What do ERW pipes mean
Inline production control of longitudinally welded pipes and experience in remote diagnosis of ultrasonic testing systemsDr.-Ing. Michael Schmeißer
Project manager for ultrasonic testing systems
NUKEM Nutronik GmbH, Alzenau
Contact: M. Schmeißer
In the production of longitudinally welded pipes, the ultrasonic testing device "scraping control" is used to monitor and visualize the weld profile. At the same time, the wall thickness in the weld seam and in the adjacent strip material is measured. Errors are marked in the correct location and the pipes are sorted after receiving the cutting signal.
The possibilities of remote diagnosis as well as the influence on ultrasonic testing systems in the commissioning phase or in production are discussed.
ERW pipes, scraping control, wall thickness, remote diagnosis US test systems
1 inline production control
The inline production control (Fig. 1) in the manufacture of resistance welded tubes includes, on the one hand, the monitoring of the strip material in the area of the forming section and the monitoring of the parameters on the welding machine.
The most important welding parameters include
- the compression pressure (semiconductor strain gauge bridge)
- the welding temperature (pyrometer)
- as well as the welding current and the welding voltage.
Standard test methods based on ultrasound and eddy currents are used for the non-destructive testing of the weld seam.
The weld seam profile, i.e. the seam contour after the external and internal scraping process, can be monitored using ultrasonic testing. At the same time, however, it is also of interest to obtain information about the wall thickness in the seam and in the adjacent strip material.
Furthermore, the weld seam is usually checked for crack defects oriented along and across the seam, as well as the detection of doublings in the heat-affected zone of the strip material.
The welding process and upsetting pressure create an outer and inner burr on the longitudinal seam, which must be removed by means of an outer and inner scraper. While the quality of the external scraping can easily be checked subjectively by the operating personnel, statements about the internal scraping are only available after the continuous tube has been cut.
In Fig. 2, the top view shows good external and internal scraping quality. In contrast, the internal scraping was not carried out sufficiently in the micrograph below. It is usually even more critical if the permissible minimum wall thickness has not been reached as a result of too deep internal scraping.
With the help of the ultrasonic testing device "scraping control", continuous monitoring of the scraping process is possible.
2 Scraping control
2.1 Measuring principle
The measuring principle of the scraping control is illustrated in Figure 3 one The signal propagation times of the surface and back wall echoes are measured by the wall thickness probe swinging along the weld seam. The pendulum frequency is 1 Hz and a range of ± 30 ° is covered. The test electronics are a digital ultrasonic test channel UTxx, which is loaded with the special software for scraping control.
In addition to the visualization of the weld seam profile and the measured wall thickness, this test device provides alarm signals if the permissible tolerance specifications for the external and internal scraping and the wall thickness are exceeded.
The Test mechanics (Fig. 4) consists essentially of a pendulum device that is vertically and transversely adjustable to the pipe, the mounting frame and the supply of coupling water. A stable installation of the test mechanics is of crucial importance for the stability and reproducibility of the measurement results.
With the contact method, the test head is located in a rotatable holder which mechanically scans the pipe contour using a sliding shoe.
The small design of the test mechanics can be used for pipe diameters from 15 to 250 mm and the larger in the range from 60 ... 600 mm. The diameter range is covered by 4 different sliding shoes with different angles on the tube.
For applications in which the pipe surface must not be scratched, the measurement of the transit times can also be carried out without contact by locking the probe holder. This method only produces reproducible results if the center point of the tube is always congruent with that of the 90 ° guide segment of the pendulum mechanism.
Regardless of the selected method, the measuring accuracy depends on the roundness of the pipe, which is characterized as a vertical oval immediately behind the welding machine. The endeavor to obtain exact information about its quality as soon as possible after the scraping process is opposed by the out-of-roundness of the pipe at this position in the welding line.
To protect the pendulum mechanism in the case of larger holes in the endless pipe, it is lifted off the pipe by an automatic lifting device with a sensor for hole detection.
The Test electronics consists of a universal ultrasonic test channel UTxx (Fig. 5), the digital signal processor of which is loaded for this application with the software for scraping control. The test channel is located in a VXI frame with additional slots. This means that the ultrasonic testing system can be upgraded for e.g. crack defect testing by adding additional UTxx channels.
2.2 visualizationof the weld seam profile
The visualization of the weld seam profile consists of the continuous display of the current measured values and a trend display (Fig. 6).
In the right half of the picture, the current outer profile is shown at the top and the inner profile below. The four thick lines represent the given ones
± Tolerances, the values of which can be seen from the numerical color scale.
The diagram on the left shows the history of the external and internal scraping.
Exceeding the permissible specifications is highlighted in red, undershooting is highlighted in blue. In the example shown, the internal scraping is not deep enough, so the excess material overhang is shown as a red line. If the weld seam profile is within the tolerances, it is displayed in gray levels according to the scaling.
An incorrect setting of the scraping tool, its wear or even breakage is thus recognized immediately, so that the production of reject pipes due to poor scraping is avoided.
In the event of tolerance violations, hardware alarm signals are also available for the operating personnel at the welding machine and for the accurate marking of the pipes concerned.
