Abstract

In several mills inspection systems for tubes and pipes based on electromagnetic acoustic transducers (EMATs) are installed. Lamb waves are used for the ultrasonic inspection of precision tubes and Rayleigh waves for the surface inspection of large diameter line pipes. Wall thickness measurement systems for precision tubes as well as for pilger mill tubes are realized. The inspection techniques that are based on powerful EMATs are presented and their advantages besides the lack of coupling medium are discussed.

Introduction

In the last years ultrasound techniques using electromagnetic transducers (EMAT-technology) have been introduced to several plants for the inspection of tubes and pipes. Unlike “classic” piezoelectric ultrasound excitation, sound is directly generated in the material. Besides the advantage that no coupling medium is necessary EMATs allow to realise automated inspection techniques using plate waves, surface waves and transverse waves. The following mill applications for ferromagnetic products will be discussed:

• Ultrasonic inspection of precision tubes (welded and seamless) using plate waves.
• Surface inspection of large diameter line pipe using Rayleigh waves.
• Wall thickness measurement for precision and hot rolled tubes using transverse waves.

All mentioned inspection techniques use in principle similar transducer constructions. Near the surface of the product is a system of transmitting and receiving coils that is protected by a thin wear plate with low electrical conductivity. Behind the coils a magnet system is placed. For the plate wave and surface wave applications and for the wall thickness measurement of precision tubes permanent magnets were sufficient; for the wall thickness measurement of seamless pilger mill tubes an electromagnet was used. The type of ultrasonic wave depends on the magnet system, the transducer coils and the frequency of the hf-currents fed to the transducers.

Ultrasonic inspection of precision tubes using plate waves

The ultrasonic inspection of precision tubes (cold drawn, welded and seamless) in the mills of MHP (Mannesmann Hoesch Präzisrohr GmbH) at Wickede and Holzhausen is realized using plate waves that run many times around the tube circumference thus interacting with a defect several times (1). Only two transducers are used in a simple mechanics and the tubes pass the inspection equipment with up to 3 m/s. Neither rotation of the pipe nor mechanic scanning of pipe surface by sensors is required because ultrasonic waves cover the whole product. The installed systems inspect tubes with diameters from 22 mm to 70 mm and wall thickness from 1 to 4.5 mm.

The inspection technique uses transmission as well as reflection signals. In the current case, each electrodynamic transducer comprises transmitting and receiving coils arranged in the field of a permanent magnet for inspection. Both coil systems are symmetrically wound on a joint coil support, which ensures exact positioning of transmitter and receiver to each other; their relative displacement equals to one quarter of wave length in circumference direction (used wavelength: 3 - 5 mm). For ultrasound excitation, the transmitting coils are supplied with a short, high-frequency current and ultrasonic wave packets run in both circumference directions. Due to the special design of the coil systems both waves interfere destructively after one and more tours around the tube at the position of the receiver coil. Thus the transducer that transmits the wave packets does not “see” them. If there are reflections due to a longitudinal defect there is no destructive interference of the reflected waves and the transducer “sees” the defect. Besides the reflection technique the system uses the transmission technique (i.e. the second transducer in the same plane picks up transmission signals) and as the transducers are separated by 90° with respect to the tube axis the peaks from both directions are equidistant. A complete inspection cycle with the transducers A and B looks like that:

1. Transducer A launches waves pulses; transducer A receives reflection signals (high amplification) and transducer B receives transmission signals (low amplification)
2. Roles of transducer A and B exchanged.

The evaluation of the signals uses two absolute channels for one inspection cycle: an integration of the reflection signals (multiple reflections!) and an evaluation of the damping of the transmission signals. Thus defects with a longitudinal orientation will be detected preferentially with the reflection channel and defects with other orientations that reflect ultrasound aside or cause mode conversion will preferentially be detected in the damping channel.

In the last year an additional powerful feature was integrated in the two mill systems: the difference technique (patent pending) that is completely realised by software. The original signals at a length position x of the tube are compared with those at a position x+.x and the difference is evaluated. Here Dx can be chosen as a multiple of the pitch distance. Thus two difference channels (for reflection signals and for transmission signals) complete the signal processing. The four values are represented in diagrams as a function of the length co-ordinate and are compared with threshold values.

