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Metals
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High Performance Bainitic Steels
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High Performance Bainitic Steels

A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Metal Casting, Forming and Heat Treatment".

Deadline for manuscript submissions: closed (30 June 2022) | Viewed by 23040

Special Issue Editors

Prof. Dr. Zhinan Yang

E-Mail Website
Guest Editor
School of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China
Interests: microstructure–property relationships of high-performance bainitic steel; accelerating method of bainite phase transformation; high-strength–high-ductility steel
Dr. Guhui Gao

E-Mail Website
Guest Editor
School of Mechatronics, Beijing Jiaotong University, Beijing 102603, China
Interests: bainitic steels; bainitic transformation; quenching and partitioning steels; fatigue of bainitic steels
Dr. Haijiang Hu

E-Mail Website
Guest Editor
State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
Interests: ausforming and transformation kinetics in advanced high-strength bainite steels; microstructure and property control of ultra-high strength steels
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Bainite steel is a well-known type of high-performance steel. However, the microstructure of bainite is complex and more sensitive to chemical decomposition and heat treatment processes compared with other traditional microstructures. The morphologies, volume fraction, stability of retained austenite, and size of ferrite and the carbon content within it all play important roles in determining the mechanical properties of bainite steel. Therefore, improving the mechanical properties of bainitic steel via control of chemical composition and microstructure is one of the main research fields of bainitic steel. The transformation rate of bainite is relatively slow, especially for high-carbon bainitic steel, which represents a barrier for its industrial application. Therefore, accelerating the transformation kinetics, via changing alloying elements, novel heat treatment process, or other methods, is an important aspect in the research of bainitic steel. Works that focus on developing new bainitic steels, novel heat treatment processes, novel microstructures, new methods to accelerate transformation processes, mechanical performance, and fatigue behavior of bainitic steel are especially encouraged. Moreover, works studying the performance of bainitic steel during its service lifetime are also encouraged.

Prof. Dr. Zhinan Yang
Dr. Guhui Gao
Dr. Haijiang Hu
Guest Editors

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Keywords

  • Bainitic Steel
  • Phase Transformation
  • Chemical Composition
  • Microstructure
  • Heat Treatment
  • Mechanical Properties

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Published Papers (8 papers)

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Research

15 pages, 6999 KiB  
Article
Effect of Two-Step High Temperature Treatment on Phase Transformation and Microstructure of V-Bearing Bainitic Steel
by Bo Lv, Dongxin Yin, Dongyun Sun, Zhinan Yang, Xiaoyan Long and Zeliang Liu
Metals 2022, 12(6), 983; https://doi.org/10.3390/met12060983 - 7 Jun 2022
Cited by 1 | Viewed by 1753
Abstract
The effects of VC precipitation on phase transformation, microstructure, and mechanical properties were studied by controlling two-step isothermal treatment, i.e., austenization followed by intercritical transformation. The results show that the bainite transformation time of 950 °C–860 °C treatment and 950 °C–848 °C treatment [...] Read more.
The effects of VC precipitation on phase transformation, microstructure, and mechanical properties were studied by controlling two-step isothermal treatment, i.e., austenization followed by intercritical transformation. The results show that the bainite transformation time of 950 °C–860 °C treatment and 950 °C–848 °C treatment is shorter than that of 950 °C single-step treatment. This is related to the isothermal ferrite transformation in the intercritical transformation range. The formation of ferrite nuclei increases the density of medium temperature bainite nucleation sites and decrease the bainite nucleation activation energy. At the same time, a large number of VC particles are precipitated. The additional VC particles provide numbers of preferential nucleation sites. The toughness of the specimen treated at 950~870 °C is improved, which is related to the large proportion of high angle grain boundaries. High angle grain boundaries can hinder crack propagation or change the direction of crack propagation. The specimen treated at 950 °C–848 °C exhibits large proportion of low angle grain boundaries, which is beneficial for the strength improvement. Full article
(This article belongs to the Special Issue High Performance Bainitic Steels)
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Figure 1

