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Thermal eye watches weld quality

   
 
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A steel maker and a research centre have developed an automated system for monitoring the quality of welds in industrial seam-welding. An optical pyrometer senses the temperature of the weld as it emerges from the machine. The weld is acceptable if the average and standard deviation of the temperatures along its length both fall within set values. Studies show excellent correlation with traditional inspection techniques. The system can also detect any gradual deterioration in the weld quality, indicating the need for preventative maintenance of the welding machine.

When done correctly, welding is a strong and reliable means of joining two pieces of metal and is widely used in manufacturing industry. However, strict quality control is essential if welds are to made to a high and consistent standard.
Traditionally, a weld is tested by visual inspection and by tapping it with a hammer to check that the two metal edges have fused properly. Though effective, these tests are qualitative and partly subjective. While it is possible to monitor automatically various process parameters such as electric current, pressure, speed, and so on, none of these factors are directly related to the quality of the weld after it is formed.
The Steel Research Project started when Belgian steel-maker Cockerill Sambre approached CRM, a metallurgical research centre in Liège, for advice on how to avoid breakages in their annealing line at Kessales. Steel strip is seam-welded together in lengths of about one metre in a continuous and automated process. Faulty welds require the process to be halted, and in some cases the plant may be shut down for up to 40 hours. Cockerill Sambre wanted a fast and objective method for assessing weld quality and also a means of monitoring the condition of the welding machine to warn of impending trouble before faulty welds started to appear.

Thermal imaging

A good weld requires the metal to be heated uniformly to melting temperature, so CRM examined the possibility of monitoring the temperature of the weld as it emerges from the machine. At the temperatures involved, 600C to 1000C, the weld would emit strong infrared radiation, so CRM decided to use an infrared camera to make a thermal image of the newly made weld.
The camera is mounted to view the weld at a point 50 mm after the melting position. It scans a strip 0.5 mm wide and 20 mm long across the width of the weld. As the metal passes out of the machine, the camera records many such 'profiles', building up a two-dimensional contour map. The brightness of the infrared emission is directly related to the temperature, so CRM was able to construct a 'thermal map' showing how the temperature varies across and along the weld. A typical map has about 300 profiles for a one-metre length of weld. The next step was to use the map to assess the quality of the process.
CRM had to establish that the temperature recorded on the map did indeed correspond to the strength of the weld. They made 150 sample welds, of varying strengths, and submitted them to the standard Erichsen test, in which a welded plate is squeezed into a cup until it breaks. The tests showed that the temperature measured by the camera was a good indicator of the strength of the weld.
Next, they compared the maps for 4,000 welds with the results of traditional visual inspection and hammer tests. They found that the thermal map for each weld could be characterised in a simple way. First, the maximum temperatures of each profile are averaged over the length of the weld to produce an average temperature for the whole weld. Second, the variation of the maximum temperature from profile to profile, defined by the standard deviation, is calculated. A weld is deemed acceptable if its average temperature and the standard deviation of the temperature fall within specified limits.
The computer keeps a record of the last 2,000 welds and alerts the operator if there is any drift in weld temperature which may indicate that the process is deteriorating. Appropriate maintenance can then take place before the quality becomes unacceptable.
Although the system worked well, there were several practical problems that needed to be overcome. The camera was bulky and cumbersome and needed mirrors to view the weld without obstructing access for maintenance. The scanning mechanism used rotating prisms which required regular attention, and the optical window became contaminated with smoke from the welding process and needed frequent cleaning. Being so close to the welding point, the camera suffered from electrical interference. It turned out to be an unnecessarily complicated and very expensive solution to the problem.

Pyrometer solution

When the original project finished in 1992, CRM sought further EU funding to come up with a cheaper and more rugged version of the system that could be developed into a marketable product. The new equipment, which has been under development since 1994, uses an optical pyrometer instead of a camera.
A pyrometer is a simple instrument that measures temperature by directly sensing the amount of thermal radiation from an object. Instead of a camera, a lens focuses radiation on to a 3-metre optical fibre which transmits it to an infrared detector housed some distance away in a strong cabinet. The signal from the detector is sent by another optical link to the computer controlling the welding process. The process operator is able to view the temperature 'signature' of each weld and observe how it compares with the acceptance criteria, which are the same as for the infrared camera system.
Unlike the camera, the pyrometer does not scan across the weld to make a profile, but receives radiation from a fixed circular spot 10 mm across. This means that an average temperature across the weld is sensed, rather than a detailed profile. One concern was that the system would be vulnerable to imprecise positioning because the electrodes on the welding machine are changed every week and this alters slightly the position of the centre line of the weld. However, this does not seem to be a problem in practice.
Not only is the pyrometer cheaper than the camera, but it is less intrusive, the optical fibres are not susceptible to electrical interference, and the whole system is simpler to install and easier to maintain.
The partners are about to start work on a similar system for use on a tinning line. Here the metal is so thin (0.1-0.3 mm) that the hammer test would damage it. The lower temperatures, 350-500C, require a different kind of detector in the pyrometer, but essentially the system will work in the same way.
Several enquiries about the system have come from interested companies, but CRM, being a research centre, is not in the manufacturing business. Instead, it intends to licence the system to the Belgian company IRM for commercial manufacture. Potential customers include not only steel makers, but a wide range of firms in the manufacturing industry where quality control of welding is important.

 

Project Title:  
Determination of the quality of strip-to-strip welds in continuous processes

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: ECSC 7210-ZZ-610

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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