Papers by Keyword: Bainite

Abstract: This research aims to investigate the effect Si on microstructures and mechanical properties of normalized pressure vessel steel A517 Grade Q. The Si contents were varied from 0.5, 0.9, 1.2, and 1.6. The normalizing process was carried out at 920 °C in 10 minutes. The microstructures were characterized by optical light microscope and Scanning Electron Microscope (SEM), and The mechanical tests were conducted by hardness, tensile, wear, and impact testing machine. The microstructures and mechanical properties were investigated. The results of the microstructures test show that the phases of the material are bainite and ferrite. The increase of Si content will refine and distribute the bainite phase in the ferrite matrix. The mechanical tests show that the Si content will affect the mechanical properties such as hardness, tensile strength, ductility, wear resistance, and toughness. The Increase of Si content from 0.5 up to 1.2 will increase the hardness, tensile strength, and wear resistance, and will decrease the elongation and toughness of the normalized pressure vessel steel A517 grade Q. The mechanical properties seem constant from 1.2 up to 1.6 of Si.

Abstract: Low alloy TRIP steel is expected to be applied to automobile bodies because of its high strength, high ductility, and excellent impact properties and press formability. It has been reported that the low alloy TRIP steel of hydrogen embrittlement resistance is improved by utilizing the hydrogen storage characteristics of highly stable retained austenite. Therefore, for the purpose of increasing the volume fraction of retained austenite, it was produced at various cooling rates below the martensite transformation start temperature. As a result, the volume fraction of retained austenite increased, and then the effect of hydrogen embrittlement decreased. The matrix phase and retained austenite is refined with decrees of the cooling rate. It is considered that the size and surface area of the retained austenite also affected the improvement of hydrogen embrittlement resistance.

Abstract: The effect of ten different combinations with various amounts of niobium (0-0.6 wt.%) and chromium (1-4 wt.%) on weldability and mechanical properties of thermomechanically rolled and direct-quenched low-carbon (0.035 wt.%) microalloyed bainitic steel were investigated. Two compositions were alloyed with boron to increase the hardenability, and two with titanium to improve the toughness properties in heat affected zone. The target of the study was to produce steel with 700 MPa yield strength combined with good impact toughness. Coarse grained heat affected zone (CGHAZ) simulations were performed using the Gleeble 3800 thermomechanical simulator to evaluate the weldability of the investigated steels using cooling time from 800 °C to 500 °C (t_8/5) of 5 s and 15 s to simulate different heat inputs in actual welding procedure. Microstructures were characterized using light optical microscopy, and hardness profiles of simulated heat affected zones were determined as well as Charpy-V impact toughness at-40 °C and-60 °C. Shorter t_8/5 time (5 s) produced generally better impact toughness properties compared to longer t_8/5 -time (15 s). Steels with 4 % Cr had the highest impact energies. Generally, more softening occurred with longer t_8/5-time (15 s). However, Cr and Nb alloying decreased the amount of softening in the CGHAZ region, especially with longer t_8/5 -time. These results indicate that even with higher t_8/5 -time, it is possible to achieve strength properties equivalent to the base material in the CGHAZ region by Cr and Nb alloying.

Abstract: The authors have developed the in situ neutron diffraction technique focusing on bainite transformation during austempering. Thanks to the features of time-of-flight type neutron diffraction, textures, phase fractions and lattice parameters can be simultaneously measured at high temperature. In this paper, the design of experimental equipment and analytical approach are mainly described.

Abstract: We have determined different phase fractions from microscopy images using semi-automated image analysis fitting technique, and in addition we have classified each phase according to its hardness. The distribution of grayscale pixels of different phases is first characterised separately for each phase, which are sampled from the microscope image. After this the distributions of the separate phases are fitted to give the corresponding distribution of the whole image. The microhardness measurement provides reliability on the classification of the different phases to ferrite, bainite or martensite. In addition to describing the applied techniques in detail, we present the results obtained from the analysis for one steel subjected to isothermal holding experiments at different temperatures.

