1.Why must the zinc layer be completely removed?
The primary purpose of sandblasting is to remove the galvanized layer, preparing the weld for welding. Successful direct welding depends on the effectiveness of the sandblasting.
Sandblasting must be thorough: After sandblasting, it is essential to ensure that the zinc layer in and around the weld area (including the back of the weld and the bevel) is completely removed, exposing the natural metal (silver-white steel substrate). Any residual zinc will cause problems during welding.
Inspection method: Visually inspect the weld area. There should be no bright silver or zinc-speckled traces of galvanized layer; the weld area should exhibit the pure gray-white metallic luster of steel.
2.What are the risks of welding zinc plating?
Porosity:
Zinc has a boiling point of approximately 906°C, while steel has a melting point of approximately 1500°C.
Under the high temperature of an electric arc, zinc rapidly evaporates into zinc vapor. If this vapor cannot escape from the molten pool in time, it becomes trapped within the weld metal during solidification, forming numerous pores that severely weaken the weld's strength and density.
Welding Cracks (Zinc Embrittlement):
Molten zinc penetrates along the grain boundaries of the steel, leading to a decrease in intergranular bonding and making the weld highly susceptible to cracking under welding stress. This is a fatal defect for welded structures.
Toxic Welding Fumes:
Evaporated zinc reacts with oxygen in the air to produce zinc oxide fumes.
Inhaling these fumes can cause "metal fume fever," with symptoms including fever, chills, nausea, and headache, posing a significant health hazard to welders.
Poor Weld Quality:
Zinc vapor interferes with the stability of the electric arc and can produce welding spatter, resulting in unsightly weld formations and poor fusion.
3.What are the advantages of welding directly after sandblasting?
Eliminates a step: If sandblasting is a necessary step in the workpiece surface treatment (such as preparing for subsequent painting), it simultaneously removes the zinc layer, eliminating the need for separate grinding.
Large and uniform treatment area: For large welded areas, sandblasting is more efficient and produces a more uniform finish than angle grinder grinding.
4.What are the disadvantages of welding directly after sandblasting?
Complete removal is difficult to guarantee: Especially in complex shapes, corners, crevices, or edges of sheet metal, sandblasting may not completely remove the zinc layer. Residual zinc can cause the aforementioned quality problems.
Secondary contamination: If workpieces are improperly placed after sandblasting or in a dusty environment, zinc-containing dust may settle on the surface. This dust can also cause problems during welding.
High timeliness requirements: The fresh steel substrate exposed after sandblasting is prone to rapid oxidation and rusting in the air. Therefore, welding should be performed as soon as possible after sandblasting; otherwise, the thin rust layer that forms will also affect the weld quality.
5.What is the correct operating procedure?
Thorough Sandblasting:
Use a suitable abrasive (such as steel grit, brown corundum, etc.) to ensure complete removal of the zinc layer from the weld bevel and at least 20-30mm on both sides, until the metal is exposed.
For butt welds, the back side also requires the same treatment.
Strict Inspection:
Before welding, carefully inspect the sandblasted area to ensure no zinc layer remains. This can be done by shining a strong light on the surface and observing from different angles.
Surface Cleaning:
After sandblasting, use clean compressed air to blow away abrasive dust and residue from the surface. Wipe with an oil-free cloth if necessary.
Immediate Welding:
Complete the welding operation before the surface re-oxidizes.
Enhanced Welding Protection:
Ventilation: Even after the zinc layer has been removed, strong local exhaust or general ventilation must be maintained during welding as a precaution to remove any trace amounts of zinc vapor or other fumes that may be generated.
Personal Protective Equipment: Welders must wear respirators suitable for protecting against metal fumes (such as P100 level dust masks) and standard welding protective clothing.
Welding Process Selection:
Reducing the heat input slightly can help reduce zinc vapor generation (although the zinc layer has been removed, it is still advisable to do so).
Using a short arc facilitates gas escape.