TWO REPAIRMEN WERE CLOSING A 12 INCH AIR OPERATED VALVE. AIR PRESSURE WAS LOW SO THEY WERE USING AN EIGHT LB HAMMER TO STRIKE VALVE. A PIECE OF STEEL BROKE OFF AND LODGED IN CHEEK OF EMPLOYEE. EE DID NOT SEEK MEDICAL TREATMENT UNTIL 5-28-01. DR REMOVED STEEL AND STITCHED WOUND.