Reasons for slag inclusion in straight seam steel pipes and cutting methods

Slag inclusion in welded steel pipes refers to the residual slag inside the weld seam. In theory, the main reasons for slag inclusion in submerged arc automatic welding seams are as follows:

 

 

① There are many inclusions in raw materials (including base metal, welding wire, and flux);
② Unclean interlayer cleaning during multi-layer welding;
③ The improper selection of welding process parameters is not conducive to the emergence of slag.

 

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

After taking measures such as pre weld inspection of the base material, replacement of welding wire and flux to address the presence of impurities in the raw materials that cause slag inclusions in the weld seam, the proportion of slag inclusions in the fusion line of the weld seam only slightly decreased, indicating that impurities in the raw materials are not the main cause of slag inclusions.

Therefore, the main reason for 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 for thick-walled straight seam submerged arc welded pipes mainly include: line energy, welding current, welding voltage, welding speed, wire spacing, groove size, etc.

Further analysis from the perspective of welding metallurgy reveals that the main reason for slag inclusion in the weld fusion line is the low temperature of the fusion line, which prevents the liquid slag from precipitating in time; The reason for the low fusion line temperature is that the peak heating temperature is too low or the cooling rate is too fast.

The 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 metal being cut at high temperatures, producing iron oxide slag. Then, high-pressure oxygen airflow is used to blow the slag away from the metal, and the straight seam steel pipe is cut off.

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

The gas cutting tool for cutting straight seam steel pipes is the cutting torch, which is divided into two types according to the different acetylene pressures: injection suction and isobaric. The one we commonly use is the injection suction cutting torch. The oxygen used for gas cutting is supplied by oxygen cylinders, while acetylene gas is supplied by acetylene cylinders or acetylene generators.

Due to the danger of gas cutting, it is necessary to strictly follow the operating procedures when cutting straight seam steel pipes using gas cutting methods. The following points should also be noted:

1. Regardless of 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 to form a 70 ° -80 ° angle with the tangent line of the cutting point.

 

2. When fixing a straight seam steel pipe with gas cutting, 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 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 recommended.

 

5. After cutting the straight seam steel pipe, the oxide slag at the cut should be smoothed and cleaned with a file or hand grinding wheel. At the same time, the end face of the straight seam steel pipe should be kept perpendicular to the centerline of the pipe.

 

6. After gas cutting, the cutting oxygen valve, acetylene valve, and preheating oxygen valve should be quickly closed.