Multipass Welding in Pressure Vessel Applications Requires Proper Prep and Cleaning
By David Jescovitch, abrasives specialist manager, Weiler Abrasives, and John Limberis, welder, H.A. Phillips & Co.
Multipass welding increases critical weld strength in applications where failure could be catastrophic, including heavy-duty structural fabrication, pipelines and pressure vessel fabrication.
Many of these welds must meet stringent code requirements and pass tests before they are put into service, making it crucial to maintain high quality standards while still focusing on safety and productivity.
To optimize results in multipass welding for pressure vessel fabrication, it’s important to start with a clean base and to make each weld pass as even as possible. Following some best practices for cleaning multi-pass welds and choosing the right abrasives can help save time, improve safety and ensure quality results.
Multipass welding challenges
ASME Section VIII – Boiler & Pressure Vessel Code 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 pound-force per square inch (PSIG). Inspection of these welds often uses hydrostatic or x-ray testing.
Because pressure vessel welds must meet code requirements and pass inspection, the welds need to 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 opportunity 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 their 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 remove slag from the commonly used
flux-cored arc welding (FCAW) 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
Some common mistakes can hinder quality and efficiency in multipass welding. Less-experienced welders may be especially prone to these missteps. Consider these tips to help optimize results.
- 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 helps ensure good weld quality and consistent weld 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 pass itself. For this reason, it important to grind down the root pass and ensure there are no defects in the root. If there is a defect, grinding it down thin enough often means it can be repaired in the hot pass.
- 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 be the cause of many headaches, so it’s important to pay close attention to detail with each weld pass. Sometimes it can be tough to reach the weld to clean it if space is tight or it’s 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 parameters. Turning them too low, for example, could result in lack of 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 to grind out the tacks used for part fitup and to clean the base material. A 1/8-inch grinding wheel is a good choice to grind the root pass once it’s completed. Specifically designed pipeline wheels by some manufacturers are ideal for grinding the root pass 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, which improves safety since chipping a wheel result 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 hot pass. When grinding after the root pass, slag residues form and are exposed at the ends of the first bead. The hot 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 filler 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 the selected wheel is heat stabilized.
- Filler brushes: After cleaning the hot pass, the remaining joint must be filled with additional weld passes. The number of weld passes needed depends on the joint size and material thickness. To ensure optimal weld penetration, a perfectly clean surface is required prior to applying each bead. Root pass brushes or encapsulated brushes can be used to clean filler 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 filler passes quickly and efficiently.
- Cones and plugs: These abrasive products can be used to clean in harder to reach areas or small spaces, such as for cleaning and prepping holes or nozzles.
Safety and efficiency in multipass welding
In pressure vessel welding, each 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 it also slows down the entire process, since continually changing over the disc or wheel wastes time.
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.
Republished from Welding Journal (December 2021) with permission from American Welding Society (AWS).
David Jescovitch, Abrasives Specialist Manager
