Sintering Experiments Research Articles

Optimisation of MgO addition in low and high silica iron ore sinter to improve sinter reducibility at JSW Steel Limited

The quality of iron ore sinter mainly depends on sinter mineralogy, which in turn depends on the chemical composition of the sinter mix. The reduction properties of the mineral phases formed in the sinter influences the sinter reducibility. MgO has a varying effect on sinter reducibility at different silica contents. A recent trend in blast furnace operation shows that there is a considerable increase in usage of dolomite as a basic flux either directly or through sinter. Recently the silica levels in the sinter product of Sinter plant 1(SP1) of JSW Steel Limited have been fluctuating in the range of 5·5–9·6% due to variation in silica content of iron ore fines. At the same time, as per blast furnace requirement, the addition of dolomite has been changed from 2·4 to greater than 3·0% at SP1, and the reducibility of the sinter decreased (<60·0%). Laboratory pot grate sintering experiments have been carried out to determine the influence of MgO addition on microstructure and reducibility of low and high silica sinter. MgO additions have been varied from 1·4 to 3·2% for low silica (4·5%), and high silica (6·3%) iron ore fines.From the studies it was found that the reducibility of both sinters decreased with increase in MgO addition due to an increase in magnetite/magnesio spinel phase and silicate/slag phase. Reducibility of low silica sinter was greater with high silica sinter. High silica with high MgO sinter had lower reducibility compared to low silica with low MgO/high MgO and high silica with low MgO sinter.

Densification and microstructure changes of micron size nickel powder during direct induction sintering

Induction heating is an attractive technique to sinter metal powders in a short time and with limited energy. A series of direct induction sintering experiments has been performed with a micron size nickel powder in a dedicated set-up with 50 or 150 kHz current frequency and several heating rates, up to 900°C min−1. With a view to better catching the specific outcome of induction sintering, conventional sintering tests have also been achieved and their results in terms of densification have been depicted by adjusting a Master Sintering Curve model. The main conclusion of this study is that nickel specimens with high density, reasonably low grain size and homogeneous microstructure can be obtained by direct induction sintering with processing times much smaller than typical conventional sintering times. The obtained data also show that powder densification is accelerated during induction sintering, which is supposedly due to the enhancement of diffusion under electric current.

Influence of MgO addition on microstructure and properties of low and high silica iron ore sinter

The quality of iron ore sinter mainly depends on sinter mineralogy, which in turn depends on the chemical composition of the sinter mix. Dolomite is the critical additive for fluxing the sinter and controlling the sinter MgO. Recently, the silica level in the sinter product of the sinter plant 1 has been varying in the range of 5·51 to 9·58% owing to variation in silica content of iron ore fines. At the same time, as per the blast furnace requirement, the rate of addition of dolomite has been changed from 2·40 to greater than 3·00% at the sinter plant 1. In iron ore sinter, MgO plays different roles on sinter properties at different silica ranges. Because of wide range of fluctuation in iron ore silica it is essential to optimise the MgO addition in sinter mix to get desired properties. Laboratory pot grate sintering experiments have been carried out to know the influence of MgO addition on microstructure and properties of low silica and high silica sinter. MgO addition has been varied from 1·40 to 3·20% for low silica (4·47%), and high silica (6·25%) iron ore fines. High silica with high MgO sinter showed lower strength, higher reduction degradation index, and lower reducibility compared to low silica with low and high MgO and high silica with low MgO sinter. To achieve better sinter properties MgO content in the sinter should not be more than 2·80% when silica is more than 6·0%.

Effect of sintering atmosphere on densification, mechanical properties and diamond stability of prealloyed diamond impregnated composites obtained by free sintering

The sintering behaviour of prealloyed powder compacts has been studied as a function of the sintering atmosphere in free sintering experiments. Atmospheres with different hydrogen/nitrogen ratios and even vacuum have been used in the sintering cycles. Powder compacts with and without diamond additions have been sintered. Three different grades of diamond were used in the experiments, all of them synthetic manmade diamond. Two had different levels of metallic inclusions and one was coated with Ti. The interaction between bond/atmosphere/diamond has been characterised analysing the density, microstructure, bend strength and degradation of the diamonds after dissolving the matrix. Diamonds from atmospheres with low hydrogen content show evidence of strong degradation. Moreover, any diamond additions strongly decrease the strength of the bonds, acting as defects. The strength is also affected by the sintering atmosphere and sintering temperature but not significantly by the type of diamond.

