How strong should a weld be?

[jweaver]

I have been doing a bit of practice today and one of my tests was to take some 30x3mm flat bar.. Cut it, join it with weld and then test the strength.

I left a couple of mm's gap between the 2 pieces and welded on one side..

When I turned it over there wasn't 100% penetration, but the weld looked sound..

I put the bar in a vice and bent it back and forth aggressively, and the entire bar bent, rather than the weld.. But when I moved the bar down in the vice so that the weld was just above the jaws and then bent the bar, I finally managed to "rip" the weld.

I then did another test by welding BOTH sides and the joint was perfectly sound.

Is this what you would expect of welding on side of a bar? Or do I still need to work on my technique.

I hope that you can follow my question.. But if needs be, I will repeat the test and post a few pictures.

I look forwards to your thoughts

Jon

 

[Wozzaaah]

When the weld broke was the edge of the metal jagged or was it back to a clean, straight edge?

 

[jweaver]

When the weld broke was the edge of the metal jagged or was it back to a clean, straight edge?

Jagged.. Hang on.. If its worth asking this question. Its worth doing it properly and giving all the details.. Let me go and repeat the tests.. Hang on.

 

[Wozzaaah]

If it's jagged it means that the parent metal failed, not the weld, in which case....it was most likely a decent weld.
If you'd said the edge was as it was when you cut it that would have indicated lack of fusion/penetration.

 

[Welderpaul]

The weld should be as strong as the parent metal.
Welding one side is not good as it will readily snap if bent in the opposite direction to the weld.

If you are welding two bar ends together you should be able to bend it back on its self (i.e form a U shape, and the weld should not snap)

 

[jweaver]

I just repeated the test and this time the joint was quite strong.. I had to bend the bar back forth at the joint 5 or 6 times before it failed.. and when it did it split with a very jagged edge.

 

[jweaver]

And here is one of the earlier failures.. Cleaner but still jagged.

I guess I could turn up the current slightly to get even more penetration? But I guess the right way is to weld it from both sides. Thats what I did in another test and the weld survived the most aggressive bending.

Jon

 

[carlos420uk]

I'm no pro, but generally for a weld like this I would bevel the edges to be joined like so \ / if I wanted 100% penetration in one go. .
A 37.5-45* bevel with a knife edge should do I'd expect.

Try a few different test joints first, with a small gap, and then without to find out what works best for you and the particular joint.

Setting up in this way will limit the amount of metal that the weld has to burn through, and therefore should get you much better penetration on the backside, which is the main reason it breaks when you bend it back on itself.

You could also try linishing and polishing the weld faces back to flush with the base metal before you bend the coupon in the vice, any defects should be easier to spot then.

How much juice does your welder have?

 

[jweaver]

I'm no pro, but generally for a weld like this I would bevel the edges to be joined like so \ / if I wanted 100% penetration in one go. .
A 37.5-45* bevel with a knife edge should do I'd expect.

Try a few different test joints first, with a small gap, and then without to find out what works best for you and the particular joint.

Setting up in this way will limit the amount of metal that the weld has to burn through, and therefore should get you much better penetration on the backside, which is the main reason it breaks when you bend it back on itself.

You could also try linishing and polishing the weld faces back to flush with the base metal before you bend the coupon in the vice, any defects should be easier to spot then.

How much juice does your welder have?

If it were a weld where strength/finish was important, i would certainly have bevelled the edges, welded both sides and ground back.. I was deliberatly trying to be as crude as possible just to see how strong a weld was with limited preperation.

But what you say is taken on board and my next plan is to do some "quality" welds to see how close to the original bar i can make it (ie whether i can grimd it back and make the weld invisible).

My welder is 160A and this was on setting 3 which is recommended for 3mm (the material i was welding was 2.5mm). I guess i could take it to max power to see if i get any more penetration?

 

[hotrodder]

http://www.mig-welding.co.uk/wiki/Weld_Testing particularly the bends and nick break, the former can be improvised with a vice and something suitable to hammer the test coupon over

Those are weld failures due to LOF (lack of fusion) from incomplete penetration. As said can weld from both sides or for single sided welds change the set up so you get complete joint penetration... settings/technique, a bigger gap, veeing things etc depending on preference and material thickness (3mm isn't something i'd bother prepping, just gap it while something thicker, say 8mm, would be prepped and gapped for a single sided butt)

 

[earthman]

I remember once seeing a test on the TV, where a lorry bag of sand was picked up by a forklift via a 1" welded bracket, they said that was a good test, a good 1" weld should be able to take 1 ton of weight.

 

[dan.taylor.1]

Yeah I'm sure i got told 1/2" of weld on a 1/4" to 1/4" join would hold 1/2 a ton, so yours is effectively double, not sure how much id trust it sat under a dumpy bag though!

Dan

 

[chevy2]

Weld strength isn't everything,
Design, thickness and placement of material all plays a vital part in overall strength,

But of course thats all in vain with cr*p welding