A simple and cost effective technique to obtain locally hardened composite layer on cast alloy was previously developed based on advantages of liquid-sintering theory. This methodology provides an ...opportunity to produce final product with hardened composite layer with good wear resistance during casting process. The current study investigated the effect of combined B4C and FeV substrates on the composite layer properties during in situ cast sintering technique for low carbon steel. Microstructure inspection, XRD phase analysis, chemical composition, thickness measurement and the hardness test on the produced composite layer were conducted. The technique produced up to 640 (±300) µm thick composite layer with maximum hardness of 481 (± 14.6) HV. The hardness of the reinforced surface layer is 3-4 times higher than the base alloy. The increased hardness of composite zone is directly attributed to the presence of the iron boron (FeB), boron vanadium (V2B3) and iron carbide (Fe3C). There were no significant effect of substrate composition on the produced thickness of the composite layer. However, an increase of FeV proportion in the powder mixture leads increased the hardness. The current study is a first attempt to produce locally hardened surface layer on the low carbon steel using simple and cost effective in situ technique with use of relatively inexpensive substrates.
Centerline segregation in steel cast products is an internal defect that can be very harmful when slabs are rolled in heavy plate mills. Consequently, anticipate its presence is a matter of ...importance to prevent future risks. The aim of this study was to obtain a predictive model able to perform an early detection of central segregation severity in continuous cast steel slabs. This study presents a novel hybrid algorithm, based on support vector machines (SVMs) in combination with the particle swarm optimization (PSO) technique, for predicting the centerline segregation from operation input parameters determined experimentally in continuous cast steel slabs. This optimization technique involves kernel parameter setting in the SVM training procedure, which significantly influences the regression accuracy. Additionally, a multilayer perceptron network (MLP) and a multivariate adaptive regression splines (MARS) approach, this last method also in combination with the particle swarm optimization (PSO) technique, were fitted to the experimental data with comparison purposes. To this end, the most important physical–chemical parameters of this industrial process are monitored and analyzed. The results of the present study are two-fold. In the first place, the significance of each physical–chemical variables on the segregation is presented through the model. Secondly, some models for predicting segregation are obtained with success. Indeed, regression with optimal hyperparameters was performed and coefficients of determination equal to 0.98 for continuity factor estimation and 0.97 for average width were obtained when this hybrid PSO–SVM-based model with RBF kernel function was applied to the experimental dataset, respectively. Furthermore, the results obtained with the MLP and PSO–MARS-based models are clearly worse than those obtained with the PSO–RBF–SVM-based model. The agreement between experimental data and the model confirmed the good performance of the latter. Finally, conclusions of this innovative research work are exposed.
The aging hardening and precipitation behavior of Fe-31.6Mn-8.8Al-1.38C austenitic cast steel under aging heat treatment were investigated by microstructures characterization and hardness tests. The ...as-quenched samples were aged at 550 °C, 700 °C, 800 °C and 900 °C for various time. After aging at 550 °C, the hardness exhibited a continuous increase and finally stabilized around 480 HB, meanwhile, the β-Mn phase nucleated at austenite grain boundaries without accompanied with α-precipitate, which is mainly related to the nano-sized κ′-carbides. When aging was conducted at 700 °C and 800 °C, the hardness was only stable at 320HB because of α precipitation and high temperature softening, and the β-Mn phase appeared through the decomposition of γ → α. According to the results of nano-indentation tests, the austenite matrix exhibited relatively low nano-hardness of the sample aged at 800 °C than that of 550 °C, due to the precipitation of micron-sized κ′-carbides. In addition, considering the size, morphology and distribution of κ phase after different aging treatments, four types of that were described in the present study.
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•Four types of κ-carbides were described after aging processes (550 °C–900 °C).•Nano-sized κ-carbides can promote the precipitation of β-Mn without α-precipitate.•The nano-hardness of austenite matrix was obtained by nano-indentation.