Figure 1
<p>Thermal expansion curve.</p> Full article ">Figure 2
<p>Schematic diagram of heat treatment process.</p> Full article ">Figure 3
<p>(<b>a</b>) V component in the austenite phase with the increase of the austenitization temperature; (<b>b</b>) and (<b>c</b>) Relationship between VC volume fraction and time.</p> Full article ">Figure 4
<p>(<b>a</b>) Temperature–expansion curves at different austenitization temperatures; (<b>b</b>) Enlarged figure of (<b>a</b>) located at the transformation point.</p> Full article ">Figure 5
<p>(<b>a</b>) Time–expansion curves at different temperatures; (<b>b</b>) changes in expansion amount under different processes; (<b>c</b>) isothermal curves of different processes; and (<b>d</b>) bainite transformation rates under different heat treatment process.</p> Full article ">Figure 6
<p>SEM micrographs of specimens with different heat treatment processes. (<b>a</b>) 950 °C; (<b>b</b>) 950 °C–890 °C; (<b>c</b>) 950 °C–870 °C; (<b>d</b>) 950 °C–860 °C; and (<b>e</b>) 950 °C–848 °C. (BF: bainitic ferrite; F-RA: film retained austenite; B-RA: blocky retained austenite).</p> Full article ">Figure 7
<p>EDS analysis of specimen treated at 950 °C–870 °C and 950 °C–848 °C (<b>a</b>,<b>b</b>).</p> Full article ">Figure 8
<p>TEM micrographs of specimens with different heat treatment processes. (<b>a</b>,<b>b</b>) 950 °C; (<b>c</b>,<b>d</b>) 950 °C–870 °C; and (<b>e</b>,<b>f</b>) 950 °C–848 °C.</p> Full article ">Figure 8 Cont.
<p>TEM micrographs of specimens with different heat treatment processes. (<b>a</b>,<b>b</b>) 950 °C; (<b>c</b>,<b>d</b>) 950 °C–870 °C; and (<b>e</b>,<b>f</b>) 950 °C–848 °C.</p> Full article ">Figure 9
<p>TEM map scanning images of different heat treatment processes (<b>a</b>,<b>d</b>) 950 °C; (<b>b</b>) 950 °C–870 °C; and (<b>c</b>) 950 °C–848 °C.</p> Full article ">Figure 9 Cont.
<p>TEM map scanning images of different heat treatment processes (<b>a</b>,<b>d</b>) 950 °C; (<b>b</b>) 950 °C–870 °C; and (<b>c</b>) 950 °C–848 °C.</p> Full article ">Figure 10
<p>(<b>a</b>) High magnification TEM image of 950–848 process; (<b>b</b>) High resolution micrographs image of 950 °C–848 °C process.</p> Full article ">Figure 11
<p>Engineering stress–strain curves of tested steel under different heat treatment processes.</p> Full article ">Figure 12
<p>XRD patterns of the specimens after high-temperature two-step isothermal treatment.</p> Full article ">Figure 13
<p>IPF maps ((<b>a</b>) 950 °C; (<b>b</b>) 950 °C–870 °C; and (<b>c</b>) 950 °C–848 °C) and misorientation angle distributions (<b>d</b>) of the samples after high-temperature two-step isothermal processes.</p> Full article ">Figure 13 Cont.
<p>IPF maps ((<b>a</b>) 950 °C; (<b>b</b>) 950 °C–870 °C; and (<b>c</b>) 950 °C–848 °C) and misorientation angle distributions (<b>d</b>) of the samples after high-temperature two-step isothermal processes.</p> Full article ">
15 pages, 5360 KiB  
Article
High-Cycle Fatigue Life and Strength Prediction for Medium-Carbon Bainitic Steels
by Yusong Fan, Xiaolu Gui, Miao Liu, Xi Wang, Chun Feng and Guhui Gao
Metals 2022, 12(5), 856; https://doi.org/10.3390/met12050856 - 17 May 2022
Cited by 6 | Viewed by 2258
Abstract
High-cycle fatigue (HCF) behaviors of medium-carbon bainitic steels with various inclusion sizes and microstructural features were studied using the rotating–bending fatigue test. Here, the medium-carbon bainitic steels with different melting processes were treated by three heat treatment routes incorporating bainite formation, namely bainite-based [...] Read more.
High-cycle fatigue (HCF) behaviors of medium-carbon bainitic steels with various inclusion sizes and microstructural features were studied using the rotating–bending fatigue test. Here, the medium-carbon bainitic steels with different melting processes were treated by three heat treatment routes incorporating bainite formation, namely bainite-based quenching plus partitioning (BQ&P), bainite austempering (BAT) and “disturbed bainite austempering, DBAT”. The interior inclusion-induced crack initiation (IICI) and noninclusion-induced crack initiation (NIICI) modes were found after fatigue failure. The fracture surface of IICI is characterized by a “fish-eye” surrounding a “fine granular area, FGA” in the vicinity of an inclusion. In contrast, a microfacet, instead of an inclusion, is found at the center of FGA for the NIICI fracture surface. The predications of fatigue strength and life were performed on the two crack initiation modes based on fracture surface analysis. The results showed that a majority of fatigue life is consumed within the FGA for both the IICI and NIICI failure modes. The fatigue strength of the NIICI-fatigued samples can be conveniently predicted via the two parameters of the hardness of the sample and the size of the microfacet. Full article
(This article belongs to the Special Issue High Performance Bainitic Steels)
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Figure 1

