welding, use an Ar+CO2 mixture as the shielding gas, with an Ar content of 80%±5% and a CO2 content of 20%±5%, and control the gas flow rate at 15L / min-25L / min. For manual arc welding, use DC reverse polarity.

[0045] During commissioning, it is necessary to ensure that the current and voltage regulation accuracy of the welding equipment reaches ±1A and ±0.1V, respectively, and the welding speed control accuracy reaches ±0.1m / min. Simultaneously, auxiliary equipment such as an infrared thermometer (measuring range 0℃-1000℃, accuracy ±1℃), a weld seam tracker, and a heat input calculator are provided to ensure parameter monitoring and precise control during the welding process.

[0046] Step 2, Welding Process Control Stage: This stage is the core process, which includes three steps: root pass welding, fill pass welding, and cover pass welding. Each step requires strict control of key elements such as heat input, weld parameters, and interpass treatment.

[0047] Step 2.1, root pass welding: The purpose of the root pass welding is to ensure good root formation and prevent defects such as incomplete penetration and porosity. A single-sided welding with double-sided forming process is used, and the welding method is selected based on the groove shape: when using a V-groove, gas metal arc welding is used; when using an X-groove or U-groove, manual arc welding is used.

[0048] The process parameters for root pass welding are as follows: ① Welding material specifications: Use welding wire with a diameter of 2.0mm-2.5mm or welding rod with a diameter of 2.5mm; ② Welding current: 80A-120A; ③ Welding voltage: 18V-22V; ④ Welding speed: 80mm / min-120mm / min; ⑤ Heat input: Controlled at 8kJ / cm-12kJ / cm, adjusted according to the thickness of the base material. The greater the thickness of the base material, the greater the heat input value; ⑥ Welding angle: The angle between the welding wire or welding rod and the base material is 70°±5° to ensure that the arc is concentrated at the root of the bevel.

[0049] After the root pass is completed, visual inspection and penetration testing (PT) are used to confirm the quality of the root formation, ensuring there are no defects such as incomplete penetration, porosity, or cracks. If defects are found, they must be removed by grinding with an angle grinder before re-welding.

[0050] Step 2.2, Filler Welding: Filler welding is a crucial step in accumulating weld thickness and requires multiple passes, each of which needs to undergo tempering. This invention employs a "segmented back-welding method" for filler welding, dividing the weld into several segments (each segment being 300mm-500mm in length) and welding from both ends of the weld towards the middle, thereby reducing welding deformation and residual stress.

[0051] Precise heat input control: The heat input is calculated using the formula Q=η×U×I / v, where Q is the heat input (kJ / cm), η is the thermal efficiency (η=0.85-0.9 for gas metal arc welding, η=0.75-0.8 for manual arc welding), U is the welding voltage (V), I is the welding current (A), and v is the welding speed (cm / min). The optimal heat input range is determined based on the microstructure and base material type of the previous weld pass. For 15CrMo and 12Cr1MoV steel welds, the heat input should be controlled at 12kJ / cm-18kJ / cm to ensure that the weld temperature reaches 550℃-650℃ (below Ac1) so that the previous coarse grain zone is fully tempered. For P91 and P92 steel welds, the heat input should be controlled at 10kJ / cm-15kJ / cm and the weld temperature should be controlled at 600℃-700℃ to avoid excessive softening of the martensite structure. For Q345R steel welds, the heat input is controlled at 15kJ / cm-20kJ / cm, and the weld temperature is controlled at 500℃-600℃ to ensure microstructure refinement and stress release.

[0052] During the welding process, the heat input is monitored in real time by a heat input calculator. When it deviates from the set range, the welding current and speed are automatically adjusted. For example, when the heat input is too high, the welding equipment automatically reduces the current or increases the welding speed; when the heat input is too low, it automatically increases the current or decreases the welding speed, ensuring that the heat input control accuracy reaches ±0.5kJ / cm.

[0053] Weld bead layout and overlap width optimization: The number and arrangement of weld bead layers are designed according to the weld thickness: when the weld thickness is 10mm-20mm, 3-5 layers of weld beads are used in a fan-shaped arrangement; when the weld thickness is 20mm-40mm, 6-10 layers of weld beads are used in a spiral arrangement; when the weld thickness is >40mm, 11-15 layers of weld beads are used in a stepped arrangement. The spacing between weld beads is controlled at 1 / 2-2 / 3 of the weld bead width to ensure that the heat-affected zones of each weld bead cover each other.

[0054] The weld overlap width adopts a quantitative standard of "weld width × 1 / 3", meaning the overlap width between the subsequent weld and the previous weld is 1 / 3 ± 0.5 mm of the previous weld width. For example, if the previous weld width is 12 mm, the overlap width should be controlled at 4 mm ± 0.5 mm. During welding, the weld position is monitored in real time using a weld tracker to ensure the overlap width meets the requirements. If the overlap is insufficient, the welding direction is adjusted promptly; if the overlap is excessive, the welding speed is appropriately increased to reduce the weld width.

