Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel

Qing Qiang Ren; Sung Il Baik; Dong An; Mingfang Zhu; Bruce W. Krakauer; David N. Seidman

doi:10.1016/j.matchar.2020.110544

Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel

Qing Qiang Ren, Sung Il Baik, Dong An, Mingfang Zhu, Bruce W. Krakauer, David N. Seidman

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Abstract

Phase-transformations and microstructural evolution during and after inter-critical annealing of a low-alloy, dual-phase steel are studied utilizing dilatometric measurements, and microstructural characterization methods, including scanning electron microscopy, synchrotron X-ray diffraction, and atom-probe tomography. Dilatometric measurements reveal that the sample contains ~72% austenite, after annealing for 5-min at 871 °C, which agrees with the water-quenched microstructure, and ~90% after further annealing to 1 h, which agrees with thermodynamic predictions. A continuous-cooling transformation (CCT) diagram is constructed, within the cooling-rate range between 1 °C/s and 70 °C/s. The incubation time for the ferrite-transformation is <1 s, after inter-critical annealing, and the secondary-phases are mainly pearlite for cooling-rates of <5 °C/s, and martensite for cooling-rates >10 °C/s, which is confirmed utilizing scanning electron-microscope observations. The microhardness increases as the cooling rate increases, and an abrupt slope-change is explained by the major secondary-phase changing from martensite to pearlite between 5 °C/s and 10 °C/s. Atom-probe tomography demonstrates the coexistence of martensite and pearlite as secondary-phases for 30 °C/s and a switch in austenite-ferrite transformation mechanisms from para-equilibrium (70 °C/s) to partitioning local-equilibrium (30 °C/s). A microstructural map is constructed to reveal the austenite decomposition products in the cooling-rate range between 1 °C/s and 70 °C/s. Based on composition measurements of the matrix, utilizing APT, the effects of cooling-rates on solid-solution-strengthening are estimated, and the results demonstrate that solid-solution strengthening, especially carbon-redistribution, accounts for the majority of the strength variations due to different cooling-rates.

Original language	English
Article number	110544
Journal	Materials Characterization
Volume	168
DOIs	https://doi.org/10.1016/j.matchar.2020.110544
State	Published - Oct 2020
Externally published	Yes

Funding

This research was supported financially by A. O. Smith Corporation , Milwaukee, WI. Atom-probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI ( DMR-0420532 ) and ONR-DURIP ( N00014-0400798 , N00014-0610539 , N00014-0910781 , N00014-1712870 ) programs. NUCAPT received support from the MRSEC program ( NSF DMR-1720139 ) at the Materials Research Center , the SHyNE Resource ( NSF ECCS-1542205 ), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University . This research also made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource ( NSF NNCI-1542205 ); the MRSEC program ( NSF DMR-1121262 ) at the Materials Research Center ; the International Institute for Nanotechnology (IIN); the Keck Foundation ; and the State of Illinois , through the IIN. QQR also acknowledges Professor Wei Zhang, Eddie Pfeifer, Ying Lu, and Jennifer Semple at the Ohio State University for helping with Gleeble testing and data analysis, and research associate professor Gautam Ghosh at Northwestern University for providing the kMart-thermodynamic-database. This research was supported financially by A. O. Smith Corporation, Milwaukee, WI. Atom-probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870) programs. NUCAPT received support from the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the SHyNE Resource (NSF ECCS-1542205), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University. This research also made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. QQR also acknowledges Professor Wei Zhang, Eddie Pfeifer, Ying Lu, and Jennifer Semple at the Ohio State University for helping with Gleeble testing and data analysis, and research associate professor Gautam Ghosh at Northwestern University for providing the kMart-thermodynamic-database. The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations and will be made available by request.

Keywords

Atom-probe tomography
CCT diagram
Cooling rate
Dual-phase steel
Inter-critical annealing
Phase transformations

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10.1016/j.matchar.2020.110544

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title = "Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel",

abstract = "Phase-transformations and microstructural evolution during and after inter-critical annealing of a low-alloy, dual-phase steel are studied utilizing dilatometric measurements, and microstructural characterization methods, including scanning electron microscopy, synchrotron X-ray diffraction, and atom-probe tomography. Dilatometric measurements reveal that the sample contains \textasciitilde{}72\% austenite, after annealing for 5-min at 871 °C, which agrees with the water-quenched microstructure, and \textasciitilde{}90\% after further annealing to 1 h, which agrees with thermodynamic predictions. A continuous-cooling transformation (CCT) diagram is constructed, within the cooling-rate range between 1 °C/s and 70 °C/s. The incubation time for the ferrite-transformation is <1 s, after inter-critical annealing, and the secondary-phases are mainly pearlite for cooling-rates of <5 °C/s, and martensite for cooling-rates >10 °C/s, which is confirmed utilizing scanning electron-microscope observations. The microhardness increases as the cooling rate increases, and an abrupt slope-change is explained by the major secondary-phase changing from martensite to pearlite between 5 °C/s and 10 °C/s. Atom-probe tomography demonstrates the coexistence of martensite and pearlite as secondary-phases for 30 °C/s and a switch in austenite-ferrite transformation mechanisms from para-equilibrium (70 °C/s) to partitioning local-equilibrium (30 °C/s). A microstructural map is constructed to reveal the austenite decomposition products in the cooling-rate range between 1 °C/s and 70 °C/s. Based on composition measurements of the matrix, utilizing APT, the effects of cooling-rates on solid-solution-strengthening are estimated, and the results demonstrate that solid-solution strengthening, especially carbon-redistribution, accounts for the majority of the strength variations due to different cooling-rates.",

