RU2547153C1 - Method to improve accuracy of eddy current non-destructive testing - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: method to improve accuracy of eddy current non-destructive testing consists in the following: prior to eddy current testing an item is affected by the load being sufficient to detect a hypothetical fault like a crack in the control point up to the value which would provide for increased detectability of the fault and make it detectable.

EFFECT: improved operation characteristics of items due to the improved accuracy of detecting cracks of operation origin or process faults with low opening like hardening cracks.

5 dwg

Description

FIELD OF THE INVENTION

The invention relates to methods for testing and operational eddy current testing (VTK) of products within the framework of scheduled preventive repairs, in particular for evaluating product quality indicators based on the results of non-destructive testing. The need for eddy current non-destructive testing of parts is especially evident in many industries, for example, in aircraft and shipbuilding, during the installation and operation of nuclear and thermal power plants, and the laying of various pipelines. Particularly high demands are placed on parts in these industries for the strength, reliability and durability of their joints due to harsh operating conditions.

The invention can be applied in transport, nuclear and traditional energy, aviation, shipbuilding, petrochemicals, oil, gas and product pipelines, agricultural machinery and other fields of technology and engineering.

State of the art

The reliability of the VTK depends on the ability to identify a defect, which in turn depends on the resolution of the method.

The detection of defects in metal parts by the eddy current method is based on the law of electromagnetic induction, according to which an alternating magnetic field excites eddy currents in them. As you know, eddy currents are closed in the thickness of the metal and therefore cannot be directly used to detect defects. Therefore, the current-eddy method is based on the observation of such processes that always accompany eddy currents and which can be observed outside the controlled part.

According to the law of electromagnetic induction, eddy currents will appear in the surface layer of the metal, the closed contours of which cover the lines of an alternating magnetic field. Eddy currents, like any electric currents, create their own magnetic field, which, in contrast to the field of the coil, is secondary. According to the Lenz rule, the secondary alternating field at each moment of time is opposite to the primary, i.e. counteracts him. The interaction of the eddy current field (secondary field) with the coil field (primary field) causes a change in its electrical parameters. Thus, the presence of eddy currents in the controlled part can be indirectly established by changing the electrical parameters of the magnetizing coil. The change in the inductance of the magnetizing coil during the control of a non-magnetic material (non-ferrous metal) occurs differently than when controlling a magnetic material. In a non-magnetic metal, the opposing eddy current field reduces the primary field.

Eddy current control is most effective when monitoring non-magnetic materials (non-ferrous metals, austenitic steel, etc.). The ability to control ferromagnetic materials and their parts is determined by the uniformity of their magnetic properties.

The most dangerous defects that occur during the operation of parts include fatigue cracks that extend to the surface of the part (through) and occur near it. When sorting products, it is necessary to take into account that the actual length of the defective region consists of the actual crack length plus the length of the plastic deformation zones at its ends, so even a short crack is unacceptable. Such cracks are the most difficult to detect, they cannot be prevented, they significantly reduce the structural strength.

As shown by studies conducted by employees of the All-Russian Research Institute of Aviation Materials (Moscow) under the leadership of E.A. Kosarina, the sensitivity of the eddy current method of control largely depends on the cleanliness of the surface treatment of the part in the manufacturing process and the degree of its preliminary preparation before control. For example, detection of a crack with an opening (width) in the range of 1-10 μm is possible only when machining a part no worse than in the fifth grade, and when the part is cleaned in grade 3-4, crack opening is comparable with traces of marks (traces from the processing tool) , which makes the crack almost undetectable. Thus, one of the important factors influencing the detection of VTK is the opening of a crack-like defect.

In real products, the probability of detecting unacceptable defects by the VTK method is in the range from 0 to, at best, 80-90%.

Thus, studies using the “blind” method of detecting defects such as cracks in the heat transfer tubes of steam generators of nuclear power plants under the PISC-III program (Program Inspection of Steel Components), carried out with the participation of 25 groups of inspectors from 16 European countries, the USA and Japan, showed that the probability of detecting defects is in the range of 20-80%.

Thus, increasing the detection of metal defects in parts and products of modern technical objects is an urgent task.

