TWI's in-depth knowledge can quickly throw light on the root cause of manufacturing problems.
A manufacturer of galvanized steel window frames used flash butt welding to produce the mitre joints at the corners of the frames. The welds were then ground flush to remove the flash and upset metal. To meet their heavy production schedule, the company then employed a sub-contractor to fabricate a sample window frame by MIG welding. After welding, the weld metal was ground flush with the frame surface and the sample considered to be acceptable.
However, when the MIG welded frame was hot dipped galvanized, the appearance of the zinc coating was dull grey colour - the rest of the frame was shiny in comparison with the typical spangle appearance of a galvanised finished product. Moreover, by running a finger over the welded metal joints it was evident that there was an excess, suggesting that the welds had not been ground flush before galvanizing.
A repeat sample where care was taken to ensure that the welds were ground effectively produced exactly the same result, and it was clear that the MIG welded frames would not be acceptable. The manufacturer approached TWI which was able to shed light on the subject.
A hot dipped galvanized coating consists of two parts, one an alloy of zinc and iron in contact with the steel base metal formed by reaction between the zinc and the steel, and the other an outer layer of unalloyed zinc. In low silicon steels containing less than about 0.12%Si the alloy layer is thinner than the pure zinc outer layer, whereas in higher silicon steels the coating may be 100% alloy.
The relative thickness of each layer will depend on a number of factors as well as the silicon content, and when the coating is composed entirely of zinc-iron alloy it is commonly referred to as a 'grey' coating. Grey coatings are claimed to have corrosion resistance comparable with that of two layer coatings of equivalent thickness.
The MIG welds in question, after dilution by parent metal, would have a silicon content of 0.3% or more which would account for the grey appearance of the zinc coated welds. The excess could also be caused by the high silicon content because the rate of reaction between zinc and iron depends on silicon content as well as a number of other factors.
In low silicon steels the alloy layer grows rapidly during the first two minutes in the galvanizing bath but the rate then falls. In high silicon steels or weld metal the alloy layer may continue to grow in thickness linearly with time so that the coating thickness will depend on the time that the work is in the bath. Therefore, the zinc coating on the weld may be thicker than that on the parent metal.
Also on thick sections that cool slowly when withdrawn from the bath, the alloy layer continues to grow and may diffuse to the surface in patches. Hot dip galvanizing of welded components is normally carried out without any problems but in cases where appearance is critical, there should be co-operation between the customer, fabricator and galvanizer with the following check list in mind:
- Composition of parent metal.
- Selection of consumables - low silicon rutile electrodes are available but MIG welding wire having less than 0.2%Si is rare.
- Edge preparation - square edge butt joints increase dilution of weld metal which may lower silicon content.
- Surface roughness of ground welds - the smoother the surface, the lower the reaction rate between zinc and iron because of smaller surface area.
- Temperature of galvanizing bath - high temperatures increase reaction rate.
- Time of immersion - should not be prolonged.