Causes and cutting methods of slag inclusion in straight seam steel pipes

Slag inclusion in welded steel pipes is slag remaining inside the weld. From a theoretical analysis, the main reasons for slag inclusion in submerged arc automatic welding welds are the following three points: ① There are many inclusions in the raw materials (including parent materials, welding wires, and fluxes); ② The interlayer cleaning is not clean during multi-layer welding; ③ The improper selection of welding process parameters is not conducive to the floating of slag.

According to the production characteristics of straight seam submerged arc welded steel pipes, the reason for slag inclusion in the weld due to the unclean cleaning between layers during multi-layer welding can be ruled out.

Because there are many inclusions in the raw materials and slag inclusion in the weld, after taking measures such as inspecting the parent material before welding, and replacing the welding wire and flux, the proportion of slag inclusion in the weld fusion line is only slightly reduced, indicating that the inclusions in the raw materials are not the main cause of slag inclusion.

Therefore, the main reason for the slag inclusion in the fusion line of thick-walled straight seam submerged arc welded steel pipes is the improper selection of welding process parameters. The welding process parameters of thick-walled straight seam submerged arc welded pipes mainly include: line energy, welding current, welding voltage, welding speed, wire spacing, groove size, etc.

From the perspective of welding metallurgy, an in-depth analysis shows that the main reason for the slag inclusion in the weld fusion line is that the fusion line temperature is too low, so the liquid slag has no time to precipitate, and the reason for the low fusion line temperature is that the peak temperature of heating is too low or the cooling speed is too fast.

The most common method for cutting straight seam steel pipes is gas cutting. Gas cutting uses the heat generated by the combustion of oxygen and acetylene to melt the cut metal at high temperatures to produce iron oxide slag, and then uses high-pressure oxygen gas flow to blow the slag away from the metal. At this time, the straight seam steel pipe is cut.

Cutting straight seam steel pipes with gas cutting is efficient and easy to operate. The incision section is also relatively neat, but a layer of oxide film will be attached to the surface of the incision. It needs to be removed before welding. In the installation process of straight seam steel pipes, gas cutting is often used to cut various straight seam steel pipes, steel plates, and profiles with larger pipe diameters.

The gas-cutting tool for cutting straight seam steel pipes is a cutting torch. According to the different acetylene pressures, it is divided into two types: injection and suction type and isobaric type. We often use the injection and suction-type cutting torch. The oxygen used for gas cutting is supplied by oxygen cylinders, and the acetylene gas is supplied by acetylene cylinders or acetylene generators.

Due to the dangers of gas cutting, the operating procedures must be strictly followed during operation. When cutting straight seam steel pipes with gas cutting, the following points should also be noted:
1. Regardless of whether the straight seam steel pipe is rotating or fixed, the cutting nozzle should be kept perpendicular to the surface of the straight seam steel pipe. After cutting through, the cutting nozzle will gradually tilt forward to an angle of 70°-80° with the tangent of the cutting point.
2. When gas-cutting fixed straight seam steel pipes, it is generally started from the lower part of the straight seam steel 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 selected according to actual conditions.
4. The distance between the cutting nozzle and the surface of the straight seam steel pipe should be determined according to the length of the preheating flame and the thickness of the straight seam steel pipe. Generally, the distance from the end of the flame core is 3-5mm.
5. After the straight seam steel pipe is cut, the iron oxide slag at the cut should be filed or hand-grinded to make it smooth and clean. At the same time, the end face of the straight seam steel pipe should be kept perpendicular to the center line of the pipe.
6. When the gas cutting is finished, the cutting oxygen valve, acetylene valve, and preheating oxygen valve should be quickly closed.