G. G. Roy

Modeling of heat transfer and fluid flow during gas tungsten arc spot welding of low carbon steel

The evolution of temperature and velocity fields during gas tungsten arc spot welding of AISI 1005 steel was studied using a transient numerical model. The calculated geometry of the weld fusion zone and heat affected zone and the weld thermal cycles were in good agreement with the corresponding experimental results. Dimensional analysis was used to understand the importance of heat transfer by conduction and convection at various stages of the evolution of the weld pool and the role of various driving forces for convection in the liquid pool. The calculated cooling rates are found to be almost independent of position between the 1073 and 773 K (800 and 500 °C) temperature range, but vary significantly at the onset of solidification at different portions of the weld pool. During solidification, the mushy zone grew significantly with time until the pure liquid region vanished. The solidification rate of the mushy zone/solid interface was shown to increase while the temperature gradient in the mushy zone at...

Numerical modelling of 3D plastic flow and heat transfer during friction stir welding of stainless steel

Abstract Three-dimensional (3D) viscoplastic flow and temperature field during friction stir welding (FSW) of 304 austenitic stainless steel were mathematically modelled. The equations of conservation of mass, momentum and energy were solved in three dimensions using spatially variable thermophysical properties using a methodology adapted from well established previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering strain rate and temperature dependent flow stress. The computed profiles of strain rate and viscosity were examined in light of the existing literature on thermomechanical processing of alloys. The computed results showed significant viscoplastic flow near the tool surface, and convective transport of heat was found to be an important mechanism of heat transfer. The computed temperature and velocity fields demonstrated strongly 3D nature of the transport of heat and mass indicating the need for 3D calculations. The computed temperature profiles agreed well with the corresponding experimentally measured values. The non-Newtonian viscosity for FSW of stainless steel was found to be of the same order of magnitude as that for the FSW of aluminium. Like FSW of aluminium, the viscosity was found to be a strong function of both strain rate and temperature, while strain rate was found to be the most dominant factor. A small region of recirculating plasticised material was found to be present near the tool pin. The size of this region was larger near the shoulder and smaller further away from it. Streamlines around the pin were influenced by the presence of the rotating shoulder, especially at higher elevations. Stream lines indicated that material was transported mainly around the pin in the retreating side.

A computationally efficient model of convective heat transfer and solidification characteristics during keyhole mode laser welding

Computationally efficient heat transfer models of keyhole mode laser welding ignore fluid flow in the gas, liquid, and the two phase solid-liquid regions. These models cannot be applied to high Peclet number systems where convective heat transfer affects weld pool geometry, cooling rate, and other weld attributes. Here we show that by synthesizing features of an existing model to determine keyhole shape and size with rigorous fluid flow and heat transfer calculations in the liquid and the two phase solid-liquid regions, important features of both high and low Peclet number systems can be satisfactorily simulated. The geometry of the keyhole is calculated by assuming thermal equilibrium at the gas/liquid interface and point by point heat balance at the keyhole wall. The heat transfer outside the vapor cavity is calculated by numerically solving the equations of conservation of mass, momentum, and energy. A vorticity based turbulence model is used to estimate the values of effective viscosity and effective ...

Dimensionless correlation to estimate peak temperature during friction stir welding

Abstract A dimensionless correlation has been developed based on Buckinghams π-theorem to estimate the peak temperature during friction stir welding (FSW). A relationship is proposed between dimensionless peak temperature and dimensionless heat input. Apart from the estimation of peak temperature, it can also be used for the selection of welding conditions to prevent melting of the workpiece during FSW. The correlation includes thermal properties of the material and the tool, the area of the tool shoulder and the rotational and translation speeds of the tool. The peak temperatures reported in the literature during FSW of various materials and welding conditions were found to be in fair agreement with the proposed correlation.

Mathematical modeling of heat transfer, fluid flow, and solidification during linear welding with a pulsed laser beam

The evolution of temperature and velocity fields during welding of 304 stainless steel with a pulsed laser beam was simulated using a three dimensional numerical heat transfer and fluid flow model. The weld pool solidified between pulses and regions of the weld bead melted and solidified several times during welding. Short laser pulses restricted the width of the weld track and velocities in the weld pool. However, convection still remained an important mechanism of heat transfer in the weld pool. The computed high cooling rates during linear welding with neodymium-doped yttrium aluminum garnet pulsed laser operated at 140W average power, 20Hz frequency, and 5ms pulse duration were consistent with those observed in typical laser welding. After the laser beam was switched off, the mushy zone expanded, reaching its maximum size when no pure liquid region remained. Calculations of solidification parameters indicated that the criterion for plane front solidification was not satisfied. The results demonstrate ...

