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 F82K-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 degreesC for 30 s to the steel in order to introduce AI to the surface near zone by an easy way. The very hard intermetallic scale Fe,AI, which forms during the immersion process gets completely transformed into FeAl(2), FeAl and alpha -Fe(Al) phases during a subsequent HIP process step at high pressure at 1040 degreesC 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 degreesC, 30 min) will be discussed in this paper. (C) 2000 Elsevier Science B.V. All rights reserved.