Preparation and investigation of aluminized coating and subsequent heat treatment on 9Cr–1Mo Grade 91 steel

• Hot dip aluminizing and heat treatment was carried out on 9Cr–1Mo Grade 91 steel.
• Sample heat treated at 650 °C showed Fe2Al5 phase and at 750 °C showed Fe2Al5/FeAl.
• Samples heat treated at 950 °C showed FeAl/α-Fe(Al).
• The scratch test showed the best result with 950 °C/5 h + 750 °C/2 h sample.
• α-Al2O3 and γ-Al2O3 were present on the surface of the samples treated at 950 °C.

Iron aluminide inner coating with alumina top layer is being considered as a potential solution for tritium permeation barrier and mitigating MHD pressure drop for liquid metal blanket concepts in the fusion reactor systems. Hot-dip aluminizing with subsequent heat treatment seems to offer a good possibility to produce aluminized coating with alumina top layer. 9Cr–1Mo Grade 91 steel samples were hot dipped in Al melt containing 2.25 wt% of Si at 750 °C for 3 min. Heat treatment was performed at 650, 750 and 950 °C for 5 h; samples were either air cooled or furnace cooled. Coatings have been evaluated by SEM, EDX, X-ray diffraction, microhardness, scratch adhesion and Raman spectroscopy. The thickness of the layers and phases formed were influenced by the heat treatment adopted. Fe2Al5 was the major phase present in the samples heat treated at 650/750 °C, whereas FeAl and α-Fe(Al) primarily made up the outer and inner layers respectively in the samples heat treated at 950 °C. Cooling method deployed affected the hardness. Air cooled samples had comparatively higher hardness than furnace cooled samples. The scratch test showed the adhesion for the samples heat treated at 950 °C was much better as compared to the samples heat treated at 650/750 °C. Raman spectroscopy analysis showed the presence of both α-Al2O3 and γ-Al2O3 on the surface of the samples heat treated at 950 °C, while Fe3O4 was present in the furnace cooled sample only.