Weldable?
 

2002 Trek 520. So I assume it's 4130 chromoly. This is where the down tube meets the head tube. Fixable?
https://cimg0.ibsrv.net/gimg/bikefor...edc3a6dc12.jpg
https://cimg1.ibsrv.net/gimg/bikefor...cd3d04685e.jpg

Thanks for the good pictures. I would say that there are too many cracks to successfully weld that. I'm amazed it held together for as long as it apparently did.

People don't usually weld cracks in steel. I'm not sure there is a good reason why, but maybe cost?

Good to see better photos. I agree with Eric, If I were to consider fixing this I would likely replace the tubes. But I'd want to check out the rest of the frame first. Andy

I would not trust that except for puttering around the neighborhood. I have NO experience with framebuilding but like to think I know more about it that most folks about the subject, having studied it and worked with a couple of folks who've built frames.

In theory it's possible as the crack is in the thick part of the tube where the butt is. You might want to gouge it out a little bit with a die grinder to make sure you get the weld all the way through, and you would want to leave the bead on the top because even if you had full penetration, the filler metal is generally not as strong as 4130. So it would be a bit ugly and a bit sketchy and I doubt a professional would want to do it :)

I'm with Andrew. It's a fairly simple repair for a framebuilder to replace the DT and HT. The cost is going to be too high relative to the cost of a replacement frame. At a local Recycle shop here in Seattle, something like this could be purchased for $100-200.

it also looks like there's another crack forming so, welding this is going to be an ongoing activity:
https://cimg1.ibsrv.net/gimg/bikefor...f827606000.png

A good welder (very hard to find) might be able to fix it, but probably better to replace the tube, and more cost effective to buy a new frame. Here's a source for tubes, etc. https://framebuildersupply.com