Electric Field Assisted Sintering of Cubic Zirconia at 390°C

Using the flash sintering technique, cubic yttria‐stabilized zirconia is shown to sinter at 390°C, more than 1000°C below nominal sintering temperatures, by using a DC electric field of 2250 V/cm. Furthermore, flash sintering experiments performed with electric fields between 60 and 2250 V/cm were used to show that the relationship of the temperature at the onset of flash sintering (TOnset) and the applied field (E) follows the power relationship TOnset (K) = 2440 E−1/5.85(V/cm). Using this relationship, and considering the sintering of the sample in the absence of an electric field, the critical electric field to enter the flash sintering regime is shown to be 24.5 V/cm. For electric fields between this critical electric field and 2250 V/cm, the onset of flash sintering occurs in the same range of critical volumetric power dissipation, between 1 and 10 mW/mm3, suggesting this is a material property. Despite the volumetric power dissipation being the critical value for the onset of flash sintering behavior, the current density achieved during sintering appears to be more critical for densification rather than maximizing power dissipation.

Two-stage master sintering curve applied to two-step sintering of oxide ceramics

Tetragonal (3 mol% Y2O3) and two cubic zirconia (8 mol% Y2O3) as well as alumina green bodies were used for the construction of the Master Sintering Curve (MSC) created from sets of constant-rate-of-heating (CRH) sintering experiments. The activation energies calculated according to the MSC theory were 770 kJ/mol for Al2O3, 1270 kJ/mol for t-ZrO2, 620 kJ/mol and 750 kJ/mol for c-ZrO2. These values were verified by an alternative approach based on an analysis of the densification rate in the intermediate sintering stage. The MSCs established from the Two-Step Sintering (TSS) experiments showed at high densities a significant deflection from those constructed from the CRH experiments. This deflection was explained by lower sintering activation energy in the closed porosity stage. A new two-stage MSC model was developed to reflect the change in sintering activation energy and to describe TSS. The efficiency of TSS of four materials under investigation was correlated with their activation energies during the final sintering stage.

Modeling kinetics of distortion in porous bi-layered structures

Abstract Shape distortions during constrained sintering experiment of bi-layer porous and dense cerium gadolinium oxide (CGO) structures have been modeled. Technologies like solid oxide fuel cells require co-firing thin layers with different green densities, which often exhibit differential shrinkage because of different sintering rates of the materials resulting in undesired distortions of the component. An analytical model based on the continuum theory of sintering has been developed to describe the kinetics of densification and distortion in the sintering processes. A new approach is used to extract the material parameters controlling shape distortion through optimizing the model to experimental data of free shrinkage strains. The significant influence of weight of the sample (gravity) on the kinetics of distortion is taken in to consideration. The modeling predictions indicate good agreement with the results of sintering of a bi-layered CGO system in terms of evolutions of bow, porosities and also layer thickness.

The influence of Si on the microstructure and sintering behavior of ultrafine WC

The microstructure of sintered nanoscale tungsten carbide powders with 1 wt % Si addition was found to be populated by an abnormally large number of elongated grains. Interrupted sintering experiments were conducted to clarify the origins of the excessive abnormal grain growth seen in the microstructure. It was observed that rapid coarsening occurred at high temperatures owing to the formation of a liquid phase. However, the grain shape evolution during this coarsening period was found to be a consequence of excessive stacking faults and micro twins on the basal planes probably generated by reaction of WC with Si. Analyses of the microstructures and the isothermal and non isothermal coarsening behaviors suggested that the platelet morphology evolved by defect-assisted nucleation and growth on faceted grains. Based on experimental evidence from samples interrupted at low temperatures and crystal growth theories, we discuss the possible mechanisms that eventually led to the rampant platelet-type morphology. Further, the influence of such rapid grain growth on the shrinkage rate during sintering is also discussed. In comparison with the cyclic coarsening-densification process of sintering in pure nanoscale WC, the addition of Si leads to only two distinct sintering stages: either densification dominated or coarsening dominated. Concurrent densification and coarsening cannot be sustained particularly in the presence of a liquid phase that significantly enhances coarsening.

