Thursday, February 28, 2013

Welding on Zinc coated plates

For zinc coated plates, we should always use a low silicone type consumable for welding. Some manufacturer have wires that is especially built for welding on these primed plates, however they are not too popular. Safro and Oerlikon has got the product used for welding of zinc surfaces, they are the Safdual Zn, or Carbofil Galva.

Or in anycase if you have difficulties looking for these wires, you could opt for AWS: 5.18:ER70S-3 designated wires. With this general wires, expect porosity in the welds due to vaporisation of zinc which forms gas bubles in the weld.

Sunday, February 24, 2013

Discolourtion of HAZ

I was working on some PLC tank welding of 304 stainless steel when i found this. A welder asked me why is there traces of blue in their welds?.

As you could see from the diagram below, blue HAZ means the thickness of the oxide layer measures at 170nm (1 x 10^-6 milimeter). At this scale its thickness enough to get a fail from the welding inspector.






Temperature
Colour
Oxide thickness



500
Jaune
50nm
750
Violet
100nm
900
Bleu
170nm
1050
jaune-vert
280nm



The reason why discolouration happens in a weld is because of oxidation that occurs in HAZ.Oxygen as much as 50ppm could result in loss of corrosion resistance. While in Titanium alloys, less than 20ppm is desirable.

If ever oxide thickness exceeds 50nm, recheck on the purging gas as there might have been an oxygen contamination either in the gas supply, or there's a leak in the purging system.