Multi pass mig welds

[BrokenBiker]

Right, this may sound like a daft question but it was asked to me earlier and I didn't know the answer.

Say, for instance, I have a 150 amp mig welder, but need to weld some 10mm steel with a fillet weld.

There is obviously no way that I am going to get full penetration, so how do I work out how many passes will be needed to make a joint as strong as a full penetration joint? Or is it not possible to get a joint as strong without actually getting full penetration?

 

[davidjohnperry]

Hmm this is interesting if that was me I'd v out the top plate one side run as hot a root as possible let it cool run a second on the toe of the weld covering half then run my third pass over the top of the root blending all 3 in together on a test piece then cut it down and test it there are real people who could tell if its truly possible but I cant see why not
Cheers
Perry

 

[Richard.]

Even by creating a single Or double V T joint your not going to get optimum strength from such low power. Penetration can easily be achieved by preps and even opening up a root Gap but fusion into the parent material is where your problems will lie. Side wall fusion will be very minimal as that thick cold plate sucks away your heat. Your more likely to get cold laps, LOF (as mentioned) and cold cracking in the HAZ. You need 250 amps realistically to weld a 10mm fillet.

 

[BrokenBiker]

The reason the question was asked was because a friend wants me to weld up a bottle jack press as he does a lot of wheel bearings and other small pressed things and liked the press I made myself, however I used a 3 phase 400 amp mig in work to weld the important bits up and used 13x75mm flat bar as a frame with some braced bolted in too. He has a Clarke 151 that he uses for the odd rust repair but isn't confident of anything serious with it so asked me to do the welding (he thought I was coded in structural mig for some reason...) and I pretty much said his welder wasn't up to it

 

[BrokenBiker]

just out of interest (as it's been a. While since I last welded anything that thick) what size mma electrode would be good for this joint, just a bog standard fillet on 10mm steel. I would usually have used a 3.2 with 3 passes but it's been a while and it's surprising how much you forget when you don't think about it for a few months

 

[Richard.]

A 3.2 with 1 pass either side will be perfect for it bb. No need to get carried away with multi runs on 10mm. It simply doesn't need a huge legged fillet. You'll get all the pen and fusion you need from 120amps on a 3.2. Much more suitable than a low powered mig.

 

[BrokenBiker]

Hmmmm maybe I was over welding before then. So realistically a 3.2mm rod at 120 amps will penetrate about 4-5mm then? I never really looked into it that much, I was mainly only making my own tools and welding up gas bottle burners so mostly just 4-5mm steel that only needed to be stuck together, except the press but I made sure I had strength in that with the works mig l

 

[Richard.]

Bb it won't penetrate 4-5 mm and adding multiple runs won't change that either . If it really must be a full pen T joint you'll still have to prep it. I would just weld it squre edged mma single hot pass both sides 3.2 rod. It will be super strong trust me.

 

[AdrianH]

Richard your post confused me somewhat, I think down to terminology could you please help me out?

You said a single Hot pass both sides, which suggests to me a slightly high amps single run both sides. But, and this is where I get the confusion, I thought a Hot pass was a second run over the initial root pass whilst the metal is still hot, it helps to draw the slag out of the weld joint and recrystallise the weld helping to prevent cracking.

So to me this would be a tack up both sides, a run on one side to get in the root of the joint, clean off with angle grinder/wire cup and whilst still hot metal up the current a bit and weld on top to as above. Repeat other side.

Is this what you were meaning, or, am I getting it wrong again.

Adrian

 

[Richard.]

Hi ad sorry not a hot pass with regards to putting over a root in a V prep just a pass with decent amount of current to shove down the sides of a squared edged fillet. I wouldn't personally v prep it or multi pass it as although it's too thick for a hobby mig it's still only 10mm. If for some specific reason it requires full pen then yes a Prep with root and "hot pass" "cap" may be needed. I wouldn't adopt that though on that job. My bad for the confusion, your terminology is dead right.

 

[Rrunnach]

What blend arc rods, 6013 or, 7018?

 

[Richard.]

Depends on what it's got to load. If it's under high stress you could go with a low hi for increased strength. If not a rutile would be sufficient I would of thought. You do quite a bit of this sort of thing Runnarch, what flavour would you suggest. I guess it depends on ability also. Low hi's are a little more tricky to get singing than a 6013. They'll also need to be fresh out the vac pack or warmed up too if it's to be a benefit. The op will benefit from your take on this too I believe mate.

 

[Rrunnach]

With material being 10 mm thick, it is fair to assume strength is required, I would use 7018's, where the mechanicals are higher. However, OP may not have MMA plant capable of firing up Lo-Hi's, or a rod oven for baking?

If I were using 6013 rod. I'd go in with hi-end amps with a 2.5 rod, then a couple of passes with 3.2 dia rod.

Correction, I just went back and read what welded component will be used for, I'd go 6013's all the way, in order I have outlined above.

 

[BrokenBiker]

It's going to be for an 8 to. Bottle jack press. I did the maths on mine but it was 20 ton and mostly used for flattening the ends of tube or bending plate. I had on of the more experienced engineers look it over and he added the bracing.

Have told him if he can get a stick welder then I will weld it for him as I prefer stick anyway and am far better at stick than mig.

Will be using fresh out of the pack 7018 probably as I have a few

 

[Richard.]

My thoughts on the low hi's exactly. I'd probably not bother with a multi pass on 10mm but it's very much dependent on where the filet is taking it's stress. If the horizontal is been pulled down away from the vertical then a longer leg would help here as runnarch suggests. More passes does equate to more heat and distortion so it's best to work out where the loading is put on the fillets and where you'll need to beef it up. No point in adding heat unnecessarily. My guess is you'll get a good 5mm leg out of a 3.2 single pass. Is that sufficient () depends on the design. If I get time today (rather snowed under ATM) I could always etch up a joint for the op to see if pen is suffice for the application.

 

[Richard.]

Ok I've knocked up a quick etch. I used 6013's 125 amps 3.2 square edged fillet. Leg length turned out at 5.5mm fusion depth around 2mm as with the pen depth.
Use it as a guide. If you want deeper pen then prep the plates. If you want further strength from downward pulling forces do it in 3 runs. Hope this helps as a guideline to work to.

 

[BrokenBiker]

That's quite helpful

Obviously you can get deeper penetration with prep but that still won't affect fusion depth will it? But a greater leg length should add strength to the weld?

 

[BrokenBiker]

Also what do you etch with? I have access to a lot of sulphuric (32%) acid (battery acid) if that will do it?

 

[Richard.]

Yes you'll get deeper pen with a prep. No your fusion depth won't be effected by prep but will by current. A 3 run will add strength to the fillet in 2 ways
1) more reinforcement
2) the multi run refines grain structure and a finer grain is a stronger joint.
Down side is if it doesn't need that volume of strength don't multi run it as distortion will worsen. That run ive put in there would have no issues in coping with been part of an 8 ton press. I've got access to a 20 press here. If you want I'll happily put it under some serious load.

 

[BrokenBiker]

I think I understand it better now. Would you mind trying to break that fillet with the press and see what it takes?