TIG question......easy and fun question....

[Jonny Hotnuts]

OK, hears the deal.....


I worked really hard to set up a chassis, made sure everything was spot on aligned and square to >.056 and clamped down and braced with temp struts before burning the sections together.

While relatively green to TIG I was taking my time but feel the welds looked really pretty good and was just tickling blowing through .120 so I know I am getting good penetration. After  burning 80% together, I re-measured the chassis and found that it warped to poop. I subsequently started over as it was beyond any tolerance I would find acceptable.

I am assuming now that when I burn the sections together I should only do maybe 1" or so at a time and use a cooling rag?
I didnt do this on my first attempt because in my research it mentions that 4130 (*note that I am using a combination of size thicknesses of mild and NOT 4130) should be cooled very gradually to prevent cracking.

Is it common to burn the joints in gradually like I am suggesting or is there another method.
Unlike projects of the past it is imperative to maintain the up most in detail with alignment and square.

~JH

[Peter Jack]

Jonny, the order in which you do the welds is critical so you don't make all the distortion in one direction. Every weld expands when the heat is applied and then shrinks more than it expanded when the operation ends. DON'T cool the welds artificially. That's the way you create extra shrinkage when you want to do that type of operation. Just letting the weld cool normally should allow you to hold tolerances as long as you think about the order in which you do the welds.

It sounds like you may be a little on the hot side if you're on the edge of making holes. Butt weld a couple of very short lengths of tubing together then cut in half length wise to see what your penetration is really like. It's usually rather easy to get full penetration in steel. Aluminum can be much more challenging.

Good luck. Lots of practice helps.

Pete

[Tman]

Yes, you need a WELD SCHEDULE as I have heard it called. If you weld one connection partway and it warps up lets say, I move to the same weld on the opposite rail while the first one cools. Then later I come back and weld the bottoms of those pulling the material back into tolerance.  I like to have a set pattern or schedule, flip parts if needed so you dont concentrate 1/2 the welding in only one area ie the top of the rails.

[fredvance]

Whe I weld a chassis I do it in a sort of x pattern. Welds 1 to 2" at a time start right front, next left rear, then right rear, them left front. By the time you get back to the right front it should be cool. If not wait for natural cooling. It is pretty time consuming, but what isnt in our prep for going fast.

  Fred

[floydjer]

JHN, You`ll also find that tacking the joints on all corners of the intersection (for sq. tube) or every 90 degrees on round, will help control movement too. JB

[Stainless1]

That was the question I started asking,

Was it all fully tacked togeter before you started or just braced.

[hotrod]

Yes lots of small tack welds to tie everything together, then skip weld the sections and as mentioned above alternate locations so you allow everything a chance to cool and stabilize. A long continuous weld will really pull things out of shape.

Instead or running a long bead, weld about 1/2 3/4 inch, skip a few inches, and do the same again, then move to another part of the chassis and repeat. Then go back and weld a short segment in between the ones you just did. In many structural situations you really don't need 100% weld, skip welds also have the added safety factor that if one develops a crack there is no way for that crack to propagate to the next weld segment.

http://www.sweethaven.com/sweethaven/BldgConst/Welding/lessonmain.asp?lesNum=3&modNum=5

I also would avoid forced cooling if possible. One other thing you can do to minimize distortion is to pre-heat the parts until they are quite warm to the touch. In the winter time it is very tempting to weld on cold material but that causes excessive cooling rates, poor penetration and lots of distortion. Better to gently warm the part with some quartz halogen lights or a forced air kerosene heater or some other method until the whole piece is up to a reasonable temperature. This will ensure slower cooling and more time to allow stresses to relieve.

I also sometimes will peen the welds with a ball peen hammer after cooling but while still hot to change tension stresses into compression stresses (same effect as shot peening a part to prevent cracking).


