Pre-hot-dip galvanizing welding

1. General guidelines
In the design stage of steel structure must consider whether the design results can meet the requirements of hot-dip galvanizing process. During hot dip galvanizing, the steel members are heated and internal stresses are created, resulting in distortion or cracking of the components during the hot dip galvanizing process. Therefore, in the welding must take some measures to ensure that the welded components of the internal stress as low as possible. The following basic guidelines need to be adhered to or followed during the manufacturing process:

1)       The structural design should minimize the workload and cost of welding, because the steel components, the greater the amount of welding work, the greater the contraction of the component stress.
2) Where possible, welding shall be arranged along the direction of the heavy vertical of the steel structure. If longitudinal distribution is not possible, the weld shall be symmetrically distributed (with the vertical axis being the axis of symmetry).
3) The welds that produce a strong hardening of the component shall be finally welded.
4) When welding components should follow the principle from the inside out, in order to avoid high shrinkage stress when welding.
5) develop and optimize the welding sequence and procedures, and take into account the above-mentioned precautions.

The welding stresses can be evenly distributed over the entire cross-section of the welded structure, without permanent deformation or deformation of the structure, under the action of a reasonable sequence of welding procedures (and, of course must be strictly followed during welding operations) To the lowest.
In the event of deformation of the steel members during the manufacture and processing of the welding, the determination of whether the steel members are to be corrected or straightened shall be made on a case-by-case basis. But the reality is that the deformation caused by welding will be further aggravated during the hot-dip galvanizing process, which is the result of the decrease in the strength of the steel members during hot-dip galvanizing.

２.The source of the defect
Hot-dip galvanizing prior to the use of flame method (heating to strengthen) or pressure method (cold correction) of steel components for correction or straightening possible. However, taking into account the cost factors, when the steel components require a higher degree of correction or a certain degree of difficulty, it is recommended not to use these corrective measures, taking into account the actual deformation of steel components, but expect its hot dip galvanized in the process of slight deformation．If the hot-dip galvanizing before welding, the product manufacturing process must be given attention. When welding, if the chemical composition of the filler material and the chemical composition of the substrate material is different, the formation of hot-dip galvanized layer in the appearance and thickness of galvanized and other locations there are significant differences, when the weld This phenomenon is particularly evident when the formation.

In a hot dip galvanized zinc bath environment, the member may be deformed due to the heating of the components in the zinc bath. Because as the temperature increases, the strength of steel decreased at 450 ℃, the strength of steel is usually only half the normal temperature and pressure.

3. Welding practice
With the support of fully optimized welding sequences and procedures, the welding stresses can be evenly distributed over the entire cross-section of the weldment so that the component will not be permanently deformed or its deformation will be minimized.
Before hot-dip galvanizing, make sure that the weld location is clean and the slag has been removed. The adhesion of the adhered residue is so high that it is difficult to remove in the pre-treatment step of the hot-dip galvanizing, and galvanized defects are caused in the galvanizing. It is also important to ensure that the filler material of the weld is consistent with the base material and that the difference in composition of the weld and matrix material may lead to differences in the appearance and thickness of the zinc coating at the weld location and at other locations.