The effects of the post-yield behaviour on the shape of small ball punch test (SBPT) curves were investigated with finite element simulations from which the load-deflection curves were extracted. We have shown the general yield load defined on the ball punch curves depends on the post-yield behaviour and that the usual linear calibration between the yield load and the yield stress may lead to erroneous estimations of the irradiation-hardening. A new calibration procedure has been proposed to determine the yield stress from a ball punch curve that makes use of empirical parameters that minimizes the effects of the post-yield behaviour which influence the overall shape of the ball punch test from the very beginning of the tests.