We've discussed the welding of hot dipped galvanized steels before. The argument for and against are numerous.
I was teaching a course at the Johnson Space Center in Houston a couple of weeks ago and the subject of liquid metal embrittlement came up during the presentation on metallurgy. I know my limitations and liquid metal embrittlement is not a subject I know well. One of the attendees, a Welding Engineer, yes my friends there are real ones out there, was kind enough to get together with their Metallurgist and copy some literature on the subject.
It was interesting reading to better understand the mechanics of the problem and what can cause it to occur. I was especially interested to see that zinc is listed as a "known" embrittling agent for a number of metals including aluminum, nickel, steels (all kinds), and titanium. An article by William R. Warke titled "Liquid Metal and Solid Metal Induced Embrittlement" included an extensive listing of related articles for additional information on the subject.
Best regards - Al
A more likely contributor is that the zinc bath has other elements in it such as tin that contribute to embrittlement. In a nutshell, the tin has a low melting point and gets into very small pre-existing cracks.
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What kind of weld process are we talking about? If we are to claim that resistance welding is included then the entire auto industry is suspect. They say it is not a problem with spot welding because the coating melts first and is displaced from the weld fusion zone. If you look at the nuggets you can even see what is called the "annular ring" of zinc coating surrounding every weld. If the zinc is displaced from the weld it is a non-issue. Since the weld is going to burn off the zinc coating, maybe the best thing to do for welds of concern is to just strip it off if that is possible and cost effective.
Tin was also listed as one of the embrittling elements.
As is the case with many things, it must require a confluence of several conditions for cracking to occur. Otherwise, how would hot dip galvanizing be used so successfully for so many years?
The subject of welding on HDG is always a good subject for discussion.
Best regards - Al
IMO, a clear understanding of the procedure for welding galvanized steel doesn't exist. I get many interpretations and quite honestly don't feel comfortable with any of them. We used to use NR-202 on Galvanized HSS in the field and it worked great, but as far as LH requirements I don't know. No matter how much you clean and grind heavy iron it still seems like the puddle gets contamination and in the vertical position undercut is always a problem, prompting multiple passes. As more Architects and engineers specify the use of galvo, there needs to be a better understanding of how to weld it. Just my $0.02
JohnJohn
Thanks for the insight. Thats what I have found to be the best method through trial and error and from input from other welders and inspectors. I will bookmark that link for future reference.
JohnJohn
By 803056 
Date 07-29-2009 17:46
Edited 07-29-2009 17:48
It's all a question of "What is good enough?"
It seems most of us are in agreement that a high level of weld integrity and high quality requires the removal of the HDG from the area to be welded. In those cases where a lower level of weld integrity is tolerated the welder may produce acceptable welds by burning off the zinc by using slow travel speeds. The decision to use that technique must recognize the weld is going to have a certain level of contamination that may or may not affect it structural integrity and mechanical properties.
I believe due consideration must be given to the fact that one welder's technique (when welding over HDG) will not provide the same properties as those welds made by a welder using a different technique. Thus, welding over HDG introduced the "crap shoot" factor, i.e., the results are not as predictable as those welds made where the HDG have been completely removed prior to welding.
Best regards - Al
