Modelling precipitation of niobium carbide in austenite: multicomponent diffusion,capillarity, and coarsening

Abstract

The growth of niobium carbide in austenite involves the diffusion of both niobium and carbon. These elements diffuse at very different rates. A model is presented for the overall transformation kinetics of niobium carbide precipitation in austenite that takes into account the multicomponent nature of the diffusion process while at the same time allows for the curvature of the transformation front. The inclusion of the curvature (capillarity) effect has, in a numerical scheme, permitted the precipitation and coarsening reactions to be treated in a single model. The model is compared with published experimental data.     

Carburisation layer evolution of Fe–Cr–Ni alloy in furnace after long term service: experimental study and numerical prediction

Abstract

Owing to high temperature comprehensive properties, Fe–Cr–Ni alloys are designed to operate in corrosive gaseous environments of ethylene pyrolysis furnace. However, most premature failed tubes were caused by carburisation. In the present study, based on the Fick’s second law and equilibrium constant method, study on the carburised layer evolution of HP40Nb and KHR45A alloys by pack carburising experimental investigation and numerical simulation by MATLAB software were carried out. The results show that the experimental and simulated data agree with each other acceptably. The carburising layer rate of KHR45A alloy is much smaller than that of HP40Nb alloy due to higher contents of Cr and Ni element in the former. With increasing operating temperature ranging from 1000 to 1100°C, the maximum service lives of the two alloys sharply decrease.     

Model for CO2 laser welding of stainless steel,titanium, and nickel: parametric study

Abstract

The present work has been carried out experimentally and theoretically with the aim of studying the effect of the operative pressure on the weld beads of different materials (AISI 304, Ti–6Al–4V, and Ni), welded using a CO₂ laser beam. At the same time, the reliability of a properly developed analytical model of the laser beam welding process has been confirmed. Such a model, taking into account the dynamics of the process itself, describes the laser induced thermal fields in terms of two heat sources, the first one representing the keyhole effect and the second one, the role played by the plume. A comparison between the experimental data and the beads predicted using the model gives satisfactory results, with average errors less than 5% for Ti–6Al–4V and ~10% for AISI 304 and Ni. The model allows the quantitative evaluation of the power distribution between the keyhole and the plume and a deeper understanding of the entire process.

MST/1591     

Three-dimensional diffusion model for austenitisation of ferritic spheroidal graphite irons

Abstract

A mathematical model has been developed for studying the diffusion kinetics of carbon during the austenitisation of a ferritic ductile iron. In modelling the diffusion process, the finite difference method was used and the diffusion equation was solved using the Crank–Nicolson (implicit) form. The results obtained were compared with those from the conventional error function estimates. For short diffusion distances, that is, spheroidal graphite irons with a high nodule count, the diffusion times required to attain carbon equilibrium suggested by the finite difference and error function methods were similar. For medium to low nodule counts, the times predicted by the error function estimate were less than those suggested by the finite difference analysis.     

A Diffusion Controlled Drug Delivery System for Theophylline

Abstract

The goal of achieving ideal attributes of a drug delivery system including reliability and predictability has led investigators to design controlled release (CR) systems based on the principles of microporous coatings, diffusion controlled coatings and various hydrogel type systems.

In this study, the critical role of “water content fraction” of a polymer in deciding its diffusion characteristics has been ascertained and the correlation between molecular size/shape, membrane thickness, pore radii and drug diffusion has also been demonstrated. The theoretical considerations, designing and engineering of a “barrier coated-reservoir” type of a delivery system for theophylline using poly (vinyl alcohol) [PVA] as the coating material are discussed. After realizing the desired theoretical in-vitro release profile, in-vivo studies were carried out on a dog model. The potential of poly (vinyl alcohol) as a barrier coating material in developing a CR system is interestingly observed.     

Structural characteristics of low temperature plasma carburised austenitic stainless steel

Abstract

A low temperature plasma carburising process has recently been developed to engineer the surfaces of austenitic stainless steels to achieve combined improvements in wear and corrosion resistance. The present paper discusses the structural characteristics of the carburised layers produced on AISI type 316 steel at temperatures between 400 and 600°C. It was found that at low temperatures (<520°C), the carburised layers produced were precipitation free and comprised a single phase, which had a face centred cubic structure and was identified as expanded austenite owing to the supersaturation of carbon in austenite. The carburised layer was in a deformed and distorted state. High densities of twins, stacking faults, and dislocations were found in the expanded austenite. The degree of lattice expansion was estimated and was found to vary with processing temperature and depth in the layer. Precipitation of carbides (mainly Cr₇ C₃ ) occurred when the carburising temperature was relatively high (for example 550 and 600°C). In addition, stress induced martensite was found, particularly in the carburised layers produced at relatively high temperatures.     

