Direct Resistance Heating Research Articles

Electromagnetic and temperature fields in direct resistance heating of compound axisymmetric bodies

A nonstationary coupled problem of thermoelectrodynamics is formulated for resistance heating of dissimilar ferro- and paramagnetic bodies by an alternating current. An iterative algorithm for solving this problem by finite-difference methods is proposed. Temperature and electromagnetic-field distributions are obtained for the processes of direct resistance heating both in air and in the region of the molding tool.

Kinetics and thermodynamics of the α→γ m massive transformation in a Ti–47.5 at.% Al alloy

Continuous cooling experiments, utilizing in situ, high-speed computer-controlled temperature and electrical resistivity measurements, were performed to study the kinetics and thermodynamics of the α→γ m massive transformation in a Ti–47.5 at.% Al alloy. Samples of the alloy were heated by controlled direct resistance heating in vacuum to the α-phase field and cooled at various rates either in vacuum or by controlling the flow of a helium jet quench. The results have shown that the α→γ transformation is sensitive to cooling rate and is accompanied by large discontinuous changes in resistivity and thermal arrest. Lamellar, feathery and massive γ m are observed with increasing cooling rate. Correlation of the resistivity–temperature–time data with microstructure allowed the determination of the cooling velocity range in which the massive transformation takes place, the reaction temperatures of this transformation as a function of cooling rate, and the growth rate of the massive γ m phase as a function of undercooling below T 0. The enthalpy and driving force associated with the formation of the massive γ m phase were determined and compared with those in other alloy systems; the experimental values of the driving force were also found to compare well with theoretically calculated values. Comparison of estimates of the activation enthalpy for boundary diffusion obtained with values for bulk diffusion established that the α→γ m massive transformation is controlled by interfacial rather than volume diffusion. Possible growth mechanisms of the massive phase, whether by a ledge process or by continuous growth, were examined and are discussed.

Near-Net Shape Forming of Functionally Graded Cemented Carbide, Sinter-Bonded on Steel

The cemented carbide coating on steel is required to have a harder and thicker coated layer to improve durability However, it is difficult to coat a hard and thick coated layer firmly, because it promotes cracks and peels the coated layer due to the thermal stress originated by the thermal expansion mismatch. To overcome this problem and to improve the property of hard material, we studied a new FGM of steel coated with cemented carbide layers. Cobalt compositionally graded cemented carbide powders stacked on steel and sinter-bonded to the steel by a new sintering process, using pressurizing and direct resistance heating by pulse current We manufactured the desired structure and confirmed the stress relief, high bonding strength and distribution of functionality of coated layers between the surface with high wear resistance and with high toughness inside. We applied this FGM to a stabilizer blade for a well drilling tool. Although the blade has a curved surface, we successfully sinter-bond graded cemented carbide layers on the near-net-shaped steel substrate with a uniformly graded structure. To fabricate the stabilizer, the blades were welded to steel. The FGM blades with-stood the heat cycle of welding and were confirmed to have welding performance equivalent to conventional blades. Because the entire surface is covered with graded cemented carbide layers, this new stabilizer is expected to have excellent performance compare to the conventional stabilizer whose blades have cemented carbide tips that are inserted partially.

A 3D numerical model of the temperature - time characteristics of specimens tested on a Gleeble thermomechanical simulator

A three-dimensional (3D) explicit finite difference model has been developed to simulate the thermal behaviour of specimens subjected to controlled thermal cycling patterns using direct resistance heating in a Gleeble thermomechanical simulator. Model predictions have been compared with temperature measurements performed on 2024 aluminium alloy, 0.2% plain carbon steel and 316 austenitic stainless steel specimens in the simulator. The specimen materials were chosen to provide widely differing thermophysical properties and, in the case of the steels, to allow a comparison of the predictions with and without an to phase transition.

Direct resistance and induction heating of a slab with convective surfaces

An analytical solution for the temperature rise in a non-insulated slab is obtained for the cases in which direct resistance heating or induction heating is applied. The solutions for both cases are carried out simultaneously for better comparison. The effects of applied current frequency and the Bi number on the temperature profile are shown. The advantages of direct resistance and induction heating for certain industrial applications are discussed.

