Cracked chainstay weld -- now what? Yesterday while riding my Surly Ogre around town I noticed that it was making a weird metallic popping noise on the left downstroke. When I got home to take a look I was rather surprised to find that the right chainstay had broken completely free of the dropout at the weld (see pic at bottom). 

The frame is out of warranty, so my options seem to be a) buy a new frame or b) try to weld it back. I know nothing about welding, and there is no frame builder in town, but a local machine shop said they could sandblast and TIG weld it back together for $120. My question is a) is this safe and b) is it worth it? The frame set me back ~$440 when I bought it new 5 years ago. So the repair option saves money, but I'd want to be sure the same problem will not arise again in a few years. Also I'm unsure whether it can be rewelded sufficiently strongly.

More generally, what I can do to prevent this from reoccurring? I weigh 175lbs, I've never crashed the bike and have used it exclusively for commuting and ferrying groceries. Back when I built it I sprayed the inside with cosmoline, and it's been kept in a dry climate, so frame rust also doesn't seem like an explanation. I'm puzzled as to how such a beefy bike (to quote Surly's own marketing copy) could fail so catastrophically in a fairly short amount of time.
Get in touch with Surly even if it out of warranty. The worst they can do is say no, and even if they won't fix it, they should be able to offer advise on the the best way to proceed. They live or die based on peoples perception of the brand, so have a vested interest in helping you resolve this kind of problem.  

Any competent engineering shop should be able to make a reliable repair on Cromo. frames.