Welding through Paint and R/P 4

[dbooker630]

I have been asked by the management at my site to consider a revision to the process of welding ASTM A27 grade 70-40 cast brackets to SAE 1022 housings with high Mn. The current process requires a fillet weld using ESAB Dualshield R-70 Ultra E70T-2 wire, 100% CO2 @ 35-40 cfh gas, which works very well for us. However the housings are now coming from overseas with paint or R/P applied in the joint areas to prevent rust. We are washing or grinding as appropriate to ready the surface for welding otherwise porosity will be present in the welds.

Now the management at my site informed me that our sister plant is welding through paint, etc., and that they have no porosity issues at all. Of course without any extra surface prep their productivity is better than ours so it's getting attention. They are using ESAB Dualshield T-63, 100% CO2 @ 60-75 cfh to weld the same materials.

I was always taught that you do not weld through paint and this just flies in my face. Even ESAB does not recommend any change, stating that I already have their superior product. I do not want to make this change and I have some support for my stand but I am very interested in my fellow metallurgists and engineers positions on the concept of welding through paint. Thanks!

 

[HgTX]

I haven't heard anything good about welding through paint. You could introduce hydrogen into your weld, not to mention lord knows what kind of toxic fumes could be created with an unknown coating.

Do you have any say in what paint your supplier uses? There are some supposedly weldable primers out there.

I haven't seen the following sources myself but they were brought up on an SSPC discussion list (I'm copying & pasting from old listserv email, with no permission whatsoever):

* SSPC-Paint 30, "Weld-through Inorganic Zinc Primer"
* Munger (Corrosion Protection by Protective Coatings, Second Edition) discusses welding inorganic zinc coated steel in detail (p 156).
* Nippe Ceramo (a kind of paint, I assume)
* discussion of "Weldable Shop Primer" in one of the SSPC red books, probably Good Painting Practice but possibly one of the specifications: 'for the most part it is up to the ship yards and fabricators to "self Approve" a coating as "weldable" based on supportive data received by the coating manufacturer, largely based on burn spread rates (how far weld burn-back spreads to either side of the weld), how thick the coating may be applied as a weldable primer, etc.'

If you can find an archive for the SSPC Coatings Talk list there might be more info there.

Hg

 

[metengr]

In general, it is always good engineering practice to have clean surfaces prior to welding to improve your chances in making a sound weld with no porosity or lack of fusion defects. However, under certain applications this practice can be waived - for example welding over galvanized carbon steel. I for one am more conservative, and would stipulate local removal of the zinc for welder health reasons, and to assure a quality weld deposit. The other approach is to use a more forgiving weld rod - like an E6010 or E6015 weld rod to essentially burn thru the zinc, and deposit a sound weld. This does work, provided the welder has adequate protection from harmful fumes, and has been used routinely.

The issue of welding over a coating needs to be carefully evaluated to assure consistent weld quality with the base material, and to assure the welders are protected from any harmful fumes that could be admitted into the air from the welding arc. If your sister company has gone thru and evaluated this risk, I see no reason why you can't use it at your facility, as well.

 

[HgTX]

And if your sister company has evaluated the risk, make sure that you're dealing with the same paint they are. Not all coatings are alike.

Hg

 

[JoeTank]

Just for fun, take a look at API-650, the design standard for petroleum storage tanks. Welding through a coating is permissible; however, the WPS/PQR treats the coating brand, formulation and thickness as an essential variable. ASME IX is used for welding qualifications.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com