Effect of Heat Treatment and Silicon Concentration on Microstructure and Formation of Intermetallic Phases on Hot Dip Aluminized Coating on Indian RAFMS

Hot dip aluminizing was tried on Indian reduced activation ferritic martensitic steel. This experiment was performed with aluminum (Al) melt, with three different silicon (Si) concentrations (3%, 5%, and 7%). Samples were dipped into the Al-Si melt, at 750°C for 30 s, which produced a hard and brittle Fe₂Al₅ intermetallic layer on the samples. These samples were subjected to two types of heat treatments: (I) 760°C for 30 h and (II) 980°C for 0.5 h, followed by 760°C for 1.5 h to convert the intermetallic layer into more ductile phases. The width of the Fe₂Al₅ layer was <10 μm for all the samples with different Si concentrations, and for the pure Al melt, it was ∼35 μm. For both the heat treatments, FeAl and α-Fe(Al) layers were observed. Out of the two heat treatments, heat treatment I gives thinner FeAl and α-Fe(Al) layers compared to heat treatment II. X-ray diffraction measurements confirmed the formation of an α-Al₂O₃ layer on the surface, for 3% and 5% Si concentrations for heat treatment I and for all Si concentrations for heat treatment II. The hardnesses of the Fe₂Al₅, FeAl, and α-Fe(Al) layers were found to be 972 to 1089 HV (hardness value)/0.01, 324 to 384 HV/0.01, and ∼200 HV/0.01, respectively.