ECONOMIC EFFICIENCY OF QUALITY CONTROL OF WELDED JOINTS

V. A. BERDANOSOV, L. M. DOBNER, Yu. S. PROKOFIEV

(Presented by the scientific seminar of the Department of Economics)

For quality control of welded joints in thick-walled products, various methods of radiation flaw detection are now widely used: inspection by X-rays and gamma rays, and betatron flaw detection. The resolving power of these methods varies. Thus, with the aid of radioscopy it is possible in practice to inspect steel products up to 110 mm thick. The penetrating capacity of gamma rays extends to 200–250 mm. Betatron flaw detection makes it possible to extend this limit to 500 mm [2].

Thus, within a certain range of inspected thicknesses, the methods under consideration are interchangeable. Each of the inspection methods has its own advantages and disadvantages. For example, the use of betatron flaw detection makes it possible to save time in carrying out inspection operations, but requires large capital investments in the main equipment and protective means. In order to reduce the particular advantages and disadvantages of different methods to a common denominator, it is necessary to carry out an economic analysis of the alternatives being compared.

The areas of effective application of each of the above-mentioned radiation inspection methods are determined by a number of factors, which may be divided into 3 groups:

1) technical,
2) organizational,
3) economic.

Technical factors include the list of technical parameters of the products: the thickness of the product being inspected, the material, the ratio between the length of circumferential and longitudinal welds, etc. Organizational factors: the production program, the type of inspection—continuous or selective, the nature of the inspection (under laboratory conditions or at the workplace). Economic factors comprise the list of cost indicators determined by the use of one or another type of inspection equipment, for example, the cost of the inspection equipment, the cost of tooling, etc.

The influence of the factors listed above on the magnitude of the overall economic indicators (capital investments and current costs) when implementing one or another inspection method is different. As studies have shown—

research carried out at NII EI showed that the determining factors are the thickness of the products, the cost of the equipment and protective means, equipment utilization, and the ratio between the length of circumferential and longitudinal welds.

Studies of the comparative economic efficiency of the variants were based on a methodology developed at NII EI. According to this methodology, the expenditure of piece-calculation time for inspecting 1 linear m of welded joints consists of the sum of the basic time for radiographing the products and the auxiliary time. Naturally, in quality control of welds in thick-walled products, the greatest specific weight in the total expenditure of control time is taken on by the basic time for radiographing, which depends on a whole series of technical characteristics of the radiation source, intensifying screens, focal distance, etc. The influence of the focal distance is especially significant on the duration of exposure and, consequently, on the norm of piece-calculation time. A series of studies carried out at NII EI made it possible to determine focal distances, optimal from the standpoint of maximum productivity, for a number of radioactive-radiation sources; these were used in determining the basic time for radiographing.

Fig. 1. Dependence of the standard time for quality control of 1 linear m of welded joints on product thickness. 1. Radiographing with a PMB-6 betatron (lead screens). 2. Radiographing with a PMB-6 betatron (fluorescent screens). 3. Radiographing with the isotope Co-60. 4. Radiographing with the RUP-400-5-1 unit.

The total uninterrupted auxiliary time spent on installing and removing cassettes, moving products, setting up the radiation source, etc., also varies over wide limits and depends on a whole series of factors: on the type of radiation source and the type of products inspected, on the degree of mechanization and automation of the control process, and on the level of organization of work in the laboratory.

In determining the auxiliary time per 1 linear m of welded joints, the data of [1], calculated for the average level of mechanization of work in a defectoscopic laboratory, were used.

In determining the amount of capital investment for the control variants, only the differing cost elements were taken into account—capital investments in inspection equipment—since all the control methods considered are used under identical conditions.

In determining the cost of inspection operations, only those expenses directly connected with radiographing the products were taken into account, since the costs of purchasing films and chemicals, as well as of decoding the radiographs, are analogous for the variants.

In calculating the economic indicators according to the methodology described above,

It is assumed that the compared inspection methods are also identical in terms of the degree of detectability of defects in welded seams. In accordance with the methodology, a study was carried out of the dependence of certain economic indicators on a number of factors for three variants of inspection: betatron flaw detection using a portable small-sized PMB-6 betatron as the radiation source; isotope radiography with a Co-60 source of activity 50 g-eq Ra; and X-ray inspection using an RUP-400-5-1 unit as the radiation source.

Thus, the graph in Fig. 1 presents the dependences of the standard time for inspection of welded seams, for different radiographic methods, on the thickness of the products being inspected.

As can be seen from the graph, the use of betatron flaw detection makes it possible to reduce the inspection time by a factor of 2–4 in the 110–200 mm range in comparison with isotope inspection. When radiography is used, already at a thickness of 100 mm the time expenditures for inspection are 4–5 times higher than with radiographic examination by betatron or isotope.

Fig. 2. Dependence of the reduced costs for quality inspection of 1 running m of welded seams on the thickness of the products. 1. Radiographic examination by PMB-6 betatron (lead screens). 2. Radiographic examination by PMB-6 betatron (fluorescent screens). 3. Radiographic examination by Co-60 isotope. 4. Radiographic examination by RUP-400-5-1 unit.

The graph in Fig. 2 presents the dependence of the reduced costs of inspection on the thickness of products for different radiographic methods. As can be seen from the graph, the reduced costs for inspection by the PMB-6 betatron with the use of lead intensifying screens are higher than with isotope flaw detection over the entire thickness interval of 50–200 mm.

When fluorescent screens, which reduce the exposure time, are used, the use of the betatron is economically more advisable at thicknesses exceeding 160–170 mm. The use of an X-ray unit is justified at thicknesses not exceeding 80–100 mm.

Thus, by analyzing the graphs shown in Figs. 1 and 2, it is possible to establish the areas of economically effective application of the compared methods of quality inspection of products.

REFERENCES

1. V. I. Postnikov. Methodology for Determining the Effectiveness of the Use of Radioactive Isotopes. Atomizdat, 1964.

2. A. A. Vorob’ev, V. A. Vorob’ev, V. I. Gorbunov. Use of betatrons in flaw detection. In: Nondestructive Methods of Inspection of Materials and Products. ONTI pribor, 1964.