Impact of Electron Beam Welding on the microstructure of PM2000 ODS steel
This work investigated the impact of electron beam (EB) welding, and in particular varying the transverse speed of the electron beam (i.e. 1200-2200 mm/min), on the resultant microstructure of PM2000 ODS steel, using analytical electron microscopy and microhardness. The resultant range of heat inputs during welding explored in this study (72-39 J/mm), induced subtle differences in the width of the fusion zone that correlated with the amount of heat input. Only minor differences in local structures and hardness of the fusion zones were detected amongst the four butt welds produced. The fusion zone is characterised by large columnar grains in the ferritic matrix, together with a coarsened Y-Al-O particle distribution and a reduction in its particle number density, as compared to the base material. There is also a sharp boarder between the fusion zone and base material, with the joint showing little evidence of a heat-affected zone. These changes in microstructure during welding led to a hardness reduction of ~30% in the fusion zone of the welds with respect to the base material. These results provide a parameter window to produce successful butt EB welds of ODS steels with sufficient retention of the nano-oxide distribution, for structural integrity of the welds operating at high temperatures and in fusion-relevant radiation environments.