Thermophysical properties of the Ni-based alloy Nimonic 80A up to 2400 K

Nimonic 80A is a nickel-chromium alloy which is strengthened by additions of titanium and aluminium. The alloy is used for high temperature, high strength applications. Wire shaped Nimonic 80A samples are resistively volume heated as part of a fast capacitor discharge circuit. Time resolved measurements with sub-μs resolution of current through the specimen are performed with a Pearson probe, voltage drop across the specimen is measured with knife-edge contacts and ohmic voltage dividers and the radiance temperature of the sample with a pyrometer. These measurements allow to determine heat of fusion as well as heat capacity and electrical resistivity at initial geometry of Nimonic 80A as a function of temperature in the solid and in the liquid phase up to 2400 K.     

Correct shielding gas selection in dual wire GMAW

K-type thermocouples are fabricated using a chromel wire and an alumel wire of 50 m by the non-contact discharge welding method. The welding is carried out at six different butt angles. The size of the hot junction tends to decrease with an increase in butt angle, when the butt angle is larger than 1.4 rad. The response rate is measured and it is almost proportional to the reciprocal of the size of the hot junction. It is shown that the size of the hot junction is an important factor for the response rate. Wire diameter is another important factor for temperature measurement of small objects. When temperature is measured at the same point by a thermocouple under the conditions that only the hot junction and its vicinity are heated, the output of electromotive force, i.e. indicated temperature, will depend on the wire diameter. As the metal wire is a good thermal conductor and the rate of heat transport is proportional to the area of the wire cross section, deviation from the true value increases with an increase in wire diameter. Our experiments showed that the indicated temperature decreased linearly with the wire diameter. Furthermore, we measured the discharge voltage and current at various setting currents and discharge gaps.     

New thermogalvanic method determines the conditions which cause dezincification of admiralty brass in field service

A newly developed laboratory test method based on thermogalvanic currents realistically simulates the dezincification of admiraly brass in fresh or sea water. It is especially useful for studying the dezincification of brass heat exchanger tubes.The method uses an internally heated tube that is exposed to a test solution while being galvanically coupled to a relatively large colder piece of the same metal. The tube is thermally insulated in such a way that hot spots and boiling are induced on its surface. The temperature difference between the hot tube and the colder piece of metal produces a thermogalvanic action that results in current densities up to 0.16 mA cm^?2. The electrode potential of the heated tube is more negative than when the tube is cold. Under these conditions, dezincification of inhibited admiralty tube occurred within 72 h in fresh water and much sooner in a 0.5% solution of sodium chloride.Under these test conditions, two types of dezincification were noted, depending on the kind of boiling and the intensity of the associated thermogalvanic current produced on the surface of the tube; localized film boiling caused a plug-type dezincification with strong evidence for the redeposition of copper, whereas localized nucleate boiling causes a layer-type dezincification with strong evidence for dissolution of the zinc. Very low water velocities, high process side temperatures, low pH or presence of a water conditioning treatment resulting in non-scaling water, high solution conductivity, and the absence of waterside corrosion inhibitors are factors which lead to dezincification in field service.     

Combined analytical-numerical approach to the voltage of a cylindrical coil with pulsed current

This paper presents a combined analytical-numerical approach to solving the pulsed eddy current problem accurately and quickly. Considering the displacement current, the analytical solution to the voltage of a cylindrical coil above a laminated conductor in the complex-frequency domain is deduced by Laplace transform. The time-domain induction voltage values of a cylindrical coil with a pulsed current are calculated by the fourth-order integro-differential FFT-based numerical inversion of Laplace transform. At the same time, the time-domain analytical solution to the induced voltage of a cylindrical coil with a pulsed current above a half-infinite non-ferromagnetic conductor is derived, and has been verified by comparison with Finite Element Method (FEM) simulation results. The calculation results prove that the adopted numerical inversion method of applying Laplace transforms to the pulsed eddy current problem has a high accuracy and fast convergence. The transient voltages produced by a square-wave current excitation when considering the displacement current in the vacuum area are higher than those when ignoring the displacement current, by as much as 27.7% at certain times. The higher the lift-off is, the smaller the voltage peak is and the faster the voltage drops. As the application of this method, the induced voltages are computed in the measurements of metal's thickness and metal coating thickness.     

