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Effect of Austenitizing Temperature and Prior Martensite on Ultra-Fine Bainite Transformation Kinetics
Open AccessArticle

Bainite Transformation-Kinetics-Microstructure Characterization of Austempered 4140 Steel

1
Technology Center, Fiat Chrysler USA LLC, 800 Chrysler Dr. Auburn Hills, MI 48326, USA
2
Department of Mechanical Engineering, Oakland University, Rochester, MI 48306, USA
*
Author to whom correspondence should be addressed.
Metals 2020, 10(2), 236; https://doi.org/10.3390/met10020236
Received: 15 December 2019 / Revised: 23 January 2020 / Accepted: 3 February 2020 / Published: 10 February 2020
(This article belongs to the Special Issue Bainite and Martensite Transformation in Steel)
Bainite transformation is a kinetic process that involves complex solid diffusion and phase structure evolution. This research systematically studies the bainite transformation of austempered 4140 steel in a wide range of isothermal temperatures, in which four bainite phases structures were generated: upper bainite; mixed upper bainite and lower bainite; lower bainite and mixed lower bainite and martensite. The kinetics of bainite transformation has been described with a linear trend using an Avrami n-value. It was found that the bainitic ferrite sheaves grow with widthwise preference. The sheaves are stable when half-grown and are variable in length, due to austenite size limit or soft/hard impingement, or autocatalytic nucleation, or these conditions combined. The full-grown upper/lower bainite sheaves were found to be 1.9 μm/1.2 μm in width under the conditions of this study. Each individual bainite sheave is lath-like instead of wedge-like. The upper bainite sheaves mostly appear as broad-short-coarse lath, while the lower bainite sheaves appear as narrow-long-fine lath. The overall bainite transformation activation energy ranges from 50–167 kJ/mol.
Keywords: upper/lower bainite; lower bainite and martensite mixture; kinetics; morphology; bainite sheaves; activation energy upper/lower bainite; lower bainite and martensite mixture; kinetics; morphology; bainite sheaves; activation energy
MDPI and ACS Style

Zhu, J.G.; Sun, X.; Barber, G.C.; Han, X.; Qin, H. Bainite Transformation-Kinetics-Microstructure Characterization of Austempered 4140 Steel. Metals 2020, 10, 236.

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