Modelling of isothermal formation of pearlite and subsequent reaustenitisation in eutectoid steel during continuous heating

Abstract

Three different morphologies of pearlite have been formed isothermally at three different temperatures in a eutectoid steel. Moreover, the interlamellar spacing of the pearlite was calculated using the Zener and Hillert theoretical method. Experimental results suggest that the growth of pearlite is mainly controlled by volume diffusion of carbon in austenite, in the temperature range studied in this steel. In addition, a model that describes pearlite to austenite transformation during continuous heating in a eutectoid steel has been developed. The influence of structural parameters, such as interlamellar spacing and edge length of pearlite colonies, on the transformation kinetics has been experimentally studied and considered in the modeling. It has been found that the kinetics of pearlite to austenite transformation are slower the coarser the initial pearlite microstructure. Experimental validation of this model has been carried out and a good agreement (an accuracy level of higher than 90% in square correlation factor) between the experimental and calculated values has been found.     

Improving Prediction Accuracy of a Rate-Based Model of an MEA-Based Carbon Capture Process for Large-Scale Commercial Deployment

Carbon capture and storage (CCS) technology will play a critical role in reducing anthropogenic carbon dioxide (CO₂) emission from fossil-fired power plants and other energy-intensive processes. However, the increment of energy cost caused by equipping a carbon capture process is the main barrier to its commercial deployment. To reduce the capital and operating costs of carbon capture, great efforts have been made to achieve optimal design and operation through process modeling, simulation, and optimization. Accurate models form an essential foundation for this purpose. This paper presents a study on developing a more accurate rate-based model in Aspen Plus^® for the monoethanolamine (MEA)-based carbon capture process by multistage model validations. The modeling framework for this process was established first. The steady-state process model was then developed and validated at three stages, which included a thermodynamic model, physical properties calculations, and a process model at the pilot plant scale, covering a wide range of pressures, temperatures, and CO₂ loadings. The calculation correlations of liquid density and interfacial area were updated by coding Fortran subroutines in Aspen Plus^®. The validation results show that the correlation combination for the thermodynamic model used in this study has higher accuracy than those of three other key publications and the model prediction of the process model has a good agreement with the pilot plant experimental data. A case study was carried out for carbon capture from a 250 MW_e combined cycle gas turbine (CCGT) power plant. Shorter packing height and lower specific duty were achieved using this accurate model.     

Simulation and application of dynamic heat transfer model for improvement of continuous casting process

Abstract

Numerical simulation is being increasingly used to improve the existing cooling systems. In order to attain highest quality strand, a two-dimensional dynamic mathematical heat transfer model of billet continuous casting of low carbon steel has been presented. This model can be used to compute the billet temperature distribution and shell thickness, especially it can be used to simulate the solidification process which is caused by frequently variational casting conditions. The fluctuation of measured temperature has been reduced to <10°C with thermal imaging system. The online model can monitor surface temperature and shell thickness in the casting process. So it provides the possibility for the online process control. For the validation of the dynamic model, a lot of billet surface temperature and shell thickness measurement were carried out on an actual casting machine. Finally, the dynamic model has been used for adjusting the operating parameters to improve the casting speed.     

Synthesis of a mesoporous carbon from peach stones for adsorption of basic dyes from wastewater: kinetics,modeling, and thermodynamic studies

The objective of this work was to evaluate the efficiency of a synthesized activated carbon for the removal of methylene blue from aqueous solution. Activated carbons synthesized from agricultural wastes precursors are an interesting alternative for their use as adsorbents and catalyst supports in the wastewater treatment field, and represent an important ecological benefit. In this work, mainly, mesoporous-activated carbon was synthesized from peach stones by chemical activation. The textural, morphological, and chemical characterizations of the material were carried out, revealing the acidic nature of the solid. The effects of different experimental conditions on the adsorption capacity, initial methylene blue concentration, pH solution, adsorbent dosage, and temperature have been investigated. The kinetic experimental data followed the pseudo-second-order model. Intra-particle diffusion model indicated that the adsorption process is controlled by intra-particle diffusion stage. Adsorption isotherms were conducted at several operation temperatures, obtaining the largest methylene blue adsorption capacity value, 444.3 mg g^?1, at the highest temperature, 333 K. The equilibrium adsorption data were tested by four isotherm models, i.e., Langmuir, Freundlich, Tóth, and Redlich–Peterson. It was found that the adsorption data fitted better to Tóth and Redlich–Peterson isotherm models. The calculated thermodynamic parameters suggested that an endothermic and spontaneous process is occurring.     

