NBA 625 is a nickel-based alloy with an incredible resistance to corrosion – as long as its technological composition is preserved. The iron content especially may have a negative effect on the material properties. It is therefore a difficult endeavour to apply an NBA 625 layer to a carbonic steel using a thermal process without mixing the various materials and maintaining the purity of the surface finish. The layers are mechanically welded in large parts, either using a MAG process and a deposition rate of 5 kilograms per hour, or using a TIG process with a deposition rate of 2.5 kilograms per hour.
The aim is to achieve the highest possible deposition rate with the lowest level of dilution. Although this seemed strange to EWM application engineers to begin with, the MAG welding process at ITAG is combined with additional wire feeding using the tigSpeed hot wire feeder. The idea behind this was to transfer more arc energy to the welding consumable. This meant that less base material was melted which is, in turn, helpful for maintaining the purity of the surface finish. As a result, both the feed and deposition rate could be doubled in one go.
Deposition rates from up to 13.8 kg per hour are more than double those of MAG welding and quadruple those of TIG hot wire welding. What’s more, the tightly-packed weld beads displayed only very little rippling (< 0.5 millimetres) thanks to extremely high consistency in the wire feeding and welding parameters. As a result, welds on the surface layer only needed to be carried out approximately 1 millimetre higher than the specified height which would then be reached through machining. This allowed ITAG to save both time and a large amount of expensive materials.