The Correlation of Radiation-Hardening with Interstitial Nitrogen Content in Mild Steels

Abstract

The interstitial nitrogen contents of annealed En2, silicon-killed, and aluminium-grain-size-controlled high-manganese mild steels have been determined quantitatively by internal-friction and chemical methods and qualitatively by strain-ageing. Comparison of the results with previously published data on the effects of neutron irradiation on the room-temperature tensile lower yield stresses of the steels shows that the magnitude of the radiation-hardening increased with increasing interstitial nitrogen content. The interstitial nitrogen and the radiation-hardening in the annealed silicon-killed mild steel were decreased by ageing at 650° C (923 K) before irradiation, presumably as a result of the nitrogen in solution combining with the silicon, forming silicon nitride or silicon–manganese nitride. The strain-ageing in the annealed and annealed and aged silicon-killed mild steel was reduced after irradiation, indicating that the interstitial nitrogen was removed from solution. It is tentatively concluded that the interstitial nitrogen atoms were trapped by the point defects or defect clusters introduced during irradiation and that the radiation-hardening was enhanced as a consequence of the effects of the nitrogen trapping on the size, density, and/or intrinsic strength of the defect clusters.