Figure 1
<p>Microstructure of (<b>a</b>) BAT [<a href="#B28-metals-12-00856" class="html-bibr">28</a>], (<b>b</b>) DABT and (<b>c</b>) BQ&amp;P samples; B: bainite, B<sub>l</sub>: leaf-shaped bainite, M/A: martensite/austenite, M<sub>2</sub>: carbon-depleted martensite, RA: retained austenite.</p> Full article ">Figure 2
<p>The geometry and dimensions of the smooth hourglass-type specimens.</p> Full article ">Figure 3
<p><span class="html-italic">S-N</span> data of steels treated by various melting and heat treatment processes; (<b>a</b>): UBAT; (<b>b</b>): EBAT; (<b>c</b>): DBAT and (<b>d</b>): BQ&amp;P; Sur: surface-defect-induced crack initiation, Sur (Inc): surface-inclusion-induced crack initiation, Inter (Inc): interior-inclusion-induced crack initiation, NIICI: interior-noninclusion-induced crack initiation (crack initiated from microstructure).</p> Full article ">Figure 4
<p>SEM images of fatigue crack initiation morphologies; (<b>a</b>): UBAT sample, surface inclusion (<span class="html-italic">σ<sub>a</sub></span> = 824 MPa, <span class="html-italic">N<sub>f</sub></span> = 1.16 × 10<sup>6</sup> cycles); (<b>b</b>): BQ&amp;P sample, surface inclusion (<span class="html-italic">σ<sub>a</sub></span> = 902 MPa, <span class="html-italic">N<sub>f</sub></span> = 3.82 × 10<sup>5</sup> cycles); (<b>c</b>): BQ&amp;P sample, interior inclusion (<span class="html-italic">σ<sub>a</sub></span> = 843 MPa, <span class="html-italic">N<sub>f</sub></span> = 8.33 × 10<sup>6</sup> cycles); (<b>d</b>) The enlarged window of (<b>c</b>); (<b>e</b>): UBAT sample, NIICI (<span class="html-italic">σ<sub>a</sub></span> = 726 MPa, <span class="html-italic">N<sub>f</sub></span> = 5.28 × 10<sup>6</sup> cycles); (<b>f</b>) The enlarged window of (<b>e</b>); FiE: fish-eye, FGA: fine granular area.</p> Full article ">Figure 4 Cont.
<p>SEM images of fatigue crack initiation morphologies; (<b>a</b>): UBAT sample, surface inclusion (<span class="html-italic">σ<sub>a</sub></span> = 824 MPa, <span class="html-italic">N<sub>f</sub></span> = 1.16 × 10<sup>6</sup> cycles); (<b>b</b>): BQ&amp;P sample, surface inclusion (<span class="html-italic">σ<sub>a</sub></span> = 902 MPa, <span class="html-italic">N<sub>f</sub></span> = 3.82 × 10<sup>5</sup> cycles); (<b>c</b>): BQ&amp;P sample, interior inclusion (<span class="html-italic">σ<sub>a</sub></span> = 843 MPa, <span class="html-italic">N<sub>f</sub></span> = 8.33 × 10<sup>6</sup> cycles); (<b>d</b>) The enlarged window of (<b>c</b>); (<b>e</b>): UBAT sample, NIICI (<span class="html-italic">σ<sub>a</sub></span> = 726 MPa, <span class="html-italic">N<sub>f</sub></span> = 5.28 × 10<sup>6</sup> cycles); (<b>f</b>) The enlarged window of (<b>e</b>); FiE: fish-eye, FGA: fine granular area.</p> Full article ">Figure 5
<p>Schematic showing whole fatigue process from crack initiation to final fracture for (<b>a</b>) inclusion-induced crack initiation and (<b>b</b>) “noninclusion-induced crack initiation”; TA: transition area, FiE: fish-eye, FGA: fine granular area, SCG: steady crack growth, FCG: fast crack growth, TP: transition point. Reprinted with permission from Refs. [<a href="#B29-metals-12-00856" class="html-bibr">29</a>,<a href="#B30-metals-12-00856" class="html-bibr">30</a>]. 2022, Elsevier.</p> Full article ">Figure 6
<p>Inclusion, microfacet and FGA size versus applied fatigue life for interior IICI and NIICI mode. Inc and I-FGA: inclusion and FGA in IICI fracture surface; Fac and NI-FGA: microfacet and FGA in NIICI fracture surface.</p> Full article ">Figure 7
<p>Stress intensity factor range of inclusion and the inclusion-induced FGA (IF) of bainitic steels.</p> Full article ">Figure 8
<p>Stress intensity factor range of microfacet and the noninclusion-induced FGA (NF) of bainitic steels.</p> Full article ">Figure 9
<p>(<b>a</b>) Variation of Δ<span class="html-italic">K</span><sub>FGA</sub> vs. <span class="html-italic">area</span><sub>FGA</sub><sup>1/6</sup> and (<b>b</b>) variation of Δ<span class="html-italic">K</span><sub>fac</sub> vs. <span class="html-italic">area</span><sub>fac</sub><sup>1/6</sup> for bainitic steels; IF: inclusion-induced FGA, NF: noninclusion-induced FGA.</p> Full article ">Figure 10
<p>Normalized crack initiation life, i.e., the ratio of fatigue life due to crack initiation within FGA, as a function of total fatigue life for four bainitic steels.</p> Full article ">Figure 11
<p>The average crack growth rate within FGA as a function of total fatigue life for four bainitic steels.</p> Full article ">Figure 12
<p>Comparison of experimental and predicted fatigue strengths of steels using exiting formulae, solid points for IICI estimation and triangle points for NIICI estimation [<a href="#B13-metals-12-00856" class="html-bibr">13</a>,<a href="#B36-metals-12-00856" class="html-bibr">36</a>,<a href="#B37-metals-12-00856" class="html-bibr">37</a>,<a href="#B38-metals-12-00856" class="html-bibr">38</a>].</p> Full article ">
14 pages, 4127 KiB  
Article
Real-Time Quality Monitoring of Laser Cladding Process on Rail Steel by an Infrared Camera
by Pornsak Srisungsitthisunti, Boonrit Kaewprachum, Zhigang Yang and Guhui Gao
Metals 2022, 12(5), 825; https://doi.org/10.3390/met12050825 - 11 May 2022
Cited by 9 | Viewed by 4692
Abstract
Laser cladding is considered to be a highly complex process to set up and control because it involves several parameters, such as laser power, laser scanning speed, powder flow rate, powder size, etc. It has been widely studied for metal-part coating and repair [...] Read more.
Laser cladding is considered to be a highly complex process to set up and control because it involves several parameters, such as laser power, laser scanning speed, powder flow rate, powder size, etc. It has been widely studied for metal-part coating and repair due to its advantage in controllable deposited materials on a small target substrate with low heat-affected distortion. In this experiment, laser cladding of U75V and U20Mn rail steels with Inconel 625 powder was captured by an infrared camera with image analysis software to monitor the laser cladding process in order to determine the quality of the cladded substrates. The cladding temperature, thermal gradient, spot profile, and cooling rate were determined from infrared imaging of the molten pool. The results showed that cladding temperature and molten pool’s spot closely related to the laser cladding process condition. Infrared imaging provided the cooling rate from a temperature gradient which was used to correctly predict the microhardness and microstructure of the HAZ region. This approach was able to effectively detect disturbance and identify geometry and microstructure of the cladded substrate. Full article
(This article belongs to the Special Issue High Performance Bainitic Steels)
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Figure 1

