Hardening of the ferritic/martensitic steels by irradiation damage processes (Chapter 12) occurs below 400 to 450°C (the temperature depends on the steel); it is caused by the irradiation-produced vacancies and interstitials and from irradiation-enhanced precipitation of new phases [1]. In general, below 0.3Tm, dislocation loops form from interstitials and vacancies and increase the flow stress, which leads to hardening and embrittlement. Between 0.3 to 0.4Tm, a void and dislocation structure forms (the dislocation loops are unstable and grow into a dislocation network), and diffusion is sufficient for the formation of precipitates. Above about 0.4 Tm, displacement damage can continuously anneal out, resulting in little change in strength (at these temperatures strength sometimes decreases because irradiation-enhanced diffusion accelerates the normal thermal aging process).