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.
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 misalign-
ment. 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.
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.
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.
Dan Hernandez, CWI, CWE, is regional sales
specialist at Miller Electric Mfg. Co., 920-734-9821,