Figure 1
<p>Experimental setup for process monitoring of laser cladding.</p> Full article ">Figure 2
<p>Online monitoring of laser-cladding experiment’s equipment.</p> Full article ">Figure 3
<p>Laser-cladding process and infrared image of the laser hot spot on the substrate taken by an IR camera.</p> Full article ">Figure 4
<p>Image-analysis steps with LabVIEW to determine the molten pool’s size from infrared image.</p> Full article ">Figure 5
<p>Parametric study of laser cladding on (<b>a</b>,<b>b</b>) U75V and (<b>c</b>,<b>d</b>) U20Mn rail steels. For all cases, the laser cladded four times (four layers) on the substrates. (<b>a</b>,<b>c</b>) Effect of scanning speed and (<b>b</b>,<b>d</b>) effect of laser power.</p> Full article ">Figure 6
<p>SEM images from cross-section of cladded rail steels at different conditions. For all cases, the laser cladded four times (four layers) on the substrates.</p> Full article ">Figure 7
<p>Temperature converted from infrared images during laser cladding for different laser powers and scanning speeds.</p> Full article ">Figure 8
<p>Laser hot-spot profiles during laser cladding on U75V substrate for different laser powers and scanning speeds.</p> Full article ">Figure 9
<p>Temperature profile of laser hot spots plotted along the dashed lines from <a href="#metals-12-00825-f008" class="html-fig">Figure 8</a>.</p> Full article ">Figure 10
<p>Microstructure of the interface zone between the cladded material and the heat-affected zone of substrate on U75V rail steels with different scanning speeds.</p> Full article ">Figure 11
<p>Correlation between IR image width (molten pool’s width) and clad width from experiment.</p> Full article ">Figure 12
<p>Correlation between IR image width (W<sub>IR</sub>) and actual clad width (W<sub>Clad</sub>) from experiment.</p> Full article ">Figure 13
<p>Laser cladding on U75V with laser power 500 W scan speed 1 mm/s with 10-layer deposition resulted in porosities. (<b>a</b>) Cladded substrate, (<b>b</b>) substrate’s cross-section, (<b>c</b>) IR image during laser cladding, (<b>d</b>,<b>e</b>) SEM images at the interface region and (<b>f</b>) SEM at the HAZ region.</p> Full article ">
14 pages, 9661 KiB  
Article
Microstructure Characteristics and Wear Performance of a Carburizing Bainitic Ferrite + Martensite Si/Al-Rich Gear Steel
by Yanhui Wang, Qingsong He, Qian Yang, Dong Xu, Zhinan Yang and Fucheng Zhang
Metals 2022, 12(5), 822; https://doi.org/10.3390/met12050822 - 10 May 2022
Cited by 4 | Viewed by 2433
Abstract
In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different [...] Read more.
In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different times, were carried out on the newly designed gear steel. After heat treatment, a series of different microstructures (superfine bainitic ferrite + retained austenite, superfine bainitic ferrite + martensite + retained austenite, and martensite + retained austenite) were obtained on the surface, whilst the low-carbon lath martensitic microstructure was obtained in the core. The microstructure of the surface was examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The phase composition was analyzed using X-ray diffraction (XRD). The hardness and wear resistance of the surface as well as the hardness distribution of carburizing layer of the samples with different microstructures were studied. The results show that the Si/Al-rich gear steel, after carburizing and austempering at 200 °C for 8 h, not only has excellent mechanical properties but also has high wear resistance, which meets the technical requirements of heavy-duty gear steel. The research work in this paper can provide a data reference for the application of carburized steel with mixed microstructures of bainitic ferrite and martensite in the design of heavy-duty gear. Full article
(This article belongs to the Special Issue High Performance Bainitic Steels)
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Figure 1

