University of Oulu

Nanoscale, 2023,15, 10004-10016,

A combined 3D-atomic/nanoscale comprehension and ab initio computation of iron carbide structures tailored in Q&P steels via Si alloying

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Author: Ghosh, Sumit1; Rakha, Khushboo2; Sasikala Devi, Assa Aravindh3;
Organizations: 1Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, Oulu, 90014, Finland
2Department of Metallurgical and Materials Engineering, Indian Institute of Technology Ropar, Rupnagar, 140001, India
3Nano and Molecular Systems Research Unit, University of Oulu, Oulu, 90014, Finland
4Materials and Metallurgical Division, Spica Research, Dhaka, 1229, Bangladesh
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 6.4 MB)
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Language: English
Published: Royal Society of Chemistry, 2023
Publish Date: 2023-11-09


The essences of the quenching and partitioning (Q&P) process are to stabilize the finely divided retained austenite (RA) via carbon (C) partitioning from supersaturated martensite during partitioning. Competitive reactions, i.e., transition carbide precipitation, C segregation, and decomposition of austenite, might take place concurrently during partitioning. In order to maintain the high volume fraction of RA, it is crucial to suppress the carbide precipitation sufficiently. Since silicon (Si) in the cementite θ (Fe₃C) is insoluble, alloying Si in adequate concentrations prolongs its precipitation during the partitioning step. Consequently, C partitioning facilitates the desired chemical stabilization of RA. To elucidate the mechanisms of formation of transition η (Fe₂C) carbides as well as cementite, θ (Fe₃C), besides the transformation of transition carbides to more stable θ during the quenching and partitioning (Q&P) process, samples of 0.4 wt% C steels tailored with different Si contents were extensively characterized for microstructural evolution at different partitioning temperatures (TP) using high resolution transmission electron microscopy (HR-TEM) and three-dimensional atom probe tomography (3D-APT). While 1.5 wt% Si in the steel allowed only the formation of η carbides even at a high TP of 300 °C, reduction in Si content to 0.75 wt% only partially stabilized η carbides, allowing limited ηθ transformation. With 0.25 wt% Si, only θ was present in the microstructure, suggesting a ηθ transition during the early partitioning stage, followed by coarsening due to enhanced growth kinetics at 300 °C. Although η carbides precipitated in martensite under paraequilibrium conditions at 200 °C, θ carbides precipitated under negligible partitioning local equilibrium conditions at 300 °C. Competition with the formation of orthorhombic η and θ precipitation further examined via ab initio (density functional theory, DFT) computation and a similar probability of formation/thermodynamic stability were obtained. With an increase in Si concentration, the cohesive energy decreased when Si atoms occupied C positions, indicating decreasing stability. Overall, the thermodynamic prediction was in accord with the HR-TEM and 3D-APT results.

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Series: Nanoscale
ISSN: 2040-3364
ISSN-E: 2040-3372
ISSN-L: 2040-3364
Volume: 15
Pages: 10004´ - 10016
DOI: 10.1039/d3nr00816a
Type of Publication: A1 Journal article – refereed
Field of Science: 114 Physical sciences
214 Mechanical engineering
119 Other natural sciences
Funding: The authors express their gratitude to Jane ja Aatos Erkon säätiö (JAES), Tiina ja Antti Herlinin säätiö (TAHS), and the Academy of Finland (grant No. #311934) for their financial support on the Advanced Steels for Green Planet project and the Genome of Steel (Profi3) project.
Copyright information: This journal is © The Royal Society of Chemistry 2023. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.