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Seamless tubes are used in oil field applications, hydraulic cylinders, power transmission components (gears and bearing), and power plants, which requires strict controls on mechanical dimension. Wall thickness sensors have been used routinely to control the production of seamless tubes. Until recently, however, the locations of on-line wall thickness sensors have been limited. Laser-ultrasonics, which combines the precision of the ultrasonic to the flexibility of optical system, has provided a new method to measure on-line wall thickness under plant conditions. The Timken Company (Timken) and the Industrial Materials Institute of the Research Council of Canada (IMI), in a project funded by the US Department of Energy, has developed and built a new sensor, called the Laser Ultrasonic Thickness (LUT) gauge. The gauge has been in use under production condition for nearly four years, providing plant operator with valuable day-to-day information.

Laser-ultrasonic wall thickness measurement of hot tube

Laser-ultrasonics is an ultrasonic method where laser light is used to both generate and detect ultrasonic pulses. It has the advantage of been fully non-contact and of having a relaxed angular alignment requirement compared to conventional ultrasonic systems.
In laser-ultrasonics, a short-pulse high-powered laser beam, the generation laser beam, is focused onto the surface of the tube. The generation beam is tightly focused as to produce local vaporization of the surface of the tube, ablating a small volume of material. In the case of hot mechanical tube, the ablated material consists only of the surface oxide, which is rapidly replaced. The ablation causes, by recoil effect, a pressure wave (ultrasonic compression wave) that is sent into bulk of the tube in a direction normal to the surface of the tube. Since the ablated material is primarily ejected in a direction normal to the surface of the tube, the ultrasonic wave is launched in the normal direction, independently of the angle of incidence of the generation laser beam with respect to the tube surface. Therefore, wall thickness measurement can be obtained without the need to align the generation laser beam with respect to the center of the tube. This allows the sensor to be used in area of rapid motion (bouncing) of the tube.
The laser-generated compression wave propagates through the thickness of the tube where it is reflected at the back wall, back towards the surface. Measuring the time of flight of the ultrasonic compression wave from the front surface to the back wall and back to the front surface yields a direct measurement of the wall thickness. Accuracy of the wall thickness measurement is determined by the accuracy of the time of flight measurement (dependent of the ultrasonic bandwidth) and by the accuracy of the velocity measurement used to convert time of flight to material thickness.
Non-contact measurement of the ultrasonic compression wave is made by using a second laser. The LUT uses a long-pulsed frequency-stabilized Nd:YAG laser for the non-contact detection of ultrasounds. The second laser is focused on the tube surface in an area near the generation laser beam point of impact.
The detection laser light can be represented as a very precise periodic cycles of electrical energy (electromagnetic wave). If a reflecting surface is moving towards the incident detection laser beam, the cycles of the reflected laser beam will be compressed: the compression factor being a function of the velocity of the surface. In the same way, if a reflecting surface is moving away from the incident detection laser beam, the cycles of the reflected beam will be stretched; the stretching factor being a function of the velocity of the surface. Therefore, any motion of the surface of the tube will be “recorded” in a variation of the cycles of the electromagnetic wave of the reflected beam. One can easily understand that high frequency stability is need in order to separate the cycle variations of the laser source itself from the ones induced by the surface motion.
Photodetectors are not fast enough to record directly these electromagnetic wave cycles, and therefore measure directly the cycle variations. To visualize the changes in the cycle, the signal beam (light beam reflected or back-scattered from the sample) is made to interfere or “beat” with a reference beam. The interference of both beams transforms the cycle variations into amplitude variations, which can then be recorded with standard photodetector.
In theory, the reference beam could be provided directly from the laser source. However, in practice, the surface of the tube being rough, the signal beam has speckle and would not interfere well with a beam provided directly from the laser source. For optimum interference, the reference beam must be created using part of the signal beam itself. Several optical systems can be used to provide such a reference beam. In the LUT gauge at Timken, the laser-interferometer is a confocal Fabry-Perot (CFP). Recently, Tecnar Automation Ltee has demonstrated that a laser-interferometer based on two-wave beam mixing in a photorefractive crystal is more efficient than the CFP for on-line wall thickness measurement.
The signal obtained at the output of the laser-interferometer is equivalent to that of a conventional ultrasonic thickness gauge. The strong signal at time equal to 1ìsec is the initial impact of the generation laser on the surface of the tube. The following pulses are the subsequent arrival of the ultrasonic compression wave on the surface of the tube. Wall thickness is obtained by measuring the time between the initial laser impact and the first ultrasonic echo.
Wall thickness measurement is obtained by multiplying the time of flight with a calibration factor, related to the acoustic velocity of the compression wave in steel. Since acoustic velocity is a function of temperature, the gauge must measure the temperature of the tube to be able to convert accurately the time of flight measurement into wall thickness measurement. In the LUT, temperature measurements are made with a two-color optical pyrometer, integrated in the gauge. Optical pyrometry consists in measuring the intensity of the light emitted from a hot target (which is a function of the temperature of the target). By using two different wavelengths (or color), the temperature measurement is independent of the emissivity of the materials. An optical pyrometer was developed for the LUT to provide accurate measurement for temperature ranging from 700 to 1200ºC, while in the presence of a high-power Nd:YAG lasers (infrared laser).