Figure 1
<p>Schematic diagrams of carburizing and heat treatment processes.</p> Full article ">Figure 2
<p>The optical micrographs of carburized surface austempered at 200 °C: (<b>a</b>) 2 h, (<b>b</b>) 4 h, (<b>c</b>) 8 h, (<b>d</b>) 12 h, (<b>e</b>) 24 h, and (<b>f</b>) 48 h.</p> Full article ">Figure 3
<p>The optical micrographs of carburized surface austempered at 230 °C: (<b>a</b>) 2 h, (<b>b</b>) 4 h, (<b>c</b>) 8 h, (<b>d</b>) 12 h, (<b>e</b>) 24 h, and (<b>f</b>) 48 h.</p> Full article ">Figure 4
<p>The SEM images of the carburized surface austempered at 200 °C: (<b>a</b>) 2 h, (<b>b</b>) 8 h, and (<b>c</b>) 48 h. Notes: BF—bainitic ferrite, RA—retained austenite, and M—martensite.</p> Full article ">Figure 5
<p>The SEM images of the carburized surface austempered at 230 °C: (<b>a</b>) 2 h and (<b>b</b>) 48 h. Notes: BF—bainitic ferrite, RA—retained austenite, and M—martensite.</p> Full article ">Figure 6
<p>The TEM micrographs of the carburized surface after isothermal treatment: (<b>a</b>) 200 °C × 8 h, (<b>b</b>) 200 °C × 48 h, (<b>c</b>) 230 °C × 8 h, and (<b>d</b>) 230 °C × 48 h. Notes: BF—bainitic ferrite, RA—retained austenite, and M—martensite.</p> Full article ">Figure 7
<p>XRD patterns of carburized surface austempered at 200 °C (<b>a</b>) and 230 °C (<b>b</b>).</p> Full article ">Figure 8
<p>Carbon content distribution curve of the specimen after carburizing.</p> Full article ">Figure 9
<p>Vickers hardness distributions of carburized layer austempered at 200 °C (<b>left</b>) and 230 °C (<b>right</b>) for different times.</p> Full article ">Figure 10
<p>Relationship curves between weight loss and wear time of the carburized surface austempered at 200 °C.</p> Full article ">Figure 11
<p>Wear morphology of carburized surface austempered at 200 °C: (<b>a</b>) 2 h, (<b>b</b>) 8 h, (<b>c</b>) 48 h.</p> Full article ">Figure 12
<p>XRD patterns of the surface at 200 °C for 8 h before and after wear.</p> Full article ">Figure 13
<p>Optical micrographs of the transition layer (<b>a</b>) and core (<b>b</b>) of the experimental steel, after carburizing and austempering at 200 °C for 8 h.</p> Full article ">
14 pages, 4139 KiB  
Article
Corrosion Behavior of Multiphase Bainitic Rail Steels
by Tanaporn Rojhirunsakool, Thammaporn Thublaor, Mohammad Hassan Shirani Bidabadi, Somrerk Chandra-ambhorn, Zhigang Yang and Guhui Gao
Metals 2022, 12(4), 694; https://doi.org/10.3390/met12040694 - 18 Apr 2022
Cited by 7 | Viewed by 2517
Abstract
Pearlitic steel experiences excessive corrosion in a hot and humid atmosphere. The multiphase bainitic/martensitic structure was developed for a better combination of strength and ductility, especially rolling contact fatigue, but little attention to corrosion has been investigated. Corrosion behaviors of multiphase steels obtained [...] Read more.
Pearlitic steel experiences excessive corrosion in a hot and humid atmosphere. The multiphase bainitic/martensitic structure was developed for a better combination of strength and ductility, especially rolling contact fatigue, but little attention to corrosion has been investigated. Corrosion behaviors of multiphase steels obtained from bainitic-austempering (BAT) and bainitic-quenching and -partitioning (BQ&P) processes were investigated via immersion and electrochemical tests in 3.5 wt.% NaCl solution. The corroded surface and rust after immersion and electrochemical tests were analyzed via electron microscopy, Fourier transform infrared spectra, and x-ray diffraction. The multiphase bainite + martensite/retained austenite island showed higher corrosion resistance than that of the pearlitic one. The acicular bainite obtained from the BQ&P process showed slightly higher corrosion resistance than the granular bainite + martensite structure obtained from the BAT process. Full article
(This article belongs to the Special Issue High Performance Bainitic Steels)
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Figure 1

