Effect of ultra-fast heat treatment on the subsequent formation of mixed martensitic/bainitic microstructure with carbides in a crmo medium carbon steel

Spyros Papaefthymiou, Alexandros Banis, Marianthi Bouzouni, Roumen H. Petrov

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    Abstract

    The current work focuses on complex multiphase microstructures gained in CrMo medium carbon steel after ultra-fast heat treatment, consisting of heating with heating rate of 300 C/s, 2 s soaking at peak temperature and subsequent quenching. In order to better understand the microstructure evolution and the phenomena that take place during rapid heating, an ultra-fast heated sample was analyzed and compared with a conventionally treated sample with a heating rate of 10 C/s and 360 s soaking. The initial microstructure of both samples consisted of ferrite and spheroidized cementite. The conventional heat treatment results in a fully martensitic microstructure as expected. On the other hand, the ultra-fast heated sample shows significant heterogeneity in the final microstructure. This is a result of insufficient time for cementite dissolution, carbon diffusion and chemical composition homogenization at the austenitization temperature. Its final microstructure consists of undissolved spheroidized cementite, nano-carbides and martensite laths in a ferritic matrix. Based on EBSD and TEM analysis, traces of bainitic ferrite are indicated. The grains and laths sizes observed offer proof that a diffusionless, massive transformation takes place for the austenite formation and growth instead of a diffusion-controlled transformation that occurs on a conventional heat treatment.

    Original languageEnglish
    Article number312
    Number of pages14
    JournalMetals
    Volume9
    Issue number3
    DOIs
    Publication statusPublished - 2019

    Keywords

    • AHSS
    • Diffusionless transformations
    • Martensite/bainite
    • Ultra-fast heating

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