Advantage of Constant Frequency Inductive Heating in Quenching Dilatometry Instruments
Keywords: Quenching Dilatometry, Inductive Heating, Constant Sinus Frequency, Amplitude Adjustment
INTRODUCTION induction as ideal heating method for industrial and analytical heat treatment processes in TA Instruments DIL 805 quenching dilatometers metallurgy [1]. allow rapid inductive heating of conductive samples with a constant frequency using PENETRATION DEPTH proprietary technology. The sophisticated powerful inductive heating with constant sinus The frequency of the alternating magnetic field frequency is power controlled by amplitude determines the depth it penetrates the object. adjustment and provides most homogeneous The penetration affects the distribution of sample heating under all conditions. induced eddy currents in the material. In the simplest case of a solid round bar, the induced INDUCTION HEATING PRINCIPLE current decreases exponentially from the surface. An "effective" depth of the current- Induction heating is the process of heating an carrying layers can be derived as electrically conducting object by 1 훿 = (1) electromagnetic induction through heat √휋∙푓∙휇∙휎 generated in the object by eddy currents. An induction heater consists basically of a coil and where δ is the standard depth of penetration, f an electronic oscillator that passes a high is the frequency of the AC field, μ is the magnetic frequency alternating current (AC) through the permeability and σ the conductivity of the coil. The coil generates a rapidly alternating material [2]. magnetic field which penetrates the object. The The strongly non-linear relation between the alternating magnetic field generates electric penetration depth and the frequency of the AC currents inside the conducting object, called field is shown in the double-logarithmic diagram eddy currents. The eddy currents flowing below for different materials. through the resistance of the material heat it by Joule heating. In ferromagnetic materials like iron, heat may also be generated by magnetic hysteresis losses. The optimum frequency of current used depends on the object size, material type, coupling between the coil and the object to be heated and the desired penetration depth of the induction of eddy currents in the object.
An important feature of induction heating is that the heat is generated inside the object itself and is not limited by heat conductivity. Thus, objects Figure 1: Influence of frequency on penetration depth can be heated very rapidly. This qualifies in different materials [3].
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FREQUENCY INFLUENE ON SAMPLE In diagram 1 above a temperature program of a TEMPERATURE Pt reference sample is recorded along with the frequency of the inductive heater in the DIL 805. If the frequency of the inductive field changes The temperature program consists of a heating during the heating process – for instance while segment up to 1000°C with 100 K/min heating the heating power is adjusted – the penetration rate, 10 min isothermal dwell time, and a quench depth of the induced current in the metal sample with 100 K/min cooling rate. changes (cp. Figure 1 above). The induced The frequency of the inductive heating field is current generates the heating of the sample. constant within ±0.3% over the entire Thus, changing the frequency changes the temperature program. penetration depth of the induced current which then changes the temperature profile in the sample. CONCLUSION
Generating a consistent, reproducible, and Inductive heating of conductive metal samples is homogeneous temperature in the sample during effective for rapid sample temperature control in the entire temperature program in a quenching quenching dilatometers. Changing frequency of test is inherent for an accurate determination of the inductive heating field is known to change the phase transition temperatures. Generating the penetration depth and the distribution of the the inductive heating of the sample at constant eddy currents induced in the sample. The frequency is therefore a key feature for alternation in current distribution leads to a generating highly accurate data in quenching changing and inhomogeneous temperature experiments. profile in the sample with the change in heating frequency. TA PROPRIETARY INDUCTIVE HEATER WITH TA Instruments’ DIL 805 quenching dilatometers CONSTANT FREQUENCY apply a proprietary high frequency generator allowing for constant frequency heating power In TA Instruments’ DIL 805 quenching control by amplitude adjustment. This constant dilatometers a proprietary high frequency frequency inductive heating is an essential generator is used. It allows heating power feature for the superior accuracy and control by amplitude adjustment at a constant reproducibility of the quenching data measured frequency. with the DIL 805.
REFERENCES
[ 1 ] Rudnev, V., Loveless, D., Cook, R.L. and Black, M.; Handbook of Induction Heating, Manufacturing Engineering and Materials Processing; Taylor & Francis; 2002. [ 2 ] Zinn, S. and Semiatin, S. L.; Elements of Induction Heating; ASM International; 1988; P. 15ff. Diagram 1: Temperature of a Pt-sample heated and [ 3 ] Ether NDE Ltd.; Definitions and Data quenched in the DIL 805 and recording of inductive heating field frequency. Tables for Eddy Current Testing; 2019.
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