Larry

[Gwillard]

Distortion is often a big problem in welding, as you have discovered. Others have pointed out the benefits of preplanning your weld sequence so the distortions tend to counteract each other. NEVER use a cooling rag on 4130! Proper fit of joint members is a big plus.
One other thing you can do that doesn't reduce the distortion directly but reduces it's influence is to save the positioning and welding of position critical items until as late in the sequence as possible. For expample, if you have a plate that serves as an engine mount, wait until you have welded as much as you can, then let the structure cool to room temperature before adding that plate.
Predicting distortion is as much an art as it is a science. Sometimes you just have to use the SWAG method.

[Tman]

Distortion is often a big problem in welding, as you have discovered. Others have pointed out the benefits of preplanning your weld sequence so the distortions tend to counteract each other. NEVER use a cooling rag on 4130! Proper fit of joint members is a big plus.
One other thing you can do that doesn't reduce the distortion directly but reduces it's influence is to save the positioning and welding of position critical items until as late in the sequence as possible. For expample, if you have a plate that serves as an engine mount, wait until you have welded as much as you can, then let the structure cool to room temperature before adding that plate.
Predicting distortion is as much an art as it is a science. Sometimes you just have to use the SWAG method.

You are a smart man!

[Jonny Hotnuts]

Thanks for the replies guys....

Basically to answer a few question and to shed some light is yes, its very cold in my work area.
All the tubes were tack welded (MIG) and used a series of temporary solid .5" stock struts to tie in the sections in a horizontal, vertical and triangulated configuration.

Just as an example one of the top tubes now makes a "W"  (3/16" space high to low on each side of the apex) in a 10' span when a section was welded in the middle. You can see a 'peak' under the vertical section.

Going to weld the joints gradually and make a weld plan.

~JH

[Rex Schimmer]

Tman,
What amperage is your welder set at? What size rod are you using? After going through a fabrication thread on an off road truck build I have completely changed my approach to welding heavy tubing. I set my welder at around 125 to 140 amps and use .045 wire. Joint fit is very critical and also if you can restrict movement with some sort of tooling it all helps. I am attaching some pics from the truck fab thread. Many of us have already seen these but the amp settings and wire size suggestions are from this site and I think the frame speaks for itself.

Rex

[hotrod]

Going to weld the joints gradually and make a weld plan.

One of the key  things to consider as you plan your welding sequence is to balance stresses. Generally the work pulls toward the weld as it cools. A Butt weld will warp toward the side the bead is on. If you can, set up the weld sequence so you balance those stresses by immediately making another weld that tries to pull things back in the opposite direction you can largely eliminate welding distortion. Then the two welds will oppose each other and lead to minimum movement.

Think about intentionally deforming the item slightly in the opposite direction in the initial fit up, so that the weld pulls it back into the neutral position.
In the case of low alloy steels, don't be afraid of heating areas to allow the stresses to dissipate. After a weld is made that distorts the piece, jack it or pull it as necessary  to apply a stress in the opposite direction then heat the weld area to a dull red and let it stress relieve back into the intended position.

As mentioned fit up and welding is as much an art as a science. My Aunts father in law worked for 20+ years at a steel fabrication company here in Denver making bridge girders and other heavy structural steel assemblies, and told endless tales of how they anticipated the movement of the steel as they fabricated it. Girders were built with a camber in mid span so when they were in place their weight would pull them down to be straight rather than sagging.

He would fit up pieces that had lots of weld on one side so the welding shrinkage would pull it back into true and they were not afraid of getting out large acetylene torches and heating areas to a dull red  after welding to let the stresses relax as the slightly plastic hot steel yielded to the welding stresses. Sometimes putting large chain fall come-a-longs on the pieces and pulling them hard in the direction opposite of the direction the welding shrinkage would act as the welds cooled.

Lots of carefully applied brute force is used in heavy fabrication to make things actually be the shape and size intended when the fabrication was done.

Larry

[manta22]

Wow, Rex-- that truck tube chassis looks like a Birdcage Maserati!

Regards, Neil   Tucson, AZ

[Rex Schimmer]

Neil,
If you haven't seen this web site it is worth the time. The level of tube fabrication skill is unbelievable! The link is below.

Rex


http://www.race-dezert.com/forum/showthread.php/31417-Something-New!!

[manta22]

Very nice stuff-- thanks, Rex.

Regards, Neil   Tucson, AZ