Zn Diffusion and α-Fe(Zn) Layer Growth During Annealing of Zn-Coated B Steel

Vit Janik, Yongjun Lan, Peter Beentjes, David Norman, Guido Hensen, Seetharaman Sridhar

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    Abstract

    Direct hot press forming of Zn-coated 22MnB5 steels is impeded by micro-cracks that occur in the substrate due to the presence of Zn during the forming process. A study was therefore undertaken to quantify concentration of Zn across the α-Fe(Zn) coating and on grain boundaries in the α-Fe(Zn) layer and the underlying γ-Fe(Zn) substrate after isothermal annealing of Zn-coated 22MnB5 at 1173 K (900 °C) and to link the Zn distribution to the amount and type of micro-cracks observed in deformed samples. Finite difference model was developed to describe Zn diffusion and the growth of the α-Fe(Zn) layer. The penetration of Zn into the γ-Fe(Zn) substrate after 600 seconds annealing at 1173 K (900 °C) through bulk diffusion is estimated to be 3 μm, and the diffusion depth of Zn on the γ-Fe(Zn) grain boundaries is estimated to be 6 μm, which is significantly shorter than the maximum length (15 to 50 μm) of the micro-cracks formed in the severely stressed conditions, indicating that the Zn diffusion into the γ-Fe(Zn) from the α-Fe(Zn) during annealing is not correlated to the depth of micro-cracks. On the other hand, the maximum amount of Zn present in α-Fe(Zn) layer decreases with annealing time as the layer grows and Zn oxidizes, and the amount of Zn-enriched areas inside the α-Fe(Zn) layer is reduced leading to reduced length of cracking. Solid-Metal-Induced Embrittlement mechanism is proposed to explain the benefit of extended annealing on reduced depth of micro-crack penetration into the γ-Fe(Zn) substrate.

    Publisher Statement: The final publication is available at Springer via http://dx.doi.org/10.1007/s11661-015-3203-y


    Original languageEnglish
    Pages (from-to)400-411
    Number of pages12
    JournalMetallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
    Volume47
    Issue number1
    Early online date28 Oct 2015
    DOIs
    Publication statusPublished - 6 Jan 2016

    Bibliographical note

    The final publication is available at Springer via http://dx.doi.org/10.1007/s11661-015-3203-y

    Keywords

    • Zn coated boron steels
    • hot press forming
    • heat treatment
    • FIB lift-out
    • Zn concentration
    • 1D FD model
    • α-Fe(Zn)
    • γ-Fe(Zn)

    ASJC Scopus subject areas

    • Condensed Matter Physics
    • Mechanics of Materials
    • Metals and Alloys

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