Experimental and computer thermodynamics evaluations of an Al-Si-Coating on a quenchable steel

Vicente Braz Trindade,Hans-Juergen Christ

doi:10.1590/0370-44672016700080

Abstract

High-strength steels are commonly used in the automobile industry in order to reduce the weight of the vehicles. However, a technical difficulty appears due to the need of hot stamping of the components, which leads to oxidation. Therefore, the application of a coating on the substrate to avoid high-temperature oxidation is used. In this work, experimental analysis and computer thermodynamic calculation were used to describe the phase transformations within an Al-Si coating on a quenchable high strength steel. The Al-Si coating was deposited by hot dipping and its characterization was done using SEM and XRD techniques. Computer thermodynamics calculations were done using the commercial software FactSage using the Calphad methodology. It demonstrated a good relationship between the experimental results and the computer calculations of phase stabilities for the as-deposited condition and after diffusion experiment at 920oC for 7 minutes, which simulates the thermal cycle of hot stamping of the quenchable steel used.

Highlights

The demand for weight reduction in mechanical structures and at the same time, car safety improvement require the use of ultra-high-strength steels of high toughness and good deformability
A generally accepted Fe-Si phase diagram is shown in Figure 4a, which was calculated using the commercial software FactSage
The Fe-Al phase diagram is as complex as the Fe-Si system

Summary

Introduction

The demand for weight reduction in mechanical structures and at the same time, car safety improvement require the use of ultra-high-strength steels of high toughness and good deformability. High-strength steel components with complex geometries are deformed at high temperatures. Iron forms very stable oxides at high temperatures, especially above 570oC due to the fast growing of wustite (FeO). Ultra-high-strength steel plates are often hot-dip coated with the Al–Si alloy (Grauer et al, 2015; Borsetto et al, 2009). The coating is deposited using an eutectic composition, which melts at 577oC (Paar et al, 2008) and the diffusion of iron from the steel substrate into the molten Al–Si layer forms intermetallic phases (Al–Si–Fe) with a melting temperature around 1200oC (Grauer et al, 2015; Engels et al, 2006; Gui et al 2014), which is higher than the temperature used during hot stamping (around 920oC)

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Conclusion