Published on 02 March 2012
The client wanted to find the best welding process for joining stainless steel box section with minimal preparation and set-up time. With the same finished quality as their existing supplier. This request was received by TWI from Mech Tool, a Darlington based fabrication company with a history in the fabrication of blast shields and similar fabrications for the oil and gas sector.
Mech Tool approached TWI for help with support from the REMTEC funding system. Currently the company buys in box section from mainland Europe. But it's anxious to cut costs and improve the quality control of its products.
After discussions between TWI and Mech Tool a number of alternative methods were short- listed for investigation. The selected welding process was to be mounted on a travelling carriage above the work piece. This would allow high levels of control over welding torch positioning and travel speed. Since this eliminated manual work the frequency of stop-starts would also be reduced as well as the chances of rejection at inspection after completion.
Using robust jigging the box section needed to be clamped into position beneath the carriage. This would reduce set up times and increase repeatability. To allow an accurate comparison test samples were produced using several arc processes.
Activated Tungsten Inert Gas (ATIG) welding could be used to achieve deep penetration welds up to 8-10mm. This allowed the use of existing welding equipment with a flux added to the joint before welding. The process was however rejected because the regularity of flux application could not be assured, and also because the cost of available fluxes was prohibitive.
Plasma welding allowed full penetration welds with high welding speeds to be achieved. The weld cap produced also dictated that little post-weld treatment was needed to remove excess weld metal. As the weld samples produced proved to meet all demands the process was compared to MIG/MAG welding.
MIG/MAG welds were trialled with different weld preparations. These included open and closed corner welds and butt welds. Using this process Mech Tool would be able to use its existing equipment. Welding could take place in a short period after the jig had been produced.
Using TWI's advice Mech Tool has subsequently bought the material and equipment needed to produce their own box section.
For further information read about services in Construction and Fabrication at TWI.