The effect of the microscale distribution of boron on the yield strength of C–Mn steels subjected to neutron irradiation

Abstract The presence of B as an impurity in C–Mn steel and weld metal can have a significant effect on the mechanical properties of these materials depending upon whether this element is present as the free atomic species or incorporated within particular microstructural features such as grain boundaries, inclusions or precipitates. This is important in the case of the steel plates and weld metals used for the construction of nuclear pressure vessels where irradiation, with a contribution from thermalised neutrons to the fast neutron spectrum, can lead to transmutation of the B. In this work we examine the distribution of B within the microstructure of a range of C–Mn steels and weld metals containing 1 ppm ≤ boron ≤48 ppm using low and high resolution microanalytical techniques. These results are compared with measured tensile and hardness properties. The distribution of the impurity B within the microstructure and the measured changes in the yield strength of these C–Mn steels following neutron irradiation are discussed. The measured changes in the yield strength are addressed using models that describe the displacement damage arising from several sources. In particular the contribution of neutron irradiation damage from B transmutation to the irradiation strengthening is related to the bulk B content and its distribution to inclusions and carbide precipitates in the overall microstructure.