DEVELOPMENT AND IMPLEMENTATION OF METHODS AND EQUIPMENT FOR AUTOMATED RADIATION INSPECTION OF WELDED JOINTS

A. V. POKROVSKII, A. K. TEMNIK, V. F. VOLKOV

(Presented by the scientific seminar of the NII EI)

The rapid development of industry, in particular power-machine construction, is characterized by a transition to machines and units with very high operating parameters.

It is sufficient to say that over the past decade boiler construction has moved from steam parameters of 40 atm at 450°C to parameters of 140–250 atm at 570–600°C. In this connection, the problem has arisen of sharply increasing the reliability of drums of boiler units, cylinders, and high-pressure vessels, which have numerous welded joints.

In the welding process, various kinds of defects may arise that are inadmissible in products of this class. For many years, quality control of welded joints has been carried out by means of ultrasound and selective (25%) gamma radiography, owing to the long exposure time.

The thickness of the weld metal in the units described reaches 200 mm and more, which is the limiting value for the radioactive isotopes used in industry.

During 1965–1972, at the NII EI, with the direct participation of the authors, a method and a set of equipment were developed that make it possible to carry out automated radiation inspection of welded joints of cylinders and drums.

For the inspection of thick-walled welded joints having a steel thickness of up to 250 mm, the BD-2 betatron flaw detector was developed.

At the same time, the problem of automated inspection of tanks and large-diameter vessels with wall thicknesses up to 80–100 mm was solved. Products having surface irregularities and reinforcement of the weld seam were also subjected to inspection.

The recording equipment of the flaw detectors contains a radiometric measuring channel, a mosaic system of scintillation sensors, and an indicator—a self-recording instrument.

To eliminate the influence of irregularities of the weld surface on inspection sensitivity, studies were carried out to determine the dimensions and orientation of the collimating apertures of the radiation receiver; a method was developed for reducing the influence of weld waviness by means of averaging the level over the surface being inspected, as well as by connecting radiation detectors in a differential circuit.

In the recording channel of the flaw detector, in order to increase the rela-

—to improve the signal-to-noise ratio, selective filters with subsequent accumulation of signals from the detector were used.

The functional diagram of the BD-2 flaw detector, shown in Fig. 1, includes the following units:

Fig. 1. Functional block diagram of a betatron flaw detector for inspecting thick-walled welded vessels

1 — indicating device,
2 — rod-displacement sensor,
3 — rod drive,
4 — drive control panel,
5 — manual-control console,
6 — radiation-source displacement sensor,
7 — inspected article,
8 — radiation source,
9 — industrial television installation,
10 — marking device,
11 — radiation-source control console,
12 — radiation receiver,
13 — cassette-feed mechanism,
14 — platform drive,
15 — tilter drive,
16 — instrument rack,
17 — central control console,
18 — indicating device.

The inspection process is carried out as follows. The article to be inspected is placed on the platform with tilters (14–15). Then, after the inspection “geometry” corresponding to the given class of articles has been established, the article is smoothly fed into the inspection zone. The mosaic system of collimation openings of the radiation receiver is arranged in such a way as to make it possible to inspect the entire weld-seam zone simultaneously.

The inspection results, synchronously with the displacement of the article, are recorded on the chart tape of a self-recording millivoltmeter. The type of flaw—

Fig. 2. Diagram with an image of a pore-type defect (A) and of a defectometer (B)

A diagram with an image of a pore-type defect (A) and of a defectometer (B) is shown in Fig. 2.

When a defect or a questionable excursion is detected on the chart tape of the recording instrument, additional gamma radiography of the defective area is carried out in order to obtain additional information on the dimensions and configuration of the defect.

The process of feeding the cassette with X-ray film into the inspection zone, exposure, and return of the cassette to storage is performed semi-automatically by means of cassette-feed mechanisms mounted on a rod with a radiation detector.

Analyzing the results obtained, it may be concluded that defects of the type of pores, slag inclusions, and fistulas are reliably detected. The sensitivity of the flaw detector to defects of this kind is 3–15 mm³ at a thickness of 10–80 mm and 30–60 mm³ at thicknesses of 150–200 mm, at an inspection speed of 1 m/min.

At present, flaw detectors having the parameters indicated above are not manufactured by industry. The BD-2 and SID-1 flaw detectors have undergone production tests and have been introduced at the Barnaul Boiler Plant.

As a result of pilot industrial operation of the flaw detectors at the plant, statistical material was accumulated that made it possible to evaluate the reliability of the results obtained by the scintillation inspection method. The identity achieved between the results obtained by radiographic and radiometric inspection methods allowed the plant to propose to the USSR Boiler Inspection Authority that inspection by the radiometric method be permitted.