Crack Formation after Diffusion Annealing of Hot-Dip Aluminized AISI 4140 Steel

Hot dip aluminizing (HDA) is a surface treatment process in which a metal substrate is coated with a layer of aluminum to enhance its corrosion ad oxidation resistance. However, crack formation can occur during the HDA process possibly due to presence of thermal stresses within the coatings arising from mismatch in thermal expansion coefficients of the aluminide layers and the substrate, brittle nature of the aluminide phases and process parameters. Therefore, optimization of the HDA process parameters such as temperature, dipping time and cooling rate from the dipping temperature might help reducing the possibility of crack formation. Additionally, subsequent diffusion annealing might have an effect on crack formation and overall integrity of the aluminized coating. In this study, an AISI 4140 low alloyed steel was subjected to the HDA process in an Al-11wt.% Si bath at 750°C for 9 minutes and subsequent annealing was performed at 750°C, 800°C and 850°C. Examination of the diffusion annealed samples indicated that there were some cracks within the coatings of the samples, which were annealed at 750°C and 800°C, while there was no cracking on the surface of sample annealed at 850°C. The results were comparatively evaluated by considering the process parameters and the characteristics of the aluminide layers, and was attributed to the formation of ductile and brittle aluminide phases depending on the applied annealing temperature.