小径管周向裂纹脉冲涡流检测仿真和试验研究

针对小径管周向裂纹缺陷,通过有限元仿真及试验,研究了利用磁导体环形激励脉冲涡流检测技术检测小径管周向裂纹缺陷的问题。仿真给出了管道在有缺陷和无缺陷状态下磁场分布、涡流分布以及接收线圈的电压值。从仿真结果可以观察出,周向裂纹端头处的磁场分布以及涡流分布会发生明显变化,产生沿管壁法向的磁场,检测线圈位于裂纹端头处正上方时检测灵敏度最高。实际检测结果与仿真结果一致,表明磁导体环形激励轴向涡流对小径管周向缺陷具有显著的检测效果。     

An investigation on surface grinding using graphite as lubricant

Grinding requires high specific energy and the consequent development of high temperature impairs workpiece quality by inducing tensile residual stress, burn, micro cracks etc. Control of grinding temperature is achieved by providing effective cooling and lubrication. Conventional flood cooling is often ineffective due to the relative inaccessibility of the fluid to the actual grinding zone, film boiling etc. Further these fluids are also a source of health hazards. Minimization and possibly the elimination of fluid coolants by substituting their functions by some other means is of current research interest. This paper deals with an investigation on using graphite as a lubricating medium to reduce the heat generated at the grinding zone. An experimental set-up has been developed for this and a detailed comparison has been done with dry and coolant flooded grinding in terms of forces, specific energy, temperature and surface finish. Results show that grinding force, energy and temperature are reduced and resultant surface finish depends on workpiece material.     

Effect of Steam Temperature on the Corrosion of Metals under Heat-Transfer Conditions*

Abstract

Previous workers have shown that the corrosion rate of metals in acid media is frequently much greater under heat-transfer conditions than it is under simple immersion conditions. In the work described here it has been found that the rate of corrosion of a metallic heating surface is critically dependent upon the temperature of the heating surface in relation to the boiling point of the liquid heated. Experiments have been made with copper and stainless steel heat-transfer surfaces over the temperature range 75°-140° in acetic and sulphuric acids. It was found that there was a pronounced increase in the corrosion rate when. the temperature of the heat-transfer surface was held atapproximately the boiling point of the liquid being heated. At this temperature the corrosion rate was some five times that of the corrosion rates at higher and lower temperatures.     

Identification of Heat Transfer Coefficients by Use Conditions of Quenching Oil

Heat transfer coefficients of the quench medium are necessary for heat-treatment simulation. Cooling characteristics of quenching oil vary with kinds and usage greatly. Users are selecting oil solutions that come up to their desired hardness and quenching distortion requirements. In particular cooling performance rises by agitation and decompression. Therefore we identified a heat transfer coefficient by usage and kinds of quenching oil. Cooling characteristics are different greatly by a kind of quenching oil. A difference of a cooling characteristic by a kind of oil depends on a temperature range of a boiling stage and the maximum heat transfer coefficient mainly. On the other hand, in a convection stage, there are few changes in a boiling stage. Even if quenching oil temperature is changed, heat transfer coefficients do not change greatly. When quenching oil stirred, heat transfer coefficients of vapor blanket stage and a convection stage rise, but there are a few changes in a boiling stage. When quenching oil is decompressed a temperature range of a high heat transfer coefficient moves to the low temperature side. In addition, a heat transfer coefficient in a vapor blanket stage comes down. For precision improvement of heat-treatment simulation, it is important that the heat transfer coefficient is calculated in conformity to the on-site use reality.     

Formation regularities of gaseous vapour plasma envelope in electrolyzer

This work focuses on the factors causing appearance of a steady and continuous vapour-gas envelope which functions as medium for plasma electrolytic saturation of metal and alloys with interstitialelements (nitrogen, carbon, and boron). It is established that second critical voltage associated with transition from the current oscillation mode to the stable heating is determined by anion emission from boiling electrolyte in the envelope and heat transfer conditions in the system. Stability of the interface electrolyte–envelope is provided by the energy liberation in the envelope due to the passage ofcurrent. Second critical voltage promoting the anion emission is calculated on the base of Gouy–Chapman model and Tonks–Frenkel aperiodic instability. Theoretical dependence of critical voltage on the electrolyte concentration is confirmed experimentally. The influence of the electrolyte concentration on the second critical voltage is explained by the ability of the electrolyte to emit anions. Effect of solution flow rate on this voltage accounts for heat transfer conditions. It should be noted that the anion emission explains the influence of electrolyte composition on the weight change of the anode sample, limit heating temperature (~1000°C) due to the limited emissivity of electrolyte, discrete current in the case of a small surface anode, and high-frequency pulse of the current.     

