Effect of aluminizing on surface microstructure of an HH309 stainless steel

Oxidation resistance of chromium and heat resistant steels is due to formation of Cr2O3 on the surface. This surface layer will be destabilized above 1000 °C and will not protect the metal. Recent investigations show that aluminizing process increases oxidation resistance of these steels by formation of Al2O3 which is more stable than Cr2O3 at high temperatures. In this investigation aluminizing process is done on austenitic heat resistant stainless steel HH309. The microstructure of the samples was then examined by SEM and EDS and phases were identified by XRD. The results show that the coating consists of two layers. The first consists of Fe–Al intermetallic phases and the second one is an interdiffusion layer consists of ferrite associated with NiAl and Ni3Al precipitates. Oxidation resistance of these samples was studied in air at 700 and 1100 °C. The results show that aluminized samples have higher oxidation resistance.