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Better preps, better pipe welds

Need to save time and money? Start with good weld prep

In pipe welding applications, proper weld preparation helps prevent problems such as weld inclusions, slag entrapments, hydrogen cracking, lack of fusion, and lack of penetration.

No matter the welding process being used, proper preparation before you get started is key to ensuring quality in the finished weld. Taking the necessary steps to prepare the weld also can reduce the risk of weld failure as well as wasting time and money on rework and consumables.

Proper weld preparation in pipe welding helps prevent problems such as weld inclusions, slag entrapments, hydrogen cracking, lack of fusion, and lack of penetration. Consider the following key points for cleaning and preparing the weld joint and avoiding some common mistakes to achieve success in pipe welding.

Cleaning and Prep

Joint preparation and cleaning go hand-in-hand. Which happens first depends on the state in which the pipe is received. Some welding operators, especially on outdoor job sites, may be responsible for cutting the pipe and beveling edges. But in some applications, often performed in pipe shops, the cutting and beveling are handled by someone else before the welder receives the pipe.

Proper joint preparation—and whether it’s beveled, grooved, or notched—is often dictated by the qualified weld procedure, which should ensure access to the joint and proper penetration and weld strength for the application. Once the pipe is cut using an oxyfuel torch, plasma cutter, cutting machine, or other tool, and the bevel is established with a grinder or by machining, be sure to clean the inside and outside of the pipe joint and the bevel.

If the pipe was cut with a machine, it’s likely a lubricant was used, so be sure to remove it during cleaning to reduce the risk of hydrogen inclusions. Cutting with an oxyfuel torch or plasma cutter typically leaves a slag or oxide layer on the cut edge. Be sure to clean this to prevent inclusions and porosity.

Remove any paint, oils, and dirt on the base material before welding; otherwise, these materials could make their way into the weld and cause inclusions or porosity that could harm weld integrity and cause it to fail. Clean the area 1 to 2 inches from the weld joint and the tie-in points, where the lacquer coating on the pipe’s outside surface meets the bevel.

While some welding processes or filler metals are more forgiving to dirt or mill scale on the material, don’t rely on the belief that dirt and oil can be burned off during welding. Any foreign material in the weld can cause problems later.

Part Fit-up and Tacking

Proper part fit-up ensures that the joint is set uniformly from start to end, resulting in weld consistency throughout the part. It helps prevent problems with lack of penetration or too much penetration, issues that can decrease the service life of the finished weld.

Part fit-up involves several key steps, including matching the inside diameter (ID) of the two pieces of pipe to be welded and making sure you have a proper gap between the pipes for the root pass.

To match the pipes’ IDs, first take measurements to ensure everything will line up properly, then do some dry runs before actually attaching the pipe with tack welds to establish the gap. The IDs should be as close as possible to perfectly aligned. If the IDs are not aligned, use a grinder on the inside of the pipe to match up the diameters. Mismatched IDs are more common with large-diameter pipes.

A welder inspects the consistency of the inside diameter, ensuring the IDs are consistent between the two pipes. Mismatched pipe IDs can cause various welding defects.

Establishing the gap, or root opening, comes next. The welding process being used and the qualified weld procedures determine what type of land the root face needs. Some welds, especially for gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW), may require a 371/2-degree bevel with a knife edge or a 1/16-in. land for easier fusion of the root pass. Shielded metal arc welding (stick, or SMAW), which has a stiffer arc (that is, more forceful and penetrating), often requires a heavier land. The land on the root face can range from a knife edge up to 1/8 in. in some applications.

Qualified procedures should specify the proper width of the root gap, which depends on the welding process, joint geometry, and material type and thickness. The gap may be as tight as 1/16 in. or up to 5/32 in. SMAW typically requires a tighter gap, while GMAW is more forgiving and allows a larger gap. The root gap should be large enough to allow the molten weld pool to fill it, but small enough so that the weld puddle doesn’t fall through.

The root gap should be uniform from beginning to end. Inconsistent gaps will affect quality and consistency throughout the entire weld. To measure and ensure proper gap fit-up, use a gap rod or a piece of filler rod that matches your desired gap size.

Once you establish a consistent gap, tack the pipes together for welding. On a large pipe, tack welds may be 1 in. long or even longer; on a small pipe, they may be between ¼ and ½ in. long. The number of tacks needed depends on the pipe diameter. Small pipes often can be welded with three tacks, while larger pipes may require four or more. The more tacks used on larger-diameter pipes, the less likely it is the gaps will shrink as they cool after welding, causing the joint to close up. No matter how many tacks are used, be sure they are evenly spaced.

Also make sure the tacks are clean on the inside. Whether tacks are cut out during welding is a matter of operator preference. They can be left in and feathered into the weld. In this case, grind each tack to a feathered edge (that is, with a smooth connection between the tack weld bead and the joint wall) before performing your root pass weld. This will ensure that you consume the tacks when completing the root pass. After welding, take care to inspect the tacks to ensure there are no defects or inclusions.

Common Mistakes

Spending time and money on rework caused by weld failure or poor aesthetics is the consequence of sloppy weld preparation. To get the best results, avoid these common mistakes:

  • Rushing part fit-up can result in part misalignment. It’s common to see a bevel with a too steep angle, which results in poor base metal penetration.
  • Establishing too much land when pipe SMAW can make it difficult to get proper penetration in the root pass. In general, don’t exceed a 1/8-in. land when stick welding.
  • Closing the root gap too much is a common mistake when welding large pieces of pipe, such as those 24 to 30 in. in diameter. In laying down the root pass, if you close in on the root too much, the gap will start to shrink. In applications that start off with a tight gap, closing the gap too much may require cutting it open again to complete the root pass. This is non-value-added time that requires extra labor, wastes consumables, and increases costs.
  • Not properly cleaning off lubricating oil, dirt, paint, or lacquer from the base material can result in hydrogen inclusions and cracking. Improper cleaning practices can also cause porosity. Using a grinder or buffing wheel is the fastest way to properly clean the weld area. Make sure to clean the joint itself and the entire area 1 to 2 in. back from the joint to prevent foreign materials from creeping into the weld.
  • Not following the weld preparation requirements set out in the welding procedure specification (WPS) may seem like a time-saving opportunity, but it can lead to significant time and money spent later in rework and failed welds. Before starting the process, get familiar with the WPS for the application. It typically specifies the proper bevel angle, land size, root gap, and other factors.

Better Prep Saves Time

Pipe welding applications require quality and consistency. Proper cleaning, joint preparation, and part fit-up help ensure access to the joint and the necessary penetration and weld strength. Taking the time to follow specified procedures and ensure proper weld preparation can save significant time and money later, and ultimately improve productivity of the entire operation.

About the Author

Dan Hernandez

Regional Sales Specialist

1635 W. Spencer St.

Appleton, WI 54912

920-734-9821