INVESTIGATION OF THE EFFECT OF TEMPERATURES ON TENSILE LOAD DROPS OF TITANIUM ALLOY

1.IntroductionLowtemperaturesmaysignificantlyaffectthemechanica1propertiesofmaterials-Basedonthelowtemperaturestrellgth-toughnesscharacteristics,cry0genicstructuralmaterial8weredividedint0threeclassesbyMorrisll].Thefirstclassincludesmaterialsthatundergoaductile-brittletransitionatlowtemperatures,thesecondclassthatremainsductileatalltemperaturesandthethirdthemetastablealloyswhoseductilityandtoughnessarelargelydeterminedbylowtemperaturephaJsetransf0rmations.Severalillterestingmechanicalphenomen…     

ENERGY STORAGE IN METAl DEVICES AND THE ADVANTAGES OF LIQUID HYDROGEN TEMPERATURES

1.IntroductionPurealuminumisappliedasaconductingmaterialforcreationofdifferentpowerfulmagnoticsystemsoperatingatfoWtemperatures.Smallmagnet0resistanceofaluminumgivespossibilitytousethismaterialasonecomp0nentofcdmpositecryoconductorsbe-ingusedforstabilizationofsuperc0nductivecablesandalsoformachinesanddevices-IncompositecryoconductorsonthebaseofaluminumasmallmagnetoresistanceisamainadVanage.Insufficientmechanicalstrength0faluminumiscompensatedbyhardermate-rialsforexamplebycopper.Howeverinthei…     

Measurement and calculation of arc power and heat transfer efficiency in pulsed gas metal arc welding

Abstract

This study investigates the heat transfer efficiency of the pulsed gas metal arc welding (GMAW-P) process. The arc power and heat input were calculated from arc current and voltage measurements and the heat input was also measured with a liquid nitrogen calorimeter. The measured heat transfer efficiency for GMAW-P varied slightly over a wide range of pulse parameters, with an average value of 70%, a maximum of 72% and a minimum of 68%. Welding heat transfer efficiency based on arc power calculated as the product of average current and voltage was too high (averaging 82%), while that calculated using the product of the root mean square (RMS) of the average current and voltage was too low (averaging 61%). Both also varied significantly with pulsing parameters. Mathematical analysis shows that average instantaneous power values must be used when current and voltage vary significantly with time. The experimental differences between the average instantaneous power and the other calculated values are explained by the relative phases of the pulsed current, voltage and arc resistance waveforms.     

NUMERICAL SIMULATION OF MECHANICAL BEHAVIOR DURING LOCAL POSTWELD HEAT TREATMENT

1.IntroductionStresscorrosioncrackingduetothecombinedeffectsofserViceenvirollrnentandlOcalizedweld-indllcedstresshajsbeenrecognizedasaseriousProblemintheweldingindustryforanumberofyearsLI].InordertoimprovetheductilityofweldssusceptibletocrackingandrelievetheresidualstressesthelOcalpostweld11eattreatmentisusuallyperformedwhenitisunpracticaltoheattreatthewholevesselinafurnace.TwoprimarybenefitsofPWHTarethetemperingandtherel~tionofresidualstresses.Benefits,suchasimprovedductilityltoughnessand…     

An experimental study of heat transfer in aluminum castings during water quenching

The influence of quenching orientation and agitation conditions on heat transfer of aluminum alloys during water quenching was experimentally investigated with a test casting. The results indicate that heat transfer in water quenching of casting aluminum alloy consists of film boiling, nucleate boiling and convection stages. The highest heat transfer coefficients (HTC) are observed in the nucleate boiling, while the lowest is in the convective cooling stage. The heat transfer coefficients on the horizontal surfaces facing down during quenching are lower than those of other surfaces regardless whether the water is agitated or not. Agitation enhances heat transfer process especially when castings are at high temperatures and heat transfer process is in the film boiling stage.     

Thermal stabilization for accurate dimensional measurement using gallium

A new method of thermal stabilization that uses gallium as an isothermal heat exchanger is presented. Gallium maintains a fixed temperature near 29 °C when it melts, and the resultant temperature sustains extremely good stability against ambient change of temperature and pressure. This thermo-physical property allows gallium to be utilized as an isothermal medium that is capable of performing precise thermal stabilization at near room temperature without intensive care on temperature sensing and heat control. In this investigation, an isothermal gallium chamber is designed and an ultraprecision level of temperature stabilization to ±1 mK is demonstrated with the aid of Fabry–Perot interferometry.     

