Stellite 21 coatings on AISI 410 martensitic stainless steel by gas tungsten arc welding

Abstract

Degradation of AISI 410 martensitic stainless steel, a typical alloy for many applications such as steam turbine blade, could impair its efficiency and lifetime. To overcome this problem, critical surfaces could be modified by weld cladding via gas tungsten arc welding technique. In the present research, a comparative study of Stellite 21 weld overlays deposited in three different thicknesses, i.e. dilutions, at various preheat and post-weld heat treatment temperatures on the surface of AISI 410 martensitic stainless steel, has been made. The surface of coatings has been examined to reveal their microstructures, phase characterisation and mechanical properties using XRD, microhardness tester and metallographic techniques. The results showed that the deposition of Stellite 21 coating on AISI 410 martensitic stainless steel improved its corrosion resistance. Moreover, the volumetric dilution had a considerable effect on the hardness, microstructure and electrochemical corrosion behaviour of Stellite 21 weld overlays.     

Three-dimensional modeling of arc plasma and metal transfer in gas metal arc welding

An integrated comprehensive 3D model has been developed to study the transport phenomena in gas metal arc welding (GMAW). This includes the arc plasma, droplet generation, transfer and impingement onto the weld pool, and weld pool dynamics. The continuum formulation is used for the conservation equations of mass, momentum, and energy in the metal zone. The free surface is tracked using the volume-of-fluid (VOF) technique. The 3D plasma arc model is solved for the electric and magnetic fields in the entire domain. The interaction and coupling between the metal zone and the plasma zone is considered. The distributions of velocity, pressure, temperature, and free surface for the metal zone and the velocity, pressure, and temperature for the plasma zone are all calculated as a function of time. The numerical results show the time-dependant distributions of arc pressure, current density, and heat transfer at the workpiece surface are different from presumed Gaussian distributions in previous models. It is also observed that these distributions for a moving arc are non-axisymmetric and the peaks shift to the arc moving direction.     

Numerical modeling of heat transfer and fluid flow in rotor–stator cavities with throughflow

The present study considers the numerical modeling of the turbulent flow in a rotor–stator cavity subjected to a superimposed throughflow with heat transfer. Numerical predictions based on one-point statistical modeling using a low Reynolds number second-order full stress transport closure are compared with experimental data available in the literature [E.M. Sparrow, J.L. Goldstein, Effect of rotation and coolant throughflow on the heat transfer and temperature field in an enclosure, J. Heat Transfer 98 (1976) 387–394; M. Djaoui, A. Dyment, R. Debuchy, Heat transfer in a rotor–stator system with a radial inflow, Eur. J. Mech. B – Fluids 20 (2001) 371–398; S. Poncet, M.P. Chauve, R. Schiestel, Batchelor versus Stewartson flow structures in a rotor–stator cavity with throughflow, Phys. Fluids, 17(7) (2005).]. Considering small temperature differences, density variations can be here neglected which leads to dissociate the dynamical effects from the heat transfer process. The fluid flow in an enclosed disk system with axial throughflow is well predicted compared to the velocity measurements performed at IRPHE [S. Poncet, M.P. Chauve, R. Schiestel, Batchelor versus Stewartson flow structures in a rotor–stator cavity with throughflow, Phys. Fluids, 17(7) (2005)] under isothermal conditions. When the shroud is heated, the effects of rotation and coolant outward throughflow on the heat transfer have been investigated and the numerical results are found to be in good agreement with the data of Sparrow and Goldstein [E.M. Sparrow, J.L. Goldstein, Effect of rotation and coolant throughflow on the heat transfer and temperature field in an enclosure, J. Heat Transfer 98 (1976) 387–394]. Their results have been extended for a wide range of the Prandtl number. We have also considered the case of an open rotor–stator cavity with a radial inward throughflow with heat transfer along the stator, which corresponds to the experiment of Djaoui et al. [M. Djaoui, A. Dyment, R. Debuchy, Heat transfer in a rotor–stator system with a radial inflow, Eur. J. Mech. B – Fluids 20 (2001) 371–398]. Our results have been compared to both their temperature measurements and their asymptotic model with a close agreement between the different approaches, showing the efficiency of the second order modeling. An empirical correlation law is given to predict the averaged Nusselt number depending on the Reynolds and Prandtl numbers and on the coolant flowrate.     

