1. Raw material preparation

LSAW Steel Pipe welding first requires the preparation of suitable steel plates, and the material of the steel plates should be selected according to the ultimate use of the steel pipe. For example, steel pipes used for transporting oil and gas are generally made of high-strength alloy steel. Before welding, the steel plate needs to be pre treated, including cutting into appropriate sizes and shapes, and ensuring the flatness and smoothness of the cutting edges to facilitate subsequent welding operations. The edges of the cut steel plate usually need to be polished to remove impurities such as iron oxide and oil, in order to prevent these impurities from entering the weld seam during the welding process and affecting the welding quality.

2. Molding process

Pre bending edge: The edge of the steel plate is pre bent, usually by mechanical cold bending, to form a certain curvature at the edge of the steel plate, preparing for subsequent rolling and forming. The angle and curvature of pre bending should be precisely controlled based on parameters such as the diameter and wall thickness of the steel pipe.

Roll forming: Steel plates are rolled into tubes using a rolling machine. During the rolling process, it is necessary to ensure that the mating edges of the steel plates are aligned and that the roundness of the rolling meets the requirements. The diameter tolerance of the rolled steel pipe should generally be controlled within a small range to ensure the quality of subsequent welding and the overall performance of the steel pipe.

3. Welding process

Bottom welding (pre welding)

Usually, carbon dioxide gas shielded welding (CO ₂ welding) is used for bottom welding. The advantages of this welding method are fast welding speed, large penetration depth, and the ability to provide good root fusion for subsequent welding. When welding, it is necessary to adjust parameters such as welding current, voltage, and welding speed. For example, the welding current is generally selected based on the diameter of the welding wire. For a welding wire with a diameter of 1.2mm, the current can be set between 100 and 150A, the voltage between 18 and 22V, and the welding speed can be controlled at around 20 and 30cm/min.

The thickness of the bottom welding seam is generally thin, mainly to ensure the welding quality at the root and prevent defects such as incomplete penetration and porosity at the root. The width of the weld seam also has certain requirements, usually ensuring that the weld seam covers the entire joint edge and smoothly transitions with the base metal.

Submerged arc welding (precision welding)

This is the key step in LSAW steel pipe welding. Submerged arc welding is a welding process that uses the arc heat generated between the welding wire and the base metal under a granular flux layer to melt the welding wire and base metal. During welding, the steel pipe is placed on specialized welding equipment, and the welding wire is automatically fed in. The flux is evenly covered in the welding area through a funnel.

The selection of welding wire should be determined based on the material of the base material of the steel pipe. For example, for steel pipes made of Q345B material steel plates, H08MnA welding wire is generally used. The control of parameters such as welding current, voltage, and welding speed is more stringent. Taking a steel pipe with a welding wall thickness of 10-20mm as an example, the welding current can be between 600 and 1000A, the voltage can be between 32 and 40V, and the welding speed can be around 30 and 50cm/min.

The weld quality of submerged arc welding is high, and the mechanical properties of the weld are good. Due to the protective effect of the flux, there are fewer defects such as porosity and slag inclusion in the weld seam. Meanwhile, submerged arc welding can perform multiple passes of welding to fill the weld seam and achieve the desired wall thickness. When performing multi pass welding, attention should be paid to good fusion between each weld seam, and the interlayer temperature should be controlled well. Generally, the interlayer temperature should not exceed 200 250 ℃ to avoid overheating of the weld seam, which may cause coarse structure and affect the performance of the weld seam.

4. Post welding treatment

Weld cleaning: After welding is completed, the welding slag and spatter on the surface of the weld should be cleaned in a timely manner. A combination of mechanical cleaning (such as wire brushes, grinding wheels, etc.) and chemical cleaning (such as acid washing, etc.) can be used. The cleaned weld surface should be smooth, without obvious defects or impurities.

Non destructive testing: In order to ensure welding quality, non-destructive testing of the weld seam is required. Common methods include ultrasonic testing (UT), radiographic testing (RT), etc. Ultrasonic testing can detect defects inside the weld seam, such as porosity, slag inclusion, and incomplete penetration, with high sensitivity; Radiographic inspection can visually display the internal defects of the weld seam and provide detailed information such as the shape, size, and location of the defects. Determine the qualified inspection level based on different standards and requirements for the use of steel pipes. For example, for some important oil transportation pipelines, the non-destructive testing qualification standards for welds are usually required to be high, such as ultrasonic testing to meet the Level I qualification standard.

Heat treatment (as needed): For some LSAW steel pipes made of high-strength alloy steel, or in cases where significant welding stresses are generated during the welding process, heat treatment may be necessary. The main methods of heat treatment include tempering, normalizing, etc. For example, tempering treatment can reduce the hardness of the weld, eliminate welding stress, and improve the toughness and plasticity of the weld. The temperature, time, and other parameters of heat treatment should be accurately set according to the material and welding conditions of the steel pipe.

