In‑Situ Observation of Martensitic Transformation in a Fe–C–Mn–Si Bainitic Steel During Austempering

Junyu Tian,Guang Xu,Zhengyi Jiang,Haijiang Hu,Qing Yuan,Xiangliang Wan 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.7

The martensitic transformation in a Fe–C–Mn–Si bainitic steel was examined by in situ high-temperature laser scanningconfocal microscopy (LSCM) and dilatometry. The phenomenon of continuous martensitic transformation during austemperingwas firstly dynamically observed by LSCM. Differing from the commonly accepted viewpoint on martensite formationin bainitic steels, the martensitic transformation in the conventional medium-carbon bainitic steel was not instantaneous andproceeded gradually when the sample was austempered below martensite starting temperature (MS). It can be attributed tothe generation of internal stresses, thermal activation, stimulating nucleation, and the segregation of Mn. In addition, apartfrom the continuous martensitic transformation, the bainitic transformation was also directly observed by LSCM duringaustempering below MS. Moreover, it was clear from the results of dilatation during austempering that the inflection point inthe dilatation curve against time was not the demarcation point between martensitic and bainitic transformation, and in situobservations confirmed that martensite was still formed after the inflection point. Therefore, the obtained results could be anexcellent reference to further understand the mechanism of bainitic and martensitic transformations in Fe–C–Mn–Si bainiticsteel during austempering below MS.

Effect of Ni Addition on Bainite Transformation and Properties in a 2000 MPa Grade Ultrahigh Strength Bainitic Steel

Junyu Tian,Guang Xu,Zhengyi Jiang,Haijiang Hu,Mingxing Zhou 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.6

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigatedby three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics,and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth ofbainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steelsdecreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant differenceat lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RAfirstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in thesample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and propertyof high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volumeof bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformationtime.

Microstructure and Properties of a Low Carbon Bainitic Steel Produced by Conventional and Inverted Two-Step Austempering Processes

Junyu Tian,Wei Wang,Guang Xu,Xiang Wang,Mingxing Zhou,Hatem Zurob 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.5

The microstructure evolution and strain hardening behaviour of a low-carbon carbide-free bainitic steel prepared by eithersingle-step austempering (420 °C or 365 °C), conventional two-step austempering (420 °C then 365 °C) or inverted two-stepaustempering (365 °C then 420 °C) treatments were investigated. The results show that when the total isothermal holdingtime was the same, the inverted two-step austempering treatment (first completing bainitic transformation at low-temperatureand then annealing at high-temperature austempering) led to the highest toughness (30.7 GPa%) due to the finer bainiticmicrostructure and higher fraction of film-like retained austenite. Grain refinement and transformation-induced plasticityallowed the material to achieve high ductility without sacrificing strength. Comparing single-step austempering at 365 °Cwith the inverted two-step austempering process indicates that annealing at a higher temperature after completion of thebainitic transformation resulted in better tensile properties because of a lower dislocation density and more stabler retainedaustenite. In addition, the samples heat-treated by the conventional two-step austempering process exhibited slower bainitetransformation kinetics and the worse tensile properties than the sample which was heat-treated using a single-step austemperingtreatment at 365 °C or the one which was heat-treated using an inverted two-step heat treatment. Through the analysisof the orientation relationships, it is observed that the original austenite and the bainitic plates mainly followed the K-Sorientation relationships regardless of whether the bainite plates were formed in the first or the second heat-treatment step.

Effects of Ni and Cr on Cryogenic Impact Toughness of Bainite/Martensite Multiphase Steels

Zishan Yao,Guang Xu,Zhengyi Jiang,Junyu Tian,Qing Yuan,Hongwei Ma 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.5

In the present research, the effects of Nickel (Ni) and Chromium (Cr) on cryogenic impact toughness (CIT) of low-carbonbainite/martensite multiphase steels [processed by two different cooling processes: isothermal transformation process (ITP)and continuous cooling process (CCP)] were investigated. It was found that due to the formation of carbides during isothermaltreatment, the addition of Ni and Cr yielded no significant improvements in CIT. However, during CCP treatment, theaddition of Ni manifested a considerable enhancement in CIT, whereas the addition of both Ni and Cr caused a decreasein CIT. Further, after ITP treatment, the microstructure of all steels consisted of bainite and martenite, while Ni + Cr steelcontained the largest amount of bainite. The microstructures of the CCP-treated steels mainly also consisted of bainite andmartensite, but no retained austenite and carbides were observed, thus resulting in a superior CIT.

Research on Improved Schwarz Arc Model Considering Dynamic Arc Length

Li Jingli,Yuan Hao,Xu Mingming,Tian Fenglan,Wang Zijian,Ren Junyue 대한전기학회 2023 Journal of Electrical Engineering & Technology Vol.18 No.3

Establishing an accurate arc model is significant for simulating arc characteristics and research on arc high impedance fault detection. This paper firstly expounds arc physical properties, and theoretically analyzes the relationship between arc length and arcing voltage. Then, the magnetohydrodynamic model of AC arc is established by COMSOL Multiphysics, and the influence of arc length on electric field intensity, temperature, voltage and current of arc plasma is analyzed. Secondly, based on the influence of arc length on arc, an improved Schwarz arc model considering dynamic arc length is proposed, which not only considers arc time constant and dissipated power as functions of arc conductance, but also introduces arc length. The comparison error of the arcing voltage of the improved Schwarz model with actual measured data is 5%. In addition, the arcing voltage is compared with calculation results of theoretical formula and simulation results of magnetohydrodynamic model, which verifies the validity of improved arc model. Finally, the improved arc model is compared with Mayr model, Schwarz model and cybernetic model from the perspectives of arc voltage and current, and analysis shows that the improved arc model is reasonable in simulating arcing voltage, arc steady-state combustion voltage, and current zero-break characteristics.

Kinetics Model of Bainitic Transformation With Stress

Mingxing Zhou,Guang Xu,Haijiang Hu,Qing Yuan,Junyu Tian 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.1

Thermal simulations were conducted on a Gleeble 3800 simulator. The main purpose is to investigate the effects ofstress on the kinetics of bainitic transformation in a Fe-C-Mn-Si advanced high strength bainitic steel. Previousstudies on modeling the kinetics of stress affected bainitic transformation only considered the stress below theyield strength of prior austenite. In the present study, the stress above the yield strength of prior austenite is takeninto account. A new kinetics model of bainitic transformation dependent on the stress (including the stresses belowand above the yield strength of prior austenite) and the transformation temperature is proposed. The new modelpresents a good agreement with experimental results. In addition, it is found that the acceleration degree of stresson bainitic transformation increases with the stress whether its magnitude is below or above the yield strength ofaustenite, but the increasing rate gradually slows down when the stress is above the yield strength of austenite.