[en] The purpose of the present work was to study the influence of different regimes of overloading of pressure vessel steels in different states which correspond to the steel properties at the beginning of the reactor operation and at different degrees of embrittlement (simulated by thermal treatment). The experiments were performed on 25-, 50-, 150-mm-thick specimens with short and long cracks of various shape in the temperature range 293-623 K corresponding to the service temperature range of those steels. The following factors were investigated: contribution of different mechanisms (residual stresses, strain hardening, crack tip blunting) into the enhancement of the brittle fracture resistance of steels after warm prestressing; stability of the warm prestressing positive effect during subsequent exposure of those steels to different service loading conditions; size effect on optimal regimes of thermo-mechanical prestressing and on the brittle fracture resistance characteristics of the steels studied after warm prestressing. An approach is proposed for the prediction of the increase in the brittle fracture resistance of steels with cracks after warm prestressing. (author)