[0055] Interpass processing: Interpass temperature control: An infrared thermometer is used to monitor the interpass temperature in real time after each weld pass. The interpass temperature range is determined according to the base material type: 15CrMo and 12Cr1MoV steels are controlled at 150℃-200℃; P91 and P92 steels are controlled at 200℃-250℃; Q345R steel is controlled at 100℃-150℃. When the interpass temperature is below the lower limit, electric heating elements are used for supplemental heating; when it is above the upper limit, compressed air is used for forced cooling to ensure that the interpass temperature control accuracy reaches ±5℃. Weld grinding: After each weld pass is completed, an angle grinder is used to grind the weld surface. The grinding thickness is 1 / 3 ± 0.2 mm of the weld thickness, covering the entire weld surface and a 2 mm area on each side. The purpose of grinding is to remove defects such as weld slag, spatter, and porosity from the weld surface, while making the weld surface smooth, providing a good foundation for subsequent weld passes. After polishing, the surface quality is checked by visual inspection to ensure that there are no defects left.

[0056] Detailed parameters for filler welding: The specifications of the welding materials for filler welding are adjusted according to the number of weld layers: the first filler layer uses 2.5mm diameter welding wire or electrode, and subsequent layers use 3.2mm-4.0mm diameter welding wire or electrode. The welding current, voltage, and speed are determined based on the welding material specifications and heat input requirements, as shown in Table 1 below: Base material type Welding material specifications (mm) Welding current (A) Welding voltage (V) Welding speed (mm / min) Heat input (kJ / cm) 15CrMo 2.5 120-150 22-25 100-130 12-15 15CrMo 3.2 180-220 25-28 80-110 15-18 P91 2.5 110-140 21-24 110-140 10-13 P91 3.2 160-200 24-27 90-120 13-15 Q345R 2.5 130-160 23-26 90-120 15-17 Q345R 3.2 190-230 26-29 70-100 17-20 Step 2.3, Cover weld: The purpose of capping welding is to ensure a beautiful weld surface while maintaining the overall performance of the weld. Two weld passes are used to complete the capping. The first pass covers the bottom two-thirds of the weld area, and the second pass covers the top one-third. The overlap width between the two passes is still one-third of the pass width.

[0057] The process parameters for cover welding are as follows: Use 3.2mm diameter welding wire or electrode; reduce the welding current by 5%-10% compared to filler welding. For example, if the filler welding current is 200A, control the cover welding current at 180A-190A; reduce the welding voltage by 1V-2V. If the filler welding voltage is 26V, control the cover welding voltage at 24V-25V; increase the welding speed by 10%-15%. If the filler welding speed is 100mm / min, control the cover welding speed at 110mm / min-115mm / min; control the heat input at 80%-90% of the filler welding heat input to ensure that no weld beads or undercut defects are generated on the weld surface.

[0058] After the cover weld is completed, an additional "strengthening tempering weld" is welded. This weld serves to perform a second tempering of the cover weld, further refining the microstructure and releasing stress. The heat input of the strengthening tempering weld is controlled at 70%-80% of that of the cover weld, with correspondingly reduced welding current and voltage, and increased welding speed. After the additional weld is completed, half the thickness of the strengthening tempering weld is ground away using an angle grinder, leaving half the thickness as the final tempering layer. This ensures a smooth weld surface that meets appearance quality requirements.

[0059] Step 3: Post-weld quality inspection and evaluation stage: This stage is crucial for ensuring welding quality. Multi-dimensional testing methods are employed to comprehensively inspect and evaluate the appearance, microstructure, mechanical properties, and residual stress of the welded joint.

[0060] Visual Inspection: After welding is completed, a visual inspection is first performed, using both visual observation and weld gauge measurement. The inspection items include: ① Weld surface smoothness: The unevenness of the weld surface ≤ 0.5mm / m; ② Weld reinforcement: The reinforcement height is controlled within 0-3mm, and the transition is smooth; ③ Undercut: Undercut depth ≤ 0.5mm, length ≤ 10% of the weld length; ④ Cracks, porosity, slag inclusions: No visible cracks, porosity, slag inclusions, or other defects on the surface. If the visual inspection fails, the defect location must be marked, removed using an angle grinder, and re-welded until the requirements are met.

[0061] Non-destructive testing (NDT): After passing visual inspection, NDT is performed, including the following items: ① Penetrant testing (PT): Detecting defects such as cracks and porosity on and near the weld surface, with a sensitivity level of II; ② Ultrasonic testing (UT): Detecting defects such as incomplete penetration, lack of fusion, and slag inclusions inside the weld, covering the entire weld thickness, with a sensitivity level of II; ③ Radiographic testing (RT): For important welds (such as high-pressure pipeline welds), radiographic testing is used to verify internal quality, with an assessment level of II or higher. Welds that fail NDT must be repaired, with no more than two repair attempts, and re-testing must be performed after repair.

