A reducing pipe expander that applies to casing deformation restoration of deep wells such as oil field and comprises a conical-nosed enclosure, a mandrel and several steel balls. The main body of the enclosure is equipped with several radial slotted holes to nest steel balls. Held by a taper, slant or curved face against insides of the slotted holes, the steel balls move axially, overhanging the enclosure. When the deformed casing pipe or patching pipe obstructs the pipe expander from moving forward, the steel balls will move in the reverse direction of the movement of the expander to directly expand the deformed pipe. When the expander can pass through the casing pipe or patching pipe, the steel balls will be free to return to their original status.
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