ACTIVE QUALITY CONTROL OF WELDED JOINTS

V. A. VERDONOSOV, V. I. GORBUNOV, Yu. V. MATVEEV, A. P. SHPAGIN

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

Recent years have been characterized by intensive development of welding science and technology. New methods have been created for welding various materials, more advanced equipment has been developed, and the physicochemical phenomena occurring during welding have been studied in depth. At the same time, the intensified development of mechanical engineering and the creation of new structural materials impose ever more stringent requirements on welding production for increasing the reliability and durability of welded structures. Ensuring that these requirements are met sometimes exceeds the physical and psychological capabilities of a person. In addition, the comparatively long interoperation interval between welding and inspection does not allow the information obtained on the quality of the weld to be used promptly for active control of the welding process. Therefore the question arises of increasing the efficiency of mechanized welding. The solution of this question lies in expanding the areas of application of automatic devices that will be able successfully to solve the problem of optimizing not only welding itself, but also the entire complex of technological operations preceding and immediately following it. In other words, the urgent problem being placed on the agenda is the creation of active-control systems with feedback to the technological process.

Despite the known achievements in the creation of physical methods of nondestructive testing, all of them still remain passive.

The tasks of defectoscopy should include not only rejection, but also active influence on the production process by monitoring the physical dimensions of defects, analyzing fluctuations in their parameters within tolerance limits, detecting the first signs of quality deterioration, and issuing control commands for correcting the process. This will make it possible not only to record defects, but also to prevent them [1]. Thus, quality control of welding will change from a passive operation into a link of the welding process itself. Timely detection of deviations in the parameters of the welding regime and proper action on it on the basis of feedback should prevent the very possibility of defects arising [2].

The quality of a weld obtained with the aid of automatic machines can be characterized by the width of the weld, the degree of penetration, and the presence of pores or cavities. In the systems currently used (control of penetration, of the position of the electrode relative to the joint, etc.), it is not the output parameter—namely the quality of the welded seam—that is regulated, but only the parameters that influence it. In other words, in the process

welding there is no active control of weld quality, which is determined not only by the electrical parameters of the arc, but also by a number of technological factors (a change in the gap in the joint, a change in the shape of the edge preparation, variability in the thickness of the materials being welded, some heterogeneity in their chemical composition, etc.). Since the indicated causes do not directly affect the change in the regulated quantities (voltage, current), systems for regulating the regime with respect to the action of such disturbances prove to be open-loop, and compensation of the errors caused by the listed factors does not occur.

It follows from the foregoing that a further improvement in the quality of the weld can be ensured by introducing into welding production automatic systems that provide not only precise orientation of the welding electrode relative to the joint and the depth of penetration, but also self-adaptation of the welding regime to arbitrarily changing external and internal production conditions.

To create systems for active control of the welding process, reliable methods are needed for recording defects at the stage of crystallization of the weld metal. The rate of crystallization of the weld metal is comparable with the welding speed, which, depending on the method, may vary over a wide range. If in electroslag welding, depending on the thicknesses being welded, the speed is on the order of several meters per hour, then in arc welding it varies in the range from 10 to 160 m/h and more. Therefore, methods for recording defects in terms of speed must likewise be comparable with the welding speed; in addition, the method must have the capability of control at the crystallization temperature of the weld metal.

The search for inspection methods that, in order to reduce the time required to obtain information on weld quality, make it possible to combine welding and inspection operations has long attracted the attention of researchers. The use of cooled cassettes in radiography at a weld temperature below 900°C made it possible to reduce the interoperation time of welding and inspection by 50% [3], which, however, does not make it possible to obtain operational information on weld quality.

In order to shorten the manufacturing cycle of products [4], it is proposed to inspect the weld by ultrasound during welding. For this purpose the welding apparatus is equipped with the necessary equipment. Three probes are positioned at a certain distance behind the arc and at different angles with respect to the weld, which makes it possible to detect defects of various shapes. Defects are recorded on a chart tape of a self-recording instrument. The temperature at the point of contact is about 20°C. Inspection is carried out 60 s after welding. The slag is removed automatically. The low temperature resistance of the probes does not make it possible to reduce the time between welding and inspection, which, in turn, does not permit the information obtained to be used for regulating the process, especially at high welding speeds.

An analysis of existing methods for control directly during the welding process shows that such control methods as X-ray, magnetic, acoustic, and infrared-radiation methods can be applied to solve the problem posed [5].

