|NDT.net - March 2002, Vol. 7 No. 03|
In a so-called Multi Test Block, steel tubes up to a diameter of 710 mm and a wall thickness of 100 mm are tested simultaneously by ultrasonic wall thickness measurement and flux leakage for surface defects.
Ultrasonic testing is done by means of a newly designed electro-dynamic wall thickness ultrasonic transducer which, in connection with the MESUS ultrasonic system by Mannesmann Forschungsinstitut, provides a colour-coded online representation of the course of wall thickness of a complete tube. The results obtained are used for data analysis and rapid optimisation of the tube rolling process.
For flux leakage testing, a high-energy AC flux leakage testing system STATOFLUX by FOERSTER is used; in particular, it detects longitudinal defects or hole-type defects and replaces hydro-testing in accordance with EN 10246-1.
In the middle of 1999, a so-called Multi Test Block was installed in the pilger mill of Vallourec & Mannesmann Deutschland GmbH in Düsseldorf Rath. The pilger mill produces seamless tubes with an outside diameter ranging from 219.1 mm to 710 mm. Their wall thicknesses range from 8 mm to 140 mm. The basis for manufacturing these seamless tubes are conventionally cast ingots made of essentially ferromagnetic steels.
|Diameter range:||219.1 mm - 710 mm|
|Wall thickness range:||8 mm - 140 mm|
|Tube length range:||5 m - 16 m|
|Surface:||Rough as rolled, scaled|
|Products:||Oil field pipes, line pipes, structural and boiler tubes|
|Testing speed:||1.5 m/s (on circumference)|
|Table 1: Tubes and Testing Characteristics|
An important aim of that investment into the realisation of the Multi Test Block was to improve production reliability as far as surface finish and tube geometry were concerned. The equipment was directly integrated into the production flow of the pilger mill in order to achieve a fast feedback to the rolling mill.
|Ingot storage||Rotary-hearth furnace||Centre-punching press|
|Pierce rolling mill||Pilger rolling mill||Hot saw|
|Reheating furnace||Sizing mill||Cold straightener|
|Multi test block||Final inspection||Dimensional check|
|Fig 1: Process Flow in the Pilger Mill of V&M Deutschland GmbH.|
The Multi Test Block is equipped with two measuring systems. An ultrasonic testing system is used for determining wall thickness, eccentricity and laminations. A flux leakage system provides for inspection of mainly longitudinal surface defects and is used as a replacement of hydro-testing in accordance with EN 10246-1. In the following, these systems are described in more detail.
Electro-Magnetic Ultrasonic Technology (EMUS technology)
The system used for measurement of wall thickness and detection of laminations is based on the dry-process electro-magnetic ultrasonic technology. Unlike conventional ultrasonic technology, ultrasound in the field of EMUS technology is directly excited in the electric conductive test piece by means of inductive and in the case of a ferromagnetic specimen additionally by means of magnetic and magnetostrictive effects.
This way of ultrasonic technology requires neither direct contact nor any coupling medium like water. Therefore, it is possible to easily combine it with other dry-process non-destructive testing technologies as this has been done for the first time at Vallourec & Mannesmann Deutschland GmbH with high-energy AC flux leakage technology.
|Fig 2: Principle of Ultrasonic Wall Thickness Measurement with and without Couplant.|
The electro-magnetic ultrasonic measurement system is required to perform a complete (without any gap) testing for laminations while simultaneously measuring the wall thickness. For the excitation and detection of the ultrasonic signal, a four-channel electromagnetic transducer equipped with an electromagnet is used. The transducer is guided over ceramic sliding blocks on the tube surface. Apart from surface unevennesses, this arrangement maintains a constant distance of more than 2 mm between probe system and tube surface.
|Fig 3: Four-Channel EMUS Transducer.|
Evaluation is performed in a central eight-channel "Measurement and Evaluation System for Ultrasonic Signals (MESUS)", equipped with four measuring channels for laminations and four measuring channels for wall thickness. This system, which excites all four channels automatically and in parallel is characterised by a high inspection performance. The system provides good availability supported by a standard remote diagnostic and maintenance feature.
|Table 2: Ultrasonic System.|
Test results may be given in several online graphs which can be selected from a menu by the testing personnel. Apart from maximum, minimum and mean values of wall thickness or the course of eccentricity, it is also possible to choose a colour-coded representation of the course of wall thickness of a complete tube (D scan). Results of lamination detection and surface inspection (possibility of C scan representation) may be combined with wall thickness graphs.
Fig 4: MESUS Online Graph for Representation of Results.
The system used for wall thickness measurement and lamination testing comprises the following components:
Specificities Concerning the Design of the System
When further developing the EMUS technology for inspection of tubes coming out of the pilger mill, the focus has been put on limiting wear and tear of the system due partly pitted and scaled surfaces resulting from the process and providing measurements of wall thickness up to 100 mm. Considering the tube geometry, strong vibrations of the testing head and the local electronics during inspection had been expected. Therefore, electronic components were partly fixed by gluing and additionally encapsulated.
