Neutron irradiation produces greater increases in room-temperature tensile lower yield stress of annealed silicon killed mild steel than in that of annealed aluminum grain size controlled steel. This difference in behavior is due primarily to the presence of free nitrogen in the silicon-killed steel. Preliminary results suggest that free carbon in the steels does not enhance the radiation hardening.
The amount of nitrogen in solution in the silicon killed mild steel is progressively reduced with increasing neutron dose. Furthermore, the nitrogen returns to solution and the radiation hardening is recovered on annealing at temperatures in the range 300 to 400 C.
These effects are attributed to the nitrogen and the irradiation-induced point defects forming complexes that restrict dislocation movement and thereby increase the radiation hardening.
Neutron irradiation has a marked effect on the dynamic strain ageing behavior and the temperature dependence of the yield and flow stresses of the annealed silicon killed mild steel in the range 90 to 250 C. The results are explained on the basis of a strain rate effect, assuming that channeling is the operative deformation mechanism in the irradiated steel, and thermal activation of the radiation damage.