BETATRON FLAW DETECTOR FOR INSPECTION OF WELDED JOINTS

V. I. GORBUNOV, A. V. POKROVSKII, A. K. TEMNIK

(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)

Nondestructive testing of welded joints using sources of penetrating radiation occupies one of the leading places among other methods of inspection.

In radiation flaw detection, radioactive isotopes, X-ray units, betatrons, and some other accelerators are now widely used as radiation sources. In recent years, the construction of large-size units and installations for various purposes has been acquiring ever greater importance. The parts used in these units as their constituent elements have great thicknesses. The requirements imposed on the quality of the welded joint are in many cases very stringent. Thus, in welded joints of boiler-unit drums and welded vessels with metal thicknesses up to 400 mm (steel), local inclusions only a few millimeters in diameter are already impermissible.

The Co-60 gamma-radiation source used for radiographing large thicknesses produces radiation with an average energy of 1.25 MeV.

With good sensitivity and exposure time per exposure, this isotope can be used to radiograph welded seams up to 150 mm thick. Reducing the exposure time is associated with increasing the activity of the source, and this leads to an increase in the emitting body of the isotope source to 4 mm or more in diameter. As a result, geometric unsharpness increases, and, correspondingly, the sensitivity of radiographic inspection decreases. To this it must be added that the problem of protection from radiation when working with isotopes of high activity under production conditions proves to be very difficult.

Substantial progress in this field has been brought about by the use of electron accelerators, and in particular betatrons.

The betatron is most widely used for flaw detection of materials and products. The source of high-energy electromagnetic radiation is an induction electron accelerator. The maximum energy of the bremsstrahlung radiation and the intensity are the principal parameters characterizing a particular type of betatron.

In betatrons manufactured in our country and abroad, the possibility is provided of adjustable variation of the maximum energy of the bremsstrahlung radiation within certain limits. Betatrons with bremsstrahlung-radiation energy in the range of 25–35 MeV have been developed for industrial radiography. Practically accessible for

inspection with the aid of betatrons, the steel thickness has increased to 400–500 mm. At the same time, the sensitivity of radiography is no worse than 1–2% of the thickness being penetrated. Although it is rather difficult to assess the economic effect of introducing betatrons into the practice of radiation flaw detection, experience in inspecting thick-walled critical products shows that even a brief shutdown of such installations and units, incorporated into a power or chemical complex, costs many times more than the funds spent on their inspection. In many cases, exposure by betatron radiation is at present the only possible method of nondestructive testing of products of great thickness. Examples include inspection of the seams of welded roll-bonded vessels, as well as inspection of welded joints made of austenitic steels, where ultrasonic methods for monitoring weld quality cannot be used.

The variety of tasks solved in radiation flaw detection has led to the use of a large number of detectors for recording information about defects. Thus, along with X-ray film, detectors of other types are now widely used: scintillation gas-discharge counters and ionization chambers. These detectors, which convert information about defects into electrical signals, have made it possible to move to automatic 100-percent inspection. Moreover, by using scintillation radiation detectors, one obtains a sensitivity commensurate with the sensitivity of the radiographic method of recording. At the same time, in a number of cases the productivity of inspection can be increased substantially, by more than an order of magnitude.

Automation of radiation flaw-detection methods through the use of scintillation detectors also has the advantage that data on inspection results arrive continuously and can be presented in the form of a record on the tape of an indicator instrument.

In radiometric systems of radiation flaw detection there is no visual representation of the shape of a defect; therefore, as a rule, duplication is carried out for the examined defective areas. Although such a combination of two recording methods leads to a certain reduction in productivity, it nevertheless makes it possible to obtain exhaustive information about the quality of the product.

The nature and size of a defect can be obtained most visually by using the radiographic method of radiation recording. In this case, the sensitivity of the radiographic method of betatron flaw detection in the range of steel thicknesses 100–400 mm is within 1–1.5%.

Figure 1 presents the dependence of the sensitivity of the radiographic method on product thickness for betatrons with energies of 25 and 35 MeV, as well as for the isotope Co-60.

Fig. 1. Dependence of the sensitivity of radiographic inspection on product thickness (steel).

