Rewelding bad welds?

admin

I have a friend that is building up an old car frame. He boxed the frame, and put various brackets for crossmembers, motor mounts, and structural components. He did a pretty questionable job on the welding. He mig'd it, and it's pretty lumpy and penetration is questionable. I am wondering the feasability of grinding all his welds flush and me going over with a good tig bead. These are mostly fillet welds, but a few butt and lap welds. I know they would look cleaner and have proper penetration after I welded them, but what about strengh? What are the shortcomings of welding something twice, brittle? I know the "right" way would be to cut it all out and do it again with the tig, but he wasn't having it.
Reply:Just a few questions to get a better picture here...Was weldor skill the problem? Were the "bad" welds mostly out-of-position (vertical, overhead, etc.)?  If not, why do you think his welds ended up not being adequate?  Was there a rust or contamination problem?  Are you using just an old I-beam frame or is it a unibody or what?  Mig is used an awful lot in building and rebuilding cars and if done right, I would think the process is appropriate.  Unless you have some specific reason for using tig, I would thing re-migging would be a better option.Smithboy...if it ain't broke, you ain't tryin'.
Reply:I was really wondering if I would be weakening the joint by grinding down the joints and laying down a better weld whatever the process. Especially the fillet welds, they really look bad. I just prefer a tig bead, I could use his mig, but this is an old car and some of these weld will be exposed. His fit up was good, and the base metals look to be pretty clean, but the mig bead was irregular and the HAZ was inconsistant. Appearance and safety are my concerns. I looked though all the welding books I have and none of them discussed rewelding an existing joint. I have stick welded with mutlple passes so I would assume grinding down and rewelding would be similar, since you are hitting the same area more than once, which is what I am looking to find out. Thanks!
Reply:No you wont be making an inferiorr weld by welding over it.  However, you will need to clean it well.  Id grind it down, and then gouge a lil groove before welding.  As smith pointed out if there was containments and such perhaps these welds should be ground out more thoroughly.  Of course I will admit most welds on cars and trucks and things like that are often p*ss poor quality.  But I agree if you can its better to make the best weld you can. Rewelding it wont bother though.  Good luckIF it Catches...Let it Burn
Reply:For the most part, on auto frames, it's a balancing act between making a good solid weld and keeping the HAZ minimized. Once you make a bad pass that all sort gets thrown out the window. If you decide it's a weld you just can't live with or trust then you have no choice but to go back and do it over. Touching up a few wouldn't bother me, but if it got to where I thought I had to go back in a redo one I wouldn't waste the heat on capping over it. I'd gouge it out deep and treat it like a first pass.I do as many bad welds as anyone and it's a rare case where I grind one out that isn't ugly to the bone. I grind untill I see good stuff.
Reply:the fact of the matter is that its an old frameoxidation rises to the top when welding to itso.... more grinding and re-welding is neededas far as going over it with a tig torch?big waste of time and moneyuse a gas mig or a stick weldermore cost effective in the long run ...zap!
Reply:Gee, most of the welding I do consists of rewelding a bad weld I did 10 minutes previously.I have to stop being so vigorous with the grinder so I don't expose the bad ones.
Reply:Thanks for all the advice everyone. I grinded them all down last night and rewelded. I first ground them flush, then put a groove down the joint line and finished it up with the tig. The HAZ is nice and even now and the welds are pretty, strong and ready for paint. As for tig being not cost effective I would agree if this was a nisson, but this is an old 30's ford. Old cars are already a money pit, and a hobby, so money is not the most important factor here, craftsmenship is. The price difference is pretty minimal here too.
Reply:I'm like Scott, re-welding bad welds that I just did. I had to look up what HAZ meant though, being the back-yard, self-taught, hobby welder that I am.  Heat Affected Zone for those like me who didn't want to ask.I'm always learning something new on this site!Sure, I can fix it... I got a welder!!!
Reply:I had to look up what HAZ meant though, being the back-yard, self-taught, hobby welder that I am