An investigation of heat transfer and fluid flow on laser micro-welding upon the thin stainless steel sheet (SUS304) using computational fluid dynamics (CFD)

A transient three-dimensional model is numerically developed using the method of computational fluid dynamics (CFD) to characterize some thermal phenomena and characterization of heat transfer and fluid flow in laser micro-welding by considering the heat source and the material interaction leads to rapid heating, melting and thermal cycles in the heating zone. The application of developed thermal models has demonstrated that the laser parameters, such as laser power, scanning velocity and spot diameter, have considerable effects on the peak temperature and resulted weld pool. The heat source model is consisted of surface heat source and adaptive volumetric heat source that could be well represented the real laser welding as the heat penetrates into the material. In the computation of melt dynamics, mass conservation, momentum and energy equations have been considered to compute the effects of melt flow and the thermo-fluid energy heat transfer. The simulation results have been compared with two sets of experimental research to predict the weld bead geometry and solidification pattern, which laser welds are made on thin stainless steel sheet (SUS304). The shape comparison describes those parameters relevant to any changes in the temperatures and melt dynamics are of great importance on the heat distribution and formation of weld pool during laser micro-welding process. The fair agreement between simulated and experimental results, demonstrates the reliability of the computed model.     

Numerical computation of fluid flow and heat transfer in microchannels

Three-dimensional fluid flow and heat transfer phenomena inside heated microchannels is investigated. The steady, laminar flow and heat transfer equations are solved using a finite-volume method. The numerical procedure is validated by comparing the predicted local thermal resistances with available experimental data. The friction factor is also predicted in this study. It was found that the heat input lowers the frictional losses, particularly at lower Reynolds numbers. At lower Reynolds numbers the temperature of the water increases, leading to a decrease in the viscosity and hence smaller frictional losses.     

Predicting the influence of groove angle on heat transfer and fluid flow for new gas metal arc welding processes

This article studies the three dimensional transient weld pool dynamics and the influence of groove angle on welding of low carbon structural steel plates using the ForceArc® process. The deformation of the weld bead is also calculated with an accurate coupling of the heat transfer with fluid flow through continuity, momentum and the energy equations combined with the effect of droplet impingement, gravity, electromagnetic force, buoyancy, drag forces and surface tension force (Marangoni effect). Different angles of V groove are employed under the same welding parameters and their influence on the weld pool behavior and weld bead geometry is calculated and analyzed, which is needed for subsequent calculations of residual stress and distortion of the workpiece.Such a simulation is an effective way to study welding processes because the influence of all welding parameters can be analyzed separately with respect to heat transfer, weld pool dynamic, and microstructure of the weld. Good agreement is found between the predicted and experimentally determined weld bead cross-section and temperature cycles. It is found that the main flow pattern is more or less the same although the groove angle increases, but it will evoke larger amount of fluid to flow downward to get deeper penetration.     

Influence of machining-induced martensite on hydrogen-assisted fracture of AISI type 304 austenitic stainless steel

Hydrogen-assisted fracture of AISI type 304 steel has been evaluated with a special focus on the strain-induced martensite that is produced below the specimen surface during standard turning operation. Two different surface conditions were investigated: one containing martensite, resulting from the machining process, and a martensite-free state which is obtained after a proper heat treatment. Additionally, chemical composition and thickness of oxide layers, occurring in both studied cases, were analyzed by secondary ion mass spectrometry. These two different conditions were tested at room temperature in air (ambient pressure) and in hydrogen gas (40 MPa) atmosphere, respectively. Experimental results reveal a detrimental effect of machining-induced martensite on AISI type 304 steel performance in hydrogen, leading to major differences in relative reduction of area (RRA) between the as-machined and the heat-treated state for the same material. In this context, an operating mechanism based on hydrogen diffusion is discussed.     

Numerical prediction of fluid flow and heat transfer in a wavy pipe

IntroductionA pipe with periodically converging-divergingcross-section is one Of the sevens devices employed forenhancing the heat and mass tusfer efficiency. Thenuid flow, to the now passages with a periodicallyvaling cross-section, attains a folly develOPed acmethat differs fundamentally from that for a convelltionalconstant-area flow channel. In the periodically vwigcross-seCtions, the ac developed VelM field repeatsitSelf at cormsponding edal locations in successivecycles. The change of…     

Numerical study of fluid flow and heat transfer in microchannel cooling passages

Numerical investigation was conducted for fluid flow and heat transfer in microchannel cooling passages. Effects of viscosity and thermal conductivity variations on characteristics of fluid flow and heat transfer were taken into account in theoretical modeling. Two-dimensional simulation was performed for low Reynolds number flow of liquid water in a 100 μm single channel subjected to localized heat flux boundary conditions. The velocity field was highly coupled with temperature distribution and distorted through the variations of viscosity and thermal conductivity. The induced cross-flow velocity had a marked contribution to the convection. The heat transfer enhancement due to viscosity-variation was pronounced, though the axial conduction introduced by thermal-conductivity-variation was insignificant unless for the cases with very low Reynolds numbers.     

