Welding steel under tension?????

[xpsled]

Im having issue with the chassis installation in my Falcon. I tacked the rails into the car and was very happy with the geometry. I then welded in a rear cross member only to notice that due to a lack of bracing the rail moved about 1/2" at the top of the wheel arch. I grabbed the bottle jack out of my engine crane and can very easily push the rail back to its correct position. However my question is should you weld when the steel is under tension or compression?? Will this reduce the strength of the weld/chassis by welding it when it when the steel is not in a neutral position? Any advice would be appreciated!!!!
Thanks
Dallas

[Jim Nilsen]

Good question?

I like to tack everything well and make sure everything lines up and didn't move. You can however weld certain areas that will pull the metal toward the load or away from it. You have to decide which way to go as to create preload if wanted for that stressed area. It takes a lot of welding to get good at which way it will be stressed or pulled ,which makes larger tacks a must in some cases and also sometimes heating the weld to relieve the stress totally is needed, kind of like what has to be done with chrome moly tubing, if it isn't stress relieved it can cause it to crack.

I bumped this up to see if we could get some other opinions about this to help others doing structural work on frames and cages.

[GetMore]

I don't see that it would be an issue. You are talking about welding the ends of the crossmember while the jack is in the center, right?
The welded areas aren't going to "see" any of that force.
Usually you place the part where you want it, tack one side, make any adjustments to the location, then tack the other side to make sure it won't move, and then do a full weld.

[ErikLS2]

You can also heat one side of the tube and let it cool naturally (not rapidly like with water), this will "pull" the end of the tubing towards the heated side. In other words, if the rail is parallel to the ground and the end of the rail needs to move up, heat the top side of the tube.

The trick is how much to heat it. That's where practice and experience comes in. Practice on a scrap piece if you have one first.

[xpsled]

Thanks heaps for your responses. I decided to make it right and i unstitched the problem rail( about 4hrs work). I was then able to move the rail to its correct position and add some extra bracing.
Really happy with the results. I still used the bottle to move the rails 1mm here or there but now i can continue without out wondering whether I did the right thing.

[Jim Nilsen]

The way to find out about the stress that can be moved around while welding is when you cut it apart. I have removed catwalks with railings that would spring apart or compress on the bandsaw blade, some would swing side to side depending on the way it was welded. It is something that takes a lot of experience to learn what works. Heating the weld until it is evenly glowing the right color will relieve the stresses all around and that is why most don't put CM cages in because of the extra work and expertise invloved.

When the concern is preload in a certain direction to help control flex it really can make a difference if you are looking for predictable movement.