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Ultrasonic Testing of the Quality of Joints Made By Flash-Butt WeldingPad'ko V.P., Troitsky V.A.
E.O.Paton Electric Welding Institute of the NAS of Ukraine
Test samples were used to conduct investigations on determination of the parameters of UT of welds to detect and determine the coordinates, dimensions and kinds of defects (lack-of-penetration of discontinuity type; lack-of-penetration of thick oxide film type; non-metallic inclusion), allowing for the influence of the geometrical parameters of the weld.
Ultrasonic testing of the above samples was conducted in two directions, in keeping with the norms and parameters of UT of the joints, made by the arc welding processes (VSN 012-88) and UT parameters developed at the E.O.Paton Electric Welding Institute for control of the joints made by FBW.
Control sensitivity setting is determined by an area (diameter) d = 2 mm of blind holes (disc reflectors) located in the upper, central and lower parts of FRB.
For adjustment of the flaw detector in control of FBW joints with combined transducers and «tandem» transducers, the reference reflector is a normal through-thickness hole 2 of 2 mm diameter.
Cylindrical horizontal holes of 2 mm diameter were also used as reference defects (reference reflectors) in the test samples. Control was performed with an angle probe in the frequency ranges of 2 to 5 MHz and incidence angles of 50 to 700. Transducers connected in a «tandem» and R/T angle probes were used.
In order to determine the kind of defect (lack-of-penetration, non-metallic inclusion) UT was conducted at different sensitivity levels (threshold levels of echo-signal detection).
The maximal sensitivity (minimal detection threshold) was determined by the average level of echo-signals from the welding zone structure of a sound joint. The ultimate (control) sensitivity was set by disc control reflectors (bottom of a horizontal drilled hole) and side surface of a vertical cylindrical drilled hole. During control with R/T UT probes the control sensitivity was determined in keeping with VSN 012-88 by angular reflectors (bench marks).
UT of the above samples was performed in keeping with the requirements of GOST 14782-86. Flaw detectors UST-50 (52) and Zipscan-3 (Great Britain) were used, which implement the method of defect sizing by the time interval between the acoustic waves diffracted at its edges, when sounding with a wide beam (TOFD method). In this case, the defect orientation, damping in the material, quality of acoustic contact make a much smaller impact on the control results, than in the pulse-echo method.
When USN-50 (52) flaw detectors were applied to detect the signals from defects against the background of signals from the noise, the following was determined: optimal angles of incidence are 65 to 70 0 in the combined mode and 50 to 650 with «tandem» connection of the transducers. Control was performed with transverse waves in the range of 2 to 5 MHz, depending on the pipe wall thickness. When Zinscan-3 flaw detector was operated in the TOFD mode, longitudinal waves in the frequency range of 1 - 10 MHz were used wit the angles of incidence of 45, 60, 700. It is shown that simultaneous application of the diffracted wave TOFD and the «tandem» procedures allows the validity of weld control to be improved, as this reduces the number of false signals from the weld reinforcement which remained after flash removal. The threshold level of echo-signals was established by the amplitude of echo-signal from a flat-bottomed hole of 2 mm diameter.
The edges misalignment leads to the increase of the dead band. In order to reduce the influence of the noise due to echo-signals from edges misalignment, it is necessary to use multi-element probes and have the equipment which enables setting the appropriate gates and control them. It has been proved that in order to improve the control validity it is necessary for the edges misalignment to be not more than 15 to 20% of the weld half thickness. In order to develop an improved technology of such joints, it is necessary to have a sufficient number of them.
When oxide films were detected, also the noise from the weld structure was taken into account. It is found that when ultrasonic waves are applied by transducers connected in «tandem» at an angle of 50 to 55 0 to the joint zone plane and frequency of ultrasonic oscillations of 2.5 MHz, the level of echo-signal amplitude from such defects is by 4 to 6 dB above the average level of echo-signals from the structure of a sound joint welding zone.
Proceeding from statistical processing of the signals from defects and structural noise, an UT procedure was developed which allows detection of thin non-metallic inclusions. This procedure was tried out both in manual and automatic control.
The main method of control of the joints made by flash-butt welding, currently is the operational control of the welding mode parameters. The essence of this control consists in comparison of effective values of the main parameters of the mode with their assigned values which are entered into the programmer. The conclusion on the correspondence or discrepancy between the effective and the assigned parameter values can be made by the welding machine operator, or the inspection unit representative who have been trained to perform analysis of the diagram with the records of the main parameters. This work can be now performed by a computerised system. In this case, the control validity becomes higher, due to elimination of the influence on the result of such subjective factors as the physical condition of the inspectors, their qualifications and experience. A computerised system has been used with success in FBW of rails.
Despite the high validity of assessment of the welding quality, operational control is a concurrent, process control. Therefore, there is a practical need for development of an independent inspection.
For joints made by electric arc processes, ultrasonic testing (UT) and X-ray inspection (RI) are usually used for independent inspection. These control methods at present are the main methods for arc welds. Therefore, it is rational to transfer the extensive gained experience to assessment of the quality of the joints made by FBW.
After performance of UT and X-ray inspection, the welded samples were cut into templates and broken up by the method of static bending in the joint zone. Examination of the welded sample fractures for the presence, nature of distribution, type and sizes of the defects was performed visually.
The Table gives comparative analysis of the results of ultrasonic and radiation control with the data of destructive testing of the joints made in different modes.
|Table : Results of control of the quality of joints made by flash-butt welding with different weld geometries|
Symbols: HH - lack-of-penetration of discontinuity type; HO - lack-of-penetration of oxide film type; BK - non-metallic inclusions.
It is shown that the use of the «tandem» method and the diffracted waves method is the most effective, as these methods of ultrasonic testing are especially sensitive to defects oriented normal to the scanning surface of ultrasonic transducers, which is characteristic of welds made by FBW.
Simultaneous use of the «tandem»-method and diffracted waves method (TOFD) allows increase of the control validity, as in this case the number of false signals due to the weld geometry (misalignment of the edges of the butts being welded, irregular shape, weld reinforcement left after flash removal) is reduced.
Comparative analysis of ultrasonic testing and X-ray inspection with the data of destructive tests showed the high reliability of detection of lacks-of-penetration of the type of discontinuities and oxide films.
The ability to detect non-metallic inclusions was demonstrated, in principle, with ultrasonic testing. The detectability of discontinuities of oxide film and non-metallic inclusion type by X-ray inspection is low.
The high validity of ultrasonic and X-ray methods in detection of lacks-of-penetration of the type of discontinuities should be noted.
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