Figure 1
<p>Schematic diagram of different heat-treatment processes: (<b>a</b>) U75V, (<b>b</b>) U20Mn-1, and (<b>c</b>) U20Mn-2 steels.</p> Full article ">Figure 2
<p>SEM micrographs of different microstructures for rail steel: (<b>a</b>) U75V, (<b>b</b>) U20Mn-1, and (<b>c</b>) U20Mn-2 steels (F: proeutectoid ferrite, P: pearlite, B: bainite, and M: martensite).</p> Full article ">Figure 3
<p>Corrosion rates of rail steels after immersion tests.</p> Full article ">Figure 4
<p>FTIR spectra of rust of (<b>a</b>) U75V, (<b>b</b>) U20Mn-1, and (<b>c</b>) U20Mn-2 steels after immersion test for 21 days.</p> Full article ">Figure 5
<p>XRD spectra of rust of (<b>a</b>) U75V, (<b>b</b>) U20Mn-1, and (<b>c</b>) U20Mn-2 steels after immersion test for 21 days.</p> Full article ">Figure 6
<p>SEM micrographs of (<b>a</b>) U75V, (<b>b</b>) U20Mn-1, and (<b>c</b>) U20Mn-2 steels after immersion for 21 days.</p> Full article ">Figure 7
<p>Polarization curves of U75V, U20Mn-1, and U20Mn-2 steels after electrochemical test in freely aerated 3.5 % NaCl solution.</p> Full article ">Figure 8
<p>The corrosion rate of U75V, U20Mn-1, and U20Mn-2 steels after electrochemical test in freely aerated 3.5 %NaCl solution.</p> Full article ">Figure 9
<p>SEM micrographs of (<b>a</b>) U75V, (<b>b</b>) U20Mn-1, and (<b>c</b>) U20Mn-2 steels after electrochemical test in freely aerated 3.5 % NaCl solution.</p> Full article ">Figure 10
<p>Tafel slope of U75V, U20Mn-1, and U20Mn-2 steels after electrochemical test in freely aerated 3.5 % NaCl solution.</p> Full article ">
15 pages, 5979 KiB  
Article
Relationship between Microstructure and Properties of 1380 MPa Grade Bainitic Rail Steel Treated by Online Bainite-Based Quenching and Partitioning Concept
by Miao Liu, Yusong Fan, Xiaolu Gui, Jie Hu, Xi Wang and Guhui Gao
Metals 2022, 12(2), 330; https://doi.org/10.3390/met12020330 - 13 Feb 2022
Cited by 13 | Viewed by 2659
Abstract
According to the concept of the bainite-based quenching and partitioning (BQ&P) process, we designed the online heat treatment routes of bainitic rail steel for heavy haul railway. The new heat treatment process reduced the fraction and size of the blocky martensite/austenite (M/A) islands [...] Read more.
According to the concept of the bainite-based quenching and partitioning (BQ&P) process, we designed the online heat treatment routes of bainitic rail steel for heavy haul railway. The new heat treatment process reduced the fraction and size of the blocky martensite/austenite (M/A) islands formed during the conventional air-cooling process. The M/A islands are coarse and undesirable for mechanical properties. A new kind of 1380 MPa grade bainitic rail steel with more uniform microstructure and better mechanical properties was produced by the online BQ&P process. We characterized the multiphase microstructures containing bainite, martensite, and retained austenite of 1380 MPa grade bainitic rail steels via optical microscope, scanning electron microscopy, transmission electron microscopy, and X-ray diffractometer. We investigated in-depth the relationship between the microstructure, retained austenite stability, and mechanical properties, particularly the resistance to wear and rolling contact fatigue, of the new 1380 MPa grade bainitic rail steels. Meanwhile, the conventional air-cooling bainitic rail steel was studied as a comparison. Full article
(This article belongs to the Special Issue High Performance Bainitic Steels)
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<p>Schematic of the microstructural evolution of bainitic rail steels during natural air-cooling process and on-line heat treatment based on BQ&amp;P concept. (<b>a</b>) Air-cooling process for 1280 G rail steel; (<b>b</b>) on-line BQ&amp;P for 1380 G rail steel; A: austenite, B: bainite, M: martensite.</p> Full article ">Figure 2
<p>Photograph of the GPM-30D twin-disc wear testing machine.</p> Full article ">Figure 3
<p>Microstructure of 1280 G and 1380 G rail steels. (<b>a</b>,<b>b</b>) 1280G; (<b>c</b>,<b>d</b>) 1380G; (<b>a</b>,<b>c</b>) OM images; (<b>b</b>,<b>d</b>) SEM images; B: bainite, M: martensite, BF: bainitic ferrite, M/A: martensite/austenite, RA: retained austenite.</p> Full article ">Figure 4
<p>TEM images of the microstructure of 1280 G and 1380 G rail steels. (<b>a</b>) 1280G; (<b>b</b>) 1380G; BF: bainite ferrite, M: martensite.</p> Full article ">Figure 5
<p>TEM images of microstructure in 1380 G rail steels. (<b>a</b>) bright field image showing lath bainite with inter-lath film-like retained austenite, where the inset image is the selected area electron diffraction patterns; (<b>b</b>) dark field image showing retained austenite; (<b>c</b>,<b>d</b>) carbide in lower bainite; (<b>e</b>,<b>f</b>) martensite lath and inter-lath film-like retained austenite.</p> Full article ">Figure 6
<p>Change of retained austenite amount with engineering strain for 1280 G and 1380 G rail steels. Note: the errors of all the results obtained by XRD are within ±5%.</p> Full article ">Figure 7
