Monitoring quality early in the process is particularly important for processes performed at high temperatures, where the long delay required to make room-temperature measurements can lead to a high volume of rejected parts. However, many conventional inspection techniques, most notably those using ultrasonics, are not suited for high-temperature operation. Rapid part motion and rough surfaces are additional obstacles to these techniques.
In this article we will describe a modern variant of ultrasonic inspection, laser ultrasonics, which is well suited for in-line inspection during modern industrial processes. As described below, laser ultrasonics combines the noncontact property, speed and spatial resolution of lasers with the ability of ultrasonics to penetrate and inspect opaque materials. We will also describe a number of applications that take advantage of the unique properties of laser ultrasonics.
Laser Ultrasonic TestingLaser ultrasonic testing (LUT) is a noncontact, remote form of ultrasonic inspection. As illustrated in Figure 1, a pulsed, high-energy laser is used to generate an ultrasonic pulse in the part under inspection. This pulse interrogates the feature of interest. When the scattered ultrasonic wave reaches the surface, the resulting high-frequency surface displacement is detected with a separate continuous-wave laser interferometer and detector. The hardware in a typical system consists of a small, fiber-delivered measurement head near the sample being tested and a base station containing both lasers and optical demodulator. The measurement head contains only simple optical components. The linking fibers can be many meters in length. A commercial receiver system is illustrated in Figure 2.
While laser-based generation and detection of ultrasonic waves are quite different from transducer-based techniques, lasers can generate and detect the full complement of ultrasonic waves (longitudinal, shear, surface and plate). Normal transducer-related configurations (pulse echo, pitch-catch and through-transmission) can be used, as well as familiar signal-interpretation methods.
The rich selection of wave types and their broad directionality enables the probing of large volumes and allows the selection of specific wave types and propagation paths to suit the application. The small size of the laser spots on the surface allows easy access to confined areas as well as broad coverage of rough surfaces.
In metals, the laser generation process is most effective in the ablation regime, in which a plasma is formed at the surface and a few monolayers of material are ejected. The recoil from this material removal produces strong longitudinal, shear and surface waves.
The most common generation laser is a Q-switched Nd:YAG laser having a pulse width of ~10 ns. Thus, at the point of generation, the ultrasonic waves have a bandwidth of ~100 MHz. Even after transit through many millimeters of steel with associated grain scattering, it is common to measure a bandwidth of 30-50 MHz at the point of detection. This bandwidth is much greater than that of typical contact transducers and enhances the depth resolution as well as the scattering amplitude from small defects.
In Table 1 we compare the attributes of the most common ultrasonic approaches with the properties of LUT, especially as they apply to real-time, in-line inspection. Laser ultrasonics has several advantages over other ultrasonic nondestructive testing techniques:
- It is noncontact, with only laser beams reaching the part.
- The small beam footprint on the part allows access to small spaces as well as rough or curved surfaces.
- It can perform measurements at process speeds on rough surfaces.
- Longitudinal, shear and surface waves are readily produced over a range of angles.
- High bandwidth enables high spatial and temporal resolution.
In-line Wall-Thickness Monitoring of Seamless Steel TubesSeamless steel tubes are produced in large quantities for oil-well drilling and for the production of certain mechanical components such as ball bearings. The manufacturing process starts with a solid steel rod and involves several milling steps of piercing and shaping. Rather tight tolerances are imposed on wall-thickness uniformity and on ovality. In conventional operation, dimensional measurements are not made until after each tube has cooled sufficiently for a conventional transducer-based ultrasonic measurement. If a tube is out of specification, it has to be reworked or scrapped, adding considerable cost. If the loss of specification is a result of improper process settings, then many tubes could be produced before the problem is detected and corrections are made. It is thus highly desirable to monitor critical dimensions earlier in the process so that more rapid corrections can be made. In the past, in-line measurement was only possible with expensive radiation-based gauges, posing a significant radiation hazard.
It was recognized some years ago that laser ultrasonics would be highly suited for in-line monitoring of the tube production process. This technique can perform wall-thickness measurements on tubes moving at process speeds and temperatures. No special surface preparation is required, and the system can be designed so that no laser eye-safety hazard exists.
