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.

Transgranular Cracking in a Liquid Zn Embrittled High Strength Steel

“…Transgranular cracking may also be facilitated by bulk diffusion of Zn from liquid infiltrated boundaries, resulting in Zn enrichment of the ferrite in vicinity of the crack tip. Bulk Zn diffusion occurring simultaneously with liquid penetration, has been confirmed by previous Zn LME studies [6][7][8] . The greater Zn diffusivity in ferrite compared to that in austenite might thus help facilitate transgranular cracking in the ferrite.…”

The influence of pre-plating on the LME phenomenon of advanced high strength steel

“…Due to the low melting temperature of Zn and Fe-Zn intermetallic compounds, most of the Zn or Zn-alloy coatings would partially turn to liquid at the austenitizing termpertature. The melted Zn appears to penetrate into the austenite grain boundaries causing an embrittlement characterized by surface crack formation under the influences of high temperature and complex stress, leading to the phase-transformation of austenite to ferrite [3]. The penetration of Zn is the main reason causing embrittlement, which called liquid metal embrittlement (LME) [4].…”

“…The micro-cracks found in the resistance spot welded AHSS could be attributed to tensile-stress corrosion by liquid zinc that penetrate into the grain boundaries of the substrate [7,8]. The LME cracks degraded the mechanical properties of the high strength steel [3,9]. The reliability and mechanical performance of the welds are affects with the growth of these cracks by the vibrations of the automobiles [10].…”

Fusion Zone Microstructural Evolution of Al-10% Si Coated Hot Stamping Steel during Laser Welding

“…To suppress LME, a diffusion annealing treatment to eliminate Fe 3 Zn 10 IMC or an indirect hot stamping process can be used. 12,13) Application of TWB is beneficial in that the mechanical properties can be locally adjusted by a proper choice of the joining partner. 14) A blank can consist of different strength grades and sheet thicknesses, a feature that cannot be used with other methods to tailor properties.…”