pre-heating for welding?

[daniel_buck]

So, I have only 110 power, but even then I still have to fight metal warping sometimes.

I've never tried pre-heating steel before welding. From what I understand, there's no real down side to pre-heating mild steel, accept that it takes fuel to do so. I have a few propane/mapp hand held torches, which I assume would be sufficient to pre-heat metals of 1/4" thick or sell.

In addition to reducing warping, it would also help with weld penetration I assume?


So, how much pre-heating is need to be to actually be of any use? Red hot? A few hundred degrees?



To control warping, I assume a large area needs to be heated, for penetration just the local area to be welded needs to be heated?

[RAT]

What metal are you welding? Sheet metal? Thin metals warp easily. Heavier gauge stuff, not so much. Preheating is not normally needed unless it is a special application or situation that calls for it. Normal welding processes do not.

[daniel_buck]

steet metal and flat bar, those are the ones I have trouble with bending most of the time.

[curtis]

You cannot do long welds on that material. You need to spot welds and hook the welds together.

[inVERt'D]

Daniel, if you're wanting better welds on thicker steel with your 110V MIG then pre-heat is essential. Whether you'll get adequate penetration and fusion even then is another question. Even with pre-heat I still wouldn't dream of welding a .120" wall roll cage with one. Normal welding processes use power levels adequate to produce welds that have adequate fusion and penetration. For mild steel, that usually results in a weld joint that's close to the strength or even stronger than the parent material.

Distortion is caused by the molten weld shrinking as it cools, which pulls on the adjacent material. The best ways to control and minimize distortion are by understanding it and designing and welding things accordingly. Joint design, where you weld and where you don't, the order and timing with which you weld things, fixtures and jigging, and even the overall design of the work piece itself have a lot of influence over distortion.

One way to reduce distortion is to get the weld itself to relax and thereby reduce its pull. One way this can be done is by peening the weld, but that can also cause brittleness. Another way is to control how fast the weld cools, and that's best done by getting the base metal to maybe 350-400 degrees (even say 100 or 200* will help some) and keeping it there while you're welding and for a period of time afterwards, hours if possible. This is also a good way to prevent or minimize cracking in or a adjacent to the weld.

Distortion is a topic worthy of a book or at least a healthy chapter or two, so perhaps if you provide some concrete examples that would be helpful to you, I or we could provide some concrete tips for dealing with it.

[daniel_buck]

Well for instance, I had a piece of flat bar with a piece of angle iron welded to it in a T shape, top piece being the flat bar. When I was done welding, the flat bar bowed a bit. It wasn't super critical, as it was only for a battery tray, but I wonder if I were to have put a torch on the back side of the flat bar before and after welding, would it have minimized the bowing?

Flat bar was 3/8" thick and 1" wide (maybe 8 inches long) , angle Iron was 1" size, and was 1/8" thick. Probably over kill for a battery tray (the 3/8" anyway), but it was what I had. and I'm sure 1/8" flat bar would have warped even more.

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[RAT]

Well for instance, I had a piece of flat bar with a piece of angle iron welded to it in a T shape, top piece being the flat bar. When I was done welding, the flat bar bowed a bit. It wasn't super critical, as it was only for a battery tray, but I wonder if I were to have put a torch on the back side of the flat bar before and after welding, would it have minimized the bowing?

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Nice drawing! That is typical behavior for the flat bar in that set up. Bout the only thing you can do to prevent it short of building a jig every time you weld something like that, is tack the pieces to a plate first so they don't pull while cooling. If you watched American Chopper, they used that method quite a bit. The black widow bike was one build they did a shit ton of welding of small round stock to flat.

[daniel_buck]

What can I say, I'm an artist ;-) haha


So if it's held in place as it cools, (with tack welds to something very sturdy, or clamped in a bench vice) It won't spring into the shape it wants to warp to when the vice is released or when the spot welds are cut? Maybe I should pick up a nice thick piece of angle iron, to tack things to to help with warping.

In this particular instance, it could have been the order in which I chose to weld the pieces that made the warping worse. If I were to have had the tray made up, and then welded the mounts that bolt to the frame, the warping might not have been as much. But, I kinda did it backwards. I've got a bunch to learn about metal

[RAT]

Daniel, if you're wanting better welds on thicker steel with your 110V MIG then pre-heat is essential.

I don't agree. Pre heating does little if nothing to prevent shrinkage at cool down. Post heating might but with small, thin metal it's probably gonna warp no matter what. Sheet metal for sure if you don't stitch weld. For the set up he drew it's easier to tack the parts to a plate first.

