How to Fix Internal Suckback in Orbital Pipe Welding

Internal suckback or concavity is one of the most frustrating—and costly—defects in orbital pipe welding. Everything looks perfect during the weld, only to discover internal concavity right at the tie-in. In high-purity, code, or critical service applications, that’s a failed weld. ASME B31.3 states that there should be no excessive internal concavity for process piping. So if we see we must make some adjustments to get rid of it!

The good news: internal suck back is easily detectable—and fixable—once you understand what’s actually happening in the weld pool.

What Is Internal Suckback (Concavity)?

Internal suckback is an internal concavity that forms inside the ID of an orbital weld, typically this will rear its ugly head about half way through the weld or towards the end of the weld. This is because later in the weld as the puddle is getting its wettest and carying more heat the puddle is more susceptible to being either pulled up by excessive heat and slow travel speed or pushed up by excessive internal purge pressure.

It’s most common in:

• Autogenous (fusion) orbital welding

• Thin-wall stainless tubing and pipe

• High-purity applications (semiconductor, pharma, food-grade)

• Wire-fed pipe welding on smaller diameter pipes

Visually, it appears as:

• A “sunken” area on the inside diameter (ID)

• Loss of root reinforcement at the tie-in

• Sometimes accompanied by slight oxidation if shielding is not sufficient

Why It Happens

Suckback is fundamentally a weld pool collapse issue. Several factors combine to cause it:

1. Excessive Heat Input for Travel Speed

As a weld is getting hotter and the puddle is becoming more fluid it is more susceptible to being pulled upwards by a slow moving electrode that is dumping too much heat on it at a time.

2. Excessive Purge Pressure

One of the most common causes for internal concavity is too much purge pressure. If you notice the suckback starts about half way or further through your weld, often times you just need to being down your purge pressure. The internal purge when too high can push upwards too much on the puddle. When the puddle is very wet and carrying a lot of heat it becomes more fluid and thus more likely to be pushed upwards bu the purge pressure.

3. Insufficient Filler (Autogenous Welding)

Without filler metal, there’s no additional material to compensate for shrinkage during solidification. This makes autogenous welds especially prone to suckback but it can certainly still happen when feeding wire.

3. Poor Overlap Programming

If the suckback is happening at the very end of the weld the overlap zone is too short—or poorly timed—and the weld doesn’t properly fuse and reinforce the tie-in area.

5. Joint Fit-Up Variations

Even slight inconsistencies in:

• Hi-lo (misalignment)

• Gap

• Wall thickness

…can amplify shrinkage effects in the root pass and mimic the appearance and affect of suckback.

How to Fix Internal Suckback

Here are the most effective corrective actions, ranked by impact:

1. Decrease Purge Pressure

This is the #1 fix in most cases.

The simplest and quickest thing to change is reducing your purge pressure. The best way to measure and adjust the ID purge pressure incrementally is with a magnahelic gauge. Magnahelic gauges are very sensitive and accurate pressure measuring devices that can detect even slight pressure changes. GSI sells custom purge kits that integrate a magnahelic gauge and a very sensitive needle value to make very fine tune purge pressure adjustments.

Pro Tip:

Make small Incremental pressure adjustments. Moving too fast can result in external concavity and you can loose your place so to speak as to how much adjustment to make if you move too quickly making large adjustments.

2. Increase Overlap Length

If the concavity is only at the overlap at the end of the root pass you only need to adjust the overlap. This can be done on our Axxair and Magnatech orbital welding power supplies. A longer overlap gives the weld more time to:

• Reheat the start point

• Build reinforcement at the tie-in

Typical adjustment:

• Increase overlap by 5–15% of circumference

Be careful not to overdo it, or you may introduce excessive reinforcement or heat buildup.

4. Add Travel Speed

Slowing travel speed slightly during the final segment:

• Increases local heat input

• Allows better fusion and reinforcement at the tie-in

This is especially effective on thin-wall tubing.

5. Increase Background Amp Pulse Time

We always use a pulsed GTAW process for root pass welding for orbital welding applications. This is for many reasons but mainly it is used for precise heat input control.

If you are seeing internal suckback and your purge pressure is good and you’re travel speed is as low as you want it to be then you can start increasing the pulse time for your background amps. Background amps are intended to carry the arc whilst allowing a short break from the hotter more intense primary amp. If we increase background pulse time we effectively reduce overall heat and reducing heat without affecting penetration too much and thus is an excellent way to combat internal concavity due to excessive heat input.

6. Evaluate Fit-Up Consistency

If suckback only happens intermittently, check:

• Pipe alignment (hi-lo)

• Gap variation

• Ovality

Even a well-tuned program can’t compensate for poor fit-up. Remember orbital welding is program based and therefore cannot detect changes in fitup. Therefore our fit up accuracy is critical in orbital welding. That’s why GSI recommends using our Axxair orbital cutting saws and facing tools to ensure your pipe or tube is properly prepped for a well-aligned joint.

Common Mistakes to Avoid

• Overcorrecting with excessive heat

  
  

  → Leads to burn-through or makes the problem worse

• Ignoring overlap settings

  
  

  → Many operators focus only on amperage and forget timing/position

• Blaming gas when it’s really programming

  
  

  → Most suckback issues are heat profile problems, not shielding problems

• Using one program for all fit-ups

  
  

  → Variability in prep requires parameter flexibility

Make one change at a time and document results. Make incremental changes so that you don’t get “lost” in the program, this takes more patience but is less likely to lead to frustration and confusion.

For more tips and advice reach out to GSI Machineey. We offer rentals, sales, training and machine repairs for orbital welding cutting and facing equipment out of Houston Texas. If you want to see a live welding demonstration reach out to us to schedule a time to come see our “Center for Automation Excellence” in Houston Texas where we will be glad to show you our solutions.