Table of Contents ECNDT '98 Session: Steel Industry

Non-destructive Ultrasonic Testing for High Frequency Inductive(HFI) Pressure Welded Tubes in the Production Line D.Lingenberg, E.Fischer Siemens AG Erlangen W.Vogel Siemens AG Siegen G.Bach, A.Kalwa Röhrenwerke Gebrüder Fuchs Siegen Corresponding Author Contact: Rainer Meier Siemens AG - Power Generation Group, KWU NP, Freyeslebenstraße 1, D-91058 Erlangen Email: Rainer.Meier@erl11.siemens.de , URL: NDTnet Exhibition - Siemens

Introduction

In a three year lasting cooperation between Gebrüder Fuchs Röhrenwerke Siegen and Siemens AG there has been realized a developement of an ultrasonic inspection system for high frequency inductive (HFI) welded tubes while passing the assembly line. Flat endless sheet steel is shaped into an endless tube. The weld joint between the bended edges is done by heating with high frequency induced eddy currents. The aim of the inspection is to prove that there is no appearence of laminar-type defects in the sheet steel material and also that there is no cracks occuring in the tube weld during every step of the production sequence.

Examination of Inspection

The inspection is done by using four separate testing devices occuring at different sections of production sequence. Diagram 1 shows a schematic diagram of the processes. The first ultrasonic inspection occurs before the flat endless sheet steel is shaped into an endless tube. This inspection takes place using eight straight beam search units in immersion testing. The aim is to detect laminar-type flaws in the sheet steel. The second ultrasonic inspection occurs immediately after the processes of bending the sheet steel and welding. The still hot welded zone of the endless tube is inspected with four 45° angle beam probes using contact testing in order to detect defects orientated lengthwise to the welded line. Simultaneously, an oscillating straight beam search unit determines the wall thickness and contour of the welded area. This oscillating probe is arranged in a squirter device, which uses a 70 mm jet of water as the couplant medium.

Fig. 1: Schematic Diagram Of The Processess

The endless tube line is cut in segments and a water pressure leak test of the single tube segments take place. The third ultrasonic inspection station is used to detect laminar-type flaws and to measure wall thickness in the straight faced ends of the tube segments using a straight beam search unit in immersion testing. Afterwards, the final ultrasonic inspection station carries out an additional inspection of the welded zone using contact testing. This inspection is done on the inactive tube segment using a moving test carriage system to scan the weld line.

The four electronic ultrasonic inspection systems consist of the same components from the SIMUS ultrasonic system. These components can be configured in such a way that they adapt to various test requirements. Modular ultrasonic-specific hardware components using VME-Bus as well as UNIX software modules are used. The requirements in the different inspection systems depend on the various ultrasonic applications. In order to satisfy the different test requirements, it is necessary to vary the computer-control of all important ultrasonic specific parameters and functions quickly from shot to shot. These functions are:

• variable receiver-amplifier gain adjustment
  Ranges: Pre-amplifier between -18 dB and 22 dB
  main amplifier between 0 dB and 60 dB
• variable flaw gate adjustment ( four gates are maximum )
• adjustable pulser-system, able to vary pulse length and pulse damping
• adjustable filter systems, able to support probes in the frequency range between 0.5 MHz and 25 MHz.
• variable trigger configurations
  e.g.
  • all gates triggered equal
  • flaw gate triggered by interface-echo-controlling gate
  • flaw gate triggered by backwall-echo-controlling gate
• variable distance amplitude correction curves
  Standard range: between 0 dB and 30 dB in every point of the curve

Fig. 2: Matrix Of The Self Test Results

All important device functions and ultrasonic functions are monitored via extensive self controlled algorithms. Diagram 2 shows the matrix of the test results whenever a test loop is done. For all configured ultrasonic channels, the algorithms verify the function of the pulser, the receiver-amplifier, the gates, and the distance amplitude corrections. Moreover, the impedances of the probes and the cable connections are determined to detect any sign of wear or defects

Inspection of Endless Flat Sheet Steel
This ultrasonic inspection system is designed for testing endless flat sheet steel in wall thicknesses ranging between 2.6 mm and 16 mm and widths ranging between 350 mm and 1250 mm. The aim of the inspection is to guarantee sheet steel free of laminar-type defects. A 100% inspection of the edge area ( minimum 25 mm according to the Stahl-Eisen-Lieferbedingung-SEL-072 ) and also a statistical 12.5% inspection of the remaining area is required.

During the inspection the endless sheet is moved at a maximum velocity of 45 m/min.

A ultrasonic shot to shot distance of 1 mm or less in the direction of material flow is required. The inspection in the edge area is accomplished by two probes ( one for each side ). Six more probes are oscillating in equal phase over the remaining inspection area with a frequency of 1/6 Hz ( see Diagram 3 ).

