HYDROGEN DIFFUSION IN 9Cr STEEL: EFFECT OF MICROSTRUCTURE

9Cr steels such as T9 are mainly used in tubes of steam generator for conventional thermal power plants. This material would not suffer hydrogen damage in normal operation. However, it could be susceptible to hydrogen ennbrittlement as it is cooled to room temperature during plant stops. From the academic point of view, 9Cr steels are interesting because they are expected to present conventional trapping ascribed to microstructural defects (dislocations, carbides, etc) as well as a possible effect of Chromium as a minority component of the metallic matrix. In this work we have studied the diffusion and trapping of hydrogen in T9 steel membranes with thickness of 0.15 to 0.30 mm. The hydrogen permeation method with gas phase charging and electrochemical detection has been used, at temperatures ranging from 30 degrees C to 70 degrees C. The steel was studied in three different metallurgical conditions: as received (quenched + tempered), annealed (775 degrees C, 4 h), and quenched (970 degrees C, 40 min, air cooling). The microstructure of these conditions has been assessed by means of optical microscopy and scanning electron microscopy. The permeation transients (rising and decreasing) have been normalized with respect to the membrane thickness, permeation coefficients, apparent hydrogen diffusion coefficients and average trapping energies have been determined. The nature of the hydrogen traps is discussed.