Fume extraction must be addressed, and a post-weld coating may be needed
Q: I have a fabrication shop and have been having difficulty welding some galvanized steel. I have been told to use a stainless steel welding electrode and then I would not have to paint after welding. What is your opinion?
A: When welding galvanized DO NOT use an austenitic stainless steel welding electrode.
The zinc in the galvanized steel coating will be swept to the center of the weld during solidification resulting in a centerline crack. The amount of cracking will be dependent on the thickness of the coating and how much is brought into the weld.
Depending on the strength requirement of the weldment, the best choice would be to use a silicon bronze (Er-CuSi-A) electrode.
The zinc is soluble in the copper-based material, and should yield a weld with minimal porosity. The minimum tensile strength will be 50 ksi according to AWS A5.7.
If slightly higher strength is required, an aluminum bronze (Er-CuAl-A2) electrode may be used. That would yield a minimum strength of 60 ksi.
If you are going to be using the gas metal arc welding process you should use 100 percent argon shielding gas with either wire.
Another benefit to using these electrodes is that on thinner materials there is a lower burn-though tendency that is due mainly to the differential in melting temperatures between the steel and the copper based fillers.
Mild steel fillers also may be used, such as an ER70S-3 filler metal for gas metal arc welding. Elevated spatter levels along with increased tendencies for porosity generally are experienced.
Weld joint fit-up that is looser, yielding an alternate path for the vapor that is generated from the zinc can be effective in reducing the tendency for porosity. Additionally, welding technique and welding procedure can be effective in reducing porosity tendencies by using the arc heat to “burn” off the zinc ahead of the weld.
The region adjacent to the weld toes will have exposed surfaces because the arc heating has removed the zinc layer, and may be prone to corrosion regardless of filler metal selection.
A weld made with the silicon bronze or aluminum bronze will be cathodic relative to the mild steel, resulting in accelerated corrosion locally along the weld toes in a moist environment. Depending on the environment, the weldment most likely will require a post weld coating such as a paint containing zinc.
When welding galvanized, it is important to use proper fume extraction.
A good article on the internet about welding galvanized, post weld coatings and welding fumes can be found at the following website: www.sperkoengineering.com/html/articles/WeldingGalvanized.pdf.
Additionally, there is excellent information on welding safety at Lincoln Electric's website at the following link: www.lincolnelectric.com/community/safety.
This column is sponsored by Penton and the Lincoln Electric Co., Cleveland. Dave Barton is a senior welding engineer in the Application Engineering Group of The Lincoln Electric Co. He oversees welding procedure development for both new technology and existing products, performs failure analyses for customers, and serves as a consultant on welding application problems. Barton has been with Lincoln Electric for 21 years. Send your questions for Mr. Barton in care of WM by e-mail to: firstname.lastname@example.org.
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