Welding Moly Tube?

[Mr Froggy]

I've read some different methods for tig welding moly tubing.  Sometimes its preheated, sometimes slow cooled and sometimes the tube is filled with the shielding gas prier to welding.  Are these methods used in chassis fabrication?  Rooman, Bruce, please explain!

[dreracecar]

Everyone has their own method. 
 The fit must be good to prevent pulling while welding
 Material must be clean inside and out, some builders go as far as to remove the mill finish on the outside of the tube prior to welding, but not a must.
Back gassing, pre heat, chamber welding is left to areospace and nucular reactors. However, one should never weld on a frame that is cold, warm is better, and for that if you can run a torch over the tube and see a shadow move,then the tube is too cold and that shadow is moisture. I have a roofing torch and sweep the tubing on cold days that way the weld does not cool down too quick. Slow welding is the best as the weld area heat goes out further and slowly draws back to the weld. It takes a very very long time to properly weld a chassis together. I've done it often, and still takes me over a week and a bottle of argon, 5 stainless brushes, and a gallon of acetone. Hot days are the worst because you can have any fans running and keep the doors closed and foot traffic to a minimum

[Mr Froggy]

After you have the frame jigged do you tack all the joints first?  And when you are welding do you turn the chassis over when possible so you are welding flat as apposed to vertical or overhead?   Or, would welding the chassis out of the jig warp the chassis?

[dreracecar]

the more tacks you do the better it holds the chassis from moving around when welding.
welding should be done off jig table to allow access and posisioning
there is a proper weld sequence and takes time to learn, do some research on airframe welding

[nostalgic371]

Welding the the tubes at a decent warm temperature is a must to help minimize distortion, and cleaning is a must as well like described above, I clean out the inside right after I notch a tube in case I forget. Following an air frame type sequence, welding from tack to tack, with at least 4 tacks minimum on a small tube, slowly heating up and cooling with the tig torch, and not letting to much heat build in any one area all seems to help minimize distortion. Some other tips are: Always grind the tapered end of a angle cut or notch to have the full thickness of the tube exposed to the seam; I don't try to do teeny little "barely there" welds, do a full weld and go slow; on a sharp inside corner like a diagonal, use a a gas lens so you can extend the tungsten as needed closer to the seam; when you get toward finishing all the endless welding, mark any last spots you see, they will be easier to find the next day. ER- 80 and ER- 70 S2 welding rods are commonly recommended for 4130cm, I like the 70 S2 rod.
 

[janjon]

I asked a similar question in the past, and got a similar answer. We got a similar answer this time. Tack it up good on the jig, and cheat every chance you get on positioning for the best possible weld joints. Thanks much to all who share their expertise and experience.