Abstract
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.
Highlights
In certain temperature ranges, isothermal heat-treated steels contain bainite [1,2]
Liu and Di developed a method to add an autocatalysis factor into the Johnson-Mehl-Kolmogorov-Avrami equation (JMKA)/Avrami to cover the nucleation on the austenite grain boundary to better reveal the bainite transformation kinetics [28]
This overall activation energy includes the energy needed for bainitic ferrite grain boundary nucleation and autocatalytic nucleation, subsequent carbon partitioning and dislocation effects, and bainitic ferrite plate growth until impingement occurs at the austenite grain boundary
Summary
In certain temperature ranges, isothermal heat-treated steels contain bainite [1,2]. By heating steels or cast irons above their eutectoid reaction temperature, austenite can be obtained through the austenitizing process. The bainite transformation volume fraction increases with isothermal holding time This is the kinetic aspect of the microstructure phase changes. The thermodynamically induced physical-chemical driving force behind the reaction is expressed well by the Arrhenius equation [7], which can be applied to evaluate the phase transformation, and to assess newly proposed kinetic models. The Arrhenius equation has been applied for more than 70 years since it was published It remains popular and important in phase transformation evaluations. The knowledge acquired in this work can help develop improved mechanical properties in terms of strength, ductility, hardness and toughness due to the microstructures achieved
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