Slag inclusions in welded steel pipes are residual slag inside the weld. Theoretically, the main causes of slag inclusions in submerged arc welded steel pipes are as follows: ① Excessive inclusions in the raw materials (including base metal, welding wire, and flux); ② Inadequate interlayer cleaning during multi-layer welding; ③ Inappropriate welding process parameters, hindering slag flotation.
Based on the production characteristics of straight seam submerged arc welded steel pipes, the cause of slag inclusions due to inadequate interlayer cleaning during multi-layer welding can be ruled out.
Regarding the issue of excessive inclusions in the raw materials causing slag inclusions, measures such as pre-welding inspection of the base metal, replacement of welding wire and flux, etc., only slightly reduced the proportion of slag inclusions at the weld fusion line, indicating that inclusions in the raw materials are not the main cause of slag inclusions.
Therefore, the main cause of slag inclusions at the fusion line in thick-walled straight seam submerged arc welded steel pipes is inappropriate welding process parameters. The main welding process parameters for thick-walled straight seam submerged arc welded pipes include: heat input, welding current, welding voltage, welding speed, wire spacing, and bevel dimensions.
Further analysis from a welding metallurgical perspective reveals that slag inclusions at the weld fusion line are primarily caused by an excessively low fusion line temperature, preventing the molten slag from precipitating. This low fusion line temperature is caused by either a low peak heating temperature or an excessively rapid cooling rate.
A common method for cutting straight seam steel pipes is gas cutting. Gas cutting utilizes the heat generated by the combustion of oxygen and acetylene to melt the metal being cut at a high temperature, producing iron oxide slag. A high-pressure oxygen stream then blows the slag away from the metal, thus cutting the straight seam steel pipe.
Gas cutting is efficient, convenient, and produces a relatively clean cut, but a thin oxide film will adhere to the cut surface. This film needs to be removed before welding. Gas cutting is commonly used in the installation of straight seam steel pipes, plates, and profiles with larger diameters.
The gas cutting tool for cutting straight seam steel pipes is a cutting torch. Based on the acetylene pressure, there are two types: the jet-suction type and the isobaric type. We commonly use jet-suction type torches. Oxygen for gas cutting is supplied by an oxygen cylinder, and acetylene gas is supplied by an acetylene cylinder or acetylene generator.
Due to the inherent dangers of gas cutting, operating procedures must be strictly followed. When cutting straight seam steel pipes using gas cutting, the following points should be noted:
1. Whether the straight seam steel pipe is rotated or fixed, the cutting nozzle should be kept perpendicular to the surface of the straight seam steel pipe. After cutting through, gradually tilt the cutting nozzle forward until it forms a 70°–80° angle with the tangent at the cutting point.
2. When gas cutting a fixed straight seam steel pipe, generally start from the lower part of the pipe.
3. The selection of the cutting nozzle and oxygen pressure is related to the thickness of the straight seam steel pipe and can be chosen according to the actual situation.
4. The distance between the cutting nozzle and the surface of the straight seam steel pipe should be determined based on the length of the preheating flame and the thickness of the straight seam steel pipe. Generally, a distance of 3–5 mm from the end of the flame core is appropriate.
5. After the straight seam steel pipe is cut, use a file or hand grinder to smooth and clean the iron oxide residue at the cut edge, ensuring the pipe end face is perpendicular to the pipe’s centerline.
6. After gas cutting, quickly close the cutting oxygen valve, acetylene valve, and preheat the oxygen valve.
Post time: Dec-25-2025