[0062] Microstructural Inspection: Samples were taken from the weld joints, and the microstructure of the weld and heat-affected zone was observed using a metallographic microscope. The inspection items included: ① Weld microstructure: It should be uniform sorbite, bainite, or fine-grained ferrite, without coarse grains; ② Coarse-grained zone of the heat-affected zone: The width of the coarse-grained zone should be ≤100μm, and the grain size ≤10μm; ③ Hardened microstructure: The content of hardened microstructure should be ≤5%. For example, the microstructure of 15CrMo steel welds should be uniform sorbite, and the grain size of the coarse-grained zone in the heat-affected zone should be ≤8μm; the microstructure of P91 steel welds should be a mixed microstructure of fine-grained martensite and bainite, with a hardened microstructure content ≤3%.

[0063] Mechanical property testing: Mechanical property tests shall be conducted in accordance with the "Test Methods for Mechanical Properties of Welded Joints of Metallic Materials" (GB / T 2651-2021), including: ① Tensile strength test: The tensile strength of the welded joint shall not be less than 90% of that of the base metal. For example, if the tensile strength of the 15CrMo steel base metal is 510 MPa, the tensile strength of the welded joint should be ≥459 MPa; ② Yield strength test: The yield strength shall not be less than 85% of that of the base metal; ③ Impact toughness test: At -20℃, the impact absorption energy of the weld and heat-affected zone shall be ≥47 J; ④ Hardness test: The microhardness of the weld and heat-affected zone shall be measured using a Vickers hardness tester. The hardness value shall be controlled between 180 HV and 280 HV, with a gentle hardness gradient and no obvious abrupt changes. Welded joints that fail the mechanical property test shall be analyzed for the cause, and the welding process parameters shall be adjusted before re-welding.

[0064] Residual stress detection: Residual stress in the weld and heat-affected zone (HAZ) is detected using an X-ray stress meter. Detection points include the weld center, the transition zone between the weld and base metal, and the HAZ. Residual stress control standards: ① Residual stress at the weld center ≤ 30% of the base metal yield strength; ② Residual stress in the transition zone ≤ 25% of the base metal yield strength; ③ Residual stress in the HAZ ≤ 20% of the base metal yield strength. For example, for Q345R steel with a base metal yield strength of 345 MPa, the residual stress at the weld center should be ≤ 103.5 MPa. If the residual stress is too high, vibration aging or local tempering should be used until the requirements are met.

[0065] Corrosion resistance test: For welds used in corrosive media, corrosion resistance tests are conducted, including: ① Salt spray test: Under a 5% NaCl salt spray environment, continuous spraying for 72 hours, the surface corrosion rate of the weld is ≤0.01mm / year; ② Electrochemical corrosion test: In a 3.5% NaCl solution, the corrosion potential of the weld is ≥-0.6V, and the corrosion current density is ≤1×10⁻ 6 A / cm². Welds that fail to meet corrosion resistance standards require the selection of new welding materials or adjustment of the welding process, followed by re-welding and testing.

[0066] To make the technical solution of the present invention clearer and more explicit, the present invention will be described in detail below with reference to specific embodiments. This embodiment takes the welding of 15CrMo low alloy steel pressure pipelines commonly used in the petrochemical industry as an example to illustrate in detail the specific implementation process and effects of the energy-saving welding method of the present invention.

[0067] Example 1: Welding of 15CrMo steel pressure pipeline (normal temperature and high pressure conditions) This embodiment focuses on a 15CrMo steel pressure pipeline for transporting naphtha in a petrochemical plant. The pipeline specifications are φ325×20mm, and the service conditions are normal temperature (25℃) and high pressure (15MPa). The method of this invention is used for circumferential welding, and the specific steps are as follows: Step 1: Preparation before welding Step 1.1, Base Material Pretreatment: The carbon equivalent of the 15CrMo steel base material, Ceq, is 0.52%, falling within the range of 0.45% < Ceq ≤ 0.6%. The preheating temperature is set at 150℃ ± 20℃. An angle grinder is used to clean the bevel and 50mm areas on both sides, removing rust, oil, and scale. The surface roughness is tested to achieve Ra 10μm. Due to the 20mm thickness of the base material, an X-shaped bevel is used, with a bevel angle of 60°, a blunt edge thickness of 2mm, and a gap of 2mm. Magnetic particle testing (MT) after beveling confirms the absence of cracks. Preheating is achieved using flexible electric heating elements surrounding the bevel, with a heating range of 100mm on both sides, a heating rate of 45℃ / h, and a holding time of 30min. Infrared thermometers are used to check the uniformity of the preheating temperature, with a maximum deviation ≤ 3℃.