Formation of nickel and copper ferrites in ceramics: a potential reaction in the reuse of iron-rich sludge incineration ash

This study investigates potential solid-state reactions for the stabilization of hazardous metals when reusing the incineration ash from chemically enhanced primary treatment (CEPT) sludge to fabricate ceramic products. Nickel and copper were used as examples of hazardous metals, and the iron content in the reaction system was found to play a major role in incorporating these hazardous metals into their ferrite phases (NiFe 2O 4 and CuFe 2O 4). The results from three-hour sintering experiments on NiO+Fe 2O 3 and CuO+Fe 2O 3 systems clearly demonstrate the potential for initiating metal incorporation mechanisms using an iron-containing precursor at attainable ceramic sintering temperatures (above 750°C ). Both ferrite phases were examined using a prolonged leaching experiment modified from the widely used toxicity characteristic leaching procedure (TCLP) to evaluate their long-term metal leachability. The leaching results indicate that both the NiFe 2O 4 and the CuFe 2O 4 products were significantly superior to their oxide forms in immobilizing hazardous metals.

Simulation of transient temperature for mixture of two materials in the selective laser sintering process

In the process of rapid prototyping by the method of selective laser sintering, transient temperature has a direct effect upon the sintering performance. In the present work, a model is developed to predict the three-dimensional transient temperature field. It uses the finite element method to model the variation of thermal properties and solid–liquid two-phase interface in sintering the mixture of Al2O3-coated ceramic powder and polystyrene powder. A high-speed charge-coupled device image temperature measurement system is used to obtain real data from laser sintering experiments for comparison. The test results validate the simulation data and imply that the modelling method is useful in predicting the transient sintering temperature when correct thermal properties and key factors of two-phase interface are main concerns. The performance characteristics of the sintering process are predicted by the modelling method.

Defect Healing and Cracking in Ceramic Films During Constrained Sintering

Cracking is a major problem during constrained sintering of ceramic films. A local variation of density in a green film, referred to as a defect in this work, is the leading cause for cracking. This article presents analytical and numerical studies of the behavior of such defects during constrained sintering. An interesting behavior of de‐sintering and healing of the defects is revealed. It is demonstrated that the healing process can be made to start earlier during sintering by using (a) larger particles, (b) lower sintering temperature, and (c) lower initial density. However, a uniform distribution of initial density in the film is shown to be the most effective means to avoid cracking. It is realized that most of the factors beneficial to defect healing are detrimental to sintering a film to high density because the driving force for sintering also drives cracking. The analytical and numerical studies also offer some simple explanations to why thin films are less susceptible to cracking than thick ones. Finally, the constitutive model has been combined with a simple empirical failure criterion, to simulate the initiation and propagation of multiple cracks in a sintering film. The cracking patterns predicted by the simulations resemble those observed during sintering experiments.

Lean alloys in PM: from design to sintering performance

Low alloy sintered steels with an optimised content of alloying elements require the use of promising candidates such as Mn and Si, which can provide improved properties with minimum contents at a lower and less volatile price. The introduction of these alloying elements in the form of a master alloy powder prevents, to some extent, the oxidation and allows a proper ‘tailoring’ of its composition to accomplish particular goals. In this sense, low melting point alloys are especially interesting since they provide the formation of a liquid phase that enhances sintering and promotes the homogeneous distribution of alloying elements within the compact. In this work, the product developing process of master alloys containing Fe–Mn–Si is described from the theoretical design to the sintering performance. The effects of the liquid phase (produced by the added master alloys) are studied by differential thermal analysis and dilatometry. Moreover, to depict the behaviour of the liquid phase during heating, interrupted sintering experiments under high cooling rates were carried out. The results reported allow us to conclude that diffusion of carbon seems to be beneficial for lowering the melting temperature of the alloying particles, and the fact that the master alloys studied can dissolve part of the iron base particles has been shown to be beneficial for wetting.