본 연구에서는 망간강을 소재로 하는 부품의 수명향상을 위해 망간강에 Ti의 첨가량을 변화하여 망간강을 주조하였으며, 주조한 소재의 특성을 확인하기 위하여 인장 및 표면 특성 그리고 베어링률 등 가공특성에 대해 조사하였다. 고망간강에 Ti 첨가 시 0.5%를 초과 시 결정립 미세화로 인한 합금의 강도가 향상되었으며 내부에 미세 탄화물이 형성시킴으로써 Mn만 ...첨가된 합금에 비해 표면의 마모에 대한 저항성을 높이는 결과를 나타냄을 확인하였다. 망간강에서 Ti의 함유량이 증가함으로 인해 인장강도가 증가함에는 큰 차이가 없었으나 마모성의 부분에서는 Ti이 Mn에 비해 마모성에는 미량이지만 더 큰 영향을 끼치며 Ti의 함량에 따라 탄화물의 크기 및 분포가 조대하고 균일하게 분포하였다. 망간강을 소재로 하는 부품의 수명향상을 위해 망간강에 Ti 첨가함으로써 강도 및 표면특성을 향상시킬수 있음을 확인하였다. Ti이 수지상정 결정립의 미세화로 인한 내마모성이 우수한 재질을 개발하는데 효과있음을 알 수 있었다. Ti가 첨가된 샘플에서 탄화물은 표면 거칠기에 대한 내성을 증가시키는 것으로 나타났으며 Mn강의 특성상 표면경화가 일어나기 시작하여 수명이 연장되는 것으로 보인다.
In this study, in order to improve the lifespan of parts made of manganese steel, manganese steel was cast by varying the amount of Ti added to the steel. In order to confirm the characteristics of the cast material, processing characteristics including tensile and surface characteristics and bearing ratio were investigated. It was confirmed that when the amount of Ti added to high manganese steel exceeds 0.5%, the strength of the alloy is improved due to grain refinement, and fine carbides are formed inside the steel. This results in increased resistance to surface wear compared to the alloy with only Mn added. There was no significant difference in the increase in tensile strength as the Ti content in manganese steel was increased. However, inclusion of Ti showed a small but greater effect on wear resistance compared to Mn, and the size and the distribution of carbides become coarse depending on the Ti content. and was evenly distributed. It was confirmed that the strength and surface properties of manganese steel can be improved by the addition of Ti to improve the lifespan of parts made with this steel. It was found that Ti is effective in developing materials with excellent wear resistance due to refinement of dendrite crystal grains. In the samples where Ti was added, the carbide appears to increase the resistance to surface roughness, and due to the nature of Mn steel, surface hardening begins to occur, which appears to extend the life.
•Addition of Ce enhances the selective oxidation of Cr.•Addition of Ce facilitates the release of stress in the oxide scale.•Addition of Ce hinders the removal of Mn to retard transformation of γ- to ...α-iron.
The effect of Ce on the oxidation behaviour and microstructure evolution of nickel-saving austenitic heat-resistant cast steel in air at 900 °C was investigated by performing the isothermal oxidation tests. After 200 h of oxidation, the oxidation resistance of Ce-containing steel was about 20 % higher than that of Ce-free steel. In addition to its role as a reactive element, alloyed Ce slowed the rate of manganese depletion from the substrate and subsurface ferrite formation, leading to the improvement of the oxidation resistance of Ce-containing steel.
This paper presents a comprehensive study on steel beam-to-column joints equipped with weld-free cast steel connectors used as replaceable energy-dissipating components. The basic idea was to use the ...connectors as the main source of deformation and energy dissipation, and to realize rapid repair of the joints by simple replacement of theses connectors after earthquakes. The research commenced with tests on five full scale specimens, including one specimen with conventional welded T-stub connectors and four specimens with the innovative cast steel connectors. The specimens with cast steel connectors were shown to have good energy dissipation capacity with stable and plump hysteretic curves, and the equivalent viscous damping could achieve up to 37.5%. These joints were also shown to meet the classification criteria of semi-rigid joints in terms of stiffness, and the ductility, which was governed by fracture of energy dissipating elements, was found to be satisfactory. The presence of shear tab connection was found to increase the initial stiffness and ultimate strength of the joints, but the energy dissipation was not significantly improved. Importantly, the yielding regions and main source of energy dissipation were limited to the cast steel connectors, while the adjacent structural members remained in the elastic range, indicating minimal repair work in the aftermath of earthquakes. For a further understanding of the proposed joints, finite element models were established, and based on test and numerical observations, preliminary design recommendations, including those on cast steel connector detailing and normal component-based joint design strategies, were finally outlined.
•Beam-to-column joints with new types of cast steel connectors were tested.•The innovative joints have satisfactory energy dissipation capacity and ductility.•Finite element modelling for the joints is developed.•Preliminary joint design strategy for the joints is proposed.
