Is it just me?

[Kent]

Set up this morning to weld some angle cleats onto a U/B with MIG ground the scale off everything nicely prepped up in front of my 250 amp MIG in the shop. Then dragged it outside and did it with A Low Hy and MMA! Is it just me I have done a stack of structural MIG even lift gear and such stuff, is it confidence or preference that makes me pull the switch to stick ? Never seen a tidy stick weld fail when the correct rod was used for purpose but I have only seen a couple of fails on Tidy MIG work.

Do other do this sort of swap from one process to another for some strange hidden voodoo?

 

[Pete.]

Dunno but I can relate to why you might do it on a loaded part. I have seen the lifting lug pull off a bottle cage, there was a perfect-looking weld stuck on the lug and the only evidence on the cage that there had ever been a weld was a patch of missing red oxide. Absolutely zero penetration yet that lug ( a loop of 1/2" rod really) had been on there for years.

 

[Windy Miller]

Very strange that you should mention this, as I've been thinking the same thing recently. I have a load of welding to do on my old box trailer soon, and I've been worrying about whether to mig or stick weld it. Just like you, I've never seen a tidy looking stick weld let go, but it seems that a nice looking mig weld wont necessarily mean that it's a sound one.

Years ago I'd have automatically grabbed the stick set as my mig was under powered for anything over 3mm, but now I've got a nice 200A ESAB Inverter, I have the choice to user either machine, but I'm still leaning toward the stick set...

 

[Kent]

my MIG is ample big enough though at 250 amp and runs on 1mm

 

[norlander]

Same here, I just cannot have confidence in a heavy duty MIG weld over the same thing done with MMA-can't explain it, probably unfounded, but its just me gut feeling.

I recall I remarked on the use of MIG for structural on the EN 1090 thread I think it was, I have since visited a big fab shop and they had some mega MIGs in there, but their main work seemed to massive gates and related stuff, not actual FoS.
One of the gents on here was doing a heavy duty job with MIG and ran some tests on test pieces and was not happy, I think he ended up buying in a bigger MIG set iirc.

So I reckon your gut feeling is not entirely unfounded.
I wonder if it comes from learning and doing MMA before MIG, with MMA, you go a bit slower, so you get more time watching the pen and fusion going on?, and you can run hotter a bit easier-in some cases- I would imagine the pro MIGs have much finer control features and settings than my little set.

 

[matt1978]

Same here, I just cannot have confidence in a heavy duty MIG weld over the same thing done with MMA-can't explain it, probably unfounded, but its just me gut feeling.

A bad weld is a bad weld regardless of the process, the difference being that its "easier" to make a bad mig weld appear better than it actually is.

Mig welding is the process of choice for pretty much all of the production of structural fabrications, JCB/Yellow Good manufacture, Rock Crushing machines, Bridges, Buildings, Boat/ship building, oil rigs, drilling equipment and the list goes on. If its done correctly with the correct parameters then there is no reason why it can't be as strong as any other process with the added benefit that its much faster and with less downtime.

 

[Kent]

you know perhaps that speed is the unspoken handicap? if it goes wrong it goes wrong quickly. Of course a good MIG weld is good and every bit as tough as MMA, its just I can look at what is happening more and see faults easier with stick. Thing is if there was 30 of them to do I would have just MIGed them with confidence, tomorrow morn I have 24 hangers to do out of 8mm and all them will get the MIG no question, one off at a critical joint though and I picked up some 7018 and went had at it.
Had the is stick stronger discussion about 25 yrs back over our chips one lunch with the other welders I worked with then only one thought it was with the caveat that it wasn't done by noddy, the others all thought equal but only if the mig was in spray transfer. These days having seen good looking failures done by good welders I still think equal and it don't have to be in spray either but looks can deceive with MIG.
I was doing some set up and tests on MIG the other week or so and I couldn't break some even with serious over abuse and a very no scientific lump hammer

 

[matt1978]

Another reason I guess why MMA could be considered "stronger" is for the fact that you can add further alloying elements to the weld pool via the make up of the Flux coating so the UTS and Charpys can be better on some materials compared to "bare" mig wire

 

[Bill Edwards]

We all know that most production things are done with MIG and that it's just as strong and capable as stick. As I recall it's also a low hydrogen process, so what's not to like...?

But kent isn't alone here, for something heavy and/or structural I think that with a 300a MIG sat to one side or a stick welder with 7018 I'd feel more confident with the stick setup.

I think that the way to have confidence with the MIG is probably to do some test runs and etch them as was done on here for some heavy work, then find out exactly what you're getting.

 

[Paul G]

I have the same memory as Bill.
I too am under the impression that MIG is a low hydrogen process.