Abstract: This work studies the surface welding parameters for a practical repair for pearlitic rail grades: R260 and R350HT. A filler metal containing low carbon (0.15 %), high silicon (0.5 %) and nickel (2.5 %), self-shielded flux-core welding electrode (FCAW-S) is the candidate in order to ensure the preferable carbide-free bainite. The film-like morphology of the retained austenite is reported to promote the wear resistance and is ensured by silicon and nickel. The effect of preheat temperature and dilution on the microstructure and resulting hardness can be concluded. Too high dilution, as a result of high current and travel speed, and the reheating during the welding of the second layer can result in martensite formation and too high hardness. Proper control of the dilution ensures satisfactorily hardness and avoids martensite formation.

Abstract: In a railway track for intermodal usage between a train station to a port, an observation on corrosion behavior of the track alloy in the coastal environment needs to be considered. In this study, Fe-Ni lateritic steel with bainite structure is observed. This alloy is developed from lateritic ores in Indonesia as an alternative to the conventionally made Fe-Ni steels. This study aims to determine the effect of cold rolling and austempering processes on the corrosion properties of the alloy. The cold-rolling reductions used are 30% and 70%, followed by an austempering process at 400°C for 30 mins with air cooling. The corrosion test was performed on four different samples-- First, a before deformation sample. Second, 30% and 70% cold-rolled samples. Third, austempered without deformation samples. Besides, fourth, deformed austempered samples. The corrosion test method implemented was the CASS Method for 2, 4, and 6 days. The highest corrosion rate obtained by 30% cold rolling, and the corrosion rate is increased as the austempering addition. The microstructures of rust are approximately porous few compact, near to voluminous coarse-grain corrosion products, and tend to crack.

Abstract: The paper presents the results of the development of a special group of cast composite materials based on nodular cast irons. The alloys were based on alloying cast iron with copper in amounts that exceed its solubility in liquid Fe-C-Si systems (Cu>6 wt.%). A new component is obtained in the structure of castings in the form of a complex mixture based on the Cu-phase. Inclusions are relatively large (up to 200 μm) and globular in shape, with a uniform allocation in the volume of the castings. This favorably distinguishes the obtained alloys from the known cast irons and gives them high antifriction properties. With isothermal hardening, it is possible to obtain the lowest values of wear for structures of lower bainite. Developed cast composite materials are recommended to be used as tribotechnical materials working in conditions of poor lubrication and corrosion.

Abstract: The Heavy-Haul railroad wheels started to use higher wear resistance steels microalloyed with niobium, vanadium, and molybdenum [1]. During continuous cooling, these elements depress the temperature of the pearlite formation, producing smaller interlamellar spacing that increases the hardness of the steel, besides to favor the precipitation hardening through the formation of carbides [2, 3]. Also, they delay the formation of difusional components like pearlite and bainite during isothermal transformation. The effects of these alloy elements on microstructure during isothermal transformation were studied in this work using a Bähr 805A/D dilatometer. Three different compositions of class C railway wheels steels (two microalloyed and one, non microalloyed) were analyzed in temperatures between 200 and 700 °C. The microstructure and hardness for each isothermal treatment were obtained after the experiments. Comparing with non microalloyed steel (7C), the vanadium addition (7V steel) did not affect the beginning of diffusion-controlled reactions (pearlite and bainite), but delayed the end of these reactions, and showed separated bays for pearlite and bainite. The Nb + Mo addition delayed the beginning and the ending of pearlite and bainite formation and also showed distinct bays for them. The delays in diffusion-controlled reactions were more intense in the 7NbMo steel than in 7V steel. The V or Nb + Mo additions decreased the start temperature for martensite formation and increased the start temperature for austenite formation.

Abstract: Railway has become more essential for both mass and goods transportation so that the rails are required to carry higher loads and exhibit longer lifetime. Thus, mechanical properties, especially strength and toughness of rail steel must be continuously increased. In the present work, microstructure, tensile properties and impact toughness of a pearlitic rail steel grade 900A were firstly characterized. It was found that the investigated steel showed high yield and tensile strengths, but moderate elongation. Subsequently, representative volume elements (RVE) model was employed to investigate the effects of bainitic phase on mechanical properties of pearlitic rail steels. The flow stress curves of the individual phases were defined with regard to the chemical composition. As a result, the relationships between predicted yield strengths and tensile strengths in dependence on the phase fraction of bainite were provided. The model can be used to identify the proper microstructure characteristic of rail steel.