Results of experiments aimed at revealing the phase-structure stability as well as mechanisms of the mechanical property changes formation in austenitic chrome-nickel and manganese steels irradiated with high-energy nuclear particles and subjected to elevated temperatures are presented and discussed. Influence of the phase-structure transformations on radiation hardening and embrittlement of steels used in nuclear power industry is analyzed. Change in the experimentally determined “true” values of critical strains and peculiarities of the ά-phase initiation in deformed steels Cr18Ni9Ti, 12Cr18Ni10Ti as a function of temperature and deformation rate, type and fluence of the bombarding particles are discussed. Influence of irradiation on the martensitic structure morphology, critical γ→ά transformation parameters, and thermal effects accompanying the nondiffusion phase transformation is analyzed. The revealed effects are discussed in terms of the internal strength field changes due to thermal and radiation effects.