Carbide-Free Bainite Transformations Above and Below Martensite Start Temperature Investigated by In-Situ High-Energy X-Ray Diffraction

Carbide-free bainitic advanced high-strength steels (AHSS) exhibits multiphase microstructures made of fine ferrite and retained austenite. The carbon partitions between these constituting phases during the bainitic transformation were studied above and below the martensite start temperature ( M S )...

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Bibliographic Details
Published in:JOM (1989) 2021-11, Vol.73 (11), p.3181-3194
Main Authors: Rampelberg, Cécile, Allain, Sébastien Yves Pierre, Geandier, Guillaume, Teixeira, Julien, Lebel, Florimonde, Sourmail, Thomas
Format: Article
Language:English
Subjects:
Advanced High-Strength Steels
Bainite
Bainitic transformations
Carbides
Carbon
Carbon content
Chemistry/Food Science
Cooling
Earth Sciences
Energy
Engineering
Engineering Sciences
Environment
Experiments
Ferrite
High strength steels
Martensite
Martensitic transformations
Materials
Microstructure
Nucleation
Physics
Retained austenite
Sensors
Steel
Temperature
X-ray diffraction
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Summary:Carbide-free bainitic advanced high-strength steels (AHSS) exhibits multiphase microstructures made of fine ferrite and retained austenite. The carbon partitions between these constituting phases during the bainitic transformation were studied above and below the martensite start temperature ( M S ) by in-situ high-energy X-ray diffraction experiments. The time-resolved analysis of the phase fraction, austenite lattice parameter, and ferrite tetragonality offer the possibility of establishing phase-scaled consistent and precise carbon balances. In the studied steel, the carbon content in ferrite ranges between 0.1 and 0.15 wt.% after isothermal treatments, meaning that the available carbon for austenite stabilization is reduced. The stasis conditions are close to the T 0 ′ criterion below 400°C but are not reached at higher temperatures. Our multi-step experiments also prove that the transformation below M S is very sensitive to the carbon content in austenite and auto-catalytic nucleation processes inherited from prior transformations (martensite and bainite at higher temperature). The bainitic transformation thus does not follow additivity rules.
ISSN:1047-4838
1543-1851
DOI:10.1007/s11837-021-04903-8