Effect of Si addition on the microstructure and oxidation behaviour of formed aluminide coating on HH309 steel by cast-aluminizing

Abstract

Cast-aluminizing (CA) of HH309 stainless steel by Al-alloy sheets containing various amounts of silicon was studied in this research. To do this, the Al-alloy sheets were inserted at the bottom of a copper mould. Then, molten HH309SS was cast into the mould. The heat transfer from the steel melt enabled the melting of Al-alloy and the high-temperature reactions with the substrate elements to result in the intermetallic compounds (IMCs) at the HH309/Al interface. FE-SEM and XRD results indicated that the (Fe,Cr,Ni)2Al5 and (Fe,Cr,Ni)Al3 phases were detectable when pure Al sheet was used. In contrast, when the sheet contained silicon, a new intermetallic layer Al7(Fe,Cr)2Si appeared at the interface. To create the aluminide coating on the cast-aluminized specimens, they were annealed at 1000 °C for 2h. The multilayer coating was formed by inter-diffusion between aluminum and substrate elements during the heat treatment. This multilayer coating included (Fe,Cr,Ni)2Al5 + (Fe,Cr,Ni)Al2, (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) sub-layers. To investigate the oxidation behaviour of the CA specimens with various Si contents, the isothermal oxidation tests were conducted at 1000 °C for 75 h. The oxidation rate constant, kp, was inversely proportional to the Si content in the aluminide layers. The kp values for CA samples with pure Al, 5, 8 and 12 wt% Si sheets were 1.5 × 10−11, 1.175 × 10−11, 6.39 × 10−12 and 4.36 × 10−12 g2 cm−4 s−1, respectively. The addition of Si significantly improved the high-temperature oxidation resistance of CA samples due to the formation of a combined Al2O3-SiO2 scale on the surface.