Welding types of LSAW steel pipes

1. Single wire submerged arc welding

Principle: Single wire submerged arc welding is the most basic welding method in submerged arc welding. It is welded by melting the welding wire and base material under the granular flux layer through the arc heat generated between the welding wire and the workpiece. During welding, the arc burns under the flux layer, and the molten slag formed by the melting of the flux covers the surface of the weld, protecting the weld metal from air damage.

characteristic:

High welding quality: Due to the protective effect of the flux, it can effectively prevent harmful gases such as oxygen and nitrogen from entering the welding area, and there are fewer defects such as pores and slag inclusions in the weld metal. The chemical composition of the weld is stable and the mechanical properties are good, such as the strength and toughness of the weld, which can meet high requirements.

Stable welding parameters: The welding current, voltage, and other parameters of single wire submerged arc welding are relatively stable and easy to control. For example, when welding some small and medium-sized LSAW steel pipes, the welding process can be made smooth by adjusting the welding current between 400 and 800A and the voltage between 30 and 38V, thereby ensuring the quality and appearance of the weld seam.

High welding efficiency: Compared to manual arc welding and other welding methods, single wire submerged arc welding has a faster welding speed. Its welding speed can reach about 30 60cm/min, which can complete the welding of longer welds in a shorter time.

2. Double wire submerged arc welding

Principle: Double wire submerged arc welding uses two welding wires to weld simultaneously. Two welding wires can be arranged in series or parallel. During the welding process, two welding wires generate arcs, with the front wire mainly used to melt the base metal and the rear wire mainly used to fill the weld seam. This welding method can achieve efficient welding by reasonably configuring the welding parameters of two welding wires.

characteristic:

Higher welding efficiency: Double wire submerged arc welding has a faster welding speed than single wire submerged arc welding, reaching around 80 120cm/min. This is because the simultaneous welding of two welding wires increases the amount of metal melted per unit time, greatly improving welding efficiency and suitable for mass production of LSAW steel pipes and other situations.

Good weld quality: The synergistic effect of two welding wires can improve the formation of the weld. The front wire can ensure good fusion of the base material, while filling the weld seam with the rear wire can make the excess height of the weld seam more uniform and reduce stress concentration in the weld seam. Meanwhile, double wire submerged arc welding can effectively reduce defects such as porosity and cracks in the weld seam.

Flexible parameter adjustment: Double wire submerged arc welding can flexibly adjust the welding current, voltage, and wire feeding speed of two welding wires according to different welding requirements. For example, a larger welding current can be used for the front wire to ensure deep penetration, while a smaller current can be used for the rear wire to control the filling speed, thereby better controlling the quality and shape of the weld seam.

3. Multi wire submerged arc welding

Principle: Multi wire submerged arc welding (usually three or more wires) is developed on the basis of double wire submerged arc welding. Multiple welding wires work simultaneously during the welding process, and the function and welding parameters of each wire can be adjusted according to specific welding requirements. They cooperate with each other to jointly complete the melting of the base material and the filling of the weld seam.

characteristic:

Ultra high welding efficiency: The welding speed of multi wire submerged arc welding is extremely fast, which can greatly improve production efficiency. The advantages are obvious when welding large-diameter and thick walled LSAW steel pipes. For example, when welding steel pipes with a wall thickness exceeding 30mm, multi wire submerged arc welding can quickly complete the welding task, and its welding speed can exceed 150cm/min.

Optimized weld performance: By setting the parameters of multiple welding wires reasonably, the performance of the weld can be optimized. For example, by adjusting the position and parameters of the welding wire, the microstructure of the fusion zone and heat affected zone of the weld can be made more uniform, and the strength, toughness, and fatigue resistance of the weld can be improved.

Complex process control: Due to the involvement of multiple welding wires, the process control of multi wire submerged arc welding is relatively complex. Accurate control of parameters such as welding current, voltage, wire feeding speed, wire spacing, and angle is required for each welding wire, which demands high technical proficiency from welding equipment and operators.

4. Carbon dioxide gas shielded welding (CO ₂ welding) is used for bottom welding

Principle: Carbon dioxide gas shielded welding is a method of welding using carbon dioxide gas as the shielding gas, welding wire as the electrode, and filler metal. During the welding process, the welding wire is continuously fed through the wire feeding mechanism, generating an arc between the workpiece and the welding wire. The heat of the arc melts the welding wire and the base material, while carbon dioxide gas is sprayed from the nozzle, forming a protective gas cover around the arc to prevent harmful gases in the air from coming into contact with the molten metal.

characteristic:

Fast welding speed: The welding speed of CO ₂ welding is relatively fast, and it can quickly complete the welding of the root weld seam during the bottom welding, providing good root conditions for subsequent submerged arc welding, etc. The welding speed for base welding can generally reach around 20 40cm/min.

Larger penetration depth: It can provide a larger penetration depth, ensuring good fusion of the root weld and preventing defects such as incomplete penetration at the root. This is very important for products like LSAW steel pipes that require high welding quality.

Low cost: The price of carbon dioxide gas is relatively cheap, and the utilization rate of welding wire is high, making this welding method cost-effective and suitable for bottom welding in large-scale production.