Understanding when you would do a multipass weld?

[Thebeast]

Just wondering when would I use a multipass weld and how many passes would you use and why?

I get it if you were doing a butt joint you would come in with a root pass then the other layers to fill in the cavity till it’s flat.

What about T joint? Let’s say I use a 2mm electrode on some thicker material (10mm mild steel) would I multi pass and how many passes would I do?

I’m guessing if I ran higher amps with a thicker electrode (4mm-5mm) I would be able to do it in one pass.

I have always struggled on understanding this and I haven’t found anywhere that explains it exactly

 

[chrismac]

In the professional world of welding some specifications state you can only weave x times the rod or filler metal thickness, yet a weld will be required to be a certain size, you would need to build it up to make that size of required weld, its far tidier in MMA to put on 2 or 3 smaller runs with smaller rods say 3.2mm than trying to wallop it on in one with a 4mm and the same goes for other processes, more smaller runs are better than one big one, less defects too.

On your description 10mm plate fillet, it would be done probably 2 runs with a 3.2mm rod, first run 2/3rds on the bottom plate, 2nd run set on top.

hope that poor wording sort of make a little sense...

 

[denlow60]

Just wondering when would I use a multipass weld and how many passes would you use and why?

I get it if you were doing a butt joint you would come in with a root pass then the other layers to fill in the cavity till it’s flat.

What about T joint? Let’s say I use a 2mm electrode on some thicker material (10mm mild steel) would I multi pass and how many passes would I do?

I’m guessing if I ran higher amps with a thicker electrode (4mm-5mm) I would be able to do it in one pass.

I have always struggled on understanding this and I haven’t found anywhere that explains it exactly

I hope I can explain to you the logic behind your questions.

first of all your 2 mm electrode is a wrong electrode with this you can get a lot of mistakes to make a fillet weld in plates of 10 thick.

you need 0.5x10=5 here so a design Throat of a5 because your plate is 10 thick. unless it is indicated otherwise by calculation of the engineers.

you can use a 5 mm electrode but there is a big chance that you will not get a5 and on edge undercutting.

you can use 4 mm electrode but you will have to do this in two layers (lay three beads) here you will come above the a5 you do not want that
because this is too much.

3.2 mm electrode is the perfect way to weld this, you do this in two layers (lay three beads) with this you get good a5.

hope that is a bit understandable.

 

[chrismac]

I hope I can explain to you the logic behind your questions.

first of all your 2 mm electrode is a wrong electrode with this you can get a lot of mistakes to make a fillet weld in plates of 10 thick.

you need 0.5x10=5 here so a design Throat of a5 because your plate is 10 thick. unless it is indicated otherwise by calculation of the engineers.

you can use a 5 mm electrode but there is a big chance that you will not get a5 and on edge undercutting.

you can use 4 mm electrode but you will have to do this in two layers (lay three beads) here you will come above the a5 you do not want that
because this is too much.

3.2 mm electrode is the perfect way to weld this, you do this in two layers (lay three beads) with this you get good a5.

hope that is a bit understandable.

I think some more information is required to help your explanation, you refer to a5 often the OP might not know what this is, or where you get your formula from, this might also help the OP out.

Good description though.

 

[Memmeddu]

Just wondering when would I use a multipass weld and how many passes would you use and why?

Generally speaking wider beads are weaker than multiple smaller ones well tied together.
For example the other day I've had to repair a cracked corner on an excavator bucket. Right where the cutting edges (side and blade) meet in a T joint
Once having gas axe it, cleaned it up, aligned...
It ended up to be like a good double bevel joint roughly 50x50mm in both directions by 120 to 130mm length.
I've run Three Root passes first one at roughly 200A and believe me it was cold and second one at 300A on both sides of the root .
Once done that I've filled it up with straight 300A beads not wider than 10 12 mm right on the edge of the previous bead.
Mig in spray transfer obviously.
No undercut and flat beads in the entire joint once finished I've removed the glass kind slag cover from the silica impurities either in the gas or the wire.
Now this one was a little extreme but essentially you run multipass on things that have to have strength or fill up a bevel or a joint.
On 10mm I would go with three passes four at the most and I would use nothing more than 3.2mm 7018 if using MMA .
I would run the first pass using 2.4mm 7018 on the hot side of things like 100A

 

[denlow60]

I think some more information is required to help your explanation, you refer to a5 often the OP might not know what this is, or where you get your formula from, this might also help the OP out.

Good description though.

yes i understand what you mean.

plate thickness is 10 thick so you take half of the plate thickness is 5

5 is then the throat height a5, if i want to express this in leg length then it becomes 1.4 x 0.5= 7 so z7

but this is not infinite because an engineer can calculate what a or z you need. for thicker plates.

