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Dissertation/Thesis Abstract

Bainite Transformation Kinetics-Microstructure Characterization of Austempered 4140/4150 Steel
by Zhu, Jian G., Ph.D., Oakland University, 2019, 177; 13810256
Abstract (Summary)

Bainite transformation in steel involves understanding of its kinetics, morphology, and the activation energy. This research studied the characteristics of all three of these aspects for 4140/4150 steel in a wide range of austempring processes, in which four bainite phase matrices were transformed: upper bainite, mixed upper bainite and lower bainite, lower bainite, and mixed lower bainite and martensite. The kinetics of 4140/4150 steel bainite transformation has been described with a linear trend using an Avrami n value. The bainitic ferrite sheaves grow with widthwise preference. The sheaves are stable when half-grown and are variable in length, due to the austenite size limit, soft/hard impingement, and autocatalytic nucleation. The full-grown upper/lower bainite sheaves were found to be 1.9 µm/1.2 µm in width. Each individual bainite sheave was lath-like instead of wedge-like as usually described. The upper bainite sheaves mostly appeared as broad-short-coarse lath, and the lower bainite sheaves appeared as narrow-long-fine lath. The overall bainite transformation activation energy ranged from 50–167KJ/mol as found from the Arrhenius equation.

4140 and 4150 steel are different only in terms of carbon content. A comparison between the kinetics and microstructure of the bainite transformation of the two steels has been carried out to determine the effect of carbon content in the transformation in terms of kinetics-morphology characteristics of the four phase matrices. The bainite reaction time span is shorter when carbon content is higher. The n values in the JMKA equation have a wider deviation when carbon content is higher, indicating more bainite reaction variations. The activation energies required were compared and it was found that overall higher activation energy is needed when carbon content is higher.

Higher dislocation density is one of the results of bainitic transformation. The average dislocation density needs to be known to relate the bainitic microstructures to the mechanical properties. Using X-ray diffraction technology and diffraction line width broadening analysis, this research quantified the average dislocation density in the four bainite phase matrices. The effect of isothermal temperatures on the average dislocation density is assessed for different thermal dynamic driving forces in terms of activation energy and cooling rate.

Indexing (document details)
Advisor: Barber, Gary, Sun, Xichen
Commitee: Schall, David, Shillor, Mier
School: Oakland University
Department: Engineering
School Location: United States -- Michigan
Source: DAI-B 80/11(E), Dissertation Abstracts International
Subjects: Mechanical engineering, Materials science
Keywords: Activation energy, Bainite microstructure, Bainite phase matrices, Transformation kinetics
Publication Number: 13810256
ISBN: 978-1-392-24465-4
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