Optimization of Process Variables in Gas Carburizing—An Experimental Investigation with AISI 3310 Steel Material

Gas carburizing is the most widely used process for surface hardening of critical components used in automobiles, heavy duty machines, etc. In this process the surface composition of the carbon steel changes by diffusion of carbon and results in a hard outer surface with good wear resistance properties. Fatigue behavior of case hardened parts depends largely on the correct combination of its magnitude and depth of hardness penetration without undue distortion. An industrial survey indicates that there is a rejection of 10-12% of case hardened components due to defects like low hardness and strength after hardening, distortion, and warping. Optimal control of the process parameters is the way to ensure the quality of the surface hardened part. This paper is the result of the investigation carried out using limit design concept called Taguchi's Technique in the optimization of process variables in gas carburizing.     

Development of process for aluminising nickel alloy 718 strips for tube bundle support structures in liquid sodium–water steam generators

Abstract

The tubes in liquid sodium–water steam generators of the Indian prototype fast breeder reactor (PFBR) will be supported by corrugated nickel alloy 718 strips. Aluminisation of nickel alloy 718 strips has been chosen for this application because of the excellent performance of aluminide coatings in reducing impact fretting wear of the tubes due to flow induced vibrations and compatibility of the coating with liquid sodium at the operating temperature of the steam generators. Aluminisation of nickel alloy 718 strips for steam generator tube bundle support structures has been developed using a procedure involving thermal spraying of aluminium followed by diffusion heat treatment in vacuum atmosphere. One of the advantages of the technique is that it will coat only the desired surfaces of the strips, whereas in conventional pack cementation process, significant precautions have to be ensured. Furthermore, this process has enabled aluminisation to be carried out at a much lower cost than the conventional process of pack aluminising. The problems encountered during the initial trials and technology development, such as coating thickness and distortion, are discussed. A process flow chart for this procedure to take the job on an industrial scale is also reported. This process (under patenting) has been adopted for the aluminisation of corrugated strips for the support structures of one steam generator module and the steam generator for a test facility during the technology development phase, as also of all the steam generators being fabricated for the PFBR.     

Time-dependent behavior of gas absorption in lubricant oil

Gas absorption behavior in liquid with time has been studied by applying a simple mathematical model. The model is based on a one-dimensional mass diffusion process due to the concentration difference in dissolving gas. In order to examine the model validity, selected experimental data in the literature have been analyzed with the present model. The model has faithfully reproduced the experimental transient behavior of dissolving gases, and physically meaningful model parameters, diffusion constant and solubility limit, have been obtained for dissolving gas (water and refrigerant) in various lubricant oils. The validity and the significance of the model are discussed.     

Modelling of diffusion bonding of metals

Abstract

Modelling of diffusion bonding has been carried out to quantify the kinetics of the bonding processes and to predict the time for achieving a sound bond. An alternative geometric assumption for the shape of the interfacial cavities to those considered previously was employed. Three subprocesses of bonding were introduced to simplify the modelling. These involved volume and interfacial diffusion coupled with creep, rigid collapse, and surface diffusion. The effects of grain size and phase ratio on diffusion bonding have also been considered. The predictions are compared with existing experimental data for copper and Ti–6Al–4V alloy and in general show good agreement.

MST/588     

EULERIAN AND LAGRANGIAN SIMULATIONS OF PARTICLE DEPOSITION FROM A POINT SOURCE IN A TURBULENT CHANNEL FLOW

ABSTRACT

Results comparing Eulerian and Lagrangian simulations of particle deposition from a point source in a channel are presented. The mean turbulent flow field is simulated using a two-equation k-? turbulence model. In the first, approach, diffusion of aerosol particles is studied by solving the corresponding advection-diffusion equation. Deposition of particles in the intermediate size range are analyzed by considering both the turbulent eddy diffusion and the eddy impaction processes, as well as the Brownian diffusion effects. In the second approach, the turbulence fluctuating velocity field are numerically simulated as a Gaussian random process. The Lagrangian trajectories of aerosol particles in the channel are then evaluated by solving the corresponding particle equation of motion. Effects of Brownian diffusion on particle motions are also included. A series of digital simulations for particles of various sizes which are released at different locations across the channel are carried out. Depositions of different size particles on the wall under a variety of conditions are analyzed. The relative significance of turbulence and Brownian effects are also discussed.     

