No it didn't. The melted core ate away at the bottom mounted in-core instrumentation guides that extend a foot or so up into the vessel, but it never exited to pressure vessel, never ate through and welds, 'stainless steel' or otherwise, and never violated the primary pressure boundary.
FYI... all the welds in a pressure vessel are on high strength carbon steel. The stainless steel inside nuclear vessels is a corrosion resistant cladding applied over top of the carbon steel. It is not intended to be part of the pressure boundary. It is just a cladding to resist corrosion and keep contamination to a minimum.
The Reactor Pressure Vessel (RPV) did not 'break,'. The pressure boundary was never violated. All of the fuel was contained inside the RPV. None of it exited. Zero.
