Welding thick metal, tips please!

[BlackBeaut]

Hi all,

 

Looks like I've set myself up for a big job with the welder in the near future. I'm looking at joining up 18mm thick pieces like this:

 

I believe the thing to do is grind away the edges to be joined to allow greater penetration, which looks pretty straight forward for the outer edge of the joint. I've heard/read that 30degrees is the magic angle to grind to:

 

But what about the inner edge of the corner? Do I just build up a big bead like this:

 

Or should I also grind away here to get better penetration, and if so what angle should it be ground to in this situation?

 

Or...?

 

Joint strength is paramount here, so what's the best way forward?

 

Cheers,

Rob

[Rival5]

I would grind the inside corner to get since its harder to get in there. I would also leave a gap between the two pieces for the best penetration.

 

[Z-TARD]

Is this going to be TIG, MIG, or Stick? What material? There are some tricks to getting good penetration and joint strenght depending on material and process.

[mrparks]

Assuming this is stick, and that you can weld from the outside of the joint:

 

 

Take your time with the root pass, burn it in. You'll need to make sure you got the pieces lined up properly, too. Then burn the root pass. After that, do two cover passes over it, making sure you go slow enough that two passes will fill it in. (The black on the picture)

 

Then, you put another set of cover passes over that, making sure you catch the corners. That should take at least 3 passes. Watch the heat build up.

 

This may not be the "right" thing to do, but it's what I would do.

[Z-TARD]

This joint design should work pretty well for your application. As you can see, it's a lot like one of your first drawings, just with a lot more material removed from the back. The yellow lines represent the approximate depth of penetration, so leaving just a small contact area between the two plates assures a full penetration weld as the weld from the inside angle penetrates all the way down into the beveled area. I'd recommend leaving just a small gap between the two plates when tacking them together, about 2 or 3 millimeters. This gives it a little room to crush down as the welds shrink. More of a gap than this is an invitation for hidden pockets of slag or other impurities. Any pockets of slag or voids in the weld create stress points from which cracks can begin. If you are using TIG or MIG, you can probably get away with using slightly more of a gap between the two, and quite a bit more if you purge the backside of the weld with argon during welding. The bevel angle on this drawing probably isn't quite correct, but I've had pretty good luck using an included angle of 45 degrees for welds like this, so about 22.5 degrees per side. Opening it up a little will make it easier to weld, but will also tend to let the weld pull the material a lot more as it shrinks.

 

 

Hope this helps,

 

Mike

[BlackBeaut]

Cool, thanks guys.

 

Should've said it'll be MIG welding on mild steel as that's what I've got and can just about cope with. Seems like I've got some major grinding ahead of me then!

 

Cheers,

Rob

[johnc]

You didn't mention what kind of MIG welder you are using. The type of transfer will determine joint preparation requirements for welding steel plate that thick. Your welder most likely uses short circuit (short arc) arc transfer which is not recommended for the thickness material you're planning to weld.

 

To get around this limitation you will need to bevel the inside of the fillet. Put a 45 degree bevel on the plate butting up against the other plate. Leave a 1/4"root unbeveled. Don't bevel the backside of the filler, just do a face pass over it when you're all done.

 

Try to complete the weld with two or three passes and you're going to have to do something to control distortion of the part.

[BlackBeaut]

Is this the sort of thing you mean John?

 

My MIG is natch not a pro model so I guess it does the short circuit arc transfer. How would you recommend coping with distortion? I was going to put a few tacks on first before giving it large. I was also thinking of doing some cunning bolt/dowel set up so that the parts are held solidly in place togther to start with. This is only one joint in the finished piece, which will be made up something like this:

So I was thinking of tacking all the parts together first in the hope that would combat distortion. Have I been sniffing too much welding fumes again?

 

Cheers,

Rob

[Z-TARD]

Tacking all the parts together first should help combat distortion, as long as you still have access to the weld joints once it's all tacked up. Welding sequence will help a lot too, weld a little on each side at a time, being careful not to put too much heat into either side all at once. Leaving that small 2 to 3mm gap will also help to keep it from distorting as much as if no gap were left. When two pieces are contacting, the weld will use the contact point as a fulcrum to pull against, resulting in a lot more warpage. How much weight does this part have to hold? Is there any chance you could post a 3d sketch of the part?

 

Mike

[BlackBeaut]

Is there any chance you could post a 3d sketch of the part?

 

GASP! And reveal my plans for world domination?!!!!

 

Well OK, 'cos you asked

 

Kinda gives the game away!

 

cheers,

Rob

[Z-TARD]

Hmmmmmm....... What could it be.........