[0068] Step 1.2, Welding Material Selection and Pretreatment: Based on the composition of 15CrMo steel and the normal temperature and high pressure operating conditions, ER5515-1CM welding wire with specifications of φ2.5mm and φ3.2mm is selected. Welding wire pretreatment: First, wipe off the oil with anhydrous alcohol, then place it in a drying oven at 350℃ for 2 hours. After drying, immediately transfer it to a 40℃ insulated container for later use. After being taken out on site, the exposure time in the air is controlled to be within 1.2 hours, not exceeding the 2-hour limit.

[0069] Step 1.3, Welding Equipment Debugging: Select the KRⅡ-500 type gas metal arc welding equipment. The shielding gas is a mixture of Ar (80%) and CO2 (20%), with a gas purity of 99.995%. The gas flow rate is 18L / min for φ2.5mm welding wire and 22L / min for φ3.2mm welding wire, with a flow rate control accuracy of ±0.8L / min. Debug the equipment to ensure current adjustment accuracy of ±1A, voltage adjustment accuracy of ±0.1V, and welding speed control accuracy of ±0.1m / min. Simultaneously connect an infrared thermometer, a heat input calculator, and a weld seam tracker.

[0070] Step 2, Welding Process Control Step 2.1, Root Pass Welding: A single-sided welding with double-sided forming process is used, employing φ2.5mm ER5515-1CM welding wire, a welding current of 110A, a voltage of 21V, and a welding speed of 105mm / min. The calculated heat input is Q=0.88×21×110 / 10.5=19.36kJ / cm (here, welding speed 105mm / min = 10.5cm / min). The welding angle is 70°, and the arc is always aligned with the root of the bevel to ensure root penetration. After the root pass welding is completed, the root formation is visually inspected for uniformity, and penetrant testing (PT) shows no defects such as porosity or incomplete penetration.

[0071] Step 2.2, Filler Welding: A segmented back-welding method is used, dividing the circumferential seam into four segments, each 400mm long. Welding proceeds from both ends of the circumferential seam towards the center, resulting in eight filler welds arranged in a spiral pattern. The heat input for the 15CrMo steel filler weld is set to 15kJ / cm ± 0.5kJ / cm. The heat input calculator monitors the input in real time. When the detected heat input rises to 15.6kJ / cm, the equipment automatically reduces the current from 200A to 195A, restoring the heat input to 15.1kJ / cm. When it drops to 14.4kJ / cm, the current is increased to 205A to ensure control accuracy. The weld bead width is 12mm, the spacing is 8mm (2 / 3 of the weld bead width), and the overlap width is 4mm (1 / 3 ± 0.5mm of the weld bead width). The weld seam tracker corrects the welding torch position in real time, ensuring an overlap width deviation of ≤0.3mm. Interpass temperature was controlled between 150℃ and 200℃. An infrared thermometer recorded the temperature every 30 seconds. When the temperature dropped to 148℃, the electric heating element replenished the heat to 152℃; when it rose to 203℃, compressed air cooled it to 198℃. After each weld pass was completed, it was ground 1mm (1 / 3 of the 3mm weld thickness) with a 100-grit angle grinder, covering the weld surface and 2mm on each side. The surface roughness after grinding was Ra 5.8μm, with no weld slag or spatter residue. (Refer to...) Figure 2 Filler welding parameters: Layers 1-3: φ2.5mm welding wire, current 130A, voltage 23V, speed 110mm / min, heat input 15kJ / cm; Layers 4-8: φ3.2mm welding wire, current 200A, voltage 26V, speed 90mm / min, heat input 17kJ / cm.

[0072] Step 2.3, Cover Weld: Two cover weld passes. The first pass covers the bottom 2 / 3 of the weld area, and the second pass covers the top 1 / 3 of the weld area, with an overlap width of 4mm. Cover weld parameters: φ3.2mm welding wire, current 180A (10% lower than the filler weld), voltage 24V (2V lower), speed 100mm / min (11% higher), heat input 14kJ / cm (82% of the filler weld). After the cover weld is completed, a strengthening tempering weld pass is added with the following parameters: current 150A, voltage 22V, speed 120mm / min, heat input 10kJ / cm (71% of the cover weld). After adding the tempering weld pass, half the thickness of the strengthening weld pass (approximately 1.5mm) is ground off using an angle grinder, leaving a 1.5mm tempered layer. The weld surface is smooth, with a 2mm excess height.

[0073] Step 3: Post-weld quality inspection and evaluation Step 3.1, Visual inspection: The surface unevenness of the weld is 0.3mm / m, the excess height is 2mm, the transition is smooth, the undercut depth is 0.3mm, the length accounts for 3% of the total weld length, there are no defects such as cracks and porosity, and the appearance is qualified.

[0074] Step 3.2, Non-destructive testing: PT test sensitivity level II, no surface defects; UT test covers the full thickness of the weld, no internal defects such as incomplete penetration or slag inclusion, sensitivity level II; RT test rating level II, internal quality qualified.

[0075] Step 3.3, Microstructure Inspection: Metallographic sampling showed that the weld microstructure was uniform sorbite, the width of the coarse grain zone in the heat-affected zone was 80 μm, the grain size was 6 μm, and the hardened microstructure content was 2%, which met the requirements.

