What should be paid attention to when welding galvanized steel pipe
December 05, 2018
1. Need to polish in the early stage
However, the galvanized layer of the weld must be polished away, otherwise bubbles, trachoma, and false welding will occur. It also makes the weld brittle and the rigidity decreases.
2. the welding characteristics of galvanized steel
Galvanized
steel is generally coated with a layer of zinc outside the low carbon
steel, and the galvanized layer is generally 20 um thick. Zinc has a melting point of 419 ° C and a boiling point of about 908 ° C. In welding, zinc melts into a liquid that floats on the surface of the bath or at the root of the weld. Zinc
has a large solid solubility in iron, zinc liquid will deeply etch the
weld metal along the grain boundary, and low melting point zinc forms
"liquid metal embrittlement".
At
the same time, zinc and iron can form intermetallic brittle compounds,
and these brittle phases reduce the plasticity of the weld metal and
cause cracks under tensile stress.
If weld fillet welds, especially fillet welds of T-joints, are most susceptible to penetration cracks. When
the galvanized steel is welded, the zinc layer on the surface and the
edge of the groove is oxidized, melted and evaporated under the action
of arc heat to volatilize white smoke and steam, which easily causes
weld pores.
3. Welding process control
The
pre-weld preparation of galvanized steel is the same as that of
ordinary low-carbon steel. It is necessary to pay attention to the
groove size and the nearby galvanized layer. For
the penetration, the groove size should be appropriate, generally
60~65°, to leave a certain gap, generally 1.5~2.5mm; in order to reduce
the penetration of zinc into the weld, the galvanized inside the groove
can be welded before welding After the layer is removed, it is soldered again.
Welding
method: When welding the first layer of welds of multi-layer welding,
the zinc layer is melted as much as possible and vaporized and
evaporated to escape the weld seam, which can greatly reduce the liquid
zinc remaining in the weld.
When welding the
fillet weld, the zinc layer is also melted as much as possible in the
first layer and vaporized and evaporated to escape the weld. The method
is to first move the end of the electrode forward about 5~7 mm, when the
zinc layer is made. After melting, return to the original position and continue to weld forward.
In
the case of cross-welding and vertical welding, if short-slag welding
rods such as J427 are used, the tendency of undercutting will be small;
if the round-trip transportation technology is used, the quality of
non-defective welding can be obtained.