Fig. 3: Inspection Of Endless Flat Sheet Steel

The probes are straight beam immersion transducers with high near surface resolution. The specialty probes are designed by Siemens for this application. The ultrasonic inspection frequency is 10 MHz. Diagram 4 shows the probe and its dimension ( 15 mm x 35 mm x 25 mm ). The active transducer area is 3 mm x 25 mm. With this type of probe, SEL-072 requires an ultrasonic inspection system that is able to support eight search units. It also is shown in Diagram 4 the characteristic soundfield curve detected in the direction lengthwise to the crystal. The test device is shown in Diagram 5. It contains the probe and a undercarriage skid that is made from special hardened steel to position the device on the sheet steel at a constant distance. Between the probe and the steel surface, the test device also contains a water chamber that provides a 6 mm water gap that is used as the couplant medium.

Fig. 4: Probe for Inspection Of Endless Flat Sheet Steel Fig. 5: Device For Inspection Of Endless Flat Sheet Steel =>

Hot Flaw Detection of the Weld


Fig. 6: Oscillating Squirter Device

Fig. 7: Wall Thickness Profiles

Fig. 8: Profile Of The Welded Zone

The hot flaw detection of the welded zone happens 3 m behind the place where some seconds before the bended edges of the sheet steel were bonded by heating with eddy currents. Before testing the welded zone, it must be cooled below 85 °C using a water cooling device. This is necessary to prepare for the ultrasonic inspection detection using water as the couplant so that there are no air bubbles caused by heat.

The test searches for defects orientated lengthwise to the welded line using four 45° - angle probes in contact testing. Two probes at a time are orientated on both sides of the welded line so that it is possible to control the coupling of the probes using simultaneous through-transmission testing. The first pair of probes is searching for defects in the weld at the inside and outside surface. The second pair is controlling the removal of welding beads at the inside tube surface.

At the same time, an oscillating straight beam probe is used to determine the wall thickness and contour of the welded zone. The probe being used is a 25 MHz straight beam search that is arranged in a squirter device using a jet of water 70 mm in length as the couplant medium. This squirter device that has been optimized for pulse-echo-testing is a special application made by Siemens. The squirter device is oscillating +/- 20 mm over the welded zone with a frequency of 0.5 Hz ( see Diagram 6 ).

The results of this contactless wall thickness measurement are used to control the weld process and also to control the removal of welding beads at the inside tube surface. Diagram 7 shows the wall thickness profiles that are updated in each half cycle of oscillation. In this way, the current profile is always seen in the front of the 3D-diagram. Diagram 8 shows the representation of the welded zone contour that is determined by the time-of-flight of the ultrasonic interface echos and back wall reflection echos.

Inspection of Straight Faced Ends
The purpose of this inspection is to guarantee that the tube segments have straight faced ends with 50 mm in length without the presence of laminar-type defects. The inspection is performed in accordance with SEL-072 and Stahl-Eisen-Prüfblatt-SEP-1917.

The tube segments are rotated so that the velocity of the outside tube surface is a maximum of 0.5 m/s. The probe being used is constructed with four 10 MHz-Transmitters and is designed to provide high near surface resolution. The inspection is done using pulse-echo immersion testing with 10 mm water distance between the probe and the tube surface. To achieve the same detection sensitivity over a distance of a minimum 60 mm ( 50 mm demanded in the SEL/SEP and additive 10 mm to compensate for inaccuracies when positioning the tube segements in the test device ), the probes' crystals are arranged out-of-line and overlapping as seen in Diagram 9. This probe is also a special ultrasonic application by Siemens.

Fig. 9: Probe For Inspection Of Straight Faced Ends

Flaw Detection of the Weld in Tube Segments
The endless tube moving forward in the assembly line is cut into segements with lengths between 6 m and 18 m. The tube diameters range from 110 mm up to 400 mm. The single tube segments are transported into a device that executes a water pressure leak test. Afterwards the tube segments travel in the fourth ultrasonic inspection station where another inspection of the weld takes place. During testing, the tube segments do not move. A moving test carriage system is used to scan the welded line in water coupling contact testing. The maximum velocity of the carriage system is 2 m/s.

Eight 45°-angle beam probes are used for flaw detection. The eight probes are arranged in four pairs. Each pair is arranged oppositely to make it possible to control the coupling of the probes by using through-transmission testing. Three pairs are searching for defects in the weld zone that are orientated lengthwise to the welded line. The first pair is optimized to provide the highest sensitivity to the inside surface zone, the second pair to the outside surface zone and the third pair to the remaining zone. The other pair of probes is used to search for cracks orientated crosswise to the welded line.

Operational Experiences and Options

The ultrasonic inspection systems for the hot inspection detection of the weld and also for testing the straight faced ends of tube segments have been operating successfully since early 1995. The system for testing the endless flat sheet steel has been running without any failure since May 1997. The system for inspecting of the weld in tube segments will be integrated into the assembly line in January 1998.

The data communication between the SIMUS ultrasonic inspection systems and the data processing system of the manufacturing company ( Röhrenwerke Gebrüder Fuchs in Siegen ) will also be installed in January 1998 so that a central operational monitoring system will be available.

The wear of ultrasonic probes has been minimized when using contactless immersion and squirter testing in cases of wall thickness und contour determinations. Optional in the future, it will be possible, without any changes in the ultrasonic electronic design, to replace the currently used inspections by contact testing with contactless methods, too.