[0076] Step 3.4 Mechanical property test: Tested according to GB / T 2651-2021 standard. The tensile strength of 15CrMo base material is 510MPa, and the tensile strength of welded joint is 480MPa (≥459MPa); the yield strength of base material is 330MPa, and the yield strength of joint is 290MPa (≥280.5MPa); the impact absorption energy at -20℃ is 55J (≥47J); the hardness of weld is 220HV, the heat-affected zone is 230HV, and the base material is 210HV, with a gentle gradient.

[0077] Step 3.5 Residual stress detection: X-ray stress meter test, the residual stress in the weld center is 95MPa (≤103.5MPa), the transition zone is 80MPa (≤86.25MPa), and the heat-affected zone is 65MPa (≤69MPa), all of which meet the standards.

[0078] Example 2: Welding of P91 martensitic heat-resistant steel pipe seams (ultra-high temperature and high pressure conditions) This embodiment focuses on the P91 steel main steam pipeline in a supercritical thermal power unit. The pipeline specifications are φ406×30mm, and the service conditions are ultra-high temperature (600℃) and ultra-high pressure (30MPa). The method of this invention is used to repair the weld, and the specific steps are as follows: Step 1: Preparation before welding Step 1.1, Base Material Pretreatment: P91 steel carbon equivalent Ceq=0.85%>0.6%, preheating temperature set at 200℃±20℃. The welding area (bevel and 60mm on each side) is cleaned with an angle grinder and wire wheel to remove oxide scale and decarburized layer, surface roughness Ra11μm, and ultrasonic testing (UT) confirms no internal defects. The base material thickness is 30mm, using a U-shaped bevel with a bottom radius of 8mm, a bevel angle of 50°, a blunt edge thickness of 3mm, and a gap of 3mm. MT testing after beveling shows no cracks. Preheating uses a tracked electric heating element, with a heating range of 150mm on each side of the bevel, a heating rate of 40℃ / h, and a holding time of 40min. Infrared thermometer readings show a preheating temperature of 195℃-205℃, with a uniformity deviation ≤5℃.

[0079] Step 1.2, Welding Material Selection and Pretreatment: Based on P91 steel and ultra-high temperature and pressure conditions, E9015-B91 welding electrodes (φ2.5mm, φ3.2mm) were selected. Electrode pretreatment: After wiping with alcohol to remove oil, the electrodes were dried at 350℃ for 2 hours, then placed in a 120℃ insulated box. After use, they were exposed to air for 1.5 hours, which did not exceed the 2-hour limit, and were not dried repeatedly.

[0080] Step 1.3, Welding Equipment Debugging: Select ZX7-630 manual arc welding equipment with DC reverse polarity. The shielding gas for high-temperature and high-pressure conditions is Ar (90%) + CO2 (10%), with a purity of 99.99%. The gas flow rate is 16L / min for φ2.5mm welding rods and 21L / min for φ3.2mm rods. Debug the equipment to ensure current accuracy ±1A and voltage accuracy ±0.1V. Equipped with a high-temperature infrared thermometer (measuring range 0-1200℃), a heat input calculator, and a weld seam tracker.

[0081] Step 2, Welding Process Control Step 2.1, Root Pass Welding: Manual arc welding, single-sided welding with double-sided forming, φ2.5mm E9015-B91 electrode, welding current 90A, voltage 20V, speed 90mm / min, heat input Q=0.78×20×90 / 9=15.6kJ / cm. Welding angle 75°, short arc operation to ensure root penetration. PT inspection after root pass welding showed no surface defects, and UT inspection showed good root penetration.

[0082] Step 2.2, Filler Welding: The circumferential weld is divided into 6 segments, each 350mm long, welded from both ends towards the middle, for a total of 12 filler welds arranged in a spiral pattern. The heat input for the P91 steel filler weld is set to 12kJ / cm ± 0.5kJ / cm, monitored in real-time by a heat input calculator. When the heat input reaches 12.6kJ / cm, the current decreases from 180A to 175A, returning to 12.1kJ / cm; when it decreases to 11.4kJ / cm, the current increases to 185A. The weld width is 11mm, the spacing is 7mm (2 / 3 of the weld width), and the overlap width is 3.5mm (11mm × 1 / 3 ± 0.5mm). The weld tracking device controls the overlap deviation to ≤0.4mm. The interpass temperature is controlled between 200℃ and 250℃, recorded every 20 seconds by a thermometer. Below 195℃, electric heating elements provide supplemental heating; above 255℃, compressed air cooling is used. Each weld pass was ground to 0.8mm using a 120-grit grinding wheel (1 / 3 of the 2.5mm weld pass thickness), resulting in a surface roughness Ra of 6.0μm with no residual defects. Filler weld parameters: Layers 1-4: φ2.5mm electrode, current 120A, voltage 22V, speed 100mm / min, heat input 12kJ / cm; Layers 5-12: φ3.2mm electrode, current 180A, voltage 25V, speed 85mm / min, heat input 14kJ / cm.

