A flexible hardrolling mandrel for expanding a sleeve within a tube of a nuclear steam generator is disclosed herein. The flexibility of the tool facilitates the insertion of the tool into tubes of the nuclear steam generator that have only a limited accessibility, and the tool is designed to transfer sufficient torque as required for hardrolling by way of the flexible design. Generally, the hardroll mandrel assembly includes an elongate housing with flex joints along its length, a roller cage rotatably mounted to the housing supporting at least one roller, a tapered actuator shaft slidable within the roller cage to extend the roller, a flexible inner shaft connected to the actuator shaft and drivingly connected to the housing near the roller cage, and an end effector and drive device for imparting torque to the housing. The driving torque is then transferred from the housing to the flexible inner shaft at the end thereof to drive the tapered actuator shaft, thereby providing the required hardrolling torque and minimizing the torque losses associated with flexible drive shafts.