The equipment comprises a local electronic, the main electronic and the mechanical block (see figure 1) with the transducers. The local electronic supplies the transducers with high-power pulses and contains low-noise preamplifiers. The timing and the signal processing is done in the main electronic that contains our “Measurement and Evaluation System for Ultrasonic Signals (MESUS)” that works under full computer control. All test results are displayed on-line on a screen and can be stored and printed. The mechanics of the system is very simple, the nominal position of the transducers is chosen according to the tube diameter and the probes are positioned on the surface pneumatically. In order to guarantee a constant distance between the electrodynamic transducers and pipe surface – which is only some tenths of a millimetre ,rollers were integrated into the probe. After diameter change no further adjustment is necessary and thus the change is very fast, all parameter values are stored. The systems are used for the ultrasonic inspection of tubes according SEP 1915 (German steel and iron testing standard) (2). The technique has been tested for other dimensions as well and it works for the inspection of the surface of peeled bars as well using surface waves instead of plate waves.

Fig 1: Mechanics for the ultrasonic inspection of precision pipes (installed at MHP) no coupling medium no rotation diameter change within few minutes no adjustment of the transducers necessary up to 3 m/s simple (maintenance!).

Surface inspection of large diameter line pipe by means of ultrasonic surface waves

At EUROPIPE, surface inspection of submerged-arc welded line pipes had been based on magnetic particle inspection including visual inspection. Besides high personnel requirements, the disadvantages of that method are its subjectivity, its poor documentation facilities and the fact that it does not provide any information about the depth of defects detected. The latter in particular may lead to a high and unnecessary amount of repair (grinding of surface). An automatic testing equipment providing full outside surface inspection and facilities to document inspection results is presented. Systems have been installed in Mülheim and Dunkerque.

Pipe surface is inspected by means of ultrasonic Rayleigh waves running in circumference direction on the pipe surface. Depth of penetration of those ultrasonic surface waves is of the same order of magnitude as the wavelength (typically 4 mm). Inspection is performed during longitudinal transport of the pipe to be inspected, maximum linear speed being 20 m/min; a rotation is not necessary, too (see above).

Fig 2: Reflection signal due to a test notch (depth: 0,3 mm, length: 30 mm, width: 0,3 mm) distance transducer – notch:500 mm.

Figure 2 shows the high frequency presentation of an reflection signal due to an artificial notch (0.3 mm in depth, 0.3 mm in width and 30 mm in length), which is 500 mm away from the transducer. The dead zone of the transducer due to the transmitting pulse is about 30 µs, the signal-to-noise ratio of reflection signal and (coherent) background is about 25 dB. For a known sound velocity, the distance between transducer and reflector can be calculated based on the difference in time of the reflection signal and the moment of triggering. The transducers are similar to those used for inspection of precision tubes, i.e. one single reflection signal does not allow to decide on which side of the transducer is the defect. Considering the typical surface quality of SAW line pipes, sound travels long distances (about 10 m) before the measurable signal amplitude reaches noise level.

Fig 3: Schematic arrangement of transducers.

Fig 4: Part of the mechanics with 2 transducers.

The diameter of pipes to be inspected ranges from 20“ to 64“. To ensure inspection sensitivity required, mainly reflection technique is employed. Furthermore, transmission signals are also measured and assessed.

To be able to continuously monitor the whole pipe circumference, six transducers (transmitter/receiver) are arranged in one plane (see figure 3). In any case, three transmitting transducers [{1,3,5} or {2,4,6}] are triggered at the same time. For a complete cycle, in total 6 reflection signals are received, and they show the same general structure. Since any location on the circumference is inspected several times, it is possible to say where a reflection indication comes from (because transducers work bi-directionally). Apart from reflection signals, 6 transmission signals with identical structure are received (amplification typically 25 dB lower than for reflection signals).

The inspection mechanical unit (see figure 4) provides for setting down the 6 combined transducers onto pipe surface according to the principle scheme (figure 3). The adjustment of the equipment after a change of dimensions is largely automatic. All schematic operations are performed by the equipment computer. There is no need of adjustments of the transducers (as required for conventional US inspection), the mechanic unit is adjusted fully automatically according to the nominal diameter entered.