 

Well now that we know what it is, things get much easier. Depending on what kind of wire you are using, the weld will have somewhere between 60,000 and 80,000 psi tensile strength. Putting this into perspective, a 1" by 1" area of weld should reach failure under about 60,000 pounds of pulling force, give or take. That is in a perfect world, in reality of course, it's probably quite a bit less than that, say 45,000 pounds. The point I'm getting at is that a full penetration weld on steel 18mm thick is WAY overkill on a 2500 pound car. You can just bevel the parts slightly, about 5 or 6 mm, and do a root pass and a cover on both sides and call it good. For the pipe going through the plates, I would recomend cutting a hole through the plate so that the pipe will pass all the way through and be welded on both sides, this will add a considerable amount of strength. Something like this:

 

[BlackBeaut]

Cool, thanks Mike, makes life a bit easier. I'll be using 0.8mm wire with gas so should get a decent enough weld with a couple of passes to fill out the ground areas.

 

Cheers,

Rob

[johnc]

I won't comment on your design but just tacking 18mm (almost 3/4") plates won't stop distortion once you start apply beads. The tacks themselves, if strong enough to limit distortion, would cause distortion themselves. You will need to clamp the parts in place while welding and as the part cools to maintain the accuracy you need for an upright.

 

You diagram above is correct in how I described how to prep the parts before welding. I got the information from an American Welding Society steel plate welding specification.

[BlackBeaut]

Feel free to comment on the design John, nothing's set in stone - or rather metal! I have slightly tweaked the smaller leg profile so that it has more meat at the top edge to help welding.

 

In theory it shouldn't be too hard to clamp it all up good and solid. I have this idea about running 2 M6 bolts through from the front in to each of the legs, basically to allow it to be held together whilst welding. I was thinking that I'd bolt it up, tack up then remove the bolts, cut their heads off, thread back in and weld the cut ends u on the front face, so they'd be basically acting as dowels at that point.

 

For the tube I was just going to spot drill the two legs a few millimeters deep to create a socket that the tube would fit in, then I could run a bolt through the legs and tube to clamp that all up tight. Mike's idea for running the tube completely through the legs and welding both sides definitely has it's merits though.

 

If it's not screamingly obvious, the idea is for the hub to accept Z32 hub and brakes (caliper and handbrake mechanism).

 

Cheers,

Rob

[johnc]

Well, if you're building it for the rear of a 10 ton truck that's doing wheelies I think you're on the right track. But, for a passenger vehicle you can expect maximum loads of 1,500 lbs. 10mm steel plate would be more then enough for the upright and I would use 8mm for the side plates. If you're worried get some high strength steel in both of those thicknesses instead of regular mild steel.

[BlackBeaut]

The 18mm for the front face was dictated by taking measurements of a Z32 hub knuckle thing, granted they're aluminium, but for me to mount the parts I got of that on to my version I thought it easier to just replicate it.

 

The 18mm leg will have a section channelled out of it, to fit the 2" dia tube for the strut, to a depth of 10mm, so actually it will only be 8mm for part of it's thickness. Erm, I can't think of a decent reason why I chose 12mm for the other leg, sounded good to me

 

There's definitely room for some weight reduction holes to be added in, but I'll look in to that at a later date. Ideally it would be made in aluminium with a bolt on strut tube. But I can't TIG weld so costs would escalate if I'd've gone that route. With good ol'mild steel at least I can screw up a few times without going in to bankruptcy

 

(At least you didn't say it was stupid! Yet )

 

Cheers,

Rob

[OlderThanMe]

MIG should be fine for a light weight car but be aware of things like Lamellar tears...not good. That type of joint shown at the top has the tendency to pull the grain of the steel apart. I have had a MIG weld pull a piece of 1/4 in steel into 2, 1/8 pieces of steel in that configuration in that section. Bad steel is out there...just to let you know.

But if you weld it on both sides it should be better off.

[SHO-Z]

Wrap your corners on the welds can reduce the possibility of cracking. Also do not put a lot of weld on one side compared to the other. As the metal cools it will draw in warping the fabrication.

[grumpyvette]

you guys just AMAZE ME with your computer drafting & image display skills

[alexideways]

Well, before I started as mecanic a few years back,I was a wellder, and use to assemble suspention parts for farme equipement and fist, I have to agree with Mr. Coffey,

Well, if you're building it for the rear of a 10 ton truck that's doing wheelies I think you're on the right track. But, for a passenger vehicle you can expect maximum loads of 1,500 lbs. 10mm steel plate would be more then enough for the upright and I would use 8mm for the side plates. If you're worried get some high strength steel in both of those thicknesses instead of regular mild steel.

your designe is good but, it's way overbuilt.

In fact HE is right on the money with his estimations.

One thing you could do, is to build one with only a few surface tacks and then, build a jig aroud it, this time, with de thickest steel you can find.

Then, you heat the whole thing with an oxy/propane/acetylene torche up to a "verry dull and dark, when the lights are dimmed red", THEN you weld it. After that, you keep heating it a little bit once in awhile so it won't cool down to fast (it'll release stress in the part and prevent cracks. Or, if the wife is ok with it, you can put it in the kitchen oven at full broil and gradually bring the temp. down.). A big plus with this solution is that it keeps distortion a a minimum + if your desing works right, you can reproduce them easily and sell them. Just MHO.