[0083] Step 2.3, Cover Weld: Two cover weld passes, with an overlap width of 3.5mm. Cover weld parameters: φ3.2mm electrode, current 162A (reduced by 10%), voltage 23V (reduced by 2V), speed 94mm / min (increased by 10.6%), heat input 11.2kJ / cm (80% of the filler weld). Add a strengthening tempering weld pass, parameters: current 130A, voltage 21V, speed 110mm / min, heat input 8.4kJ / cm (75% of the cover weld). Grind to retain 1 / 2 tempered layer (approximately 1.2mm), weld surface reinforcement 2.5mm, smooth transition.

[0084] Step 3: Post-weld quality inspection and evaluation Step 3.1, Visual inspection: Weld unevenness 0.4mm / m, excess height 2.5mm, undercut depth 0.4mm, length ratio 5%, no visible defects, visually qualified.

[0085] Step 3.2, Non-destructive testing: PT and UT sensitivity level II, no surface or internal defects; RT test rating level II, meeting the requirements of the main steam pipeline.

[0086] Step 3.3, Microstructure Inspection: Metallographic images show that the weld microstructure is fine-grained martensite + bainite, the width of the coarse-grained zone in the heat-affected zone is 90μm, the grain size is 8μm, and the content of hardened microstructure is 2.5%≤3%, which meets the standards.

[0087] Step 3.4 Mechanical property test: P91 base material tensile strength 620MPa, joint tensile strength 570MPa (≥558MPa); base material yield strength 440MPa, joint yield strength 374MPa (≥374MPa); -20℃ impact absorption energy 52J ≥47J; weld hardness 240HV, heat-affected zone 250HV, base material 230HV, with a gentle gradient.

[0088] Step 3.5, Residual stress test: The residual stress at the weld center is 120MPa (≤440×30%=132MPa), the stress in the transition zone is 100MPa (≤110MPa), and the stress in the heat-affected zone is 80MPa (≤88MPa), which meets the standard.

[0089] Example 3: Welding of Q345R steel pressure vessel (corrosive media condition) This embodiment focuses on the Q345R steel tank wall of a sulfur-containing wastewater storage tank in an oil refinery. The tank wall is 16mm thick and operates under normal temperature (20℃) and medium pressure (8MPa). The medium contains 5% H2S + 3% NaCl. The longitudinal seam welding is performed using the method of this invention. The specific steps are as follows: Step 1: Preparation before welding Step 1.1, Base Material Pretreatment: Q345R steel carbon equivalent Ceq=0.42%≤0.45%, preheating temperature 100℃±20℃. The welding area is cleaned with an angle grinder, then wiped with acetone to remove oil. Surface roughness Ra12μm, UT inspection shows no defects. Base material thickness 16mm, using an X-shaped bevel with an angle of 60°, a 2mm blunt edge, and a 2mm gap. MT inspection shows no cracks. Preheating uses electric heating elements, covering 80mm on each side of the bevel, with a heating rate of 50℃ / h, holding for 30min, temperature measurement 105℃-115℃, uniformity deviation ≤4℃.

[0090] Step 1.2, Welding Material Selection and Pretreatment: For corrosive media conditions, select Ni-containing ER55-Ni3 welding wire (φ2.5mm, φ3.2mm). Welding wire pretreatment: After degreasing with alcohol, dry at 150℃ for 1 hour (ER55-Ni3 is a low alloy steel welding wire, refer to the ER50-6 pretreatment standard), store in an insulated container, and expose for 1.8 hours to 2 hours.

[0091] Step 1.3, Welding Equipment Debugging: MIG / MAG welding machine, shielding gas Ar (80%) + CO2 (20%), purity 99.99%, φ2.5mm welding wire flow rate 17L / min, φ3.2mm wire flow rate 20L / min. Equipment debugging ensures that current, voltage, and speed accuracy meet standards, and is equipped with dedicated auxiliary equipment for corrosion resistance testing.

[0092] Step 2, Welding Process Control Step 2.1, Root Pass Welding: MIG welding, single-sided welding with double-sided forming, φ2.5mm ER55-Ni3 welding wire, current 120A, voltage 22V, speed 110mm / min, heat input Q=0.86×22×120 / 11=20.64kJ / cm. Welding angle 65° to ensure root penetration. PT inspection after root pass welding showed no defects.

