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Welding butt joints... crack in weld after grindin


Jedi Knight
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Hi all,

I am replacing the lower part of the outer sills on my TR4A. I bought new sills from TRF. Using a contour gauge I marked a line on the old and on the new, and cut the parts to match.

I then butt-welded the new piece in place. And it looks fantastic, except...

When I grind down the weld to be flush with the metal, some areas have a crack in the joint. I have been grinding very carefully not to grind the surrounding metal, using an air die grinder plus a 36 (?) grit sanding disk. In parts I use a cut-off wheel and carefully hit the top of the weld.

What did I do wrong? I'm thinking either I did not get sufficient penetration of the weld through the metal, or the welded joint is brittle and I have cracket it during grinding, or ...?

I have access to the back of the sill and will weld the joint from the back if needed. But then I will have a bunch of "pockets" along the welded joint for water to collect on... I'm open to suggestions.

TD Skip is carrying out a similar repair on the Triumph forum; https://www.britishcarforum.com/bcforum/ubbthreads.php/topics/563439#Post563439


Jedi Knight
It would be pertinent to know, what weld process are you using? IE. Mig, Tig, Stick Or oxy acetylene.
The next question is, IF Mig what type wire IE. Solid core or Flux core?
Solid core wires have a tenancy to be more brittle Than the Flux cored ones.
The next thing is IF solid core what type of gas and how much C.F.H.? Should be 75/25 Argon/co2 @ around 20 C.F.H.
BOTH sides of the material to be welded should be free of paint,rust,scale, or any other foreign substances if at all possible.
For the best results IF at all possible use a purge gas {75/25 mix} on the back side of the weld @ around 5 - 10 C.F.M.
Make sure that your replacement parts are NOT Stainless Steel! Stainless will NOT weld to Mild Steel with Mild Steel weld wire without cracking!IF the replacement parts are S.S. you will need to use S.S. welding wire.
A backup strip {sometimes called a chill strip} tacked to the back side of the weld can be advantageous In allowing for A slightly hotter heat range and better penetration without fear of burn through.
Due to the nature of stress related issues caused by welding, any weld will crack if ground thin enough.
So the key issues here are
1. Penetration 2. good clean weld {no porosity no inclusions accomplished by cleanliness and adequate gas coverage.} 3. The third thing is, when grinding your weld { especially on thin sheet metal} The heat generated by the grinding process will create shrinkage in the surrounding metal which in turn creates even more stress in the weld which in turn will cause the thin areas to crack.
I hope this information gives you some insight as to what you need to do to accomplish the task at hand.


Jedi Knight
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Hi Kerry,

Thanks for the quick reply! I am slow with my response because my internet at home has been on the funk for the past week (believe it is the wireless router). I am sneaking on to the BCF today while at work (shhh...).

Anyhow, I am using MIG, solid core, 75/25 CO2/Argon shielding gas; Hobart 140 welder. Good comments... I'd have to think about the purge gas on the backside (how to set that up). I will try, first simply welding on the backside as I can get to the area, and secondly, a strip on the backside.

In general, I am having trouble with balancing sufficient heat penetration and laying down excessive weld. Sometimes I will get a what appears to me a perfect shallow hill with a dimple in the middle. Genearlly I get a larger "blob"... requiring a lot of grinding.

Practice, practice, practice!! Thanks for your suggestions, to this post and previous questions of mine!


Jedi Knight
You can make a gas backup cover out of a piece of tin {tin can Or other thin tin type material}. Tape it on with Aluminum tape { make it wide enough that the heat from the weld doesnt effect the glue on the tape} <span style="font-weight: bold">Let it purge for a few minutes before making your weld.</span> A Y connector and valve on your gas line {Up by the regulator} {can be purchased at your local weld supply shop} so you can run continuous gas coverage on the backup cover. {Surgical tubing or other suitable rubber hosing from your Y and valve to the gas backup cover}. The C.F.M. you run in the backup cover really does`t matter {I usually use 5 to 10 C.F.M} an educated guess is close enough as long as there is gas inside of the cover with enough flow to ensure that air does not enter.
<span style="font-weight: bold">Hint: add the gas from the top side of the backup cover, leave a small purge hole on the bottom of the cover { argon is heavier than air} other than that make the cover as air tight as possible { tape all gaps cracks and stuff shut with the aluminum tape}</span>
If you decide to try the chill strip on the back side of the weld no gas is needed But can be used as added insurance if desired. However .... you want to attach the chill strip as tightly to the parent material as possible {someone to hold it tight while you tack it on will help}
A word about dressing the weld .... as I mentioned grinding the weld will generate heat, this residual heat will cause the surrounding metal to try and shrink which in turn will add additional stress to the weld area. Go slow when grinding don`t be too aggressive! Perhaps a helper with a spray bottle of water to cool the area being ground will help. The trick is to minimize the heat as much as possible.
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