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

Welded steel pipe slag inclusions are slag that remains inside the weld. From a theoretical analysis, the main reasons for the slag inclusion in the submerged arc automatic welding seam are as follows: ①There are many inclusions in the raw materials (including base metal, welding wire, and flux); ②The cleaning between layers is not clean during multi-layer welding; ③ The selected welding process parameters are improper, which is not conducive to the floating of slag.

According to the production characteristics of straight seam submerged arc welded steel pipes, the cause of slag inclusion in the weld due to unclean interlayer cleaning during multilayer welding can be ruled out. Given the large number of inclusions in the raw materials that cause slag inclusions in the weld, after taking measures such as inspecting the base metal before welding, and replacing the welding wire and flux, the proportion of slag inclusions in the weld fusion line is only slightly reduced, indicating that the inclusions in the raw materials Substances are not the main cause of slag inclusions.

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

From the in-depth analysis of welding metallurgy, it can be seen that the main reason for the slag inclusion in the weld fusion line is that the temperature of the fusion line is too low so that the liquid slag has no time to precipitate, and the reason for the low temperature of the fusion line is that the peak temperature of the heating is too high. The low or cooling rate is too fast.

The most common way to cut straight seam steel pipe is gas cutting. Gas cutting uses the heat generated by oxygen and acetylene combustion to melt the metal to be cut at a high temperature to produce iron oxide slag, and then blows the slag away from the metal with high-pressure oxygen flow. At this time, the straight seam steel pipe is cut off.

Cutting straight seam steel pipes with gas cutting is highly efficient and easy to operate, and the cut section is relatively neat, but a layer of oxide film will adhere to the cut surface. Need to be removed before welding. In the installation process of straight seam steel pipes, the gas-cutting method is often used to cut various straight seam steel pipes, steel plates, and profiles with large pipe diameters.

Gas cutting tools for cutting straight seam steel pipes are cutting torches, which are divided into two types: jet-suction and isobaric according to the pressure of acetylene. We often use jet-suction cutting torches. Oxygen for gas cutting is supplied by oxygen cylinders, and acetylene gas is supplied by acetylene cylinders or acetylene generators.

Due to the danger of gas cutting, the operating procedures must be strictly followed when cutting straight seam steel pipes by gas cutting. The following points should also be paid attention to:
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 the cut is through, the cutting nozzle is gradually tilted forward to an angle of 70°-80° to the tangent line of the cutting point.
2. When gas cutting and fixing the straight seam steel pipe, it generally starts 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 the actual situation.
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 between the end of the flame center is 3--5mm.
5. After the straight seam steel pipe is cut, use the iron oxide slag at the incision of the file or hand-grinding wheel 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. After gas cutting, the cutting oxygen valve and acetylene valve should be quickly closed to preheat the oxygen valve.