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·Methods and Instrumentation
Elevated Frequencies in Eddy Currents - New Possibilities of Thin Surface Layer EvaluationDr. Valentin Uchanin
Generala Gritzaja str., 11-5, Lviv 290017, Ukraine.
For metal constructions and components quality estimation it is very needed to evaluate the parameters of thin surface layers. With reference to non-magnetic materials an eddy current (EC) method based on the electric conductivity measurement in effective zone of the EC probe is widely utilized for this purpose. The effective zone depth is a function of real (or effective) depth of eddy currents penetration and depends on work frequency and EC probe parameters. This zone is more than 0.5 mm for the main nonferrous metal materials when traditional frequencies (less than 10 MHz) are applied. So, when a specific electrical conductivity is measured on these frequencies we obtain some value averaged in the eddy currents distribution volume (more 0,3 - 0,5 mm). When the material is homogeneous on depth, it does not limit the eddy current method possibilities. The problems arise if it is necessary to evaluate the inhomogeneous on depth surface layers in which the structural changes and changes of specific electrical conductivity happens in the depths of few tens micron, for example gas-filled (diffusion) alpha layers on titanium alloys. For detection and estimation of thin (less than 0.1 mm) surface layers parameters the application of elevated frequencies to localize eddy currents it was suggested. The desirable frequencies can be estimated from the relations for electromagnetic fields depth of penetration. To concentrate eddy currents in 100 microns depth surface layer in a titanium alloy workpiece (an electrical conductivity about 1 MSm/m) it is necessary to operate on frequencies more than 100 MHz. Despite of importance of a problem we cannot term many investigations conducted in this direction. It is possible to mention only the results obtained by Ju. Grigulis, V. Gavrilin, A. Dorofejev, V. Ostapenko and N. Kalinin [1 - 3] and some our works [4 - 7, 9, 10]. All this works are published in Russian and are not well presented for world NDT community. In this paper eddy current elevated frequency method and technique for selective thin surface layer material testing developed by the author are presented.
For estimation of quality of components made from titanium alloys it is necessary to detect and to evaluate the brittle and hard gas-filled (alpha) case on a surface of titanium alloys , such as the mechanical properties is determined by the ratio of alpha and beta phases. Alpha case is surface gas contamination that results during the heat treatment in the air presence. If the alpha cases are not removed before the service, the brittle layer can result the surface cracking and ultimately the failure of component. Now alpha cases are investigated destructively by etching technique or hardness measurement of inclined microsections of model samples, which have passed similar heat treatment. For titanium alloy structure nondestructive evaluation acoustic methods with ultrasonic velocity and attenuation measurements are applied, because beta phase is more attenuative than the alpha phase. This work is in progress. But according to our reckoning the acoustic methods does not allow to investigate modified structures localized in thin surface layers . That is why the most promising technique for thin surface alpha cases (and another structural changes) evaluation is EC method based on the fact that the alpha phase has lower electric conductivity.
For detection of gas-filled (alpha) surface cases in titanium alloys EC method and devices "Alpha" and "Delta" were designed [4, 7]. Such as this cases are situated in thin surface layer (10 - 200 microns) the elevated operational frequencies (more than 100 MHz) were applied to receive good sensitivity. The method is based on the application of the high-frequency double-circuit self-oscillator with the sensing EC coil included in a working circuit. The device operation grounded on registration of failure of oscillation of the self-oscillator when probe is situated on surface with gas-filled (alpha) surface layer.
These devices are provided with sensitive EC coil (0.5 mm in diameter) mounted in hand-held EC probe together with generator and primary signal processing circuit. The working frequencies are 100, 200 and 400 MHz in dependence of the needed sensitivity and tested material electric conductivity. The scheme of EC self-oscillator double-circuit EC probe is shown in figure 1. The EC sensing coil L2 and capacitor C4 make the first working circuit of the self-oscillator and the elements L1, C1, C5 1 and control varicap VD2 make the secondary circuit. The working and second circuits are connected with coupling capacity C3 .The secondary circuit includes a control varicap and is asic in a double-circuit system. Resonant frequency of a reference circuit is selected less operational frequency of the self-oscillator, and the resonant frequency of the first working circuit on the contrary exceeds it.
|Fig 1:||Fig 2:|
Thus, the equivalent simplified scheme of the self-oscillator can be submitted as a capacity three-point system, in which one the reference circuit is represented as equivalent capacity, and primary working circuit as equivalent inductance. Changes of capacity of a control varicap VD2 is carried out at the expense of change of a control voltage UContr and reduces in change of a resonant frequency of a reference circuit and to change of a relation of frequencies of measuring and basic circuits. With the help of a varicap VD2 by change of a control voltage UContr attune the self-oscillator at installation of an EC probe on an inspected sample with initial electric conductivity. When EC probes with sensing coil L2 is installed on an inspected surface the added resistance is imported to primary circuit and requirements of oscillation are changed. The added resistance depends on an inspected material electrical conductivity. For signal processing the amplitude detection of high-frequency oscillations of the self-oscillator with the help of the diode VD1 and capacity C6 will be applied. Such as elevated operational frequencies are applied, all control and signal processing elements are mounted directly in hand-held EC probe very closely to sensing coil and active element. Such way eliminates problems connected with strong influence of distributed parameters (mainly a capacity) of connective conductors and circuits on elevated frequencies.