Surface inspection takes place as follows: Pipes are moved into the inspection equipment in longitudinal transport, the weld being pre-positioned approximately in a 6 o’clock position. After moving the transducers down pneumatically onto the pipe surface, the pipe is longitudinally transported and at the end of the pipe, the transducers are lifted while the pipe moves on. During inspection, reflection indications and transmission indications as well as the longitudinal co-ordinate for each testing shot are recorded, all information from the 6 channels is put together, and the locations of indications are recorded in a map corresponding to the pipe in process. The evaluation of reflection indications is automatically performed in several steps. What is detected first is the weld causing relatively small indications which often do not exceed the recording threshold for reflection indications. By this way, the circumference co- ordinate of each indication can be referred to the weld. Since due to the weld bead, the weld zone frequently generates small indications, that area is submitted to a particular assessment. Indications detected are submitted to defect evaluation based on various criteria (for instance minimum length, minimum amplitude).

The defect list including co-ordinates and the parameters characterising the defect in question are transferred to the internal IT network of the pipe mill; furthermore, for the inspector, defects are recorded in the surface map, and list and map may be printed. Defects are treated in the sector of general pipe inspection where several working places are equipped with automatic display systems. Those systems get the defect list of the pipe from the IT system, turn the pipe into an appropriate working position and point on the defect location on the pipe surface by means of a laser pointer so that the defect can be ground manually. Wall thickness is checked, the repair is confirmed, and then the system displays the next defect, if appropriate.

The sensitivity of the equipment is usually adjusted by means of longitudinal notches according to SEP 1913 (3). Figure 5 shows an example of an inspection result as seen by the inspector on his screen for artificial notches on a reference pipe. The upper chart presents the „map“ corresponding to the pipe surface, including entries of defects detected. The chart in the middle presents the result of attenuation for the 6 transducer clearances (no indication in this case), and the last chart refers to the weld zone, those indications are compared with a threshold.

Fig 5: Test result (screen image). Pipe diameter: 56“, line of indications from test notches, depth/length (mm): 0,4/75 , 0,3/75 , 0,2/75 , 0,4/50 , 0,3/50 , 0,2/50 , 0,4/25 , 0,3/25 , 0,2/25

Wall Thickness Measurement and Lamination Check

Systems for the wall thickness measurement of seamless precision tubes (diameter: 30 – 120 mm) as well as seamless tubes with wall thickness up to 100 mm (diameter: 219 – 710 mm) have been installed. The wall thickness of precision tubes is measured in two lines during linear transport of the tubes with 2 m/s whereas the testing of pilger mill tubes with regard to wall thickness and laminations is carried out during rotation of the product with 1.5 m/sec. EMATs reveal big advantage with products that exhibit a texture or if the inspection has to be carried out in combination with other techniques that need no coupling medium.

An EMAT system for measurement of wall thickness and detection of laminations has been combined for the first time at Vallourec & Mannesmann Deutschland GmbH with high-energy AC flux leakage technology (STATOFLUX by FOERSTER) (4).

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. 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. 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.

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 (figure 6). 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 6: EMAT transducer for wall thickness measurement and lamination check (four channels).

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.

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.

When further developing the EMAT 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 thickness 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 and 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. 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.

A high-performance electromagnet is used for the required high magnetisation of the tube surface – there is a magnetic induction 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 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.

References

1. F. Schlawne, H. Schneider, F-W. Schoch, “Ultraschallprüfung von Präzisrohren mit elektrodynamischen Wandlern”, DGZfP-Jahrestagung 1994, Zerstörungsfreie Materialprüfung, p. 347-354
2. SEP 1915, “Ultraschallprüfung von Stahlrohren auf Längsfehler”, STAHL-EISEN- Prüfblatt, Verlag Stahleisen, September 1994
3. SEP 1913, “Ultraschall-Oberflächenprüfung von nahtlosen und längsnahtgeschweißten Stahlrohren mit Oberflächenwellen”, STAHL-EISEN-Prüfblatt, Verlag Stahleisen, September 1997
4. A. Graff, N .Arzt, S. Nitsche, D. Sy, “Multi Test Block for Inspection of Seamless Tubes", ITA Conference, Bilbao, Spain, 23.- 26. Oct. 2001

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