When the material is magnetized, magnetic arc blow is caused in cases of direct current welding, making it difficult to perform welding. While this is not a problem with ordinary 400 to 490 N/mm2 class steel plates, steel with high nickel content is easily magnetized, and the occurrence of magnetic arc blow can become marked with 780 and 950 N/mm2 class steel plates and 9% nickel steel.
Countermeasures for the occurrence of magnetic arc blow during welding at the job site are shown below.
(1) Changing the welding method
• Change from direct current to alternating current With direct current welding, the effect of magnetic arc blow is great.
If manual welding is to be used, consider whether alternating current can be used.
• Change the slant of the welding rod
Magnetic arc blow can be alleviated depending on the slant of the welding arc.
(2) Changing the grounding method
• The remanent magnetism in the steel plate changes depending on the grounding method.
• Increase the number of ground points on the steel plate from 1 to 2.
• Change the attachment location of the ground (Magnetic arc blow occurs in the direction opposite the ground point)
(3) Short circuiting the magnetism of the subject material (installing tab plates)
Bridge the material to be welded with other steel plates (tab plates) to short circuit the magnetism. The magnetism will flow through the bridged locations, so the remanent magnetism of the groove face will be alleviated.
(4) Alleviating remanent magnetism with magnets
The magnetism of the groove face can be alleviated when a permanent magnet or a magnetic particle inspection magnet is brought close to the steel plate to change the direction of the remanent magnetism.
(5) Installing supplemental material (steel)
Install supplemental material (steel) on the opposite side of the magnetic arc blow. In the case of a lap joint, the magnetic arc blow will occur in the lapped material, so the remanent magnetism can be alleviated by placing a steel plate on the opposite side as well.
6) Heating the groove face
The remanent magnetism can decrease even as a result of heating at or below the magnetic transformation point.
(7) Placement of the subject material
The remanent magnetism of the steel plate is affected by the power supply and the like.
Changing the way the material is placed can result in a change in the remanent magnetism of the groove face.
Source: nssmc.com
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