TPi January 2011

pass, most of the energy in the laser beam and the assist gas are used to cut material originally contacted by the beam. Only laser energy and gas which have passed through the upper section of the cut are available to address the lower section and this is cut less effectively. For the second pass, a kerf has been previously opened in the top section and now the majority of the laser energy passes through this and acts more effectively on the lower section of the tube. As an example of performance, Figure 2 shows the dependence of the maximum cutting speed at which the tube is severed as a function of laser power. The results shown are for a tube of 155mm diameter and 1.5mm wall thickness, for two pass cutting, but similar trends were observed for other diameters and walls. Note that the cutting speed is fairly linear with applied power, at least up to 5kW. The optimum assist gas pressure appeared to be about 8bar.

Figure 6: Cutting demonstrator, before and after cutting

Figure 3 shows cut sections from 60mm diameter tube, with wall thicknesses from 1.5 to 11mm, again for two pass cutting. Process parameters can be found on the figure caption. In this series of cuts, the only parameter to be varied was the process speed, which was ten times faster on the 1.5mm wall, compared to the 11mm wall. However, it should be noted that all the different wall thicknesses could also be cut at a speed of 100mm/min, thereby affecting cutting of all these tubes without any change at all in the process parameters. The largest tube to be cut in this work had a diameter of 170mm, with a 7mm wall thickness (Figure 4). This is not believed to be the limit for the laser in use. To go above this diameter, a shorter nozzle assembly would be needed. Using a three pass technique, this tube was severed in a time of 7min, using 4.8kW of laser power. Another possibility demonstrated was the cutting of concentric tubes, for example, a 25mm diameter tube located inside a 60mm diameter tube. In this case, a two pass technique was effective in severing both tubes at once (Figure 5).

In order to simulate a possible arrangement of different diameter tubes in different locations and with different packing density, the assembly of tubes shown on the left of Figure 6 was constructed. Over 50 cuts were employed to demolish this assembly of tubes ranging from 25 to 155mm in diameter, including tube severance, fixture severance and hole cutting (in the larger diameter tubes). The assembly was reduced to the state shown on the right of Figure 6 in one continuous cutting sequence lasting just over 15 minutes. Conclusions A very effective and efficient system for cutting of stainless steel pipes and other fixtures/fittings has been developed. The cutting head is both lightweight and has a significant stand-off tolerance and so is relatively simple to remotely deploy and operate. Although this work was commissioned by the NDA, primarily with a view to nuclear decommissioning, the techniques developed could be useful in a wide range of industries for cutting up pipework and vessels during demolition. Acknowledgements The authors are grateful to the Nuclear Decommissioning Authority for funding the work reported in this article and for giving permission for its publication.

Figure 5: Cutting of concentric tubes, in this case a 25mm tube inside a 60mm diameter tube

TWI Ltd – UK paul.hilton@twi.co.uk ali.khan@twi.co.uk www.twi.co.uk

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Tube Products International January 2011

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