RU2109276C1 - Process of nondestructive test of surface layer of metal - Google Patents

Abstract

FIELD: nondestructive testing. SUBSTANCE: specified current is passed through tested object and standard and voltage emerging on section of surface is measured. Then second current is additionally passed in opposite direction and voltage is measured. Value of second current is selected so that zero resulting voltage is obtained across standard. Thickness and electric conductivity of surface layer are determined by obtained values of voltage and diagram plotted with allowance for location of points of current input and voltage pick-up on surface of tested object and standard. EFFECT: enhanced authenticity of process. 2 dwg

Description

 The invention relates to non-destructive testing of electrically conductive products and can be used in mechanical engineering to control the thickness and quality of hardened layers of structural steels obtained by thermal and chemical-thermal treatment, as well as to control metallization and galvanic coatings.

 A known method of non-destructive testing of electrically conductive objects with excitation of oppositely directed currents in them and inductive measurement of magnetic field perturbation over a controlled surface [1]. The method is intended to detect defects representing a violation of the integrity of the object.

 This method is also applicable to control the thickness of the surface layer of the metal, but only in the case when the electromagnetic properties of the surface layers and the core of the controlled products are constant.

The closest in technical essence is a way to control the quality of adhesion of electrically conductive coatings to metal [2]. In this method, a direct current is passed through a controlled product and a reference using a pair of electrodes, using the second pair of electrodes, the voltage U is measured on the surface. Next, the resistivities of the coating ρ ₁ and substrate ρ _{2 are} determined, and the coating thickness h. The quality of the coating is estimated using calculated graphs of the dependence of U on the degree of adhesion of the coating to the substrate for various ρ ₁ , ρ ₂ , h.

 The disadvantage of this method is the need to first determine the specific resistance of the coating, which makes this method unsuitable for controlling the layers of chemical-thermal treatment, since their electrical conductivity is very sensitive to deviations from the technology for producing these layers, and their peeling is difficult.

 To expand the functionality, to increase the information content and reliability of the control, in the proposed method, the thickness and electrical conductivity of the surface layer are simultaneously determined.

 To do this, through a test object and a standard identical in properties to the core of the object, a current of a given value is passed through one pair of electrodes and the voltages arising on the surface area of the test object and the reference are measured, respectively. Then, a second current of the opposite direction and a value that gives zero voltage on the reference is passed through a reference and a test object simultaneously with the first current using another pair of electrodes, and the resulting voltage is measured on the same surface area of the test object. The obtained voltage values are used to find the thickness and electrical conductivity of the surface layer according to a diagram constructed taking into account the location of the points of current input and stress relief on the surface of the samples.

 The possibility of two-parameter control is connected with the fact that the voltage measured on a surface area is determined mainly by the properties of that part of the object where the probing current lines mainly lie. Taking the interelectronic distance for one current pair as much larger than the layer thickness, and for another as comparable to it, we will introduce the first current mainly into the core of the object, and the second into the layer. With the oncoming and simultaneous switching on of the currents, the difference between the generated voltages will depend on the properties, layer and core. Aligning the magnitude of these voltages on the standard plays the role of calibration, after which the voltage difference at the controlled object will be directly related to the difference in the conductivities of the layer and core. In this case, the dependence of the indicated voltage difference on the thickness d and electrical conductivity σ of the layer is qualitatively different from the same dependence of the voltage generated only by the first current, which allows us to determine the parameters d and σ.

 In FIG. 1 shows a device that implements the proposed method of two-parameter control. In FIG. 2 is a diagram of the converter described below, used to determine the controlled parameters d and σ.

The device contains a six-electrode transducer 1 (indicated by a dotted line), consisting of potential and two current pairs lying in the same plane, movable, independently spring loaded, needle electrodes, the mutual arrangement of which corresponds to the thickness range of the layers being monitored. The electrodes 1 ⁺ , 1 ^{- of the} first current pair are spaced wider than the electrodes 2 ⁺ , 2 ^{- of the} second current pair; electrodes a, b of the potential pair are located between the electrodes of the second current pair. The device also includes a generator 2 (shown by a dotted line) with two independent output stages operating in the floating load mode (in this case, an output transformer with two independent secondary windings is used). In addition, the device contains rheostats 3, 4, current meters 5. 6, a disconnecting switch 7, a differential amplifier 8, a selective voltmeter 9 and an oscilloscope 10.

 The method is as follows.

The converter is brought into contact with the surface of the standard — the sample, whose properties are identical to those of the core of the processed products. When the switch 7 is open, some current I _{1 is} passed through the first current pair and the corresponding voltage U ₀ on the potential pair is measured. The key 7 is closed and, using rheostats 34, 4, a current I _{2 is} established in the circuit of the second current pair, giving at the same current value I ₁ zero voltage on the potential pair. The transducer is transferred to the surface of the controlled product and, with the switch 7 open and the same current value I _1, the voltage U _{1 is} measured on a potential pair. Close the key 7 and when restored by rheostats 3, 4 current values I ₁ , I ₂ measure the voltage U ₂ on a potential pair.

