Best practice for GTAW (TIG welding) of pipe joints

At Technoweld, we are often approached about the best practice for welding, in particular, GTAW (TIG welding) of a pipe.

We’ve compiled some best practices around pipe joints. This blog is important for those completing the welding tickets as it is specifically required to be completed as part of AS1796 Welders Certification Certificate 7.

The material used for this discussion is A106 Grade B, 100NB Sch40 pipe (WT 6mm).

The joint setup used is:

• 60 degree included angle,
• 1.5-2mm root face
• 3.2mm root gap

Everyone prefers to set their pipe joints up differently. We recommend this setup as best practice for the reasons outlined below:

• The bigger landing closes up the prep so there is less to fill and makes completion of the joint much faster.
• A more consistent weld is achieved by applying a root face. If the edge is feathered, it burns away when you arc up and turns a 3.2 into a 5 or 6mm gap
• Your wire must have the ability to penetrate the joint in order to prevent suck back. We recommend using a 2.4mm wire which requires a 3.2mm gap.

 

Preparation

It is all about the preparation. Ensure the inside of the pipe has been cleaned with a die grinder to reduce internal undercut, clean it back about 5mm. Ensure the root face is consistent.

 

 

 

Bend a piece of 3.2mm wire in half to use as a spacer for the gap. As required by AS1796, tack in three places, 120 degrees apart. Ensure the bore is not misaligned. In production, we recommend using four tacks rather than three. The reason for this is that you then have complete control over joint setup and deal with one axis at a time.

 

 

 

 

Root tacks are always the preference rather than bridge or bullet tacks with GTAW. Using root tacks makes it much quicker than having to stop and cut tacks out, like you would with bridge or bullet tacks. Tacks do not need to be feathered for those who are highly competent at GTAW, but for the purpose of the welding ticket test, feel free to feather your tacks.

 

5G test

Set the test piece up so the tacks straddle the centre line at the bottom and weld from one to the other. The bottom tack, which is the starting position, should be at about the 5 o’clock position for a right hander. This gives you more time on your preferred side and minimal time on your less-preferred side.

 

 

 

Talking about your dominant side, always weld the less-preferred side first, in a 6G whilst it’s at the coldest. This makes it a little easier to weld.

There is a trick to stopping the bottom suck-back, there are two ways to do this:

• Allow for gravity
• Surface tension effect.

 

Allow for gravity

Add the wire above the root so when it sucks back, it ends up flat. Basically, allow for gravity.

 

Surface tension effect

The second option is about maintaining the surface tension of the weld pool. Essentially, you get suck back every time you pull the wire out of the weld pool. This is the surface tension effect.

Rather than pulling it out, slowly maintain the amount of wire in the pool. Avoid overfeeding the wire, just concentrate on maintaining the surface tension of the weld pool.

We have found this works very well. Put simply, do not pull the wire out and it will not suck back. Simply maintain the wire length. It is difficult to get excess wire underneath, so don’t be shy with the wire. You will need pretty high amps to achieve this method successfuly, we recommend about 120-160 amps to start even hotter.

To prevent unburnt wire, ensure your wire angle is shallow, not steep. It is impossible to have unburnt wire if your wire is at the right angle, it will simply push out of the pipe instead of into it.

 

 

 

 

 

Remember that the weld pool is a liquid and gravity will affect it. As you weld up the sides, add the wire in the middle of the root rather than behind it. When you reach the top, add it slightly higher so when it falls in you do not get excess pen.

For the most part, welding is just logical thinking and practice

For your fill pass, crank it up and fill it to 1mm below the top of the material. Before capping, buff the pipe, this lets you move faster and stops the undercut at the toes of the weld. Make sure it is full enough to ensure you do not get any underfill, particularly on the bottom.

When starting your runs, we recommend a right-hander to start about 4.30 – 5 o’clock. This makes the non-dominant side slightly easier.

Additionally, weld from the bottom and then go to the non-dominant side while the pipe is still fairly cool. Do your preferred side while the pipe is at it’s hottest. This applies to all runs.

We recommend avoiding walking the dog where possible. Freehand weld is the best method to use. The reason for this is that it is impractical to walk the dog in a pipe rack, for example, whilst crammed against other pipes.

 

2G test

It is hard to get excess on the root, it is much the same as the bottom of the pipe in 5G. Follow the same techniques as described for the fill and cap at it at the top and pull it down with the arc.

The fill should aim to be full at the top of the prep and 1mm away from the bottom. Allow room for the gravity effect on the cap run.

Some recommend completing the cap in two runs but we prefer one run for brevity and efficiency.

The secret to the weave is the weave pattern. Do not weave up and down, the trick is to weave backwards and diagonal, like this: ///////////.

The liquid at the top has a solid base (shelf) at the bottom to sit on. If you weave in a vertical motion (| | | | | |), the whole vertical plane will be liquid and fall down into a heap.

 

 

Other useful tips

Do not forget to practice the flame cut of the bevel and the arc air gouge.

Pace yourself and do some timed practice tests the night before your test.

Sharpen a heap of tungstens so you do not waste any time. Make sure they are sharpened correctly. Generally what is taught during your apprenticeship is based on welding thin materials. In practice, we rarely ever sharpen an electrode to 2 to 3 times the diameter. Normally 1.5 times the diameter is sufficient. The steeper the angle the better the penetration.

 

Remember that you can cut out any defects and errors. You can also even out the cap. You cannot, however, dress the final weld. So, even if you make an error with the cap, carefully grind it out and reweld. As long as you do not dress the final weld, you are within the requirements of AS1796 and most welder qualification test standards around the world.

Finally, our best advice, do not stress; relax and let it flow.

If you found this advice beneficial, please let us know! If you wish to further your welder training, we offer welding inspector and supervisor training. Get in touch to book your spot!