Causes of slag inclusion in longitudinally welded steel pipes and cutting methods
Slag inclusions in longitudinally welded steel pipes are slag remaining inside the weld. From theoretical analysis, the main reasons for slag inclusion in submerged arc automatic welding are as follows: ①There are more inclusions in raw materials (including base metal, welding wire, and flux); ②The cleaning between layers is not clean during multi-layer welding; ③Improper welding process parameters are selected, which is not conducive to the emergence of slag.
According to the production characteristics of LSAW steel pipes, it can be ruled out that slag inclusions in the welds due to poor cleaning between layers during multi-layer welding can be eliminated.
Because there are many inclusions in the raw materials that cause slag inclusions in the weld, after taking a pre-weld inspection of the base material, replacing the welding wire and flux, the proportion of slag inclusions in the weld fusion line is only slightly reduced, indicating the inclusions in the raw materials Substances are not the main cause of slag inclusions.
Therefore, the main reason for slag inclusion in the fusion line of thick-walled LSAW steel pipes is improper welding process parameters. The welding process parameters of thick-wall LSAW pipes mainly include heat input, welding current, welding voltage, welding speed, welding wire spacing, groove size, etc.
From the perspective of welding metallurgy, we can see that the main cause of slag inclusion in the weld fusion line is that the temperature of the fusion line is too low, which makes the liquid slag too late to precipitate; the reason for the too low temperature of the fusion line is that the peak temperature of heating is too low. Low or too fast cooling rate.
The well-known method of 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 a high temperature to produce iron oxide slag, and then blow the slag away from the metal with a high-pressure oxygen gas stream. When the straight seam steel pipe is cut.
Cutting the straight seam steel pipe with gas cutting is efficient and easy to operate. The cut section is also relatively neat, but a layer of oxide film will adhere to the surface of the cut. Need to be removed before welding. In the installation process of straight seam steel pipes, gas cutting methods are commonly 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 difference of acetylene pressure, it is divided into two types: jet suction type and equal pressure type. The jet suction type cutting torch is commonly used. The oxygen for gas cutting is supplied by an oxygen cylinder, and the acetylene gas is supplied by an acetylene gas cylinder or an acetylene generator.
Due to the danger of gas cutting, the operating procedures must be strictly followed during the operation. When cutting the straight seam steel pipe with the gas cutting method, you should also pay attention to the following points:
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, the cutting nozzle is gradually tilted forward to an angle of 70°-80° with the tangent line of the cutting point.
2. Gas cutting generally starts from the lower part of the straight seam steel pipe when fixing the straight seam steel pipe.
3. The selection of cutting nozzle and oxygen pressure is related to the thickness of the straight seam steel pipe, which 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 from the end of the flame core is 3-5mm.
5. After the straight seam steel pipe is cut, apply the iron oxide slag at the cut 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 centerline of the pipe.
6. At the end of the gas cutting, the cutting oxygen valve, and acetylene valve should be quickly closed to preheat the oxygen valve.