On-line sensor

Any sensor used on a plant floor must be customized, to some extent, for the particular production line. Knowledge of the production process is therefore essential to determine the best location for the sensor in order to achieve the targeted level of control. In our case, the tube making process begins with a cylindrical billet that is heated in a rotary furnace to a temperature near 1300ºC. At the following piercing process, the heated billet is formed into a hollow bloom by cross rolling of the billet over a piercing plug. At the elongation process, the hollow is stretch into a shell by cross rolling or longitudinal rolling the hollow over a mandrel bar, followed by a corresponding thinning of the wall. The shell can then be re-heated. After re-heating, the shell outer dimension is reduced to the targeted outer diameter. Finally, the tube may be passed through a Rotary Sizer to enhance the “roundness” of the tube, i.e., to have a very good circular shape of the outer surface of the tube. This is of particular importance when the produced tubes are to be used for bearing parts.
Although penetrating radiation techniques have been developed and used for thickness gauging tubes, such techniques have limitations on the location where it may be installed. Penetrating radiation gauges cannot measure wall thickness with a mandrel inside and cannot adapt easily to rapid side-motion of the tube. Laser-ultrasonics, on the other hand, has the flexibility to be used throughout the tube production line. The presence of a mandrel does not affect the wall thickness measurement. Also, since the laser-generated compression wave is always launched in a direction normal to the surface, large tube motion can be tolerated without affecting the accuracy of the wall thickness.
At Timken, the gauge was installed immediately after the rotary sizer. At that location, the gauge uses the ability of laser-ultrasonics to adapt to fast and rapid motion of the tube. Movement of several centimeters can be observed in side-to-side and up-down direction while the gauge is taking data.
At the rotary sizer, the tube is rotated as it moves in front of the gauge. The LUT probe, which is at a fixed position, therefore collects data along a spiral that follows the tube's surface. Knowing the location on the tube where data is taken, the wall thickness data is used to measure the eccentricity of the tube. The measurement of the location is made using a laser-velocimeter system, integrated in the gauge.

Description of the gauge

The gauge includes the most advanced laser-ultrasonic technology available, as well as advanced data acquisition, data processing, laser velocimetry and optical pyrometer technologies. The gauge consists of five units: namely the generation laser unit, the detection laser unit, the detection unit, the control unit and the inspection probe.
The generation laser unit is composed on the high-power short-pulse laser for non-contact generation of ultrasounds. The laser is a Q-switch Nd:YAG operating at a wavelength of 1064nm (infrared). The laser is based on robust flashlamp technology. The firing rate of the laser is 100Hz. The gauge therefore provides up to 100 measurements per second. The laser generation unit is an industrial-grade system with easy access for quick servicing, in particular, for replacement of the flashlamp.
The detection laser unit is composed of the frequency-stabilized laser source for non-contact detection of ultrasound. The detection laser is based on flashlamp technology, providing a high-power output for detection that cannot be achieved with continuous wave (CW) laser. Such high power is needed for the detection of ultrasounds on highly scattering surfaces, such as encountered for on-line tubes. As with the generation laser, the firing rate of the detection laser is 100Hz and the optical wavelength of operation is 1064nm.
The detection unit used in the gauge at Timken is composed of a confocal Fabry-Perot for optical processing of the laser-ultrasonic signal. The detection unit also contains the optical pyrometer and the light-processing module of the laser-velocimeter. All signal beams, i.e., the backscattered detection laser beam for laser-ultrasonic measurement, the thermal emissivity signal for temperature measurement, and the laser-velocimeter signal, are transmitted from the inspection probe to the detection unit by optical fibers.
The control unit consists of a computer system to digitize the various signals, to process the signals and to display and store the data. The extensive use of computers in the gauge allows for easy operation of the gauge by plant personnel. Very little interaction between the gauge and the operator is needed. The gauge is activated with a single key. No operator interaction is required to setup the gauge. The data is automatically stored via TCPIP network and can be easily recalled.
The inspection probe consists of the optical components, located on the plant floor, near the hot tube, that focuses the generation laser beam and the laser-velocimeter beams onto the surface of the tube, and that collects all of the relevant light signals. As the inspection probe may be submitted to high temperature and vibrations, the probe was design to contain only passive optical components and very little electronic components (mainly for laser safety). The casing of the inspection probe was build to survive the harsh environment of a steel mill plant.
The inspection probe rests upon a mechanical structure, the laser light shield, through which passes the tube. The function of the laser-shield is to hold the probe above the tube and to provide laser light shielding. The section of the shield encircling the tube is sufficiently long to block scattered light, thus ensuring eye safe operation. The probe has its own cooling unit and its temperature is monitored. It also has an output window that is protected from water splash and fumes deposits by a strong airflow.
For each data acquisition location on the tube, time of flight of the ultrasonic compression wave, the temperature, and the location along the length of the tube are recorded. The data for a tube is stored in a single file. Additional production information, such as target values and standard deviation, are also displayed, giving plant personnel information to quickly evaluate each produced tube.
For more detailed analysis, the data can be viewed either in real-time or off-line on any computer linked to the data storage area with an application. The application can be used to view the same information as on the real-time display, or more detailed information such as average wall temperature, eccentricity, or average wall thickness.

