IE Product Research Paper Summary

Research Paper Summary on

AUTOMATED ULTRASONIC PIPE WELD INSPECTION

By.Wolfram A. Karl DEUTSCH, Peter SCHULTE,

Michael JOSWIG, Rainer KATTWINKEL

17th World Conference on Nondestructive Testing, 25-28 Oct 2008, Shanghai, China

Presented By:

Saket Wankhede

PGDIE 42

Roll NO. 84

1.      Introduction

This paper discusses various applications where the ultrasonic weld inspection is carried out in an automated manner. The highest throughput rates are required in pipe mills where the testing systems are part of the production line so as not to limit the capacity of mill. This is mostly the case for Electric Resistance Welding (ERW)-pipe mills, where a large number of pipes is produced in dependence of the weld speed (typical 10-35 m /min). The systems often work in 3-shift operation with a rate of availability of more than 90%. Therefore, robust testing machines are required in which case ultrasonic testing equipment is used.

2.       Ultrasonic Coupling techniques

Water is commonly used for the ultrasonic coupling. This usually governs the design of every testing system. Small-diameter ERW-pipes (or seamless pipes) can be tested with such systems. The pipe diameters range up to approximately 170 mm. If the weld position is known, the immersion chamber can also be partially equipped with ultrasonic probes to cover only the weld seam area. For larger pipe diameters one technique is commonly used called water gap coupling. The probe is mounted into a probe holder and the distance of the probe face to the pipe surface is in the order of 0.5 mm. The probe holder is guided along the pipe surface by rollers or by shoes.

                   

Fig. 1: Immersion Testing. a) Immersion testing for component test, b) HRP-immersion high-speed testing for long profiles (bars & pipes), c) HRP-immersion testing of small diameter ERW-pipes with setup for seamless pipe, and d) HRP-immersion setup with probes only for weld inspection in 12 o’clock position.

The disadvantage of rather high probe and shoe wear (especially for rough or black pipe surface), the need of many sets of curved shoes and probe angles and the limited test speed (typically 0.5 m/sec) have led to the introduction of water jet coupling. Water jet coupling offers a higher near-field resolution and a longer lifetime of the probes. A water jet is guided within a plastic nozzle towards the component surface. The jet diameter has to be large enough to carry the entire ultrasonic beam and has to be without air bubbles and turbulences for a good ultrasonic signal to- noise ratio.

Straight-beam immersion probes are used. A fairly long water column (30 – 50 mm) between probe and component guides the ultrasound. In order to produce angular incidence, the entire probe holder is mechanically tilted with respect to the component surface. This technique is almost free of wear. Only the shoes or rollers which guide the probe holder along the pipe surface have to be changed from time to time but in general, they don’t have to be changed for different pipe curvatures (diameters).

                 

Fig. 2: Gap and Jet Coupling, a) gap coupling, typically used for lamination and strip testing with dual-element probes, b) wear-free jet coupling with straight-beam incidence, c) jet coupling with angular incidence for high-speed weld inspection and convenient angle adjustment.

3.       Common Test Tasks for the Ultrasonic Inspection of Welded Pipes

          3.1 Detection of Longitudinal Defects

Lack of fusion is the most dangerous and most common defect in a pipe weld and therefore makes the testing for longitudinal defects the most important test. Longitudinal defect detection requires approximately one probe pair for every 7-10 mm of pipe wall thickness. Thin walled ERW pipes can therefore be inspected with one probe pair. Heavy-walled pipes might require additional probes for the detection of longitudinal mid-seam defects.

                    

Fig. 3: Longitudinal Defect Detection. a) top view of probe pair with angular incidence with respect to weld, b) cross-sectional view of probe pair and pipe, here shown for the detection of external defects.

3.2  Detection of Transverse Defects

Transverse defects are less often during the production of welded pipes. Inclusions within submerged-arc welds might occur and therefore, most SAW-pipe testing systems include probes for the transverse defect detection.

Conventional testing systems for SAW-pipes use the K or X-configuration where two or respectively four probes are mounted next to the weld. Two probes work in transmitter – receiver arrangement and their V-reflection signal is used to detect the transverse defects. This setup then requires perfect positioning of two probes with respect to the weld and also a rather complicated mechanical adjustment with respect to the pipe geometry (wall thickness, pipe diameter and respective curvature).

Good ultrasonic coupling can be achieved by proper design of the probe holders and the water nozzles. The incidence angle is typically 45 degrees. Only the distance of the two probes might need adjustment for coupling check purposes (V-transmission) in dependence of the respective pipe wall thickness.