インサート材を用いた鋼とアルミニウムの異種金属接合 （第１報） アルミクラッド鋼と鋼およびアルミニウムとのスポット溶接

This paper deals with the resistance spot weldability of an aluminum clad steel sheet to a steel or an aluminum sheet, and the dissimilar metal joining of a steel to an aluminum sheet with an aluminum clad sheet as an intermediate layer. The clad sheet was produced by hot rolling of steel and aluminum sheets with a direct resistance heating process. The mechanical properties of the clad sheet were between those of the steel and the aluminum sheet, and the peel strength was satisfactorily high.Materials used in spot welding were a 0.8 mm-thick EDDQ steel sheet, a 1.0 mm-thick aluminum-magnesium alloy sheet and the 0.8 mm-thick clad sheet which has a thickness ratio of aluminum/steel 1.1. The spot welding of the steel sheet to the steel side of the clad sheet and the aluminum sheet to the aluminum side of the clad sheet were successful. The spot welding of the steel sheet to the aluminum sheet by using the clad sheet as intermediate layer was also successful. However, thin intermetallic compound layer was formed at the steel/aluminum interface of the clad sheet due to the spot welding thermal cycle. The suitable welding current ranged between the values suitable for steel to steel and for aluminum to aluminum.The tensile shear strength and the U-tension strength of steel-clad-aluminum joints were higher than those of steel-aluminum direct joints and close to those of aluminum-aluminum joints. These results from mechanical tests suggest that the existence of intermetallic compound layers at steel/aluminum interfaces of the clad sheets were not detrimental to the strength of the steel-clad-aluminum joints. The fracture mode of these joints varied with the welding current.

Spot welding of aluminium clad steel to steel or aluminium: Joining steel to aluminium with an intermediate layer (1st report)

Summary This paper describes an investigation of the resistance spot weldability of aluminium clad steel sheet to steel or aluminium sheet as well as joining of a steel to an aluminium sheet with an aluminium clad steel sheet as an intermediate layer. The clad steel sheet was produced by hot‐rolling of steel and aluminium sheet by a direct resistance heating process. The mechanical properties of the clad steel sheet were between those of the steel and aluminium sheet, and the peel strength was satisfactorily high. The materials used in spot welding were a 0.8 mm thick EDDQ steel sheet, a 1.0 mm thick aluminium‐magnesium alloy sheet, and a 0.8 mm thick clad steel sheet which had an aluminium/steel thickness ratio of 1.1:1. Spot welding of steel sheet to the steel side of the clad steel sheet and aluminium sheet to the aluminium side of the clad steel sheet was successful. Spot welding of steel sheet to aluminium sheet using the clad steel sheet as an intermediate layer was also successful. The spot welding thermal cycle, however, led to a thin intermetallic compound layer being formed at the steel/aluminium interface of the clad steel sheet. The suitable welding current ranged between the values suitable for steel to steel and aluminium to aluminium. The tensile shear strength and U‐tension strength of steel/clad‐aluminium joints were higher than those of steel‐aluminium direct joints and near those of aluminium‐aluminium joints. These mechanical test results suggest that the presence of an intermetallic compound layer at the steel/aluminium interface of the clad steel sheets is not detrimental to the strength of the steel‐clad‐aluminium joints. The fracture mode of these joints varied with the welding current.

Influence of the mould temperature on the properties of melt-cast Bi-Sr-Ca-Cu-O precursors

The melt casting route has been shown to be a promising synthesis technique for the fabrication of bulk Bi-Sr-Ca-Cu-O superconductor. Precursors produced via this technique may be converted into superconducting material through conventional radiative heating or, as has been shown, through direct electrical resistance heating. For processing via the latter technique, a precursor with homogeneous electrical properties is preferred. In this paper melt cast precursor rods of composition Bi/sub 1.6/Pb/sub 0.4/Sr/sub 2/Ca/sub 3/Cu/sub 4/O/sub y/ have been prepared through casting a melt into a preheated copper mould and the influence of the temperature to which the mould was preheated on the electrical resistance and homogeneity of the precursor was studied. >