From the standpoint of the prospects of the method for active quality control of welded joints, the X-ray television method is the most suitable in terms of its merits and the relative simplicity of its technical implementation.

An example of the successful application of the X-ray television method for monitoring the weld of products made of steel up to 10 mm thick during welding is the system [6], in which the main unit is

X-ray vidicon. The quality of the welded seam is judged from the image on the monitor, or a graph of the defect’s occurrence is traced both in the horizontal plane (the first monitor) and in the vertical plane (on the second monitor). However, the shortcomings inherent in X-ray vidicons (limited effective range of X-ray radiation energies, inertia) substantially restrict the range of application of the installation.

At NIIZI a layout has been developed for an installation for active quality control of welded seams during consumable-electrode welding (Fig. 1).

Fig. 1. Layout of a laboratory installation for active quality control of welded seams: 1 — radiation source; 2 — specimen being welded; 3 — single-crystal screen; 4 — objective; 5 — television transmitting camera; 6 — communication channel; 7 — video monitoring device; 8 — automatic control unit; 9 — information-processing unit; 10 — remote-control panel; 11 — mirror; 12 — welding-apparatus control cabinet; 13 — arc power supply; 14 — welding head.

The installation consists of a set of apparatus for consumable-electrode welding, an X-ray television introscope, and a remote-control panel for the welding process 10. The specimen being welded 2 is moved at the welding speed \((V_{\text{w}})\) from 15 to 100 m/h. Information on the quality of the welded seam is taken from a scintillation single-crystal screen 3, 130 mm in diameter and 3 mm thick. Turning the optical axis through \(90^\circ\) by means of a mirror 11 with an external coating reduces the effect of X-ray radiation on the photocathode of the supericonoscope of the transmitting television camera 5.

At the first stage the scheme provides for closing the links 7—10 of the feedback circuit for controlling the quality of the welded joint through the operator.

The experimenter judges the quality of the welded seam from the image on the screen of the video monitoring device 7 and, by means of remote control of the welding process 10, according to the readings of the instruments, records with a photographic camera the indications of the numerical values of the process parameters. Thus, this will make it possible to accumulate the necessary information on the technological process. Processing the information and optimizing the decisions issued, using in a number of cases electronic computing devices, will make it possible to create a variant of an active-control system [7] that permits objective control of the quality of welded seams in products (the links of the circuit 7—10 are closed through the links 8, 9).

On the basis of the studies in [8] assessing the influence of various factors characterizing the technology of the arc welding process (input) on the quality characteristics of the welded joint (output of the arc process), conclusions were drawn that, although the quality of the welded joint is influenced by the preparatory stage (development of the product design, selection of material and welding technology), its share in the final outcome is estimated at 80%. The quality of the joint during welding is associated with deviations of the parameters of the design, technology, and materials from the accepted mean values. At the same time, purposeful influence on the quality of the joint can be exerted only by regulating the welding mode, as the most mobile component of the arc welding process.

Conclusions

1. The control systems used for tracking the joint, penetration depth, and stabilization of welding modes predict the quality of the weld, but do not guarantee it.

2. The relatively long period of time between welding and inspection does not make it possible to use the obtained information on weld quality to control the welding process.

3. The proposed control scheme makes it possible to obtain information on weld quality during the welding process.

4. Work in the field of welding automation that provides for control of its quality by acting on the mode parameters with the aid of information on the quality of the welded joint obtained at the stage of the welding process should be regarded as a promising direction.

LITERATURE

1. E. N. Ermolov, V. T. Shcherbinskii. The present state of nondestructive methods for inspecting welded seams. Welding Production, 1966, No. 2.

2. B. E. Paton. New frontiers of welding technology. Welding Production, 1970, No. 4.

3. Schiebold E., Becker E. Prüfung von Schweißnähten bei hohen Temperaturen mit der Kühlekassette. Schweißtechnik (DDR), N 11, 1961.

4. Werner Günther. Qualitätssicherung und Qualitätsentwicklung im mechanisierten Schweißbetrieb. Schweißtechnik (DDR), N 3, 1966.

5. New controls and in-process detectors improve weld quality. Iron Age Metalworking International, N 6, 1967.

6. Marikava Yasuhiro. X-ray inspection of welded seams using television. Hihakai KENSA, Journal N.D.I., 1968, No. 11.

7. B. E. Paton. Some forecasts for the development of welding. Automatic Welding, 1971, No. 5.

8. V. F. Davidenko. Influence of initial factors of the arc process on the quality of a welded joint. Automatic Welding, 1972, No. 2.