50mm lens and you'll be far enough back not to be hit by sparks and the heat from the weld pool is very localised.
The tricky part is really balancing the exposure. You have a really bright weld arc, and the camera in auto mode will expose for this leaving the rest of the scene dark. which can be atmospheric ! http://www.flickr.com/photos/tonycro/3051438290/
You need to balance this with fill in flash/strong daylight or hide the arc behind a convenient block (actually flame cutting in this shot but it shows the principle);
http://www.flickr.com/photos/tonycro/3050599913/
hope this helps.
tony LensCapTony on Flickr