Splitting phenomenon of martensitic transformation in a F82H reduced neutron activation steel

ABSTRACT One of the most promising solutions to global energy supply is nuclear fusion. This process, which is implemented in nuclear fusion reactors (from which one of the first designs was the Tokamak), involves deuterium-tritium plasmas at very high temperature, confined in vacuum chambers of toroidal shape under the influence of strong magnetic fields. The hostile conditions underwent by structural components of these reactors drives the development of new materials with metallurgical properties adapted to these conditions. In particular, the so-called reduced activation martensitic-ferritic steels (R.A.F.M.’s) are strong candidates for constructing these reactors components, due its excellent physical properties. In this contribution 6 samples of the alloy F82H, a steel belonging to the R.A.F.M. group, were analyzed. They were extracted from a plate normalized at a temperature of 1040ºC for 38 minutes and tempered at 750ºC for 60 minutes (as- received condition). The specimens were tested in a high resolution dilatometer Bähr model DIL 805-A. The samples were heated at a controlled rate of 5ºC/min. up to 1050ºC, followed by an isothermal holding at the same temperature for 15 minutes, to finally be cooled continuously to room temperature at different rates: 1.5, 2, 3, 5, 10 and 50 ºC/min. The dilatometric curves obtained after the tests showed the presence of a splitting phenomenon of the martensitic transformation at low cooling rates. By simple mathematical models, based on experimental data, curves giving the fraction of transformed phase (austenite-martensita) and their respective temporal derivatives as a function of temperature were obtained for each cooling rate. High resolution images of the tested samples were also obtained using a FEG-SEM, as a complement of metallographic analysis.