The sensitivity setting of eddy current flaw detectors is carried out on control samples, an example of which is shown in FIG. 1 and FIG. 2 in accordance with RD-13-03-2006. For heat transfer pipes of steam generators of VVER-type reactor plants, a life-size pipe of the same material as the pipes at the nuclear power plant (FIG. 3) is used, with a wall drilling of 2 mm in diameter (MVTK-EK-2000-01 method).

Control samples are designed to determine the performance and threshold sensitivity of eddy current flaw detectors. Figure 1 and Figure 2 shows that the adjustment is carried out on defects with an opening equal to or more than 50 μm.

During the control, there are no measures aimed at increasing the detection of crack-like defects by increasing their disclosure (for example, guidance document RD-13-03-2006 “Methodological recommendations on the procedure for eddy current monitoring of technical devices and structures used and operated at hazardous production facilities”. - Moscow, NTC Industrial Safety OJSC, 2007; “Methods of eddy current control of heat exchange tubes of steam generators of nuclear power plants with VVER reactors MVTK-EK-2000-01).

As a prototype can be used GOST 24289-80 "Non-destructive eddy current control. Terms and Definitions"; RD RD-13-03-2006 "Methodological recommendations on the procedure for eddy current monitoring of technical devices and structures used and operated at hazardous production facilities." - Moscow, NTC Industrial Safety OJSC, 2007; “Methods of eddy current control of heat transfer tubes of steam generators of nuclear power plants with VVER reactors” MVTK-EK-2000-01.

Disclosure of invention

The problem that this invention solves is to improve the operational properties of products based on increasing the reliability of identifying cracks of an operational nature such as fatigue cracks or stress corrosion cracking or technological defects with a small opening of quenching, welding or other types of cracks.

The technical result to which this invention is directed is that it allows the quality control of the product to be carried out at the stage of factory outlet control or during operation of the product with higher reliability, which will allow for the timely detection of dangerous defects and repair of the product based on the results of this control

A way to increase the reliability of eddy current non-destructive flaw detection testing is to load the product with a load sufficient to open a hypothetical crack type defect at the test site to a value that would provide the best effect of the defect on secondary eddy currents (currents in the product) and their effect to the primary currents of the flaw detector and would make the defect detectable.

Thus, it is known from fracture mechanics that when a load is applied to an elastoplastic body with a crack, the magnitude of which is such that stresses arise in the body below the yield strength, however, plastic (i.e., irreversible) deformations arise at the crack tip. After removing the load, the body recovers its shape, with the exception of the crack tip zones, which ensures residual crack opening after the load is removed from the body.

The residual opening of the crack is the higher, the higher the stress created in the body with the crack, since the value of the opening δ at the mouth of the crack (FIG. 4) is equal to (for example, Panasyuk VV Limit equilibrium of brittle bodies with cracks. - Kiev, Naukova Dumka , 1968):

, $$\delta =\frac{8R_{P0.2}C}{\pi E}\ln \sec \frac{\pi \sigma }{2R_{P0.2}}$$

,

where R _p0,2 is the yield strength, E is the elastic modulus, C is the half-length of the crack, σ is the effective stress.

Wherein:

- determine the voltage in the controlled product during operation - voltage σ _e ;

- determine the maximum allowable voltage in the controlled part - the maximum allowable voltage in the product σ _add ;

- before the VTK load the product with a voltage that exceeds the operational, but not higher than the maximum permissible voltage, for example, from 1.25σ _e to 0.97σ _additional ;

- conduct VTK products;

- according to the results of the inspection, repair the identified defects.

Brief Description of the Drawings

FIG. 1. Control samples KO, designed to determine the performance and threshold sensitivity of eddy current flaw detectors:

sample KO 1 for determining the sensitivity when monitoring a flat surface.

FIG. 2. Sample KO 2 for determining the sensitivity when controlling the ribs.

FIG. 3. A control sample for a tube for determining the threshold sensitivity of an eddy current flaw detector.

FIG. 4. The stress diagram at the ends of the cracks during tension in a plate of elastoplastic material: in unloaded (a) and loaded state (b).

FIG. 5. Opening at the apex δ and in the middle v of a crack 2c long under the action of stresses σ and pressure p.

