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

• Melting of Al–Si coating is investigated via differential scanning calorimetry.
• Melting cannot be avoided by reducing sample heating rate.
• Blank pre-treatment at 650 °C prevents further melting of Al–Si coating.

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.