Toughening of zirconia/alumina composites by the addition of graphene platelets

Abstract A study on graphene platelet/zirconia-toughened alumina (GPL/ZTA) composites was carried out to evaluate the potential of the new structural materials. GPL–ZrO2–Al2O3 powders were obtained by ball milling of graphene platelets and alumina powders using yttria stabilized ZrO2 balls. Samples were sintered at different temperatures using spark plasma sintering. Fracture toughness was determined by the single-edge notched beam method. The results show that the GPLs are uniformly distributed in the ceramic matrix and have survived high temperature sintering processes. Several sintering experiments were carried out. It is found that at 1550 °C, GPL/ZTA composites were obtained with nearly full density, maximum hardness and fracture toughness. A 40% increase in fracture toughness in the ZTA composite has been achieved by adding graphene platelets. The toughening mechanisms, such as pull out, bridging and crack deflection, were observed and are discussed.

Experimental Research on Sintering Amorphous Based Material with Iron-Tailings

The homogeneous mixture of iron tailings powder and glass powder is as the experimental material to sinter the amorphous based material. The particle size of the glass powder is less than 0.15 millimeter, and the particle size of the iron tailings powder is from 0.07 millimeter to 2 millimeter. The weight percentages of glass powder are 10% and 15% and 22% and 25% in the experimental material, respectively. The heating rate is about 170°C•min-1 and the maximum sintering temperature is 1150°C.They are proved by the analysis results of thermo-gravimetric analysis and differential thermal analysis with the experimental material. And the sintering temperature and the heating rate of the sintering experiments were based on them. The facts that the sintered material is mostly made up of amorphous substance and contains a certain amount of crystals and it’s micromechanism is amorphous are proved out by the experiments of scanning electron microscope. The testing results of the sintered material are these that the maximum density is 2.6g.cm-1, the maximum compressive strength is 94MPa, the maximum tensile strength is 5MPa, the maximum bending strength is 28MPa, the hydroscopicity and the radioactivity is zero, and it can not be eroded by acids and alkalis. When the sintered material contains 22% glass powder, the mechanical properties of the sintered material sample is same as marble.

Diffusion Behaviour of the Grain-Growth Inhibitor Vc in Hardmetals

Diffusion couples of the type WC-Co/VC-Co were employed to observe the diffusion behaviour of the grain-growth inhibitor (GGI) VC within nanoand ultrafine-grained WC-10wt.% Co samples. Since diffusion of the GGI during heating occurs already in the solid-state regime, interrupted sintering experiments were performed up to the liquid phase formation temperatures. By the use of light-optical and scanning electron microscopy the impact of GGI concentration on the microstructure as a function of depth from the interface was investigated. Electron Probe Microanalysis (EPMA) was used in order to quantify the amount of vanadium diffused into WC-Co as a function of distance. In first approximation, an activation energy of 3.45 eV (332,6 kJ/mol) for vanadium transport was determined.

Reducing sintering temperature of yttria stabilized zirconia through addition of lithium nitrate and alumina

Reducing sintering temperature of yttria stabilized zirconia (YSZ) has been achieved through doping with alumina and lithium nitrate at levels below 1mol%. Sintering experiments of pure and doped samples have been conducted with the same profile using an optical dilatometer. All samples exhibited anisotropic sintering over a wide range of temperatures but final shrinkage values were comparable in axial and radial directions. Sintering temperature has been reduced by as much as 110°C. We believe that the reduction in sintering temperatures is due to viscous flow in the first stage sintering. Bimetallic doping (mixture of alumina and lithium nitrate doping) is more effective in reducing sintering temperature than single doping possibly due to better distribution of doping material throughout the matrix material. Separate sintering experiments for 5h have been conducted at 1250°C and 1170°C on doped 8mol% and 3mol% YSZ, respectively, and have shown that near full density (∼96%) is reachable.