Three-dimensional modeling of heat transfer and fluid flow in laminar-plasma material re-melting processing

Modeling study is performed concerning the heat transfer and fluid flow for a laminar argon plasma jet impinging normally upon a flat workpiece exposed to the ambient air. The diffusion of the air into the plasma jet is handled by using the combined-diffusion-coefficient approach. The heat flux density and jet shear stress distributions at the workpiece surface obtained from the plasma jet modeling are then used to study the re-melting process of a carbon steel workpiece. Besides the heat conduction within the workpiece, the effects of the plasma-jet inlet parameters (temperature and velocity), workpiece moving speed, Marangoni convection, natural convection etc. on the re-melting process are considered. The modeling results demonstrate that the shapes and sizes of the molten pool in the workpiece are influenced appreciably by the plasma-jet inlet parameters, workpiece moving speed and Marangoni convection. The jet shear stress manifests its effect at higher plasma-jet inlet velocities, while the natural convection effect can be ignored. The modeling results of the molten pool sizes agree reasonably with available experimental data.     

Effect of alloying elements on hydrogen environment embrittlement of AISI type 304 austenitic stainless steel

The chemical composition of an AISI type 304 austenitic stainless was systematically modified in order to evaluate the influence of the elements Mo, Ni, Si, S, Cr and Mn on the material’s susceptibility to hydrogen environment embrittlement (HEE). Mechanical properties were evaluated by tensile testing at room temperature in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere. For every chemical composition, the corresponding austenite stability was evaluated by magnetic response measurements and thermodynamic calculations based on the Calphad method. Tensile test results show that yield and tensile strength are negligibly affected by the presence of hydrogen, whereas measurements of elongation to rupture and reduction of area indicate an increasing ductility loss with decreasing austenite stability. Concerning modifications of alloy composition, an increase in Si, Mn and Cr content showed a significant improvement of material’s ductility compared to other alloying elements.     

Numerical investigation of fluid flow and heat transfer under high heat flux using rectangular micro-channels

A 3D-conjugate numerical investigation was conducted to predict heat transfer characteristics in a rectangular cross-sectional micro-channel employing simultaneously developing single-phase flows. The numerical code was validated by comparison with previous experimental and numerical results for the same micro-channel dimensions and classical correlations based on conventional sized channels. High heat fluxes up to 130 W/cm² were applied to investigate micro-channel thermal characteristics. The entire computational domain was discretized using a 120 × 160 × 100 grid for the micro-channel with an aspect ratio of (α = 4.56) and examined for Reynolds numbers in the laminar range (Re 500–2000) using FLUENT. De-ionized water served as the cooling fluid while the micro-channel substrate used was made of copper. Validation results were found to be in good agreement with previous experimental and numerical data [1] with an average deviation of less than 4.2%. As the applied heat flux increased, an increase in heat transfer coefficient values was observed. Also, the Reynolds number required for transition from single-phase fluid to two-phase was found to increase. A correlation is proposed for the results of average Nusselt numbers for the heat transfer characteristics in micro-channels with simultaneously developing, single-phase flows.     

Numerical investigation of fluid flow and heat transfer over louvered fins in compact heat exchanger

Numerical investigation of fluid flow and heat transfer characteristics over louvered fins and flat tube in compact heat exchangers is presented in this study. Three-dimensional simulations of single and double row tubes with louvered fins have been conducted. Simulations are performed for different geometries with varying louver pitch, louver angle, fin pitch and tube pitch and for different Reynolds number. Conjugate heat transfer and conduction through the fins are considered. The air-side performance of heat exchanger is evaluated by calculating Stanton number and friction factor. The results are compared with experiment and a good agreement is observed. The local Nusselt number variation along the top surface of the louver is calculated and effects of geometrical parameters on the average heat transfer coefficient is computed. Design curves are obtained which can used to predict the heat transfer and the pressure drop for a given louver geometry.     

