hydrogen bubbles

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I tend to be anal about preheat.  One of the things that convinced me was actually seeing hydrogen bubbles coming out of a weld bead.  In a laboratory situation they submerge a recent weld in glycerin and capture the hydrogen bubbles and measure the amount of hydrogen related to the type of process/electrode.    What I did was weld some short beads on some small pieces of steel that would fit in an aluminum pie plate.  I filled up the pie plate with compressor oil because it was the lightest and clearest oil I could find.  I then ran beads with 7018, and 6010.  One 6010 was soaked in water first before welding to increase the effect.  I then quench the samples enough to get the major heat out then allowed to air dry for a few seconds before putting them in the oil.   I left the strips for around five or ten minutes expecting some bubbles to float up.  I was disappointed until I noticed that my 6010 welds had fine peach fuzz on them and when I disturbed the surface I could see that the fuzz was actually very tiny bubbles clinging to the surface.  The point here is that this migration of hydrogen to the surface takes hours and if the metal temp is elevated more hydrogen will have a chance to escape.  Preheat in itself is not important.  What is important is that by preheating a weldment before welding the cooling rate is slower allowing the hydrogen time to escape.  It is an interesting shop experiment to do.  I could not see any bubbles off the 7018 weld.  I imagine there could be some escaping hydrogen but I could not see it with the naked eye.  Underbead cracking is a real thing and does not happen until hours or even days after the welding operation.  The solution is preheat.... and of course Low Hydrogen procedures.   Rule of thumb in local structural shops is that anything over 3/4 thick should be preheated!  Anything over 1 inch MUST be preheated.
Reply:That's a great experiment, thanks for sharing.  Very cool that you saw results with your welds.  This would be a great demonstration for a training course.  I think there were several articles over the years, probably AWS Welding Journal, that used similar techniques to measure hydrogen pickup from different processes.
Reply:Writing that down in my ol' what and what not to do book.Thanks for the cool info.
Reply:Here's a pretty good article on hydrogen cracking.http://files.aws.org/wj/supplement/A...1/ARTICLE1.pdf