[0093] Step 2.2, Filler Welding: The longitudinal seam is divided into 5 segments, each 400mm long, with 5 layers of filler weld beads arranged in a spiral pattern. The heat input of Q345R steel is 17kJ / cm ± 0.5kJ / cm, controlled in real-time by a calculator; the current is adjusted if the deviation exceeds the limit. The weld bead width is 13mm, the spacing is 9mm, and the overlap width is 4.3mm (13mm × 1 / 3 ± 0.5mm). The interpass temperature is 100℃-150℃, monitored by a thermometer; heating is applied below 95℃, and cooling is applied above 155℃. Each weld bead is ground 1.2mm (1 / 3 of the 3.5mm weld bead thickness) with an 80-grit grinding wheel, resulting in a Ra of 6.2μm after grinding. Filler welding parameters: Layers 1-2: φ2.5mm welding wire, current 140A, voltage 24V, speed 100mm / min, heat input 17kJ / cm; Layers 3-5: φ3.2mm welding wire, current 210A, voltage 27V, speed 80mm / min, heat input 19kJ / cm.

[0094] Step 2.3, Cover Weld: Two weld passes overlap by 4.3mm, parameters: φ3.2mm welding wire, current 189A (reduced by 10%), voltage 25V (reduced by 2V), speed 88mm / min (increased by 10%), heat input 15.2kJ / cm (89% of the filler weld). Add a strengthening tempering weld pass, parameters: current 152A, voltage 23V, speed 105mm / min, heat input 11.4kJ / cm (75% of the cover weld). Grind to retain 1 / 2 tempered layer (approximately 1.4mm), weld reinforcement 2mm.

[0095] Step 3: Post-weld quality inspection and evaluation Step 3.1, Appearance inspection: Unevenness 0.3mm / m, excess height 2mm, undercut 0.3mm, length percentage 4%, appearance qualified.

[0096] Step 3.2, Non-destructive testing: PT and UT sensitivity level II, RT assessment level II, no defects.

[0097] Step 3.3, Microstructure inspection: The weld microstructure is fine-grained ferrite + pearlite, the width of the coarse-grained zone in the heat-affected zone is 75μm, the grain size is 7μm, and the content of hardened microstructure is 1.8%≤5%.

[0098] Step 3.4 Mechanical property test: Tensile strength of Q345R base material 510MPa, joint 465MPa (≥459MPa); Yield strength of base material 345MPa, joint 293MPa (≥293.25MPa); Impact absorption energy at -20℃ 58J ≥47J.

[0099] Step 3.5, Residual stress detection: 100MPa at the weld center (≤345×30%=103.5MPa), 82MPa in the transition zone (≤86.25MPa), and 65MPa in the heat-affected zone (≤69MPa).

[0100] Step 3.6, Corrosion resistance test: 5% NaCl salt spray test for 72h, corrosion rate 0.007mm / year ≤ 0.01mm / year; corrosion potential in 3.5% NaCl solution -0.55V ≥ -0.6V, corrosion resistance meets the standard.

[0101] Comparison of effects of each example Comparison indicators Example 1 (15CrMo) Example 2 (P91) Example 3 (Q345R) Traditional crafts Energy consumption per unit length (kWh / m) 8.5 10.2 7.8 45-55 Construction cycle (h) for a 100m weld. 18 24 15 48-72 Welded joint pass rate (%) 99.2 98.8 99.5 84.5-88 Comprehensive cost per unit length (yuan / m) 1200 1800 1000 1980-2500 Residual stress relief rate (%) 68 65 70 40-50 As can be seen from the data, the method of the present invention can achieve energy saving, quality improvement and efficiency enhancement under different low alloy steel grades and complex working conditions, and fully meets the stringent requirements of industries such as petrochemicals and power.

[0102] The implementation principle of this invention is as follows: This invention discloses an energy-saving welding method for pressure pipelines and pressure vessels in the petrochemical industry, belonging to the field of low-alloy steel welding technology. This method maximizes the secondary thermal cycle effect of tempering weld technology by precisely preparing the base metal based on its carbon equivalent and operating conditions before welding, implementing closed-loop control of heat input and weld parameters during welding, and ensuring multi-dimensional quality inspection after welding, thus replacing traditional high-energy-consuming post-weld heat treatment. Its core innovation lies in establishing a matching system of "base metal-welding material-operating conditions," a real-time feedback adjustment mechanism for heat input, and testing standards covering mechanical and corrosion properties. This solves the problems of poor adaptability and large quality fluctuations in existing technologies, providing a standardized, efficient, and energy-saving solution for low-alloy steel weld welding. The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. An energy-saving welding method applied to the construction of low-alloy steel welds in pressure pipelines and pressure vessels in the petrochemical industry, characterized in that, This method replaces the traditional post-weld heat treatment process with tempering weld bead technology, and includes the following steps: Step 1, Pre-welding preparation stage: This includes base metal pretreatment, welding material selection and pretreatment, and welding equipment debugging. The base metal pretreatment temperature is determined based on the carbon equivalent of the base metal: 100℃±20℃ when carbon equivalent Ceq≤0.45%, 150℃±20℃ when 0.45%<Ceq≤0.6%, and 200℃±20℃ when Ceq>0.6%. Welding material selection is based on a matching system between the base metal composition and operating conditions, determining the welding material type according to the low-alloy steel type, service temperature, pressure, and media corrosivity. Step 2, Welding Process Control Stage: Perform root pass welding, fill pass welding, and cover pass welding in sequence. The fill pass welding adopts a spiral weld bead arrangement, and the overlap width of the weld bead is 1 / 3 ± 0.5 mm. The interpass temperature is monitored and controlled in real time at 150-220℃ using an infrared thermometer. After the cover pass welding is completed, an additional layer of strengthening tempering weld bead is welded, with a heat input of 70%-80% of that of the cover pass weld. After the additional weld, the tempering layer with a thickness of 1 / 2 is ground off. Step 3, Post-weld quality inspection and evaluation stage: A multi-dimensional inspection system is adopted, including visual inspection, non-destructive testing, microstructure inspection, mechanical property testing, residual stress testing, and corrosion resistance testing, to ensure that the quality of the welded joint meets the standards.

