Investigation of alumina film formed over aluminized RAFM steel by plasma assisted heat treatment

Abstract

Reduced Activation Ferritic Martensitic (RAFM) steel is currently under intense consideration as a structural material for the blanket applications in fusion reactor. The concept of blanket module utilizes both solid i.e. Li2TiO3 and liquid breeder material, i.e. Eutectic Pb-17Li operating at 320–480 °C. The critical issues like liquid metal corrosion of RAFM steel, tritium permeation into RAFM steel and magneto hydrodynamic drag generation due to flowing Pb–17Li is already reported. αAl2O3 + FeAl coatings have been found promising to mitigate these challenges & reported with a substrate of P91 steels. However, coatings with hot dipping process on RAFM are scarcely reported as RAFM steel is in under development. Hence, an experimental investigation is done to examine the effects of heat treatments to form FeAl with a top layer of Al2O3. In-RAFM steel (9Cr-1.4W-0.06Ta) samples were hot dipped in a molten bath of 93%Al & 7%Si at 730 °C. These hot dipped samples were subjected to various heat treatments in 3 different routes. In the 1st route, normalizing heat treatment has been carried out at 980 °C for 30 min followed by thermal tempering in a muffle furnace at 760 °C for 90 min. 2nd route consists of plasma tempering along with normalizing in which O2 was used at low pressure (5 mbar) at 760 °C/90 min with pulsed DC at −520 V. Whereas in the third route, to form α-Al2O3, plasma assisted heat treatments were conducted directly without normalizing up to 24 h. The transformations of these phases have been analyzed through X-ray diffraction. Moreover, to confirm Al2O3 coatings, its thickness as well as the case depth of diffused FeAl, cross-sections of coated samples were investigated by mapping through SEM equipped with energy dispersive x-rays. Among these heat treatments, α-Al2O3 + FeAl formation with a thickness of ∼60–70 μm was observed in route 2. Route 1 resulted in to the formation of θ-Al2O3 + FeAl while route 3 indicated less diffusion of Al in to the substrate with defects.