Welding was being performed on the underside of the inlet plenum of the No. 1 Megatex. The plenum is constructed of stainless steel that is internally lined with Tivar 1000 which is a UHMW polyethylene material. The heat created from the welding process ignited the UHMW material inside the plenum. Once ignited, fire propagated through the UHMW material and A side of the Megatex.