Spring steel doesn't weld very good at all. Once inside the seat you see the factory used strips of steel to strap it together. Upholstery shops have spring material for repairing. A poor man or do it yourselfer like some of us would relocate weak or patched springs to the center of the seat and use those good ones where the driver sits. Another important part of the construction is a piece of burlap like material with small wires running lengthwise through it.
That helps spread the pressure across a span of springs, until those little wires break anyway. That same material can be purchased also and not terribley expensive either.