The nominal wall thickness, the ± tolerances and the specifications for statistical interference suppression are entered in the "Scraping control" setup menu (Fig. 7).
To facilitate or check the settings, the current signal transit times of the surface echo (upper signal diagram), converted into millimeters, and below that of the back wall echo, are shown here.
To check the pendulum mechanism, its two reversal points are also displayed in the upper signal diagram in the form of blue pulses.
To calculate the outer contour, it is necessary to know the distance from the probe to the pipe surface. This will be determined automatically with a click of the mouse.
2.3 wall thickness
The wall thickness in the weld seam as well as in the adjacent strip material is recorded by means of an additional evaluation without any additional hardware expenditure. The probe track is divided into three parameterizable wall thickness measurement windows, left and right of the weld seam and the seam itself (Fig. 8).
In addition to the profile display, three wall thickness plots are available. In Figure 9, the wall thickness in the left strip material was recorded above, in the middle that in the weld seam and below that in the right strip material.
The wall thickness plot stored here under production conditions at a welding speed of 1 m / s has a size of approx. 16 MB with a running time of the coil of approx. 30 minutes.
The wall thickness can be used to identify details to of the data recording as desired (Fig. 10). On its right-hand side, the maximum, average and minimum values of the respective wall thickness range are displayed for the current section. The permissible tolerances and the distribution of the pendulum stroke over the seam are entered separately for the three wall thickness measurement windows in the "Wall thickness" setting menu (Fig. 11) for the weld seam and together for the strip material on the left and right.
If necessary, the speed of sound for the coupling medium can be automatically corrected for temperature compensation.
3. Master Control Unit
The Master Control Unit (MCU) is configurable software with a process interface for coordinating the test sequence. It runs with the ultrasonic software UTxx on the same test PC.
Figure 12 shows the main mask of the MCU, which was used for the first time at Opel in the welding line for the production of tubes for front wheel carriers. The MCU is superordinate to the eddy current and scraping control test systems as well as the measuring devices for upsetting pressure and welding temperature.
During the fully automatic test, the most important information is displayed on the main screen:
- in the upper area information on the production order and the parameter sets of the test and measurement systems involved are selected
- in the middle area the test results and the batch statistics are displayed, such as the number of pipes tested, the distribution of the types of defects among the rejected pipes and their respective total lengths
- in the lower range The main mask shows the test section and the detected defects as well as the current cutting position in a graphically accurate manner. Furthermore, the current operating states and the error signals of the test and measuring devices are displayed there in the form of alarm flags.
The MCU generates individual pipe graphics with localized error display for each test lot,
a test report of the entire lot as well as a result report with an error graphic (Fig. 13) are made available. Faulty pipes receive an entry in the alphanumeric pipe section graphic according to the legend given below.
In the production control of resistance-welded endless tubes, the MCU is characterized by the following essential functional features:
- Acquisition and visualization of the error signals from the test systems
- Locally accurate marking of the fault positions on the pipe
- Make the decision about sorting the pipe too good, bad or reworking only after receiving the saw cut signal
- Determination of the total pipe lengths related to the types of defects
- Output of test reports including statistical evaluation.
4 Experiences with remote diagnosis of ultrasonic testing systems
By means of remote data transmission and a PC duo, it is possible to provide the personnel on site with the necessary support both during commissioning and in production use of the test system. In addition to avoiding delays during the commissioning phase, it is primarily the later reduction of production downtimes due to malfunctions and incorrect settings in the setup menus.
Figure 14 shows an overview of the options for remote diagnosis and influencing of such test systems.
The variety of these diagnostic and influencing possibilities can be divided into the main groups test process and test system.
To the Inspection process include setting the test parameters and analyzing the test results. The test system comprises the areas of test mechanics, test electronics and test software.
If problems occur on site, they can be caused, for example, by the incorrect setting of the ultrasonic test parameters. While static settings are easy to control, the evaluation of dynamic processes is only possible to a limited extent due to the limited data transfer rate. With regard to dynamic settings, there are currently still limits to remote diagnosis. Due to the availability of the envelope function available in the ultrasonic oscilloscope from UTxx, the envelope curve of the ultrasonic signal can be frozen so that even fast events can be analyzed via the modem connection.
Static specifications for the test sequence, on the other hand, can be checked and adjusted without any problems. Remote diagnosis of the test results, which are available as recorder writing, error graphics or lot protocol, is also easily possible.
Regarding the Testing system Modifications of the control software, the monitoring of drives and temperature curves in areas of test mechanics or test electronics can be easily evaluated.
With regard to the test PC, general settings can be made and event logs can be evaluated. An analysis of the logged mains voltage is often an important point of reference when troubleshooting or troubleshooting. The correct functioning of the test software can be easily checked using individual control and log files. This may result in an update that can be quickly transferred to the test system on site by means of remote data transmission.
With the help of the aforementioned remote diagnosis and influencing options for the ultrasonic testing systems in use in production, the commissioning and operating personnel on site have been supported in a very targeted manner and without any significant delay, which in turn has led to a reduction in commissioning times, production downtimes and travel expenses.
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