I don't think the metallurgy of spring steel lends itself to casual welding. Specific annealing temperatures and times would be needed to get the spring back to 'springy-ness'.
I have seen upholstery shops lap extra pieces of spring on to the broken one using hog rings. Sort of like making a splint on a broken leg.
I will also note that I have no idea what a "seast spring" might be!
Stuart