Whether you'll get adequate penetration and fusion even then is another question. Even with pre-heat I still wouldn't dream of welding a .120" wall roll cage with one. Normal welding processes use power levels adequate to produce welds that have adequate fusion and penetration. For mild steel, that usually results in a weld joint that's close to the strength or even stronger than the parent material.

I do agree here.

Distortion is caused by the molten weld shrinking as it cools, which pulls on the adjacent material. The best ways to control and minimize distortion are by understanding it and designing and welding things accordingly. Joint design, where you weld and where you don't, the order and timing with which you weld things, fixtures and jigging, and even the overall design of the work piece itself have a lot of influence over distortion.

One way to reduce distortion is to get the weld itself to relax and thereby reduce its pull. One way this can be done is by peening the weld, but that can also cause brittleness. Another way is to control how fast the weld cools, and that's best done by getting the base metal to maybe 350-400 degrees (even say 100 or 200* will help some) and keeping it there while you're welding and for a period of time afterwards, hours if possible. This is also a good way to prevent or minimize cracking in or a adjacent to the weld.

All true, but he's welding little pieces of metal together. What you describe above is a lot of pre work for little or no gain on a part like he is making.

Distortion is a topic worthy of a book or at least a healthy chapter or two, so perhaps if you provide some concrete examples that would be helpful to you, I or we could provide some concrete tips for dealing with it.

Again, I agree with you. Learning to weld in a school is not the same as doing it on your own. Most of the techniques and technology taught in a trade school don't come into play for the home garage welder. Especially one with a cat food can welder... Sorry daniel.
You're doing what yo need to do. Practice a lot and ask butt loads of questions. You'll get a lot of different answers to the same question because there are so many different schools of thought on the subject.

[inVERt'D]

Wow, I'm really surprised you managed to significantly warp the 3/8" flat bar with the 110V MIG. How many passes and how much warpage? Got a photo?

Using those materials and that design, I'd put it in a fixture or brace it until it was cooled, or perhaps even add a little permanent brace if it's really a problem. On the other hand, if you get it close to red-hot the metal will lose its spring and it will be easier to bend it back without tweaking the angle iron.

For something like that I'd be happy to take a little warpage rather than a cold weld. As long as the battery fits and the welds hold, it's a 110 success .

[RAT]

I've got a bunch to learn about metal

Same way I am about wood. I need to work with a real wood worker to get some tips.

[RAT]

As long as the battery fits and the welds hold, it's a 110 success .

LMFAO! It's a 110 success! My new tag line for Ahole Fab. Thanks!

[inVERt'D]

I don't agree. Pre heating does little if nothing to prevent shrinkage at cool down. Post heating might but with small, thin metal it's probably gonna warp no matter what. Sheet metal for sure if you don't stitch weld. For the set up he drew it's easier to tack the parts to a plate first.

I had no idea what he was welding or if he was even thinking of something in particular or things generally. Also I wasn't talking about distortion here, just penetration and fusion, which would be my main concern with a 110 MIG. As I explained further down, the reason a little pre-heat can be good with regard to distortion is to keep the metal from being too much of a heat sink as you're welding. Keeping the weld hot for a period of time afterwards can help it to relax and reduce the stresses that cause the distortion.

I do agree here.

All true, but he's welding little pieces of metal together. What you describe above is a lot of pre work for little or no gain on a part like he is making.

Like I said, I didn't have a target for the subject matter. His question was phrased pretty generally, so I was responding in kind. Maybe his battery tray is no big deal (as long as it doesn't drop the battery), but for future projects a little technique can prevent some disappointing results. I'm still guessing that's more his point. Daniel?

Again, I agree with you. Learning to weld in a school is not the same as doing it on your own. Most of the techniques and technology taught in a trade school don't come into play for the home garage welder. Especially one with a cat food can welder... Sorry daniel.
You're doing what yo need to do. Practice a lot and ask butt loads of questions. You'll get a lot of different answers to the same question because there are so many different schools of thought on the subject.

Yeah, the home garage weldor definitely doesn't have the technology that a production welding facility or even a good fab shop would have, so some of the techniques don't apply. The basic principles are the same though, so a thinking person can apply those to what he has available. I don't even have a real welding table or a good place to put one, so I have to avail myself of other means of keeping things from wonking out, or at least of fixing them.

I've learned a lot about distortion, and I'm still learning. From time to time I get a little surprise.

[inVERt'D]

Well sorry guys, I started my first reply around noon, was busy with other things for a few hours, and missed the posts in-between. I missed a little pertinent info. Was slow with the subsequent posts too.

LMFAO! It's a 110 success! My new tag line for Ahole Fab. Thanks!

You're most welcome, although I don't know how it applies?

[curtis]

I did welding in a fab shop for a few years. I think you guys are over thinking this. Or is it over dissecting.