Beta ray monitoring technique for control of resin content in continuous fibre prepreg materials

Abstract

A beta ray monitoring technique has been developed to measure the resin content during the manufacture of preimpregnated fibre (prepreg) unidirectional tapes by a solution impregnation process. A series of calibration relationships have been obtained for aramid, carbon and glass fibre reinforced epoxy prepreg tapes, and the effect of variation of the fibre areal weight on measuring accuracy has been analysed. Good agreements with resin content have been demonstrated using the beta ray system and conventional solvent extraction method. Moreover, online control of the resin content has been carried out by adjusting the concentration of resin in the impregnation bath.     

CFD modelling and simulation of an industrial scale continuous fluidized bed roaster

In the present work, computational fluid dynamics (CFD) based modelling of an industrial scale continuous fluidised bed roaster (FBR) has been carried out to study its performance at different operating conditions, so that the sulphide-sulphur content in the product is within 0.4% at the designed feed rate of 39.75 DMT/h. Eulerian-Eulerian multiphase model, considering four granular phases and one gas phase has been implemented to investigate the velocity and mass fraction profile of the particles in the FBR. The heat and species mass balance calculations have been performed external to CFD, by dividing the roaster into several sections. The conversion of ZnS to ZnO at various sections of the roaster has been estimated using reaction kinetics under isothermal condition (1203 K). The heat liberated and possible temperature rise at each section was predicted based on the heat of reaction and sensible heat of the solid and gaseous products. The CFD model was validated with the plant data for a feed rate of 36.5 DMT/h, air flow rate of 65,000 Nm³/h and O₂ content of 21%. The proposed model predicted the sulphide-sulphur content in the product to be 0.4% for the designed feed rate of 39.75 DMT/h, when the O₂ content in the inlet air was increased to 25%.     

Quantitative X-ray diffraction analysis of development of Z phase in 12%Cr–Nb–V–N steel

Abstract

To study the evolution of nitrides (Nb,V)N, (V,Nb)N and Cr₂N, and in particular the formation of the Z phase Cr(V, Nb)N, a model alloy with composition similar to that of 12%Cr steels for high temperature applications, microalloyed with Nb and V but with a very low carbon content, has been designed. A quantitative determination of the volume fractions of the extracted nitrides that had formed after aging treatments at 650°C for up to 10 000 h was carried out by an X-ray diffraction procedure, based on the Rietveld approach. The investigation of the Z phase evolution by the Johnson–Mehl–Avrami–Kolmogorov kinetics at 650 and 700°C revealed that, as the kinetic exponent is very close to unity, the formation mechanism of this phase is not associated with a conventional process of nucleation but hints at a gradual diffusion controlled transformation of the pre-existing V and Nb nitrides.     

Effect of current density on distribution and roughness of nanocrystallites for duplex treatment

Abstract

Up to fourth moment distribution of carbide nanocrystallites produced by duplex treatments of surface nanocrystallisation and pulsed plasma electrolytic carburising on AISI 316 stainless steel was investigated by means of figure analysis with high precision. Skewness and kurtosis study of Gaussian distribution have been studied, and the effect of current density has been determined. The usage of higher current densities is more suitable for achieving lower sizes of carbide nanocrystallites. Surface roughness of treated samples was measured, and it has been observed that there is an optimum level of current density for surface roughness increasing (difference between two measured data).     

Microstructural evolution of Inconel* 718 during ingot breakdown: process modelling and validation

Abstract

A computer model is presented that describes microstructural evolution during the ingot breakdown of nickel base superalloy Inconel 718 via the open die cogging operation. To support the development of the model, a compression testing programme has been carried out which covers the ranges of temperatures, strains, and strain rates experienced during thermomechanical processing. Analysis of the flow curves has allowed the identification of the regimes in which the various deformation mechanisms take place. Logic based rules have been incorporated into the model, and this has allowed predictions of the microstructural evolution to be made. Where possible, the results have been compared with the available experimental data and it is shown that theory and experiment are in reasonable agreement. A number of computational experiments have been carried out, to study the effects of changing the forging procedure.