In 2002, when the author was president of Lasson Technologies, Lasson developed a laser ultrasonic inspection system for demonstration at a Japanese seamless-tube mill. As shown schematically in Figure 3, the system was installed at the exit of one of the forming mills. At the exit of the mill, the tube was moving at 2-5 m/s, with a temperature around 1000°C (1832°F). The installed system had one channel for measuring wall thickness at one position around the tube. The generation laser was a pulsed Nd:YAG laser at 1064 nm, with a repetition rate of 10 Hz. Thus, at a tube speed of 3 m/s, wall-thickness measurements were made every 30 cm.
Of course a generation laser with a higher repetition rate would have a smaller measurement spacing. The wall thickness was measured by acquiring a series of longitudinal-wave wall echoes and measuring the time-of-flight between adjacent arrival temporal peaks, corresponding to one round trip through the tube wall. The wall thickness was then determined by multiplying the longitudinal wave velocity by the measured time-of-flight.
It is well known that the longitudinal-wave velocity in steel is a function of temperature. Thus, in order to determine the correct value of velocity in calculating wall thickness, it is necessary to first measure the wall temperature at the position of measurement and then use a lookup table to obtain the wave velocity.
A plot of measured wall thickness as a function of position along a 16-meter tube is shown in Figure 4. The nominal wall thickness was 3.2 mm. We see that there is considerable variation in the thickness along the length of the tube. We have also plotted the thickness values measured after cooling using a conventional ultrasonic thickness gauge. There is a small offset between the two sets of measurements, which is likely due to the thermal expansion at the measurement temperature.
In recent years, Tecnar Automation has developed and marketed a commercial laser ultrasonic system for this same application. It has been widely adopted in the seamless-tube industry.
In-line Inspection of Girth Welds Used in Gas/Oil Pipeline AssemblyOne challenge in the development of new gas reserves is the high cost of pipeline construction, of which welding costs are a major component. Industry continues to seek advanced pipeline welding technologies to achieve additional improvements in productivity and enable significant cost savings. Based on recent improvements of weld lasers and laser-welding technology, it is likely that the next generation of automated pipeline welding equipment will be built around hybrid laser arc welding (HLAW). HLAW enables butt welding of pipe sections with fewer weld passes. In addition, less heat is produced and the weld volume is much smaller than that obtained with conventional arc welding.
As this new technique is developed and implemented for pipeline construction, concurrent development of an in-process technique for defect detection, location and sizing is required. Such an in-line technique would replace current post-process ultrasonic inspection techniques, which require a fluid couplant and add time and cost to the process. By contrast, in-line inspection would allow defects to be detected earlier in the process, thereby allowing the adjustment of process parameters to limit these defects.
LUT offers the advantage of true in-process measurement, thereby providing immediate information on weld integrity and saving the cost of an extra crew for later inspection, as required with phased array UT. A small fiber-delivered measurement head can be integrated into the weld head so that testing can be performed in tandem immediately as the weld is formed.
New TechnologyIntelligent Optical Systems has developed an automated laser ultrasonic testing (ALUT) system to inspect new HLAW welds in the field. The system was developed with co-funding from the U.S. Department of Transportation and in coordination with a parallel project intended to develop and qualify the HLAW process. In our technique, we generate ultrasonic waves on the pipe OD near the weld and detect reflected and scattered waves from defects with a detection laser at a separate position. A measurement of the arrival time of defect signals allows us to determine the depth of the defect within the pipe wall. With careful signal discrimination we can also measure the height of a crack that extends partway through the wall. It is well known that defects very near the OD or the ID wall are hard to detect in the presence of strong signals associated with the surface itself. The high bandwidth of laser ultrasonics allows us to detect very small defect signals that are only slightly shifted in time from the very large surface signals.
In Figure 4 we show a photo of our ALUT measurement head integrated into the weld head of a demonstration HLAW system for welding 36-inch pipe. A close-up is shown in Figure 5. The ALUT head is about 12 inches downstream from the weld head. This distance is sufficient to ensure that the welded material has solidified. In fact, the low heat input associated with the HLAW weld process leads to a surface temperature at the point of measurement of only about 300°C (572°F). IH
For more information: Marvin Klein, manager, Laser Ultrasonics Products Group, Intelligent Optical Systems, 2520 West 237th Street, Torrance, CA 90272; tel: 424-263-6361; e-mail: Mklein@intopsys.com; web: http://www.intopsys.com
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