The effect of heating rate and coating weight on the intermetallic growth of Al[sbnd]Si coated hot stamping steel

Abstract

The effect of furnace heating rate on the intermetallic growth of AlSi coatings for 22MnB5 hot stamping steel was characterized and quantified for two different commercial coating weights, AS80 and AS150. Characterizing and quantifying the formation of the AlFe and Al-Fe-Si intermetallic layers is crucial to the optimization of the austenitization process, which continually strives to reduce furnace time while maintaining acceptable intermetallic development. Initial nominal heating rates up to the AlSi melting temperature (577 °C) varied from Low (7 °C/s) to Very High (23 °C/s) and then proceed to reduce which resulted in the nominal heating rates to vary from Low (1 °C/s) to Very High (7 °C/s) within a temperature range of 600 to 900 °C. The coating composition was similar at 600 °C for all the heating rate tests and suggests that there is potential to save furnace time by imposing high initial heating rates. SEM and EDS analysis was conducted on the coating at 50 °C intervals. The overall intermetallic transformation occurred significantly faster at the higher heating rates due to the accelerated diffusion of Fe into the coating. The intermetallic transformation also occurred faster for the thinner AS80 coating due to the shorter diffusion path of Fe. A simple empirical model was developed to predict the coating transformation percentage for any temperature and time from 600 °C. The formation of FeAl and Fe-Al-Si intermetallics was characterized. For each coating weight, the evolution of each intermetallic species was consistent for each heating rate, but at a delayed rate for each subsequently higher heating rate. Additionally, the tests conducted in this work showed that full austenitization of the base metal can be achieved during the heating phase (72 s for Very High heating rate).