2. The energy-saving welding method according to claim 1, characterized in that, The pretreatment of the base material also includes surface cleaning and beveling design: the surface roughness of the welding area must reach Ra12.5μm or less, and ultrasonic testing is used to confirm the removal of defects; when the base material thickness is ≤10mm, a V-shaped beveling is used; when the thickness is 10mm < ≤20mm, an X-shaped beveling is used; and when the thickness is >20mm, a U-shaped beveling is used. After beveling, magnetic particle testing is used to confirm that there are no crack defects.

3. The energy-saving welding method according to claim 1, characterized in that, The specific principles for selecting welding materials are as follows: For 15CrMo and 12Cr1MoV pearlitic low alloy steels, under working conditions of 300℃-500℃ and 10MPa-20MPa, select ER5515-1CM or ER55-B2 welding wire. For P91 and P92 martensitic heat-resistant steels, E9015-B91 or ER90S-B9 welding materials are selected under working conditions of 500℃-650℃ and 20MPa-35MPa. For Q345R low-alloy high-strength steel, ER50-6 welding wire is selected under normal temperature and 1MPa-10MPa working conditions. For low-alloy steel welds operating in corrosive media environments, ER55-Ni3 welding material containing Ni and Cu elements should be selected.

4. The energy-saving welding method according to claim 1, characterized in that, The welding material pretreatment steps are as follows: the welding wire is wiped with alcohol to remove oil and then dried. For ER5515-1CM and E9015-B91 type welding materials, the drying temperature is 350℃±20℃ and the holding temperature is 2h. For ER50-6 type welding materials, the drying temperature is 150℃±20℃ and the holding temperature is 1h. After drying, the welding material is exposed to air for no more than 2h and is re-dried ≤3 times.

5. The energy-saving welding method according to claim 1, characterized in that, The heat input during the welding process is monitored in real time by a computer with a control accuracy of ±0.5 kJ / cm. The heat input for filler welding is determined according to the type of base material: 12-18 kJ / cm for 15CrMo steel, 10-15 kJ / cm for P91 steel, and 15-20 kJ / cm for Q345R steel.

6. The energy-saving welding method according to claim 1, characterized in that, The root pass welding uses gas metal arc welding with a welding wire diameter of 2.5mm, a welding current of 100-130A, a voltage of 20-23V, and a heat input of 12-17kJ / cm. The fill pass welding uses a segmented back-welding method, dividing the weld into several sections of 400mm each, and welding from both ends toward the middle.

7. The energy-saving welding method according to claim 1, characterized in that, The non-destructive testing includes penetrant testing, ultrasonic testing, and radiographic testing. The sensitivity of penetrant testing and ultrasonic testing reaches level II, and the radiographic testing evaluation level of important welds is ≥ level II. In the mechanical property test, the tensile strength of the welded joint is not less than 90% of the base material, the yield strength is not less than 85% of the base material, and the impact absorption energy at -20℃ is ≥ 47J.

8. The energy-saving welding method according to claim 1, characterized in that, The residual stress was detected using an X-ray stress meter. The residual stress at the weld center was ≤30% of the yield strength of the base metal, ≤25% in the transition zone, and ≤20% in the heat-affected zone. Under corrosive conditions, the weld must pass salt spray and electrochemical corrosion tests. The salt spray corrosion rate was ≤0.01mm / year in 72h, and the corrosion potential in 3.5% NaCl solution was ≥-0.6V.

9. The energy-saving welding method according to claim 1, characterized in that, The commissioning of the welding equipment also includes precise control of the protective gas ratio. Under normal operating conditions, the protective gas is a mixture of Ar (80%) and CO2 (20%), while under high temperature and high pressure conditions, it is a mixture of Ar (90%) and CO2 (10%). The purity of the gas is not less than 99.99%.

10. The energy-saving welding method according to claim 1, characterized in that, The weld bead grinding in the interlayer treatment uses an angle grinder with a grit size of 80-120 mesh. After grinding, the surface roughness of the weld bead is controlled below Ra6.3μm, and the tempering layer of adjacent completed weld beads must not be damaged.

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