A model of irradiation damage in high nickel submerged arc welds

Abstract A model has been produced to describe irradiation damage in high nickel submerged arc welds (1.6% Ni, 1.5% Mn, 0.5% Mo). It is fitted to a database of hardness change results obtained for 12 such welds with copper contents from about 0.02 to about 0.6% irradiated in materials test reactors. Irradiation dose rates ranged from about 6×10−9 dpa/s down to about 1×10−10 dpa/s. In addition to the hardness change data, Charpy shift data were also available. For some specimens irradiation induced changes in microstructure have been characterised using small angle neutron scattering (SANS) and energy compensated position sensitive atom probe (ECoPoSAP) techniques. The model was empirically fitted to the data, but available physical understanding and the evidence from the microstructural studies were used to guide selection of the equations fitted. Physical understanding and microstructural evidence were also used to help choose between alternative models with similar statistical goodness of fit. The model has two components. First, ‘matrix’ damage, which is insensitive to copper content and irradiation dose rate, and which appears to increase with phosphorus and sulphur. Second, ‘precipitation damage’, which increases with copper and manganese content, and reduces with increasing silicon. Precipitation damage also increases with reducing irradiation dose. The ECoPoSAP data show that the precipitates are composed of primarily of Ni, Mn, Cu and Si, with a substantial amount of iron entrained within them. In the low copper welds the same instrument shows significant compositional fluctuations in these elements, but no visible clusters. The Charpy data have been used to develop an empirical correlation between transition shift and hardness change. This is linear up to about 250 °C shift.