Effect of plasma fluctuations on in-flight particle parameters

The influence of arc root fluctuations in direct current (DC) plasma spraying on the physical state of the particle jet is investigated by correlating individual in-flight particle temperature and velocity measurements with the instantaneous voltage difference between the electrodes. In-flight diagnostics with the DPV-2000 sensing device involve two-color pyrometry and time-of-flight technique for the determination of temperature and velocity. Synchronization of particle diagnostics with the torch voltage fluctuations are performed using an electronic circuit that generates a pulse when the voltage reaches some specific level; this pulse, which can be shifted by an arbitrary period of time, is used to trigger the acquisition of the pyrometric signals. Contrary to predictions obtained by numerical modeling, time-dependent variations in particle temperature and velocity due to power fluctuations induced by the arc movement can be very large. Periodic variations of the mean particle temperature and velocity, up to ΔT=600 °C and Δv=200 m/s, are recorded in the middle of the particle jet during a voltage cycle. To our knowledge, this is the first time that large time-dependent effects of the arc root fluctuations on the particle state (temperature and velocity) are experimentally demonstrated. Moreover, large fluctuations in the number of detected particles are observed throughout a voltage cycle; very few particles are detected during parts of the cycle. The existence of quiet periods suggests that particles injected at some specific moments in the plasma are not heated sufficiently to be detected.     

Arc power and efficiency in gas tungsten arc welding of aluminium

Abstract

A calorimetric study of gas tungsten arc welding of aluminium is described. The present study comprised experiments in which autogenous welding runs were each made on a block of electrical conductor grade aluminium. The blocks were all approximately cubic in shape which, when combined with the high thermal conductivity of aluminium, ensured that their temperature equalised soon after the completion of a run. Each sample was immersed in insulating material before welding so that heat losses to the surroundings were minimised. Thermocouples were attached to the block in each experiment and the bulk temperature rise was related to the energy input associated with the welding run. The effects of arc polarity, alternating current balance, shielding gas composition, arc length and welding current on the arc power and arc efficiency were investigated. The results obtained with alternating current are compared to those for direct current, and the differences are explained.     

Control of Ni-P and Co-P Chemical Deposition with Transient Voltage Curves and Structure of the Deposits

Aqueous chemical deposition (ACD) of Ni-P and Co-P films can be controlled with a secondary current pulse (SCP) technique, measuring the voltage variation at an electrode on which electrodeposition is already occurring at constant current density as a consequence of short square current pulses, in the ms range. The transient voltage curves are interpreted and three parameters obtained: the transient Tafelslope, related to metal ion electrochemical discharge mechanism, the surface adsorption capacity and the surface ohmic resistance. The influence of H₂ or 02 saturation in the bath on the deposition kinetics and on the structure of thin layers is shown, and the mechanism of bath stabilization by several additives is interpreted.     

On temperature dependence of the parameters of corona discharge current–voltage characteristics

The dependences of the parameters of corona discharge current–voltage characteristics on the gas temperature are considered in this work. It is shown that the corona discharge ignition voltage U_c(T) of helium and nitrogen decreases with the increase in temperature. The dependence of the parameter A(T) is complicated. It increases at the initial and final sections of the studied temperature range (20–369°C) and decreases at its central portion with two extrema. As the discharge in nitrogen is unstable, we failed to obtain any definite regularity. Generalized dependences that make assertions about the presence of a discharge different from a corona are presented. We made an assumption of an important role of two effects in the observed processes: the absorption of ions by the surface of electrodes and the continuous change in ion mobility due to the ion mass variation in the course of clustering and declustering of ions. It is supposed that there is a dynamic equilibrium between them in a steady state.     

Estimation of heat flux in array of jets quenching using experimental and inverse finite element method

This work presents the combined experimental and numerical technique to estimate the heat flux in the metal quenching process by array of jets. Experimental techniques are explained for the measurement of temperature. A two-dimensional inverse heat conduction problem (IHCP) is solved through the Ling et al. [Ling, X., Keanini, R.G., Cherukuri, H.P., 2003. A non-iterative finite element method for inverse heat conduction problems. International Journal for Numerical Methods in Engineering 56 (9), 1315–1334] non-iterative finite element method (FEM) using the experimental temperature data. Wetting front which separates the film boiling and nucleate boiling zone, changes the order of the heat flux. Maximum heat flux position and its propagation velocity are plotted as a function of time. It is demonstrated that increase in water velocity decreases the maximum heat flux and delays the wetting front movement.