The study of the graphitization behavior for polyimide and polyamide films

The structure transformation for the carbonized polyimide and polyamide films during their resistance heat-treatments has been examined. Two polymer precursor films, Kapton (here after Kp) (polyimide polymer) film, and Mk (polyamide polymer) film, were carbonized to 900°C under Ar in furnace. A direct resistance heating approach, a DC current passed through the film via electrodes, was utilized for the further heat-treatment of these carbonized polymer films under inert atmosphere. The heating processes were carried out by step increasing either the supply voltage, or the supply current to their destined values. The processed films have been characterized by X-ray diffraction (XRD), Raman, SEM, and four probes conductivity measurement. The results of XRD seem to suggest an earlier d-spacing dropping for the carbonized Kp films from 1400 to 1600°C under the resistance heating process. The carbonized Mk film gives a different XRD pattern than the Kapton's during its heat-treatment process.

Phase transformations in two-phase [formula omitted] alloys during continuous heating and cooling, studied by electrical resistivity measurements

The α → γ transformation in a Ti-47.5at.%Al alloy and the α → α 2 transformation in a Ti-40at.%Al alloy was studied by a novel computer-interfaced control-cum-data acquisition technique. In-situ, real-time, high-speed simultaneous measurements of resistivity and temperature were made in the system designed and constructed for this purpose. The experiments were made in the system designed and constructed for this purpose. The experiments were conducted in high vacuum. The samples were heated to the completely α region by controlled direct resistance heating, and cooled at various rates utilizing either controlled cooling algorithms or by varying the gas-jet quench. Temperature and resistivity changes were monitored during heating and cooling, and correlated with corresponding phase/microstructural changes. The results have shown that the γ phase is characterized by a low resistivity (about 50 μΩ cm at room temperature), whereas both α and α 2 phases have high resistivities (about 190 to 200 μΩ cm at room temperature). Because of the large difference in resistivities of the α and γ phases, large and measurable changes in resistivity can be observed when one of these phases transforms to the other. In the Ti-47.5Al alloy, the lamellar γ morphology was observed at slower cooling rates, the Widmanstatten-like at intermediate cooling rates, and a massive reaction at very high cooling rates. Preliminary results of the transformation start and completion temperatures for the various reactions as a function of cooling rates are presented.

Transient heat conduction in a slab during direct resistance and induction heating

Transient one dimensional conduction in a slab during direct resistance and induction heating is studied analiticaly. Solutions are caried out simultaneously because of their similarity. Effect of applied current frequency on the temperature variation is discussed. Advantages of direct resistance heating and induction heating for certain industrial applications are pointed out.

Liquefaction of coals and maceral concentrates in a flowing-solvent reactor

A flowing-solvent liquefaction reactor with a continuous solvent sweep through a fixed bed of coal has been developed. The reactor is heated by computer-controlled direct electrical resistance heating, allowing the application of variable heating rates between 0.1 and 10 K/s to temperatures up to 450 °C and holding times at peak temperature between zero and 1600 s. In contrast to liquefaction experiments using closed (batch) reactors, where products remain within the reactor for the duration of the experiment, solubilised liquefaction products are continuously swept out of the reaction zone, thus minimising the probability of secondary reactions. The apparatus has been proved by performing liquefaction experiments on a selection of European and American coals and maceral concentrates

The α→γ Transformation During Continuous Cooling in Ti-48 At% Al Alloys

The α→ γ transformation in a Ti-48 at% Al alloy was studied by a novel computer interfaced control-cum-data acquisition technique. In situ ,real time, high speed measurements of temperature were made in the system engineered for this purpose. The samples were heated by controlled direct resistance heating and gas-jet quenched. Various cooling rates were achieved by control algorithms and controlling the pressure of the quenching medium. Cooling curves and thermal arrest data were used to determine the transformation temperatures and characteristics as a function of cooling rate. Subsequent microstructural analysis by optical microscopy was used to confirm the occurrence of the various transformation modes, namely, lamellar, Widmanstatten and massive. The transformation start and completion temperatures for the various reactions were determined for different cooling rates, and continuous cooling transformation diagrams were determined. The various results are presented and discussed in light of thermodynamic and phase diagram considerations.