As can be seen from Fig. 1, beginning at 150 mm, when isotope sources are used, owing to the insufficient radiation energy, the inspection sensitivity becomes 1.5–2 times worse than when bremsstrahlung with a maximum energy of 25–35 MeV is used. The irradiation time for radiography is determined, for a given level of mechanization and automation of inspection, by the exposure time. If one compares the exposure time for Co-60 with an activity of 50 g-eq radium and for the above-mentioned betatrons, which provide dose rates of, respectively, 40 and 250 roentgens per minute at one meter from the target, then even at a steel-layer thickness of 100 mm the exposure for the Co-60 isotope is 25 min. For betatrons this time is several times shorter. It is advisable to reduce the exposure time to the limit at which the exposure time is approximately equal to the auxiliary time [5]. The auxiliary time for one exposure ranges from 2 to 6 min. Then, from the standpoint of productivity, the effectiveness of using betatrons in radiation flaw detection begins to predominate when irradiating products more than 120–150 mm thick.

The works of the authors [3, 4, 5] have shown that, in this case, the costs of inspection, despite the high cost of betatrons in comparison with gamma-radiation sources, will be several times lower.

The choice of the type and parameters of a betatron for the purposes of radiation flaw detection is determined primarily by the requirements for inspection sensitivity, the specifics of production (mass or individual radiographs), and by the dimensions, shapes, and material of the products to be inspected.

The effectiveness of using one source or another is determined primarily by the radiation intensity and its energy composition, since from the standpoint of sensitivity, inspection costs, and productivity, for each thickness range there exists an optimum energy and intensity. The choice of the radiation source and its parameters must be made with allowance for the method of radiation recording.

For the radiometric method of recording in betatron flaw detection, the radiation energy is not a determining parameter, since in practice the work is carried out in the direct beam, and the effect of the scattered component on the inspection sensitivity is eliminated by collimating the detectors. With a maximum bremsstrahlung energy of 25–35 MeV and a dose rate of not less than 150–250 R/min, welded joints up to 400 mm thick and more can be successfully inspected at speeds of up to 3 m per minute. The principal requirement imposed on the source in the radiometric recording method is high stability of the radiation intensity, not worse than 1–2%. In this case the inspection sensitivity reaches one percent and better. The required stability of the radiation intensity can be ensured by autonomous powering of the betatron from a motor-generator system and by employing a special stabilization circuit.

For the radiographic recording method, the sensitivity, especially in the region of large thicknesses, depends on the maximum energy, since in radiography scattered radiation makes a significant contribution.

Thus, for a weld thickness of 300 mm, the sensitivity for a bremsstrahlung energy of 30 MeV is higher than for an energy of 15 MeV by approximately a factor of one and a half.

Increasing the betatron energy to 30–35 MeV leads to an increase in dose rate to 250–400 R/min. At such radiation energy, the sensitivity of radiography is no worse than one percent for steel thicknesses up to 350 mm, with exposure times within several minutes. More-

In addition, betatron radiation with a maximum energy of 25–35 MeV has the greatest penetrating power when radiographing steel. The half-value layer thickness for attenuation of the intensity of bremsstrahlung from a betatron with an energy of 30 MeV differs only slightly from the analogous thickness for a radiation energy of 25 MeV. For an energy of 30 MeV, the value of the buildup factor is smaller than for 20 MeV and, consequently, the contribution of scattered radiation is smaller. Final recommendations on the choice of the type and parameters of the betatron are made on the basis of an analysis of the specific production conditions; nevertheless, they must be based on the considerations set out above.

As already noted, modern power engineering uses welded vessels (drums) operating under pressures of several hundred atmospheres at superheated-steam temperatures above 600°C. The thickness of the welds in such products reaches 200 mm and more. Quality inspection of welded seams under production conditions until recently was carried out by selective radiography with the Co-60 isotope, with recording on film, and also by the ultrasonic flaw-detection method. However, the sensitivity and productivity of the gamma-radiographic method when radiographing steel more than 150 mm thick with radioactive isotopes do not meet the requirements of modern production. The exposure time when radiographing a 155-mm-thick seam with a GUP-Co-60 apparatus is 25–30 minutes; the total inspection time for a drum (cylinder) with welds 35–40 m long is about one hundred hours.

