High Temperature Oxidation Behaviour of Ferritic Stainless Steel SUS 430 in Humid Air

Xiawei Cheng,Zhengyi Jiang,Dongbin Wei,Jingwei Zhao,Brian J. Monaghan,Raymond J. Longbottom,Laizhu Jiang 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.2

The high temperature oxidation behaviour of ferritic stainless steel SUS 430 was investigated over the temperaturerange from 1000 to 1150 °C in humid air containing 18% water vapour. Isothermal thermogravimetricanalyses were performed to study the oxidation kinetics. The microstructure, composition and thickness ofthe oxide scale formed were investigated via optical microscopy (OM), X-ray diffraction and a scanning electronmicroscope equipped with an energy dispersive spectrometer. The results indicate that breakaway oxidationoccurs at all temperatures and that its onset is accelerated by increasing temperature. The growth rate of themultilayer oxide scale follows a parabolic law with apparent activation energy of 240.69 kJ/mol, and theformation of FeO is decreased when the temperature is higher than 1120°C. The inner oxide scale, Fe-Cr spinel,grows mainly inward and internal oxidation is observed even in a short oxidation test at 1150°C for 105 s. The mechanism of high temperature oxidation of SUS 430 in humid air containing 18% water vapour is discussed.

Flow Behaviour and Constitutive Modelling of a Ferritic Stainless Steel at Elevated Temperatures

Jingwei Zhao,Zhengyi Jiang,Guoqing Zu,Wei Du,Xin Zhang,Laizhu Jiang 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.3

The flow behaviour of a ferritic stainless steel (FSS) was investigated by a Gleeble 3500 thermal-mechanical test simulator over the temperature range of 900-1100 °C and strain rate range of 1-50 s -1 . Empirical and phenomenological constitutive models were established, and a comparative study was made on the predictability of them. The results indicate that the flow stress decreases with increasing the temperature and decreasing the strain rate. High strain rate may cause a drop in flow stress after a peak value due to the adiabatic heating. The Zener-Hollomon parameter depends linearly on the flow stress, and decreases with raising the temperature and reducing the strain rate. Significant deviations occur in the prediction of flow stress by the Johnson-Cook (JC) model, indicating that the JC model cannot accurately track the flow behaviour of the FSS during hot deformation. Both the multiple-linear and the Arrhenius-type models can track the flow behaviour very well under the whole hot working conditions, and have much higher accuracy in predicting the flow behaviour than that of the JC model. The multiple-linear model is recommended in the current work due to its simpler structure and less time needed for solving the equations relative to the Arrhenius-type model.

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.

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.

Effect of Temperature and Strain Rate on the Hot Deformation Behaviour of Ferritic Stainless Steel

Guoqing Zu,Yukuan Lu,Yi Yan,Xiaoming Zhang,Jingwei Zhao,Wei Du,Xu Ran,Zhengyi Jiang 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.2

The fow behaviour and microstructure characteristics of a ferritic stainless steel were investigated using plain strain compression test on a Gleeble 3500 thermo-mechanical test simulator with a hydrawedge system in the temperature range of850–1100 °C and strain rate range of 0.1–50 s−1. The phenomenological constitutive model and the relationship betweenthe Zener–Hollomon (Z) parameter and fow stress were established. The results reveal that the fow softening phenomenonoccurs at high strain rate, which is caused by the coupling efect of the adiabatic heating and dynamic recrystallisation (DRX). New grains nucleate preferentially at the original grain boundaries by strain-induced grain boundary migration. With anincrease of temperature or strain rate, a part of new grains form in the interior of deformed grains. The DRX grain size andfraction increase with the increase of temperature, however, exhibit a non-linear relationship with strain rate.