Microstructure evolution and hardness of hot dip aluminized coating on pure iron and EUROFER 97 steel: Effect of substrate chemistry and heat treatment

• Published in: Surface & coatings technology 2021-03, Vol.409, p.126783, Article 126783
• Main Authors: Kishore, Kaushal, Chhangani, Sumit, Prasad, M.J.N.V., Bhanumurthy, K.
• Format: Article
• Language: English
• Subjects: Chromium; Diffusion barriers; Diffusion coatings; Electron backscatter diffraction; Electron probes; EUROFER 97 steel; Evolution; Ferritic stainless steels; Ferrous alloys; Hardness; Heat treating; Hot dip aluminizing; Immersion coating; Interdiffusion; Intermetallic compounds; Intermetallic phases; Intermetallics; Iron aluminides; Martensitic stainless steels; Metallurgical analysis; Microstructure; Nanoindentation; Solution heat treatment; Substrates; Tritium
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2020.126783

EUROFER 97 steel, a variation of reduced activation ferritic-martensitic (RAFM) steel, is considered to be a potential material for the first wall of fusion reactor. Aluminide diffusion barrier coatings offer an effective solution to mitigate corrosion and tritium permeation in the first wall. However, the performance of the coating is influenced by the nature of intermetallic phases and their microstructure. In order to critically understand the phase evolution during both hot dip aluminizing (HDA) process and subsequent heat treatment, experiments were performed on (i) pure iron and (ii) EUROFER 97 steel. The samples were hot dip aluminized at 973 K in an inert Ar-5% H2 atmosphere. Detailed metallurgical analysis using backscattered electron detector, electron probe microanalyzer and electron backscattered diffraction technique was performed to examine the evolution of phases and microstructure. It was noted that Fe2Al5 is the major phase forming in both the cases during HDA. Subsequent heat treatment involving solution treatment at 1253 K and ageing at 1033 K led to transformation of Fe2Al5 to FeAl2, FeAl(Cr) and α-Fe(Al, Cr) in HDA EUROFER steel due to interdiffusion of Fe, Al and Cr. However, no trace of FeAl2 could be detected in HDA iron. Thicker and wavy interdiffusion layers were formed in HDA iron, whereas in HDA EUROFER steel near-planar interfaces were present. Micro-pores were observed in all heat treated samples. It was found that the hardness of the phases determined using nanoindentation are in the order of FeAl2(Cr)>Fe2Al5(Cr)>Fe2Al5>FeAl(Cr)>FeAl>α-Fe(Al, Cr)>α-Fe(Al). •Pure iron and EUROFER 97 steel were hot dip aluminized (HDA) and then heat treated.•Fe2Al5 is the major phase forming during HDA and exhibits strong [001]||GD texture.•The interdiffusion zones are thicker and wavy on iron than on EUROFER steel.•FeAl2(Cr) formed during heat treatment exhibits maximum nanohardness of ~13 GPa.