The implementation of the invention

Prior to and during operation of critical products, for example, in the field of nuclear energy, in accordance with regulatory documents, for example, “Rules for the Construction and Safe Operation of Equipment and Pipelines of Nuclear Power Plants” PNAEG-7-008-89; “Equipment and pipelines of nuclear power plants. Welded joints and surfacing. Control Rules ”PNAEG-7-010-89, Gosatomnadzor of Russia, Energoatomizdat, 1991 and others, conduct non-destructive testing of the condition of products. Moreover, in operation, as a rule, during each inspection, defects of either technological nature or operational are detected. This is mainly due to the lack of reliability of non-destructive testing, including eddy current (VTK).

A way to increase the reliability of ultrasonic non-destructive flaw detection inspection is to load the product with a load sufficient to expose a hypothetical crack type defect at the test site to a value that would ensure a higher detection of the crack type defect.

Wherein:

1) Determine the voltage in the controlled product during operation - voltage σ _e ; the current voltage can be determined from the verification calculation of strength, which is mandatory for critical products.

2) Determine the maximum allowable voltage in the controlled part - the maximum allowable voltage in the product σ _add ; these stresses, as a rule, can also be taken from the verification calculation of the strength of the product. In the absence of a verification calculation, the maximum allowable voltage can be determined from the normative document for strength calculations, where safety factors are normalized. For example, in nuclear energy there are the Norms for calculating the strength of equipment and pipelines of PNAEG-7-002-86 nuclear plants, in which the permissible voltage [σ] is set for membrane stresses as the smaller of the two values:

R _p0,2 / n _t or R _m / n _m , where n _t - margin of yield strength, n _m - margin of ultimate strength R _m .

3) Before the VTK load the product with a voltage that exceeds the operational, but not higher than the maximum allowable voltage [σ] = σ _add , for example, from 1.25σ _e to 0.97σ _add .

4) Relieve the load to zero and conduct VTK products; the second option, if conditions allow, the load is dropped by 10% and conduct VTK.

5) Based on the results of the inspection, repair the identified defects.

The invention is illustrated by the following example.

There are tubes made of austenitic steel, the geometric dimensions of which are shown in FIG. 3. During operation, cracks appeared in the tubes, including through ones (FIG. 5). The working voltage in the tubes is created by an internal pressure of 8 MPa and, in accordance with the Laplace formula, is 40 MPa × 7.5 mm / 1.5 mm = 40 MPA, the maximum allowable voltage is 120 MPA (determined from the ratio: R _p0,2 / n _t = 180 MPa / 1.5).

Cuttings of pipes with through cracks with subsequent investigation of the probability of identifying such defects of P _water by the VTK method showed that P _water = 38%.

In this case, the disclosure of a through defect 10 mm long after loading is relieved is approximately:

. $$\begin{array}{l}\delta =\frac{8R_{P0.2}C}{\pi E}\ln \sec \frac{\pi \sigma }{2R_{P0.2}}=8*180*\mathrm{one\; hundred}/2*3.14*2*10\mathrm{<figure-callout\; id="5"\; label="E"\; filenames="00000005.png"\; state="\{\{state\}\}">E</figure-callout>}5*\ln [\mathrm{one}|\cos (3.14*40/2*180)]\\ =0.006mm\end{array}$$

.

After the tubes were loaded with an internal pressure of 24 MPa (the stress in the tubes was 24 MPa × 7.5 mm / 1.5 mm = 120 MPa), the residual crack opening increased and amounted to approximately:

δ = 8 * 180 * 100/2 * 3.14 * 2 * 10E5 * ln [1 | cos (3.14 * 120/2 * 180)] = 0.07 mm,

that is, increased by about 10 times.

Moreover, the probability of detecting end-to-end defects doubled and amounted to P _water = 79%.

After completion of the inspection, the defective tubes were repaired according to the inspection results.

Claims (1)

A way to increase the reliability of eddy current non-destructive flaw detection testing of products, consisting in the fact that the criteria for defect rejection are determined for the product, characterized in that they determine the voltage σ _e acting during operation of the product, determine the maximum allowable voltage in the product, σ _add , load the product and create voltage from 1.25σ _e to 0.97σ _dop , discharge the load by at least 10% and conduct eddy current control of the product.

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