The battleship Iowa will return to sea duty next month while awaiting repairs on its badly damaged No. 2 turret, defense officials said in Washington. They said the Iowa would serve a four- to six-month tour in the North Atlantic and Mediterranean before returning for repair of the turret next winter. Except for the disabled turret, where an explosion killed 47 sailors during practice firing on April 19, "the ship is sound," one official said. The ship could use its two other turrets, as well as other weapons systems, if it became involved in hostilities, but a moratorium against test-firing the 16-inch guns remains in effect until the cause of the accident is determined. The officials said the decision to return the Iowa to sea before repairing the turret, at an estimated cost of $15 million to $20 million, was made to avoid disrupting fleet and shipyard schedules.