Welding methods for steel plate spiral pipes

　　 Steel plate rolled pipe It refers to seamless tubes formed by bending steel strips or carbon steel plates into circular or square shapes, with welded joints on the surface. The billets for these welded seamless tubes are either carbon steel plates or steel strips. How can steel strips or carbon steel plates be joined together using electric welding? What welding method is employed?

　　1. Depending on their application scope, steel plate‑rolled pipes exhibit both versatility and high‑pressure resistance; the latter are typically fabricated from various carbon steels. It is essential to carefully address the challenge of weld‑induced shrinkage cracking throughout the entire welding process, while ensuring that the mechanical properties of the heat‑affected zone remain largely unaffected by damage caused during welding.

　　2. Depending on wall thickness, pipelines made of thin-walled materials such as steel straps may be welded using a high-frequency welding machine; certain thin-walled pipes can be welded with gas-shielded arc welding. For pipes with wall thicknesses exceeding 3 mm, manual arc welding is recommended.

　　3. Depending on the pipe diameter, for smaller diameters, welding is performed on one side only; skilled technicians can drill into the pipe to carry out welding on both sides. Single-sided welding must be executed by a qualified welder. Both single-sided and double-sided welding can be carried out by two operators. Based on bearing steel ball testing, combined with professional expertise and weld bead formation, it can be ensured that the cross-sectional area meets the design requirements.

　　4. If the pipe material is a high‑wear‑resistant carbon steel—such as heat‑resistant, pressure‑resistant, wear‑resistant, or low‑temperature alloy steel—it is easier to prevent the formation of martensitic microstructure and the tendency for cracking caused by high‑temperature welding. Therefore, proper implementation of the welding procedure is crucial. General requirements for this manufacturing process include: selecting appropriate electrodes, baking them as specified, and issuing them to welders for use; thoroughly cleaning oil and contaminants from both sides of the weld joint prior to welding, and removing weld spatter, rust, and other foreign matter; minimizing the vibration of the welding equipment throughout the welding process to reduce sway; arranging subsequent weld passes in such a way as to provide heat‑treatment effects to the thermally affected zone whenever possible; and, after welding, allowing the joint to cool slowly and applying preventive measures such as covering it with asbestos cloth.

　　5. To ensure the quality of pipeline welding, welders should undergo training and pass the relevant assessments; only those who are qualified may be assigned to work. In addition to fundamental theoretical knowledge and technical skills, welders must also give full consideration to the pipeline’s welding procedures. The entire process of electric arc welding for pipelines can be divided into overhead, vertical‑up, and horizontal‑up positions.

　　The microstructure of rolled steel pipe is prone to shrinkage and loosening, and it is relatively sensitive to solidification cracks (a type of hot cracking). During welding, arc craters and depressions are highly susceptible to cracking. When the carbon content is high, rapid cooling can readily produce a hardened microstructure—such as austenite—that is sensitive to cold cracking. Moreover, in regions subjected to overheating, if the cooling rate is very high, hard and brittle high‑nitrogen austenite may form, rendering those overheated zones brittle.

　　Key points of the welding method for steel plate rolled pipes:

　　1. Arc welding is generally performed under quenching conditions.

　　2. The welding process is unrestricted.

　　3. Employ relatively high linear kinetic energy to moderately increase the heating temperature; typically, both the heating temperature and the solid-layer temperature can be maintained within the range of 250–300°C.

　　4. Welding consumables shall ensure that the composition of the deposited alloy is essentially identical to that of the base material, such as J107-Cr.

　　5. Heat treatment should be performed immediately after welding. If quenching and tempering cannot be carried out right away, an intermediate quench or a holding period at a temperature above the ambient can be implemented to prevent hydrogen embrittlement and softening. For components with complex geometries and extensive welds, an intermediate quench may be conducted after a specified number of weld passes.