TPT July 2008

New strategies for wall thickness measurement in the hot seamless tube production plant Holger Gurski-Schramm, Ingenieurbüro Gurski-Schramm & Partner, Germany and Dr Marc Choquet, Tecnar Automation Ltee, Canada

1. Introduction The competitive nature of the current world economy has placed new emphasis on productivity, quality control and energy consumption of production facilities. Market demands have placed added pressure on manufacturers to better control production and minimize fabrication and rejection of out-of-specifications products. In particular, the tube steel industry has felt such market pressure. Production efficiency of high quality product such as mechanical tubing has been constantly increasing in the past few years. Low volume productions per batch have also increased the demands on production teams to rapidly achieve final specifications of products with low numbers of ‘test’ units. Sensors have been an integral part of this process. Sensors placed on the production line allow the plant operator to control the production parameters as well as quickly react to unexpected situations. A large part of the increase in productivity is the result of an extensive use of sensors. Mechanical tubes are used in applications such as hydraulic cylinders and power transmission components (gears and bearing), which place strict controls on mechanical dimension. Wall thickness sensors have been used routinely to control the production of mechanical tube. Until recently, however, the availability for online wall thickness sensors has been limited. Penetrating radiation ( γ -rays) techniques have been developed and used for thickness gauging of tubes. However, such techniques have limitations on the location where they can be installed. Penetrating radiation gauges cannot measure wall thickness with a mandrel inside or adapt easily to rapid side-motion of the tube. In addition, there is a limit to the range of wall thickness and outer diameter sizes that a penetration radiation system can measure. Laser-ultrasonics, which combines the precision of ultrasonics with the flexibility of optical systems, has provided an advanced method

to measure online wall thickness under plant conditions [1] . With laser-ultrasonics, the presence of a mandrel does not affect the wall thickness measurement. In addition, since the laser-generated probing pulse is always launched in a direction normal to the surface, large tube motion cannot be tolerated without affecting the accuracy of the wall thickness measurement. Finally, the size of the outer diameter of a tube does not impose any limitations on the ability to measure. The flexibility of laser-ultrasonic allows for wall thickness measurement at the output of processing tools. It therefore permits ‘real-time’ data for automated feedback control on location, which was not possible in the past. 2. Online ultrasonic wall thickness measurement with ultrasonics Standard ultrasonics inspection is a renowned non-destructive technique that provides several parameters of interest for materials and process control. Ultrasonic wall thickness gauges are used in several industries, such as aircraft inspection and metallic thickness gauging, because it provides high accuracy measurements. Minute changes in wall thickness are easily detected and quantitatively measured. Conventional ultrasonics (UT) utilise a piezoelectric transducer (PZT) to generate and detect the sound waves used to probe the material. A PZT, stimulated with an appropriate electrical signal, will impact the outer surface of the tube to which it is attached. The resulting pulse (ultrasonic pulse) will then travel to the inner wall of the tube, where it will be reflected back towards the outer surface. The reflected signal is called the echo. Measurement of the travel time of the probing pulse directly provides the thickness of the tube, based on the velocity of sound in the alloy of the tube (which is a physical property of the alloy). Conventional UT requires a good mechanical contact between the PZT and the inspected tube to be able to have a measurable signal. Such a method is therefore difficult and often impossible to use when the tube is at high temperature or moving rapidly, such as encountered in a steel mill production line. Commercial UT systems are available for ‘offline’ tube dimensioning, but require a wait-period for product cooling, which may take several hours. Non-contact ultrasonic sensors are needed for inspection of high-temperature moving materials. Conventional UT also requires a strict orientation of the PZT with respect to the surface of the material, in order to get a strong signal into the bulk of the material and achieve true wall thickness. This involves measurement of the travel-time solely in the direction normal to the surface. Angular tolerance for proper operation is only about a few degrees. Any deviation from the angular tolerance results in a rapid decrease of signal amplitude. Strict angular

 Figure 1 : Typical on-line laser-ultrasonic signal from hot tube (WT=15mm)

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J uly 2008

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