A flowing solvent reactor for coal liquefaction with direct electrical resistance heating

A flowing solvent reactor for coal liquefaction has been developed, which uses direct electrical resistance heating of the reactor tube wall to achieve rapid, controllable heating. A computerized control system previously developed for a wire-mesh pyrolysis apparatus is used for temperature control; heating rates between 0.1 and 10 K/s, peak temperatures up to 450 °C, and isothermal holding times from 5 to 1600 s have been obtained. Alternating current is used for heating, allowing a simple thyristor bridge to be used for power control. Solvent is fed from a pressurized reservoir at up to 100 bars, obviating the need for a supply pump. Using this apparatus the effects of a variety of parameters on primary coal liquefaction processes can be examined; the design of the system and typical results are presented.

Liquefaction of coal in a flowing-solvent reactor

A flowing-solvent liquefaction reactor has been used to examine the primary solubilization potential of a range of coals. The reactor configuration allows the suppression of secondary reactions by sweeping the heated zone with a continuous flow of solvent. The design of the system combines low thermal inertia and direct electrical resistance heating to provide closely controlled time-temperature ramping, and a variable preset holding time at the target temperature. Heating rates of 0.1–15K s −1 up to temperatures of 450 °C and hold times of 0–1600 s can be achieved. Data are reported for a range of European and US coals with elemental carbon contents ranging from 74 to 91%. The effect on conversion of reactor pressure, heating rate, temperature, holding time at temperature and solvent flow rate have been determined. In contrast with pyrolysis, results suggest that liquefaction reactivities are relatively insensitive to changes in the heating rate. Comparison with results from micro-bomb reactors has shown that the continuous solvent sweep allows the relatively intact removal of primary solubilization products from the reaction zone. Molecular mass distributions of solubilized products thus obtained have been found to increase with increasing carbon content of the substrate coal (up to ~85%), and to decline with further increases up to 91% carbon content. In general, this reactor configuration appears to offer a promising avenue for analytically probing the more reactive structures of coals without interference from effects of secondary reactions.

On the importance of direct resistance heating in thermo-mechanical fatigue

A computationally efficient method of determining the temperature distribution and stress intensity factor induced by the resistance heating method of cycling temperature in thermo-mechanical fatigue is presented. It is further used to evaluate the importance of the singularity of the heat source caused by the electric current. Results indicate the thermally induced stress intensity factor of cracked components cycled by the direct resistance method of heating increases with the frequency of the thermal loading and may become quite large at higher frequencies.

The use of direct resistance heating in the food industry

Direct resistance heating (DRH) offers the chance to process solid and liquid foods at the same rate, avoiding the delay due to thermal conduction which prevents the use of HTST technologies on paniculate foods. The attempts to exploit the advantages of DRH in food processing over the last century are reviewed. A successful DRH unit requires non-contaminating electrodes which have a good contact with the food material, control of the food heating rate and, if sterilisation is required, an efficient aseptic packaging process. Recent developments in these three areas mean that the advantages of direct resistance heating can now be commercially exploited.

High-temperature strength testing of directly resistance-heated metal specimens

Direct resistance heating of specimens was used for high-temperature strength testing in a protective atmosphere. The details of the design of temperature control and loading equipment are described. By using long specimens the test temperature of the central gau~e length can be kept satisfactorily constant. The method is well suited for creep testmg provided the cross sectwn decreases homogeneously during primary and steadystate creep. Htgh-temperature tenstle tests can also be performed, ptoviding correct data on yield and ultimate tensile stresses provided the strength of the material decreases monotonically with temperature.

A Unified Approach to the Teaching of Induction and Resistance Heating of Steel Conductors

The step-function approximation of the magnetisation characteristic is used systematically to develop equations describing direct resistance heating of a steel bar. The equations for the inductively heated thick plate follow as a limiting case. Sinusoidal applied current (field) or sinusoidal applied voltage conditions are considered.

Ultrahigh vacuum measurements in China

Research and development on UHV gauges were started in China in 1953. Since then incessant efforts have been devoted to improvements of Bayard–Alpert gauges, suppressor gauges, extractor gauges, deflector gauges, and modulated Bayard–Alpert gauges. Performances of various gauges, from a special gauge with all electrodes degassed by direct resistance heating, to a shielding gauge with no photocurrent effect are briefly reviewed in the present paper. Finally, a method for UHV gauge calibration is suggested and a new quadrupole structure for partial pressure measurement is described.