 

[denlow60]

yes i understand what you mean.

plate thickness is 10 thick so you take half of the plate thickness is 5

5 is then the throat height a5, if i want to express this in leg length then it becomes 1.4 x 0.5= 7 so z7

but this is not infinite because an engineer can calculate what a or z you need. for thicker plates.

View attachment 488366
View attachment 488368


View attachment 488371

5 is then the throat height a5, if i want to express this in leg length then it becomes 1.4 x 0.5= 7 so z7

must be 1.4 x 5 = 7 z7

 

[aesmith]

yes i understand what you mean.

plate thickness is 10 thick so you take half of the plate thickness is 5

5 is then the throat height a5, if i want to express this in leg length then it becomes 1.4 x 0.5= 7 so z7

but this is not infinite because an engineer can calculate what a or z you need. for thicker plates.

Can I just ask in plain terms, is "a" the dimension labelled "Design Throat" in your first picture, and "z" the one labelled "Leg Length"?

 

[denlow60]

Can I just ask in plain terms, is "a" the dimension labelled "Design Throat" in your first picture, and "z" the one labelled "Leg Length"?

Design Thoat = a
Leg Length = z

example:

a = 10 then z is 1.4 x 10 = 14

z = 10 then a is 0.7 x 10 = 7

 

[cocker]

Well , that's that then .

 

[chrismac]

it

I'd like to offer my explanation to answer your question. Long winded, this answer comes from Canada so keep that in mind.

It seems everyone else is trying to tell you how they equate the strength of a fillet weld to its size. Fair enough. While it's a textbook reply to " how to measure a fillet weld", it doesn't really answer the question of when and why to weld multiple passes, does it?

I'm not going to address that welding symbol stuff. Because one, it doesn't answer your question. Two, it's practical use isn't of value other than conveying information. It's usefulness is passing information from white hats to blue collars on the weld size and it's placement in the form of a drawing. Three, the value it brings to attesting to strength is quickly diminished by a less skilled operator.

Size isn't as important, what is, is fusion and dilution of the materials, and the strength of the granular bonding of elements from molten mixing and solidification cooling reducing the transition of stress through the heat affected zone. Rarely does the weld break.

Butt two end of two square bars together and ask how much material is actually missing to make one bar? Look at the tee of one bar on top a flat plate, how much is missing to make it a solid bar? Not much. In both instances you could glue them to stay together, unless you add in leverage, strain, or a load.

When would you use a multipass weld...One answer might /could be when you have small rods and require lots of fill. Because a bevel or gap is needed to penetrate deeper than the depth the filler metal will fuse and dilute into. That's one answer. That said, one large deposit yields a coarse grain structure and more ductile deposit. Multiple welds tend to refine each previous weld increasing tensile strength with a slighter loss of ductility.

If the interest is in reducing arc on time, use the largest rod possible. Want a weld firm and rigid, lacking ductility but with weld toughness, use smaller rods more passes.
Why use multiple passes...
Another might be to switch chemistry and mechanical properties of dissimilar materials. A transition from one to another. Cast to carbon, low carbon to higher carbon. If you can understand how an avalanche occurs, you can guess about grain boundaries in the HAZ of a weldment. A refined grain adds toughness.
Also, a large rod requires greater puddle management skills. Greater power source current and duty cycle. How many rod sizes can one guy stock, afford to keep on hand? Multipass welds allow you to you stock less variety in sizing.

So what's creates a strong weld? Is it the size? Single or multipass? Is it the application knowledge? Looking around seeing what you have an using it? What or where is the concern of over welding with multipass welds?

Knowing how to build a castle wall with the material you have for the threat you expect to guard against about describes it. A little good weld, well placed, is always better than a whole lot of bad welding. Amps volts and a steady hand, what can go wrong? 70,000 psi sure seems like a lot until you discover notching, reversal of stress, lack of fusion. In again comes the multipass weld.

The multipass weld, the purpose of which is to compensate. To cover, to make the appearance of stronger by the definition of largeness. More, many. Sometime you do it for looks. To fill in a overly beveled plate, to fill a corner or blend contour a plate edge, it improves the appearance, becomes more pleasing. Other times, building up without risking burn thru from large rods and higher currents. Improved operator control using smaller rods, smaller wires. While covering up with a multipass weld is a popular thing to compensate for a lack of skill and understand, it's requirement is the comparable difference between weld penetration and weld fusion.

Unless directed by weld procedure, weld size doesn't dictate the difference between singular or a multipass deposit. What does is welding knowledge, welding hand skills and the confidence necessary to understanding and apply all three as required, when needed, knowing the expected outcome from doing so.

At the end of the day, one big weld, a bunch of little welds, or a tacking festival of weld tacks, all still can yield a successful weld.

Sorry but you spoiled it with your last sentence,

I dont have the time or inclination to respond to other parts I have a different view on, but one bit that stands out is where you said some times you do it for looks, aesthetics is the end product, we are driven how we get there by some type of guidelines not just how we fancy doing it, unless your referring to back yard welding, where you can do what you want.