•Mechanical properties of G20Mn5QT cast steel at low temperatures were determined.•Parameters in VGM/SMCS were calibrated for G20Mn5QT cast steel at low temperatures.•VGM/SMCS are applicable for ...G20Mn5QT cast steel at low temperatures.•Characteristic length of G20Mn5QT cast steel decreases with the drop of the temperature.
With superior structural performances, G20Mn5QT cast steel has been extensively used in fabricating complex joints in steel constructions. Fracture failure of G20Mn5QT cast steel is then an important issue, especially for low-temperature situations. In this paper, mechanical properties of G20Mn5QT cast steel at low temperatures were studied by tensile coupon tests and Charpy impact tests. The effects of the low temperature on mechanical properties and the ductile-brittle transition temperature of G20Mn5QT cast steel were obtained based on the test results. The void growth model (VGM) and the stress modified critical strain (SMCS) model, which are two typical micromechanical fracture models, were calibrated for G20Mn5QT cast steel by experiments and complementary FEA on smooth notched tensile (SNT) specimens at four temperature levels, 20°C, −20°C, −40°C and −60°C. For each temperature level, the material parameters in VGM and SMCS calculated based on test results of all six different specimens are in very good consistence. This consistency indicates the applicability of VGM and SMCS for predicting the ductile fracture of G20Mn5QT cast steel, even at the temperature of −60°C, which is much lower than the transition temperature of the material. The calibrated material parameters representing the resistance to ductile fracture of the material decrease linearly with the drop of the temperature. The characteristic length of G20Mn5QT cast steel was also determined by observation of fractured surfaces of test specimens with the scanning electron microscope, which has the tendency to decrease with the drop of the temperature.
The austenitic cast steel HK30, used in exhaust manifolds of heavy-duty truck engines, was studied with respect to creep, creep damage, oxidation and precipitation at 750 ◦C in air. Even though creep ...clearly affects the lifetime of the manifolds, there are very few studies available on the topic. The results of the present study provide a basis for materials selection as well as creep data input to component lifetime simulations. In the present study, five specimens were subjected to constant loads resulting in creep rupture times between 1 and 94 days. The minimum creep rate follows Norton’s creep law with a Norton parameter (n) of 8.9 and the logarithmic creep rupture time increases linearly with reduced stress. At low stresses secondary creep is predominant observing oxide intrusions from the specimen surface, precipitation of sigma and G-phases, creep cavitation at grain boundaries and weak dislocation networks. At high stresses direct transitions from primary to tertiary creep are observed as well as stronger dislocation networks, but no oxide intrusions, no precipitation, or cavitation are observed. The material has grains of cm size and intergranular fracture is predominant at all stress levels. The segregation of elements during solidification was studied by Scheil/Gulliver simulations and precipitations of sigma and G-phases in the last solidified regions were predicted using Thermo-Calc.
•The changes in microstructure after low cycle fatigue (LCF) have been examined.•LCF of the cast steel at three temperatures has been investigated.•Cycle softening of cast steel without the ...stabilization of loop parameters was observed.•Subgrain/carbide growth, dislocation density fall, laths morphology loss were shown.•The changes in microstructure depend on the temperature and level of strain.
The paper presents the results of research on fatigue properties in a small number of cycles to failure of high-chromium martensitic GX12CrMoVNbN9-1 (GP91) cast steel designed for the casts working under the so-called supercritical parameters. The tests of fatigue life were carried out at three temperatures: room temperature, 550°C and 600°C, for five levels of controlled amplitude of total strain ɛac: 0.25%; 0.30%; 0.35%; 0.50% and 0.60%. Cyclic softening was observed in the fatigue tests at all temperatures without a stabilization period. The fatigue lifetime curves at each temperature were obtained based on Basquin and Coffin–Manson equations. It has been shown that the influence of temperature on fatigue life depends on the level of strain and is the biggest for the lowest levels of strain covered in the research. The results of fatigue tests are connected with the quantitative changes in the microstructure of the cast steel. Quantitative tests were carried out using thin foils by means of TEM. Disappearance of martensitic laths, increase in the subgrains size, decrease in the dislocations density and coagulation of carbides were observed after low cycle fatigue. The scale of these changes in the microstructure is dependent on the temperature and level of strain.
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