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Ultrasonic Testing of the Quality of Joints Made By Flash-Butt Welding Pad'ko V.P., Troitsky V.A. E.O.Paton Electric Welding Institute of the NAS of Ukraine Contact

The technology of automatic flash-butt welding of pipelines developed in the E.O.Paton Electric Welding Institute of the NAS of Ukraine, was widely introduced in the CIS countries. Over the recent years this technology was used to weld tens of thousand kilometers of various pipelines of 2 to 56 inch diameter, including approximately 4,000 km of 56 inch pipelines in the Extreme North.
The Institute has performed a large scope of theoretical and experimental studies aimed at improvement of the operational reliability of the welded joints. Considerable statistical material has been accumulated on the joint properties, nature of the possible defects and influence of these defects on the mechanical properties of the joints.
The defects which can develop in flash-butt welding, can be divided into two categories.
The first category covers defects of the joint shape. These are misalignment of the edges welded, flash left after its cutting-off, irregular shape of the welded joint. Such defects the dimensions of which exceed the admissible limits, can lower the strength properties of the joint.
The second category includes defects located in the joint zone (in the weld), these are lacks-of-penetration (lacks-of-fusion) and non-metallic inclusions.
The lacks-of-penetration (lacks-of-fusion) are sections of the joint zone which completely or partially lack the metallic bond between the parts being welded. Such sections feature the absence of crystalline fracture on the surface of samples which failed in the welded butt.
The lacks-of-penetration (lacks-of-fusion) are unfilled craters which form in flashing on the end faces of the pipes being welded, and can be light or dark, depending on their location in the butt. If lacks-of-penetration are located at a certain distance from the surface of the items being welded, they have a light-coloured surface in the fractures in the butt. If the lacks-of-penetration come to the surface of the items being welded, their surface is covered with oxides and for this reason, they are dark.
Non-metallic inclusions of a relatively small area can be found in flash-butt welding of large-diameter pipes of gas pipeline range, in the modes close to the optimal ones.
A feature of non-metallic inclusions on the fracture surface, is their fine-crystalline structure.
For this reason they are classified as inadmissible defects in a number of cases. Such defects, however, have a well-developed profile close to a plastic fracture in its structure. The dimensions of such defects usually do not exceed 30 mm² and only in individual cases they can be larger. Appearance of non-metallic inclusions in the butt, is of a random nature. Proceeding from experimental data it was established that, for instance, a weld section about 300 mm long can have up to 1 - 2 non-metallic inclusions, the majority of them being not more than 10 to 15 mm² in area.
Appearance of non-metallic inclusions in the welded butt is due to very thin films of non-metallic inclusions, destroyed during plastic deformation of the metal of the surfaces being welded in upsetting. A large number of non-metallic inclusions whose dimensions are smaller than those of the crystallites, are found on the fracture surfaces in the vicinity of such defects. Nonetheless, neither the dimensions of these crystallites, nor their number, affect the nature of fracture.
Non-metallic inclusions can contain oxides of iron and other metals present in the composition of the steel being welded. In the majority of cases, however, these are complex inclusions, incorporating iron, manganese and silicon oxides. The appearance of such inclusions located in the butt zone, is associated with the inclusions present in the steel along the rolling bands.
Ultrasonic testing of welded joints made by FBW, with the wall thickness of 15 to 26 mm was performed with the developed and made at Paton Institute factory reference blocks (FRB) with disc-shaped reflectors of 2 mm diameter, and FRB with vertical and horizontal cylindrical holes.
Experimental investigations were conducted in specially developed and made steel test samples of pipes of the main pipelines of 1420 mm diameter with the wall thickness d = 19 mm, welded joint length l = 2000 mm and different geometry, namely
• with reinforcement beads (2 to 4 mm height);
• with misalignment of the welded samples cut out of pipes;
• with reinforcement beads of 2 to 3 mm height and misalignment of 1 to 4 mm;
• without the reinforcement bead or misalignment
• with flash and misalignment.

Three batches of welded samples were prepared to study the influence of the geometrical shape of the welded butt on detectability of defects in the joint zone, namely I - with the reinforcement removed to the base metal level; II - with the reinforcement not exceeding 3 mm from the base metal level; III - with 2 to 2.5 mm misalignment of the edges samples and reinforcement similar to batch II.
Samples for investigations were welded of sectors 200 mm wide, cut out of gas pipeline pipes of 1420 mm diameter with the wall thickness of 18.7 mm, of X70 class of strength in keeping with API 1104. Samples welding was performed in a laboratory machine MSTU-500 in a mode with a low intensity of the flashing process prior to upsetting, high welding voltage and short welding time. Such a welding mode guaranteed the formation in the joint zone of such defects as lacks-of-penetration, which were pores and thick oxide films. After welding the flash was removed and the reinforcement was machined to the required dimensions.
In the welds horizontal cylindrical holes were made at the sample edges and vertical (perpendicular) cylindrical holes were made inside the samples on the fusion line. These reflectors were used as reference defects and as reference reflectors during performance of ultrasonic testing of the test samples, in addition to the natural defects incorporated into them.
It should be noted that misalignment of the edges of the parts being welded leads to deviation of the weld fusion line by angle g from to the vertical. This circumstance was taken into account in selection of the ultrasonic testing parameters.

Determination of UT parameter

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.

Conclusion.

The high operational reliability of the pipelines made with FBW, is due to the high quality of welded joints. The welding defects can appear only in the case of violation of the technology of preparatory work or welding. With this welding process dangerous defects are revealed in welding or after it by means of operational control of the welding mode parameters. In order to improve the validity of quality assessment, the welded joints should be subjected to non-destructive testing. Comparative estimation of industrial methods of non-destructive testing showed ultrasonic testing to be the most effective for flash-butt welding of pipes. This paper describes further investigation of ultrasonic testing of FBW pipes for the case of thick-walled pipes (15 to 26 mm thickness).

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.

References

1. Mazur I.I., Serafin O.M., Karpenko M.P. Electric resistance welding of pipelines: ways of its improvement. - «Stroitel'stvo truboprovodov» j., #4, 1988.
2. Wagner M.J., Patcheff. Girth weld detection in mechanized GMA field-welded pipelines. - Welding Journal, #6, 1991.
3. Trufiakov V.I., Mazur V.G., Zhemchuzhnikov V.G. et al. Influence of some defects on the strength of welded joints made by flash-butt welding. - Avtomaticheskaya Svarka j., #2, 1987.
4. Kuchuk-Yatsenko S.I., Kazymov B.I., Rad'ko V.P. Integrated control of the quality of joints made by automatic flash-butt welding. - Tekhnicheskaya Diagnostika i Nerzrushajushchii Kontrol, #4, 1996.

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