 

[indykid]

In my non specialist Engineering understanding, you need to generate a given cross sectional area in the weld throat to support the load and make the stress at the joint acceptable (stress raisers, weld defects, material property changes in the HAZ etc). Depending on the welder you have at hand, you may be able to do it in one pass if for instance you have a submerged arc setup or electron beam or something, but if you only have TIG, MIG or stick, you might need to build the weld deposit up in bits. Even with TIG/MIG/stick, if you only have a small welder, you might need to approach it in smaller bites.

A smaller weld bead will cool faster creating a smaller more refined crystal structure, so the weld will be stronger for a given cross section of filler material, but if you can blast enough in in one go to give you the required fill and also some extra to account for the loss in material properties of the weld, that would likely be the route heavy industry would take.

 

[denlow60]

How does one define the definition of a good weld? By the words of Omer.
View attachment 489067View attachment 489070

Sure, we all turn our noses up at the over lapping tacking so popular in foreign content videos, expecting to see a continuous rod melting, but at the end of the day, if it meets the requirements, it's a good weld.

View attachment 489071
Now some want/ expect something more for service requirements, it then comes down to qualifying a weld procedure if you are using that as the basis of your definition for a good weld.
View attachment 489072

This may not be the venue to discuss this " require more perfect radiographic soundness than the specifications demand" criteria in any detail, but this is where my basis of reply comes from. A little good is better than a lot of bad when discussing size.

View attachment 489073


That's saying a lot of nothing wasn't it? Neither the time or inclination you say, pity. Well move along then.


I respectfully agree with minor clarification to some of what you say, and disagree maybe with some in how you say it.

It's my understanding that in dendritic grain formation, it's prolonged cooling that allows for refinement. A settling of elements. The gripping edge of the HAZ transition. This cross section of material...Is that of what you address? Freezing quickly offers a limited refinement? That is how I see it. Small welds that cool quickly due to conduction or convection, quenched, while exhibiting a degree of rigid hard toughness, are more brittle, fracture sharply , and like a slate slab, easy to break off. A broken tack with a flat leg face as an example.

Single or multipass, that's what this is about right? Well, I posted additional support material to aid discussion or clear the air.

While you did mentioned "required fill", it seems that is this discussion? How much is required? Well to increase the clarity of my position, it depends.

But you don't need 100% JP in all cases, or to weld 360 when 3 - 40 degree length solid stitches will do. It's done for looks or to appear complete.

So back again to industry and welding symbols, other than to convey information, a weld symbol holds little value to quality or strength, and the size specified, is arbitrary to one man's idea of requirement and allowance for both. Back yard or otherwise. And... since most error to caution, they over weld, They say yeah, make that super sized.
If you can't do it in one pass you do more till you achieve it. Or you don't because you couldn't and they get someone in who can.

Heavy industry... yes and no. Truth is, more no than yes. Ask a welding sales person, what's your best seller and what's my cheapest price. He's not going to say how big a weld do you plan on making?

So to further sum that up, depending on the rated output of the welding power source, you may not supply enough power to squeeze unpopular but economical big oranges to make juice, but you can buy the popular small oranges more often to achieve the same full glass of juice.

Are we going to talk about metallurgy here?

then no weld is the best weld.

 

[indykid]

Heavy industry... yes and no. Truth is, more no than yes. Ask a welding sales person, what's your best seller and what's my cheapest price. He's not going to say how big a weld do you plan on making?

So to further sum that up, depending on the rated output of the welding power source, you may not supply enough power to squeeze unpopular but economical big oranges to make juice, but you can buy the popular small oranges more often to achieve the same full glass of juice.

Excavator booms and bridge beams are welded single pass from the factory, but field repairs are almost certainly multipass.

Perhaps our definitions of heavy industry are misaligned. I'm only talking about OE, not any field/re work.

I find your writing style quite hard to follow.

 

[denlow60]

My point, convoluted as it was, was attempting to distinguish weld quality over quantity and the need for encompassing discussion of weld strength relating to mechanical properties, metallurgical chemistry, as well applying such knowledge with relation to singular or multipass welds, there is no one size fits all answer.

I think the reason my writing style is hard to follow has to do with over thinking, over learning, over understanding and over applying. Maybe some crazy?

Most see the act of welding like hot glue sticking stuff together. For the most part it is. But when it isn't, that's when they seek answers and explanation. Most just offer answers.

So, when you learn and can do those different welding processes, how they work, react, can be manipulated...then the challenge isn't controlling them anymore, it's explaining them. That takes a lot of blah, blah, blah, and some of it gives the impression of crazy, some well that makes sense.

And some just need more practice.

May I ask what training you did to make you so crazy?