Investigation of melting in the Al-Si coating of a boron steel sheet by differential scanning calorimetry

This paper investigates the veracity of claims, reported in the literature, that slow heating of Al-Si coated boron steel blanks in a roller hearth furnace can allow sufficient diffusion of iron from the steel substrate to the coating and prevent liquefaction of the Al-Si layer. The results of differential scanning calorimetry experiments conducted at various heating rates indicate that melting cannot be avoided under realistic industrial heating conditions. Heating rates as low as 0.08 K/s as well as isothermal heat treatments at 550 °C for 60 min are insufficient to prevent melting of the binary Al-Si layer at the eutectic temperature. A two-stage treatment in which the coating was pre-treated up to 650 °C to form solid Al-Si-Fe intermetallics is investigated. Intermetallic formation at 650 °C is shown to occur very quickly due to the enhanced iron diffusion in the liquid Al-Si phase. The coating of pre-treated blanks does not melt during the second stage of the heat treatment; however the properties of the final component obtained via this two-stage heat treatment remain to be assessed.