      3.3 Detection of Laminations

Most international specifications allow for either inspection of the strip edges before pipe forming OR the lamination inspection on the welded pipe. Dependent on the type of pipe and the used test specification, a test trace of 15 – 50 mm on both sides of the weld is inspected. Dual-element probes are used in order to ensure small dead zones on internal and external pipe surface, which is important especially for a small pipe wall thickness (e.g. s < 10 mm). If high testing speeds are required, jet coupling with straight-beam probes can be employed.

                       

Fig. 4: Lamination testing within heat-affected zone. a) Top view of probe pair position with respect to weld, b) cross-sectional view of probe pair and pipe.

      3.4 Pipe End Testing

            The pipe ends deserve special attention. In most cases, an inspection for laminations is carried out covering a test track of 50 mm. The portions of the weld which were not covered by the automated weld testing system must be further inspected. This could be done with a portable ultrasonic flaw detector.

4.       Ultrasonic Inspections of ERW-Pipes

The production of ERW-pipes includes several steps of Non Destructive Testing (NDT). Up to four ultrasonic systems are typically encountered during the production process. Directly after welding, a first online weld test is carried out with ultrasound. It is common to check for longitudinal defects only. One probe holder with a straight-beam probe oscillates across the weld while the pipe is linearly moved. The oscillating range covers the weld and the region besides the weld (parent strip material). If the internal pipe wall is not parallel to the external pipe wall due to wear of the descarfing tool, no ultrasonic signal is received. In that case, this method only produces a good/poor-information about the deburring process.

                   

 Fig. 5: Typical Probe Configuration for ERW-pipe inspection. a) Strip inspection with edge probes and oscillating strip middle probes, b) online weld test with 4 probes for longitudinal defect detection and an oscillating deburring check, and c) offline weld inspection with 4 probes for longitudinal defect detection, 2 on-bead probes for transverse defect detection and 2 probes for lamination testing in the heat-affected zone.

After pipe cutting and the hydrostatic test, the final weld inspection is carried out (offline weld testing). A testing portal with moveable carriage is commonly used. The testing portal shows the advantage that the weld is inspected without pipe movement, thus avoiding vibrations which could degrade the test results.

5.       Ultrasonic Inspections of Helical Submerged Arc welding (HSAW)-Pipes

            The first ultrasonic weld inspection is carried out directly after welding (online weld test). The probes are mounted to a stationary machine stand which is height-adjustable in accordance to the pipe diameter. The test position is in 12 o’clock.

                                    

Fig. 6: Testing mechanics for spiral pipe inspection. a) Machine frame (stand), b) cantilever beam (horizontal boom) with vertical position adjustment, c) probe holders with horizontal position adjustment, d) spiral SAW-pipe, and e) foundation with water drainage (closed water circuit).

           

After pipe cutting, a second ultrasonic test is performed (offline weld test). The number or probes is equal or higher than during the first inspection, because this inspection is important for the final customer of the pipe. Since the testing mechanics have to be adjustable in accordance to the weld angle, space is rather limited and for more than four probe pairs, a second weld testing mechanics and a second machine stand is employed.

Since water is critical to the welding process, the strip inspection is often carried out after the hydrostatic test and is combined with the offline weld test in one common testing system. To increase the coverage for each probe, oscillating probe movements are typical. The weld and strip inspection requires a smooth helical pipe movement with respect to the probes, making the seam tracking a difficult and important task. The probe holders are typically guided by rollers on the pipe end for short untested ends.

6.       Ultrasonic Inspections of Submerged Arc Welding with Longitudinal Seam (LSAW)-Pipes

            Once the hydrostatic test is performed, a defective pipe cannot be saved. NDT on both sides looks rather similar. An initial ultrasonic test produces early feedback about the production and welding quality. Sometimes, the first ultrasonic test is carried out with higher sensitivity than the second and final test. Defective areas are double-checked with X-rays. The same sequence of non-destructive tests (UT and RT) is carried out after the hydrostatic test. Finally, the pipe ends are checked for laminations with ultrasound, X-rays and sometimes also with magnetic particles. The inspection for longitudinal and transverse defects within the weld is always mandatory.

7.       Conclusion

           

The pipe geometry, the production process, and the pipe usage determine the number of required probes. Since seamless pipes are sometimes replaced by ERW pipes and LSAW pipes by SSAW pipes (in both cases to save production cost), the inspection methods change gradually between the various pipe types. Each testing system is unique and shows its specialties which have to be discussed by supplier, testing system user and final customer of the pipe.