The effect of neutron irradiation on the strength of ferritic/martensitic steels depends on the irradiation temperature. Below 400 to 500°C (depending on the steel), irradiation-induced microstructural changes lead to lattice hardening, which causes an increase in the yield stress and ultimate tensile strength and a decrease in the uniform and total elongation (Fig. 12.1). The magnitude of the hardening (increase in strength) decreases with increasing temperature until it disappears between 400 to 500°C.