Research On Growth Mechanism And Antioxidant Properties Of Intermetallic Layer Of Hot Dip Aluminized Ductile Cast Iron

Hot dip aluminized process can significantly improve the high temperature oxidation and corrosion-resistant properties of ductile cast iron, and because of low cost which is compared to aluminum, silicon, chromium alloyed heat-resistant cast iron, the hot dip aluminized ductile cast iron will be increasingly used as thermal barrier coatings in the fields of petrochemical, boiler accessories etc..The commercial ductile cast iron QT400-18was used in this study to research the effects of process parameters including hot-dip temperature, hot-dip time etc. on the thickness of intermetallic layer of hot dip aluminized ductile cast iron. The growth mechanism and high temperature antioxidant properties of intermetallic layer were also investigated in the present work.Statistical software DPS and SPSS were used to make regression analysis between thickness of intermetallic layer and the two parameters hot-dip temperature, hot-dip time in hot dip aluminizing process; the relationship between intermetallic layer thickness and the four parameters hot-dip temperature, hot-dip time, diffusion temperature, diffusion time is established using the above statistical software. Based on the metallographic analysis and regression analysis, the process parameters of intermetallic layer thickness were optimized: the hot-dip temperature is750℃～780℃; the hot-dip time is10min; the diffusion temperature is1000℃～1150℃; the diffusion time is3～4h; concentration of passivation agent had no significant effect on intermetallic layer thickness.The thickness changing of intermetallic layer with increase of the hot-dip time was measured to investigate the growth kinetics of intermetallic layer. The results suggest that the kinetics curves approximately obeyed the parabolic rate law. Metallographic observation and EDS analysis show that intermetallic layer consists of "tongue-sharp" Fe2Al5layer of which the thickness is large, FeAl3layer of which the thickness is very small and which located between the pure aluminum layer and the Fe2Al5layer, as well as the rod-sharp FeAl3which distributed in the pure aluminum layer.Based on observation of the microstructure of intermetallic layer and measurement of intermetallic layer thickness of hot dip aluminized ductile cast iron with Ni addition, the effect of Ni addition on the intermetallic layer was studied, the results show that with the increase of Ni content, the intermetallic layer thickness becomes uniform, and the interface between substrate and Fe2Al5phase becomes smooth; however, the intermetallic layer thickness decreases with the increasement of Ni content, and the decrease extent of layer thickness becomes small when Ni content is high than Ni-0.7wt.%.Isothermal oxidation test was conducted at800℃for120h to investigate oxidation kinetics of aluminid coating, the result shows that the rate of weight gain per surface area with increasing oxidation time approximately obeyed the parabolic rate law, the parabolic rate constant kp is about7.15×10-4mg2cm-4s-1, which is two orders of magnitude smaller compared with the rate constant kp1.54×10-2mg2cm-4s-1of the ductile iron matrix, which reveals the effective oxidation resistance of the intermetallic layer for substrate. Thermal cycling test was conducted at800℃for65cycles to investigate thermal fatigue resistance of aluminid coating, the result shows that the phenomenon of spallation of aluminid coating from the substrate which leads to the sudden reduction of weight did not appear in this study. The results fully explain the larger bond strength between the aluminized coating and ductile cast iron substrate.