The condition of full lift-off effect suppression is realized by the choice of self-oscillator parameters, such as the mode of nonlinear element (transistor)VT1 , the difference of resonant frequencies of working and basic circuits and coupling capacity. It is possible when working point on a basic hodograph is selected higher the extremum (i.e. at high frequencies) where the increment of defect added resistance and lift-off added resistance have different sign. The optimization of the indicated parameters is carried out experimentally so that at a selected relation of parameters the presence alpha case results the failure of oscillation and probe lift-off on the contrary to magnification of amplitude.
In fig. 2 the schematic standard dependences of EC probe output voltage UOut on a control voltage UContr for EC probe arranging in air (1), installation EC probe on a surface of a sample without an alpha case (2) and on a sample with an alpha case (3) are shown. From dependences it is visible, that at set-up of an EC probe it is necessary to tune it in a condition relevant to a segment A- B of performance 2 for a sample without alpha case. The alpha case occurrence will reduce to the oscillation failure (transition from a point O to a point O1 ), and the magnification of a clearance (lift-off) will reduce to magnification of amplitude of oscillation (transition from a point O to a point O2 on a curve 1).
The mode selection of an auto generating EC probe is carried out automatically with the help of a digital control unit. Operation of an autotuning is realized at installation of an EC probe on a sample without an alpha case. The block diagram of a device is shown on fig. 3.
The block diagram contains the self-oscillator 1, regulated reference supply sources 2 and 4, amplitude detector 3, comparators 5 and 9, digital/analog transformers 6 and 8, control unit 7, direct-current amplifier, clock-pulse oscillator 12, units of pointer 13, light 14 and acoustical 15 indications.
The sensitivity threshold corresponds to an alpha case with 20 microns thickness. The power supply is carried out from a standard alternating current circuit 220 V or from an independent direct current source 12 V. Overall dimensions of the control unit is 170x90x45 mm. A weight - 0.5 kg. The devices are utilized on a series of industry firms for a detection of alpha cases on a surface of workpieces made from titanium alloys (in particular such as BT-6C,BT-14,BT-20) with the purpose to estimate the heat treatment quality. The application of an autotuning allow to increase productivity of monitoring and its reliability at the expense of exception of the subjective factors, bound with proficiency of an operator at device set-up.
Sometimes it is necessary not only to determine the presence of modified structures in thin surface layers, but also to have an opportunity for quantitative determination of their parameters, for example the case thickness or surface layer specific electrical conductivity. The application of the conventional approach, founded on measuring of signal amplitude or phase, on elevated frequencies is connected with considerable technical difficulties and complicating of instrumentation. More fruitful has appeared the approach, at which the device analogue part did not become complicated. Our researches have shown, that it is possible to utilize an introduced above self-generating EC probe working in a mode of failure of oscillation. Has appeared, that values of a control voltage UContr , at which one there is a failure of oscillation depends on tested objects parameters. For affirming it we shall reduce results of experiments conducted on samples made from titanium alloy B-14 with different alpha case thickness (15, 20, 40, 60, 70 and 100 microns). The thickness was determined on a standard procedure by a microhardness testing. In fig. 4 the dependences of voltage output UOut of the auto generating EC probe on a control voltage UContr on a varicap for an alpha case with different thickness are submitted (the initial segments of dependences only in the field of failure of oscillation are shown).
|Fig 4:||Fig 5:|
From shown in a fig. 4 dependences it is visible, that values of a critical control voltage UContr (at which one there is a failure of oscillation), well correlates with thickness of alpha case. The EC testing method was implemented, in which one the difference between critical values UContr -Uo_contr at installation on a sample without an alpha case and with an inspected alpha case will be utilized as information parameter. Special method and experimental device for quantitative eddy current testing on elevated work frequencies based on shown results was developed and investigated [9,10]. On fig.5 we can see related dependences obtained with developed device (control voltage presented in reference units on device monitor). The possibility to determine specific electric conductivity of thin surface layers was also investigated. This technique can be used for quantitative eddy current testing of dimensional parameters of thin surface layers and related mechanical properties.
The relation between the thickness of alpha (gas-filled) layer and hardness is determined for some titanium alloys. So, for this titanium alloys it is possible to test the hardness of the component material directly. The devices are also used to solve related problems - to detect micro cracks in titanium components, to estimate the wear of alitized layers on the surface of super steel components, to detect the intercrystalline corrosion in austenitic steel at the initiative stage, to estimate the stresses in surface layers in nonferrous components, etc. [5 -7].
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