One of the issues is the steel the tube is made form if it is 1-1/2 cr and you weld it with standard buz box rod, then it will crack again in short order. Take a magnifying glass and find the end of the crack. Drill a small hole at each end. If you miss by a little bit, you wont put this on the moon so dont worry. There are two process you can use. TIG or braze it. If you tig the crack, take a dremell and open the crack up with the small grinding disk. Be sure the paint is removed from the weld area. Warm the steel up by using a propane torch. Do not use a cutting torch with acetylene. This can affect 1 1/4 cr steel. After the repair area is warmed up, 200 to 250 degrees is you want to be specific, weld up the crack being sure to fill the holes at the end of the crack. Let it cool for 10-15 minutes and add another pass on top and extending 1/2 inch past the holes. Let it cool and then check the end of the weld with a magnifying glass to be sure the crack has not returned. Do not overweld this thinking more is better. The cap should not be any higher than the thickness of the tube. Try to keep the weld small.
If you are going to braze the crack, again, drill stress holes at the end of the crack. Flux the crack area and butter with brass rod. After the crack is buttered, braze a bead over the crack. Again limit the height of the reinforcing material. Again extend 1/2 inch pass the stress holes. If you have the skills or know a BFG welder who knows how to put a bead down with brass, I would recommend brazing instead of TIG. But if you know a jam up TIG welder they can fix this problem.