Relationship of Post Treatment Temperature and Tensile Strength for Selective Laser Sintering

Though SLS technology have got well development, the strength of the sintering prototype parts still need a lot of research. The lower strength of prototype parts mightl seriously affect the quality. The past research result shows that intensity of coated sand specimens of SLS sintering method is lower and cannot be used for metal casting, the sintered parts must be post-processed. In this study, author used the coated sand casting as the material of selective laser sintering experiment, studied the processing temperature and time on the mechanical properties of post-processing, and the influence on gas-forming property and collapsibility of the pieces. And combined with casting experimental to analyze the accuracy influence factors of precoated sand sintering parts and other performance assessment. Through the above analysis, we concluded that the influence law the post-processing of temperature on tensile strength of coated sand specimens under three different insulation temperature conditions.

Microstructure–mechanical properties relations in SiC–TiB2 composite

Densification and mechanical properties (fracture toughness, flexural strength and hardness) of SiC–TiB2 composite were studied. Pressureless sintering experiments were carried out on samples containing 0–50vol% of TiB2 created by in situ reaction between TiO2, B4C and carbon. Al2O3 and Y2O3 were used as sintering additives to create liquid phase and promote densification at sintering temperature of 1940°C. The sintered samples were subsequently heat treated at 1970°C. It was found that the presence of TiB2 serves as an effective obstacle to SiC grain growth as well as crack propagation thus increasing both strength and fracture toughness of sintered SiC–TiB2 composite. The subsequent heat treatment of sintered samples promoted the elongation of SiC matrix and further improved mechanical properties of the composite. The best mechanical properties were measured in heat-treated samples containing 12–24vol% TiB2. The maximum flexural strength of ∼600MPa was obtained in samples with 12vol% TiB2 whereas the maximum fracture toughness of 6.6MPam1/2 was obtained in samples with 24vol% TiB2. Typical microstructures of samples with the mentioned volume fractions of TiB2 consist of TiB2 particles (<5μm) uniformly dispersed in a matrix of elongated SiC plates.

Nitridation sintering of WC–Ti(C,N)–(Ta,Nb)C–Co hardmetals

Gradient sintering of WC–Ti(C,N)–(Ta,Nb)C–Co hardmetals in vacuum and nitrogen atmosphere was investigated via interrupted sintering experiments and evolving gas analysis. Reduction of surface oxides with corresponding gas evolution was followed by spreading of the binder, shown with coercivity measurements. With low carbon balance, η phases were formed during the intermediate stages of sintering. By introducing nitrogen gas at a later stage all of the η phases could be decomposed again and a graded near-surface microstructure could be formed. If nitrogen is introduced at an early stage when porosity is still open, the TiC powder particles are nitrided, leading to a considerable uptake of nitrogen. Carbon released by this reaction must be balanced by addition of W metal. W and Co binder promote the reaction of TiC with N, whereas TaC and NbC do not. The concept of reaction sintering hardmetal by bulk nitridation was applied on two hardmetal grades and was found an effective way of introducing the nitrogen into such hardmetals.

Hot Airflow Ignition with Microwave Heating for Iron Ore Sintering

At present coal gas with high calorific value is used in ignition (CGI) of iron ore sintering, which brings about problems of high energy consumption and environmental pollution. In this paper, the ignition model using microwave heating (MHI) was developed. Took raw materials of a domestic sintering plant as sample, model calculations showed that the minimum ignition temperature was 638.54°C and the energy consumption was 42.03 MJ/m 2 for MHI process in ignition time of 1.5 min. According to the sintering experiments, with ignition at 660°C for 1.5 min the main indexes of sinter qualities and production in MHI were better than those of CGI (1 050°C) process. And the ignition energy consumption (IEC) in MHI was 43.77 MJ/m 2 , which was far less than 185.13 MJ/m 2 of CGI. Results also show other advantages of suffi