<p>Change of wear mass loss of 1280 G and 1380 G rail steels with rolling cycles.</p> Full article ">Figure 8
<p>Worn surface of 1280 G rail steel under different rolling cycles. (<b>a</b>,<b>b</b>) 1.0 × 10<sup>5</sup> cycles; (<b>c</b>,<b>d</b>) 2.5 × 10<sup>5</sup> cycles; (<b>e</b>,<b>f</b>) 5.0 × 10<sup>5</sup> cycles; (<b>g</b>,<b>h</b>) 7.5 × 10<sup>5</sup> cycles.</p> Full article ">Figure 9
<p>Worn surface of 1380 G rail steel under different rolling cycles. (<b>a</b>) 1.0 × 10<sup>5</sup> cycles; (<b>b</b>) 2.5 × 10<sup>5</sup> cycles; (<b>c</b>) 5.0 × 10<sup>5</sup> cycles; (<b>d</b>) 7.5 × 10<sup>5</sup> cycles.</p> Full article ">Figure 10
<p>Hardness distribution of worn samples with the depth from top surface. (<b>a</b>) 1280 G rail steel; (<b>b</b>) 1380 G rail steel.</p> Full article ">Figure 11
<p>Microstructure of the longitudinal section of the 1280 G and 1380 G rail steels under different rolling cycles. (<b>a</b>) 1280 G after 1 × 10<sup>4</sup> cycles; (<b>b</b>) 1280 G after 7.5 × 10<sup>4</sup> cycles, where yellow dashed lines show the uneven deformation of segregation bands; (<b>c</b>) 1380 G after 1 × 10<sup>4</sup> cycles; (<b>d</b>) 1380 G after 7.5 × 10<sup>4</sup> cycles.</p> Full article ">Figure 11 Cont.
<p>Microstructure of the longitudinal section of the 1280 G and 1380 G rail steels under different rolling cycles. (<b>a</b>) 1280 G after 1 × 10<sup>4</sup> cycles; (<b>b</b>) 1280 G after 7.5 × 10<sup>4</sup> cycles, where yellow dashed lines show the uneven deformation of segregation bands; (<b>c</b>) 1380 G after 1 × 10<sup>4</sup> cycles; (<b>d</b>) 1380 G after 7.5 × 10<sup>4</sup> cycles.</p> Full article ">Figure 12
<p>(<b>a</b>) Photograph of a 1380 G bainitic rail steel after the experience of ~500 million gross tons at the curved track of heavy haul railway; (<b>b</b>) the cross-section profile, where the blue and red lines show the original and worn profiles, respectively.</p> Full article ">
11 pages, 6161 KiB  
Article
Effect of Austempering below and above Ms on the Microstructure and Wear Performance of a Low-Carbon Bainitic Steel
by Zhirui Wei, Haijiang Hu, Man Liu, Junyu Tian and Guang Xu
Metals 2022, 12(1), 104; https://doi.org/10.3390/met12010104 - 5 Jan 2022
Cited by 5 | Viewed by 3153
Abstract
The microstructure and wear performance of a low-carbon steel treated by austempering below and above martensite start temperature (Ms) were investigated. The results show that the bainite, fresh martensite (FM) and retained austenite (RA) were observed in samples austempered above Ms. Except for [...] Read more.
The microstructure and wear performance of a low-carbon steel treated by austempering below and above martensite start temperature (Ms) were investigated. The results show that the bainite, fresh martensite (FM) and retained austenite (RA) were observed in samples austempered above Ms. Except for the three above phases, the athermal martensite (AM) was also observed in samples austempered below Ms. The bainite transformation was accelerated and finer bainite was obtained due to the AM formation in samples austempered below Ms. In addition, the strength and hardness were improved with the decrease of the isothermal temperature and time, whereas the total elongation decreased with the increasing isothermal time and the decreasing isothermal temperature. Moreover, the materials austempered below Ms exhibited better wear performance than the ones treated above Ms, which is attributed to the improved impact toughness by the finer bainite and the enhanced hardness by AM. The best wear resistance was obtained in the samples austempered at 300 °C below Ms for 200 s, due to the highest hardness and considerable impact toughness. Full article
(This article belongs to the Special Issue High Performance Bainitic Steels)
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<p>Temperature verses dilation during cooling process.</p> Full article ">Figure 2
<p>The experimental procedure.</p> Full article ">Figure 3
<p>Schematic diagram of the impact-abrasive wear machine.</p> Full article ">Figure 4
<p>Dilation–temperature curves of cooling process: (<b>a</b>) isothermal temperature above Ms; and (<b>b</b>) isothermal temperature below Ms.</p> Full article ">Figure 5
<p>Dilation–time curves: (<b>a</b>) the total dilation of bainite and martensite; (<b>b</b>) the dilation of isothermal transformation; and (<b>c</b>) bainite transformation rate.</p> Full article ">Figure 6
<p>SEM micrograph of different samples: (<b>a</b>) B200, (<b>b</b>) B400, (<b>c</b>) A200 and (<b>d</b>) A400.</p> Full article ">Figure 6 Cont.
<p>SEM micrograph of different samples: (<b>a</b>) B200, (<b>b</b>) B400, (<b>c</b>) A200 and (<b>d</b>) A400.</p> Full article ">Figure 7
<p>(<b>a</b>) The dilation curves of direct quenching sample; (<b>b</b>) the example of lever rule; (<b>c</b>) the example of net dilation of B200 and B400; and (<b>d</b>) the volume fraction of phases.</p> Full article ">Figure 7 Cont.
<p>(<b>a</b>) The dilation curves of direct quenching sample; (<b>b</b>) the example of lever rule; (<b>c</b>) the example of net dilation of B200 and B400; and (<b>d</b>) the volume fraction of phases.</p> Full article ">Figure 8
