Formation of aluminide coatings on low alloy steels at 650°C by pack cementation process

The pack aluminising process is normally conducted on alloy steels at temperatures higher than 900°C at which mechanical properties of steels would degrade. This study aims to investigate the feasibility of pack aluminising a commercial 9Cr – 1Mo alloy steel at 650°C in an attempt to increase its high temperature oxidation and corrosion resistance without adversely affecting its mechanical properties and consequently to increase its long-term structural operating temperatures to up to 700°C. It was demonstrated that this could be achieved using packs containing AlCl₃ as an activator and elemental Al as a depositing source. The coatings formed under these conditions consisted of an outer Fe₁₄ Al₈₆ layer and an inner FeAl₃ layer with an abrupt interface between the coating and substrate, suggesting that the coating is formed via a mechanism of the inward Al reaction – diffusion. The pack Al content was varied from 1 to 6 wt-% to investigate its effects on the coating formation process. It was found that the pack Al content in this range affected only the coating thickness and therefore the growth rate of the coating, but not the surface Al concentration. A post-aluminising heat treatment study was also undertaken for an aluminised specimen at 650°C under an argon atmosphere to investigate the kinetics of converting the brittle Fe₁₄ Al₈₆ and FeAl₃ phase layers to a more ductile FeAl phase layer. It was observed that this was a slow process requiring 1132 h for an initial coating layer thickness of 33μm. The coating after the conversion consisted of a uniform top FeAl layer with all other alloying elements in the solid solution and a diffusion zone underneath.