TPT July 2008

orientation with conventional ultrasonics is very difficult to sustain on moving or bouncing materials, such as tubes under production line conditions.

3. Online ultrasonic wall thickness measurement with laser-ultrasonics

 Figure 2 : On-line wall thickness measurement using LUT gauge

The above limitations are eliminated by laser-ultrasonics. In laser- ultrasonics, laser light is used for generation and detection, at a distance, of the ultrasonic pulse. Since no mechanical contact is made between the sensor and the tube, the temperature of the tube does not affect the performance of the sensor. Furthermore, in laser-ultrasonics the source of ultrasonic generation is located directly on the surface of the tube. The detection of ultrasounds is made by monitoring the motion of the same surface at the same location. In effect, the surface of the tube becomes the sensor in laser- ultrasonics. It can be shown that the orientation of the laser light with respect to the normal direction to the surface of the material does not affect the generation and detection of the ultrasonic signal. Hence, no particular orientation between the laser-ultrasonic sensor and the surface of the inspected material is needed. This allows laser-ultrasonic inspection to be insensitive to the bouncing of the tube. Laser-ultrasonics is a combination of two separate laser methods: laser generation of ultrasounds and laser detection of ultrasounds. Laser generation of ultrasounds is a technique that has been in use since the early development of pulsed lasers [2] . Efficient laser generation of ultrasounds on metal is performed by applying a strong laser pulse onto the surface of the material, which causes ablation or vaporization of a small quantity of the material at the point of impact of the laser. Following the ablation, a recoil force is produced, which is the source of a compression ultrasonic pulse. The compression pulse is always launched in a direction normal to the free surface of the tube (ie the surface of impact of the laser beam) independently of the angle of incidence of the laser light. Hence, travel-time of the ultrasonic pulse is always recorded in the direction normal to the surface, giving the correct wall thickness. It should be recognised that for high temperature steel, the blown- off material of the ablation is the oxide covering the surface. The oxide is rapidly regenerated at these elevated temperatures and therefore no visible mark is seen once the material is cooled to an ambient temperature. Laser detection of ultrasound is based on frequency-demodulation by an optical interferometer of the laser light reflected or backscattered from the surface of the material [3] . A laser light is focused on (or near) the point of impact of the generation laser beam on the surface of the material. Any surface motion at the point of impact of the detection laser is recorded in the reflected light as an optical frequency variation (slight change of ‘colour’). The ultrasonic surface displacement is therefore ‘encoded’ in the laser light, called the signal beam. After ‘extraction’ from the signal beam, the resulting information is identical to a conventional ultrasonics waveform, often called A-scan in non-destructive inspection, as shown in figure 1. The

signal contains a strong initial pulse, called the ‘surface signal’, corresponding to the initial impact of the generation laser beam on the surface of the tube, followed by several pulses, which are the echoes resulting from the forward propagation and reflection from the back wall of the initial ultrasonic pulse. The travel-time of the ultrasonic pulse within the bulk of the tube is obtained by measuring the time between the initial surface impact and the time of arrival of the first echo. Numerical signal enhancement methods are used to get a high accuracy measurement of the travel-time. The values recorded are then scaled with the appropriate velocity of sound for the material of the tube and corrected for thermal expansion. It should be recognised that since velocity of sound in a material is a function of temperature, the temperature of the tube must also be recorded simultaneously with the measurement of the laser-ultrasonic signal. The LUT gauge makes this simultaneous measurement using an optical pyrometer. The temperature is measured at the same location as the laser-ultrasonic signal. The scaling value, used to convert travel-time to thickness, is adjusted using the temperature measurement and the steel alloy category of the produced tube. The heart of a laser-ultrasonic system is the optical interferometer. Several optical interferometers are available, each with their own advantages and limitations. Tecnar Automation Ltee (Tecnar) is a world leader in the development and manufacture of laser- ultrasonics equipment, including laser-ultrasonic demodulators. Following developments made at Tecnar, an active interferometer approach has been developed, based on two-wave beam mixing [4] , specifically for online wall thickness measurement of hot tubes in the harsh operation of steel mill. By taking wall thickness measurement as function of the length of the tube, wall thickness length-profiles are obtained, as shown in the figure below. The ‘signature’ of the wall thickness length-profile is identical to the length-profile obtained with a conventional UT system for tube, with the exception that the data shown in the figure was obtained online and displayed in real-time. The shape of the length-profile is used by operators to identify the source of ‘out of specifications’ production and appropriate correction measures can be taken. The length-profile shown in figure 2 is typical of a tube with large eccentricity. The rapid variation (cycle) of the wall thickness comes for the detection of minimum and maximum wall thicknesses of the tube as it is rotated before the LUT probe. 4. The LUT (laser ultrasonic thickness) gauge The LUT gauge is the latest implementation of laser-ultrasonic technology for online wall thickness measurement of hot tube. The LUT includes the most advanced laser-ultrasonic technology

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