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Using a TIG Torch to Heat-Straighten Weldments (Hotspot Metal)

dgfoster

Diamond
Joined
Jun 14, 2008
Location
Bellingham, WA
I think most folks that have an interest in fabrication have seen folks hotspot shafts, weldments, sheet metal, and other items as a means of, in most cases, straightening them or in some cases intentionally producing a controlled amount of bend into them. Usually this is done with a fuel/oxygen torch. I have never seen or heard of it being done with a TIG torch.

Today I needed to straighten some welded foundry flasks I made a year or so ago that I use to cast my 48" straight edges. They were constructed mainly from 1/8" x 1 x 1 " angle and 1/8 X 3" flat bar for the cope and 1/8 x 1 x1" angle and 1/8 x 2" flat bar for the drag. You can see in the before photo that there was about 1/2" of bow on the length of the flask side with it bowing inward---concave. The angle and bar were originally tack welded every 3 to 4 inches with TIG so as to minimize expected warping. But because the assembly is asymmetric along the vertical axis, warpage in the x-direction in the x-y plane (right ot left in the "before' photo) was unavoidable. No appreciable warping occured in the y-z plane as the assembly was symmetric in that plane. The planes referenced are the same as those used on a common knee mill.

I had tolerated the bowing of the flask sides as the flasks worked OK as shaped. But, today I had to lengthen them by sectioning the long sides of the flask and inserting 5 more inches of combined angle iron and flat bar so that they were stretched from their original 52" length to 57 inches. At the same time I wanted to see if I could take the 1/2" of bow out of each of the four long sides. I had originally thought of getting out my oxy/acetylene torch. But I already had the TIG torch setup and running. And heat is heat----right. So, I decided to try straightening with the TIG. It worked and was considerably faster and likely a lot cheaper in consumables than burning acetylene and O2. In fact I used a phone stopwatch to time straightening two sides and found it took 7 minutes total to straighten two side faces. I was able to slide the torch on 200 amps at a rate of an inch every 2 or so seconds. It was just causing the surface to bubble but was not deeply melting the metal. As the metal cooled you could see it straightening as you watched. I just sighted down the length and touched up spots with residual bend. In a few cases I overshot and had to heat the opposite side. But the net result was rapid straightening to near perfect---perfect enough for my purposes.

Here is a sketch of the construction of a flask side that I show here as the photos may not be clear as to construction details with tack welds represented by the small black dots.:

1670046322537-png.380876


Here is the "before" picture where a piece of 1.5 x .125" flat bar has been placed near the inside edge of the top of the flask side to help illustrate the curve.

before-jpg.380877


And here is the after view showing that the side now lays pretty closely to the flat bar. You can see where the mill scale on the angle is disrupted by heating.

after-jpg.380878

And here is ashot just showing the general construciton of a flask side.
construction-of-flask-jpg.380879



Why bother to write this up? Just to suggest that this is a straightening method that might be useful. If this is commonly done already and this post is just useless noise, please just skip it but also realize my intentions were good. ;-)

Denis
 

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Straightening with weld spots isnt new ,.......I d say not done with TIG because of cost.......back in the 80s ,welding corrosion damage of alloy car heads was big business,and the heads were straigtened by welding spots and short beads on the top,inside the rocker area.
 
Straightening with weld spots isnt new ,.......I d say not done with TIG because of cost.......back in the 80s ,welding corrosion damage of alloy car heads was big business,and the heads were straigtened by welding spots and short beads on the top,inside the rocker area.
First paragraph: "I think most folks that have an interest in fabrication have seen folks hotspot shafts, weldments, sheet metal, and other items as a means of, in most cases, straightening them or in some cases intentionally producing a controlled amount of bend into them. Usually this is done with a fuel/oxygen torch. I have never seen or heard of it being done with a TIG torch."

Second paragraph: "It worked and was considerably faster and likely a lot cheaper in consumables than burning acetylene and O2. In fact I used a phone stopwatch to time straightening two sides and found it took 7 minutes total to straighten two side faces."

