In pipeline construction, oil pipeline and gas pipeline is transported toward the large diameter high pressure and the development of submarine pipeline thick direction more and more pipelines require the use of LSAW steel pipe. With the introduction of of several large-seam submerged arc welded pipe production line is put into operation, the master LSAW advanced welding technology is especially important.
Galvanized steel is generally in the low carbon steel outer layer of zinc-plated galvanized layer typically 20um thick. Melting point of zinc is 419 ° C, with a boiling point of about 908 ° C. Steel pipe welding, the zinc melt into liquid floating in the pool surface or in the position of the root of the weld. Zinc having a large degree of solid solution in iron, zinc liquid depth along the grain boundary etching the weld metal, the low melting point of zinc is formed “liquid metal embrittlement.
Meanwhile, zinc and iron may be formed on the intermetallic brittle compounds, these brittle plastic lower phase of the weld metal, and cracks under tensile stress. Welding fillet welds, especially the T-shaped connector fillet weld is most likely to produce through crack. Plastic-lined steel pipe Galvanized steel welding, surface and edge of the groove at the zinc layer, in the arc thermal effect, resulting in oxidation, melting, evaporation of volatile as well as the white smoke and steam, can easily cause the pore in the weld. ZnO, is formed due to oxidation of the higher melting point of about 1800 ° C or more, if, in the welding process parameters are too small, the slag will cause the ZnO simultaneously. Zn become Deoxidizer. Produce a low melting point oxides FeO-MnO or FeO-MnO-SiO2 slag. Secondly, due to the evaporation of zinc volatilized lot of white fumes on the human body have a stimulating, antinociception, therefore, must be polished welding galvanized layer disposed.
Galvanized steel welding and low carbon steel and general preparation is the same, you need to pay attention to the size of the groove and the galvanized layer near to seriously handle. Penetration groove size should be appropriate, generally 60 to 65 °, to leave a certain gap, generally is 1.5 – 2.5mm; to reduce zinc weld penetration welding before the groove galvanized welding layer clear later. In the actual supervision, with a focus on playing the groove, without leaving a blunt edge technology of centralized control, two layers of welding process, to reduce the possibility of incomplete penetration. Electrode material selection should be based on the matrix of galvanized pipe, generally low carbon steel due to consider the ease, the choice of J422 is more common. Welding techniques: the weld seam in the welding of the first layer of the multilayer welding, as far as possible so that the zinc layer is melted and vaporized, evaporated escape weld, can greatly reduce the liquid zinc to remain in the weld.
Welding fillet, and also try to make the first layer of zinc layer melting and vaporization, evaporation and escape the weld, its first electrode end forward out of about 5 ~ 7mm, when the zinc layer melted and then move back to the original position welding. Again when cross welding and vertical welding, such as the selection of short slag electrodes such as J427 undercut tendency will be very small; if the front and rear and from transport technology, the more you can get the welding quality defects.
Post time: Oct-25-2019