.Based on the measurement results, the relative stresses E ₁ = U ₁ / U ₀ , E ₂ = U ₂ / U ₀ and the values of the leveling functions are calculated
F ₁ = 1 / E ₁ ,

.

Using a special diagram (see Fig. 2) corresponding to the geometry of the given converter, the relative thickness d 'and the electrical conductivity σ ′ of the layer are determined. Using the known half-distance l ₁ between the electrodes of the first current pair and the electrical conductivity σ _{o of the} core of the product, the absolute values of the thickness d and electrical conductivity σ of the surface layer are found. The electrical conductivity of the core σ _o is determined by the voltage U ₀ obtained on the surface of the standard.

.To build a converter diagram, the above voltage U ₁ , U _{2 were} calculated by the formulas

.

,
представляющее потенциала поля, порождаемого постоянным током 1 на поверхности двухслойного проводящего полупространства, на расстоянии r от точки ввода тока. Здесь k = (σ-σ_o)/(σ+σ_o) , а величина I имеет знак (+) или (-) соответственно тому, как направлен зондирующий электрический ток.The potentials of the fields standing on the right side of the formulas of the magnitude V are excited at the points a, b of the sample surface by the incoming (+) and outgoing (-) currents of the first and second pairs of current electrodes, respectively, to the indicated indices. To calculate these quantities, the expression was used (see, for example, [3])

,
representing the potential of the field generated by direct current 1 on the surface of a two-layer conducting half-space, at a distance r from the current input point. Here k = (σ-σ _o ) / (σ + σ _o ), and the quantity I has the sign (+) or (-), respectively, as the probe electric current is directed.

,
где
h, l₁, l₂ представляют собой полурасстояния между электродами соответственно потенциальной, первой и второй токовой пар. В частности, нормирующее напряжение U₀ для такого преобразователя выражается в виде

.For the symmetric transducer shown in FIG. 1, the distances between the contact points of the respective electrodes are determined by the formulas

,
Where
h, l ₁ , l ₂ are half-distances between the electrodes of the potential, first and second current pairs, respectively. In particular, the normalizing voltage U ₀ for such a converter is expressed as

.

Depicted in FIG. Figure 2 is a map of the isolines of the leveling functions, F ₁ , F _{2 of the} relative voltages E ₁ , E _{2 of a} symmetrical transducer with inter-electrode distance ratios h / l ₂ constructed on the plane of variables σ ′ = σ / σ _o , d '= d / l ₁ = 1/2, l ₂ / l ₁ = 1/3. In the region where the contours of the functions F ₁ (σ ′, d ′), F ₂ (σ ′, d ′) intersect at angles close to 90 degrees, the method provides good resolution of the controlled parameters σ; d.

Analysis of the diagrams constructed for transducers with different ratios h / l ₂ and l ₂ / l ₁ gives, in particular, the following ranges of good resolution of the thicknesses of the controlled layers:
0.6 l ₂ <d <0.6 l ₁ for h <l ₂ ,
0,45 l ₂ <d <0,6 l _1, with h <l _2/2.

A comparison of the boundaries of these ranges with the boundaries of the range of thicknesses of the layers that are subject to real control determines the interelectrode distances of the transducers that have the highest resolution. For example, when choosing the ratio h / l ₂ = 1/2, a transducer with dimensions l ₁ = 3 mm, l ₂ = 1 mm, h will have the highest resolution when controlling layers with a thickness of 0.45 mm <d <1.8 mm = 0.5 mm.

 The above calculation formulas (1), (2) apply to the case of direct current. When probing products with alternating current, their use is permissible at sufficiently low frequencies, when current scanning can still be neglected. When working at higher frequencies in the indicated sense, for constructing the transducer diagram, instead of (1), (2), formulas taking into account the skin effect should be used.

The magnitudes of the currents I ₁ , I _{2 are} limited by the range in which the heating of the sample in the vicinity of the contact points can still be neglected. The upper limit of this range is determined by the shape of the electrodes, their hardness and pressure force. The belonging of the currents to the range is verified experimentally by the linearity of the current dependence of the measured voltages.

 Sources of information.

 1. Rogachev V.I. Trachtenbeng L.I., Shkatov P.N. The method of non-destructive testing of electrically conductive objects and a device for its implementation - and.with. N 746278, Bl N 25, 1980.

 2. Brainin E.I. Control of elements of electric machines and apparatuses by the electropotential method. M .: Energy, 1980, p. 4-9, 20-22.

 3. Tikhonov A. N., Samarsky A. A. Equations of mathematical physics. M .: Nauka, 1972, p. 383.

Claims (1)

 The method of non-destructive testing of the surface layer of the metal, which consists in passing through a test object and a standard identical in properties to the core of the object a current of a given magnitude and measuring the stresses arising on the surface area of the test object and reference, respectively, characterized in that then through the standard and the control object passes simultaneously with the first current a second current of the opposite direction and such a value that provides zero voltage value on the standard, while the voltage is measured and arising on the same site of the surface of the test object, and then use the obtained voltage values and a diagram constructed taking into account the location of the points of current input and stress relief on the surface of the test object and the reference, to simultaneously determine the thickness and electrical conductivity of the surface layer.

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