Transmission electron microscopy studies of thermomechanically control processed multiphase medium-carbon microalloyed steel: Forged, rolled, and low-cycle fatigued microstructures |
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Authors: | S Sankaran Gouthama S Sangal K A Padmanabhan |
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Affiliation: | (1) Center for Structural and Functional Materials and the Department of Chemical Engineering, University of Louisiana at Lafayette, 70503 Lafayette, LA;(2) Department of Materials and Metallurgical Engineering, Indian Institute of Technology, 208 016 Kanpur, India;(3) School of Physics, University of Hyderabad, 500 046 Hyderabad, India |
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Abstract: | A ferrite-bainite-martensite (F-B-M) microstructure was produced in a medium-carbon microalloyed (MA) steel through two routes,
namely, low-temperature finish forging and rolling, followed by a two-step cooling (TSC) and annealing. Transmission electron
microscopy (TEM) was employed to study the microstructural evolution in control forged and rolled material after TSC followed
by annealing (TSCA). A TEM investigation was also carried out on samples low-cycle fatigue (LCF) tested at low and high total
strain amplitudes of 0.4 and 0.7 pct in case of the forged steel (F-B-M(F)TSCA) and 0.55 and 0.8 pct for the rolled steel
(F-B-M(R)TSCA), respectively. Microstructural changes accompanying the LCF testing were identified. The two-step cooled microstructure
processed through forging (F-B-M(F)TSC) as well as rolling (F-B-M(R)TSC) revealed a complex multiphase microstructure, along
with films and blocks of retained austenite. In both microstructural conditions, vanadium carbide precipitates were too fine
to be identified after the TSC treatment. Annealing after TSC produced a stress-free microstructure. The F-B-M(F)TSCA microstructure
predominantly consisted of granular/lower bainite, lath martensite, and polygonal ferrite with interlath films as well as
blocks of retained austenite, while the F-B-M(R)TSCA microstructure predominantly consisted of lath martensite, granular/lower
bainite, and polygonal ferrite with interlath strips/films of retained austenite. Lath martensite content was higher in the
F-B-M(R)TSCA condition than in the F-B-M(R)TSCA condition. In both conditions, vanadium carbide precipitates could be seen
after annealing. Fatigue-tested F-B-M(F)TSCA microstructure up to a total strain amplitude of 0.4 pct and F-B-M(F)TSCA microstructure
up to a total strain amplitude of 0.55 pct were stable. Lath martensite did not undergo deformation and in both microstructural
conditions dislocation cell structures were not observed in the ferrite or bainite regions. The interlath retained austenite
strips/films played a significant role in preventing the softening during fatigue loading. First, it was stable up to a total
strain amplitude of 0.4 and 0.55 pct in the respective microstructures. Second, it underwent heavy deformation during fatigue
loading at high total strain amplitudes, thereby accommodating the strain. Fatigue-tested F-B-M(F)TSCA microstructure at a
total strain amplitude of 0.7 pct and F-B-M(R)TSCA microstructure at a total strain amplitude of 0.8 pct revealed deformed
bainite/martensite laths, dislocation cells, and slip bands in the ferrite regions, which are characteristic features of cyclic
softening. The retained austenite transformed to martensite through a strain-induced transformation mechanism and, at that
stage, the microstructure contained in addition dislocation-rich bainite and ferrite. |
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