Chris,
We have had a lot of galvanizing done for us over the years. Cracking has not been a major problem for us. When we do have troubles it's usually something like a long, thin angle welded to the flange of a thick beam, or similar with a high restraint situation.
I can't recall ever having a beam crack in a cope area except where someone had nicked into the radius and then improperly repair welded it.
Some people believe liquid metal embrittlement (LME) is a problem with galvanizing but I am not totally convinced due to a high success rate. Lots of pieces are dipped in molten zinc everyday with no problems. Now if some says LME is the reason for cracking in a high restraint situation, I can't argue.
We had some beams crack severely through the flange and several feet of web (one long crack, on 10 or 12 different beams. I think they were
W21 x 50s or W18 x 40s, about 30 feet long and rolled the hard way (camber) to about a 300 foot radius. The metal had been stretched a lot in those.
Another time we had 3 feet long 12 x 12 x 1/4 square tube, blow open so they resembled a plugged up shotgun barrel that had been fired. The odd part is the tubes were plain material and completely open, no restrictions in the ends whatsoever! Replacement pieces had no problems.
My standard advice regarding galvanizing is to talk with your galvanizer and let them know what you are making. I'm sure they have seen more than most of us and have worked out a good list of solutions and precautions to follow.
Chet Guilford
I have only just discovered this discussion, and coincidentally I have just mounted a 'knowledge summary' on the subject on the TWI website (Join IT) http://www.twi.co.uk/j32k/protected/pdfs/ksrjp003.pdf (Don't worry about registration if you are not already a user - it's free, and the article of interest is available to all, free, once you have registered.)
Liquid Metal Assisted Cracking (LMAC) is a real phenomenon, and has been around for years - I have a reference dating back to the 1950s. It is not frequent, but can be devastating when it occurs. The usual reaction, as I have detected in some of the correspondence here, is "are you sure it's not fabrication cracking?" or at least "are you sure there weren't fabrication cracks there, which caused propagation during galvanising?". I'm sure that poor details do at least encourage cracking, and we mustn't suddenly start blaming everything on galvanising either. Nevertheless, the Industry must recognise this as a cracking mechanism in its own right, which doesn't always require a defective weld or cut to start it. I've seen it on cold bent bar, with no weld or stress concentration present in the cracked region, and from well drilled holes.
I wish I could give definitive advice on how to avoid LMAC. Currently, however, there is not a sufficient understanding of all the contributory factors. (TWI has examined several occurrences of LMAC at first hand, and has also carried out some limited research into the subject. If anyone would like to co-operate in some further research into this, please let me know, as we are planning to do some more work in this area.) Nevertheless, we do know some factors which apparently increase the risk, and please follow the link above for my current understanding in more detail. However, one thing I would strongly advise is to inspect after galvanising. Most of the serious cases I have been involved with have only been discovered accidentally, during some other operation, in at least three cases after erection of major steel work.
I hope this helps.
Richard Pargeter
Consultant, Ferritic Steels and Sour Service,
Metallurgy, Corrosion, Arcs & Surfacing Technology Group
TWI, Granta Park, Great Abington, Cambridge, CB1 6AL, UK
richard.pargeter@twi.co.uk
+44(0)1223 891162 Ext.2358(phone) 894717fax)
TWI website: http://www.twi.co.uk/j32k/index.xtp