keywords = "Atom-probe tomography, CCT diagram, Cooling rate, Dual-phase steel, Inter-critical annealing, Phase transformations",

author = "Ren, \{Qing Qiang\} and Baik, \{Sung Il\} and Dong An and Mingfang Zhu and Krakauer, \{Bruce W.\} and Seidman, \{David N.\}",

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year = "2020",

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doi = "10.1016/j.matchar.2020.110544",

language = "English",

volume = "168",

journal = "Materials Characterization",

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T1 - Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel

AU - Ren, Qing Qiang

AU - Baik, Sung Il

AU - An, Dong

AU - Zhu, Mingfang

AU - Krakauer, Bruce W.

AU - Seidman, David N.

PY - 2020/10

Y1 - 2020/10

N2 - Phase-transformations and microstructural evolution during and after inter-critical annealing of a low-alloy, dual-phase steel are studied utilizing dilatometric measurements, and microstructural characterization methods, including scanning electron microscopy, synchrotron X-ray diffraction, and atom-probe tomography. Dilatometric measurements reveal that the sample contains ~72% austenite, after annealing for 5-min at 871 °C, which agrees with the water-quenched microstructure, and ~90% after further annealing to 1 h, which agrees with thermodynamic predictions. A continuous-cooling transformation (CCT) diagram is constructed, within the cooling-rate range between 1 °C/s and 70 °C/s. The incubation time for the ferrite-transformation is <1 s, after inter-critical annealing, and the secondary-phases are mainly pearlite for cooling-rates of <5 °C/s, and martensite for cooling-rates >10 °C/s, which is confirmed utilizing scanning electron-microscope observations. The microhardness increases as the cooling rate increases, and an abrupt slope-change is explained by the major secondary-phase changing from martensite to pearlite between 5 °C/s and 10 °C/s. Atom-probe tomography demonstrates the coexistence of martensite and pearlite as secondary-phases for 30 °C/s and a switch in austenite-ferrite transformation mechanisms from para-equilibrium (70 °C/s) to partitioning local-equilibrium (30 °C/s). A microstructural map is constructed to reveal the austenite decomposition products in the cooling-rate range between 1 °C/s and 70 °C/s. Based on composition measurements of the matrix, utilizing APT, the effects of cooling-rates on solid-solution-strengthening are estimated, and the results demonstrate that solid-solution strengthening, especially carbon-redistribution, accounts for the majority of the strength variations due to different cooling-rates.

AB - Phase-transformations and microstructural evolution during and after inter-critical annealing of a low-alloy, dual-phase steel are studied utilizing dilatometric measurements, and microstructural characterization methods, including scanning electron microscopy, synchrotron X-ray diffraction, and atom-probe tomography. Dilatometric measurements reveal that the sample contains ~72% austenite, after annealing for 5-min at 871 °C, which agrees with the water-quenched microstructure, and ~90% after further annealing to 1 h, which agrees with thermodynamic predictions. A continuous-cooling transformation (CCT) diagram is constructed, within the cooling-rate range between 1 °C/s and 70 °C/s. The incubation time for the ferrite-transformation is <1 s, after inter-critical annealing, and the secondary-phases are mainly pearlite for cooling-rates of <5 °C/s, and martensite for cooling-rates >10 °C/s, which is confirmed utilizing scanning electron-microscope observations. The microhardness increases as the cooling rate increases, and an abrupt slope-change is explained by the major secondary-phase changing from martensite to pearlite between 5 °C/s and 10 °C/s. Atom-probe tomography demonstrates the coexistence of martensite and pearlite as secondary-phases for 30 °C/s and a switch in austenite-ferrite transformation mechanisms from para-equilibrium (70 °C/s) to partitioning local-equilibrium (30 °C/s). A microstructural map is constructed to reveal the austenite decomposition products in the cooling-rate range between 1 °C/s and 70 °C/s. Based on composition measurements of the matrix, utilizing APT, the effects of cooling-rates on solid-solution-strengthening are estimated, and the results demonstrate that solid-solution strengthening, especially carbon-redistribution, accounts for the majority of the strength variations due to different cooling-rates.

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KW - CCT diagram

KW - Cooling rate

KW - Dual-phase steel

KW - Inter-critical annealing

KW - Phase transformations

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Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel

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