Example of production use of system

Although there have been previous in-plant demonstrations of laser-ultrasonics for on-line tube gauging, the gauge is the first time a system has been continuously used in a production mode. The gauge has been in production operation since February 2002, and reached the 1,000,000th inspected tube in August 2004.
The system provides, in real time, wall thickness information over the whole tube length (tube profile) allowing adjustment of mill to get a product quickly within specifications. Since the information is comparable to that obtained with off-line conventional ultrasonic system, plant operator can use their previous knowledge to identify known problems from the profile data. For examples, worn or defective mechanical parts elements can be identify by observation of the displayed data.
The first profile (top) shows a case where high eccentricity was observed. After remedial action by plant personnel, the production was brought back into specifications as shown in the second profile. The gauge has allowed plant personnel to adjust rapidly production parameters and has therefore reduced the number of tubes needed to setup a production run.
The first profile (top) shows a case of bulging of the wall thickness in the center section of the tube. The second (bottom) profile shows the tube produced once remedial action had been taken. Without the gauge, conventional method of cutting the tubes endings and measuring them would not have detected the extent of the defect in produced tube.

Recent applications

After the success of the gauge at Timken, the gauge was introduced to the market in 2003 by Tecnar Automation Ltee (Tecnar), the sole licensee of the LUT technology patented by Timken and IMI. Since its introduction, tube makers from all over the world have shown an extremely big interest in the gauge.
Starting with the basic features of the first LUT installed at Timken, Tecnar has developed several configurations of the gauge to provide optimum performance of the gauge for different locations in a seamless tube. The gauge can be beneficially installed behind the piercing process (cross roll piercer or piercing-/extrusion press), behind the elongator (mandrel mill, push bench, cross roll elongator, plug mill or pilger mill), and/or behind the reducer or stretch reducer.
Recent orders for the gauge have been received by Tecnar for different locations in seamless tube mills: LUT-002 behind a free floating mandrel mill, LUT-003 behind a stretch reducer, LUT 004 behind the extracting mill in a retained mandrel mill, LUT 005 again behind a stretch reducer, LUT 006 behind the extracting mill of a 3-roll retained mandrel mill.
The LUT are built in different configurations to be optimum for the relevant application. The LUT has been designed with one or multiple measuring probes, with fixed probes or a mobile probe mounted on a robotic arm or on a scanning device. Tailor made gauge configurations are elaborated based on the tube making process and on the location of interest to guarantee maximum information about the tube dimension and characteristics. In particular, the LUT gauge can be coupled to an outer diameter gauge to provide complete dimensioning of each tube produced.

Conclusion

The team formed by The Timken Company, the Industrial Materials Institute of the National Research Council Canada and Tecnar Automation Ltee have developed and implemented a laser ultrasonic system to gauge on-line seamless tubes at high temperature. The gauge includes an optical pyrometer to measure tube temperature and a coordinate measuring device to determine the locations of each wall thickness and temperature measurements. The gauge provides thickness information all along the full length of the tube, unlike the conventional technique of cutting and measuring end sections. It contributes to increased productivity by the time saved. It allows very quickly and reliably better mill setups, thus reducing out-of-tolerance products (less scrap and rework) and troubleshooting time. The first implementation of the gauge has been in operation since February 2002, and has reached the level of 1,000,000 inspected tubes in August 2004. The gauge has demonstrated its reliability and usefulness and is leading to significant productivity increase. The LUT technology has been licensed to Tecnar Automation Ltee and made commercially available. Commercialization has resulted in orders for five additional LUT gauges with many more under discussion and consideration. Such a gauge, although specially designed for hot tubes thickness gauging, could also be used elsewhere in the steel and metals industry, and eventually in other industries.

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