Research

15 pages, 6999 KiB

Open AccessEditor’s ChoiceArticle

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 - 07 Jun 2022

Viewed by 1487

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)

► Show Figures

Figure 1

15 pages, 5360 KiB

Open AccessArticle

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 4 | Viewed by 1837

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)

► Show Figures

Figure 1

14 pages, 4127 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

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 5 | Viewed by 4071

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)

► Show Figures

Figure 1

14 pages, 9661 KiB

Open AccessEditor’s ChoiceArticle

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 3 | Viewed by 2054

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)

► Show Figures

Figure 1

14 pages, 4139 KiB

Open AccessArticle

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 6 | Viewed by 2156

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)

► Show Figures

Figure 1

15 pages, 5979 KiB

Open AccessArticle

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 12 | Viewed by 2271

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)

► Show Figures

Figure 1

11 pages, 6161 KiB

Open AccessArticle

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 - 05 Jan 2022

Cited by 5 | Viewed by 1450

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)

► Show Figures

Figure 1

15 pages, 7824 KiB

Open AccessArticle

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 - 06 Dec 2021

Cited by 3 | Viewed by 1738

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)

► Show Figures

Figure 1

Journal Menu

Journal Browser

High Performance Bainitic Steels

Share This Special Issue

Special Issue Editors

Special Issue Information

Keywords

Published Papers (8 papers)

Research

Further Information

Guidelines

MDPI Initiatives

Follow MDPI