<p>Stress–strain curves of different samples.</p> Full article ">Figure 9
<p>Worn surface of different samples: (<b>a</b>) A200; (<b>b</b>) A400; (<b>c</b>) B200; and (<b>d</b>) B400.</p> Full article ">Figure 9 Cont.
<p>Worn surface of different samples: (<b>a</b>) A200; (<b>b</b>) A400; (<b>c</b>) B200; and (<b>d</b>) B400.</p> Full article ">Figure 10
<p>(<b>a</b>) Mass loss during six wear cycles; and (<b>b</b>) total mass loss verses wear time curves.</p> Full article ">
15 pages, 7824 KiB  
Article
The Corrosion and Wear Behaviors of a Medium-Carbon Bainitic Steel Treated by Boro-Austempering Process
by Man Liu, Wei Wang, Haijiang Hu, Feng Cai, Sheng Liu and Guang Xu
Metals 2021, 11(12), 1959; https://doi.org/10.3390/met11121959 - 6 Dec 2021
Cited by 3 | Viewed by 1954
Abstract
The effects of boro-austempering treatment on growth kinetics of borided layers, microstructure, and properties in a medium-carbon bainitic steel were investigated. The microstructure, distribution in coatings, corrosion, and wear properties of boro-austempered steels were characterized by a microscope, field-emission electron probe micro analyzer, [...] Read more.
The effects of boro-austempering treatment on growth kinetics of borided layers, microstructure, and properties in a medium-carbon bainitic steel were investigated. The microstructure, distribution in coatings, corrosion, and wear properties of boro-austempered steels were characterized by a microscope, field-emission electron probe micro analyzer, scanning vibrating electrode technique system and wear resistance machine. The results show that the corrosion resistance of steels in different corrosive mediums was significantly enhanced by boro-austempering treatment. In addition, the wear performance of borided layers was improved by more than two times compared to bainitic substrates, proving a better wear property of samples treated through the boro-austempering route. The solubility of carbon and silicon in borides is very little. In addition, the dual-phase coating of FeB and Fe2B was observed, and the internal stress induced during the growth of Fe2B and FeB was almost eliminated. The preferential crystallographic growth directions of Fe2B and FeB are [001] and [010], respectively, which belongs to the (100) plane. Finally, the kinetics equation d2 = 0.125·t of the borided layers at 1223 K was established. Full article
(This article belongs to the Special Issue High Performance Bainitic Steels)
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<p>Schematic experimental procedures: (<b>a</b>) boro-austempering treatment; (<b>b</b>) merely austempering.</p> Full article ">Figure 2
<p>OM microstructure of substrates in samples (<b>a</b>) BA-2 and (<b>b</b>) NA-2.</p> Full article ">Figure 3
<p>SEM images to measure the prior austenite grains of boro-austempered samples with varied boriding time: (<b>a</b>) 1 h; (<b>b</b>) 2 h; (<b>c</b>) 6 h; (<b>d</b>) 8 h.</p> Full article ">Figure 4
<p>Cross-sectional OM morphologies of boro-austempered samples with different boriding times: (<b>a</b>) 0.5 h; (<b>b</b>) 1 h; (<b>c</b>) 2 h; (<b>d</b>) 6 h; (<b>e</b>) 8 h.</p> Full article ">Figure 5
<p>Square of the coating thickness versus boriding time and fitting curve.</p> Full article ">Figure 6
<p>EBSD images of borided coating of sample BA-8: (<b>a</b>) KAM map; (<b>b</b>) IPF map; (<b>c</b>) phase map; (<b>d</b>) IPF of Fe<sub>2</sub>B; (<b>e</b>) IPF of FeB.</p> Full article ">Figure 7
<p>Map scanning of elements along the depth of the produced layer from the surface of sample BA-8: (<b>a</b>) origin image; (<b>b</b>) distribution of C content; (<b>c</b>) distribution of B content; (<b>d</b>) distribution of Si content.</p> Full article ">Figure 8
<p>(<b>a</b>) OM indentation morphology of produced layer on bainitic steel; (<b>b</b>) the VDI-3198 standard of indentation ratings.</p> Full article ">Figure 9
<p>Hardness profiles from the outermost produced layer to the substrate.</p> Full article ">Figure 10
<p>Effect of the boriding process (BA-2) on the potentiodynamic polarization curves in different corrosive mediums: (<b>a</b>) 0.5 mol/L HCl; (<b>b</b>) 10% NaOH; (<b>c</b>) 3.5% NaCl.</p> Full article ">Figure 11
<p>Effect of the immersion time on in situ SVET images of boro-austempered steel in 0.5% NaCl solution: (<b>a</b>) 0.5 h; (<b>b</b>) 1.5 h; (<b>c</b>) 2.5 h; (<b>d</b>) 3.5 h.</p> Full article ">Figure 12
<p>Friction coefficients recorded under dry sliding for different samples.</p> Full article ">Figure 13
<p>Morphologies of wear tracks of samples: (<b>a</b>) and (<b>b</b>) NA-2; (<b>c</b>) and (<b>d</b>) BA-2.</p> Full article ">Figure 14
<p>(<b>a</b>) wear loss and (<b>b</b>) wear rate of samples BA-2 and NA-2 in different wear durations.</p> Full article ">Figure 15
<p>SEM wear morphologies of samples (<b>a</b>) BA-2 and (<b>b</b>) NA-2 after sliding abrasion.</p> Full article ">
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