 

[Kent]

is the Low Hy really even a factor in steelwork up to 8mm thick? I cant remember a 6013 failing from hydrogen faults on 8mm. likewise the extra 10 on the yield strength (for most things as it must exceed the parent metal in some cases).

Just to confuse things I just did 24 joist hangers in 8mm black, all MIGed (not spray either)

 

[Bill Edwards]

Probably not, and I think that the hydrogen leaves the weld anyway over time (can take up to about 48 hours).

But what I do know is I prefer the way 7018 rods weld, I gain a little reassurance from the weld metal being stronger and just from trying to break tack welds I have to say that they seem pretty strong, and they are what I feel most confident with.

 

[Paul G]

I think that the hydrogen leaves the weld anyway over time

Indeed it does.

Lets imagine a brick wall. No cement, just dry bricks.
All the bricks are whole and fit neatly together.
This is like the crystal structure of metal.


Now lets imagine that the bricks have broken corners, gaps between them.
If this was a weld crystal structure then these voids would be the hydrogen.


Over time the hydrogen seeps out, but the voids are still there. Obviously a weakness.



A workshop test is to totaly immerse the suspect weld in a pan filled with paraffin.
Over several days, I think it takes days, tiny bubbles appear on the weld surface,
like CO2 forms on the side of a lemonade glass, only in this case the bubbles on
the weld are of hydrogen.


Now where's that hook to hang up my anorak?
Paul G

 

[J.B. Welding]

Gotta say I love spraying mig at 10mm+ stock, the silence of it and the ability to see the fusion and filling back all happening right there in front of you in a pool the size of a 50p. When laying on dip runs there is always the danger of 'the solder effect' as the pool is less visible under the running bead. I akin spraying mig at high amps to a tig welding process, which is the process I go to where strength is needed within a small area.
Having welded on towing eyes to dump trailers and the like with the old 500amp migatronic set I used as an apprentice, it really showed me there is no substitute for raw power when it comes to migging up. I just really go to arc now when its a special rod or outside, or there's just nothing else handy.

 

[Rig Pig]

Gotta say I love spraying mig at 10mm+ stock, the silence of it and the ability to see the fusion and filling back all happening right there in front of you in a pool the size of a 50p. When laying on dip runs there is always the danger of 'the solder effect' as the pool is less visible under the running bead. I akin spraying mig at high amps to a tig welding process, which is the process I go to where strength is needed within a small area.
Having welded on towing eyes to dump trailers and the like with the old 500amp migatronic set I used as an apprentice, it really showed me there is no substitute for raw power when it comes to migging up. I just really go to arc now when its a special rod or outside, or there's just nothing else handy.

I think that sums it up well, most of us have relatively puny migs that only drip and are amazingly good at producing cold welds on thicker material that look like nice welds!

 

[Bill Edwards]

Indeed it does.

Lets imagine a brick wall. No cement, just dry bricks.
All the bricks are whole and fit neatly together.
This is like the crystal structure of metal.


Now lets imagine that the bricks have broken corners, gaps between them.
If this was a weld crystal structure then these voids would be the hydrogen.


Over time the hydrogen seeps out, but the voids are still there. Obviously a weakness.

So presumably this means that although the hydrogen will disappear having less in the first place is going to result in a stronger weld, because the weld metal will be more 'solid'?

 

[Kent]

Probably not, and I think that the hydrogen leaves the weld anyway over time (can take up to about 48 hours).

But what I do know is I prefer the way 7018 rods weld, I gain a little reassurance from the weld metal being stronger and just from trying to break tack welds I have to say that they seem pretty strong, and they are what I feel most confident with.

yes I see that but look at the yield strength of the parent metals in so many cases, is it relevant? Theory really does 70 hold up past the strength of the parent? if so how is it relevant ?

 

[Bill Edwards]

Perhaps in that case the only advantage might be that if there's a fault in the weld and it isn't as strong as it should be, it still has a better chance of holding as it's made of stronger stuff?

 

[Paul G]

OK. Anorak back on.

We may not like to admit it but weld metal is seldom as strong as the parent plate.
The small equal sized uniform structure of the base metal is so much stronger than the larger crystals of the inner weld,
and even more so than the columnar crystals of the cap.
We make up for this loss in strength by using thicker plate with correspondingly bigger welds, or by using a rod or wire
that gives a better quality weld deposit, or often a bit of both.

The diagram below is from A C Davies, the welders bible, and sorry if it's a bit yellow round the edges, but it's not done
bad for the £2/11/6 that it cost me.

It's worth pointing out that grain structure reforms when subjected to another weld being layer on top so often a multi run
weld will have a more desirable structure than a single huge run. I'm thinking in particular of JB's remark of a 50p size
pool in post 14. In reality I think he was probably exaggerating.


Cheers
Paul G