WHILE WORKING ON MAIN FRAME INSERTS ON #2 SHORTHEAD CRUSHER, EMPLOYEES CUT A PAD EYE WITH A WELDER; THE SLAG DROPPED ON A TERTIARY SCREEN PANEL, CAUSING A FIRE ON THE TERTIARY SCREEN. EMPLOYE ES BELIEVED THE FIRE WAS OUT BY THE TIME RURAL FIRE DEPT. ARRIVED (9:30 A.M.), BUT IT WAS DISCOVERED THAT FIRE HADN'T BEEN EXTINGUISHED IN THE FEEDER AND SURGE BIN NEARBY. FIRE THEN EXTINGUIS