Liquid-Metal-Induced Embrittlement Related Microcrack Propagation on Zn-coated Press Hardening Steel

Abstract

The coatings on press hardening steel are needed to suppress high temperature oxidation and decarburization. Additional corrosion protection is provided by Zn coatings which provide cathodic protection to press hardened parts. Due to the low melting temperature of Zn and Zn–Fe intermetallic compounds, the Zn-coated PHSs are susceptible to LMIE during the die-quenching process. In the present work, the mechanical properties of Zn coated PHS were evaluated in terms of tensile properties and bending properties. A deterioration of the room temperature bendability, due to microcrack formation and propagation, was observed. The presence of Γ-Fe3Zn10 observed in the microcracks at room temperature correspond to liquid Zn at the die-quenching temperature, making it possible to trace the progress of the liquid Zn phase during microcrack formation. The results suggest that Zn-grain boundary diffusion causes the phase transformation to ferrite of the austenite grain boundary region. This results in intergranular cracking due to the lower strength of ferrite. The LMIE microcrack formation is most severe in areas where the applied stress and the friction are highest during the forming process, making the occurrence of LMIE-mitigated microcrack propagation dependent on the local deformation conditions.