At the Research Institute of Electronic Introscopy at Tomsk Polytechnic Institute, during 1962–1972, a betatron flaw detector for automatic inspection of welded joints with a seam thickness of up to 350 mm was developed and introduced into production.

Welded vessels up to 15 m long and up to 2 m in diameter are subject to inspection. The radiation receiver is a block of scintillation counters, the number of which has been selected so that, with an inspection-zone width of 100 mm, both the longitudinal and the circumferential seam are covered. The detectors are connected in a differential circuit, which made it possible, with a three- to five-percent instability in the radiation intensity of the betatron, to measure a one-percent change in intensity caused by the detector. The indicator is a five-channel self-recording milliammeter N-320, on whose chart tape the inspection results are continuously recorded. A betatron with a maximum energy of 25 MeV and an exposure dose rate of 40 R/min was chosen as the radiation source.

With this flaw detector, one-hundred-percent automatic inspection of welded seams is carried out; in this case, the results of radiometric inspection at the location where a defect is detected are duplicated by radiography. X-ray film is fed into the inspection zone automatically within 30 seconds. The flaw detector is installed in a specially equipped laboratory, which combines a bay, a betatron hangar with a capacitor battery, and a control room. The bay is a concreted trench 5.5 m deep and about 150 m² in area. In the bay, the product, mounted on a platform with tilters, moves along rails. At the opposite end of the bay, along the direction of platform travel, a rod 320 mm in diameter and 12 m long is fastened to a vertical-travel mechanism. When longitudinal seams are inspected, the vessel moves onto the rod, at the end of which the radiation receiver is installed. The rod moves in a vertical plane and is positioned along the axis of the manhole opening. The diameter of the vessels varies from 200 to 1000 mm. Above the bay, at the location of the radiation receiver, the betatron is situated. A special mechanism makes it possible to move it up and down and to rotate it about its axis.

Radiographic inspection of vessels is carried out with bremsstrahlung directed vertically downward.

The recording equipment of the radiometric channel of the flaw detector, in addition to a multichannel system of scintillation detectors and a recording instrument, contains a unit for processing the signal from the detector, a commutator, and a power-supply unit. All measuring instruments and control equipment are brought out to a common console.

When a defect is detected, this location is marked, the product is stopped, and radiography is performed. The size of the defect recorded by the radiometric method is determined by comparing the peak amplitude on the chart tape from the defect and from the standard; the standard also serves for periodic checking of the sensitivity of the flaw detector.

The sensitivity of the flaw detector at a speed of 1 m per minute is better than one percent for practically all inspected thicknesses. The sensitivity for channels remote from the beam axis is worse by 10–15% owing to the higher noise level.

In multichannel systems using the radiometric recording method, one of the principal questions is the choice of the number of channels. The choice of the number of channels is inseparably connected with the choice of the focal distance. When working with an equalized beam or a beam limited to small angles of radiation, where the sensitivity of detectors installed at the center of the field and at the edge differs only slightly, it is expedient to choose inspection productivity as the optimizing parameter, since the sensitivity achieved is better than that required by the inspection standards. In this case, productivity can be increased in two ways.

The first way is to increase productivity by increasing the inspection speed. As the inspection speed is increased, the sensitivity decreases in inverse proportion to this parameter. At a certain speed, the actual and required sensitivities coincide. In the present case, this will be the maximum inspection speed. The second way is to carry out line-by-line inspection, reducing the number of channels; in this case the focal distance can be reduced, and the inspection speed increased.

Studies of the radiometric method of betatron flaw detection, carried out in the inspection of welded joints, made it possible to find optimal combinations among the main parameters of the flaw detector.

Production tests confirmed the sensitivity data in detecting real defects. Thus, the sensitivity of the radiometric method of betatron flaw detection when radiographing vessels of 92–155 mm is within 1–1.5% according to GOST 7512-69 standards. By this method, individual pores 3–3.5 mm in diameter, blowholes about 1.5 mm in diameter, and lack of fusion 2 mm deep were detected. The results of radiometric inspection were verified by radiography.