A heat transfer study of the continuous caster mold using a finite volume approach coupled with genetic algorithms

The mold region of the Continuous Caster was studied using a heat transfer formulation coupled with an optimization scheme. The pertinent transport equations were solved using a finite volume approach and the optimization calculations were conducted using biologically inspired Genetic Algorithms. The results are compared with the data available in the literature and significant improvements in terms of the casting velocity and the solidified shell thickness are observed and reported.

Efficacy and recovery of calcium during CaSi cored wire injection in steel melts

Abstract Deoxidation, desulphurisation and inclusion modification are some of the essential features in secondary steel refining through injection of calcium alloy in the form of a cored wire. It is imperative that the filling material must be consumed by the melt to the maximum extent in order to make this cored wire alloy addition cost effective and efficient. In this connection study of calcium recovery is an important issue, which may provide guidelines to select the optimum operating parameters from a fundamental basis. The present study is based on plant data from Tata Steel, Jamshedpur, India. The results demonstrate that maximum calcium recovery is achievable at an optimum speed of the cored wire. Dimensional analysis was carried out to analyse the plant data and the calcium recovery was found to correlate well with physically sound dimensionless numbers such as Biot number, the dimensionless bath temperature and the relative rate of silicon to sulphur transfer from/to the added calcium silicide in the melt. A correlation has also been obtained between the calcium recovery and various dimensionless parameters. The efficacy of calcium addition is also assessed through characterisation of inclusions by scanning electron microscopy and energy dispersive X-ray spectroscopy.

Mathematical modelling of iron ore sintering process using genetic algorithm

Abstract A mathematical model of the iron ore sintering process in a fixed sinter bed and optimisation of the process parameters using real valued genetic algorithm is described. The mathematical model is formulated based on mass and energy balances in gas and solid phases, which are represented by simple partial differential equations. The model has a number of process controlling parameters, which are optimised using a heuristic optimisation tool, genetic algorithm. Predictions of temperatures at different bed heights are found to be in reasonable agreement with the experimental data obtained in a laboratory scale pot sintering machine. The model could be used as a potential tool to predict temperature and progress of fire line in industrial sinter strand after being tuned sufficiently with plant trials.

Estimation of Average Spot Diameter and Bead Penetration Using Process Model During Electron Beam Welding of AISI 304 Stainless Steel

A mathematical model based on macroscopic heat balance for surface ablation phenomenon and analytical temperature profile below the vapor cavity, has been developed to estimate the average effective spot diameter and predict depth of bead penetration as a function of the processing parameters during electron beam welding of AISI 304 stainless steel. The average beam spot diameter on the work piece surface has been estimated by minimizing the difference between the model prediction and experimental depth of bead penetration. Present experimental data and data from relevant literature have been used to obtain the correlations for beam spot diameter as a function of heat input (i.e. energy input per unit length). Subsequently, a close agreement between the predicted and experimental bead depths is observed.

Study of reduction behaviour of prefabricated iron ore–graphite/coal composite pellets in rotary hearth furnace

Abstract In the present study, the reduction kinetics of prefabricated iron ore–graphite/coal composite pellets of different shapes has been studied in a rotary hearth furnace (RHF). Commercial processes involving the RHF such as ITmk3/FASTMET have major problems of low productivity owing to significant heat and mass transfer resistance through the multilayer bed and consequently limited pellet layers over the hearth. In the present investigation, an attempt has been made to improve the heat and mass transfer in such system by increasing the specific surface area of individual pellets. Both the iron ore–graphite and iron ore–coal composite pellets have been reduced in an RHF at a maximum temperature of 1200°C. The ore–coal composite showed much higher degree of reduction (81%) over ore–graphite composite pellets (61%). The tablet shaped pellet with the highest specific surface area displayed a higher degree of reduction than the cylinder or sphere shaped pellets. Although no physical slag metal separation was visible, X-ray diffraction and SEM/EDX of reduced particles indicated separation at the microlevel. Higher amount of reduction and liquid silicate formation for tablet shaped pellets, in comparison with spherical and cylindrical shaped pellets, lay the foundation of a novel process flow scheme involving the use of prefabricated pellets in the RHF.