High frequency straight seam welded steel pipe also know as straight seam resistance welding pipe. The outside diameter is small, usually is under DN 600. For the outside diameter is larger than DN 600, the production process will use double sided submerged arc welding process. The high frequency straight seam welded steel pipe is mainly used in water engineering, petrochemical industry, chemical industry, power industry, agriculture irrigation and urban construction. For the transportation of liquids, the pipe can be applied for water supply, drainage. For gas transportation, the pipe can be used for transmission gas, steam, liquefied petroleum gas. For structural purposes, the pipe can be used as piling pipe fro bridges, piers, roads, buildings and other structures.
The characteristics for high frequency straight seam welded steel pipe is below:
1. the welding speed is large.
2. the welding area is small.
3. clean up is not necessary after welding
4. the welding process can produce thin walled tube, and also metal pipe.
The high frequency straight seam welded steel pipe process is below:
Slitting---Uncoiling---Strips Leveling---Head and Tail Cutting---Strips Welding---Looper Accumulator---Forming---Welding---Burr Removing---Sizing---Testing---Cutting---Inspection---Straightening---Processing---Hydrostatic Test---Flow Detection---Coating and Marking---Finished.
1. Test Process
Sampling from the defective LSAW steel pipe, and then confirm the location of the defect in the weld seam by manual ultrasonic flaw detector, then processing the weld cross defect section by the method of machining by grinding, polishing, etching after a metallurgical sample. Through the metallurgical test, we can notice the location and quantity of weld slag.
2. Cause Analysis
Theoretically, submerged arc welding seam slag is produced mainly due to the following three points:
A. raw materials inclusions (including base metal, wire, flux)
B. the multilayer welding interlayer is not clean up
C. improper selection of welding parameters is not conductive to slag floats.
According LSAW pipe production characteristics can be excluded when the layer of multilayer welding clean up is not clean leaving weld slag produced for this reason.After the raw material for many inclusions leaving produce weld slag, weld inspection before taking the base material, replaced with new wire and flux and other measures, the proportion of weld fusion clamp slag produced only a slight decrease, indicating that the inclusion of raw materials the main reason was not the cause of the slag. Thus, the main reason for the fusion thick wall LSAW pipe clamps slag is produced by improper selection of welding parameters.
Thick wall LSAW pipe welding parameters are: power lines, welding current, welding voltage, welding speed, wire spacing, groove dimensions and so on. Then do in-depth analysis from the welding metallurgy aspects seen: Weld fusion line produced slag is the main reason for the fusion line temperature is too low, the liquid slag late precipitation; and cause fusion line temperature is too low because the peak temperature of the heating or cooling too low too fast.