Last pargraph: "Why bother to write this up? Just to suggest that this is a straightening method that might be useful."

Denis
 
Great info! If anything, you can probably get more consistent movement with TIG, being able to control heat/time a little more accurately.

Thanks
 
I sometimes use TIG for straightening but it is very difficult not to mar the surface. If its ornamental bending or straightening I use O/A exclusively, if its not, I find the TIG fast and cheap.
Gordon,

Your comments regarding surface marring are "spot-on" so to speak. Since I was working on steel flasks used in my small foundry, the appearance of the steel was not a consideration. Indeed I did raise very small ridges of iron along the lines I heated. Then again, those were so slight that dusting them off with a coarse flap disk took only moments. I removed them mostly because I was flattening some welds so they did not mechanically interfere with the fit up of the flasks. Running the disk down the face of the straightened angle was incidental. But, yes, if working on a face where the finish could not be disturbed, O/A would be my choice. Thanks for pointing that out.

Denis
 
Ditto. I've always used O/A when it's a finished item, no muss no fuss. I've done the TIG method a few times too, works just fine, but can start melts very easily.
 
Backbeading (what y'all call some sorta "spot" method) is a valid process. Always has been, always will be. As a matter of fact, it's the foundation of controlling distortion, by sequencing welds in order to apply equal heat to opposite sides of a weldment.

Backbeading, as a sole means of straightening something, is either a last resort, or the only way someone without proper heating apparatus can do the work. Doing it with TIG sounds a bit better I guess........until you start to mess up the parent metal.

I suppose a lot of people read the literature on heat straightening (aka heat shrinking),, and assume it's a simple process. In fact, without properly applied restraint, in the form of clamps, weight of the weldment in the right axis, or jacking, is a prescription for disappointment. In the real world, you have to utilize applied restraint to move the metal. It's extremely satisfying when done right.

If the restraint is applied in a precision manner, to limit excess movement when the metal cools, it's literally able to move the metal within thousands of the desired shape/location.

Restraint causes the shrinking process, and if used right, limits the shrinking process. It's a fine balance. It's very very cool stuff. It literally takes distortion out of the equation when welding. I can put all of the welds on one side of a member, bow it like a banana, and not worry about it. It's a game changer.

The heart of the process, is the noodle time prior to doing the work. Look at the damage you caused, analyze where your heat input stretched the metal, then set about shrinking that localized area.

Depending on the metal you're working with, you need to pay attention to the critical heat range. Mild steel does well at around 1200*. Heat it into the dull red range. Either let it air cool...........if you feel like standing there with your thumb up your butt for a protracted period...........or hit it with water mist to get it done before Christmas.

To prevent over shrink, fixture the restraint setup so the metal won't move past where you want it to move.

Very simple example:

The weld is on one side of the tubingloader crane41.JPG
This weld draws the tubing. The apex of the curve is on the opposite side of the weld. So, we need to bring it back to straight.

loader crane43.JPG
Notice that the fixture is a straight piece of tubing, with 3 pieces of short tubing under it. The mid piece is directly opposite the weld. This is your "stop".

loader crane44.JPG
That gap tells you just how much your weld warped the piece. The idea is to bring the warped piece to exactly EXACTLY where you want it to be. This bit of short tubing is the limiter, the "stop". The two pieces of tubing are clamped firmly together. Sometimes you won't close the gap just by applying clamp pressure, but the restraint applied by the clamps during the heating process is what keeps the metal from just expanding, and simply contracting without doing any work. This is the fundamental concept of restraint. This particular job..............the clamps by themselves, didn't move the metal one iota(no movement at all)............it was the shrink that did it

loader crane45.JPG
The heat is placed where the metal hinged during welding. I favor a 500,000Btu propane heating tip. Not a weed burner LOL, an actual heating tip. This is the area you want to shrink. It's been stretched......causing the banana. Heat it in a wedge.........one side of the tubing (the flange in this orientation), and a wedge shape in the web (the sides in this configuration). I seldom go any further into the web than the central axis. Run her up to dull red, put your torch up, sit a bit and pick yer nose until the metal just goes below dull red, then hit it with the garden hose. I'd call that a precision shrink (smile).

loader crane46.JPG
Stand back, admire it, and convince yerself that yer the best welder on the planet. ROFLMAO.

