We all know that the preparation of the tubes before welding is essential. Thanks to a good preparation you can achieve very good quality with a minimum of scrap loss. We strongly advise you to master all the steps of the welding process, and especially the fit up and tacking of the workpieces.
This is an important step; so do not neglect it in order to achieve best quality welds.
#1 Necessity of the fit up whilst holding the pieces
Most of the time you have to present two workpieces that have to be welded, for example a tube and a T-piece. You will have to fit up both pieces. You will easily understand that the two diameters and the thicknesses must match and must be perfectly rounded. But this is not the case most of the time and a compromise must be found.
This is the step that we call fit-up/tacking. The degree of difficulty varies depending on if you prepare the pieces in a workshop or on site.
It is easy to understand that there are high tensions on important lengths or complex geometries of the workpieces that you wish to assemble. So it is very important to properly fit up the pieces and proceed to a tack welding; the clamping of welding machines being sometimes not enough to correct this kind of constraints.
#2 Manual fit-up is usual
The fit-up operation of the two edges is usually done manually, so the experience and the dexterity of the pipe fitter is extremely important. This means that the repeatability cannot be guaranteed and finally it is the welding operation that has to compensate the defaults. If the difference is too important, even the most expensive and performing welding equipment cannot compensate enough.
Sometimes people are tempted to invest a lot of money in welding equipment, but this is not sufficient if the upstream process is not mastered.
In most of the cases the fit-up can only be realized by the outer diameter with the exception of gas or petrol pipelines where the centring concerns mostly tube to tube junctions and where it is possible to centre the piece from the inside diameter (with centering mandrels for example)
There are just a few centering machines or accessories on the market with limited possibilities as it is really difficult to position an elbow on a tube by means of a machine. The hands and the eye of the operator remain the central skills.
In the case of manual welding this is acceptable as it is precise enough, otherwise the manual welder will compensate the fault. In the case of automatic welding, the fit-up has to be precise in order to get the best result whilst welding.
#3 The importance of shielding gases
Tack welding has to be carried out by mastering shielding gases of the inner and outer diameter, using the settings of pre-gas and backing gas, which will restrict the effect of corrosion during the tacking operation.
In the case of orbital welding, a tacking with corrosion will induce deviations of the welding seam. In fact, the weld puddle will avoid this tacking point, as it is easier to liquefy a non-corroded material. The welding machine cannot compensate this effect at that stage.
These are the consequences of a bad protection level at this stage:
- possible leakage at the point where the welding puddle has been deviated
- low penetration as the thicknesses are higher than expected
This list might look pessimistic but if you want to achieve excellent welding results you will have to take care of all the stages of the preparation.
Even very high-performance welding equipment cannot compensate a bad tube preparation before welding.
It is very important to take care of the fit-up and tack welding if you want to achieve a high quality weld!
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