Numerical investigation on synthetical performances of fluid flow and heat transfer of semiattached rib-channels

Although the conventional internal ribs or turbulators can significantly improve the performances of the convective heat transfer within a channel, the added ribs can also cause two demerits, a larger friction factor and some lower heat transfer areas (LHTA) than that in the corresponding smooth channel, especially behind fully attached (solid-type) ribs and at the corners formed by bottom and side walls. This paper presents a novel design of the ribbed channel, which is here called semiattached rib-design. The ribs are perforated at the rib corners to form two rectangular holes, so a portion of the fluid can pass through the holes. The characteristics of the semiattached rib-design are numerically investigated by the commercial software Fluent 6.3 in a Reynolds number range from 10⁴ to 2.5 × 10⁴. Five different structures of the rib (width ratios of channel to hole) and two positions (transverse rib and 45° angled ribs) are analyzed. The numerical results show that the semiattached rib-design can significantly improve local heat transfer and fluid flow performances; the semiattached ribs with 45° angle of attack can even achieve a higher efficiency of synthetical heat transfer than that of the fully attached and detached rib-channels, at the same time eliminate the LHTA; although the average Nusselt number over a pitch in the transverse ribbed channel is lower than that of fully attached and detached rib-channels, this semiattached ribs can also fully eliminate the LHTA.     

Numerical simulation of fluid flow and convection heat transfer in sintered porous plate channels

Fluid flow and convective heat transfer of water in sintered bronze porous plate channels was investigated numerically. The numerical simulations assumed a simple cubic structure formed by uniformly sized particles with small contact areas and a finite-thickness wall subject to a constant heat flux at the surface which mirrors the experimental setup. The permeability and inertia coefficient were calculated numerically according to the modified Darcy’s model. The numerical calculation results are in agreement with well-known correlation results. The calculated local heat transfer coefficients on the plate channel surface, which agreed well with the experimental data, increased with mass flow rate and decreased slightly along the axial direction. The convection heat transfer coefficients between the solid particles and the fluid and the volumetric heat transfer coefficients in the porous media predicted by the numerical results increase with mass flow rate and decrease with increasing particle diameter. The numerical results also illustrate the temperature difference between the solid particles and the fluid which indicates the local thermal non-equilibrium in porous media.     

Numerical investigation of the heat transfer and fluid flow in a Carbon Monoxide boiler

We present a numerical investigation of the heat transfer and fluid flow in a Carbon Monoxide (CO) boiler. The influences of some important parameters related to the geometry of a CO boiler are also discussed, including the refractory thickness and the insertion of an ellipsoidal cone. The purpose is to improve the performance of a CO boiler, especially to alleviate hot spots, which may lead to deterioration of the refractory. It is found that the skin friction coefficient arises earlier in the DeNOx section when the refractory is thinner. Refractory thickening leads to a lower temperature in the DeNOx section. After leaving the DeNOx section, the temperature reduces due to the cooling pipes and the skin friction coefficient arises abruptly near the cooling pipes. It is also found that insertion of an ellipsoidal cone can lower the temperature in the DeNOx section and there is a larger skin friction coefficient with a larger ellipsoidal cone angle. After leaving the DeNOx section, the skin friction coefficient and the temperature both decrease. Finally, based on the result of this research and from a practical point of view, refractory thickening is a preferable choice for reducing friction, pressure drop and temperature in a CO boiler.     

Numerical simulation of turbulent fluid flow and heat transfer characteristics in heat exchangers fitted with porous media

Numerical simulations have been carried out to investigate the turbulent heat transfer enhancement in the pipe filled with porous media. Two-dimensional axisymmetric numerical simulations using the k–? turbulent model is used to calculate the fluid flow and heat transfer characteristics in a pipe filled with porous media. The parameters studied include the Reynolds number (Re = 5000–15,000), the Darcy number (Da = 10^?1–10^?6), and the porous radius ratio (e = 0.0–1.0). The numerical results show that the flow field can be adjusted and the thickness of boundary layer can be decreased by the inserted porous medium so that the heat transfer can be enhanced in the pipe. The local distributions of the Nusselt number along the flow direction increase with the increase of the Reynolds number and thickness of the porous layer, but increase with the decreasing Darcy number. For a porous radius ratio less than about 0.6, the effect of the Darcy number on the pressure drop is not that significant. The optimum porous radius ratio is around 0.8 for the range of the parameters investigated, which can be used to enhance heat transfer in heat exchangers.     

Numerical analysis of heat flow in contact heat transfer

A finite difference analysis of heat conduction problem in a cylinder terminating in a frustum of a cone is presented. The constriction can be either in vacuum or in a gaseous environment. A fine mesh of 2500 × 800 was used for the construction of the grid such that very small constrictions could be analysed sufficiently accurately. Small constrictions i.e., small contact areas separated by large voids filled with a gas are typical of most practical applications involving contact heat transfer. The result of the finite difference analysis shows that gap conductance is predominant for all the gases considered. Gap-to-solid conductance ratio increases as the cone angle decreases due to the decrease of gap thickness. It also indicates that increase of conductance ratio is less significant at higher constriction angles. Finally, predicted conductance parameters are compared with the experimental results for different interfacial gases and a very good agreement is obtained.