You can do some serious damage trying to get into one of those. It sounds as though you broke the spear which is the tube that runs down to the bottom of the keg. You wouldn't be able to fix that with welding it anway. I bet they would still give you your deposit back though. It's a simple fix for the brewery, far less expensive than replacing the whole keg. If they won't give you your deposit back you can sell it to a metal recycling plant and recoup most, if not all of your deposit.
http://www.micromatic.com/forum/us-e...con4.gif*Those who read this, please only do this with damaged kegs. Selling a perfectly good keg to a metal recycler is a waste of our valuable resources, is a crime and will ultimately make the cost of your beer go up.*