Since the range of wall thicknesses to be measured significantly exceeds the requirements of applications already existing, new coil systems have been developed with regard to the necessary excitation energy and frequency. The single coils with a measuring frequency of about 2 MHz cover an area of 20 x 30 mm and consist of separate transmitter und receiver coils. To provide an advance of tube of 120 mm per rotation while preferably ensuring an inspection without any gap, four probes sit closely beside each other. For economic and technical reasons, two probes have been arranged on a magnetic field concentrator in each case. The concentrator material consists of a poorly electric conductive powder composite material in order to prevent eddy current excitation and thus sound excitation in the concentrator as far as possible. The way in which the double probe has been designed contributes to improved stability of the probe and holds it firmly on the probe guide. The plugging of probes is convenient for the operator and lets the probes be replaced quickly.
Probe coils are protected by a protection cap covering all probes; the protection cap, for its turn, attaches both the probes and the cap to the probe guide by a frame flush mounted with itself.
Due to the state of the tube surface, a new approach for protecting the probes had to be taken. Normally, small EMUS probes are protected by a ceramic material, larger probes are protected by plastic composite materials as for instance carbon fibre reinforced plastics. Because of the probe surface (120 x 20 mm˛), ceramic material alone was out of question. For carbon fibre reinforced plastics, however, wear was to important and economically not acceptable. The solution of the problem was a carbon fibre reinforced material with a ceramic inlay of about 0.5 mm thickness. Compared to previous solutions, considerably longer life times could be achieved.
A high-performance electromagnet is used for the required high magnetisation of the tube surface there are magnetic inductions of about 2 Tesla achieved for an air gap of 2 mm over a length of 120 mm. However, a very efficient air cooling of the electromagnet coil and an appropriately chosen yoke material made it possible to design the electromagnet very compact.
The developments concerning the probe and the wear protection systems combined with the high-performance magnet allow the system to meet the requirements it was designed for. To improve its reliability in an industrial environment which, for a NDT technology is fairly rough, the air coming out from the air cooling system of the magnet is purposefully used to keep the sensitive probe area free from scale and pollution. Another purpose is to dissipate the thermal energy generated by friction of sliding blocks on the tube surface in order to prevent heat transfer to the probes.
STATOFLUX High-Energy AC Flux Leakage Testing
For steel tubes which have been manufactured in a hot pilger mill and present the scaled surface resulting from hot rolling, high-energy AC flux leakage testing is the only method to reliably detect narrow cracks to a minimum depth of 0.3 mm. The rotating ferromagnetic test material is magnetised with high power via a magnetic yoke with an alternating field of high frequency (3kHz). The magnetic flux generated in the yoke is transferred contactlessly into the testing material. Due to the skin effect occurring at the high magnetisation frequency, a thin magnetically saturated layer is created between the yoke legs along the tube surface.
|At the top: Magnetic flux in the tube|
|At the bottom: Magnetic flux and flux leakage at the defect|
|Fig 5: Principle of Flux Leakage Testing.|
Magnetic inhomogeneities in the surface structure resulting from surface scale are considerably reduced by magnetic saturation and thus allow sensitive detection of cracks in the surface.
Drill hole 3.2 mm
Outside longitudinal defect with 5 % defect depth
|Table 3: Flux Leakage Testing System|
|Fig 6: Testing Head of the Surface Testing System|
The controller- or micro-processor based evaluation electronics comprises 16 evaluation channels. Testing signals are evaluated by means of three defect thresholds, events are recorded and generate appropriate marking and sorting signals. Via an interface to the host computer, device settings can be archived and retrieved and test results can be recorded.
The probe systems used for surface inspection as well as wall thickness measurement and lamination detection are mounted in supports which are attached to a traverse beam. This allows a movement parallel to the tube axis. When the testing procedure begins, the supports are lowered, one after the other, onto the rotating tube. Once lowered on the tube surface, the probe systems, which are supported by sliding blocks, move in the direction of the tube axis and inspect the whole tube following a helical track of 120 mm width until the end of the tube; there they are lifted one after the other now in the opposite order from the tube again. To reduce uninspected tube ends, the respective probe system inspects both tube ends for at least one tube rotation without advancing in the direction of the tube axis. The maximum testing speed in the circumference direction of the tube is 1,5 m/s in the course of inspection.
|Fig 7: Multi Test Block during Tube Testing|
The Multi Test Block has been tried and tested over more than two years of multi-shift operation. It provides results which give a comprehensive quality description of the manufactured tubes. Therefore, the aim of that investment has been achieved and the test results delivered by the Multi Test Block are now an important tool for monitoring the production and the quality of tubes from the pilger mill.
The measuring systems work trouble-free to which for sure the experiences gathered and improvements made during the two-year operational period have contributed. So areas of the EMUS transducer highly affected by wear and tear, as for instance probe protection, could be efficiently enhanced, leading to considerably increased life times of the probe system.
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