Scale structure of aluminised F82H-mod. steel after HIP treatment

Coatings on low activation steels are required in fusion technology in order to reduce the tritium permeation rate through the steel into the cooling water system by a factor of at least 100. Alumina seems to be a promising coating material. However, an appropriate coating system must also have the potential for self-healing since the ceramic alumina scale tends to fail if mechanical stress is applied. A technology is introduced to form a ductile Al enriched surface scale on F82H-mod steel (Fe-7.7% Cr) and on top of it alumina. This technology consists of two main process steps. Hot dip aluminising has been performed at 700°C for 30 s to the steel in order to introduce Al to the surface near zone by an easy way. The very hard intermetallic scale Fe2Al5 which forms during the immersion process gets completely transformed into FeAl2, FeAl and α-Fe(Al) phases during a subsequent HIP process step at high pressure at 1040°C and 30 min. The pressure chosen for the HIP experiment was 3000 bar. Compared to a heat treatment without superimposed high pressure pores formation due to the Kirkendall effect could be suppressed successfully. The influence of the high pressure on the heat treatment (1040°C, 30 min) will be discussed in this paper.