Multipass Welding in Pressure Vessel Applications

Multipass welding increases critical weld strength in applications where failure could be catastrophic, including heavy-duty structural fabrication and pipelines and pressure vessel fabrication. Proper preparation and cleaning are critical for attaining quality welds.

Multipass Welding Challenges

The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section VIII, guides the process of pressure vessel welding. It details requirements as they apply to the design, fabrication, inspection, testing, and certification of fired or unfired pressure vessels, specifically those operating at internal or external pressures exceeding 15 lb/sq in. Inspection of these welds often uses hydrostatic or x-ray testing.

The AMSE Section VIII guides the process of pressure vessel welding, detailing requirements as they apply to the design, fabrication, inspection, testing, and certification of fired or unfired pressure vessels.

Because pressure vessel welds must meet code requirements and pass inspection, the welds must be free of defects. The more weld passes required — such as for large-diameter workpieces or thicker materials — the harder it is to make every pass look seamless. There is also a greater risk for weld defects that require rework.

Not only does rework and its associated downtime cost time and money but too many failed welds could also result in a fabrication shop losing its ASME U Stamp. This stamp certifies that the shop is producing ASME-compliant pressure vessels.

Proper cleaning between weld passes is a key step to removing slag produced from flux cored arc welding, which is commonly used in pressure vessel fabrication. It can also help reduce the risk of slag inclusions and weld defects reaching the inspection stage.

Tips for Overcoming Common Mistakes

-Pay attention to part fitup. The first step is to clean both surfaces of the joint to prepare them for welding. Proper joint preparation and fitup help ensure good weld quality and consistent root penetration.

-Watch the root pass. The root pass is often where a welding operator has the most stops and starts, and these stops and starts are typically much thicker than the root bead itself.

For this reason, it is important to grind down the root bead to ensure there are no defects and to prepare for the second weld bead. If there is a defect, grinding it down thin enough often means it can be repaired in the second pass.

The root pass is often where a welding operator has the most stops and starts. It’s important to properly grind the weld after the root pass to ensure there are no defects and prepare for the second weld bead.

-Clean properly between passes. Leaving in slag or discontinuities and simply welding over them with the next pass makes it harder to produce a quality weld on each subsequent pass. With every slag inclusion, the likelihood of passing inspection decreases. Improper weld cleaning can cause many headaches, so it’s important to pay close attention to detail with each weld pass. Sometimes reaching the weld to clean it can be difficult if space is tight or it is at a difficult angle. Abrasive products come in numerous shapes and sizes, including some designed specifically for hard-to-reach areas.

-Stay within code parameters. For many pressure vessel applications, the code being followed dictates a range of parameters for making a successful weld. Even if the operator is having a hard time making a weld under those parameters — perhaps the weld is very hot or hard to control — it’s important to stay within those guidelines. For example, turning the parameters too low could result in incomplete fusion or slag inclusions.

-Avoid cross contamination. If mild steel and stainless steel are being welded in the same facility, it’s crucial to take the proper steps to avoid cross contamination between the materials. Store filler metals and consumables separately. When prepping and cleaning stainless steel, be sure to use abrasive products specifically designed for stainless and don’t use a product that has previously been used on mild steel or another material. Otherwise, it can result in cross contamination that can lead to rust.

Abrasives for Multipass Welding

Wire brushes and abrasives play a critical role in the finished weld quality, so it’s important to choose the abrasive product that is most appropriate for the task being completed. Different products are designed for different steps in the preparation and cleaning process for multipass welds.

-Grinding or notching wheels. These products can be used for many steps in multipass weld cleaning, including grinding out the tacks used for part fitup and cleaning the base material. An 1⁄8-in. grinding wheel is a good choice to grind the root bead once it’s completed. Specifically designed pipeline wheels made by some manufacturers are ideal for grinding the root bead because they are designed to grind hot welds without glazing or wheel loading. Operators don’t need to chip these pipeline wheels to get optimal performance. This improves safety since chipping a wheel results in a safety hazard. Also, some grinding wheels are designed to notch and bevel, which is safer and more efficient. Using a notching-only wheel for grinding can result in unsafe operating conditions.

-Stringer bead brushes and encapsulated brushes. These are often used to clean the weld pass subsequent to the root pass. When grinding after the root pass, slag residues form and are exposed at the ends of the first bead. The next pass is done as soon as possible to burn these residues and flow them out of the bead. That is why notching with a pipeline wheel and surface cleaning with a bead brush must be done quickly. Wider-face versions of these brushes can also be used for intermediate weld passes. Stringer bead brushes have narrow, tightly twisted knots for high-impact cleaning and aggression. The narrow profile (face) and aggressive cleaning make them popular for cleaning in narrow gaps between weld passes.

Encapsulated wheels, which feature a heat-stabilized encapsulation that gradually wears away to expose a consistent short trim, provide higher aggression, controlled brushing action, and longer life, making them ideal for cleaning hot welds. Note: All encapsulated wire wheels are not heat stabilized, so be sure to confirm that the selected wheel is heat stabilized.

-Filler brushes. After cleaning the second weld bead, the remaining joint must be filled with additional weld passes and cleaned with a wire brush for all subsequent passes. The number of weld passes needed depends on the joint size and material thickness. To ensure optimal joint penetration, a perfectly clean surface is required prior to applying each weld bead. Root pass brushes or encapsulated brushes can be used to clean intermediate weld passes, but a filler pass brush (sometimes referred to as a cable twist brush) is a better choice for wider welds. Filler brushes have additional wire strands and a wider profile for aggressive brushing and increased durability. Their wider contact area makes them an excellent choice for cleaning intermediate weld passes quickly and efficiently.

-Cones and plugs. These abrasive products can be used to clean in hard-to-reach areas or small spaces, such as cleaning and prepping holes or nozzles.

Safety and Efficiency in Multipass Welding

In pressure vessel welding, each weld pass must be thoroughly cleaned to avoid weld inclusions and defects to help ensure the weld meets stringent quality standards. Choosing the wrong abrasive product for the job not only increases potential safety risks but also slows the entire process because continually changing over the disc or wheel wastes time.

For multipass welding, as completed on this ammonia re-circulating vessel for industrial refrigeration, each pass must be thoroughly cleaned to meet stringent quality standards.

Using the proper abrasive product designed for the job helps operators finish work quickly and safely so less time is spent grinding and cleaning and more time is spent welding.

This article was written by David Jescovitch (abrasives specialist manager of Weiler Abrasives, Cresco, Pa.) and John Limberis (welder at H.A. Phillips & Co., DeKalb, Ill.) for the American Welding Society.