Characterization, growth kinetics and high-temperature oxidation behavior of aluminide coating formed on HH309 stainless steel by casting and subsequent heat treatment

摘要

In the present work, a novel technique has been introduced to obtain an aluminide coating by casting process and subsequent heat treatment. To do so, the aluminum sheet was placed at the bottom of a copper mold, then HH309 SS melt was poured into the mold. This technique was named Cast-Aluminizing (CA). The CA samples were heat-treated at the temperature range of 900-1050 degrees C for 0.5-5 h. The FE-SEM, XRD, and EDS were utilized to characterize the microstructure, phase analysis and chemical composition of cast-aluminized samples, respectively. Results showed that (Fe,Cr,Ni)Al-3 and (Fe,Cr,Ni)(2)Al-5 layers were formed at the Al/HH309 interface. FE-SEM analysis demonstrated a multi-layer aluminide coating on the heat-treated specimens. This coating consisted of (Fe,Cr,Ni)(2)Al-5+(Fe,Cr,Ni)Al-2, (Fe,Cr,Ni)Al and alpha-Fe,Cr,Ni(Al) sub-layers. The growth kinetics investigation showed that the thickness of layers increased with the increase of the annealing temperature and time. The growth rate of layers obeyed a parabolic law. The activation energies for the growth of (Fe,Cr,Ni)(2)Al-5+(Fe,Cr,Ni)Al-2, (Fe,Cr,Ni)Al and alpha-Fe,Cr,Ni(Al) layers were about 203, 250 and 247 kJ/mol, respectively. Microhardness measurements revealed that (Fe,Cr,Ni)(2)Al-5+(Fe,Cr,Ni)Al-2, (Fe,Cr,Ni)Al and alpha-Fe,Cr,Ni(Al) layers had a hardness of about 820-1040, 580-710 and 380-470 HV, respectively. The resistance to oxidation of cast-aluminized and heat-treated (CA + HT) samples in the air at 1000 degrees C was studied. The CA + HT samples exhibited higher oxidation resistance than uncoated samples due to the formation of a protective Al2O3 layer on the surface.