Question on Welding

In my opinion, You must grind not only back but also surface area around welding zone ( approx. 2" ). It to clean out rust, oil, ...which will penetrate in welding bead to form cavity, slag (bad quality)...

Welding proces:

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When you need to apply a high- end photo weld, you need to have a uniform melting track. Both materials have to come to the melting points, together with your stick or electrode. When tack welding you have a cold start that has to be removed again, the weld is not uniform because the initial temperature and spark did not create a perfect melting bath. That appears only with long continued strikes. Start and end, whether mantle is enclosed or not do not comply with perfect welding and will be seen as cold or impure spots later.

Back grinding or chipping is done to remove slag on multiple pass welds. You do not want slag or any other impurities to be embeded in the weld.

Back grinding or chipping is done to remove slag on multiple pass welds. You do not want slag or any other impurities to be embeded in the weld.

Is technically correct but likely to give error signals.

The back grinding/ chipping is done in full penetration butt welds, that are welded from bot sides.

After atleast the root layer of the first side and some weld build up (usually we complete the first side weld), the other side is ground , gouged and ground (back gouging) , machined with milling cutter (to make a groove on the other side) etc.

This is to remove the defects in the root- the most problable area for slag entrapment (though that is not the case in TIG) , High level of oxidation (unless back purging is done), lack of fusion etc.

Once the defect is removed, the new weld layer is now no more open on the back (the side which is already welded) and thus is much less prone to defect.

While doing the back grinding/chipping/milling/gouging, care must be taken so that no notches are created, that itself can create the defect (lack of fusion, ...). Ensure relatively smooth and rounded profile of the groove.

Thanks for the replies! As you will often need a number of runs for a weld, do you usually back grind every pass to level it before the next run? And since the half bead temper bead welding technique back grind half of the root weld beads off, does it mean that literally all welds are half bead temper bead weldings because you usually do back grindings? Thanks!

Of course not, in this acse 90% of the time will be spent on grinding and 10% on welding, with the same ratio of metal removal Vs deposition.

Back grinding/ chipping is done only once (ref #5)

Back gouging / grinding is a process cutting a groove in the back side of a joint that has been welded. Back gouging can ensure 100% fusion at the root and remove discontinues of the root pass.

This process can also remove the root pass metal if the properties of the metal are not desirable to the finished weld.

After back gouging, the groove is then welded to get a sound metal.

a. Back gouging sequence for a weld to ensure 100% joint penetration on plates up to 6 mm

Step 1. Weld on one side Step.

Step 2. Gouge out from the other side.

Step 3. Final weld after gouging .

b. Back gouging sequence to remove all the weld metal for the root pass on plates beyond 8 mm.

Step 1. Finish the weld on one side.

Step 2. Gouge out from the other side.

Step 3. Final weld after gouging.

Sridhar.

But what would you do when the dimension or orientation is such that you will not be able to back grind and weld on the root (eg: welding 2 small NPS pipe together)? Then how to ensure that the root weld has no defects?

Thanks.

In cases, where it is not possible to back grind and weld on the root, the only option left out is to take more precautions and proper parameter selection, for right size of electrode / filler metal to ensure adequate root penetration, fusion and defect free welds in the initial stage itself.

Basically, back grinding/gouging is an optional/additional design requirement to ensure sound weld metal at the root and this does not mean that, welds done without back grinding /seal pass does not pass radiography or will not withstand pressure test.

For example, all pressure pipe joints right from 2 to 30 inches diameter cannot be back gouged and re-welded. Any repair seen at the root side on the film, are rectified by grinding from top/face side and welded from the same side only.

IT is always better to avoid repairs in the joints as far as possible, by employing skilled welders, right parameter & procedural selection, good positional (preferred in flat / 1G position) approach where ever it is possible.

This point is much more important, when welding Alloy (to avoid grain growth and hardened metal structure) & stainless steel pipes where more than 2 attempted repairs generally not encouraged.

Sridhar.