I might laugh about this crap, and make it sort of a joke, but it's one of the most important tools in the toolbox. The piece of tubing used as the strongback for this particular heat shrinking session HAS TO SLIDE INSIDE THE TUBING I STRAIGHTENED. So,, yeah, this stuff is cool, and it's mondo important.

So I don't crap on a guy that is interested in the process, and has "discovered" a way to move metal. TIG, backbeading, heat shrinking, and yes in some cases heat working(although I frown on it). I try to bring the guy along by showing ways to do it, and giving some insight into why it works.

Ain't machinin', it's fabrication. Whole different animal.
 
I forgot to show the clamping method.

Remember............the clamps, themselves, are not moving the metal. They prevent the metal from expanding when heated.

We have another weldment with all of the heat applied to one side. A banana in the making.

heavy duty hinges44.jpg
And......................by golly..............she's not quite straight anymore.

heavy duty hinges45.jpg
The fixturing setup. Look at it carefully, and think about how the force lines are applied.

heavy duty hinges46.jpg
The heat signatures show how the process works.

heavy duty hinges47.jpg
And.......................there ya go...............................

heavy duty hinges48.jpg
Without line boring, or any other crap...........you have hinges made to a .005 tolerance, line up. This is cool stuff.
 
I forgot to show the clamping method.

Remember............the clamps, themselves, are not moving the metal. They prevent the metal from expanding when heated.

We have another weldment with all of the heat applied to one side. A banana in the making.

View attachment 381059
And......................by golly..............she's not quite straight anymore.

View attachment 381060
The fixturing setup. Look at it carefully, and think about how the force lines are applied.

View attachment 381061
The heat signatures show how the process works.

View attachment 381062
And.......................there ya go...............................

View attachment 381063
Without line boring, or any other crap...........you have hinges made to a .005 tolerance, line up. This is cool stuff.
damn nice "coaxing" there farmer!!
 
The problem with Tig is the very high temperature of the arc and its concentration. Carbon arc has a much more diffused arc so it better for heating. My dad used carbon arc for heating long before we owned an oxy-acetylene torch.
 
There was a local to me blacksmith where I used to live in the lower mainland of BC Canada, John Adolf I believe was his name, who is long long retired now, and I am not sure if he is still with us, but he specialized in straightening large I beams and other large structural steel members. He did demonstrations at quite few blacksmithing conferences over the years. It was magical how he knew just how much heat and air/water to use to move the metal where he wanted.

He had a torch with an pressurized air/water setup he used. He could take away a 8" bow in a 12' I beam or put one in it if needed. Its a real art.

I have another friend who machines and fabricates dirt moving buckets and attachments for various machines. He to knows how to use the torch, or the welding process to moves things where he wants them instinctually. Purposely not squaring up heavy tabs, know that things will pull and line up just where he wants them based on his welding. an amazing time saver for sure, vs squaring it all up perfect, welding it and having it move, just to try and straighten it back out.
 
In something like the casting moulding box the HAZ of the tig would be irrelevant, if you were making a finished product, that spot that got heated to probably 1000 plus would definitely be somthing to consider, a pressure vessel or some highly stressed component, gas and oxygen heats from zero or ambient up to red, a tig blast of plasma is a cooling heat site, so it’s regarded as a stray arc on say pipework, mostly undesirable, but can be used on an old gate or some such with impunity.
I visited a firm making spherical gas tanks once, they used gas torches to heat spots to bend the plate, the plates were then welded into the vessels, they were very touchy about welders hitting the plates with stray arcs and inspectors marked them, I beleive they were ground off but could result in cutting a section out of the tank

Mark
 








 
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