Valk Welding: 2002

Together with its Belgian partner Victor Buyck, the construction company Hollandia in Krimpen aan den IJssel received an order from Swiss Reinsurance Company UK , to construct the frame for the Swiss Re-Tower. Hollandia and Victor Buyck will each do half of the job.

For this project, Hollandia had to solve a number of problems. The material to be welded S 355 J2G3, 40 to 60 mm thick, with a Ceq (carbon equivalent) of 0.43 to 0.45 %, causes the risk of cold ripping. Pre-heating was essential. The mass of the workpieces, 3000 kg to 5000 kg per knot further caused manipulation problems. The required a-height of 7 mm is another problem. In order to achieve this a-height, welding would have to be done in two or three strings. Hollandia chose the first solution and the highest level of manual welding, whereby the required a-height was obtained by welding in three strings The chance of cold ripping was minimised by pre-heating. Manual welding took about 22 hours per knot. Welding was done with filled wire with a diameter of 1.2 mm.

In view of their size, the smaller knots of the following levels could be welded by robot. Hollandia already has a robot installation, without however using it for various reasons. For the Swiss Re-Tower project, the robot might be an alternative for manual welding. Together with supplier Valk Welding from Alblasserdam the problem of adjusting the existing robot installation to the current production requirements of Hollandia was tackled. It was decided to use two freely programmable external axes and to replace the old 350 A power source by a new Panasonic 500 A Dip Pulse power source.

For the revised robot installation, a number of other problems had to be solved. The treatment of heavy knots seemed to be a problem. Finally, it was decided to make the knots in two steps, thus also largely improving the accessibility of the workpiece.

Because the robot had not been used for some time, Hollandia's programming know-how had somewhat disappeared. After consultation with Valk Welding, it was decided to let Valk Welding do the programming for the first 'level'. These programming activities took a week to complete. The following 'levels' were programmed by Hollandia itself by adjusting the existing. This was done with a combination of off-line and on-line programming techniques. The adjustment of the basic programme took just one and a half days.

Solid wire with high heat-input prevents cold ripping. With this robot, there was again the chance of cold ripping. That is why they chose to weld with a solid wire (G3Si1) of 1.6 mm and high heat-input. Due to the high heat-input (25.8 Kj/cm) there was no need for pre-heating, and the production increase became a fact. With parameters of 410 A and 33 V, a welding speed of 33 cm/min and a welding duty of 80% could be obtained now.

Another advantage was the deep burning-in that could be realised with the solid wire, due to which the a-height could be reduced externally from 7 to 6 mm (aeff = 12 mm) and the number of strings per fillet weld could be reduced to one. Procedure welding proved that the heat-input of 25.8 Kj/cm for a fillet weld with one layer was sufficient. The limit value for hardness (HV10) for a one layer fillet weld according to EN288-3 is 380; tests have shown that a value of 333 HV10 was reached as maximum hardness. With a lower heat-input, the hardness would increase, as well as the risk for cold ripping.

The robotised welding of solid wire of Ø1.6 mm, with an amperage of 410 A and a heat-input of 25, 8 KJ/cm, seemed the best compromise to get a good welding result with high productivity and favourable working conditions. Moreover, due to deeper root penetration, the customer allowed a lower a-height: 6 in stead of 7 mm. As the quantity of melted welding metal is in proportion to the square of the a-height, a saving of (49-36)/49 = 0.265 or almost 27% could be achieved.

Cheaper and higher welding duty. All in all the chosen welding process offers different possibilities to cost savings. Not only the welding volume is lower, but solid wire is also cheaper than filled wire In addition, the lower welding volume means saving on material but also gain of time; the process duration decreases. But the most important economic advantage is of course that even with 'high risk' materials, a robot can be employed.

The welding duty of the robot at Hollandia is minimum 80%. When welding one knot, without the effects of product exchange, the precentage goes up to just under 90%. For manual welding, the welding duty is at about 20% mainly due to the many manipulations. Moreover, manual welding with amperages of about 400 A is no longer justifiable from a healthy and safety point of view. For Hollandia, the employment of the robot lead to a productivity increase of 500%!

(Source: Metaal & Kunststof no. 10 - 24 June 2002)