Figure 2 shows a defectogram of a welded seam in which a defect in the form of a pore and a slag inclusion of the indicated dimensions was recorded. Interpretation of the inspection results—distinguishing a defect at the noise level even close to the sensitivity threshold, as can be seen in Fig. 2—does not present great difficulties, since the nature of the defect representation (twofold recording) differs from the nature of the “white noise.” The reliability of recording—the reproducibility of results for a threshold defect—is practically close to 100%. Its calculated value is 0.995. The nature of the recording of defects on the tape of the electronic potentiometer makes it possible to determine the length of the defect. The length of the defect in the direction of motion of the product is determined by the product of the ratio of the speeds of the product and the chart tape by the length

Fig. 2. Defectogram of a weld seam: a — defect; b — wire flaw detector.

throw from the defect on the chart tape and by the sensitivity of inspection.

On the basis of the practical material obtained, it is possible to give certain recommendations for the development and operation of betatron flaw detectors in radiometry and radiography. As production operation of the flaw detector has shown for inspection of the overwhelming range of products (excluding fast-running technological processes), the inspection productivity should be selected so that the exposure time is at least 50% in relation to the auxiliary time. From this it is not difficult to proceed to the limiting speed and number of channels. Taking into account that, at a sensitivity within 1% for \(E_{\max}=25\) MeV and a steel thickness of up to 250 mm, an increase in productivity by going from 2–3 channels to 5–10 does not exceed 20–30%, it is obvious that in these cases one should not follow the path of increasing the number of channels and complicating the installation, and it is advisable to limit oneself to 3–5 channels with a collimator area of 1.5 cm\(^2\). In this case the productivity lies within 6 m\(^2\)/h. Such productivity is on average 3–4 times higher than in radiography at approximately the same sensitivity. The design parameters of the flaw detector, such as the collimator dimensions and the diameter and height of the scintillator, can be calculated with the accuracy required in practice (20–30)% for a specified volume of the defect to be detected and the probability of its detection. In view of the significant spatio-temporal fluctuations of the betatron radiation intensity, achieving high sensitivity and reliability of defect detection in radiometry is possible only with autonomous power supply of the entire installation from a stabilized voltage source, or by using in the installation’s power-supply system a motor-generator that maintains the voltage with an accuracy of 0.5%. The reliability with which the flaw detector performs the entire task can be brought to 0.995 and higher; however, for this it is necessary, first, to select as identical a number of the main elements of the channel recording circuit as possible and, second, to increase the reliability of the betatron and of the measurement circuit by introducing a hot standby reserve of the radiometric channel.

In radiography, maximum detectability of defects has been achieved with RT-5 film and ZPZ,5 screens. With a vessel thickness of 155 mm, pores 1.5 mm in diameter and lack of fusion with an opening of 1 mm and more are reliably recorded. Optimization of the radiographic exposure mode for the chosen film and screens must be carried out with respect to productivity, since productivity as a function of focal distance has a maximum. A comparison of the betatron flaw detector for the conditions considered with the isotope unit GUP-Co-50 shows that the latter is inferior on average by a factor of 3–4 in productivity and costs, and by a factor of 2–2.5 in sensitivity.

It should be noted that the introduction of such installations in machine building, shipbuilding, and other industries is also economically expedient.

REFERENCES

1. S. V. Rumyantsev. Radiation Flaw Detection. Moscow, Atomizdat, 1968.

2. A. A. Vorob’ev, V. I. Gorbunov, A. V. Pokrovskii. Inspection of welded joints of large thicknesses using a betatron flaw detector, Welding Production, 1968, No. 1.

3. V. A. Berdonosov, V. A. Vorob’ev, A. V. Pokrovskii. Evaluation of the effectiveness of using a betatron in the radiometric method of flaw detection of welded joints, Welding Production, 1968, No. 2.

4. Betatron flaw detector BD-2 for inspection of thick-walled welded joints. Information Bulletin No. 9 (39), 1970.

5. A. K. Temnik. Candidate dissertation. Tomsk, 1968.

6. L. M. Anan’ev, A. A. Vorob’ev, V. I. Gorbunov. Induction Electron Accelerator—Betatron. Moscow, Atomizdat, 1961.